Analysis of well stress with the effect of natural fracture nearby wellbore during hydraulic fracturing in shale gas wells

Abstract

Sustained Casing Pressure (SCP) and production casing deformation in shale gas wells have been of growing interest in the industry, which should be attributed to the failure of cement sheath and casing respectively during the hydraulic fracturing process. Natural fractures are widely developed in shale formation and show tight connection with casing deformation. However, the effect of natural fracture nearby wellbore on casing deformation have hardly been investigated in previous references. Based on this situation, a staged analytical model for stress calculation of casing-cement-formation system (CCFS) is established, which fully incorporates the effect of natural fracture nearby wellbore during hydraulic fracturing. The calculation results of new analytical model are in good agreement with the numerical method by staged finite element. Moreover, the stress variation of CCFS under the effect of natural fracture is revealed by employing the simulation of field data from Weiyuan shale gas field. Results illustrate that the circumference stress at interface between casing and cement (interface I) turns to compression from tension conditions once the natural fracture effect is generated, moreover, both the radial and circumference stress increase greatly considering the effect of natural fracture. Further, optimizing suggestions for the design of fracturing, casing and cement sheath are provided through sensitivity analysis. It implies that the well stages with close distance fractures (the limit distance of calculation example in this research is 1.4 m) should be avoided hydraulic fracturing in the well design for the integrity of casing and cement sheath, Nevertheless, the increasement of production casing thickness and fracturing fluid temperature can dramatically reduce the risk of casing deformation and failure of cement sheath, if the well stages with fractures can not be avoided rightly.