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Engineering geological models for efficient development of high-rank coalbed methane and their application – Taking the Qinshui Basin for example
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ScienceDirect Natural Gas Industry B 5 (2018) 185e192 www.elsevier.com/locate/ngib
Research Article
Engineering geological models for efficient development of high-rank coalbed methane and their application e Taking the Qinshui Basin for example*,** Zhu Qingzhong a, Yang Yanhui b, Wang Yuting b,* & Shao Guoliang b b
a PetroChina Huabei Oilfield Company, Renqiu, Hebei 062552, China Exploration and Development Research Institute of PetroChina Huabei Oilfield Company, Renqiu, Hebei 062552, China
Received 20 July 2017; accepted 25 October 2017 Available online 6 June 2018
Abstract Low average single-well production resulting in low economic benefit has become the main bottleneck of the CBM gas development in China. So it is significant to choose suitable efficient development technologies based on CBM geological factors for high rank CBM recovery enhancement. In view of this, CBM geological factors were analyzed, different geological models were established and the corresponding models of development engineering technologies were thus put forward. It was proposed that the four main factors affecting high rank CBM recovery from a lower degree to a higher degree respectively include coal texture, rank of coal metamorphism, in-situ stress, and the ratio of critical desorption pressure to initial reservoir pressure. On this basis, four engineering geological models were classified as follows: vertical well, open-hole multilateral horizontal well, U-shaped and roof tree-like horizontal wells, and fish-bone and L-shaped wells. It is concluded that the former two models are more adaptable in such areas with better coal texture and high degree of thermal maturity, while the latter two are commonly applied in a wide range of areas. © 2018 Sichuan Petroleum Administration. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Keywords: Qinshui Basin; High-rank coalbed methane (CBM); Coal texture; Rank of coal metamorphism; In-situ stress; Ratio of critical desorption pressure to initial reservoir pressure; Engineering technology; Geological model; Optimization model; Development benefit
Commercial production of coalbed methane (CBM) has been successfully realized in the southern Qinshui Basin, the most important high rank coal reservoir development area in *
Project supported by the National Major Science and Technology Project “Demonstrative horizontal well development of CBM in Qinshui Basin, Shanxi” (No.: 2011ZX05061), PetroChina Major Science and Technology Project “Demonstrative exploration and development of CBM in Qinshui Basin” (No.: 2010E-2208), and PetroChina Huabei Oilfield Science and Technology Project “Research on efficient development technologies for Mabe East block” (No.: 2016-HB-M06). ** This is the English version of the originally published article in Natural Gas Industry (in Chinese), which can be found at https://doi.org/10.3787/j.issn. 1000-0976.2017.10.004. * Corresponding author. E-mail address: [email protected] (Wang YT.). Peer review under responsibility of Sichuan Petroleum Administration.
China [1e3]. However, affected by such geologic characteristics as low pressure, low porosity, low permeability, undersaturation and high heterogeneity, the development effect of vertical well development engineering technology dominated by active water fracturing is largely different in the blocks with different geologic conditions: the daily gas flow rate per well exceeds 3000 m3 in some areas, or lower than 1000 m3 in other areas, and lots of low yield and low efficiency wells even occur. Taking the ZhengzhuangeFanzhuang block as an example, the daily gas flow rate of 1196 vertical wells is lower than 1000 m3, accounting for 64.5% total vertical wells (1853 vertical wells); however, about 10000 m3 daily gas flow rate is obtained in the vertical well low productivity area by roof treelike horizontal wells. Therefore, the clarification, classification and evaluation of geologic factors that affect the development
https://doi.org/10.1016/j.ngib.2018.04.005 2352-8540/© 2018 Sichuan Petroleum Administration. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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effect of high-rank coalbed methane and the selection of suitable and efficient development engineering technologies are of great significance strategically for enhancing the single well gas flow rate and the realization of large-scale efficient development of high-rank coalbed methane in China [4]. 1. Geologic factors affecting development effect of highrank coalbed methane Numerous researches and practices have been implemented abroad on mediumelow rank coalbed gas. Usually, appropriate development engineering technologies are selected based on such geologic factors as gas content, permeability, water saturation, in-situ stress, reservoir mechanical strength and thickness. For low rank coal reservoirs, coiled tubing drilling technology and high pumping rate nitrogen foam fracturing technology have been developed in Canada, and some drilling technologies like MRD, TRD and U-shaped horizontal well drilling as well as drilling along high angle coalbed have been developed in Australia. For medium rank coal reservoirs, open hole cavity completion technique has been developed in the San Juan Basin, and casing completion þ fracturing technology has been developed in the Black Warrior Basin and the Raton Basin [5e9]. However, there are rare studies dealing with the geologic factors affecting the development effect of high-rank coalbed methane. In this paper, after reviewing the previous studies, and the development practices of high-rank coalbed methane in southern Qinshui Basin, the authors believe that the coal texture, rank of coal metamorphism, in-situ stress and ratio of critical desorption pressure to initial reservoir pressure are the major geologic factors affecting the development effect of high-rank coalbed methane. 1.1. Coal texture Coal texture refers to the grain size, morphologic characteristics and interrelationship of coalbed components [10]. It reflects the damage level of in-situ stress to coalbed or coal seam in the course of geologic evolution. Coal is divided into 4 types according to its damage level: primary texture coal, cataclastic coal, flax seed coal and mylonitic coal [11,12]. Specifically, flax seed coal and mylonitic coal are severely damaged, with poor permeability and poor fracturability; during fracturing, the coal surrounding the wellbore is compacted, leading to the failure of normal fractures generation. When this type of reservoir is developed by horizontal well drilling, the wellbore is apt to collapse in the course of drilling or gas recovery by water drainage, bringing extreme challenges to operations. Meanwhile, very low permeability makes it impossible to obtain high production rates. Therefore, this type of reservoir can hardly be developed beneficially using the available engineering technologies. Cataclastic coal still contains endogenic fissures and also more exogenous fissures and inherited fissures. For high metamorphic coal without endogenic fissures, the permeability is much higher, but the fracturability is lower, with no fractures generated by fracturing; wellbore is also apt to collapse in the course of drilling
or gas recovery by water drainage when this type of coal is developed by horizontal well, and instead the horizontal well with support is an effective technique to realize beneficial development of this type of reservoir. The development engineering technology suitable for this type of reservoir is still under consideration. Primary texture coal is preserved completely, which is favorable for creating long fractures by fracturing, and the success ratio of horizontal well drilling is high. Both vertical and horizontal wells are suitable for the development of this type of reservoirs. According to the statistics for the ZhengzhuangeFanzhuang block, more than 90% of vertical wells with a daily gas flow rate higher than 1000 m3 distribute in the area where the thickness of primary texture coal accounts for more than 2/3. Therefore, the primary texture coal development area is the preferred development target, and it is also the basic geologic factor for the efficient development of coalbed methane. 1.2. Rank of coal metamorphism At the same effective confining pressure, the compressive strength of anthracite tends to drop with the increase of metamorphic rank, and the firm coefficient of coal sample decreases correspondingly [13]. Therefore, the fracturability of high rank coal reservoir strengthens with the increase of rank of coal metamorphism, and the fracturing stimulation effect also gets better. The relation between gas flow effect and rank of coal metamorphism in vertical wells of the ZhengzhuangeFanzhuang block in southern Qinshui Basin is shown in Fig. 1. It can be seen that the single well gas flow rate of high-rank coalbed methane vertical well increases with the increase of vitrinite reflectance (Ro). When Ro>3.8%, the daily gas flow rate exceeds 1800 m3, efficient development is realized. When Ro<3.8%, the daily gas flow rate is lower than 1000 m3, beneficial development is not realized. Therefore, Ro ¼ 3.8% can act as a key criterion to judge whether vertical well development is suitable. 1.3. In-situ stress The present in-situ stress field not only controls the openness of cleat, but also affects the morphology and length of
Fig. 1. Vitrinite reflectance vs. single well daily gas flow rate of vertical wells.
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Fig. 2. Stress areas vs. single vertical well gas flow rate in the ZhengzhuangeFanzhuang block.
hydraulic fractures. Theoretically, the collocation relationship among vertical stress (sv), horizontal minimum principal stress (shmin) and horizontal maximum principal stress (shmax) decides the in-situ stress state. In practical application, it is hard to get their values, and even if their values are obtained, there are large errors. Moreover, high testing cost makes it hard to acquire the values in the whole study area. Generally, the in-situ stress field state is determined based on the fault attribute and difference of coalbed occurrence resulted from the mentioned collocation relationship. In the tensional stress state, sv > shmax > shmin, only normal faults are developed, the coalbed occurrence is gentle, and small or micro structures are not developed. In the transitional stress state, shmax > sv > shmin, a few reverse faults are developed, and small or micro structures of coalbed are extremely developed. In the compressional stress state, shmax > shmin > sv, reverse faults are intensively developed, and the dip angle of coalbed is high [14e16]. In the tensional stress state, the breakdown pressure gradient of coal reservoir is low; as a result, the pressure rise range of coal reservoir is small during fracturing, easily giving rise to long vertical fractures extending in the maximum horizontal principal stress direction, thus the postfracture response is good and the fracturing fluid has little damage to coalbed. In the transitional stress and compressional stress states, the breakdown pressure gradient of coal reservoir is high; as a result, the pressure rise range of coal
reservoir is large during fracturing, and the damage to reservoir is large, easily giving rise to complicated and horizontal fractures with short extension, thus the post-fracture response is poor. The relation between the in-situ stress field distribution and the single well gas flow rate of vertical wells in the ZhengzhuangeFanzhuang block is shown in Fig. 2. Obviously, the average daily gas flow rate per vertical well of coalbed in transitional stress and compressional stress areas is below 1000 m3. The vertical well þ active water fracturing stimulation development mode is inapplicable; instead, effective measures should be taken to relieve stress, induce cleat fissures to open and improve the coalbed permeability. 1.4. Ratio of critical desorption pressure to initial reservoir pressure The ratio of critical desorption pressure to initial reservoir pressure is an index to visually expressing the dynamic strength of methane from desorption and diffusion to flowing to the bottom hole. The higher the ratio of critical desorption pressure to initial reservoir pressure is, the higher the gas saturation of coalbed, the stronger the dynamic field of methane, and the easier the gas flows due to desorption under the same pressure drawdown; meanwhile, the negative effect of drilling and fracturing fluids on the critical desorption
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are determined to be the main control parameters for building engineering geological model of high-rank coalbed methane. The area where primary coal is developed is selected as the prospect. The prospect is firstly divided into 2 types of zones with the vitrinite reflectance of 3.8% as a limit. Then, the prospect is divide into tensional and transitionalcompressional stress areas based on the fault attribute and triaxial stress combined features. Strong and weak dynamic fields are classified given the ratio of critical desorption pressure to initial reservoir pressure of 0.57. Finally, 4 engineering geological models are built using a progressive mode based on the classification criteria of the 4 main control geologic parameters (Table 1). Fig. 3. Ratio of critical desorption pressure to initial reservoir pressure vs. daily gas outputs of coalbed methane vertical wells in the ZhengzhuangeFanzhuang block.
pressure of reservoir can also be weakened, and accordingly, the gas flow dynamic becomes stronger [17,18]. Therefore, it is very significant practically that the ratio of critical desorption pressure to initial reservoir pressure is used as one of the key parameters for optimizing the main development engineering technologies of coal reservoirs. The relation between ratio of critical desorption pressure to initial reservoir pressure and vertical well gas flow rate of coalbed methane is shown in Fig. 3. It is seen that two areas are divided given the ratio of critical desorption pressure to initial reservoir pressure is 0.57. When the ratio of critical desorption pressure to initial reservoir pressure is higher than 0.57, majority of vertical wells have the daily gas flow rate per well exceeding 1000 m3, basically suggesting the beneficial development of a CBM field. When the ratio of critical desorption pressure to initial reservoir pressure is lower than 0.57, majority of vertical wells have the daily gas flow rate per well of lower than 1000 m3. This means that when the ratio of critical desorption pressure to initial reservoir pressure is lower than 0.57, vertical well fracturing cannot contribute to the efficient development of a CBM field, and new development engineering technologies must be adopted. 2. Engineering geological model of high-rank coalbed methane 2.1. Engineering geological model construction Coal texture, rank of coal metamorphism, in-situ stress and ratio of critical desorption pressure to initial reservoir pressure
2.2. Demand of different engineering geological models for main development engineering technologies Demands of different engineering geological models for main development engineering technologies are different. For Model 1, a coal reservoir is hard and brittle with good fracturability and low requirements for development engineering technologies; in its development flexible well type is needed for an overall control on high-quality reserves. For Model 2, a coal seam is poor in fracturability, but relatively favorable in the in-situ stress state and a methane dynamic field; in its development a well type with a large drainage area is needed, and application of fracturing stimulation is decided by the demand of productivity. For Model 3, a coal seam is poor in fracturability and in a weak methane dynamic field, but relatively favorable in the in-situ stress state; in its development a well type with a large drainage area is needed, and lowpollution stimulation will be adopted based on the demand of productivity. For Model 4, a coal seam is poor in fracturability, unfavorable in the in-situ stress state and poor permeability; in developing this coal seam, specific efforts should be made to further increase the exposure of wellbore to coal seams, and release of stress need to be solved. 2.3. Applicability of current development engineering technologies The development engineering technology dominated by vertical wells & active water fracturing and aided by open hole multilateral horizontal wells is adopted in the ZhengzhuangeFanzhuang block. Development wells in the study area were compared for production performance,
Table 1 Engineering geological models of high-rank coalbed methane. Coal texture
Primary
Rank of metamorphism
Ro Ro Ro Ro
> < < <
3.8% 3.8% 3.8% 3.8%
State of stress
Ratio of critical desorption pressure to initial reservoir pressure
Engineering technical geological model
e Tensional Tensional Transitional compressional
e >0.57 <0.57 e
Model Model Model Model
1 2 3 4
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Table 2 Statistics of production performance of vertical wells in different engineering geological models.
Table 3 Statistics of production performance of open hole multilateral horizontal wells in different engineering geological models.
Level of Items compared output/(m3$d1)
Model 1 Model 2 Model 3 Model 4
Level of output/ Items compared (m3$d1)
Model 1 Model 2 Model 3 Model 4
>2000
127 3857
58 2881
10 2419
30 2898
>10000
9 14 859
3 15 850
41.78% 85 1473
9.42% 181 1362
8.93% 36 1359
3.65% 130 1376
5000e10000
32.14% 8 7405
8.11% 2 5484
27.96% 45 747
29.38% 151 746
32.14% 23 733
15.84% 144 727
3000e5000
28.57% 5 3612
5.41% 2 3923
3 4740
14.80% 47 234
24.51% 226 176
20.54% 43 181
17.54% 517 129
<3000
17.86% 6 969
5.41% 30 802
9.38% 29 816
15.46%
36.69%
38.39%
62.97%
21.43%
81.08%
90.63%
1000e2000
500e1000
<500
Number of wells Average daily gas production/m3 Proportion Number of wells Average daily gas production/m3 Proportion Number of wells Average daily gas production/m3 Proportion Number of wells Average daily gas production/m3 Proportion
indicating that the applicability of development engineering technologies varies for engineering geological models. 2.3.1. Vertical well fracturing Vertical wells can be deployed flexibly. According to the statistics of production performance of vertical wells in different geological models (Table 2), the vertical wells in Models 1e4 areas with average daily gas flow rates more than 1000 m3 account for 69.74%, 38.80%, 41.07% and 19.49% of total wells, respectively. Therefore, vertical wells are very applicable in the Model 1 area, but not applicable in Models 2e4 areas with low ranks of coal metamorphism, soft coal, and poor fracturability. 2.3.2. Open hole multilateral horizontal wells When open hole completion is conducted in horizontal section of multilateral horizontal wells, the effective coal seam footage is long, the drainage area is large, and the output is high. According to the statistics of production performance of open hole multilateral horizontal wells in different geological models (Table 3), the open hole multilateral horizontal wells in Model 1 area with an average daily gas flow rate per well of more than 5000 m3 and 10000 m3 account for 60.71% and 32.14% of total wells respectively in the area. The open hole multilateral horizontal wells in Model 2 area with an average daily gas flow rate per well of more than 5000 m3 only account for 13.52% of total wells in the area. Sidewall collapse occurred in drilling and gas recovery by water drainage is the major cause for a large number of stripper wells in Model 2 area. Except for the sidewall collapse wells, the average daily gas flow rate of the 5 wells with stable sidewalls in the area can reach 11700 m3. In Model 4 area, 32 open hole multilateral horizontal wells in production mostly have low yields due to sidewall collapse. It is found that the open hole multilateral horizontal well has high requirements for reservoir and structure. In the structure-complex areas, drilling operations are exposed to huge risks due to the change of coalbed
Number of wells Average daily gas production/m3 Proportion Number of wells Average daily gas production/m3 Proportion Number of wells Average daily gas production/m3 Proportion Number of wells Average daily gas production/m3 Proportion
occurrence and the geologic conditions like structural complexity. Especially in the area with Ro less than 3.8%, sidewall collapse may easily occur, which will usually lead to low yield or shut down, and make subsequent wellbore maintenance or entry via stimulation a challenging task [19]. Therefore, the open hole multilateral horizontal wells cannot be widely applied but will not possibly become the major well type. 2.3.3. U-shaped and roof tree-like horizontal wells For avoiding the collapse of borehole, U-shaped horizontal wells and roof tree-like horizontal wells are creatively proposed. The U-shaped horizontal well is characterized by the combination of engineering well and cavity well for threespud-in drilling, PE screen completion and gas recovery from cavity well. Its field application shows that the wellbore is small, leading to the failure of stress release for improving stress field and fissure field. In the area where the coal seam is soft, the resulted slack coal may block the wellbore, making the reentry impossible for drifting and stimulation. Currently, the daily gas flow rate per well is lower than 3000 m3, and high investment cannot meet the demand of efficient development. The roof tree-like horizontal well is designed with the main hole for communication, branch hole for controlling the drainage area, and laterals for realizing higher deliverability. The main hole is drilled in the roof and its sidewall is fixed by chemicals, so the wellbore may not collapse, the footage is long, and the drainage area is large. This well type has contributed a high yield in the area where the thermal evolution level and the ratio of critical desorption pressure to initial reservoir pressure are relatively low. For instance, the peak gas flow rate of Well Zhengshi 1-P5 is 10748 m3/d, while only 5 of 14 adjacent vertical wells produce gas, with an average daily gas flow rate of 243 m3. However, this type of well is costly, with single-well cost of more than CNY20 million. Moreover, it needs more well sites, and has very high requirements for
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Fig. 4. Optimization process of efficient development engineering technology for high-rank coalbed methane.
geologic conditions and coal seam roof [20]. Roof tree-like horizontal well practices prove that effective development can be realized by horizontal wells with stable main holes, but the technological design must be improved to reduce the cost and investment. 2.3.4. Fish-bone and L-shaped horizontal wells The fish-bone and L-shaped horizontal wells are new types of horizontal wells upgraded on the basis of U-shaped and roof tree-like horizontal wells. They are drilled using two-spud-in mode, and completed with Ø139.7 mm casing or screen. They have the following advantages. ① The drilling period is as short as 13 days at the most. ② The hole size is large, which is favorable for stress release, inducing the opening of cleat fissures, and improving the stress field. ③ Wells can be deployed in a parallel laneway style perpendicular to the direction of the maximum horizontal principal stress, so that more fractures can be communicated. Furthermore, vertical wells can be deployed among horizontal well groups to organically connect the horizontal wells, thus forming a balanced pressure drawdown field. ④ The drilling cost is low and can be controlled below CNY3.5 million, and the economic limit rate is only 2460 m3/d. ⑤ Well deployment is relatively flexible, and the wells can be maintained and operated at the late stage. L-shaped horizontal wells can be deployed in the primary texture coal areas and tensional stress areas to enhance the production by staged fracturing. Nitrogen unclogging or sectional cavity creating can be used in the transitional stress and compressional stress areas with primary coal. In the areas where a coal seam is relatively soft, fishbone horizontal wells can be drilled to enlarge the pressure relief area of the coal seam. 3. Building and application of optimization model for efficient development engineering technology of highrank coalbed methane 3.1. Building of optimization model for efficient development engineering technology The optimization model for efficient development engineering technology (Fig. 4) for high-rank coalbed methane is built based on the above-mentioned engineering geological
models of high-rank coalbed methane and the available development engineering technologies. Model 1: A flexible vertical well cluster is adopted to realize an overall control on high-quality reserves, and active water fracturing stimulation is adopted to enhance the production rates. Model 2: An L-shaped horizontal well cluster is adopted. The footage in horizontal section is 1000 m. The coal exposure length is 7.14 times that of vertical wells (the artificial fracture length of a vertical well is deemed as 140 m). The single-well drainage area is 5 times that of vertical well. Screen or casing completion is adopted. The sidewall is stable. Stimulation treatment can be realized (Table 4). Model 3: An L-shaped horizontal well cluster is adopted. Low-damage cavity fracturing or nitrogen reaming is adopted to stimulate coal seams. Model 4: Fish-bone horizontal wells are adopted, with the footage of 2800 m in horizontal section, which further increases the coal exposure length to be 20 times that of vertical wells. The drainage area is 7.3 times that of vertical wells. Low-damage cavity fracturing or nitrogen reaming is adopted to stimulate the main hole, increase the hole size, relieve the stress, induce the cleat fissures to open and increase the permeability of coal seams (Table 4). 3.2. Application effects The horizontal distribution map of engineering geological models for the ZhengzhuangeFanzhuang block is shown in Fig. 5. Pertinent efficient development engineering technology is selected based on the optimization model for efficient development engineering technology for high-rank coalbed methane, and its application has shown preliminary effects. In Table 4 Main parameters of different well types. Well type
Length of horizontal section or artificial fracture/m
Single well drainage area/km2
Vertical wells L-shaped horizontal wells Fish-bone horizontal wells
140 1000 2800
0.078 0.390 0.568
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Fig. 5. Horizontal distribution map of engineering geological models for the ZhengzhuangeFanzhuang block.
Model 1 area, a total of 304 vertical wells were put into production, active water was used for fracturing, and the average daily gas flow rate per well reached 2170 m3, recording an efficient development. In Model 2 area, a total of 6 L-shaped horizontal wells were drilled, of which 2 wells failed to produce gas due to geologic and engineering factors, while 4 wells achieved high yields, with the average daily gas flow rate and the average gas producing index of pure coal footage of 3224 m3 and 5.3 m3/m respectively, considerably higher than the expected levels, and the gas flow rate can be further enhanced. 4. Conclusions 1) The major geologic factors affecting the development effect of high-rank coalbed methane are coal texture, rank of coal metamorphism, in-situ stress and ratio of critical desorption pressure to initial reservoir pressure. Accordingly, 4 engineering geological models are built. The vertical well fracturing and open hole multilateral horizontal wells are less applicable to the 4 models. Therefore, roof tree-like horizontal well and U-shaped horizontal wells are developed, and finally, they are upgraded to the L-shaped horizontal wells and fish-bone horizontal wells that can be drilled at a low cost, maintained at the late stage, cover a small area. The Lshaped horizontal wells and fish-bone horizontal wells can be widely applied in the whole block.
2) Based on different geologic demands of the 4 engineering geological models, suitable well types and stimulation techniques are determined, and the optimization method and process of efficient development engineering technology for high-rank coalbed methane are defined. Vertical wells & active water fracturing development engineering technology is proposed for Model 1 areas. L-shaped horizontal wells or L-shaped horizontal wells & sectional fracturing development engineering technology is proposed for Model 2 areas. L-shaped horizontal well cavity fracturing or nitrogen reaming development engineering technology is proposed for Model 3 areas. Fish-bone horizontal well cavity building or nitrogen reaming development engineering technology is proposed for Model 4 areas. 3) The geologic conditions of high-rank coalbed methane are complex and largely different in various regions of China. Therefore, the classification criteria of optimization methods can be properly adjusted depending on the actual conditions and further optimized in practical application.
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Research Article
Engineering geological models for efficient development of high-rank coalbed methane and their application e Taking the Qinshui Basin for example*,** Zhu Qingzhong a, Yang Yanhui b, Wang Yuting b,* & Shao Guoliang b b
a PetroChina Huabei Oilfield Company, Renqiu, Hebei 062552, China Exploration and Development Research Institute of PetroChina Huabei Oilfield Company, Renqiu, Hebei 062552, China
Received 20 July 2017; accepted 25 October 2017 Available online 6 June 2018
Abstract Low average single-well production resulting in low economic benefit has become the main bottleneck of the CBM gas development in China. So it is significant to choose suitable efficient development technologies based on CBM geological factors for high rank CBM recovery enhancement. In view of this, CBM geological factors were analyzed, different geological models were established and the corresponding models of development engineering technologies were thus put forward. It was proposed that the four main factors affecting high rank CBM recovery from a lower degree to a higher degree respectively include coal texture, rank of coal metamorphism, in-situ stress, and the ratio of critical desorption pressure to initial reservoir pressure. On this basis, four engineering geological models were classified as follows: vertical well, open-hole multilateral horizontal well, U-shaped and roof tree-like horizontal wells, and fish-bone and L-shaped wells. It is concluded that the former two models are more adaptable in such areas with better coal texture and high degree of thermal maturity, while the latter two are commonly applied in a wide range of areas. © 2018 Sichuan Petroleum Administration. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Keywords: Qinshui Basin; High-rank coalbed methane (CBM); Coal texture; Rank of coal metamorphism; In-situ stress; Ratio of critical desorption pressure to initial reservoir pressure; Engineering technology; Geological model; Optimization model; Development benefit
Commercial production of coalbed methane (CBM) has been successfully realized in the southern Qinshui Basin, the most important high rank coal reservoir development area in *
Project supported by the National Major Science and Technology Project “Demonstrative horizontal well development of CBM in Qinshui Basin, Shanxi” (No.: 2011ZX05061), PetroChina Major Science and Technology Project “Demonstrative exploration and development of CBM in Qinshui Basin” (No.: 2010E-2208), and PetroChina Huabei Oilfield Science and Technology Project “Research on efficient development technologies for Mabe East block” (No.: 2016-HB-M06). ** This is the English version of the originally published article in Natural Gas Industry (in Chinese), which can be found at https://doi.org/10.3787/j.issn. 1000-0976.2017.10.004. * Corresponding author. E-mail address: [email protected] (Wang YT.). Peer review under responsibility of Sichuan Petroleum Administration.
China [1e3]. However, affected by such geologic characteristics as low pressure, low porosity, low permeability, undersaturation and high heterogeneity, the development effect of vertical well development engineering technology dominated by active water fracturing is largely different in the blocks with different geologic conditions: the daily gas flow rate per well exceeds 3000 m3 in some areas, or lower than 1000 m3 in other areas, and lots of low yield and low efficiency wells even occur. Taking the ZhengzhuangeFanzhuang block as an example, the daily gas flow rate of 1196 vertical wells is lower than 1000 m3, accounting for 64.5% total vertical wells (1853 vertical wells); however, about 10000 m3 daily gas flow rate is obtained in the vertical well low productivity area by roof treelike horizontal wells. Therefore, the clarification, classification and evaluation of geologic factors that affect the development
https://doi.org/10.1016/j.ngib.2018.04.005 2352-8540/© 2018 Sichuan Petroleum Administration. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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effect of high-rank coalbed methane and the selection of suitable and efficient development engineering technologies are of great significance strategically for enhancing the single well gas flow rate and the realization of large-scale efficient development of high-rank coalbed methane in China [4]. 1. Geologic factors affecting development effect of highrank coalbed methane Numerous researches and practices have been implemented abroad on mediumelow rank coalbed gas. Usually, appropriate development engineering technologies are selected based on such geologic factors as gas content, permeability, water saturation, in-situ stress, reservoir mechanical strength and thickness. For low rank coal reservoirs, coiled tubing drilling technology and high pumping rate nitrogen foam fracturing technology have been developed in Canada, and some drilling technologies like MRD, TRD and U-shaped horizontal well drilling as well as drilling along high angle coalbed have been developed in Australia. For medium rank coal reservoirs, open hole cavity completion technique has been developed in the San Juan Basin, and casing completion þ fracturing technology has been developed in the Black Warrior Basin and the Raton Basin [5e9]. However, there are rare studies dealing with the geologic factors affecting the development effect of high-rank coalbed methane. In this paper, after reviewing the previous studies, and the development practices of high-rank coalbed methane in southern Qinshui Basin, the authors believe that the coal texture, rank of coal metamorphism, in-situ stress and ratio of critical desorption pressure to initial reservoir pressure are the major geologic factors affecting the development effect of high-rank coalbed methane. 1.1. Coal texture Coal texture refers to the grain size, morphologic characteristics and interrelationship of coalbed components [10]. It reflects the damage level of in-situ stress to coalbed or coal seam in the course of geologic evolution. Coal is divided into 4 types according to its damage level: primary texture coal, cataclastic coal, flax seed coal and mylonitic coal [11,12]. Specifically, flax seed coal and mylonitic coal are severely damaged, with poor permeability and poor fracturability; during fracturing, the coal surrounding the wellbore is compacted, leading to the failure of normal fractures generation. When this type of reservoir is developed by horizontal well drilling, the wellbore is apt to collapse in the course of drilling or gas recovery by water drainage, bringing extreme challenges to operations. Meanwhile, very low permeability makes it impossible to obtain high production rates. Therefore, this type of reservoir can hardly be developed beneficially using the available engineering technologies. Cataclastic coal still contains endogenic fissures and also more exogenous fissures and inherited fissures. For high metamorphic coal without endogenic fissures, the permeability is much higher, but the fracturability is lower, with no fractures generated by fracturing; wellbore is also apt to collapse in the course of drilling
or gas recovery by water drainage when this type of coal is developed by horizontal well, and instead the horizontal well with support is an effective technique to realize beneficial development of this type of reservoir. The development engineering technology suitable for this type of reservoir is still under consideration. Primary texture coal is preserved completely, which is favorable for creating long fractures by fracturing, and the success ratio of horizontal well drilling is high. Both vertical and horizontal wells are suitable for the development of this type of reservoirs. According to the statistics for the ZhengzhuangeFanzhuang block, more than 90% of vertical wells with a daily gas flow rate higher than 1000 m3 distribute in the area where the thickness of primary texture coal accounts for more than 2/3. Therefore, the primary texture coal development area is the preferred development target, and it is also the basic geologic factor for the efficient development of coalbed methane. 1.2. Rank of coal metamorphism At the same effective confining pressure, the compressive strength of anthracite tends to drop with the increase of metamorphic rank, and the firm coefficient of coal sample decreases correspondingly [13]. Therefore, the fracturability of high rank coal reservoir strengthens with the increase of rank of coal metamorphism, and the fracturing stimulation effect also gets better. The relation between gas flow effect and rank of coal metamorphism in vertical wells of the ZhengzhuangeFanzhuang block in southern Qinshui Basin is shown in Fig. 1. It can be seen that the single well gas flow rate of high-rank coalbed methane vertical well increases with the increase of vitrinite reflectance (Ro). When Ro>3.8%, the daily gas flow rate exceeds 1800 m3, efficient development is realized. When Ro<3.8%, the daily gas flow rate is lower than 1000 m3, beneficial development is not realized. Therefore, Ro ¼ 3.8% can act as a key criterion to judge whether vertical well development is suitable. 1.3. In-situ stress The present in-situ stress field not only controls the openness of cleat, but also affects the morphology and length of
Fig. 1. Vitrinite reflectance vs. single well daily gas flow rate of vertical wells.
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Fig. 2. Stress areas vs. single vertical well gas flow rate in the ZhengzhuangeFanzhuang block.
hydraulic fractures. Theoretically, the collocation relationship among vertical stress (sv), horizontal minimum principal stress (shmin) and horizontal maximum principal stress (shmax) decides the in-situ stress state. In practical application, it is hard to get their values, and even if their values are obtained, there are large errors. Moreover, high testing cost makes it hard to acquire the values in the whole study area. Generally, the in-situ stress field state is determined based on the fault attribute and difference of coalbed occurrence resulted from the mentioned collocation relationship. In the tensional stress state, sv > shmax > shmin, only normal faults are developed, the coalbed occurrence is gentle, and small or micro structures are not developed. In the transitional stress state, shmax > sv > shmin, a few reverse faults are developed, and small or micro structures of coalbed are extremely developed. In the compressional stress state, shmax > shmin > sv, reverse faults are intensively developed, and the dip angle of coalbed is high [14e16]. In the tensional stress state, the breakdown pressure gradient of coal reservoir is low; as a result, the pressure rise range of coal reservoir is small during fracturing, easily giving rise to long vertical fractures extending in the maximum horizontal principal stress direction, thus the postfracture response is good and the fracturing fluid has little damage to coalbed. In the transitional stress and compressional stress states, the breakdown pressure gradient of coal reservoir is high; as a result, the pressure rise range of coal
reservoir is large during fracturing, and the damage to reservoir is large, easily giving rise to complicated and horizontal fractures with short extension, thus the post-fracture response is poor. The relation between the in-situ stress field distribution and the single well gas flow rate of vertical wells in the ZhengzhuangeFanzhuang block is shown in Fig. 2. Obviously, the average daily gas flow rate per vertical well of coalbed in transitional stress and compressional stress areas is below 1000 m3. The vertical well þ active water fracturing stimulation development mode is inapplicable; instead, effective measures should be taken to relieve stress, induce cleat fissures to open and improve the coalbed permeability. 1.4. Ratio of critical desorption pressure to initial reservoir pressure The ratio of critical desorption pressure to initial reservoir pressure is an index to visually expressing the dynamic strength of methane from desorption and diffusion to flowing to the bottom hole. The higher the ratio of critical desorption pressure to initial reservoir pressure is, the higher the gas saturation of coalbed, the stronger the dynamic field of methane, and the easier the gas flows due to desorption under the same pressure drawdown; meanwhile, the negative effect of drilling and fracturing fluids on the critical desorption
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are determined to be the main control parameters for building engineering geological model of high-rank coalbed methane. The area where primary coal is developed is selected as the prospect. The prospect is firstly divided into 2 types of zones with the vitrinite reflectance of 3.8% as a limit. Then, the prospect is divide into tensional and transitionalcompressional stress areas based on the fault attribute and triaxial stress combined features. Strong and weak dynamic fields are classified given the ratio of critical desorption pressure to initial reservoir pressure of 0.57. Finally, 4 engineering geological models are built using a progressive mode based on the classification criteria of the 4 main control geologic parameters (Table 1). Fig. 3. Ratio of critical desorption pressure to initial reservoir pressure vs. daily gas outputs of coalbed methane vertical wells in the ZhengzhuangeFanzhuang block.
pressure of reservoir can also be weakened, and accordingly, the gas flow dynamic becomes stronger [17,18]. Therefore, it is very significant practically that the ratio of critical desorption pressure to initial reservoir pressure is used as one of the key parameters for optimizing the main development engineering technologies of coal reservoirs. The relation between ratio of critical desorption pressure to initial reservoir pressure and vertical well gas flow rate of coalbed methane is shown in Fig. 3. It is seen that two areas are divided given the ratio of critical desorption pressure to initial reservoir pressure is 0.57. When the ratio of critical desorption pressure to initial reservoir pressure is higher than 0.57, majority of vertical wells have the daily gas flow rate per well exceeding 1000 m3, basically suggesting the beneficial development of a CBM field. When the ratio of critical desorption pressure to initial reservoir pressure is lower than 0.57, majority of vertical wells have the daily gas flow rate per well of lower than 1000 m3. This means that when the ratio of critical desorption pressure to initial reservoir pressure is lower than 0.57, vertical well fracturing cannot contribute to the efficient development of a CBM field, and new development engineering technologies must be adopted. 2. Engineering geological model of high-rank coalbed methane 2.1. Engineering geological model construction Coal texture, rank of coal metamorphism, in-situ stress and ratio of critical desorption pressure to initial reservoir pressure
2.2. Demand of different engineering geological models for main development engineering technologies Demands of different engineering geological models for main development engineering technologies are different. For Model 1, a coal reservoir is hard and brittle with good fracturability and low requirements for development engineering technologies; in its development flexible well type is needed for an overall control on high-quality reserves. For Model 2, a coal seam is poor in fracturability, but relatively favorable in the in-situ stress state and a methane dynamic field; in its development a well type with a large drainage area is needed, and application of fracturing stimulation is decided by the demand of productivity. For Model 3, a coal seam is poor in fracturability and in a weak methane dynamic field, but relatively favorable in the in-situ stress state; in its development a well type with a large drainage area is needed, and lowpollution stimulation will be adopted based on the demand of productivity. For Model 4, a coal seam is poor in fracturability, unfavorable in the in-situ stress state and poor permeability; in developing this coal seam, specific efforts should be made to further increase the exposure of wellbore to coal seams, and release of stress need to be solved. 2.3. Applicability of current development engineering technologies The development engineering technology dominated by vertical wells & active water fracturing and aided by open hole multilateral horizontal wells is adopted in the ZhengzhuangeFanzhuang block. Development wells in the study area were compared for production performance,
Table 1 Engineering geological models of high-rank coalbed methane. Coal texture
Primary
Rank of metamorphism
Ro Ro Ro Ro
> < < <
3.8% 3.8% 3.8% 3.8%
State of stress
Ratio of critical desorption pressure to initial reservoir pressure
Engineering technical geological model
e Tensional Tensional Transitional compressional
e >0.57 <0.57 e
Model Model Model Model
1 2 3 4
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Table 2 Statistics of production performance of vertical wells in different engineering geological models.
Table 3 Statistics of production performance of open hole multilateral horizontal wells in different engineering geological models.
Level of Items compared output/(m3$d1)
Model 1 Model 2 Model 3 Model 4
Level of output/ Items compared (m3$d1)
Model 1 Model 2 Model 3 Model 4
>2000
127 3857
58 2881
10 2419
30 2898
>10000
9 14 859
3 15 850
41.78% 85 1473
9.42% 181 1362
8.93% 36 1359
3.65% 130 1376
5000e10000
32.14% 8 7405
8.11% 2 5484
27.96% 45 747
29.38% 151 746
32.14% 23 733
15.84% 144 727
3000e5000
28.57% 5 3612
5.41% 2 3923
3 4740
14.80% 47 234
24.51% 226 176
20.54% 43 181
17.54% 517 129
<3000
17.86% 6 969
5.41% 30 802
9.38% 29 816
15.46%
36.69%
38.39%
62.97%
21.43%
81.08%
90.63%
1000e2000
500e1000
<500
Number of wells Average daily gas production/m3 Proportion Number of wells Average daily gas production/m3 Proportion Number of wells Average daily gas production/m3 Proportion Number of wells Average daily gas production/m3 Proportion
indicating that the applicability of development engineering technologies varies for engineering geological models. 2.3.1. Vertical well fracturing Vertical wells can be deployed flexibly. According to the statistics of production performance of vertical wells in different geological models (Table 2), the vertical wells in Models 1e4 areas with average daily gas flow rates more than 1000 m3 account for 69.74%, 38.80%, 41.07% and 19.49% of total wells, respectively. Therefore, vertical wells are very applicable in the Model 1 area, but not applicable in Models 2e4 areas with low ranks of coal metamorphism, soft coal, and poor fracturability. 2.3.2. Open hole multilateral horizontal wells When open hole completion is conducted in horizontal section of multilateral horizontal wells, the effective coal seam footage is long, the drainage area is large, and the output is high. According to the statistics of production performance of open hole multilateral horizontal wells in different geological models (Table 3), the open hole multilateral horizontal wells in Model 1 area with an average daily gas flow rate per well of more than 5000 m3 and 10000 m3 account for 60.71% and 32.14% of total wells respectively in the area. The open hole multilateral horizontal wells in Model 2 area with an average daily gas flow rate per well of more than 5000 m3 only account for 13.52% of total wells in the area. Sidewall collapse occurred in drilling and gas recovery by water drainage is the major cause for a large number of stripper wells in Model 2 area. Except for the sidewall collapse wells, the average daily gas flow rate of the 5 wells with stable sidewalls in the area can reach 11700 m3. In Model 4 area, 32 open hole multilateral horizontal wells in production mostly have low yields due to sidewall collapse. It is found that the open hole multilateral horizontal well has high requirements for reservoir and structure. In the structure-complex areas, drilling operations are exposed to huge risks due to the change of coalbed
Number of wells Average daily gas production/m3 Proportion Number of wells Average daily gas production/m3 Proportion Number of wells Average daily gas production/m3 Proportion Number of wells Average daily gas production/m3 Proportion
occurrence and the geologic conditions like structural complexity. Especially in the area with Ro less than 3.8%, sidewall collapse may easily occur, which will usually lead to low yield or shut down, and make subsequent wellbore maintenance or entry via stimulation a challenging task [19]. Therefore, the open hole multilateral horizontal wells cannot be widely applied but will not possibly become the major well type. 2.3.3. U-shaped and roof tree-like horizontal wells For avoiding the collapse of borehole, U-shaped horizontal wells and roof tree-like horizontal wells are creatively proposed. The U-shaped horizontal well is characterized by the combination of engineering well and cavity well for threespud-in drilling, PE screen completion and gas recovery from cavity well. Its field application shows that the wellbore is small, leading to the failure of stress release for improving stress field and fissure field. In the area where the coal seam is soft, the resulted slack coal may block the wellbore, making the reentry impossible for drifting and stimulation. Currently, the daily gas flow rate per well is lower than 3000 m3, and high investment cannot meet the demand of efficient development. The roof tree-like horizontal well is designed with the main hole for communication, branch hole for controlling the drainage area, and laterals for realizing higher deliverability. The main hole is drilled in the roof and its sidewall is fixed by chemicals, so the wellbore may not collapse, the footage is long, and the drainage area is large. This well type has contributed a high yield in the area where the thermal evolution level and the ratio of critical desorption pressure to initial reservoir pressure are relatively low. For instance, the peak gas flow rate of Well Zhengshi 1-P5 is 10748 m3/d, while only 5 of 14 adjacent vertical wells produce gas, with an average daily gas flow rate of 243 m3. However, this type of well is costly, with single-well cost of more than CNY20 million. Moreover, it needs more well sites, and has very high requirements for
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Fig. 4. Optimization process of efficient development engineering technology for high-rank coalbed methane.
geologic conditions and coal seam roof [20]. Roof tree-like horizontal well practices prove that effective development can be realized by horizontal wells with stable main holes, but the technological design must be improved to reduce the cost and investment. 2.3.4. Fish-bone and L-shaped horizontal wells The fish-bone and L-shaped horizontal wells are new types of horizontal wells upgraded on the basis of U-shaped and roof tree-like horizontal wells. They are drilled using two-spud-in mode, and completed with Ø139.7 mm casing or screen. They have the following advantages. ① The drilling period is as short as 13 days at the most. ② The hole size is large, which is favorable for stress release, inducing the opening of cleat fissures, and improving the stress field. ③ Wells can be deployed in a parallel laneway style perpendicular to the direction of the maximum horizontal principal stress, so that more fractures can be communicated. Furthermore, vertical wells can be deployed among horizontal well groups to organically connect the horizontal wells, thus forming a balanced pressure drawdown field. ④ The drilling cost is low and can be controlled below CNY3.5 million, and the economic limit rate is only 2460 m3/d. ⑤ Well deployment is relatively flexible, and the wells can be maintained and operated at the late stage. L-shaped horizontal wells can be deployed in the primary texture coal areas and tensional stress areas to enhance the production by staged fracturing. Nitrogen unclogging or sectional cavity creating can be used in the transitional stress and compressional stress areas with primary coal. In the areas where a coal seam is relatively soft, fishbone horizontal wells can be drilled to enlarge the pressure relief area of the coal seam. 3. Building and application of optimization model for efficient development engineering technology of highrank coalbed methane 3.1. Building of optimization model for efficient development engineering technology The optimization model for efficient development engineering technology (Fig. 4) for high-rank coalbed methane is built based on the above-mentioned engineering geological
models of high-rank coalbed methane and the available development engineering technologies. Model 1: A flexible vertical well cluster is adopted to realize an overall control on high-quality reserves, and active water fracturing stimulation is adopted to enhance the production rates. Model 2: An L-shaped horizontal well cluster is adopted. The footage in horizontal section is 1000 m. The coal exposure length is 7.14 times that of vertical wells (the artificial fracture length of a vertical well is deemed as 140 m). The single-well drainage area is 5 times that of vertical well. Screen or casing completion is adopted. The sidewall is stable. Stimulation treatment can be realized (Table 4). Model 3: An L-shaped horizontal well cluster is adopted. Low-damage cavity fracturing or nitrogen reaming is adopted to stimulate coal seams. Model 4: Fish-bone horizontal wells are adopted, with the footage of 2800 m in horizontal section, which further increases the coal exposure length to be 20 times that of vertical wells. The drainage area is 7.3 times that of vertical wells. Low-damage cavity fracturing or nitrogen reaming is adopted to stimulate the main hole, increase the hole size, relieve the stress, induce the cleat fissures to open and increase the permeability of coal seams (Table 4). 3.2. Application effects The horizontal distribution map of engineering geological models for the ZhengzhuangeFanzhuang block is shown in Fig. 5. Pertinent efficient development engineering technology is selected based on the optimization model for efficient development engineering technology for high-rank coalbed methane, and its application has shown preliminary effects. In Table 4 Main parameters of different well types. Well type
Length of horizontal section or artificial fracture/m
Single well drainage area/km2
Vertical wells L-shaped horizontal wells Fish-bone horizontal wells
140 1000 2800
0.078 0.390 0.568
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Fig. 5. Horizontal distribution map of engineering geological models for the ZhengzhuangeFanzhuang block.
Model 1 area, a total of 304 vertical wells were put into production, active water was used for fracturing, and the average daily gas flow rate per well reached 2170 m3, recording an efficient development. In Model 2 area, a total of 6 L-shaped horizontal wells were drilled, of which 2 wells failed to produce gas due to geologic and engineering factors, while 4 wells achieved high yields, with the average daily gas flow rate and the average gas producing index of pure coal footage of 3224 m3 and 5.3 m3/m respectively, considerably higher than the expected levels, and the gas flow rate can be further enhanced. 4. Conclusions 1) The major geologic factors affecting the development effect of high-rank coalbed methane are coal texture, rank of coal metamorphism, in-situ stress and ratio of critical desorption pressure to initial reservoir pressure. Accordingly, 4 engineering geological models are built. The vertical well fracturing and open hole multilateral horizontal wells are less applicable to the 4 models. Therefore, roof tree-like horizontal well and U-shaped horizontal wells are developed, and finally, they are upgraded to the L-shaped horizontal wells and fish-bone horizontal wells that can be drilled at a low cost, maintained at the late stage, cover a small area. The Lshaped horizontal wells and fish-bone horizontal wells can be widely applied in the whole block.
2) Based on different geologic demands of the 4 engineering geological models, suitable well types and stimulation techniques are determined, and the optimization method and process of efficient development engineering technology for high-rank coalbed methane are defined. Vertical wells & active water fracturing development engineering technology is proposed for Model 1 areas. L-shaped horizontal wells or L-shaped horizontal wells & sectional fracturing development engineering technology is proposed for Model 2 areas. L-shaped horizontal well cavity fracturing or nitrogen reaming development engineering technology is proposed for Model 3 areas. Fish-bone horizontal well cavity building or nitrogen reaming development engineering technology is proposed for Model 4 areas. 3) The geologic conditions of high-rank coalbed methane are complex and largely different in various regions of China. Therefore, the classification criteria of optimization methods can be properly adjusted depending on the actual conditions and further optimized in practical application.
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