页岩水力压裂微裂纹扩展演化试验研究及力学机制分析

Experimental study on micro-crack propagation and mechanical mechanism analysis of hydraulic fracturing in shale

水力压裂技术在页岩气开采行业中应用广泛。然而,到目前为止压裂条件下裂纹扩展及渗透的力学机制尚不清楚。基于此,对页岩试样的水力压裂缝扩展演化及层理面激活的室内试验进行统计分析及力学机制的探讨。试验结果表明,页岩试样断面所观察到的微观尺度上的微裂纹并非宏观流体压力所致,因在试验中发现在微裂纹张开处产生压应力,与之相反的是:微裂纹是由扩展断裂前的拉应力集中产生的,这也意味着层理面激活是由沿层理面的裂缝扩展引起的。此外,对微裂纹长度的统计分析表明,页岩在断裂韧性方面表现出各向异性,并且垂直于层面的断裂扩展阻力比平行于层理面的断裂扩展阻力要大2倍之多。

Hydraulic fracturing is commonly applied into shale gas exploitation industry.However mechanical mechanism of permeability under the fracturing has been unclear so far.In view of this,a statistical analysis and discussion of the mechanical mechanism by the laboratory experiments on the hydraulic fracturing cracks propagation,and the activation of bedding plane of shale specimen are presented in the study.It is demonstrated that the microscopic observed micro-cracks could not be produced by fluid pressure as the latter generated compressive stresses on the places of micro-cracks.On the contrary,the micro-cracks are produced by tensile stress concentration in front of the propagating fracture.This implies that the bedding plane reactivation is caused by fracture propagation along the plane.An analysis of micro-crack lengths shows that shale exhibits anisotropy in fracture toughness with the resistance to fracture propagation parallel to bedding planes being as twice a small as compared with the resistance to fracture propagation in the directions normal to bedding planes.