Recent progress in understanding mechanisms of shale gas wellbore integrity failure

Although significant progress has been made in shale gas exploitation in China,the industry is confronted with a prominent problem of wellbore integrity failure that mainly manifests as casing deformation and sustained annulus pressure(SCP).In the current paper,we present an analysis of failure mechanisms of shale gas wellbore integrity.Our results demonstrate that casing deformation is strongly linked to tension-compression cycling loads arising in multistage fracturing,rock heterogeneity,anisotropy,lithological interface,and natural fractures and faults.Among those factors,fault slip caused by fracturing fluid flowing into natural fractures is suggested to be the principal one that may give rise to casing deformation.SCP,however,is induced by micro-annulus or micro-fissures arising from multiple drastic changes of pressure and temperature during fracturing.Finally,we discuss approaches to maintaining shale gas wellbore integrity,including lowering the elastic modulus of cement sheath,casing optimization,staged cementing,fault slip prediction,and control of fracturing parameters.