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In-situ stress measurements by hydraulic fracturing and its implication on coalbed methane development in Western Guizhou, SW China
Journal of Unconventional Oil and Gas Resources 15 (2016) 1–10
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In-situ stress measurements by hydraulic fracturing and its implication on coalbed methane development in Western Guizhou, SW China Hongjie Xu a, Shuxun Sang b,⇑, Jingfen Yang a, Jun Jin c, Youbiao Hu a, Huihu Liu a, Ping Ren c, Wei Gao c a
School of Earth Science and Environmental Engineering, Anhui University of Science and Technology, Huainan 232001, China School of Resources and Geoscience, China University of Mining and Technology, Xuzhou 221008, China c Guizhou Engineering Research Center for Coalbed Methane (CBM) and Shale Gas, Guiyang 550008, China b
a r t i c l e
i n f o
Article history: Received 11 January 2016 Revised 22 March 2016 Accepted 19 April 2016 Available online 5 May 2016 Keywords: In-situ stress measurement Hydraulic fracturing Coalbed methane Permeability Pore pressure
a b s t r a c t Based on data independently measured and collected within depth from 135.9 to 1243.6 m in Western Guizhou, SW China, the distribution of in-situ stress was analyzed systematically. Maximum horizontal principal stress (rHmax), minimum horizontal principal stress (rHmin), vertical stress (rv) and lateral pressure ratio variations with depth were obtained by regression analysis. Results show that the growth rate of horizontal stresses is higher than that of vertical ones. Three types of stress field distribution have been noted that rv P rHmax P rHmin mainly occurs in shallow and intermediate to deep coal seams (<400 m and 600–1000 m), the rHmax P rv P rHmin mainly occurs in deep and shallow to intermediate coal seams (400–600 m and >1000 m). The ratio of maximum and minimum horizontal principal stress versus depth shows linear relationships with a correlation coefficient of 0.77 and 0.85, separately. The ratio of the maximum horizontal principal stresses to vertical stress is usually between 0.5 and 2.0 in coal seams, and decreases as the depth increases and approaches 1.0. The coefficient of average lateral stress versus depth (k) is also illustrated, which shows a wide range at shallow sites from 0.48 to 1.80, and then gradually decreases to a fixed value as the depth increases. Coal permeability obtained during injection/falloff tests shows that the permeability is damaged with a trend difference under a depth of 550–750 m for the in-situ stress belting change and other reasons. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction China has the world’s third-largest coalbed methane (CBM) resources, behind only to Russia and Canada (Meng et al., 2011). Coalbed methane is an important alternative energy for China, and the development of coalbed methane can also be helpful to avoid coal mine accidents and to reduce the emission of methane. The CBM area within the buried depth of 2000 m in China is 41.5 104 km2 with resources of about 36.8 1012 m3 (Che et al., 2008). According to the report by Guizhou Coal Geology Survey in 1997, there are 3.15 1012 m3 of CBM resources in minable coal seams with more than 4 m3/t coal of CBM content, with an average recoverability of 40.93%, in which the 2.3 1012 m3 of CBM occur in the seam with less than 1500 m of depth. The CBM resources ranks the secondary of the CBM resources in all ⇑ Corresponding author at: Institute of Energy Geology, School of Mineral Resources and Geoscience, China University of Mining and Technology, Sanhuan S. Road, Quanshan District, Xuzhou, Jiangsu 221116, P.R. China. E-mail addresses: [email protected], [email protected] (H. Xu), shuxunsang@ gmail.com (S. Sang). http://dx.doi.org/10.1016/j.juogr.2016.04.001 2213-3976/Ó 2016 Elsevier Ltd. All rights reserved.
provinces of China, and accounts for 76.34% of total CBM resources of South China under same evaluating standard (Yi, 1997). China National Development & Reform Committee and Land & Resources Ministry put in the data of new national CBM resources evaluation in 2006. Result showed that there is the 2.88 1012 m3 of CBM resources in Guizhou, accounting for 49.94% of CBM resources in whole South China. The Liupanshui and Zhina coal fields have about 2.23 1012 m3 of CBM resources that accounts for 76% of total CBM resources in Guizhou (Gao et al., 2009). Although the results above mentioned are very various, they indicated basic characteristics with rich CBM resources and high CBM abundance in Guizhou. Coalbed methane reservoir is an unconventional gas reservoir and located at shallow depths, compared to the conventional gas reservoir. The Western Guizhou should be important to the national developing tactic of CBM industry in China. But the coalfield is deposited in a typical multi-coalbed reservoir with complicated reservoir formations featured by many unique CBM geological characteristics. Those particular reservoir properties are of vital significance for CBM development and have not yet been fully understood. Therefore, in-situ stress, permeability and
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H. Xu et al. / Journal of Unconventional Oil and Gas Resources 15 (2016) 1–10
Nomenclature
rHmax rHmin rv
k CBM IFOT UNDP RAHVG
maximum horizontal principal stress minimum horizontal principal stress vertical stress coefficient of average lateral stress versus depth coalbed methane injection fall-off test United Nations Development Programme average horizontal geostresses and the vertical geostresses
pore/reservoir pressure need to be investigated to better understand and develop coalbed methane. Estimation of in-situ stress for coal bearing strata has been applied widely in underground mines, as well as in CBM exploration in many coal basins of USA, China, Australia and Canada (Bell, 2006; Brooke-Barnett et al., 2015; Enever et al., 1999; Gentzis, 2009; Li et al., 2014b). Injection fall-off test (IFOT) technology has been employed for CBM reservoir tests in China since 1992, when the United Nations Development Programme (UNDP) sponsored the first training courses in China. So far, the technique had been successfully used for in-situ stress measurement in China and was widely used (Cai and Peng, 2011; Cai et al., 2008; Li et al., 2014b; Tan and Cai, 2006; Tian et al., 2015). In-situ horizontal stress is a key factor for roof stability in underground coal mines and it also facilitates permeability predicting and fluid flow in CBM reservoirs (Bell and Bachu, 2003; Jasinge et al., 2011; Meng et al., 2011). The vertical stress (generally overburden load data) is significant in the numerical simulation of faulted zones, rock stiffness, and rock falls (Connell et al., 2010; Karacan et al., 2008). The estimation of coal properties, such as cleat volume, cleat spacing, porosity and permeability, along with the in-situ stress, controls coal mining operation and coal mine gas drainage pattern. Moreover, in-situ stress is deeply affected by gravitational and tectonic forces, and is particularly associated with horizontal tectonic movements. A gravitational stress field is relatively simple, which is mainly influenced by the overlying rock mass, while the causes for a tectonic stress field are much more complicated. The tectonic stress is extremely irregular and almost impossible to be described by precise analytical solutions, for it is constant changing as with time (Kang et al., 2010). Accurate prediction of in-situ stress distribution plays a principal role in estimating the production potential for CBM reservoirs, which is closely bound up with the permeable fracture aperture and direction. Permeability is one of the most important factors in determination of CBM productivity. During CBM production or enhanced CBM recovery, the pore pressure is placed in a variation state with a constant and almost simultaneous influence on permeability. To date, it has been generally accepted that coal permeability exponentially declines with an increase in effective stress, and this decline is potentially offset by the permeability enhancement because of matrix shrinkage (Bustin et al., 2008; Connell et al., 2016; Li et al., 2014a; White et al., 2005). Commonly, matrix swelling/shrinkage is accompanied by a volumetric strain in coals, which further impacts the permeability of the coal (Chen et al., 2012; Meng and Li, 2013; Wang et al., 2014). Moreover, the coal permeability is influenced by the transition of in-situ stress magnitude and direction (Li et al., 2014b; Qin, 2012). Focused on CBM development and its relationship with in-situ stress field under different coal burial depth, this study shall generate valuable data for
Pb Ps Po H k
r
k0
a
breakdown pressure closure pressure pore pressure depth below surface permeability (103 lm2) effective stress value changed from initial to some stressed state (MPa) coefficient depending on stress type coefficient depending on stress type
various uses, (a) a combination of injection/falloff and in-situ tests parameters together showing the stress-permeability properties of coal strata, (b) to study the variation of in-situ stresses relative to the depth of coal seams and (c) to generate stress distribution characteristic graphs related to depth, showing the stress field types. This paper, based on well testing data, analyzes the in-situ stress state and relationship of permeability and in-situ stress in the Western Guizhou, SW China. The analysis may be applicable in developing strategies in exploration, well completion and production of coalbed methane.
2. Principles and methods of hydraulic fracturing stress measurement Injection/falloff method was used for the well test and data acquisition, and the wells were tested after completion and before production. To conduct a full cycle of injection/fall off test, water was injected at a constant rate for a period of time, causing a higher pressure distribution near the well bore, and then shut in the well. During both the injection and shut-in periods, the bottom-hole pressure was measured using a down-hole pressure gauge. Through processing pressure-data from both the injection period and the fall off period independently the permeability was estimated. The detailed test procedure has been discussed by Zuber et al. (1990) and Hopkins et al. (1998). The surface facilities for the in-situ stress measurement and a schematic drawing of the measurement system with its main components are presented in Fig. 1a. It can be observed from Fig. 1a that the system utilizes the drilling rods as the only highpressure pipeline to pressurize both the sealing packers and test interval in a borehole. The design of the single-loop system can effectively avoid rotation and twist of the measurement pipelines when the instrument is lowered down to deep boreholes. Rotation and twist of the measurement pipelines can occur in a double-loop hydro-fracturing system. Before the injection/fall off test, a minifrac test was conducted to get the breakdown pressure of the coal (Fig. 1b), which was used as a direct evidence for the maximum injection pressure. To reduce the impact of stress on permeability, the injection pressure at the surface was started from relatively low value, while the maximum injection pressure was kept lower than the fracture pressure in the designed injection time. The flow injection with a constant rate was continued for more than 12 h and then shutted in for more than 24 h to get the pressure falls off (Fig. 1c). The closure pressure was recorded. In order to accurately estimate the closure stress, four pressurization cycles to finish the in-situ stress test was performed with about 6 h (Fig. 1d). The stress test was performed by injecting water at a rate sufficient to create a hydraulic fracture. After the hydraulic fracture has been established (pump time of
H. Xu et al. / Journal of Unconventional Oil and Gas Resources 15 (2016) 1–10
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Fig. 1. Schematic drawing of the used injection fall-off test string of hydro-fracturing system (a), pressure vs. time for short-breakdown test (b), pressure and time curve of injection fall-off (c) and pressure vs. time for in-situ stress test (d) of typical well.
2–3 min), injection was stopped and the bottom hole pressure fall off was recorded for 20–30 min or until the fracture closes. The data acquisition during different test periods was preset for different intervals, ranging from 1 to 30 s. During the whole test process, the formation pressure was higher than the gas desorption pressure and the fracture system was saturated with water, and the methane was held in the coal by water pressure. The well testing interpretation is based on an assumption of coal reservoir saturated condition, and the fluid flow is kept as a single-phase flow, which flows in the cleat system, for the permeability of the coal matrix was far lower than that of the coal cleats. The tested permeability is a combined permeability, dominated by the cleat’s permeability. The gauges (made by Dub Design Industries, Canada) typically capture 140–180 h of data, and were used to ensure the reliable and accurate data. The data of short-breakdown test were obtained with a data point per second, one data point was recorded per five seconds in injection fall-off stage, and in-situ stress test data were recorded per second. Surface pressure and flow rate were captured by a laptop that provided control during the injection cycle. These data were then synchronized with the bottom hole gauge data in order to match the injection and fall-off periods. The tested data were interpretative by Saphir 4.02 software and PanSystem V3.4.0 developed by France ESSCA Group and British EPS company, respectively. There are usually two analysis methods for well testing: straight line analysis and chart board matching analysis. The data from these tested wells were mainly analyzed by the good use of both the methods, combining the semi-log curve analysis (Horner method), log–log curve, straight line and chart match analysis, which have been discussed by Hopkins et al. (1998). One of the necessary data for well test designing is the minimum in-situ stress, i.e. fracture closure pressure. The fracture closure pressure can be acquired through analyzing in-situ pressure fall-off data in the shut-in period of 2–3 cycles selected from the 4 in-situ stress test cycles. The Log–Log method and tine square
root method were included. For the tests in this paper, the time square root method was used in the closure pressure analysis, and the Log–Log method was supplied purpose of comparison. While analyzing the well test data some basic physical data was firstly determined referred to domestic and international data and usage experiences (Hopkins et al., 1998). Then, the necessary parameters such as permeability, formation temperature, breakdown pressure, closure pressure and formation pressure are recorded and calculated. During in-situ stress analysis, the minimum principal stress (rHmin) is considered to be equal to the closure pressure (Haimson and Cornet, 2003).
rHmin ¼ Ps
ð1Þ
Hydro-fracturing is a 2D stress measurement approach only applicable to the determination of the maximum and minimum stresses in the horizontal plane. The vertical stress (rv) is calculated from the overburden weight, as follows (Haimson, 1993):
rv ¼ cH
ð2Þ
where c is the unit weight of the overlying rock, and H is the depth below surface. Hoek (2007) summarized some measurement values of vertical stress at various mining and civil engineering sites around the world, and the data, as illustrated by black line in Fig. 3, show a simple linear expression (i.e. rv = 0.027H) which can be used to estimate the vertical stress when in-situ measurement is not available. The average unit weight of the overlying rock is also calculated by other ways, such as density test at different depth along the boreholes, fitting analogy method, etc. But for this paper, the logging data were lacked. So the fitted value estimated by Hoek (2007) was applied for the vertical stress calculation in this paper. The maximum principal stress (rHmax) can be estimated as (Haimson and Fairhurst, 1969; Zhang and Roegiers, 2010)
rHmax ¼ 3Ps Pb Po
ð3Þ
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H. Xu et al. / Journal of Unconventional Oil and Gas Resources 15 (2016) 1–10
where Ps is the measured average closure pressure, Pb is the pressure recorded at break down in fracture and Po is the pore pressure, which is determined by the measurement of the water level in the borehole. Statistical analysis and regression analysis were adopted to discuss the relationship between in-situ stress, pore pressure and coal permeability.
3. Results and discussion 3.1. Test parameters A total of 60 injection/falloff and in-situ stress test data in 26 boreholes were conducted and collected successfully at depth ranging from 135.9 to 1243.6 m and the data is plotted in Table 1.
Table 1 Injection/falloff and in-situ stress test parameters in Western Guizhou. Sample
Coal seam ID
Depth (m)
K (mD)
Po (MPa)
Ps (MPa)
Pb (MPa)
rHmax (MPa)
rHmin (MPa)
rv (MPa)
Well 1
12 24 15 10 7 3 9 17 19 29 19 19 26 7 132 18 19 3 9 12 171 172 1+3 9 16 271 2 5 6 2 6 23 16 16 4 9 11 4 15 3 4 6 16 27 61 62 27 4 7 3 7 10 4 9 15 20 M29 M51 M73 M78
1133.90 1243.60 1080.00 1139.70 554.24 359.09 408.53 292.22 329.40 426.95 665.02 493.29 628.61 807.89 869.48 341.95 366.94 647.82 712.64 726.63 742.84 752.04 620.49 661.98 739.97 920.71 464.04 502.26 523.35 520.17 577.76 431.38 379.70 736.98 555.42 599.74 645.70 890.41 964.82 135.90 142.78 220.77 325.29 399.94 269.99 280.87 569.90 795.70 826.90 299.86 362.57 383.68 338.07 352.12 408.16 215.25 321.96 368.69 439.51 464.27
0.001 0.006 0.0096 0.002 0.426 0.0173 0.0044 0.0078 0.011 0.013 0.062 0.011 0.201 0.0459 0.0434 0.000723 0.000203 0.000173 0.0081 0.0106 0.0027 0.00052 0.106 0.0934 0.21 0.0437 0.5002 0.3228 0.3 0.1074 0.1682 0.000164 0.0179 0.000514 0.23 0.021 0.043 0.0268 0.0276 1.5621 1.3103 0.01 0.0314 0.0857 0.0607 0.05114 0.061 0.0319 0.0291 0.1758 0.0734 0.0928 0.0874 0.0758 0.0025 0.0133 0.0088 0.0638 0.0706 0.0391
12.89 11.28 12.28 10.13 6.54 3.95 5.27 3.54 3.66 4.57 7.51 4.09 6.37 6.30 6.96 3.93 4.11 10.35 8.66 9.37 8.97 2.08 6.94 7.02 10.11 12.35 2.97 4.41 4.68 5.12 5.69 3.04 2.95 6.86 4.60 5.01 5.20 8.04 8.11 0.72 0.78 1.04 3.43 2.24 2.54 2.59 5.25 5.46 5.54 2.79 4.41 4.68 4.47 3.70 6.45 2.29 4.56 6.31 6.51 5.36
27.21 27.36 23.76 23.93 15.68 10.40 13.33 6.28 8.29 9.56 15.12 11.53 11.53 9.56 13.33 8.20 11.01 15.79 12.95 15.36 20.65 8.07 10.67 11.11 13.14 21.01 8.09 8.75 9.31 8.90 11.75 15.59 8.01 17.56 11.60 16.47 16.48 12.41 17.52 2.14 2.40 5.04 5.41 6.63 4.25 4.25 8.52 11.63 13.76 6.62 6.73 7.17 7.02 8.56 11.19 7.19 9.14 12.58 11.42 11.56
28.35 32.33 29.16 28.49 17.90 11.41 16.92 7.11 10.53 10.19 17.89 13.00 13.00 11.09 13.95 10.28 11.57 19.64 15.21 19.81 22.81 9.73 12.37 13.03 15.77 25.26 8.77 9.28 9.73 9.74 12.81 17.49 10.79 19.31 11.16 16.94 16.80 16.52 21.41 3.00 3.41 5.96 6.65 9.34 5.41 5.81 9.56 14.32 15.74 7.39 7.84 7.94 8.15 9.30 13.56 9.95 10.20 12.91 11.84 13.51
40.39 38.47 29.84 33.17 22.60 15.84 17.80 8.19 10.68 13.92 19.96 17.50 15.22 11.29 19.08 10.39 17.35 17.38 14.98 16.90 30.17 12.40 12.70 13.28 13.54 25.42 12.53 12.56 13.52 11.84 16.75 26.24 10.29 26.51 19.04 27.46 27.44 12.67 23.04 2.70 3.01 8.12 6.15 8.31 4.80 4.35 10.75 15.11 20.00 9.68 7.94 8.90 8.44 12.68 13.56 9.33 12.66 18.52 15.91 15.81
27.21 27.36 23.76 23.93 15.68 10.40 13.33 6.28 8.29 9.56 15.12 11.53 11.53 9.56 13.33 8.20 11.01 15.79 12.95 15.36 20.65 8.07 10.67 11.11 13.14 21.01 8.09 8.75 9.31 8.90 11.75 15.59 8.01 17.56 11.60 16.47 16.48 12.41 17.52 2.14 2.40 5.04 5.41 6.63 4.25 4.25 8.52 11.63 13.76 6.62 6.73 7.17 7.02 8.56 11.19 7.19 9.14 12.58 11.42 11.56
30.62 33.58 29.16 30.77 14.96 9.70 11.03 7.89 8.89 11.53 17.96 13.32 16.97 21.81 23.48 9.23 9.91 17.49 19.24 19.62 20.06 20.31 16.75 17.87 19.98 24.86 12.53 13.56 14.13 14.04 15.60 11.65 10.25 19.90 15.00 16.19 17.43 24.04 26.05 3.67 3.86 5.96 8.78 10.80 7.29 7.58 15.39 21.48 22.33 8.10 9.79 10.36 9.13 9.51 11.02 5.81 8.69 9.95 11.87 12.54
Well Well Well Well
2 3 4 5
Well 6
Well 7 Well 8 Well 9 Well 10 Well 11
Well 12
Well 13
Well 14 Well 15 Well 16 Well 17
Well 18 Well 19 Well 20
Well 21
Well 22 Well 23
Well 24
Well 25 Well 26
Note: Injection/falloff testing data of Well 19 modified from Zhang et al. (2015); Well 22-Well 26 modified from Yang and Qin (2015).
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H. Xu et al. / Journal of Unconventional Oil and Gas Resources 15 (2016) 1–10
The breakdown pressure (Pb) and closure pressure (Ps) are attained in the pressure cycles of in-situ stress test record curves. Pore pressure (Po) includes lithostatic and hydrostatic components, and each of these components is of critical concern in CBM production. CBM production is induced by dewatering, and therefore, lowered hydrostatic pressure is the principal mechanism by which gas flows to the well bore. The three pressures are closely correlated with the CBM reservoir state and production capability. Statistics show that the breakdown pressure, closure pressure and pore pressure of coal seams in Western Guizhou are similar to that in Qinshui Basin of China (Meng et al., 2010), and those parameters show linear relations compared with burial depth as follows: The curve fitting of break down pressure and depth is
Pb ¼ 0:0213h þ 1:6248
ð4Þ
The correlation coefficient is 0.86, with available values N = 60. The break down pressure in increased with depth gradually (Fig. 2a). The closure pressure could be described by the relationship
Ps ¼ 0:0184h þ 1:4215
ð5Þ
where the correlation coefficient is 0.85, which is similar to the coefficient of break down pressure and depth (Fig. 2b). Relationship between break down pressure and closure pressure is fitted as
Pb ¼ 1:129P s þ 0:3079
ð6Þ
The correlation coefficient is 0.98 (Fig. 2c). This relationship can give an estimate of the closure pressure (minimum horizontal stress) in the Western Guizhou, when measured data are not available. The pore pressure (including water and gas pressure) versus depth is also shown and could be written as a linear relationship
Po ¼ 0:0094h þ 0:3798
ð7Þ
The correlation coefficient is 0.84 (Fig. 2d). With the depth increasing, the strata compaction becomes obvious. The measured pore pressure data in Table 1 shows that some pore pressures are higher than the hydrostatic pressures. In this case, these pressures indicate the gas pressure is overpressured compared to the hydrostatic water pressure. At relatively shallow depth, pore pressure
generally is hydrostatic, indicating that a continuous, interconnected column of pore fluid extends form surface to that depth. The compaction disequilibrium usually is the primary mechanism of overpressure generation (Zhang, 2011). The stress distribution type rHmax P rv P rHmin mainly occurs in shallow to intermediate coal seams in Western Guizhou (Table 2). In this case, the high insitu stress state may provide an easy way to generate weak coal reservoir properties, such as widely existed mylonitized coal. Thus, it is possible that the local sealed fluid influenced by high stress may be the main reason for the overpressure.
3.2. In-situ stress varies with depth 3.2.1. Stress regime and vertical belting The relationship of maximum, interim and minimum stress is an important indicator for stress field characteristics. According to the modality and correlative stress state of the fault, Anderson (1951) categorized the in-situ stress as a normal faulting stress regime, a strike-slip faulting stress regime and a reverse (or thrust) faulting stress regime. The normal faulting stress regime means that gravity or vertical stress drives normal faulting, and fault slip occurs when the minimum stress reaches a sufficiently low value. In this stress state, the vertical stress is the greatest principal stress, i.e. rv P rHmax P rHmin. The strike-slip faulting stress regime means that the vertical stress is the intermediate principal stress. In this stress state, one has rHmax P rv P rHmin. The reverse (or thrust) faulting stress regime means that the vertical stress is the least principal stress, i.e. rHmax P rHmin P rv. Yu (1991) categorized in-situ stress as a static stress field, a dynamic field and a quasi-static stress field, based on the relationship of horizontal and vertical stresses. Usually the normal stress regime corresponds to gravitational loading only; strike-slip and thrust regimes involve lateral neotectonic compression. According to Fig. 3, major principal stress, interim principal stress and minor principal stress are of different variation trend in different belts. The depth range was divided into four groups for a clearer analysis of the variation of the three principal stresses with depth (Fig. 3).
30
y = 0.0213x + 1.6248 R² = 0.7401
30 25
pressure (MPa)
pressure (MPa)
35
20 15 10
20 15 10 5
(a)
5
y = 0.0184x + 1.4215 R² = 0.7263
25
0
(b)
0
0
500
1000
0
1500
500
1000
1500
depth (m)
35
14
pore pressure (MPa)
break down pressure (MPa)
depth (m) y = 1.129x + 0.3079 R² = 0.9702
30 25 20 15 10
(c)
5 0 0
10
20
shut-in pressure (MPa)
30
(d)
12 10 8 6 4
y = 0.0094x + 0.3798 R² = 0.7068
2 0 0
500
1000
1500
depth (m)
Fig. 2. Curves fitting the relationship between break down pressure vs. depth (a), closure pressure vs. depth (b), break down pressure vs. closure pressure (c), pore pressure vs. depth (d).
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Table 2 Distribution of in-situ stress in Western Guizhou. Depth (m)
rv P rHmax P rHmin
rHmax P rv P rHmin
rHmax P rHmin P rv
Total
<400 400–600 600–800 800–1000 >1000 Total
9 (42.9%) 4 (25.0%) 9 (69.2%) 5 (83.3%) 0 (0) 27 (45.0%)
7 (33.3%) 9 (56.3%) 3 (23.1%) 1 (16.7%) 4 (100%) 24 (40.0%)
5 (23.8%) 3 (18.8%) 1 (7.7%) 0 (0) 0 (0) 9 (15.0%)
21 16 13 6 4 60
Note: Values within parentheses indicate the percentage of the magnitude of the three principal stresses for each depth range.
(4) There are 6 sites in the 800–1000 m depth range; at 1 sites, rHmax is larger than rv, and at 5 sites, rv, is larger than rHmax, accounting for 16.7% and 83.3% of the total sites, respectively. At these sites, the rates of rHmax and rHmin increase with depth are reduced continuously. Most of the 800– 1000 m test sites are below the regression lines in Fig. 3. (5) There are 4 sites below a depth of 1000 m; at all sites, rHmax is larger than rv, and rv, is larger than rHmin. Nearly all the sites are below the regression lines except one site in Fig. 3. Table 2 illustrates the types of in-situ stress state and shows the stress distribution type rv P rHmax P rHmin mainly occurs in shallow and intermediate to deep coal seams, and the type rHmax P rv P rHmin mainly occurs in deep and shallow to intermediate coal seams. For deep coal seams under strong influence of geological structures, the rHmax P rv P rHmin type can also occur (Kang et al., 2010). 3.2.2. The ratio of maximum and minimum horizontal principal stress versus depth Analyzing the measured maximum horizontal stress data, we obtain that the maximum horizontal stress and the burial depth has the following relationship in the Western Guizhou, as shown in Fig. 3. Fig. 3. Values of in-situ principal stresses vs. depth in coal districts. Red and purple lines are straight lines obtained by least square fitting for minimum and maximum principal stresses, black line is vertical stress calculated form weight of voerburden. In the figure, commonly assumed minimum horizontal stress (70% of the vertical stress magnitude, OBP) is also showed with a red line. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
(1) There are 21 test sites in the shallow coal formations (<400 m); at 12 sites, rHmax is larger than rv, and at 9 sites, rv, is larger than rHmax, accounting for 57.1% and 42.9% of the total sites, respectively. At these sites, rHmax and rHmin increase faster than rv with depth. Nearly all the test sites are above the red line (minimum horizontal principal stress) and purple line (maximum horizontal principal stress) in Fig. 3. The red and purple lines were obtained by least square straight-line fitting for the minimum and maximum horizontal principal stresses. (2) There are 16 test sites in the 400–600 m, at 12 sites, rHmax is larger than rv, and at 4 sites, rv, is larger than rHmax, accounting for 75.0% and 25.0% of the total sites, respectively. At these sites, the rates of rHmax and rHmin increase with depth are reduced. Nearly a half of values of rHmin and most of rHmax test sites from 400 m to 600 m are above the regression lines in Fig. 3. (3) There are 13 sites in the 600–800 m depth range; at 4 sites, rHmax is larger than rv, and at 9 sites, rv, is larger than rHmax, accounting for 30.8% and 69.2% of the total sites, respectively. At these sites, the rates of rHmax and rHmin increase with depth are also reduced. Most of the 600–800 m test sites are below the regression lines in Fig. 3.
rHmax ¼ 0:0246h þ 2:2598
ð8Þ
The correlation coefficient is 0.77. The minimum horizontal stress is a primary control on the fracture gradient and a major constraint on propagation of hydraulic fractures. Therefore, the minimum horizontal stress is a key parameter in designs of well drilling and reservoir stimulation. The minimum horizontal stress is commonly assumed to be approximately 70% of the vertical stress magnitude in sedimentary basins. The red line and blue line in Fig. 3 present the minimum horizontal stress measured from the hydraulic fracture tests and 70% of the vertical stress magnitude in the coal seams of the Western Guizhou. It shows that the blue line cannot fairly describe the minimum stress in the coal seam. The red line is above the blue line when the depth is over 400 m. Especially when the depth is deeper than 1000 m, the measured minimum stresses are higher than the 70% of the vertical stress magnitude. The measured minimum horizontal stress can be equivalent to the closure pressure, so the Eq. (5) indicates the relationship of the minimum horizontal stress and depth. This relationship can also give an estimate of the minimum horizontal stress in Western Guizhou, when measured data are not available. 3.2.3. The ratio of maximum horizontal principal stress with vertical stress versus depth The ratio of the maximum horizontal principal stress to vertical stress rHmax/rv, is of great significance representing a key characteristic of in-situ stress distribution. As shown in Fig. 4, the ratio decreases as the depth increases and approaches 1.0, though the ratio is of great difference in different burial depths, implying that hydrostatic pressure may occur in deep strata.
H. Xu et al. / Journal of Unconventional Oil and Gas Resources 15 (2016) 1–10
σHmax/σv 0
0.5
1
1.5
2
Brown and Hoek (1978) summarized in-situ stress measurement results from around the world and found that the ratio of average horizontal to vertical stress, k, generally lies within limits defined by:
2.5
0
depth (m)
200
100 1500 þ 0:3 6 k 6 þ 0:5 H H
400 600
800 þ 0:4 H
1000
k¼
1200
It can be found that k has a linear relation with the reciprocal of the depth, i.e. when x = 1/H, we have
Fig. 4. Ration of maximum horizontal principal stress to vertical stress vs. depth.
There is only 1 site with a rHmax/rv greater than 2.5, 9 sites with a rHmax/rv in the range of 1.5–2.0, these sites are located in the relatively shallow coal seams. 27 sites with a rHmax/rv in the range of 0.5–1.0 are found, as shown in Fig. 4. The rHmax/rv at all the other 23 sites are in the range of 1.0–1.5. It can be summarized that the rHmax/rv in different coals are predominantly within the range of 0.5–2.0. 3.2.4. Coefficient of average lateral stress versus depth The average horizontal principal stress is the average magnitude of maximum and minimum horizontal principal stresses. The coefficient of average lateral stress, k, the ratio of the average horizontal principal stress to the vertical stress, varies in different areas of the world, with the values generally range between 0.5 and 5 and most of them distributed from 0.8 to 1.5 (Han et al., 2012). The k value versus depth in Western Guizhou is plotted in Fig. 5. It can be seen that the ratio shows a wide change at shallow sites, and then gradually decreases to a fixed value as the depth increases. The measured results in the Western Guizhou show that the k ranges generally from 0.48 to 1.80 with an average value of 0.94. k is defined as follows:
k ¼ ðrHmax þ rHmin Þ=2rv
ð9Þ
rao/λ 0.5
1
1.5
0 200 400
depth (m)
ð10Þ
The average value of k can be deduced as follows:
800
1400
0
7
600 800
2
k ¼ ax þ b
ð11Þ
ð12Þ
where a and b are fitting coefficients and H is the depth below surface, and a, b can be obtained from the linear regression analysis of the stress measurement results. After calculation. a = 12.37 and b = 0.9152, then Eq. (11) can be written as:
k¼
12:37 þ 0:9152 H
ð13Þ
As shown in Fig. 5, k decreases gradually with the increasing depth, and rate of the decrease reduces simultaneously. The blue curve is the characteristic curve of the left part of Eq. (10) and is called the Hoek–Brown outer envelope; the red curve is the characteristic curve of Eq. (11) and is called the Hoek–Brown median envelope; and the yellow curve is the characteristic curve of Eq. (13). It is clear that the yellow curve is located between the other two characteristic curves, and its variation trend is similar to that of the blue curve. Comparing Eqs. (10) with (11) and (13), the relationship between k and H obtained from underground coal seams is similar to the Southern Qinshui basin (Meng et al., 2011) and Liulin area, eastern Ordos basin in China (Li et al., 2014b), and also to Brown and Hoek (1978) result in the general trend, even though there are significant differences in magnitude values. Brown and Hoek (1978) results were based on stress data records not only from sedimentary rocks but also from magmatic and metamorphic rocks with a depth of 2806 m. The maximum value (k = 5.56) with values greater than 3 are all located in granite and gneiss. The value of k from sedimentary rocks, such as siltstone, shale, mudstone and sandstone, mentioned by Brown and Hoek, are between 0.85 and 2.56 with the most less than 2, which are close to that of coal seams in this study. Furthermore, the ratios of the average horizontal geostresses and the vertical geostresses (RAHVG) curve and the maximum envelope curve as well as the minimum envelope curve of the sedimentary rock varying with depth in China were obtained following the Brown and Hoek’s method by Zhao et al. (2007). The ratio of average horizontal to vertical stress, k, of China sedimentary rock, generally lies within limits defined by:
1000
41 673 þ 0:4 6 k 6 þ 1:3 H H
1200
The average value of k of China sedimentary rock can be deduced as follows:
1400 Hoek-Brown outer envelope Hoek-Brown median envelope Median envelope of sedimentary rock in China Minimum envelope of sedimentary rock in China Median envelope in Western Guizhou Maximum envelope of sedimentary rock in China Fig. 5. Coefficient of the average lateral stress variation with depth.
k¼
104 þ 0:9 H
ð14Þ
ð15Þ
The curve of outer and inner envelope characterized by Eq. (14) and median envelope characterized by Eq. (15) are shown by black line, purple line and green line in Fig. 5, respectively. It clear that the yellow line is between the black line and green line, also located closely at the left of the green line, and has a similar variation trend with the green line.
8
H. Xu et al. / Journal of Unconventional Oil and Gas Resources 15 (2016) 1–10
2
1.5 1 0.5
2
(b)
1.5
permeability (mD)
(a)
permeability (mD)
permeability (mD)
2
1 0.5 0
0 0
20
40
σv (MPa)
(c) 1.5 1 0.5 0
0
10
20
30
40
50
0
10
20
30
σHmin (MPa)
σHmax (MPa)
Fig. 6. Correlation between the coal permeability and in-situ stress, including rv (vertical stress), rHmax (maximum horizontal principal stress), and rHmin (minimum horizontal principal stress).
permeability (mD) 0
0.5
1
1.5
2
0 200
depth (m)
But there is no tested value when the coal seam depth reaches 1500 m in this study, further tests will be needed to investigate the in-situ stress distribution in the deep coal seams. Moreover, the data obtained by Brown and Hoek’s derived from many countries worldwide with different geological environments, tectonic movements and rock types. But the data collected in the study area is quite small, and the relationship between vertical stress and horizontal stress is complex, especially in shallow coal seams, as shown in Table 2 and Fig. 5.
400 600 800 1000
3.3. In-situ stress influence on permeability
1200
Permeability is not a property of coal but a condition that is affected deeply by the function of stress (Meng and Li, 2013; Wang et al., 2014). Under normal virgin conditions the permeability of solid coal to gases is very low, and significant gas flow rates only occurs in seams with an open cleat or fracture network. Permeability, as well as CBM productivity, correlates strongly with the magnitude of the stress acting on the rocks. The in-situ stress magnitude is meaningful indicator of coal permeability. Regression analysis has been conducted and revealed the best fit exponential relationship between coal seam permeability and stress (Somerton et al., 1975). As shown in Eq. (16):
1400
k ¼ k0 ear
ð16Þ
where k refers to permeability (10 lm ), r is the effective stress value changed from initial to some stressed state (MPa). k0 and a presents coefficients depending on stress types, among which k0 refers to the permeability on the initial stress conditions, where the unit is 103 lm2. This permeability and an in-situ stress relationship fit the data in some coalbed basins in the world, such as the Bowen Basins in Australia (Enever et al., 1999), the Black Warrior Basin, Alabama in the U.SA. (Sparks et al., 1995). Although the relationship between permeability of a coal reservoir and different horizontal principal stresses is similar, there is no obvious correlation of the two parameters in Western Guizhou (Fig. 6). Fig. 6 demonstrates that a higher in-situ stress corresponds to a lower permeability because of a three-dimensional compressive stress state. But the permeability changes markedly as the in-situ stress increases in Western Guizhou. This is mainly induced by the high effective stress in the deep reservoir, which causes the reduction in cleat apertures. The effective stress-dependent permeability is significant for dual-porosity and dual-permeability porous media. The reason is that a rapid change in an effective stress can induce the closure in cleats, which may cause the cleats to lose permeability permanently (Meng et al., 2011). But it seems to be no obvious relationship between permeability with stress, when vertical stress and maximum horizontal principal stress are less than 15–20 MPa, and the minimum horizontal 3
2
Fig. 7. Coal permeability characteristics versus depth of the study area.
principal stress is less than 10–15 MPa, corresponding to a depth of 550–750 m (Fig. 7). This is the fact that the stress state is complex in Western Guizhou. The permeability magnitudes range mostly from 0.00173 to 0.21 mD with an average of 0.0519 mD in Western Guizhou, when the burial depth is 600–1000 m with a state of normal faulting stress regime (refer to Table 2). In this state, the permeability decreases exponentially with an increasing stress and burial depth. But, when the burial depth is between 400 m and 600 m, the vertical stress is the intermediate principal stress (refer to Table 2), which means the strike-slip faulting stress regime. This indicates that the strike-slip faulting stress regime has the higher compressive stress than the state of normal faulting stress regime. Moreover, when the depth is less than 400 m, the stress state translates into the normal faulting stress regime again (refer to Table 2). Especially, the other possible reasons for the large variability of permeability values in shallow depth are that the different coal districts are influenced by tectonic movements and geological environments with different coal reservoirs characteristics. This reason could also be responsible for the very wide range of permeability values when the depth is less than 550–750 m. 4. Conclusions In response to the CBM development in Western Guizhou, hydraulic fracturing tests were carried out for permeability and in-situ stress measurements in CBM wells. A data quality assurance system was also developed to ensure the quality and reliability of test data. About 60 high quality data of injection/falloff and in-situ stress in 26 CBM wells were collected and used to investigate the distribution characteristics of in-situ stresses and coal reservoir permeability. The following conclusions can be drawn. (1) In Western Guizhou, the breakdown pressure, closure pressure and pore pressure of coal seams tested during
H. Xu et al. / Journal of Unconventional Oil and Gas Resources 15 (2016) 1–10
injection/fall and in-situ stress tests are positively correlated with the coal burial depth, of which the break down pressure versus depth shows the highest related coefficient. (2) The rv P rHmax P rHmin stress field type appears at relatively shallow and intermediate to deep coal seams with a widespread distribution, and the rHmax P rv P rHmin type usually appears in deep and shallow to intermediate coal seams. For deep coal seams under strong influence of geological structures, the rHmax P rv P rHmin type can also occur. (3) The ratio of maximum and minimum horizontal principal stress versus depth tends to increase with a good correlation coefficient. The ratio of maximum horizontal to vertical stress tends to decrease and approaches 1.0 with increasing depth. The ratio of average horizontal principal stress versus depth in Western Guizhou ranges from 0.48 to 1.80 and decrease gradually with depth, also having a trend approaching 1.0. The relationship between depth and coefficient of average lateral stress versus depth (k), can be expressed as 0.9152 + 12.37/H, the characteristic curve of which is located between the Hoek–Brown outer envelope and the Hoek–Brown median envelope, and located at the right of the outer envelope of Chinese sedimentary rock. The variation trend of the characteristic curve is similar to that of Hoek–Brown outer envelope and median envelope of sedimentary rock in China. (4) Coal reservoir permeability decreases markedly as the in-situ stress increases in moderately deep to deep coal seams in the Western Guizhou. This is mainly induced by the high effective stress in the deep reservoir, which causes the reduction in cleat apertures. There is no obvious relationship between permeability with stress, when vertical stress and maximum horizontal principal stress are less than 15–20 MPa, and the minimum horizontal principal stress is less than 10-15 MPa, corresponding to a depth of 550–750 m. This is maybe caused by stress regime and vertical belting, and other reasons like tectonic movements and geological environments with different coal reservoirs characteristics.
Acknowledgements The research work was supported by National Natural Science Foundation of China under Grant Nos. 41330638, 41402140, 41272154, 41302129, 41372167, 41302125 and Natural Science Foundation of Anhui Province under Grant No. 1408085QE88. References Anderson, E.M., 1951. The Dynamics of Faulting and Dyke Formation with Application to Britain. Oliver and Boyd, London. Bell, J., 2006. In-situ stress and coal bed methane potential in Western Canada. Bull. Can. Pet. Geol. 54, 197–220. Bell, J., Bachu, S., 2003. In situ stress magnitude and orientation estimates for cretaceous coal-bearing strata beneath the plains area of central and southern Alberta. Bull. Can. Pet. Geol. 51, 1–28. Brooke-Barnett, S., Flottmann, T., Paul, P.K., Busetti, S., Hennings, P., Reid, R., Rosenbaum, G., 2015. Influence of basement structures on in situ stresses over the Surat basin, southeast Queensland. J. Geophys. Res.: Solid Earth 120, 4946– 4965. Brown, E.T., Hock, E., 1978. Trends in relationships between measured in situ stresses and depth. Int. J. Rock Mech. Geomech. Abstr. 15, 211–215. Bustin, R.M., Cui, X., Chikatamarla, L., 2008. Impacts of volumetric strain on CO2 sequestration in coals and enhanced CH4 recovery. AAPG Bull. 92, 15–29. Cai, M., Peng, H., 2011. Advance of in-situ stress measurement in China. J. Rock Mech. Geotech. Eng. 3, 373–384. Cai, M., Peng, H., Ji, H., 2008. New development of hydraulic fracturing technique for in-situ stress measurement at great depth of mines. J. Univ. Sci. Tech. Beijing Miner. Metall. Mater. 15, 665–670.
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Regular Articles
In-situ stress measurements by hydraulic fracturing and its implication on coalbed methane development in Western Guizhou, SW China Hongjie Xu a, Shuxun Sang b,⇑, Jingfen Yang a, Jun Jin c, Youbiao Hu a, Huihu Liu a, Ping Ren c, Wei Gao c a
School of Earth Science and Environmental Engineering, Anhui University of Science and Technology, Huainan 232001, China School of Resources and Geoscience, China University of Mining and Technology, Xuzhou 221008, China c Guizhou Engineering Research Center for Coalbed Methane (CBM) and Shale Gas, Guiyang 550008, China b
a r t i c l e
i n f o
Article history: Received 11 January 2016 Revised 22 March 2016 Accepted 19 April 2016 Available online 5 May 2016 Keywords: In-situ stress measurement Hydraulic fracturing Coalbed methane Permeability Pore pressure
a b s t r a c t Based on data independently measured and collected within depth from 135.9 to 1243.6 m in Western Guizhou, SW China, the distribution of in-situ stress was analyzed systematically. Maximum horizontal principal stress (rHmax), minimum horizontal principal stress (rHmin), vertical stress (rv) and lateral pressure ratio variations with depth were obtained by regression analysis. Results show that the growth rate of horizontal stresses is higher than that of vertical ones. Three types of stress field distribution have been noted that rv P rHmax P rHmin mainly occurs in shallow and intermediate to deep coal seams (<400 m and 600–1000 m), the rHmax P rv P rHmin mainly occurs in deep and shallow to intermediate coal seams (400–600 m and >1000 m). The ratio of maximum and minimum horizontal principal stress versus depth shows linear relationships with a correlation coefficient of 0.77 and 0.85, separately. The ratio of the maximum horizontal principal stresses to vertical stress is usually between 0.5 and 2.0 in coal seams, and decreases as the depth increases and approaches 1.0. The coefficient of average lateral stress versus depth (k) is also illustrated, which shows a wide range at shallow sites from 0.48 to 1.80, and then gradually decreases to a fixed value as the depth increases. Coal permeability obtained during injection/falloff tests shows that the permeability is damaged with a trend difference under a depth of 550–750 m for the in-situ stress belting change and other reasons. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction China has the world’s third-largest coalbed methane (CBM) resources, behind only to Russia and Canada (Meng et al., 2011). Coalbed methane is an important alternative energy for China, and the development of coalbed methane can also be helpful to avoid coal mine accidents and to reduce the emission of methane. The CBM area within the buried depth of 2000 m in China is 41.5 104 km2 with resources of about 36.8 1012 m3 (Che et al., 2008). According to the report by Guizhou Coal Geology Survey in 1997, there are 3.15 1012 m3 of CBM resources in minable coal seams with more than 4 m3/t coal of CBM content, with an average recoverability of 40.93%, in which the 2.3 1012 m3 of CBM occur in the seam with less than 1500 m of depth. The CBM resources ranks the secondary of the CBM resources in all ⇑ Corresponding author at: Institute of Energy Geology, School of Mineral Resources and Geoscience, China University of Mining and Technology, Sanhuan S. Road, Quanshan District, Xuzhou, Jiangsu 221116, P.R. China. E-mail addresses: [email protected], [email protected] (H. Xu), shuxunsang@ gmail.com (S. Sang). http://dx.doi.org/10.1016/j.juogr.2016.04.001 2213-3976/Ó 2016 Elsevier Ltd. All rights reserved.
provinces of China, and accounts for 76.34% of total CBM resources of South China under same evaluating standard (Yi, 1997). China National Development & Reform Committee and Land & Resources Ministry put in the data of new national CBM resources evaluation in 2006. Result showed that there is the 2.88 1012 m3 of CBM resources in Guizhou, accounting for 49.94% of CBM resources in whole South China. The Liupanshui and Zhina coal fields have about 2.23 1012 m3 of CBM resources that accounts for 76% of total CBM resources in Guizhou (Gao et al., 2009). Although the results above mentioned are very various, they indicated basic characteristics with rich CBM resources and high CBM abundance in Guizhou. Coalbed methane reservoir is an unconventional gas reservoir and located at shallow depths, compared to the conventional gas reservoir. The Western Guizhou should be important to the national developing tactic of CBM industry in China. But the coalfield is deposited in a typical multi-coalbed reservoir with complicated reservoir formations featured by many unique CBM geological characteristics. Those particular reservoir properties are of vital significance for CBM development and have not yet been fully understood. Therefore, in-situ stress, permeability and
2
H. Xu et al. / Journal of Unconventional Oil and Gas Resources 15 (2016) 1–10
Nomenclature
rHmax rHmin rv
k CBM IFOT UNDP RAHVG
maximum horizontal principal stress minimum horizontal principal stress vertical stress coefficient of average lateral stress versus depth coalbed methane injection fall-off test United Nations Development Programme average horizontal geostresses and the vertical geostresses
pore/reservoir pressure need to be investigated to better understand and develop coalbed methane. Estimation of in-situ stress for coal bearing strata has been applied widely in underground mines, as well as in CBM exploration in many coal basins of USA, China, Australia and Canada (Bell, 2006; Brooke-Barnett et al., 2015; Enever et al., 1999; Gentzis, 2009; Li et al., 2014b). Injection fall-off test (IFOT) technology has been employed for CBM reservoir tests in China since 1992, when the United Nations Development Programme (UNDP) sponsored the first training courses in China. So far, the technique had been successfully used for in-situ stress measurement in China and was widely used (Cai and Peng, 2011; Cai et al., 2008; Li et al., 2014b; Tan and Cai, 2006; Tian et al., 2015). In-situ horizontal stress is a key factor for roof stability in underground coal mines and it also facilitates permeability predicting and fluid flow in CBM reservoirs (Bell and Bachu, 2003; Jasinge et al., 2011; Meng et al., 2011). The vertical stress (generally overburden load data) is significant in the numerical simulation of faulted zones, rock stiffness, and rock falls (Connell et al., 2010; Karacan et al., 2008). The estimation of coal properties, such as cleat volume, cleat spacing, porosity and permeability, along with the in-situ stress, controls coal mining operation and coal mine gas drainage pattern. Moreover, in-situ stress is deeply affected by gravitational and tectonic forces, and is particularly associated with horizontal tectonic movements. A gravitational stress field is relatively simple, which is mainly influenced by the overlying rock mass, while the causes for a tectonic stress field are much more complicated. The tectonic stress is extremely irregular and almost impossible to be described by precise analytical solutions, for it is constant changing as with time (Kang et al., 2010). Accurate prediction of in-situ stress distribution plays a principal role in estimating the production potential for CBM reservoirs, which is closely bound up with the permeable fracture aperture and direction. Permeability is one of the most important factors in determination of CBM productivity. During CBM production or enhanced CBM recovery, the pore pressure is placed in a variation state with a constant and almost simultaneous influence on permeability. To date, it has been generally accepted that coal permeability exponentially declines with an increase in effective stress, and this decline is potentially offset by the permeability enhancement because of matrix shrinkage (Bustin et al., 2008; Connell et al., 2016; Li et al., 2014a; White et al., 2005). Commonly, matrix swelling/shrinkage is accompanied by a volumetric strain in coals, which further impacts the permeability of the coal (Chen et al., 2012; Meng and Li, 2013; Wang et al., 2014). Moreover, the coal permeability is influenced by the transition of in-situ stress magnitude and direction (Li et al., 2014b; Qin, 2012). Focused on CBM development and its relationship with in-situ stress field under different coal burial depth, this study shall generate valuable data for
Pb Ps Po H k
r
k0
a
breakdown pressure closure pressure pore pressure depth below surface permeability (103 lm2) effective stress value changed from initial to some stressed state (MPa) coefficient depending on stress type coefficient depending on stress type
various uses, (a) a combination of injection/falloff and in-situ tests parameters together showing the stress-permeability properties of coal strata, (b) to study the variation of in-situ stresses relative to the depth of coal seams and (c) to generate stress distribution characteristic graphs related to depth, showing the stress field types. This paper, based on well testing data, analyzes the in-situ stress state and relationship of permeability and in-situ stress in the Western Guizhou, SW China. The analysis may be applicable in developing strategies in exploration, well completion and production of coalbed methane.
2. Principles and methods of hydraulic fracturing stress measurement Injection/falloff method was used for the well test and data acquisition, and the wells were tested after completion and before production. To conduct a full cycle of injection/fall off test, water was injected at a constant rate for a period of time, causing a higher pressure distribution near the well bore, and then shut in the well. During both the injection and shut-in periods, the bottom-hole pressure was measured using a down-hole pressure gauge. Through processing pressure-data from both the injection period and the fall off period independently the permeability was estimated. The detailed test procedure has been discussed by Zuber et al. (1990) and Hopkins et al. (1998). The surface facilities for the in-situ stress measurement and a schematic drawing of the measurement system with its main components are presented in Fig. 1a. It can be observed from Fig. 1a that the system utilizes the drilling rods as the only highpressure pipeline to pressurize both the sealing packers and test interval in a borehole. The design of the single-loop system can effectively avoid rotation and twist of the measurement pipelines when the instrument is lowered down to deep boreholes. Rotation and twist of the measurement pipelines can occur in a double-loop hydro-fracturing system. Before the injection/fall off test, a minifrac test was conducted to get the breakdown pressure of the coal (Fig. 1b), which was used as a direct evidence for the maximum injection pressure. To reduce the impact of stress on permeability, the injection pressure at the surface was started from relatively low value, while the maximum injection pressure was kept lower than the fracture pressure in the designed injection time. The flow injection with a constant rate was continued for more than 12 h and then shutted in for more than 24 h to get the pressure falls off (Fig. 1c). The closure pressure was recorded. In order to accurately estimate the closure stress, four pressurization cycles to finish the in-situ stress test was performed with about 6 h (Fig. 1d). The stress test was performed by injecting water at a rate sufficient to create a hydraulic fracture. After the hydraulic fracture has been established (pump time of
H. Xu et al. / Journal of Unconventional Oil and Gas Resources 15 (2016) 1–10
3
Fig. 1. Schematic drawing of the used injection fall-off test string of hydro-fracturing system (a), pressure vs. time for short-breakdown test (b), pressure and time curve of injection fall-off (c) and pressure vs. time for in-situ stress test (d) of typical well.
2–3 min), injection was stopped and the bottom hole pressure fall off was recorded for 20–30 min or until the fracture closes. The data acquisition during different test periods was preset for different intervals, ranging from 1 to 30 s. During the whole test process, the formation pressure was higher than the gas desorption pressure and the fracture system was saturated with water, and the methane was held in the coal by water pressure. The well testing interpretation is based on an assumption of coal reservoir saturated condition, and the fluid flow is kept as a single-phase flow, which flows in the cleat system, for the permeability of the coal matrix was far lower than that of the coal cleats. The tested permeability is a combined permeability, dominated by the cleat’s permeability. The gauges (made by Dub Design Industries, Canada) typically capture 140–180 h of data, and were used to ensure the reliable and accurate data. The data of short-breakdown test were obtained with a data point per second, one data point was recorded per five seconds in injection fall-off stage, and in-situ stress test data were recorded per second. Surface pressure and flow rate were captured by a laptop that provided control during the injection cycle. These data were then synchronized with the bottom hole gauge data in order to match the injection and fall-off periods. The tested data were interpretative by Saphir 4.02 software and PanSystem V3.4.0 developed by France ESSCA Group and British EPS company, respectively. There are usually two analysis methods for well testing: straight line analysis and chart board matching analysis. The data from these tested wells were mainly analyzed by the good use of both the methods, combining the semi-log curve analysis (Horner method), log–log curve, straight line and chart match analysis, which have been discussed by Hopkins et al. (1998). One of the necessary data for well test designing is the minimum in-situ stress, i.e. fracture closure pressure. The fracture closure pressure can be acquired through analyzing in-situ pressure fall-off data in the shut-in period of 2–3 cycles selected from the 4 in-situ stress test cycles. The Log–Log method and tine square
root method were included. For the tests in this paper, the time square root method was used in the closure pressure analysis, and the Log–Log method was supplied purpose of comparison. While analyzing the well test data some basic physical data was firstly determined referred to domestic and international data and usage experiences (Hopkins et al., 1998). Then, the necessary parameters such as permeability, formation temperature, breakdown pressure, closure pressure and formation pressure are recorded and calculated. During in-situ stress analysis, the minimum principal stress (rHmin) is considered to be equal to the closure pressure (Haimson and Cornet, 2003).
rHmin ¼ Ps
ð1Þ
Hydro-fracturing is a 2D stress measurement approach only applicable to the determination of the maximum and minimum stresses in the horizontal plane. The vertical stress (rv) is calculated from the overburden weight, as follows (Haimson, 1993):
rv ¼ cH
ð2Þ
where c is the unit weight of the overlying rock, and H is the depth below surface. Hoek (2007) summarized some measurement values of vertical stress at various mining and civil engineering sites around the world, and the data, as illustrated by black line in Fig. 3, show a simple linear expression (i.e. rv = 0.027H) which can be used to estimate the vertical stress when in-situ measurement is not available. The average unit weight of the overlying rock is also calculated by other ways, such as density test at different depth along the boreholes, fitting analogy method, etc. But for this paper, the logging data were lacked. So the fitted value estimated by Hoek (2007) was applied for the vertical stress calculation in this paper. The maximum principal stress (rHmax) can be estimated as (Haimson and Fairhurst, 1969; Zhang and Roegiers, 2010)
rHmax ¼ 3Ps Pb Po
ð3Þ
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H. Xu et al. / Journal of Unconventional Oil and Gas Resources 15 (2016) 1–10
where Ps is the measured average closure pressure, Pb is the pressure recorded at break down in fracture and Po is the pore pressure, which is determined by the measurement of the water level in the borehole. Statistical analysis and regression analysis were adopted to discuss the relationship between in-situ stress, pore pressure and coal permeability.
3. Results and discussion 3.1. Test parameters A total of 60 injection/falloff and in-situ stress test data in 26 boreholes were conducted and collected successfully at depth ranging from 135.9 to 1243.6 m and the data is plotted in Table 1.
Table 1 Injection/falloff and in-situ stress test parameters in Western Guizhou. Sample
Coal seam ID
Depth (m)
K (mD)
Po (MPa)
Ps (MPa)
Pb (MPa)
rHmax (MPa)
rHmin (MPa)
rv (MPa)
Well 1
12 24 15 10 7 3 9 17 19 29 19 19 26 7 132 18 19 3 9 12 171 172 1+3 9 16 271 2 5 6 2 6 23 16 16 4 9 11 4 15 3 4 6 16 27 61 62 27 4 7 3 7 10 4 9 15 20 M29 M51 M73 M78
1133.90 1243.60 1080.00 1139.70 554.24 359.09 408.53 292.22 329.40 426.95 665.02 493.29 628.61 807.89 869.48 341.95 366.94 647.82 712.64 726.63 742.84 752.04 620.49 661.98 739.97 920.71 464.04 502.26 523.35 520.17 577.76 431.38 379.70 736.98 555.42 599.74 645.70 890.41 964.82 135.90 142.78 220.77 325.29 399.94 269.99 280.87 569.90 795.70 826.90 299.86 362.57 383.68 338.07 352.12 408.16 215.25 321.96 368.69 439.51 464.27
0.001 0.006 0.0096 0.002 0.426 0.0173 0.0044 0.0078 0.011 0.013 0.062 0.011 0.201 0.0459 0.0434 0.000723 0.000203 0.000173 0.0081 0.0106 0.0027 0.00052 0.106 0.0934 0.21 0.0437 0.5002 0.3228 0.3 0.1074 0.1682 0.000164 0.0179 0.000514 0.23 0.021 0.043 0.0268 0.0276 1.5621 1.3103 0.01 0.0314 0.0857 0.0607 0.05114 0.061 0.0319 0.0291 0.1758 0.0734 0.0928 0.0874 0.0758 0.0025 0.0133 0.0088 0.0638 0.0706 0.0391
12.89 11.28 12.28 10.13 6.54 3.95 5.27 3.54 3.66 4.57 7.51 4.09 6.37 6.30 6.96 3.93 4.11 10.35 8.66 9.37 8.97 2.08 6.94 7.02 10.11 12.35 2.97 4.41 4.68 5.12 5.69 3.04 2.95 6.86 4.60 5.01 5.20 8.04 8.11 0.72 0.78 1.04 3.43 2.24 2.54 2.59 5.25 5.46 5.54 2.79 4.41 4.68 4.47 3.70 6.45 2.29 4.56 6.31 6.51 5.36
27.21 27.36 23.76 23.93 15.68 10.40 13.33 6.28 8.29 9.56 15.12 11.53 11.53 9.56 13.33 8.20 11.01 15.79 12.95 15.36 20.65 8.07 10.67 11.11 13.14 21.01 8.09 8.75 9.31 8.90 11.75 15.59 8.01 17.56 11.60 16.47 16.48 12.41 17.52 2.14 2.40 5.04 5.41 6.63 4.25 4.25 8.52 11.63 13.76 6.62 6.73 7.17 7.02 8.56 11.19 7.19 9.14 12.58 11.42 11.56
28.35 32.33 29.16 28.49 17.90 11.41 16.92 7.11 10.53 10.19 17.89 13.00 13.00 11.09 13.95 10.28 11.57 19.64 15.21 19.81 22.81 9.73 12.37 13.03 15.77 25.26 8.77 9.28 9.73 9.74 12.81 17.49 10.79 19.31 11.16 16.94 16.80 16.52 21.41 3.00 3.41 5.96 6.65 9.34 5.41 5.81 9.56 14.32 15.74 7.39 7.84 7.94 8.15 9.30 13.56 9.95 10.20 12.91 11.84 13.51
40.39 38.47 29.84 33.17 22.60 15.84 17.80 8.19 10.68 13.92 19.96 17.50 15.22 11.29 19.08 10.39 17.35 17.38 14.98 16.90 30.17 12.40 12.70 13.28 13.54 25.42 12.53 12.56 13.52 11.84 16.75 26.24 10.29 26.51 19.04 27.46 27.44 12.67 23.04 2.70 3.01 8.12 6.15 8.31 4.80 4.35 10.75 15.11 20.00 9.68 7.94 8.90 8.44 12.68 13.56 9.33 12.66 18.52 15.91 15.81
27.21 27.36 23.76 23.93 15.68 10.40 13.33 6.28 8.29 9.56 15.12 11.53 11.53 9.56 13.33 8.20 11.01 15.79 12.95 15.36 20.65 8.07 10.67 11.11 13.14 21.01 8.09 8.75 9.31 8.90 11.75 15.59 8.01 17.56 11.60 16.47 16.48 12.41 17.52 2.14 2.40 5.04 5.41 6.63 4.25 4.25 8.52 11.63 13.76 6.62 6.73 7.17 7.02 8.56 11.19 7.19 9.14 12.58 11.42 11.56
30.62 33.58 29.16 30.77 14.96 9.70 11.03 7.89 8.89 11.53 17.96 13.32 16.97 21.81 23.48 9.23 9.91 17.49 19.24 19.62 20.06 20.31 16.75 17.87 19.98 24.86 12.53 13.56 14.13 14.04 15.60 11.65 10.25 19.90 15.00 16.19 17.43 24.04 26.05 3.67 3.86 5.96 8.78 10.80 7.29 7.58 15.39 21.48 22.33 8.10 9.79 10.36 9.13 9.51 11.02 5.81 8.69 9.95 11.87 12.54
Well Well Well Well
2 3 4 5
Well 6
Well 7 Well 8 Well 9 Well 10 Well 11
Well 12
Well 13
Well 14 Well 15 Well 16 Well 17
Well 18 Well 19 Well 20
Well 21
Well 22 Well 23
Well 24
Well 25 Well 26
Note: Injection/falloff testing data of Well 19 modified from Zhang et al. (2015); Well 22-Well 26 modified from Yang and Qin (2015).
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H. Xu et al. / Journal of Unconventional Oil and Gas Resources 15 (2016) 1–10
The breakdown pressure (Pb) and closure pressure (Ps) are attained in the pressure cycles of in-situ stress test record curves. Pore pressure (Po) includes lithostatic and hydrostatic components, and each of these components is of critical concern in CBM production. CBM production is induced by dewatering, and therefore, lowered hydrostatic pressure is the principal mechanism by which gas flows to the well bore. The three pressures are closely correlated with the CBM reservoir state and production capability. Statistics show that the breakdown pressure, closure pressure and pore pressure of coal seams in Western Guizhou are similar to that in Qinshui Basin of China (Meng et al., 2010), and those parameters show linear relations compared with burial depth as follows: The curve fitting of break down pressure and depth is
Pb ¼ 0:0213h þ 1:6248
ð4Þ
The correlation coefficient is 0.86, with available values N = 60. The break down pressure in increased with depth gradually (Fig. 2a). The closure pressure could be described by the relationship
Ps ¼ 0:0184h þ 1:4215
ð5Þ
where the correlation coefficient is 0.85, which is similar to the coefficient of break down pressure and depth (Fig. 2b). Relationship between break down pressure and closure pressure is fitted as
Pb ¼ 1:129P s þ 0:3079
ð6Þ
The correlation coefficient is 0.98 (Fig. 2c). This relationship can give an estimate of the closure pressure (minimum horizontal stress) in the Western Guizhou, when measured data are not available. The pore pressure (including water and gas pressure) versus depth is also shown and could be written as a linear relationship
Po ¼ 0:0094h þ 0:3798
ð7Þ
The correlation coefficient is 0.84 (Fig. 2d). With the depth increasing, the strata compaction becomes obvious. The measured pore pressure data in Table 1 shows that some pore pressures are higher than the hydrostatic pressures. In this case, these pressures indicate the gas pressure is overpressured compared to the hydrostatic water pressure. At relatively shallow depth, pore pressure
generally is hydrostatic, indicating that a continuous, interconnected column of pore fluid extends form surface to that depth. The compaction disequilibrium usually is the primary mechanism of overpressure generation (Zhang, 2011). The stress distribution type rHmax P rv P rHmin mainly occurs in shallow to intermediate coal seams in Western Guizhou (Table 2). In this case, the high insitu stress state may provide an easy way to generate weak coal reservoir properties, such as widely existed mylonitized coal. Thus, it is possible that the local sealed fluid influenced by high stress may be the main reason for the overpressure.
3.2. In-situ stress varies with depth 3.2.1. Stress regime and vertical belting The relationship of maximum, interim and minimum stress is an important indicator for stress field characteristics. According to the modality and correlative stress state of the fault, Anderson (1951) categorized the in-situ stress as a normal faulting stress regime, a strike-slip faulting stress regime and a reverse (or thrust) faulting stress regime. The normal faulting stress regime means that gravity or vertical stress drives normal faulting, and fault slip occurs when the minimum stress reaches a sufficiently low value. In this stress state, the vertical stress is the greatest principal stress, i.e. rv P rHmax P rHmin. The strike-slip faulting stress regime means that the vertical stress is the intermediate principal stress. In this stress state, one has rHmax P rv P rHmin. The reverse (or thrust) faulting stress regime means that the vertical stress is the least principal stress, i.e. rHmax P rHmin P rv. Yu (1991) categorized in-situ stress as a static stress field, a dynamic field and a quasi-static stress field, based on the relationship of horizontal and vertical stresses. Usually the normal stress regime corresponds to gravitational loading only; strike-slip and thrust regimes involve lateral neotectonic compression. According to Fig. 3, major principal stress, interim principal stress and minor principal stress are of different variation trend in different belts. The depth range was divided into four groups for a clearer analysis of the variation of the three principal stresses with depth (Fig. 3).
30
y = 0.0213x + 1.6248 R² = 0.7401
30 25
pressure (MPa)
pressure (MPa)
35
20 15 10
20 15 10 5
(a)
5
y = 0.0184x + 1.4215 R² = 0.7263
25
0
(b)
0
0
500
1000
0
1500
500
1000
1500
depth (m)
35
14
pore pressure (MPa)
break down pressure (MPa)
depth (m) y = 1.129x + 0.3079 R² = 0.9702
30 25 20 15 10
(c)
5 0 0
10
20
shut-in pressure (MPa)
30
(d)
12 10 8 6 4
y = 0.0094x + 0.3798 R² = 0.7068
2 0 0
500
1000
1500
depth (m)
Fig. 2. Curves fitting the relationship between break down pressure vs. depth (a), closure pressure vs. depth (b), break down pressure vs. closure pressure (c), pore pressure vs. depth (d).
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H. Xu et al. / Journal of Unconventional Oil and Gas Resources 15 (2016) 1–10
Table 2 Distribution of in-situ stress in Western Guizhou. Depth (m)
rv P rHmax P rHmin
rHmax P rv P rHmin
rHmax P rHmin P rv
Total
<400 400–600 600–800 800–1000 >1000 Total
9 (42.9%) 4 (25.0%) 9 (69.2%) 5 (83.3%) 0 (0) 27 (45.0%)
7 (33.3%) 9 (56.3%) 3 (23.1%) 1 (16.7%) 4 (100%) 24 (40.0%)
5 (23.8%) 3 (18.8%) 1 (7.7%) 0 (0) 0 (0) 9 (15.0%)
21 16 13 6 4 60
Note: Values within parentheses indicate the percentage of the magnitude of the three principal stresses for each depth range.
(4) There are 6 sites in the 800–1000 m depth range; at 1 sites, rHmax is larger than rv, and at 5 sites, rv, is larger than rHmax, accounting for 16.7% and 83.3% of the total sites, respectively. At these sites, the rates of rHmax and rHmin increase with depth are reduced continuously. Most of the 800– 1000 m test sites are below the regression lines in Fig. 3. (5) There are 4 sites below a depth of 1000 m; at all sites, rHmax is larger than rv, and rv, is larger than rHmin. Nearly all the sites are below the regression lines except one site in Fig. 3. Table 2 illustrates the types of in-situ stress state and shows the stress distribution type rv P rHmax P rHmin mainly occurs in shallow and intermediate to deep coal seams, and the type rHmax P rv P rHmin mainly occurs in deep and shallow to intermediate coal seams. For deep coal seams under strong influence of geological structures, the rHmax P rv P rHmin type can also occur (Kang et al., 2010). 3.2.2. The ratio of maximum and minimum horizontal principal stress versus depth Analyzing the measured maximum horizontal stress data, we obtain that the maximum horizontal stress and the burial depth has the following relationship in the Western Guizhou, as shown in Fig. 3. Fig. 3. Values of in-situ principal stresses vs. depth in coal districts. Red and purple lines are straight lines obtained by least square fitting for minimum and maximum principal stresses, black line is vertical stress calculated form weight of voerburden. In the figure, commonly assumed minimum horizontal stress (70% of the vertical stress magnitude, OBP) is also showed with a red line. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
(1) There are 21 test sites in the shallow coal formations (<400 m); at 12 sites, rHmax is larger than rv, and at 9 sites, rv, is larger than rHmax, accounting for 57.1% and 42.9% of the total sites, respectively. At these sites, rHmax and rHmin increase faster than rv with depth. Nearly all the test sites are above the red line (minimum horizontal principal stress) and purple line (maximum horizontal principal stress) in Fig. 3. The red and purple lines were obtained by least square straight-line fitting for the minimum and maximum horizontal principal stresses. (2) There are 16 test sites in the 400–600 m, at 12 sites, rHmax is larger than rv, and at 4 sites, rv, is larger than rHmax, accounting for 75.0% and 25.0% of the total sites, respectively. At these sites, the rates of rHmax and rHmin increase with depth are reduced. Nearly a half of values of rHmin and most of rHmax test sites from 400 m to 600 m are above the regression lines in Fig. 3. (3) There are 13 sites in the 600–800 m depth range; at 4 sites, rHmax is larger than rv, and at 9 sites, rv, is larger than rHmax, accounting for 30.8% and 69.2% of the total sites, respectively. At these sites, the rates of rHmax and rHmin increase with depth are also reduced. Most of the 600–800 m test sites are below the regression lines in Fig. 3.
rHmax ¼ 0:0246h þ 2:2598
ð8Þ
The correlation coefficient is 0.77. The minimum horizontal stress is a primary control on the fracture gradient and a major constraint on propagation of hydraulic fractures. Therefore, the minimum horizontal stress is a key parameter in designs of well drilling and reservoir stimulation. The minimum horizontal stress is commonly assumed to be approximately 70% of the vertical stress magnitude in sedimentary basins. The red line and blue line in Fig. 3 present the minimum horizontal stress measured from the hydraulic fracture tests and 70% of the vertical stress magnitude in the coal seams of the Western Guizhou. It shows that the blue line cannot fairly describe the minimum stress in the coal seam. The red line is above the blue line when the depth is over 400 m. Especially when the depth is deeper than 1000 m, the measured minimum stresses are higher than the 70% of the vertical stress magnitude. The measured minimum horizontal stress can be equivalent to the closure pressure, so the Eq. (5) indicates the relationship of the minimum horizontal stress and depth. This relationship can also give an estimate of the minimum horizontal stress in Western Guizhou, when measured data are not available. 3.2.3. The ratio of maximum horizontal principal stress with vertical stress versus depth The ratio of the maximum horizontal principal stress to vertical stress rHmax/rv, is of great significance representing a key characteristic of in-situ stress distribution. As shown in Fig. 4, the ratio decreases as the depth increases and approaches 1.0, though the ratio is of great difference in different burial depths, implying that hydrostatic pressure may occur in deep strata.
H. Xu et al. / Journal of Unconventional Oil and Gas Resources 15 (2016) 1–10
σHmax/σv 0
0.5
1
1.5
2
Brown and Hoek (1978) summarized in-situ stress measurement results from around the world and found that the ratio of average horizontal to vertical stress, k, generally lies within limits defined by:
2.5
0
depth (m)
200
100 1500 þ 0:3 6 k 6 þ 0:5 H H
400 600
800 þ 0:4 H
1000
k¼
1200
It can be found that k has a linear relation with the reciprocal of the depth, i.e. when x = 1/H, we have
Fig. 4. Ration of maximum horizontal principal stress to vertical stress vs. depth.
There is only 1 site with a rHmax/rv greater than 2.5, 9 sites with a rHmax/rv in the range of 1.5–2.0, these sites are located in the relatively shallow coal seams. 27 sites with a rHmax/rv in the range of 0.5–1.0 are found, as shown in Fig. 4. The rHmax/rv at all the other 23 sites are in the range of 1.0–1.5. It can be summarized that the rHmax/rv in different coals are predominantly within the range of 0.5–2.0. 3.2.4. Coefficient of average lateral stress versus depth The average horizontal principal stress is the average magnitude of maximum and minimum horizontal principal stresses. The coefficient of average lateral stress, k, the ratio of the average horizontal principal stress to the vertical stress, varies in different areas of the world, with the values generally range between 0.5 and 5 and most of them distributed from 0.8 to 1.5 (Han et al., 2012). The k value versus depth in Western Guizhou is plotted in Fig. 5. It can be seen that the ratio shows a wide change at shallow sites, and then gradually decreases to a fixed value as the depth increases. The measured results in the Western Guizhou show that the k ranges generally from 0.48 to 1.80 with an average value of 0.94. k is defined as follows:
k ¼ ðrHmax þ rHmin Þ=2rv
ð9Þ
rao/λ 0.5
1
1.5
0 200 400
depth (m)
ð10Þ
The average value of k can be deduced as follows:
800
1400
0
7
600 800
2
k ¼ ax þ b
ð11Þ
ð12Þ
where a and b are fitting coefficients and H is the depth below surface, and a, b can be obtained from the linear regression analysis of the stress measurement results. After calculation. a = 12.37 and b = 0.9152, then Eq. (11) can be written as:
k¼
12:37 þ 0:9152 H
ð13Þ
As shown in Fig. 5, k decreases gradually with the increasing depth, and rate of the decrease reduces simultaneously. The blue curve is the characteristic curve of the left part of Eq. (10) and is called the Hoek–Brown outer envelope; the red curve is the characteristic curve of Eq. (11) and is called the Hoek–Brown median envelope; and the yellow curve is the characteristic curve of Eq. (13). It is clear that the yellow curve is located between the other two characteristic curves, and its variation trend is similar to that of the blue curve. Comparing Eqs. (10) with (11) and (13), the relationship between k and H obtained from underground coal seams is similar to the Southern Qinshui basin (Meng et al., 2011) and Liulin area, eastern Ordos basin in China (Li et al., 2014b), and also to Brown and Hoek (1978) result in the general trend, even though there are significant differences in magnitude values. Brown and Hoek (1978) results were based on stress data records not only from sedimentary rocks but also from magmatic and metamorphic rocks with a depth of 2806 m. The maximum value (k = 5.56) with values greater than 3 are all located in granite and gneiss. The value of k from sedimentary rocks, such as siltstone, shale, mudstone and sandstone, mentioned by Brown and Hoek, are between 0.85 and 2.56 with the most less than 2, which are close to that of coal seams in this study. Furthermore, the ratios of the average horizontal geostresses and the vertical geostresses (RAHVG) curve and the maximum envelope curve as well as the minimum envelope curve of the sedimentary rock varying with depth in China were obtained following the Brown and Hoek’s method by Zhao et al. (2007). The ratio of average horizontal to vertical stress, k, of China sedimentary rock, generally lies within limits defined by:
1000
41 673 þ 0:4 6 k 6 þ 1:3 H H
1200
The average value of k of China sedimentary rock can be deduced as follows:
1400 Hoek-Brown outer envelope Hoek-Brown median envelope Median envelope of sedimentary rock in China Minimum envelope of sedimentary rock in China Median envelope in Western Guizhou Maximum envelope of sedimentary rock in China Fig. 5. Coefficient of the average lateral stress variation with depth.
k¼
104 þ 0:9 H
ð14Þ
ð15Þ
The curve of outer and inner envelope characterized by Eq. (14) and median envelope characterized by Eq. (15) are shown by black line, purple line and green line in Fig. 5, respectively. It clear that the yellow line is between the black line and green line, also located closely at the left of the green line, and has a similar variation trend with the green line.
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H. Xu et al. / Journal of Unconventional Oil and Gas Resources 15 (2016) 1–10
2
1.5 1 0.5
2
(b)
1.5
permeability (mD)
(a)
permeability (mD)
permeability (mD)
2
1 0.5 0
0 0
20
40
σv (MPa)
(c) 1.5 1 0.5 0
0
10
20
30
40
50
0
10
20
30
σHmin (MPa)
σHmax (MPa)
Fig. 6. Correlation between the coal permeability and in-situ stress, including rv (vertical stress), rHmax (maximum horizontal principal stress), and rHmin (minimum horizontal principal stress).
permeability (mD) 0
0.5
1
1.5
2
0 200
depth (m)
But there is no tested value when the coal seam depth reaches 1500 m in this study, further tests will be needed to investigate the in-situ stress distribution in the deep coal seams. Moreover, the data obtained by Brown and Hoek’s derived from many countries worldwide with different geological environments, tectonic movements and rock types. But the data collected in the study area is quite small, and the relationship between vertical stress and horizontal stress is complex, especially in shallow coal seams, as shown in Table 2 and Fig. 5.
400 600 800 1000
3.3. In-situ stress influence on permeability
1200
Permeability is not a property of coal but a condition that is affected deeply by the function of stress (Meng and Li, 2013; Wang et al., 2014). Under normal virgin conditions the permeability of solid coal to gases is very low, and significant gas flow rates only occurs in seams with an open cleat or fracture network. Permeability, as well as CBM productivity, correlates strongly with the magnitude of the stress acting on the rocks. The in-situ stress magnitude is meaningful indicator of coal permeability. Regression analysis has been conducted and revealed the best fit exponential relationship between coal seam permeability and stress (Somerton et al., 1975). As shown in Eq. (16):
1400
k ¼ k0 ear
ð16Þ
where k refers to permeability (10 lm ), r is the effective stress value changed from initial to some stressed state (MPa). k0 and a presents coefficients depending on stress types, among which k0 refers to the permeability on the initial stress conditions, where the unit is 103 lm2. This permeability and an in-situ stress relationship fit the data in some coalbed basins in the world, such as the Bowen Basins in Australia (Enever et al., 1999), the Black Warrior Basin, Alabama in the U.SA. (Sparks et al., 1995). Although the relationship between permeability of a coal reservoir and different horizontal principal stresses is similar, there is no obvious correlation of the two parameters in Western Guizhou (Fig. 6). Fig. 6 demonstrates that a higher in-situ stress corresponds to a lower permeability because of a three-dimensional compressive stress state. But the permeability changes markedly as the in-situ stress increases in Western Guizhou. This is mainly induced by the high effective stress in the deep reservoir, which causes the reduction in cleat apertures. The effective stress-dependent permeability is significant for dual-porosity and dual-permeability porous media. The reason is that a rapid change in an effective stress can induce the closure in cleats, which may cause the cleats to lose permeability permanently (Meng et al., 2011). But it seems to be no obvious relationship between permeability with stress, when vertical stress and maximum horizontal principal stress are less than 15–20 MPa, and the minimum horizontal 3
2
Fig. 7. Coal permeability characteristics versus depth of the study area.
principal stress is less than 10–15 MPa, corresponding to a depth of 550–750 m (Fig. 7). This is the fact that the stress state is complex in Western Guizhou. The permeability magnitudes range mostly from 0.00173 to 0.21 mD with an average of 0.0519 mD in Western Guizhou, when the burial depth is 600–1000 m with a state of normal faulting stress regime (refer to Table 2). In this state, the permeability decreases exponentially with an increasing stress and burial depth. But, when the burial depth is between 400 m and 600 m, the vertical stress is the intermediate principal stress (refer to Table 2), which means the strike-slip faulting stress regime. This indicates that the strike-slip faulting stress regime has the higher compressive stress than the state of normal faulting stress regime. Moreover, when the depth is less than 400 m, the stress state translates into the normal faulting stress regime again (refer to Table 2). Especially, the other possible reasons for the large variability of permeability values in shallow depth are that the different coal districts are influenced by tectonic movements and geological environments with different coal reservoirs characteristics. This reason could also be responsible for the very wide range of permeability values when the depth is less than 550–750 m. 4. Conclusions In response to the CBM development in Western Guizhou, hydraulic fracturing tests were carried out for permeability and in-situ stress measurements in CBM wells. A data quality assurance system was also developed to ensure the quality and reliability of test data. About 60 high quality data of injection/falloff and in-situ stress in 26 CBM wells were collected and used to investigate the distribution characteristics of in-situ stresses and coal reservoir permeability. The following conclusions can be drawn. (1) In Western Guizhou, the breakdown pressure, closure pressure and pore pressure of coal seams tested during
H. Xu et al. / Journal of Unconventional Oil and Gas Resources 15 (2016) 1–10
injection/fall and in-situ stress tests are positively correlated with the coal burial depth, of which the break down pressure versus depth shows the highest related coefficient. (2) The rv P rHmax P rHmin stress field type appears at relatively shallow and intermediate to deep coal seams with a widespread distribution, and the rHmax P rv P rHmin type usually appears in deep and shallow to intermediate coal seams. For deep coal seams under strong influence of geological structures, the rHmax P rv P rHmin type can also occur. (3) The ratio of maximum and minimum horizontal principal stress versus depth tends to increase with a good correlation coefficient. The ratio of maximum horizontal to vertical stress tends to decrease and approaches 1.0 with increasing depth. The ratio of average horizontal principal stress versus depth in Western Guizhou ranges from 0.48 to 1.80 and decrease gradually with depth, also having a trend approaching 1.0. The relationship between depth and coefficient of average lateral stress versus depth (k), can be expressed as 0.9152 + 12.37/H, the characteristic curve of which is located between the Hoek–Brown outer envelope and the Hoek–Brown median envelope, and located at the right of the outer envelope of Chinese sedimentary rock. The variation trend of the characteristic curve is similar to that of Hoek–Brown outer envelope and median envelope of sedimentary rock in China. (4) Coal reservoir permeability decreases markedly as the in-situ stress increases in moderately deep to deep coal seams in the Western Guizhou. This is mainly induced by the high effective stress in the deep reservoir, which causes the reduction in cleat apertures. There is no obvious relationship between permeability with stress, when vertical stress and maximum horizontal principal stress are less than 15–20 MPa, and the minimum horizontal principal stress is less than 10-15 MPa, corresponding to a depth of 550–750 m. This is maybe caused by stress regime and vertical belting, and other reasons like tectonic movements and geological environments with different coal reservoirs characteristics.
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