ABSTRACT:

Casing deformation during hydraulic fracturing process dramatically decreases the number of fracturing stages and impacts the efficient exploitation of shale gas. As shear deformation is the main type of casing deformation in shale gas wells, reactivation and slippage of dominant natural fractures caused by injected fluid is treated as the main reason for casing deformation problems. Generalized shear activation criterion is used to analyze the reactivation of natural fracture. According to the calculated results, the slippage of natural fracture ranges from 0mm to 60mm. A higher fluid pressure and larger reactivated area can induce larger slippage of natural fracture. Through the finite element modelling (FEM), non-uniform loads are distributing around casing due to the slippage of fracture. The largest load on casing reaches to 100MPa when the slippage equals to 40mm. The larger slippage of fracture causes higher loads on casing and ultimately, the loads on casing reaches to a constant value because of the plastic deformation of casing. The elastic modulus of cement sheath has little effect on the loads of casing. But on the contrary, compressive strength of cement sheath can significantly affect the loads of casing. Appropriate plastic deformation or failure of cement sheath is meaningful for reducing the loads on casing.

1. Introduction

Papers should be Large number of casing deformation problems have seriously affect the safe and efficient exploitation of oil and gas all around the world by now (Gao and Liu, 2019; Xi et al., 2019). Casing deformation will occur in the whole life of a well. Some reasons for casing deformation has been studied in previous. Like the creep of rock, water absorption of rock, formation compaction, shear deformation of rock, formation sand production, poor cementing work or poor casing quality and so on (Hu et al., 1998; Jo et al., 2010; Nester et al., 1955). But the casing deformation problems in shale gas wells occur during the hydraulic fracturing process and it is at the short time after the well is drilled. This phenomenon is different from the traditional casing deformation in wells. Casing deformation occurs in more than thirty percent of wells in Weiyuan and Changning shale gas field in Sichuan, China. Therefore, more and more researchers focus on this topic.

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