考虑有效应力和煤基质收缩效应的渗透率模型

A PERMEABILITY MODEL CONSIDERING THE EFFECTIVE STRESS AND COAL MATRIX SHRINKING EFFECT

针对煤层气的生产过程中所存在的两个相反的效应:(1)储层压力下降,有效应力增加,煤层裂隙压缩闭合,渗透率降低;(2)煤层气解吸,煤基质收缩,煤层气流动路径张开,渗透率升高;建立了包含煤基质收缩效应的煤层孔隙度和渗透率理论模型,模型与已有的一些研究结果反映的规律一致。根据模型得出在体积应力恒定条件下,渗透率随孔隙压力变化存在一临界压力,孔隙压力小于临界压力时,渗透系数随孔隙压力的增加而减少,孔隙压力大于临界压力时,渗透系数随孔隙压力的增加而增大,给出了该临界压力的计算式。对不同情形下渗透率对孔隙压力的变化响应进行了讨论,结果表明,临界压力的存在与否与影响渗透率的多种因素有关,应对影响煤层渗透性的众多因素进行动态耦合研究。

Two distinct effects are associated with the reduction of reservoir pressure during coal bed methane production,i.e.,the increase in effective stress,which causes the decrease in the permeability of the coal bed owing to mechanical closure of flow paths,and the release of gas leading to the coal-matrix shrinking,which widens the fractures that are primarily responsible for gas flow in coal bed reservoirs,and thus increases permeability.Based on these two simultaneously occurred effects,a general porosity and permeability model considering the effect of matrix shrinkage is developed,according to the model,some results can be attained.under the condition of constant overburden load,for the permeability change with pore pressure,there is a critical pore pressure pc,for pore pressure p<pc,the permeability decreases as the p increases,when p>pc,the permeability increases as the p increases,and the expression of pc is introduced.Some possible scenarios are also discussed with regard to the response of pore pressure and initial reservoir pressure to permeability.The result suggests whether the threshold pore pressure exists or not is dependent on the factors which influence the permeability,so a coupled multi-factors analysis on a coal bed methane production operation seems to be essential for optimizing the production.