Analysis of depleted gas reservoir underground gas storage wellbore integrity change during gas injection and production processes

Depleted gas reservoir type underground gas storage is the most widely distributed gas storage in the world. Due to the complex geological conditions and frequent gas injection and production operations, it is prone to the wellbore integrity failures leading to hazards such as annular pressure. To protect the wellbore integrity and contain the escape of gas to subterranean formations or surface, a finite element model is developed to study the wellbore integrity change. This model simulates the typical gas injection and production processes of gas storage with complex geological conditions and non-uniform ground stress. A new comprehensive method considering complex well conditions was developed to simulate the wellbore integrity failure process more realistically. This model can predict the wellbore integrity failure and high-risk areas during the gas injection and production processes. The simulation results indicate that the cement shear failure sections are mainly concentrated near the upper and lower boundaries of the formation interlayer during the gas injection process, and the distribution area and trend of the shear failure areas are the same as the gas production process. The most common cement tensile failure sections during the gas injection process all occur on the inner side of the cement in the direction of maximum ground stress during the gas injection process. The highest effective (Von Mises) stress area of the casing is mainly concentrated in the upper and lower boundary zone of the formation interlayer during the gas injection process. And the highest Von Mises stress sections are mainly distributed in the direction of the minimum ground stress near the formation interlayer lower boundary inside the casing, which is similar to the distribution in gas production process. The first cohesive failure position and final failure distribution result of cement inner interface both appear in the depth range of the formation interlayer. The cement outer interface's first and final failure areas are all concentrated on the lower boundary of the formation interlayer and consistent with the direction of the maximum ground stress. The cement inner interface failure occurred earlier but not as significant as the outer interface failure, which occurred later but the failure range was large in this case. Practical guidelines and solutions derived from the simulation results can be useful for improved wellbore integrity protection in field applications.