International Journal of Coal Geology 93 (2012) 16–22

Contents lists available at SciVerse ScienceDirect

International Journal of Coal Geology

j ourna l homepage: www.e lsev ie r .com/ locate / i j coa lgeo

Dynamic variation effects of coal permeability during the coalbed methanedevelopment process in the Qinshui Basin, China

Shu Tao a,b,⁎, Yanbin Wang a, Dazhen Tang c, Hao Xu c, Yumin Lv c, Wei He c, Yong Li c

a State Key Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology, Beijing 100083, Chinab Coalbed Methane Resources and Reservoir Formation Process Key Laboratory of Ministry of Education, China University of Mining and Technology, Xuzhou 221116, Chinac Coal Reservoir Laboratory of National CBM Engineering Center; School of Energy Resources, China University of Geosciences(Beijing), Beijing100083, China

⁎ Corresponding author at: State Key Laboratory of CChina University of Mining and Technology, Beijing 100

E-mail address: peach888@163.com (S. Tao).

0166-5162/$ – see front matter © 2012 Elsevier B.V. Alldoi:10.1016/j.coal.2012.01.006

a b s t r a c t

a r t i c l e i n f o

Article history:Received 9 January 2012Received in revised form 11 January 2012Accepted 11 January 2012Available online 17 January 2012

Keywords:PermeabilityProductivityCBMCoal reservoirQinshui Basin

The commercial exploitation of coalbed methane (CBM) has been achieved in the Qinshui Basin, China. However,the productivity of wells varies greatly because of their different geological settings and production rules, and thepermeability damage to coal reservoirs caused during the development process is one of themost important factorsaffecting productivity. In this study, the patterns of permeability variation in coal seamNo. 3 and their influence onCBMrecoverywere analyzed, and thedegree of permeability damage causedbydifferent factorswas calculated. Theresults show that the burial depth, the bottom hole pressure and the dewatering rate affected the effective stressimposedon the coal reservoir,which further induced changes in permeability andproductivity. The elasticmodulusof the coal body reflects its ability to withstand compression: the greater the elastic modulus is, the less the per-meability decreases. When the pressure drops from 7.05 MPa to 0.20 MPa, the relatively low elastic modulus ofcoal seamNo. 3 can cause a 10% decrease in permeability. Coals at different burial depths have different formationpressures and elastic moduli, causing different levels of permeability damage during the development process.Calculations indicate that the absolute permeability will decrease by 5.5% and 14.3% at depths of 500 m and1000 m, respectively. If the bottom hole pressure drops too fast or the dewatering rate is too high, there will bea strong stress response, resulting in decreasing permeability and productivity. This study found that, in high-productivity wells, a larger pressure drop (approximately 0.022 MPa/d) and stroke (3.0/min) are favorable inthe early drainage period, but a smaller pressure drop (0.002 MPa/d) and stroke (0.4/min) should be appliedwhen the gas peak appears.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

The permeability of a coal seam, a key parameter reflecting theability of fluids to flow inside the seam, determines the migrationand output ability of coalbed methane (CBM). Many studies andproduction practices have shown that permeability is one of themain reservoir parameters controlling the extraction of CBM(Durucan and Edwards, 1986; Harpalani and Chen, 1992; Wang,2004, 2007). Due to the special mechanical properties and gasproduction mechanisms of coal reservoirs, the changes in thephysical properties during the development process are differentfrom those in conventional reservoirs. The most significant ofthese changes is the dynamic change in permeability (Durucanet al., 2009).

The permeability model of coal seams has been a hot topic over thelast 30 years, andmany researchers have advanced the study of dynamicpermeability. Gray (1987)first attempted to quantify the role of stresses

oal Resources and Safe Mining,083, China.

rights reserved.

in the evolution of coal-reservoir permeability, computing permeabilityas coal-matrix shrinkage induced by reservoir pressure while assumingthat the shrinkage is directly proportional to changes in the equivalentsorption pressure. Sawyer et al. (1990) used gas concentration, porecompressibility, and the coefficient of matrix compression to expresschanges in the permeability of a coal seam during CBM development.Levine (1996) proposed a heuristic model in which coal strain changescaused by various factors are calculated by summing cleat widthchangeswithout regarding the constraints imposed by a geomechanicalframework. The Palmer–Mansoori (P and M) model has proved to beuseful for predicting permeability; it was the first model to be derivedfrom a rock mechanics approach, which clarifies the fundamentalparameters (Palmer and Mansoori, 1998). The Advanced ResourcesInternational (ARI) model (Pekot and Reeves, 2003) has no geome-chanical framework, but instead extracts matrix strain changesfrom a Langmuir curve of strain versus reservoir pressure. Thiscurve is assumed to be proportional to the gas concentration curve.Based on the Gray model, Shi and Durucan (2005) proposed a modelthat transforms the various effects into vertical-fracture surface-effective stress. This model uses a stress-based formulation to correlatechanges in the effective horizontal stress caused by the volumetric

17S. Tao et al. / International Journal of Coal Geology 93 (2012) 16–22

deformation and the cleat or pore compressibility. Wang et al. (2009)proposed a means to improve permeability that predicted permeabilitymore accurately than other conventional coal permeability models.Their model also showed reasonable agreement between the predictedand measured stress-strains and directional permeabilities under labo-ratory conditions. Based on the concept of internal swelling, Liu andRutqvist (2010) developed a new coal-permeability model for uniaxialstrain and constant confining-stress conditions. A coal swelling modelproposed by Pan and Connell (2011) can describe anisotropic swellingmeasurements and the permeability behavior for primary and enhancedCBM recovery.

Methane production or CO2 injection can also cause dynamicchanges in coal permeability. Compared with methane gas, CO2 has astronger affinity to coals; therefore,more CO2 can be adsorbed on the in-ternal surface of coal than CH4 for an equivalent area. On the one hand,with the injection of CO2, the sequestration into coals displaces themethane and thus gives rise to the enhanced production of CBM (Cuiet al., 2007; Stevens et al., 2001; Wang et al., 2010); on the other hand,coal reservoirs can suffer serious permeability damage due to the subse-quent swelling of the coal matrix (St. George and Barakat, 2001). Seidleand Huitt (1995) have reported that swelling or shrinkage of the coalmatrix associated with CO2 adsorption or desorption is typically two tofive times greater than that associated with methane. Therefore, theexpected reduction in field coal permeability caused by such matrixswelling could be more significant than the increase in permeabilityalready experienced due to matrix shrinkage during primary meth-ane production. Mazumder and Wolf (2008) noted that on a unitconcentration basis, CO2 causes a greater degree of coal-matrixswelling than CH4, and much of this difference is attributable to thediffering capacity of coal to absorb CO2 and CH4 (Huy et al., 2009;Kross et al., 2002). Kiyama et al. (2011) performed several laboratorytests to understand the changes in the physical properties of a coalspecimen with the continuous injection of liquid/supercritical CO2

and N2 by measuring strain, elastic wave velocity and permeabilitychange under a stress-constrained condition.

Overall, most research focuses on the dynamic change in absolutepermeability by building permeability models, including methane pro-duction and CO2 injection. However, most models rely primarily on lab-oratory simulations, which may not resemble the features of actualproductionwells. The current study considered the productivity featuresof CBM wells in the southern Qinshui Basin, offering a semiquantitativeto quantitative description of the dynamic changes in coal permeabilitycaused by geological and engineering factors. An evaluation model ofthe geological and engineering factors that drive dynamic changes inpermeability, which in turn drive the dynamic response in terms ofproductivity, was built.

2. Dynamic permeability changes — a review

It is increasingly recognized that the permeability of a reservoir maychangewith pressure depletion during CBMproduction. The variation inthe permeability of a coal seam results in serious problems for reservoirperformance analysis, productivity prediction, reserve estimation anddevelopment optimization (Wang and Ward, 2009). It is necessary toclarify the dynamic variation mechanisms and the factors that can in-duce the change in permeability with production.

Stress sensitivity and coal matrix shrinkage are the main factorsleading to absolute permeability variation. During CBM development,a series of geological effects take place in the coalbed and its fluidswith the discharge of water and gas, which leads to a change in absolutepermeability (Harpalani and Schraufnagel, 1990; Liu and Rutqvist,2010; Wang, 2007; Wang and Ward, 2009; White et al., 2005).

A coal reservoir has a dual porosity structure, namely, a porousmatrix surrounded by fractures (Harpalani and Schraufnagel, 1990;Liu et al., 2011). Matrix porosity provides the main storage space ofCBM, and fracture porosity is the main channel for gas flow, the latter

being the principle determinant of the absolute permeability of a coalreservoir. When the pore fluid pressure of the coalbed drops becauseof drainage from CBM production, the effective overburden pressureon the coalbed framework will increase. Consequently, the reservoirwill become compressed and will thus reduce coal permeability bynarrowing and even closing fracture apertures. At the same time,shrinkage of the coal matrix due to desorption of gas tends to openthe cleat fractures and increase the coal permeability (Mavor et al.,1990; Scott and Kaiser, 1995; Somerton et al., 1975; St. George andBarakat, 2001; Yao et al., 2009a;Wang andWard, 2009). These positiveand negative effects influence the entire process of CBMdesorption, dif-fusion, seepage, output, and ultimately the methane productivity of thecoalbed. Meanwhile, both positive and negative effects are influencedby numerous geological and engineering factors, amongwhich effectivestress and coal mechanical properties are the most direct.

2.1. Effective stress

Effective stress directly impacts the coal matrix and reveals thedissimilarity between the total stress perpendicular to the fractures'direction and the fluid stress within the holes and fractures (Qinet al., 2005). Coal reservoirs have specialmechanical properties that dif-ferentiate them from conventional reservoirs. The compressive strength,tensile strength, and elastic modulus of coal reservoirs are always lowerthan those of conventional reservoirs; therefore, coal reservoirs aremore sensitive to stress. When the effective stress increases, physicalparameters such as rock porosity and permeability will decrease.

The relationship between coal permeability and stress has drawnincreasing attention in recent decades. Scholars have applied variousmethods to research the relationship between coal permeability andstress. These studies have concluded that coal reservoir permeabilitychanges exponentially in relation to stress (Brace et al., 1968;Enever and Henning, 1997; Gangi, 1978; McKeeC et al., 1998; Tang,2001; Walsh, 1981; Yao et al., 2008; Yao et al., 2009b).

According to previous studies, when the effect of matrix shrinkageis disregarded, the dynamic changes in coal reservoir permeabilityduring the development process can be calculated as

k ¼ ko 1þ p−poϕoM

� �3; ð1Þ

and the elastic modulus is calculated as

ME

¼ 1−v1þ vð Þ 1−2vð Þ ; ð2Þ

where k and ko are permeability and initial permeability, p and po are res-ervoir pressure and initial reservoir pressure, φo denotes initial effectiveporosity, M is the axial modulus, E is the elastic modulus, and v is thePoisson's ratio of the axial modulus (Jaeger and Cook, 1979; Palmerand Mansoori, 1996).

Eqs. (1) and (2) demonstrate that when the original reservoirporosity and the axial modulus are fixed, greater pressure dropsresult in greater effective stress in the reservoir, causing the absolute per-meability to decrease sharply. If the pressure drops by 10 MPa, the initialpermeability will decrease by 13.6% (13.6% ko), while a 5 MPa reductionin pressure will cause a 7% decline in permeability. Low gas saturationand abnormally high reservoir pressure, in particular, will substantiallyreduce the permeability of the coalbed reservoir.

2.2. Elastic modulus

The elastic modulus is an index used to characterize coal's tendencyto be deformed elastically when a force is applied to it, and it determinesthe stress sensitivity of damage in coal reservoirs (Karacan, 2009). Manystudies have indicated that the variation in permeability with pressure

18 S. Tao et al. / International Journal of Coal Geology 93 (2012) 16–22

depletion is affected by the elastic-plastic properties of the coalmaterial (Harpalani and Schraufnagel, 1990; Levine, 1996; Seidleand Huitt, 1995).

The greater the elastic modulus is, the greater the stress requiredfor elastic deformation in a coalseam. In other words, the greaterthe coal's stiffness is, the less elastic deformation takes place undercertain levels of stress. Thus, the absolute permeability changes lessduring initial CBM development when the coal reservoir has a higherelastic modulus.

3. Location and geological setting of the Qinshui Basin

The Qinshui Basin is located in the southeast of Shanxi, NorthChina (Fig. 1). The strata in the basin include Cambrian, Ordovician,Pennsylvanian, Permian, Triassic, Jurassic, Neogene and Quaternaryunits. The main coal bearing strata belong to the Taiyuan and ShanxiFormations in the lower Permian System (see the stratigraphic columninWei et al., 2007). Coal seamNo. 3 in the Shanxi Formation is themaintarget zone for CBM development. By the end of 2010, more than 2600CBM wells had been drilled in the basin.

The gas content of coal seam No.3 in the southern Qinshui Basin ishigher than that of the central and northern areas (including thethree blocks Fanzhuang, Panzhuang, and Zhengzhuang), with a totalgas-bearing area of 3630 km2. The gas content in the Panzhuang-

Fig. 1. Burial depth contour map of coal seam No. 3 and the loca

Fangzhuang zone in particular is extremely high, ranging from 10 to30 m3/ton (Wang andWard, 2009). The total CBMresource is extreme-ly large, possibly as high as 4500×108 m3 (Cao, 2005). The presentdepths below the surface of coal seam No. 3 mainly vary from 500 to1000 m in the southern Qinshui Basin (Wei et al., 2007; Fig. 1). A totalof 15 samples were collected from coal seamNo. 3 from different under-ground coal mines, namely, Duanshi, Houcun, Sihe, Wangyun, andChangcun, at depths of 500–700 m. Nine of these samples were chosenfor the present study.

4. Data availability and methodology

The data involved in this paper, such as the formation pressuregradient, formation burial depth, bottom hole pressure, dewateringrate, and production data, were collected from research institutionsand production plants.

To determine the strain feature of coal samples under different confin-ing pressures, the elastic modulus and Poisson's ratio were measuredusing a MTS 815 load frame from the American MTS company followingthe Chinese Standard Method GB/T 23561.7 (2009). Nine cylinder coalsamples (diameter approximately 50 mm, height approximately100 mm) were prepared and divided into two groups (C and P, contain-ing 4 and 5 samples, respectively). The initial porosity was measured byan Ultrapore-200A helium porosimeter under a confining pressure of

tion of the Qinshui Basin (modified from Wei et al., 2007).

Table 1Elastic modulus of samples from coal seam No. 3 in the southern Qinshui Basin.

Confiningpressure,MPa

Elastic modulus (average), GPa

C group samples P group samples All samples

0 3.65 6.45 5.055 6.78 10.99 8.8910 6.28 7.66 6.7415 8.10 8.88 8.6220 7.28 6.39 6.8425 7.13 6.70 6.92

Fig. 2. Productivity features for well 17.

19S. Tao et al. / International Journal of Coal Geology 93 (2012) 16–22

7MPa (average formation pressure) according to Chinese PetroleumIndustryStandard SY/T5336 (2006). The test results are summarized inTables 1 and 2.

5. Semiquantitative to quantitative characterization of changes incoal permeability

5.1. Elastic modulus

Based on the formation pressure gradient of 0.94 MPa/100 m (ChinaUniversity of Mining and Technology, 1999) and the burial depth(500–1000 m, Fig. 1) of coal seam No. 3 in the southern Qinshui Basin,the coal reservoir pressure is 4.70 MPa (close to 5 MPa) at a burialdepth of 500 m and 9.40 MPa (close to 10 MPa) at a depth of 1000 m,with the average being 7.05 MPa (po). Table 1 shows the elasticmodulusof the samples from coal seam No. 3 under different triaxial stresses.Under the confining pressures of 5 MPa and 10MPa, the elasticmodulusvalues of all samples are 8.89 GPa and 6.74 GPa, respectively. Therefore,the average value of 7.81 GPa (E) was considered as the elastic modulusof coal seam No. 3 in the southern Qinshui Basin.

As shown in Table 2, the average measured porosity and Poisson'sratio of the coal samples are 2.0% (φo) and 0.3 (v), respectively (Wanget al., 2001). According to Eqs. (1) and (2), assuming that the minimumbottomhole pressure of a CBMwell ismaintained at 0.20 MPa (p), the de-crease in absolute permeability can be calculated as approximately 10.0%of the coal reservoir during the development process, whichmay indicatesevere damage to coal reservoirs in the southern Qinshui Basin, whoseinitial porosity and permeability are relatively low (Meng et al., 2011).

5.2. Depth of burial

The burial depth of a coal reservoir indirectly influences changes inthe absolute permeability during the development process. A reservoiralways has a higher vertical stress when buried at a greater depth. Be-cause pore fluid can only take a certain pressure, all the extra stresswill be converted to effective stress,which is takenup by the coalmatrix.Moreover, as the burial depth increases, the range of the pressure dropalso increases along with the effective stress.

Table 2Test results of porosity and Poisson's ratio.

Samplenumber

Diameter(mm)

Height(mm)

Porosity(%)

Confiningpressure (MPa)

Poisson'sratio

QS01 49.80 101.35 1.63 7 0.284QS02 50.10 100.44 1.40 7 0.261QS03 50.02 101.05 1.98 7 0.273QS04 49.73 99.87 2.15 7 0.294QS05 49.89 99.94 1.87 7 0.321QS06 49.82 101.15 2.50 7 0.235QS07 49.93 100.66 2.42 7 0.253QS08 50.04 100.23 2.25 7 0.276QS09 49.68 101.46 1.79 7 0.313Average 2.00 0.279

As mentioned above, the elastic modulus measured under the con-fining pressures of 5 MPa and 10MPa were considered as the elasticmodulus of a coal reservoir at burial depths of approximately 500 mand 1000 m, respectively. According to Eqs. (1) and (2), during CBMproduction, the reduction in the absolute permeability of a coal reservoirin the southern Qinshui Basin can be calculated for various depths. Theresults show that the absolute permeability of a coal reservoir decreasesby 5.5% and 14.4% at depths of 500 m and 1000 m, respectively.

To providemore examples, consider the productivity ofwells 17 and34. The depths of coal seam No. 3 in wells 17 and 34 are approximately626 m and 962 m, respectively. A comparison of the gas productiontrends shown in Figs. 2 and 3 indicates that the original reservoir pressureof well 17 was approximately 5 MPa. After dewatering for 100 days, thebottom hole pressure was between 0.5 and 1MPa; therefore, the pres-sure drop was only approximately 4 to 4.5 MPa, causing a 6.3% decreaseof initial permeability. The original reservoir pressure of well 34, howev-er, was approximately 10 MPa. After dewatering for 30 days, the bottomhole pressure was between 0.5 and 2 MPa, a much larger reduction ofapproximately 8 to 9.5 MPa, which can result in a permeability damageof 13%. This analysis indicates thatwells of relatively shallow depthmayhave a smaller pressure drop and permeability decrease, leading tohigher productivity.

5.3. Bottom hole pressure

During CBM field development, changes in the bottom hole pressurewill control changes in the effective stress of a coal reservoir, altering theabsolute permeability and production levels of gas wells.

The Langmuir isotherm adsorption model and Darcy's law indicatethat a drop in bottom hole pressure will increase gas desorption in areservoir. At the same time, the gas relative permeability will increase,improving the production of gas. Moreover, the drop in bottom holepressure will increase the production pressure difference, whichwill favorably affect the CBM flow in the coal reservoir.

However, if the bottom hole pressure drops too fast, too much gasis desorbed, causing gas locking in the reservoir, which would resultin far less than optimal outgassing. Furthermore, the effective stresson the coal reservoir will increase, creating stress sensitivity andthus decreasing permeability. Therefore, the bottom hole pressureshould be controlled; it cannot simply be reduced to obtain a largeproduction pressure difference without considering the potentialimpacts of stress sensitivity and gas locking.

Fig. 3. Productivity features for well 34.

Fig. 4. Relationship between the bottom hole pressure and gas production for well 22.

Fig. 5. Relationship between the bottom hole pressure and gas production for well 27.

20 S. Tao et al. / International Journal of Coal Geology 93 (2012) 16–22

The relationship between bottom hole pressure and daily gas pro-duction for well 22 in the southern Qinshui Basin is shown in Fig. 4.The bottom hole pressure dropped rapidly, from 3.72 MPa on the 125thday to 1.18 MPa on the 136th day, resulting in the permeability suddenlydecreasing by 3.6%. At the same time, gas production decreased until the180th day, from approximately 9000 m3/d to approximately 5000 m3/d.

In contrast, as shown in Fig. 5, well 27 began to produce gas after alonger period of drainage (approximately 200 days). At approximately100 days, the bottom hole pressure started to be controlled, decliningslightly and steadily. In this case, the gas production increased slowly.After the initial detection of gas, the bottom hole pressure remained rel-atively stable; therefore, even if the reservoir were highly pressure-sensitive, its absolute permeability would not drop significantly.

Table 3 shows the production data of selected gas wells. It can beseen that high productivity wells always show a larger pressure dropof approximately 0.022 MPa/d during the early drainage period, andthe pressure drops slowly to 0.002 MPa/d after the gas peak appears.This pressure drop produces a stable and reasonable displacement. The

Table 3Production data of gas wells (examples).

Wells(examples)

Productiondepth(m)

Rate of bottom hole pressure drop (MPa/d)

Early drainage period Gas peak period

12 542.05 0.028 0.003514 534.70 0.017 0.001218 525.60 0.023 0.002419 668.40 0.025 0.002725 701.20 0.021 0.002170 605.75 0.020 0.0017Average 0.022 0.0021 464.50 0.031 0.05720 692.80 0.027 0.04642 477.15 0.027 0.05128 577.00 0.019 0.03517 626.80 0.025 0.04511 664.20 0.025 0.04946 698.45 0.033 0.05547 726.95 0.024 0.04857 503.80 0.022 0.046Average 0.026 0.048

pressure drop funnel/cone expands continuously, which indicates thatthe pressure decline lessens with distance from the well, resulting in afunnel/cone-shaped depression radiating away from the well, and thestable production period is extended.

5.4. Dewatering rate

The dewatering rate has a significant impact on the pressure reduc-tion rate, causing both stress sensitivity andmatrix shrinkage; therefore,it can either damage or improve the absolute permeability of coal reser-voirs. If the dewatering rate is low, the CBM desorption rate will be lowand therefore unfavorable for matrix shrinkage. Here, stress sensitivityalways plays a leading role. When the dewatering rate is high, the CBMdesorption rate will be high and thus favorable for matrix shrinkage,whichmayplay a leading role in coal seamswith a large elasticmodulus.These observations indicate that a moderate dewatering rate must bemaintained during the development process to avoid reservoir damageand to promote matrix shrinkage.

The dewatering rate of a CBMwell controls the bottomhole pressuredirectly. If the dewatering rate is too high, inducing a very low bottomhole pressure, the absolute permeability can be decreased significantlynear the sidewall. These conditionswould hinder the pressure propaga-tion, which would cause only a slight pressure drop in reservoirs farfrom thewells, where little gas could be desorbed. During CBM produc-tion, the fluid level drawdown rate can be directly correlated to thedewatering rate. The relationship between the fluid level and produc-tivity in well 17 is shown in Fig. 6. The dewatering rate of this well ac-celerated suddenly after 100 days, and the production fluid leveldropped rapidly at a speed of approximately 30 m/d. As a result, thecoal permeability immediately decreased by approximately 2.9%, con-straining the flow capacity of CBM in the reservoir and thus causing asudden decline in gas production.

As shown in Table 3, during the early drainage period in high pro-ductivity wells, the pump stroke remains high (3.0/min) and drops to0.4/min after the gas peak appears, thus forming a stable and reasonabledisplacement, expanding the pressure drop funnel/cone continuously,and greatly extending the period of stable production.

6. Conclusions

(1) Many key factors determinewhether a CBMwellwill achieve highand stable production, including suitable coal structure, appropri-ate burial depth and reservoir pressure, and other geological con-ditions. Furthermore, certain engineering factors, such as the

Dewatering rate (stroke) (min-1) Averagegasproduction(m3/d)

Averagewaterproduction(m3/d)

Early drainage period Gas peak period

3.7 0.62 3143.44 2.592.4 0.26 5532.68 1.693.1 0.51 4249.09 3.173.5 0.55 3226.37 1.712.8 0.34 4998.32 1.172.7 0.31 4959.28 1.113.0 0.43.0 4.21 1901.91 1.143.5 5.23 559.16 1.432.9 4.64 793.50 2.093.3 4.92 407.57 4.213.1 4.72 2188.00 0.592.7 4.50 832.45 1.763.7 5.65 389.59 4.913.2 4.67 921.99 1.132.6 4.52 759.03 1.203.1 4.8

Fig. 6. Relationship between the fluid level and gas production for well 17.

21S. Tao et al. / International Journal of Coal Geology 93 (2012) 16–22

dewatering rate and bottom hole pressure, are also important.Most of these factors change the coal reservoir permeability firstand then the CBM production.

(2) This study indicates that coal reservoirs will be subject to dif-ferent degrees of permeability damage during CBM production.The damage cannot be avoided, but it can be mitigated some-what. This study uses actual production curves of CBM wells inthe Qinshui Basin to show the gas production change under thecontrol of the bottom hole pressure and the dewatering rate.The results show that a significant decrease in permeabilityoccurs in a well with a fast pressure drop/ high dewateringrate, with a concomitant decrease in gas production.

(3) In the early production period of a CBMwell, only water is pro-duced. To shorten this single-phase water period and achievegas breakthrough more quickly, a relatively high drainagespeed is favorable. However, when the well starts to producegas, the dewatering rate should be decreased slowly andsteadily, especially after the gas peak appears.

Acknowledgements

Thisworkwas subsidized by the Key Project of the National Science &Technology (2011ZX05034-001), theNational Basic Research ProgramofChina (973) (2009CB219604), the Postdoctoral Science Fund of China(2011M500433) and the Research Fund of Coalbed Methane Resourcesand Reservoir Formation Process Key Laboratory, China Ministry ofEducation.

References

Brace, W.F., Walsh, J.B., Frangos, W.T., 1968. Permeability of granite under high pressure.Journal of Geophysical Research 73, 2225–2236.

Cao,W., 2005. Economic analysis of various exploitationways in Fanzhuang block of Qinshuicoal-bed gas field. Natural Gas Industry 25, 174–176 (in Chinese with English abstract).

China University of Mining and Technology, 1999. Chinese coalbed methane resources.China University of Mining and Technology Press, Xuzhou. (in Chinese).

Cui, X., Bustin, R.M., Chikatamarla, L., 2007. Adsorption-induced coal swelling andstress: implications for methane production and acid gas sequestration into coalseams. Journal of Geophysical Research 112, B10202.

Durucan, S., Edwards, J.S., 1986. The effects of stress and fracturing on permeability ofcoal. Mining Science and Technology 3, 205–216.

Durucan, S., Ahsan, M., Shi, J.Q., 2009. Matrix shrinkage and swelling characteristics ofEuropean coals. Energy Procedia 1, 3055–3062.

Enever, J.R.E., Henning, A., 1997. The relationship between permeability and effectivestress for Australian coal and its implications with respect to coalbed methaneexploration and reservoir modeling. Proceedings of the 1997 InternationalCoalbed Methane Symposium, pp. 13–22.

Gangi, A.F., 1978. Variation of whole and fractured porous rock permeability with confiningpressure. International Journal of RockMechanics andMining Sciences & GeomechanicsAbstracts 15, 249–257.

GB/T 23561.7, 2009. Chinese national standard. Coal and rock, Method of determiningphysical and mechanical properties.Gray, I., 1987. Reservoir engineering in coalseams: Part 1 – the physical process of gas storage and movement in coal seams.SPE 28–34.

Harpalani, S., Schraufnagel, R.A., 1990. Shrinkage of coal matrix with release of gas andits impact on permeability of coal. Fuel 69, 551–556.

Harpalani, S., Chen, G., 1992. In: Beamish, B.B., Gamson, P.D. (Eds.), Effect of gasproduction on porosity and permeability of coal. : Symposium on CoalbedMethane, 4. Townsville, Australia, p. 67.

Huy, P.Q., Sasaki, K., Sugai, Y., Kiga, T., Fujioka, M., Adachi, T., 2009. Effects of SO2 andpH concentration on CO2 adsorption capacity in coal seams for CO2 sequestration

with considerations for flue gas from coal-fired power plants. Journal of CanadianPetroleum Technology 48, 58–63.

Jaeger, J.C., Cook, N.G.W., 1979. Fundamentals of Rock Mechanics, 3 rd Ed. Chapmanand Hall Press, London.

Karacan, C.O., 2009. Elastic and shear moduli of coal measure rocks derived from basicwell logs using fractal statistics and radial basis functions. International Journal ofRock Mechanics and Mining Sciences & Geomechanics Abstracts 48,1281–1296.

Kiyama, T., Nishimoto, S., Fujioka, M., Xue, Z.Q., Ishijima, Y., Pan, Z.J., Connell, L.D., 2011.Coal swelling strain and permeability change with injecting liquid/supercriticalCO2 and N2 at stress-constrained conditions. International Journal of Coal Geology85, 56–64.

Kross, B.M., Van Bergen, F., Gensterblum, Y., Siemons, N., Pagnier, H.J.M., David, P.,2002. High-pressure methane and carbon dioxide adsorption on dry andmoisture-equilibrated Pennsylvanian coals. International Journal of Coal Geology51, 69–92.

Levine, J.R., 1996. Model study of the influence of matrix shrinkage on absolute permeabilityof coalbed reservoirs. Geological Society Publication, pp. 197–212.

Liu, H.H., Rutqvist, J., 2010. A new coal-permeability model: internal swellingstress and fracture–matrix interaction. Transport in Porous Media 82,157–172.

Liu, J.S., Chen, Z.W., Elsworth, D., Qu, H.Y., Chen, D., 2011. Interactions ofmultiple processes dur-ing CBM extraction: a critical review. International Journal of Coal Geology 87, 175–189.

Mavor, M.J., Owen, L.B., Pratt, T.J., 1990. Measurement and evaluation of coal sorptionisotherm data. SPE 1–14.

Mazumder, S.,Wolf, K.H., 2008. Differential swelling and permeability change of coal inresponse to CO2 injection for ECBM. International Journal of Coal Geology 74,123–138.

McKeeC, R., Bumb, A.C., Koenig, A., 1998. Stress dependent permeability and porosity ofCoal. Rocky Mountain Association of Geologist 143–153.

Meng, Z.P., Zhang, J.C., Wang, R., 2011. In-situ stress, pore pressure and stress-dependentpermeability in the Southern Qinshui Basin. International Journal of Rock Mechanicsand Mining Sciences & Geomechanics Abstracts 48, 122–131.

Palmer, I., Mansoori, J., 1996. How permeability depends on stress and pore pressure inCoalbed: a new model. SPE 557–563.

Palmer, I., Mansoori, J., 1998. How permeability depends upon stress and pore pressurein coalbed: a new model. SPE REE 539–544.

Pan, Z.J., Connell, L.D., 2011. Modelling of anisotropic coal swelling and its impact onpermeability behaviour for primary and enhanced coalbed methane recovery.International Journal of Coal Geology 85, 257–267.

Pekot, L.J., Reeves, S.R., 2003. Modeling the effects of matrix shrinkage and differentialswelling on coalbed methane recovery and carbon sequestration. Proceedings of the2003 International Coalbed Methane Symposium. University of Alabama, Tuscaloosa,Alabama. Paper 0328.

Qin, Y., Fu, X.H.,Wu, C.F., Fu, G.Y., Bu, Y.Y., 2005. Self-adjusted elastic action and its CBMpool-forming effect of the high rank coal reservoir. Chinese Science Bulletin 50, 99–103.

Sawyer, W.K., Paul, G.W. and Schraufnagel, R.A.,1990. Development and application ofa 3D Coalbed simulator, Paper CIM/SPE 90-119, Proceedings of the petroleum soci-ety CIM, Calgary, Canada.

Scott, A.R., Kaiser, W.R., 1995. Hydrogeologic factor affecting dynamic open-hole cavitycomplections in the San Juan Basin, U.S.A. Proceedings of the 1995 Coalbed MethaneSymposium, the University of Alabama/Tuscaloosa, pp. 139–147.

Seidle, J.P., Huitt, L.G., 1995. Experimental measurement of coal matrix shrinkage dueto gas desorption and implications for cleat matrix increases. SPE–30010.

Shi, J.Q., Durucan, S., 2005. A model for changes in coalbed permeability during primaryand enhanced methane recovery. SPE Reservoir Evaluation and Engineering 291.

Somerton, W.H., Soyletaezoglu, I.M., Dudley, R.C., 1975. Effect of stress on permeability ofcoal. International Journal of Rock Mechanics and Mining Sciences & GeomechanicsAbstracts 12, 129–145.St. George, J.D., Barakat, M.A., 2001. The change in effectivestresswith shrinkage fromgas desorption in coal. International Journal of Coal Geology45, 105–113.

St. George, J.D., Barakat, M.A., 2001. The change in effective stress with shrinkage fromgas desorption in coal. International Journal of Coal Geology 45, 105–113.

Stevens, S.H., Kuuskra, V.A., Gale, J., Beecy, D., 2001. CO2 injection and sequestration indepleted oil and gas fields and deep coal seams: world wide potentials and costs.Environmental Geosciences 8, 200–209.

SY/T5336, 2006. Chinese petroleum industry standard. Core analysis method.Tang, S.H.,2001. Probe into the influence factors on permeability of coal reservoirs. Coal geologyof China 13, 28–30 (in Chinese with English abstract).

Tang, S.H., 2001. Probe into the influence factors on permeability of coal reservoirs.Coal geology of China 13, 28–30 (in Chinese with English abstract).

Walsh, J.B., 1981. Effect of pore pressure and confining pressure on fracture permeability.International Journal of Rock Mechanics and Mining Sciences & GeomechanicsAbstracts 18, 429–435.

Wang, X.J., 2004. Effect of variation in coal permeability with pressure drop on coalbedmethane production. 2004 International Coalbed Methane Symposium proceedings.The University of Alabama, USA. No. 0401.

Wang, X.J., 2007. Influence of Coal Quality Factors on Seam Permeability Associatedwith Coalbed Methane Production. PhD Thesis, University of New South Wales,Sydney, Australia, 32-69.

Wang, X.J., Ward, C.R., 2009. Experimental investigation of permeability changes withpressure depletion in relation to coal quality. 2009 International Coalbed MethaneSymposium. University of Alabama, Tuscaloosa, Alabama, pp. 1–31.

Wang, X.Z., Liu, W.Q., Sun, Y.Z., Ma, Y.J., 2001. Field testing of fracturing technology incoal formed gas well. Oil Drilling & Production Technology 23, 58–61 (in Chinesewith English abstract).

22 S. Tao et al. / International Journal of Coal Geology 93 (2012) 16–22

Wang, G.X., Massarotto, P., Rudolph, V., 2009. An improved permeability model of coalfor coalbed methane recovery and CO2 geosequestration. International Journal ofCoal Geology 77, 127–136.

Wang, G.X., Wei, X.R., Wang, K., Massarotto, P., Rudolph, V., 2010. Sorption-inducedswelling/shrinkage and permeability of coal under stressed adsorption/desorptionconditions. International Journal of Coal Geology 83, 46–54.

Wei, C.T., Qin, Y., Wang, G.G.X., Fu, X.H., Jiang, B., Zhang, Z.Q., 2007. Simulation study onevolution of coalbed methane reservoir in Qinshui basin, China. International Journalof Coal Geology 72, 53–69.

White, C.M., Smith, D.H., Jones, K.L., Goodman, A.L., Jikich, S.A., LaCount, R.B., DuBose,S.B., Ozdemir, E., Morsi, B.I., Schroeder, K.T., 2005. Sequestration of carbon dioxidein coal with enhanced coalbed methane recovery—a review. Energy & Fuels 19,559–724.

Yao, Y.B., Liu, D.M., Tang, D.Z., Huang, W.H., Tang, S.H., Che, Y., 2008. A comprehensivemodel for evaluating coalbed methane reservoirs in China. Acta Geologica Sinica82, 1253–1270.

Yao, Y.B., Liu, D.M., Tang, D.Z., Tang, S.H., Che, Y., Huang, W.H., 2009a. Preliminary evaluationof the coalbed methane production potential and its geological controls in theWeibei Coalfield, Southeastern Ordos Basin, China. International Journal ofCoal Geology 78, 1–15.

Yao, Y.B., Liu, D.M., Tang, D.Z., Tang, S.H., Huang, W.H., 2009b. Fractal characterizationof seepage-pores of coals from China: an investigation on permeability of coals.Computer & Geosciences 35, 1159–1166.