致密储层压裂缝网渗透性变异的影响因素分析

Influence factors of the permeability variation of fracture network caused by fracking in tight reservoir

页岩储层的渗透性是页岩气开发过程中的最为重要的指标之一。在页岩气储层压裂后的排采过程中,储层渗透性会随储层压力、孔隙压力等的降低而发生变化。本文从微观层次上对储层基质微纳孔隙收缩效应、气体滑脱效应对页岩储层渗透性影响的力学机制进行了分析,从宏观层次上分析了储层岩体蠕变、裂缝缝面闭合蠕变、储层衰减应力变化对压裂缝网渗透性影响的力学机制。结果表明:在压裂排采期,基质微纳孔隙页岩吸附气体解吸造成的基质收缩效应和气体滑脱效应会在一定程度上增大压裂储层的渗透率,但基质收缩效应对储层岩体渗透性的影响十分微弱,气体滑脱效应在低压条件下会增加储层岩体的渗透性。由于储层水力改造增加了岩体破碎程度,提高了岩体含水率,水力改造区内储层岩体长期蠕变会极大的压缩缝网空间,降低导流裂缝的渗透性。此外,与储层压力减小有关的裂缝壁面闭合蠕变、地层压实变形会致使水力裂隙开度和渗透性进一步降低。

Shale reservoir permeability is one of the most important indicators in shale gas development.Reservoir permeability is bound to change with the decrease of reservoir pressure and pore pressure during the process of shale gas extraction.This analysis aims to the mechanism of microporous pore shrinkage effect and the gas slip effect on the shale reservoir permeability from the microscopic level.From the macroscopic level,the influence mechanism of creep deformation of shale gas reservoir,creep deformation of fracture surface and reservoir attenuation stress changes on fractured network permeability were analyzed.The results show that the matrix shrinkage effect and gas slip effect caused by shale gas desorption in microporous pore will increase the permeability of fractured reservoir to some extent,but the effect of matrix shrinkage on the permeability of reservoir rocks is relatively very weak.Yet,the effect of gas slippage could obviously increase the permeability of shale gas reservoir under low reservoir pressure.Reservoir hydraulic stimulation can greatly increase the fragmentation degree and water content of shale gas reservoir.Thus,the long-term creep deformation of the fractured rock mass will greatly compress the space of fracture aperture and reduce their permeability.In addition,the closure creep deformation of cracks associated with reservoir pressure reduction and formation compaction will further cause crack closure and permeability reduction.