致密砂岩多段分簇压裂中孔隙压力场对多裂缝扩展的影响

Influences of pore-pressure field on multi-fracture propagation during the multi-stage cluster fracturing of tight sandstone

为了研究致密砂岩多段分簇压裂中孔隙压力场对多裂缝扩展的影响,采用300 mm×300 mm×600 mm天然砂岩进行真三轴水力压裂实验,实验过程中采用2套压裂液注入系统分别独立控制2条水力裂缝,实验后将试件连续切片分析实验结果,同时采用三维流固耦合数值模拟方法研究孔隙压力场对水力裂缝扩展的影响。结果表明:在致密砂岩压裂中,由于孔隙压力场的存在,无论是簇内裂缝还是簇间裂缝扩展,当缝间距与裂缝半长的比值较小时,裂缝之间表现为相互吸引;当缝间距与裂缝半长的比值较大时,裂缝之间的相互影响较弱,多裂缝之间未出现相互排斥现象;除缝内净压力、缝间距、应力差外,裂缝周围孔隙压力场也是影响多裂缝扩展的重要因素;孔隙压力的增大减小了有效应力,岩石更容易发生剪切破坏,从而加速了裂缝的非平面、非对称双翼扩展;现场致密砂岩多段分簇压裂应增大簇间和簇内射孔间距,可避免压裂液滤失导致裂缝长度的减小,增大储层的改造体积。

To study the influences of pore-pressure field on multi-fracture propagation during the multi-stage cluster fracturing of tight sandstone,the natural sandstones of 300 mm×300 mm×600 mm were used for true tri-axial hydraulic fracturing experiment.In the experiment process,two sets of fracturing fluid pumping system were employed to independently control two hydraulic fractures.After the experiment,the specimens were sliced consecutively to analyze the experimental results.Meanwhile,the 3D coupled fluidsolid numerical simulation method was developed to study the influences of pore-pressure field on hydraulic fracture propagation.The results show that because of the existence of pore-pressure field in the fracturing process of tight sandstone,when the ratio of fracture spacing to fracture half-length is small,the fractures show mutual attraction when intra-cluster fracture or inter-cluster fracture are expanded.When the ratio of fracture spacing to fracture half-length is large,the interaction between fractures is weak,and no rejection occurs among multi-fractures.Beside intra-fracture net pressure,fracture spacing and stress difference,the pore-pressure field around fractures also plays a vital role in affecting multi-fracture propagation.The increase of pore pressure can reduce the effective stress,and thus is more likely to cause rock shear failures in sandstone;meanwhile,the non-planar and asymmetric bi-wing fracture expansion is accelarated.For the multi-stage cluster fracturing of tight sandstone,the inter-cluster and intra-cluster perforation spacing should be increased to avoid the reduction of fracture length caused by the filtration of fracturing fluid and increase the reservoir reformation volume.