Critical Conditions for Wellbore Failure during CO2-ECBM Considering Sorption Stress

Abstract

Significant stress changes caused by sorption-induced swelling raise the coal wellbore failure potential, which directly impacts the safety and sustainability of CO2 enhanced coalbed methane (CO2-ECBM). Additionally, a mixture gas (CO2/N2) injection is recommended due to the sharp decline of permeability with pure CO2 injection. In this study, incorporating the impacts of mixture gas adsorption and poroelastic effects, a semi-analytical model of coal wellbore stability during mixture gas injection is proposed. Model results indicate that the stress field is significantly influenced by the boundary condition and sorption effect. In addition, parametric studies are performed to determine the influence of adsorption parameters, mechanical properties, and gas composition on the stress distribution and then on the wellbore failure index. Furthermore, mixture gas injection with a large proportion of CO2 or N2 both cause wellbore instability. Significant compressive hoop stress and shear failure are caused by the mixture gas injection with a large proportion of CO2. In contrast, the displacement of CH4 with weakly adsorptive N2 will result in less compressive and shear or tensile failure, which occurs near the wellbore. Thus, mixture gas (including pure CO2/N2) injection must be controlled by coal wellbore failure, providing an accurate estimation of in-situ coal seams’ CO2 storage capacity from the perspective of wellbore stability.

Research on Fluid–Solid Coupling Mechanism around Openhole Wellbore under Transient Seepage Conditions

“…We investigated the variations in the transient stress field surrounding the wellbore as the reservoir's permeability gradually decreased, eventually reaching impermeability. As depicted in Figure 4, the results obtained from the numerical solution are compared with those derived from the analytical solution provided by Equation (17). In the case of an impermeable reservoir, the seepage force term in Equation ( 1) is negligible, and the fluid primarily exerts a surface force on the rocks surrounding the wellbore.…”

“…This model allowed for a parametric analysis of the effects of elastic anisotropy, fracture volume fraction, fracture permeability, mass transfer coefficient, and fluid pressure on wellbore stress. Xiao et al [17] proposed a semi-analytical model for coal wellbore stability during mixture gas injection. Additionally, Rim et al [18] developed a new seepage model that employs the method of the separation of variables and the Bessel function theory to predict variations in the pore pressure and seepage force around a vertical wellbore during hydraulic fracturing.…”