Experimental and Numerical Simulation on the Generation and Development of Cumulative Plastic Strain of Cement Sheath in Salt Cavern Gas Storage Well

ABSTRACT Salt cavern gas storage is an important technical means to balance the demand for staggered energy supply. Due to the repeated injection and extraction of natural gas in gas storage facilities, sealing integrity failure in the wellbore of gas storage facilities frequently occurs. In response to this, considering the cyclic loading and unloading of the pressure load inside the casing, mechanical tests of set cement were carried out under alternating loads, quantifying the cumulative plastic strain change law of set cement and revealing the deterioration characteristics of its mechanical properties. A numerical model of cumulative plastic strain of casing cement sheath formation under alternating load was established based on the obtained experimental parameters. Comparative verification was conducted using experimental data, and the variation law of cumulative plastic strain of cement sheath was analyzed. The distribution of cumulative plastic strain on the cement sheath bonding surface of the entire wellbore was quantified. The research results indicate that the higher the internal pressure value of the casing, the earlier the plastic strain appears, and with the increase in the number of alternating loads, the cumulative plastic strain increases approximately linearly. After the internal pressure increased by 30 MPa, the cumulative plastic strain increased by a maximum of 46.75%. When the number of loading and unloading cycles under alternating loads is small, reducing the elastic modulus (6 GPa) of the cement sheath can effectively reduce its cumulative plastic strain. However, as the number of loading and unloading cycles under alternating loads exceeds a specific value, the cumulative plastic strain produced by high elastic modulus (15 GPa) cement sheaths decreases. Finally, the distribution pattern of cumulative plastic strain along the wellbore under different gas injection times and complex formation conditions was analyzed. Suggestions for establishing well barriers in salt cavern gas storage during cementing were proposed. The research results can provide theoretical and engineering references for evaluating the sealing integrity of gas storage wells.