Wellbore Stability Evaluation during Gas Drilling through Low Permeability Gas Reservoirs in Western Sichuan

In gas drilling through low permeability gas reservoirs in Western Sichuan, borehole collapse occurs frequently in the tight sandstone sections. To develop a protocol for anti-collapse technical measures during gas drilling of such as in the kinds of gas reservoirs found in western Sichuan, effective stress around the wellbore under the influence of radial drag force has been analyzed based on the analysis of non-Darcy seepage and reservoir sensitivity on pore pressure, and radial drag force during gas production. The wellbore collapse mechanism of gas drilling was analyzed by using practical drilling data of Well X3 in Western Sichuan. Then a wellbore stability evaluation method during gas drilling through low permeability gas reservoirs in western Sichuan was proposed in combination with Mohr-Coulomb criterion and tensile failure model. Results showed that the high-speed non-Darcy seepage and the radial drag force generated during gas production in low permeability reservoir will increase the borehole collapse pressure, so damage borehole stability. Tensile failure occurs first under radial drag force, and then shear failure happens due to the decrease of formation pressure. Those rock failures result in wellbore collapse. Wellbore stability of Well X3 in the low permeability reservoir was evaluated using this method, and the results were consistent with the practical drilling situations. It demonstrated that this method could be used to evaluate wellbore stability during gas drilling in low permeability gas reservoir in western Sichuan, and it would provide a protocol and recommended best practices for the development of anti-collapse measures during gas drilling in low permeability gas reservoir in Western Sichuan.