ABAQUS Numerical Simulation Study on the Shear Instability of a Wellbore Induced by a Slip of the Natural Gas Hydrate Layer

Abstract

To study the shear deformation and failure characteristics of a wellbore and the interaction mechanism with its surrounding rocks induced by a layer slip during natural gas hydrates (NGHs) extraction, this paper conducted a numerical simulation study of wellbore shear induced by a layer slip using ABAQUS software and carried out a laboratory experiment of wellbore shear to verify the accuracy of the numerical model. The results show that the shear force–displacement curves obtained from the laboratory experiments and numerical simulations are consistent with five stages, including the compaction stage, linear stage, plastic stage, strain-softening stage and residual stage. The wellbore shows a “Z”-shaped deformation characteristic after its shear breakage. The shear force of the wellbore is maximum at the shear surface, and it is distributed in an approximate “M” shape along the shear surface. The axial force of the wellbore is small and uniformly distributed in the initial stage of the shear. The wellbore bending moment is minimum at the shear surface, with a value of 0, and it is distributed in a skew–symmetric wave shape along the shear surface. During the shearing, the evolution of the wellbore axial force and shear force can be classified into the distribution pattern along the radial direction on the shear surface and the pattern along the axial direction. The combination of the wellbore axial force and shear force causes the tensile–shear compound failure of the wellbore. During shearing, the wellbore and rock body gradually enter the plastic state with the increase in the shear displacement. When the entire cross-section of the wellbore is in the plastic state, a “necking” phenomenon of the wellbore begins to appear. During the shearing, the frictional dissipation energy and plastic dissipation energy increase constantly. In addition, the elastic strain energy increases to a peak and then decreases to a certain value, which remains unchanged along with the work conducted by the shear force.