Abstract

Multi-stage hydraulic fracturing operations are widely used in long horizontal wells to enhance production in deep shale gas reservoirs in Sichuan Basin. During the stimulation process, casing deformation often occurs. One of the main mechanisms of casing deformation is shear slip on weak planes, i.e. natural fracture reactivation during injection operations. The formation pore pressure near wellbore increases as fracturing fluid is injected during hydraulic fracturing, and the stress state near wellbore in reservoir is changed. In such conditions, natural fractures in the vicinity of the treatment well may exceed slip tolerances in critical stress states. The shear slip of fractures in turn could lead to casing deformation events. In this study, a numerical simulation workflow was setup, based on the comprehensive modeling of a time lapse stress field, hydraulic fracturing simulation, natural fractures and other weak interfaces; analyzing the casing deformation mechanism of shale gas horizontal wells during the multi-stage hydraulic fracturing processes. We aim to guide the prediction and mitigating solutions to reduce the casing deformation risks and improve stimulation efficiency.

Introduction

The load exerted on a casing by the formation underground always changes. Usually, a casing has a maximum load bearing capacity, which if exceeded, the casing will deform and potentially fail. Researchers have extensively studied the phenomenon of casing deformation problems (see Al Hosani, 2019, and references therein), and there is consensus that most casing deformation occurrences can be attributed to three main causes:1) geological factors; 2) engineering factors; and 3) material defects. The geological factors that cause casing deformation include differences in formation lithology and bedding plane weaknesses (Dusseault et al. 2001; Chipperfield et al. 2007; Hossain et al. 2010; Tian et al. 2015; Chen et al. 2020), the development of natural fractures and faults (Liu et al. 2017; Han et al. 2020), and stress changes in the near wellbore region (Lian et al. 2015; Yu et al. 2016). The engineering factors such as hydraulic fracturing are generally labeled as induced factors for casing deformation. The in-situ stress field can be altered by hydraulic fracturing, those pre-exiting stable faults, natural fractures and other weak surfaces can therefore be reactivated with shear slip, which in turn may lead to a casing deformation (Li et al. 2019).

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