Predicting the Stability of the Rock Around a Reconstructed Reservoir for Gas Injection and Production in an Old Brine Salt Mine Cavity

Abstract

With the increasing demand for oil and gas by the country, the resources of salt cavern type gas storage sites are becoming increasingly scarce, and there is an urgent need to open up new areas for reservoir construction. China has abundant underground space resources in abandoned salt mines, and thousands of abandoned salt mines have been closed in the past 20 years. Therefore, abandoned salt mines can be used to transform underground gas storage. This method has a large gas storage space and high economic benefits, which can effectively accelerate China’s natural gas reserves. The stability of the cavity is the key to gas storage, which will directly affect the investment cost of salt rock mining and reconstruction. Therefore, in order to ensure the long-term safe operation of gas storage, it is necessary to grasp the deformation and damage of surrounding rock and the variation of storage capacity of the old chamber under the action of high and low pressure gas storage. Considering the geological conditions of a previous horizontally drilled well in a mining area, we created a three-dimensional geomechanical gas storage model utilizing the material balance approach. We developed six evaluation indices, namely, rock deformation in the vicinity, dilatancy safety factor, volumetric shrinkage rate, plastic zone, effective strain, and non-tensile stress, for forecasting the long-term stability of the surrounding rock of the old borehole when natural gas is injected or extracted. The results suggest that the original geometry of the old cavity’s horizontal section can be ascertained by integrating the sonar detection data, original drilling trajectory, and production data. After 30 years of injection-production operation, the maximum vertical deformation of the surrounding rock in the old cavity occurs at the top of the cavity and is equivalent to 6.48% of the maximum diameter of the cavity. The effective deformation on the wall and bottom of the cavity is larger and exceeds 3%. The volume shrinkage rate is 7.85%, which has a larger safety margin compared to other indicators, and all three gradually increase with the operation duration; The expansion safety factor and tensile stress tend to stabilize with continuous injection and extraction, and the plastic zone is distributed on the top interlayer, cavity body, and both sides of the cavity bottom. The expansion of the plastic zone range and volume increment are relatively small during operation. The above three indicators should be taken seriously in the early stage of operation. It is recommended to focus on the physical, mechanical properties, and void characteristics of the sediment within the brine-mining cavity. The research findings can serve as a basis for assessing the stability of the surrounding rock in the old cavity of horizontal directional butted wells. Additionally, the results may be applicable to the use of the old cavity in the salt mine for carbon storage, hydrogen storage, helium storage, and compressed gas storage.