Numerical simulation of coalbed methane generation, dissipation and retention in SE edge of Ordos Basin, China

International Journal of Coal Geology 82 (2010) 147–159

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International Journal of Coal Geology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / i j c o a l g e o

Numerical simulation of coalbed methane generation, dissipation and retention in SE edge of Ordos Basin, China Chongtao Wei a,b,⁎, Yong Qin a,b, Geoff G.X. Wang c, Xuehai Fu a,b, Zhiqing Zhang a a b c

School of Resource & Geoscience, China University of Mining and Technology, Xuzhou Jiangsu 221008, China Key Laboratory of CBM Resource and Reservoir-generating Process, China Ministry of Education, China School of Chemical Engineering, The University of Queensland, Qld 4072, Australia

a r t i c l e

i n f o

Article history: Received 11 May 2009 Received in revised form 2 December 2009 Accepted 4 December 2009 Available online 6 January 2010 Keywords: Coalbed methane Reservoir formation history Modelling Methane accumulation Gas content prediction

a b s t r a c t This paper presents a numerical study on the formation history of coalbed methane (CBM) reservoir in the southeast edge of Ordos Basin, China. The coal seams studied belong to the Late Palaeozoic coal-bearing series. These coal seams have a burial history and experienced the process of subsidence, rapid subsidence alternated with uplift and then uplift, sequentially, and underwent the geothermal actions at normal, extremely high, and then normal temperatures, respectively. Coal organic matter of the coal seams matured in the Triassic Period and in the Late Jurassic to Early Cretaceous Period. The results from numerical simulation reveal that CBM reservoir evolution history can be classified into five stages, namely primary, initial, stagnant, active and dissipative stages. In the first (primary) stage, coal rank was very low and there was little methane generated and stored in the coal seams. In the second (initial) stage, the coal was converted to middle-high volatile bituminous coal. As a result, a certain amount of methane was generated and began to accumulate in coal seams except part of it escaped from coal seams by diffusion and cap outburst. In the third (stagnant) stage, generation of methane was almost stagnant due to the temperature of the coal seam that dropped slightly and the maturation of coal organic matter stopped. Meanwhile CBM would keep dissipating through diffusion. In the fourth (active) stage, coal rank varied from high volatile bituminous coal A to semianthracite which accelerated pyrolysis gas formation and resulted in a large amount of methane generated at a high speed. During this period, CBM was increasingly accumulated in coal seams although there would be considerable amounts of gas dissipated from the coal seams. In the last (dissipative) stage, due to coal seams uplifted at various rates and no more methane generated, CBM was continuously dissipated by diffusion throughout the whole coal seams and by permeation at some local areas. The simulation provides insights for further interpretation of how many factors that control or affect the CBM reservoir formation history and CBM accumulation. These factors include features of coal-bearing series, characteristics of coal seams, physical properties of coal reservoir, tectonic evolution history, burial history and geothermal conditions, etc. In particular, tectonic evolution history and gas generation are critical. Under given conditions, CBM reservoirs in the study area were developed in different ways and the CBM was accumulated in the reservoirs at different levels. For example, the west part of study area is favourable for CBM accumulation. As a result, the gas content of the main coal seams in this region has a maximum of about 28 m3/t at depths of 900–1100 m, and generally increases with the increasing of burial depth from the east to the west. The coal reservoir is under-saturated in the east part where the burial depth is shallower than about 500 m while the west part is saturated. There is a close correlation of the lateral distribution of both gas content and saturation to the gas generation in the geological history. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Numerical simulation is an effective method widely used to study coalbed methane (CBM) well performance, CBM well parameters optimisation, CO2 Enhanced CBM (CO2-ECBM), CO2 geosequestration

⁎ Corresponding author. Tel./fax: +86 516 83591000. E-mail address: [email protected] (C. Wei). 0166-5162/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.coal.2009.12.005

and coal mine gas migration and emission (David et al., 2003; Karacan, 2008; Young, 1998; Wei et al., 2007b). Although there are considerable literatures about modelling the formation of petroleum and gas bearing reservoirs, studies on formation history of CBM gas reservoirs are very limited. For example, Payne and Ortoleva (2001) proposed a numerical model to simulate gas generation from lignin-derived sedimentary organic matter associated with coal seams of the Piceance Basin, Alsaab et al. (2007) studied the coalification history, including generation, expulsion and migration of hydrocarbons in coals from

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Fig. 1. Location of the study area.

Fig. 2. Burial depth (a) and Ro,

max

(b) contour map of coal seam No. 9 (or No. 8 + 9 + 10) (modified after Gui, 1993).

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The Ordos Basin locates in the west of Shanxi Province, China, and the SE edge of Ordos Basin, which is the study area in this paper, covered several coal seams is one of the important CBM exploitation areas in China (Fig. 1). The study area is about 2700 km2, with the coal seam out croup line as its east boundary and Yellow River as its west boundary. In the past few decades, several coal geology survey projects have been carried out based on this area (Gui, 1993). More recently, the study area has become one of the hottest areas for CBM development in China, resulted in a series of research and exploitation projects undertaken by national and international companies. These projects not only aimed at the CBM reservoirs in the SE edge of Ordos Basin (Ye et al., 1998; Wei et al., 1998; Chi, 1998; Su et al., 2003; Li, 2004) but also at those in neighbouring area such as Hancheng, southwest to the study area in this paper (Yao et al., 2009). A large amount of geological data related to coal and CBM gas have been generated and analysed through these projects, providing a primary database of geological structure, burial history and thermal evolution history of the CBM reservoirs in this particular area. Such a database makes it possible to numerically simulate the formation history of the CBM reservoirs using the geological model previously developed by the authors. This numerical simulation provides a tool to identify the characteristics of CBM gas accumulation in coal and hence to develop conceptual models for CBM exploitation in the target area. 2. Geological settings 2.1. Coal-bearing series and coal seams

Fig. 3. Strata column of study area (Note: 5d — coal seam No. 5 down, 6u — coal seam No. 6 upper, 8u — coal seam No. 8 upper, 10u — coal seam No. 10 upper) (modified after Gui, 1993).

Donets Basin, Ukraine, by using the software of PetroMod. Several studies have been reported in open literature to investigate the effect of burial history, geothermal history and maturity etc. on CBM gas accumulation and potential. However, all these studies were limited in the analytical discussion which was in general qualitative and did not deal with numerical simulation at all (Johnson and Flores, 1998; Boreham et al., 1998; Faiz et al., 2007; Hackley et al., 2009). The authors of this paper have developed a geological model and relevant computer program, which can be used for numerical simulation of CBM gas generation, retention and dissipation in geological history under given reservoir conditions (Wei and Sang, 1997; Wei et al., 1998; Wei, 1999, Wei et al., 2007a). In this study, the model will be extended to apply to the CBM reservoirs in the southeast (SE) edge of Ordos Basin, China, in order to provide comprehensive information for a better understanding of the formation history of the CBM reservoirs.

In the study area, strata include the Ordovician System, the upper Pennsylvanian System, the Permian System, the Triassic System and the Quaternary System. Rock layers generally dips westward with the dip angle of 5–15° (Fig. 2a). The coal-bearing series includes Taiyuan Formation of the Pennsylvanian–Lower Permian System and Shanxi Formation of the Lower Permian System (Fig. 3). Average thickness of coal-bearing strata is 157.02 m, total net coal thickness is 19.42 m. Average thickness of Taiyuan Formation is 96.22 m with 10.93 m net coal seams. Coal seam No. 9 or No. 8 + 9 + 10 is the main coal seam, which occurs in middle or lower part of the Formation. The roof of coal seam No. 9 is limestone while the floor is mudstone or sandy mudstone. Average thickness of Shanxi Formation is 60.80 m with 8.4 m net coal seams. Coal seam No. 5 is the main coal seam, which occurs in the middle of the Formation. Both of the roof and floor of coal seam No. 5 are mudstone or sandy mudstone (Gui, 1993). Table 1 shows the features of the two main coal seams. All the coals occurring in the study area are classified as humus coal, which mainly bears vitrinite and inertinite and there is very rare liptinite. Coal rank ranges from bituminous coal to semianthracite (Ro, max = 0.85–2.45%, refer to Fig. 2b). 2.2. Tectonic, burial and geothermal evolution history 2.2.1. Tectonic and burial evolution history Since the Pennsylvanian Period, the SE edge of Ordos Basin experienced a series of tectonic movements. Slow subsidence of the Hercynian Movement resulted in the wide spread deposition of the

Table 1 Properties of two main coal seams in the southeast edge of Ordos Basin. Coal seam

5 9/(8 + 9 + 10)

Coal rank (Ro, max/%)

Bituminous (0.84–2.44) Bituminous-anthracite 1.05–2.45

Thickness/m

1.20–6.46 2.77 2.95–6.39 4.14

Coal quality Mad/%

Ad/%

Vdaf/%

0.46–5.78 1.14 0.46–1.22 0.62

4.88–36.17 15.68 5.82–37.11 14.67

12.42–36.16 20.63 14.32–32.73 19.72

Note: Mad: moisture, air dry basis; Ad: ash yield, dry basis; Vdaf: volatile content, dry ash-free basis. The format of data: min–max; min–max/average (data mainly come from Gui, 1993).

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Fig. 4. Burial and geo-temperature curves of coal seam No. 9.

Late Pennsylvanian–Early Permian System of coal-bearing series throughout the whole North China. From the Late Hercynian to the Indo-Chinese Epoch, the Kula-Pacific plate was formed and began to dive west, which led to the subsidence of whole Ordos Basin. The subsidence rate rapidly increased in the Indo-Chinese Epoch. This caused a large amount of fluvial and lacustrine facies clastic rock of the Triassic System deposited and formed a buried rock bed with a thickness of about 3000 m. From the Late Indo-Chinese to the Early Yanshanian Epoch, Ordos Basin reduced in size from the east to the west, which resulted in the erosion of the Triassic System. In the late Yanshanian Epoch, the Kula-Pacific Plate dived westward, which led to the development of a strong NW palaeo-stress field in the North China. Then, the whole SE edge of Ordos Basin was subjected to uplift and erode. In some areas, magmatic intrusions were formed. In this period, the regional tectonic framework of the study area was formed. In the Himalayan Movement, some parts of the coal-bearing series were uplifted nearly to the surface. Fig. 4a shows the burial history of coal seam No. 9 in various regions of study area obtained by using the residual strata thickness information (Tang et al., 1992; Chen et al., 2006; Peng and Wu, 2006).

2.2.2. Paleo-geothermal field evolution history From the Late Palaeozoic to the Early Mesozoic Era, the SE edge of Ordos Basin changed from littoral-shallow sea basin to inland craton basin, and later to a foreland basin. In that period, its geothermal gradient varied in a range of 2.2–2.4 °C per 100 m. While in the Late Mesozoic Era, it rose to 3.3–5.7 °C per 100 m because of the strong effects of the Middle Yanshanian Movement. In the Cenozoic Era, uplift was continued and the earth's crust became thicker and thicker. As a result, the geothermal gradient dropped back to 2.0–3.2 °C per 100 m (Tang et al., 1992; Ren, 1996; Zhou et al., 1998). Based on the above data and burial history data, the geothermal field evolution history of the SE edge of Ordos Basin can be determined as illustrated in Fig. 4b. According to the geothermal field evolution history, the maturation history of coal organic matter has been reconstructed (Fig. 5). 3. Physical and chemical properties of coal seams Experimental work presented here is to provide necessary information about the physical and chemical properties of main coal seams which are required for numerical simulation to be described later. A limited experiment was conducted because this study only focuses on numerical simulation, including mining site observations and laboratory tests. Coal mines such as Dujiagou, Maozequ and Taitou coal mine in the south, and Shaqu Coal Mine in the north of the study area were chosen for site observation and sampling, referring to Figs. 1 and 2b. A total of 30 samples were collected for laboratory tests. Coal rank of all samples from the observation sites was analysed and can be classified to low volatile bituminous coal. Cleat structure of the

Table 2 Cleat of the two main coal seams in SE edge of Ordos Basin.

Fig. 5. Maturation curves of coal seam No. 9.

Coal mine

Coal seam

Cleat

Strike/°

Dip angle/°

Density/sets per m

Dujiagou

9

Zhangjieta

9

Maozequ

9

TaiTou

5

Shaqu

5

Face Butt Face Butt Face Butt Face Butt Face Butt

33 130 30 160 35 115 40 115 80–90 15

87 77 80 Nearly vertical 86 76–87 Nearly vertical 85 70–90 70–90

62 62 75 – 61–79 26–40 250 101 200 180

(Modified after Wei et al., 1998).

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coal samples can be clearly observed. The face cleat nearly exhibits in the NW–EW direction with the dip angle from 70° to 90° and the density from 60 to 200 sets per m. Butt cleat is nearly perpendicular to the face cleat and its density is a little lower than that of face cleat (Wei et al., 1998). The results are summarised in Table 2. Mercury intrusion and gas adsorption tests were carried out using the samples collected from coal mines above in laboratory. The results indicate that the porosity of coal reservoir varies at 12.1–48.9 mm3/g and the porosity of coal seam No. 5 is generally a little higher than that of coal seam No. 9. Adsorption Isothermal tests show that the Langmuir volume ranges from 12.40 to 25.67 m3/t and Langmuir pressure from 1.21 to 1.94 MPa. The results are typically listed in Table 3 (Wei et al., 1998; Yao and Yin, 2006). Note that the data listed in Tables 2 and 3 do not cover the whole study area because of the limited sampling spots. The physical and chemical properties of coal seams excluded in this experimental work and required for numerical simulation were obtained from internal geological survey reports. 4. Model and simulation 4.1. Model description The model used in this study for numerical simulation of CBM reservoir formation consists of sets of geological and mathematical sub-models. The model has been developed with a set of computer program through previous studies and successfully applied to simulation of the geological evolution history of CBM reservoirs (Wei and Sang, 1997; Wei et al., 1998; Wei, 1999; Wei et al., 2007a). A brief description about the model is discussed as follows and more details can be found elsewhere (Wei et al., 2007a). The key principle of the model is the mass conservation under which a dynamic equilibrium of CBM generation, retention and dissipation has to be maintained. In other words, at any random time and for any portion of coal body, the quantity of methane preserved in coal reservoir must equal to difference between the quantity of methane generated in coal and the amount of CBM gas dissipated from the coal reservoir. The model includes two types of parameters, i.e. physical characteristics of CBM bearing system and geological evolution history of coal reservoirs, respectively. These parameters are dynamic and strongly reservoir-dependent. The parameters related to physics of CBM bearing system are coal seam dimensions (thickness, distance of neighbouring coal seam and burial depth etc.), coal properties (density, moisture, ash yield and sulphur content etc.) and coal reservoir physics (porosity, cleat, Langmuir volume, and Langmuir pressure constant etc.). The parameters associated with geological evolution history include the tectonic evolution history, burial history and coal organic matter maturation history. Followed by the principle as described above, a CBM bearing basin model can be established with aforementioned model parameters, consisting of three components as follows. 4.1.1. Generation of CBM Gas generation potential of the coals and various organic matters is commonly estimated with thermal simulation tests and varies from

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one coal reservoir to other, depending on coal types and geologic conditions under which the coal reservoir was formed (Hunt, 1991; Liu et al., 1997; Zhu et al., 2004). Methane generation amount used for simulation in this study is described by using a function defined by the gas generation on pure organic material basis, which is mainly depended on the vitrinite reflectance (Ro, max). It is obtained by coal characterisation and thermal simulating tests of coal samples. A socalled Easy%Ro method (Sweeney and Burnham, 1990; Littke et al., 1994) was adopted to determine the key parameter of Ro, max, providing the methane generation amount as a function of on the vitrinite reflectance Ro, max. Note that biogenic gas was neglected in the model because it is not the dominant component in the whole process of methane generation of coal organic matter. This is believed to be acceptable based on studies on formation of CBM in China. For instance, Qin et al. (2000) studied various CBM cross over China using Carbon isotope and revealed that biogenic gas is seldom found in Chinese coals during the coalification stage of pyrogenation, although CBM in China sometimes includes few mixture of biogenic, thermolytic and pyrolysis gas. 4.1.2. Retention of CBM Most of gas stored in coal reservoirs is in adsorbed state and the rest could be stored within fractures and macro-pores in the form of free gas or aquatic dissolving phase. Usually the aquatic dissolving phase gas can be ignored because of its limited quantity in coal reservoirs. To determine the retention of CBM gas, the model used the Langmuir equation to estimate the quantity of adsorbed gas, and the modified gas equation of state to calculate the amount of free phase gas (treated as non-real gas). In order to solve the two equations above, dynamic parameters of reservoir pressure and temperature are necessary. In the simulation, the temperature for chosen reservoir was obtained by temperature distribution of palaeo-geothermal field, and the reservoir pressure was approximately estimated by evaluating if the reservoir pressure system is opening or close (Wei et al., 2007a,b). For opening system, there is opening passage such as fault or outcrop of coal seam between coal reservoir and open atmosphere. In this case, reservoir pressure will be defined as hydrostatic pressure. For close system, coal reservoir is confined by cap rock, and swelling energy of gas in the pores of coal reservoir is the source of reservoir pressure. Gas equation of state will be adopted to calculate reservoir pressure in this case. If coal seam is deep buried and hydrostatic pressure is higher than the pressure caused by gas swelling, the hydrostatic pressure will be used as reservoir pressure. 4.1.3. Dissipation of CBM In the evolution process, gas dissipation can occur in three forms, i.e. diffusion, cap outburst and permeation. Diffusion of CBM is basically caused by the difference of gas concentration between coal reservoir and cap rock. Fickian's first Law was used in the model to calculate diffusion quantity by assuming that methane transport through cap rock in vertical direction only. The diffusive dissipation strength, defined as the diffusion quantity of 1 m2 horizontal area, was used to describe the development degree of diffusion dissipation.

Table 3 Porosity and displacement pressure of the two main coal seams in SE edge of Ordos Basin. Coal mine

Maozequ Taitou Zhangjieta Shaqu

Coal seam

5 5 5 5

Coal rock

Dull coal Dull coal Semi dull coal Semi dull coal

Ro, max/%

1.41 1.43 1.49 0.84

Pore volume/mm3 g− 1

Ratio of pore volume/% V1/Vt

V2/Vt

V3/Vt

V4/Vt

48.9 36.1 66.7 12.1

57.5 45.2 5.1 53.42

6.7 3.3 9.9 6.28

27.8 39.3 29.8 4.81

8.0 12.2 9.3 35.49

Displacement pressure/MPa

Languir volume/m3/t

Languir volume/MPa− 1

41.1 64.4 34.9 –

12.40 23.23 25.67 19.52

1.21 1.77 1.94 1.88

Note: V1: macro-pore, Φ N 1000 nm; V2: medium-pore, Φ100–1000 nm; V3: small-pore, Φ10–100 nm; V4: micro-pore, Φ b 10 nm; Vt: total pore volume (modified after Wei et al., 1998).

Fig. 6. CBM geological evolution history curves of coal seams Nos. 5 and 9 at simulation spot No. 69.

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Cap outburst is usually caused by abnormally high reservoir pressure resulting from high-speed gas generation. Micro-fissures may occur in cap rock when abnormally high pressure emerges and a large amount of gas is dissipated through the fissures by means of permeation. This would cause the considerable pressure drop in reservoir as well. As a result, the micro-fissures would close, which will in turn lead to the stop of permeation. Generally speaking, gas generation in coal is continuous in evolution of coal reservoir, while the cap outburst is periodic due to alternation of the opening and closing of micro-fissures in cap rock. The gas dissipation amount of cap outburst cannot be calculated by the Darcy's Law because of the difficulty in getting basic parameters such as pressure gradient and permeability of micro-fissure. Therefore the model introduced a pressure-determines-gas-content approach to estimate gas amount of cap outburst. The model firstly calculates the pressures associated with the micro-fissure opened and closed and then the pressures will be used to calculate the gas content under given geo-temperatures using modified gas equation of state. The gas dissipation amount of cap outburst can finally be determined by the difference of gas contents at the two pressures. Under a condition that reservoir pressure is abnormally high or/ and tensile geo-stress field emerges, cleat system might open and gas permeated within coal seam through the cleat system. As a result, part of methane was dissipated through outcrops of coal seams or from extensive faults cross the coal seams. The gas quantity dissipated by permeation can also be obtained using the similar method to the calculating dissipation amount of cap outburst. 4.2. Simulation procedure A computer program, named as Coalbed Methane History Simulator (CBMHS) developed based on the model described above (Wei and Sang, 1997; Wei et al., 1998; Wei, 1999; Wei et al., 2007a), was used for simulation in this study. CBMHS consists of four parts, i.e. computation module, data interface module, basic and resultant databases. The input parameters required for simulation include data of CBM bearing system and geological evolution history of coal reservoirs, which have been discussed above. The key output variables include gas generation, gas content, reservoir pressure and dissipating quantity of gas, etc. Before simulation, a grid net of 1000 × 1000 m covering the whole study area were constructed through a built-in pre-processor in CBMHS, providing the 109 simulation spots in total. The characteristic parameters of coal seam, coal properties, coal-bearing series and the tectonic, burial, and thermal evolution parameters were collected for each spot from experimental measurements or literature as described previously, providing the input parameters for CBMHS simulation. Every spot has been evaluated by running CBMHS and the evolution parameters such as gas generating amount, gas content, reservoir pressure, and dissipation strengths of diffusion, cap outburst and permeation at each spot were obtained and output to the resultant database in the course of simulations at each run. The database can be accessed off-line, providing numerical information for further analyses on formation of CBM reservoir such as CBM generation, retention and dissipation history in given location, and even the lateral distribution of these evolution parameters. In order to obtain the detailed information about gas generation, retention and dissipation quantities, the so-called “block method” was implemented here. As mentioned above, the study area was divided into 109 blocks according to the grid net. The borderlines of each internal block are formed by grid lines, while the borderlines of each boundary block are consisted of 3 sections of grid lines and margin line of the study area. The area of each block can be calculated by the software such as Auto CAD. The thickness, the density and the dip angle of main coal seams for each block were extracted from the pre-built basic database. The mass of coals for the block can be calculated from these data and then used for computation of various output parameters as mentioned above.

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5. Discussion 5.1. CBM reservoir evolution history 5.1.1. CBM geological evolution stages Fig. 6 shows the evolution of cumulative gas generation, gas content, reservoir pressure, and cumulative dissipation strength (including diffusion, cap outburst and permeation) for coal seams Nos. 5 and 9 at a representative simulation spot No. 69. This spot locates at about 22 km west of Yonghe (refer to Fig. 2a). Based on these simulation results, CBM geological evolution history can be divided into 5 stages as follows. Stage 1 (primary stage) occurred in the Late Permian Period (evolution time: 0–48 Ma). In this stage, both gas generation and gas content were extremely low as shown in Fig. 6a and b. Reservoir pressure increased with the increasing of burial depth. Gas diffusive strength was also weak. Cap outburst and permeation dissipation did not happen. This is because of the shallow burying and low coalification degree of coal seams. Stage 2 (initial stage) occurred in the Triassic Period (evolution time: 48–99.4 Ma). In this stage, coal seams subsided to 2700–4400 m in depth (refer to Fig. 4a); the highest temperature that coal seams bearing reached to 93 °C and coal rank reached to middle-high volatile bituminous coal (Ro, max 0.65–1.35%, refer to Figs. 4b and 5) because of the geothermal metamorphism. All cumulative gas generation, cumulative gas content and cumulative diffusion strength increased to some extent and reservoir pressure reached its maximum value because of the deep burying. Cap outburst happened in some areas, featured by the serration change of gas content and reservoir pressure (refer to Fig. 6b, c and e). There was no permeation dissipation occurred in this stage. As a result, CBM began to accumulate in main coal seams. Stage 3 (stagnant stage) lasted in the Early and Middle Jurassic Period (evolution time: 99.4–137.8 Ma). In this stage, coal seams uplifted and subsided repeatedly but generally uplifted (Chen et al., 2006). Gas generation in coal organic matter was almost stagnant, resulted in the gas content decreased and reservoir pressure dropped. Dissipation mainly occurred by means of diffusion during this period. Stage 4 (active stage) lasted from the Late Jurassic Period to the Early Cretaceous period (evolution time: 137.8–199.4 Ma). In this stage, the temperature of coal seams raised again followed by the drop at Stage 3. For example, the highest temperature of coal seam No. 9 at spot No. 69 increased to about 180 °C from less than 100°°C (refer to Fig. 4b). Coal rank in this stage varied from high volatile bituminous coal A to semianthracite (Ro, max 0.95–2.25%, refer to Fig. 5). Secondary hydrocarbon generation took place (refer to Fig. 6a) and hence formed large amount of gas in coal reservoir. This led to the increasing dissipation due to the increasing concentration difference of gases between coal seam and cap rock (refer to Fig. 6d). High gas content simultaneously resulted in high reservoir pressure or high swelling energy in coal reservoir pore system and hence accelerated the cap outburst and permeation dissipations. The sudden drop of the gas content and reservoir pressure curves in Fig. 6b, c, e and f indicates the increasing permeation dissipation. However, gas content still kept in a high level in the stage. It is obvious that active evolutions resulted in the high speed formation of CBM reservoir. Stage 5 (dissipative stage) occurred in the period of 199.4–299 Ma. Coal seams uplifted continuously and coal organic matter was mature and no longer generates any CBM gas. CBM dissipation occurred mainly through diffusion and in some areas, by permeation as well. 5.1.2. CBM reservoir evolution history Evolution history of simulation spot No. 69 discussed above shows the general feature of CBM evolution history. This feature would be variable, depended on the location, coal seam and coal-bearing series, and tectonic, burial and thermal evolution history. For example, Figs. 7 and 8 show the regional features of CBM evolution history in terms of distributions for the output parameters of interest.

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Fig. 7 presents the evolution history of coal seam No. 5. The cumulative gas generation amount of this coal seam varies in a range from 100 to 340 m3/t. The highest value locates on the southwest

region of the study area, changing to lower toward the north and east. This is quite similar to the distribution of coal seam burial depth (refer to Fig. 2a). Fig 7b shows the distribution of diffusion dissipation

Fig. 7. Lateral distribution of cumulative gas generation (a), diffusion (b), cap outburst (c) and permeation (d) for coal seam No. 5.

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Fig. 8. Lateral distribution of cumulative gas generation (a), diffusion (b), cap outburst (c) and permeation (d) for coal seam No. 9.

strength (defined as dissipation amount of gas per m2 from given coal seam). As can be seen, the diffusion dissipation strength ranges from 100 to 360 m3/m2. The lateral variation of diffusion dissipation

strength is similar to that of gas generation. This variable also has an obvious negative correlation with burial depth. Fig. 7c exhibits the distribution of cap outburst dissipation strength (defined as the cap

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Fig. 9. Contour map of recent gas content for coal seams No. 5 (a) and No. 9 (b).

outburst dissipation amount per m2 from given coal seam). It can be seen in the figure that outburst dissipation strength ranges from 100 to 1100 m3/m2, with the highest value located in the region in the southwest corner and between Puxian, Daning and Jixian. Fig. 7d illustrates the distribution of permeation dissipation strength (i.e. permeation dissipation amount per m2 from given coal seam). It shows that the permeation dissipation strength ranges from 10 to 80 m3/m2 and has the similar distribution as that of cap outburst dissipation strength. Fig. 8 indicates the evolution history of coal seam No. 9. The cumulative gas generation amount of this coal seam varies in a range from 100 to 362 m3/t. The regional distribution of other variables is similar to that as shown in Fig. 7. However the contour map looks more complex compared with Fig. 7. Fig. 9 illustrates gas content of coal seams No. 5 and 9 after about 300 Ma evolution. As can be seen, the highest values of gas content for coal seams Nos. 5 and 9 are a little higher than 20 and 28 m3/t,

respectively. Gas content of these two main coal seams has the lateral distribution that is generally corresponding to distribution of burial depth. Table 4 summarises the quantity of CBM generation, dissipation and retention of the whole study area. In Table 4, the gas generation amount, the gas content and dissipation (diffusion, cap outburst, permeation and total) gas amount at the end of each evolution stage were statistically obtained by analysing the various variables in the CBMHS database from simulation. As can be seen, both main coal seams has total gas generation of 3.20 × 1013 m3, total dissipation quantity is 2.93 × 1013 m3, among them, diffusion, cap outburst and permeation is 7.30 × 1012 m3, 1.12 × 1012 m3, and 2.09 × 1013 m3, respectively, and total gas-in-place resources is 2.67 × 1012 m3. 5.2. Factors affecting CBM reservoir formation Simulation study indicates that the key factor that controls CBM reservoir formation is the quantity and speed of methane generated

Table 4 Statistics data of CBM reservoir evolution history in the southeast edge of Ordos Basin. Stage

Coal seam

Stage gas generation/108 m3

CBM resource at the end of the stage/108 m3

Dissipation quantity/108 m3 Diffusion

Permeation

Cap outburst

Total

1

5 9 5 9 5 9 5 9 5 9

Little Little 16,776.84 104,975.16 0.00 0.00 28,050.43 169,721.78 0.00 0.00 319,524.2

Little Little 4329.68 25,176.20 2565.17 20,834.04 4680.80 28,091.25 3461.66 23,282.61 26,744.27

Little Little 9586.40 20,399.40 1764.51 3980.69 17,924.90 14,512.98 1115.15 3696.06 72,980.09

Little Little 0.00 0.00 0.00 0.00 885.86 9115.38 103.99 1112.58 11,217.81

Little Little 2860.76 59,399.56 0.00 361.47 7124.04 138,836.21 0.00 0.00 208,582.04

Little Little 12,447.16 79,798.97 1764.51 4342.16 25,934.80 162,464.57 1219.14 4808.64 292,779.95

2 3 4 5 Sum

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by coal organic matter. Among five stages, Stage 4 played the most important role for CBM reservoir formation and gas-in-place content. This was generally controlled by secondary hydrocarbon generation and caused by abnormally high geothermal field at the late Yanshanian Movement. Stage 2 also significantly contributed to CBM accumulation, depended on deep geothermal metamorphism. Taking Fig. 6a as an example, the speed of methane generation at Spot 69 is 1.07 and 4.94 m3/t per million years at Stages 2 and 4, respectively. This implies that the characteristics of geothermal field largely control the maturation degree of coal reservoir and hence amount and speed of gas generation in coal reservoir. It can be seen from Table 4 and Figs. 6–8 and 10 that at Stage 4, while a large amount of gas was generated and gas content rose rapidly, dissipation was also highly developed. In Stages 3 and 5, coal organic matter almost stopped gas generation and dissipation was weaker than that at Stages 2 and 4. As aforementioned, a large amount and highspeed gas generation would result in the development of dissipation and high gas content. This is the essential characteristic of dynamic equilibrium evolution of gas generation, dissipation and retention. It also implies that the gas generation process plays a significant role in controlling the evolution of dissipation and retention. However, methane generation is not independent. It is controlled by geothermal field and features of coal organic matter as well. The geothermal field has close relationship with burial history and magmatism, while coal organic matter with its deposition. The methane generation, geothermal field and coal organic matter all are basically controlled by tectonic movement history. The tectonic movement history also indirectly affects gas dissipation because the thickness and lithology of overlying strata are strongly affected by the tectonic movement history which controls the diffusion dissipation rate. Reservoir pressure also affected by the tectonic movement history, which would determine whether the cap outburst and/or permeation dissipations occur or not. Therefore the tectonic movement history is the fundamental factor that controls the whole process of CBM reservoir formation. Other factors, such as coal thickness, ash yield, water and sulphur content, small scale fault and fold, local variation of cap rock, etc., have relatively weak effect to the CBM reservoir formation, and the region affected by these factors is relatively small. It can be seen from Figs. 2 and 7–9 that burial depth, cumulative gas generation amount, diffusion dissipation strength and gas content have the generally similar lateral distribution. However magnitudes of these parameters vary in different regions, which implies their effects on CBM reservoir formation would be different. In the region surrounded by Daning, Puxian and Jixian, coal seam No. 5 has a thickness of about 4 m and the background value of about 2 m (Fig. 10a). It can be seen from Figs. 7c and 8d that the cap outburst and permeation dissipation took place stronger in this region than in

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the surrounding area. The same phenomenon can be found at the southwest corner as shown in Fig. 10a. The main reason is that thick coal led to the more organic matter and more gas generation, thus the stronger cap outburst and permeation dissipation. Similarly, Fig 10b shows that the ash yield distribution of coal seam No. 9 in the north of the study area. As can be seen, there is a low value belt striking in the NE direction. The gas generation, cap outburst and permeation dissipations in this region show higher values than in the neighbouring area, as illustrated in Fig. 8b, c and d. These results might be caused by the lower ash yield in given region, which directly led to the more organic matter concentration in the region. Fig. 11 summarises the overall effect of various variables on the CBM reservoir formation. Due to the complex action of these variables, CBM reservoirs in the study area developed in different ways and CBM gas accumulated in different degrees. Among these variables, the tectonic movement, burial history, geothermal field and coal seams are essential. The features of these variables dominate the whole CBM reservoir formation process. In addition, other variables also affect this process to some extent in different time. Higher CBM accumulation will be formed when the favourable factors take effect in particular area at given periods. 5.3. CBM gas accumulation Followed by the analysis on the results from CBMHS simulation, the recent geological conditions that are favourable for CBM reservoir formation in the study area can be figured out. They are thick coal seam, high coal rank, low ash yield, deep burying, and compact and thick cap rock. The west part of the study area, i.e., west of the Shilou–Yonghe– Daning–Jixian, is favourable for CBM accumulation (refer to Fig. 9). It can be seen in Fig. 9 that the gas content of main coal seams generally increases as the burial depth increases. In shallow part (burial depth of coalbed b500 m) of the study area, gas content is lower than 10 m3/t. In deep coal seams with a burial depth of coal bed of N1500 m, the gas content reaches about 20 m3/t. Simulation results reveal the lateral gas content distribution which has the following features. Firstly, it is very much similar to the distribution of cumulative gas generation (Figs. 7a and 8a), implying that gas generation controls CBM accumulation. Secondly, the CBM gas in shallow coal seams is easier to dissipate than that from deeper coal seams, implying that burial and dissipation control CBM accumulation. Moreover, the lateral variation of main coal seams, cap rock, and geological structure etc. would be a key factor that leads to the local changes of CBM accumulation, as discussed previously. Note that the CBM reservoir in the area where the burial depth of coal bed is shallower than about 500 m might be under saturation CBM reservoir. It can be evidenced by adsorption isotherm measurements. For example Langmuir volume used in simulation are 19.01 m3/t for the

Fig. 10. Local changes of coal thickness (a) and ash yield (b) of coal seams Nos. 5 and 8 (modified from Gui, 1993). a: Coal seam No. 5 at the south of the study area; b: Coal seam No. 8 at the north of the study area.

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Fig. 11. Factors controlling or affecting CBM reservoir formation and their relationship.

coal seams in the east of Shilou and 19.52 m3/t for the coal seams in the southwest of Liulin, respectively (Su and Cai, 2004). However the gas contents measured from CBM test wellbores in corresponding areas indicated about 12.0 m3/t and 17.0 m3/t, respectively, which are both lower than the theoretical capacity. To minimize the uncertainty about the measured gas content versus the theoretical capacity (i.e. an isotherm at a stated pressure), an improved cumulative gas measurement approach was adopted in gas measurement of the wellbore coal samples and details can be found elsewhere (Fu et al., 2009). This conclusion also gives a quantitative prediction of CBM accumulation degree especially in unexplored region locates in the west of the study area, i.e., in the west of Yonghe–Daning–Jixian (refer to Fig. 9).

CBM gas was continuously dissipated by diffusion throughout the whole coal seams and by permeation at some local areas. There are many factors that control the CBM reservoir evolution history and CBM accumulation. These factors include features of coalbearing series, characteristics of coal beds, physical properties of coal reservoir, tectonic evolution history, burial history, geothermal history, etc. Among these factors, the tectonic movement, burial history, geothermal field and coal seams essentially control the whole CBM reservoir formation process. The simulation results show that, after evolution of about 300 million years, the highest values of gas content of coal seams Nos. 5 and 9 are 20 m3/t and 28 m3/t, respectively. The gas content increases with the increasing of burial depth generally. The west part of the study area, i.e. the west of Wupu–Shilou–Jixian region, is favourable for CBM accumulation.

6. Conclusions Acknowledgements A numerical simulation study has been carried out by means of a coalbed methane history simulator (CBMHS) recently developed to simulate the evolution of CBM reservoir formation. The study chose the main coal seams located in the SE edge of Ordos Basin, China as the target CBM reservoirs. Under given geological settings, various variables during evolution of CBM reservoir formation have been obtained through the numerical simulation, providing the historical information for a better understanding of CBM reservoirs formation. Based on the simulation results, the formation history of the CBM reservoirs above can typically be classified into five stages. In the first (primary) stage, there was almost no methane generated and stored in main coal seams. In the second (initial) stage, a certain amount of methane was generated and partially stored in coal seams after part of the gas escaped from coal reservoirs through diffusion and cap outburst. As a result, CBM gas began to accumulate in main coal seams. In the third (stagnant) stage, coal organic matter no longer generated methane. CBM gas content in coal reservoir slightly decreased because of gas dissipation from coal seams by diffusion. In the fourth (active) stage, a large amount of methane was generated at a high speed in the background of abnormally high geothermal field. CBM gas was accumulated in coal reservoir to a high level although considerable of the gas dissipated from coal seams by diffusion and/or cap outburst. In the last (dissipative) stage, due to coal seams uplifted at various rates and no more methane generated,

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