甲烷气渗流作用对煤岩强度的影响

煤层气井在排采生产阶段，由于甲烷的不断脱附、渗流，煤层孔隙压力和有效应力逐渐发生变化，煤岩力学强度减弱，进而对储层孔隙度、渗透率，甚至井壁稳定性均有一定影响。对煤层气在煤层间的渗流过程与煤岩的力学性质间的耦合关系进行了室内实验，为煤层气井完井、开发等方案制定提供了理论支持。声波发射实验和单轴/三轴抗压实验结果表明，随着CH4流经煤岩岩心时间的延长，煤岩弹性模量逐渐降低，表明煤岩力学强度下降。单轴/三轴抗压实验表明，煤岩的弹性模量和最大抗压强度随围压的下降而减小，且在高围压条件下下降较慢，低围压条件下下降较快。该结果表明，在煤层气生产一定时间后，井下煤层可能产生坍塌、掉块而影响生产作业。

Effect of methane gas seepage on coal rock strength

In the production stage of coalbed methane (CBM) wells, the pore pressure and effective stress of coal seams change gradually with the continuous methane desorption and seepage, and correspondingly the mechanical strength of coal rock decreases.Consequently, the porosity and permeability of reservoirs and even the wellbore stability are, in a way impacted.In this paper, the coupling relationship between the seepage process of coalbed methane in coal seam and the mechanical property of coal rock was tested indoors.And it provides the theoretical basis for formulating CBM well completion and development schemes.Then, acoustic emission test and uniaxial/triaxial compression test were carried out.It is shown that as the flowing period of CH4 through coal rock cores extends and the elastic modulus of coal rocks decreases gradually, indicating the decrease of its mechanical strength.The uniaxial/ triaxial compression test reveals that the elastic modulus and maximum compression strength of coal rocks decrease with the decreasing of confining pressure.And they decrease slowly (fast) when the confining pressure is high (low).It is demonstrated that downhole coal seams may collapse and drop after coalbed methane is produced for a while, and consequently the production and operation is impacted adversely.