Evaluation on the dynamic sealing capacity of underground gas storages rebuilt from gas reservoirs: A case study of Xinjiang H underground gas storage

Abstract

In order to provide effective guidance for optimizing and adjusting the maximum operation pressure and dynamic monitoring well pattern of underground gas storages (hereinafter referred as gas storages) and ensure the operation safety of gas storages, this paper takes Xinjiang H gas storage whose storage capacity and peak shaving capacity are the highest in China as the research object to establish a 3D fine geological model and a 3D dynamic geomechanical model in the regional scale of H gas storage based on the alternating change characteristics of geostress field during the high-speed cyclic injection and production of gas storage, combined with the geological, seismic, logging and various laboratory core experiment results. Then, the trap dynamic sealing capacity of H gas storage under the action of alternating stress after 14 years of depletion development of its original gas reservoir and long-term high-speed injection and production after its reconstruction from the gas reservoir was evaluated comprehensively. And the following research results were obtained. First, the change of the formation pressure in H gas storage has a significant effect on the regional geostress field. Second, after the development of the original H gas reservoir, the geostress on the two sides of the fault are greatly different, but the cap rock is not deformed and failed and the reservoir controlling fault as the gas bearing boundary doesn't glide, which ensures the safety of reconstructing H gas reservoir into gas storage. Third, due to the high-speed injection and production of H gas storage, the geostress field becomes more uneven, which has a potential negative impact on the integrity of the injection and production wells of the gas storage. Fourth, during the long-term high-speed injection and production operation of H gas reservoir, the cap rock does not undergo shear and tensile failure, but the numerical simulation of geomechanics shows that the long-term high-speed injection and production of the gas storage results in the relative sliding deformation (maximum about 5 cm) on both sides of H fault in the south of the reservoir, so the sealing capacity of H fault is a weak point impacting the integrity of the gas storage. Fifth, it is recommended to deploy monitoring wells at the upper block of H fault in the south of the gas storage to strengthen the injection and production dynamic monitoring. In conclusion, this study performs a qualitative and quantitative evaluation on the trap dynamic sealing capacity of gas storages rebuilt from gas reservoirs under the action of alternating stress, and it plays an important guiding role in ensuring the long-term injection and production safety of H gas storage.