煤储层粗糙割理中煤层气运移机理数值分析

Numerical investigation of migration mechanism for coal-bed methane flow through cleats with rough surfaces in coal reservoir

有效描述粗糙割理中煤层气运移机理是实现其产能准确评估的基本前提。结合裂隙的立方定律以及广义Kozeny-Carman孔-渗方程,于理论层面推导了考虑内、外摩擦所致的裂-渗关系模型。其中内摩擦效应以煤层气运移的水文弯曲度来描述,外摩擦则通过引入端面曲折率来定义。借助分形理论实现了割理端面粗糙几何的定量描述,并采用格子Boltzmann方法模拟了煤层气运移过程,分析了端面分形维、绝对粗糙度以及相对粗糙度对煤层气输运特征的影响。在此基础上,对比分析了新裂-渗方程解析值同数值模拟渗透率之间的关系。结果表明,考虑了内、外摩擦效应的新方程能有效描述微观粗糙割理中煤层气的运移规律,并且物理意义明确。

To estimate the productivity of coal-bed methane(CBM),the premise is the effective description of CBM migration mechanism. The authors have developed a new fracture-permeability relative model by coupling the general Kozeny-Carman equation and the classical cubic law of fracture flow,which takes into account the effects of external and internal friction. In the model developed,the hydraulic tortuosity is responsible for the internal friction,and the surface tortuosity for external friction effect,which is defined as the area ratio between cleat rough surface and the projected one on the macro-flow plane. Consequently,the authors modelled the cleats composed of rough surfaces characterized by the theory of fractal geometry,and simulated the CBM flow through them by lattice Boltzmann method. In addition,the authors investigated the effects,such as surface fractal dimension,absolute surface roughness,as well as the relative surface roughness,on the CBM flow. Based on the results obtained,the validity of the newly derived fracturepermeability relationship is verified by comparative analyses. The results reveal that the newly developed model accounting for the effects of external and internal friction,can effectively describe the migration law of CBM through rough cleats.