页岩气储层迂曲微裂缝二维重构及多点起裂分析

Two-dimensional reconstruction and multipoint initiation of tortuous micro-fractures in shale gas reservoirs

在复杂的地质条件下,页岩气储层中的天然裂缝往往呈现迂曲状几何形态,为了研究迂曲微裂缝在水力压裂过程中的起裂位置,根据描述真实裂缝几何形态的相关参数,利用自主开发的MATLAB程序,实现了对不同长度、孔隙度、迂曲度和方位角的裂缝的二维重构,借助COMSOL Multiphisics中"自由和多孔介质流动模块"以及"多孔弹性模块"建立了裂缝—基质耦合渗流模型及岩石基质应力—应变模型,对不同迂曲度、方位角以及带天然裂缝的迂曲裂缝周围应力及起裂位置进行分析。结果表明,压裂液流速在迂曲微裂缝中呈现波动下降,其压力平缓下降且压力梯度沿裂缝长度方向逐渐减小;迂曲微裂缝会在裂缝曲率较大的位置出现多个起裂点,且起裂位置数量和区域大小随裂缝迂曲度的增大而增加,而裂缝方位角的变化对起裂位置影响较小;在主裂缝中流体压力作用下,未胶结的天然裂缝两端出现破裂,更有利于页岩气储层中复杂缝网的形成。

In order to study the initiation points of tortuous micro-fractures which normally exist in shale gas reservoir,the MATLAB program was used to reconstruct micro-fractures of different lengths,porosity,tortuosity and azimuth,based on the parameters describing the geometry of the real fractures.The fracture-matrix coupled seepage model and a rock matrix stress-strain model were established by the module of'Free and Porous Media Flow'and'Pore Elasticity'in COMSOL Multiphisics to analyze stress distribution and fracture initiation positions with different tortuosity,azimuths,and micro-fractures with natural fractures.The results show that the fracturing fluid flow velocity decreases in the tortuous micro-fractures with fluctuation,its pressure decreases gently and the pressure gradient decreases with the length of the tortuous micro-fracture.The tortuous micro-fractures initiate at multiply points where the fracture curvature is large,and the number and area of crack initiation sites increase with the increase of the crack curvature.The azimuth angle has little effect on the initiation points.For the natural fractures,initiations occur at both tips of the unconsolidated natural cracks and the cement in the cemented natural cracks will first break under the pressure of the main fractures.