New Analytical Method to Evaluate Casing Integrity during Hydraulic Fracturing of Shale Gas Wells

Jun Li,Gonghui Liu,Xueli Guo,Shuoqiong Liu,Yan Xi

doi:10.1155/2019/4253241

Abstract

An accurate analysis of casing stress distribution and its variation regularities present several challenges during hydraulic fracturing of shale gas wells. In this paper, a new analytical mechanical-thermal coupling method was provided to evaluate casing stress. For this new method, the casing, cement sheath, and formation (CCF) system was divided into three parts such as initial stress field, wellbore disturbance field, and thermal stress field to simulate the processes of drilling, casing, cementing, and fracturing. The analytical results reached a good agreement with a numerical approach and were in-line with the actual boundary condition of shale gas wells. Based on this new model, the parametric sensitivity analyses of casing stress such as mechanical and geometry properties, operation parameters, and geostress were conducted during multifracturing. Conclusions were drawn from the comparison between new and existing models. The results indicated that the existing model underestimated casing stress under the conditions of the geostress heterogeneity index at the range of 0.5–2.25, the fracturing pressure larger than 25 MPa, and a formation with large elastic modulus or small Poisson’s ratio. The casing stress increased dramatically with the increase of in situ stress nonuniformity degree. The stress decreased first and then increased with the increase of fracturing pressure. Thicker casing, higher fluid temperature, and cement sheath with small modulus, large Poisson’s ratio, and thinner wall were effective to decrease the casing stress. This new method was able to accurately predict casing stress, which can become an alternative approach of casing integrity evaluation for shale gas wells.

Highlights

During the multistage fracturing process, fracturing fluids are pressed into a borehole with a high pump rate and pressure. e complex downhole environments—high pressure and large temperature variation—increase the risk of casing deformation. e volume fracturing technique effectively reconstructs shale reservoirs; frequent and serious casing deformation failures occur [1]. ere were over 36 wells with casing deformation during fracturing process in some shale gas plays
A low temperature Tn and the fracturing pressure Pf were assigned in the internal casing wall. e wellbore stress field was obtained under the condition of thermalmechanical coupling
A new analytical model considering drilling construction was established to assess the casing stress under di erent conditions considering thermal-pressure coupling. e solutions were obtained by dividing the model into three parts such as initial stress eld, wellbore disturbance eld, and thermal stress eld

Summary

Introduction

During the multistage fracturing process, fracturing fluids are pressed into a borehole with a high pump rate and pressure. e complex downhole environments—high pressure and large temperature variation—increase the risk of casing deformation. e volume fracturing technique effectively reconstructs shale reservoirs; frequent and serious casing deformation failures occur [1]. ere were over 36 wells with casing deformation (including 112 horizontal wells by 2017) during fracturing process in some shale gas plays. Based on the stress function method, a three-dimensional model of the casing-cement sheath-formation (CCF) system was proposed subjected to linear crustal stress, and an analytical solution of the model was obtained [8]. Liu et al [20] presented an analytical method for evaluating the stress field within a casing-cementformation system of oil/gas wells under anisotropic in situ stresses in the rock formation and uniform pressure within the casing. To evaluate the thermal and mechanical stresses of casing, an analytical model of casing-cement sheath-formation system was established considering wellbore construction. Sensitivity analyses were conducted to estimate the influence of different factors on casing stress such as property of cement sheath and casing, fracturing pressure, fluid temperature, and initial geostress. Useful countermeasures were put forward to decrease casing stress during the fracturing operation

Method Comparison

Stress Distribution around Wellbore

Model Validation

Conclusions