Effective migration system of coalbed methane reservoirs in the ...

SCIENCE CHINA Earth Sciences • RESEARCH PAPER •

December 2014 Vol.57 No.12: 2978–2984 doi: 10.1007/s11430-014-4988-z

Effective migration system of coalbed methane reservoirs in the southern Qinshui Basin WU CaiFang1,2*, QIN Yong1,2 & ZHOU LongGang1,2 1

School of Resource and Earth Sciences, China University of Mining & Technology, Xuzhou 221116, China; 2 Key Laboratory of CBM Resource and Formation History, Ministry of Education, Xuzhou 221008, China Received August 12, 2013; accepted November 28, 2013; published online November 4, 2014

Effective migration system of coalbed methane (CBM) reservoir, which was controlled by development degree and opening-closing degree of fractures, determines the permeability of coal reservoir and can be characterized by the pore-fracture system in the extrinsic form. In this paper, based on coal matrix elastic self-regulating effect theory and coal reservoir combined elastic energy theory, the fracture opening-closing degree parameter Δ and the fracture development degree parameter ξ are suggested for the quantitative study of the effective migration system of CBM reservoir in southern Qinshui Basin. Further, the control functions of ξ and Δ to CBM enrichment and high production are discussed. The results show that in present stage the area with high ξ value is located in Anze and Qinyuan, and then Zhengzhuang and Fangzhuang, where fracture development degree is high. The area with high Δ value is located in Zhengzhuang and Fanzhuang, and then Anze and Qinyuan, indicating where coal matrix elastic self-regulating positive effect dominates and fractures tend to be open. Through the comprehensive analysis on ξ and Δ, it can be found that their best match area is located in Zhengzhuang and Fanzhuang, with high values for fracture development degree and opening-closing degree probably bringing about high fluid pressure and good permeability of reservoirs, which are advantageous to an abundant CBM production. CBM reservoirs, effective migration system, opening-closing degree of fracture, development degree of fracture, permeability Citation:

Wu C F, Qin Y, Zhou L G. 2014. Effective migration system of coalbed methane reservoirs in the southern Qinshui Basin. Science China: Earth Sciences, 57: 2978–2984, doi: 10.1007/s11430-014-4988-z

Coal reservoir is ternary system possessing macro-cleats, micro-fractures, and pores (Fu, 2001). The accumulative or runaway micro-environment of coalbed methane (CBM) is determined by the coupling relationship among coal, gas, and water, which form a three-phase medium. Effective migration system (EMS) refers to the favorable relation of three-phase media for CBM migration and occurrence. EMS whose extrinsic form is pore-fracture characteristic of coal reservoir comes from CBM formation micro kinetic power and depends on the fracture development degree and open-

*Corresponding author (email: [email protected])

© Science China Press and Springer-Verlag Berlin Heidelberg 2014

ing-closing degree of coal reservoir, and is the physical characteristics manifestation pattern of coal-bearing strata including coal reservoir in a basin and determines the permeability of coal reservoir (Wu, 2004). In macroscopic point, EMS is controlled by tectonic differentiation function in the process of basin evolution and closely related to generation history of heat-hydrocarbon in coal reservoir. In microscopic point, EMS is controlled by coal matrix self-regulating effect and self-closing effect and is closely related to physical characteristics and differences in evolution of elastic energy of coal reservoir. Among the geological factors, tectonic stress field, hydrodynamic field, thermal field, and condition of strata can contribute to the earth.scichina.com

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“closed” environment to some extent. However, the role of these factors reflected in the ability of coal reservoir itself first, allowing CBM diffusion seepage or division (Qin, 2003). Because gas diffusion in coal bed is very low, the development characteristics of natural fractures e.g., especially the development degree and opening-closing degree of natural fractures in geological history, can be one of the key elements to decide the CBM reservoir’s production (Wu et al., 2007, 2012; Jiang et al., 2011). In the 1900s, researchers started to pay attention to the coal matrix self-regulating effect on permeability of coal reservoir and carried on some related research work (Gash et al., 1993; Puri et al., 1991; Harpalani et al., 1990; Geonge et al., 2001). But majority of them were focused on experimental methods of the coal matrix shrinkage rules, the apparent relationship between cleat porosity, the absolute permeability, the relative permeability and confining pressure, reservoir pressure, cleat frequency and coal matrix shrinkage ratio (Harpalani et al., 1990; Geonge et al., 2001). Therefore, their research ideas were limited to mostly the changes physical and mechanical properties of coal-rock mass and thus the geological control mechanism to coal matrix self-regulating combined effect was neglected. In terms of reservoir formation energy, those scholars focused mainly on macroscopic field qualitative analysis of oil and gas reservoir and a direct and effective method to quantitative calculation was not developed (Amyx et al., 1960; Abdullah et al., 2001), especially for coal reservoir energy quantization, and thus the specific control function is unknown about combined elastic energy of coal reservoir to CBM reservoir exploration and development. Therefore, two quantitative characterization parameters for EMS of CBM reservoir, fracture opening-closing degree parameter Δ and fracture development degree parameter ξ, are suggested in this paper on the basis of coal matrix elastic self-regulating effect theory and combined elastic energy theory of coal reservoirs. Taking the Southern Qinshui Basin as an example, the controlling effects of Δ and ξ on CBM enrichment and production are discussed.

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self-regulating effect induced in different geological circumstances, called coal matrix self-regulating effect (Chen, 2003). Under strata conditions, the natural fractures’ change of opening or closing in coal reservoirs is the result of the two effects together, known as coal matrix self-regulating combined effect, which can indicate fracture openingclosing degree. Based on the models of coal matrix elastic self-regulating combined effect (Figures 1–4), the following regular patterns were obtained: Firstly, when coal rank is constant, the combined effect declines gradually due to the increasing of reservoir porosity and fracture fluid pressure (Figure 1). It can be found that up to 3 MPa of the fluid pressure, the decreasing rate of the combined effect is fast and after 3 MPa it becomes slow. In other words, coal matrix elastic deformation is inversely proportional to the fluid pressure (Qin et al., 2005). Secondly, when fluid pressure is constant, the coal matrix elastic self-regulating combined effect becomes weak with the increasing coal rank (Figure 2). Obviously, the degree of coal coalification is the key factor to determine the elastic deformation characteristics of coal reservoir. Further, the higher the coal rank, the weaker the capacity of coal reservoir elastic deformation when CBM adsorption or desorption, especially the latter, forms better internal storage condition of CBM for natural fractures in coal reservoir difficult to get tension. It is found that when max reflection of vitrinite (Ro,max) is higher than 2.1% within the scope of the experimental fluid pressure, the combined effect can become independent of the fluid pressure and is changed from positivity to negativity. The turning point is not only the boundary of lean coal and meager coal, but the jump point of third coalification. As described above, Δ is always negative when Ro,max is higher than 2.8%, which indicates coal matrix self-regulating negative effect is always superior to positive effect for high rank coal. Therefore, for high rank CBM reservoirs, coal matrix self-regulating negative effect causes high adsorption and difficult desorption capacities of coal reservoir, so the high gas content and high gas saturation at all the time.

1 Coal matrix self-regulating combined effect Coal matrix can expand itself after gas adsorption during the CBM accumulation process, which can form extrusion effect to natural fractures surface in coal reservoir and reduce natural fractures to some extent. The increasing fluid pressure is the direct cause for CBM adsorption and reduces the effective stress of coal reservoir under the same tectonic stress, which will emerge stretching role to natural fracture surface and result in opening natural fractures. In other words, the increase of effective stress can decrease reservoir permeability (negative effect); however, the contraction of coal matrix can increase the permeability (positive effect). These two effects both originate from the coal matrix

Figure 1 The relationship between reservoir fluid pressure and elastic self-regulating combined effect.

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Figure 2 The relationship between reservoir coal rank and elastic selfregulating combined effect.


sure and coal matrix adsorption expansion on the opening-closing degree of fractures can cause the apparent elastic self-closing effect when reservoir pressure is high (above 5.9 MPa), and the elastic self-closing effect also becomes more pronouncedly with the increase of the gas content. Fourthly, when reservoir pressure is constant, the coal matrix self-regulating combined effect is decreased with the increase of reservoir permeability (Figure 4). The higher the fluid pressure, the more obvious the coal matrix elastic self-regulating negative effect increases with the permeability. The elastic self-regulating effect is decreased quickly when permeability is lower than 1×10−3 μm2, and slowly when permeability higher than 1×10−3 μm2. Therefore, with the change of fluid pressure, the permeability of reservoir decreases when the closing of fractures is obvious, which means that the change of reservoir permeability is controlled by the opening-closing of reservoir fracture system and there is a positive correlation between them.

2 Fracture opening-closing degree parameter for coal reservoir

Figure 3 The relationship between reservoir gas content and elastic self-regulating combined effect

In this paper, “” is used to characterize the openingclosing degree of fractures in coal reservoir, whose main influence factors include reservoir burial depth, fluid pressure, effective pressure, coal rank (coalification degree), reservoir temperature, gas content and permeability (Table 1). Taking the main coalbed of southern Qinshui Basin as an example, all these factors can be obtained in numerical values based on the laboratory test data and the model of coal matrix elastic self-regulating combined effect. After using the quantification data processing software, the quantitative equation for opening-closing degree Δ can be obtained as follows:

=−0.23748x(1)−0.12635x(2)−0.92177x(3)+0.12221x(4) +0.31783x(5),

Figure 4 The relationship between reservoir permeability and elastic self-regulating combined effect.

Thirdly, when reservoir pressure is constant, the coal matrix elastic self-regulating combined effect decays gradually with the increasing of gas content (Figure 3). The coal matrix elastic self-regulating negative effect is enhanced more pronouncedly with the gas content when the fluid pressure is higher. Coal matrix elastic self-regulating combined effect is always negative when the fluid pressure exceeds 5.9 MPa. It indicates that the influence of fluid pres-

(1)

where  is coal matrix self-regulating combined effect; x(1) to x(5) mean reservoir fluid pressure, effective pressure, coalification degree (coal rank), reservoir temperature and permeability respectively. The max deviation of the equation is 0.429410, statistic F is 52.67, correlation coefficient R is 0.976196, and residual standard deviation S is 0.226112, which indicated the correlation of variables for equation is obvious. The Δ values in the studied area were listed in Table 1. In the middle and south of Qinshui Basin,  is completely negative in the axial of main syncline, especially in Zhongcun-Fengyi and Yangcheng area; however, selfregulating positive effect becomes stronger to the two wings of syncline (Figure 5). It is found that the degree of coalification of two wings region is low and Ro,max is lower than 2.8%, and thus influence on coal matrix decreases with the

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Table 1

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Parameters of self-regulating effect in the present stage

Location Yangcheng Tingdian Jincheng Qinshui Duanshi Zhengzhuang Fanzhuang Gaoping Changzi Anze Fengyi Changzhi Guxian Tunliu Zhongcun Lu’an Qinyuan Xiangyuan Qinxian

Burial depth Fluid pressure Effective (m) (MPa) pressure (MPa) 200 1 4.2 400 2 8.4 100 1 1.6 500 2.5 10.5 500 4 9 600 5 10.6 400 3 7.4 300 1 6.8 400 3 7.4 800 7 13.8 800 5 15.8 300 1 6.8 600 2 13.6 400 4 6.4 900 10 13.4 500 3 10 800 7 13.8 700 3 15.2 1100 10 18.6

Ro,max 3.8 4.0 3.5 3.1 3.6 3.0 3.4 2.0 2.5 2.1 3.1 1.1 1.4 1.5 3.1 1.9 2.3 1.9 2.9

Temperature (°C) 26 32 23 35 35 38 32 29 32 44 44 29 38 32 47 35 44 41 53

Gas content (m3/t) 8 10 14 14 16 18 12 2 13 16 18 6 4 12 16 10 8 4 20

Permeability (10−3 μm2) 0.5 0.4 0.5 0.5 1.0 0.1 0.3 0.4 0.8 0.1 0.1 0.8 0.4 0.2 0.07 0.3 0.1 0.5 0.6

 −0.87 −1.05 −0.68 −0.52 −0.81 −0.59 −0.82 0.63 −0.12 −0.16 −0.64 2.04 1.49 0.67 −0.73 0.75 −0.33 0.75 −0.76

strong elastic self-closing effect of coal reservoir, a big adsorption capacity, and a high gas content, a difficulty for diffusion-escaping, which bring advantages for CBM formation and disadvantages for increasing CBM production. In contrast, in ring-inclination of the basin wings with moderate burial depth, the gas content is high and self-regulating positive effect dominates with an opening tendency of fractures, a high permeability, a weak coal reservoir elastic self-closing effect and a big desorption capacity may exist in this region, which means advantages to CBM formation and high productions.

3 Coal reservoir combined elastic energy and fracture development degree parameter 3.1

Figure 5 Isoline of fracture opening-closing degree parameter  in the study area.

burial depth and effective pressure. This shows that high permeability may not exist in axis of main syncline possessing a deep burial depth where self-regulating negative effect dominates with a closing tendency of fractures, a

Coal reservoir combined elastic energy

In the Qinshui Basin, high structure curvature indicates a strong deformation of coal reservoir and a developed area with structural fractures (Qin et al., 1999). Obviously, the strong deformation of coal reservoir is due to the high strength of combined function by different stress, and it can be measured in energy factor by combined elastic energy of coal reservoir (E) which is parameter combining the major different kinds of system energies stored in coal seams, such as thermodynamics energy, macroscopic kinetic energy, stress strain energy and groundwater dynamic energy. As coal reservoir is a weak elastic sponge, the energy stored in coal seams is shown as a combined elastic energy. Combined elastic energy of coal reservoir (E) consists of coal matrix elastic energy (Ec), coal seam water elastic energy (Ew), and gas elastic energy (Eg) (Wu et al., 2007, 2012),

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which can be calculated as follows: E＝Ec＋Ew＋Eg.

(2)

Among them, coal matrix elastic energy can be shown as Ec 

Cv 2  1   22   32  2   1 2   2 3   1 3  ， (3) 2 

where Cv is coal matrix coefficient of volume compressibility;  is Poisson’s ratio; 1, 2 and 3 are triaxial stress. Water elastic energy can be shown as P  Ew  RT0  1 1  T 1   P   ,  P0 

(4)

where P1 is developed fluid pressure; P0 is initial fluid pressure before developing; T0 is initial water temperature;  is initial water coefficient of thermal expansion;  is initial water coefficient of compressibility; T is temperature variation; P is pressure variation; R is molar gas constant, 8.314 J/(mol K);  is irreducible water saturation in 1 m3 coal matrix. Gas elastic energy includes free gas energy (Ef) and adsorption gas energy (Ea):  a Eg  Ef  Ea  Ef 1   v





 P0  P  , 

(5)

where  is CH4 coefficient of thermal expansion when temperature is changed from T0 to T, v is molar volume of CH4 under standard state, 22.4×10−3 m3/mol. Free CH4 elastic energy can be shown as Ef 

 RT0 1  T 1  P  P T k 1

P0 T

,


be developed only when coal reservoir elastic energy is strong enough to make coal-rock mass rupture. The structural fracture development zone, where fracture system grows greatly, was caused by coal-rock mass breach through reservoir combined elastic energy. So the fracture development degree in reservoir can be shown by the ratio of reservoir combined elastic energy and coal breach energy (Wu, 2004, 2007): ξ=E/Eb, 2

Eb=3P0 (1−2μ)/2Em,

P0

P

a 2v P

dP  Ef

a v





P0  P ,

(6)

(7)

where P is fluid pressure of coal reservoir;  is CH4 content coefficient, 3.16×10−3 m3/(t Pa0.5); Ef is free CH4 elastic energy. 3.2 Fracture development degree parameter ξ for coal reservoir It is obvious that fracture system can be formed and further

(9)

where ξ is fracture development degree in coal reservoir; E is combined elastic energy of coal reservoir; Eb is required energy for coal breach; P0 is confining pressure; μ is Poisson’s ratio; Em is elastic modulus. When ξ>1, the combined elastic energy of coal reservoir is enough to make coal-rock burst to form fractures, and the development degree of fracture is increased with the value of ξ; on the contrary, when ξ1 area and the intensity of fractures and the development of EMS is increased with this value, which is advantageous to forming a high production of CBM reservoir. In the present stage, the area with a high value of ξ is located in Fengyi-QinxianTunliu, the middle of the basin (Figure 7), but the values are mostly less than 1 and are smaller than those in the Late Jurassic-Early Cretaceous because of the shallow burial depth and the weak tectonic stress of coalbed reservoir. In other words, combined elastic energy is not strong enough to make coal-rock burst, so fracture system of reservoir cannot get further development and improvement in the present stage.


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above 2.8%, under effective stress from burial depth and tectonic stress, coal matrix self-regulating combined effect is negative no matter how fluid pressure changes. Therefore, reservoir formation energy field is one of the key controlling factors for EMS in the present stage. (2) ξ, the parameter of fracture development degree in coal reservoir, which is closely related to the paleo-tectonic stress field, is the reflection of the ability for combined elastic energy to burst coal-rock. ξ>1 means reservoir combined elastic energy can exceed the limit of coal-rock. The ability of bursting coal-rock for reservoir combined elastic energy, the development degree of fracture system and the existence of EMS are enhanced with the increase in the value of ξ. Therefore, ancient reservoir formation energy field is one of the key controlling factors for EMS, especially at the important stage. (3) Taking the middle and southern Qinshui Basin as an example, in the present stage the area of high values ξ (ξ>0.9) is located in Anze and Qinyuan, then Zhengzhuang and Fanzhuang, so the development degree of fractures is high in these two areas. The area of low values of Δ is located in Yanghcheng-Fengyi-Zhongcun (middle of the basin), where coal matrix elastic self-regulating negative effect dominates and fractures tend to be closed and permeability of coal reservoir is low. The area of high values of Δ is located in Zhengzhuang and Fanzhuang area, then Anze and Qinyuan, where coal matrix elastic self-regulating positive effect dominates and fractures tend to be open and permeability of coal reservoir is high. Through the comprehensive analysis of the ξ and Δ, it can be found that their best match area is located in Zhengzhuang and Fanzhuang area. It indicates that fracture development degree and opening-closing degree are relatively high in these areas, probably together with the high fluid pressure and good permeability of reservoirs, which is usually advantageous to a high CBM production. The CBM development practices in the study area have confirmed this point.

4 Conclusions

We thank reviewers who carefully reviewed this paper and provided some valuable suggestions. This research was jointly supported by National Natural Science Foundation of China (Grant No. 41272178), the Major Projects of National Science and Technology of China (Grant No. 2011ZX05034), National Basic Research Program of China (Grant No. 2009CB219605), and “Qinglan” Project of Jiangsu Province.

(1) , the parameter of fracture opening-closing degree, which is closely related to modern tectonic stress field, coalification degree and burial depth, is the reflection of coal matrix elastic self-regulating combined effect. The opening-closing degree of fractures and the permeability of reservoirs increases with the value of . >0 indicate that coal matrix self-regulating positive effect dominates and fractures are in open state and permeability is improved as the decrease of fluid pressure. But this phenomenon only occurs at Ro,max of coal reservoir under 2.8%. Once Ro,max is

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