Study on Stress Variation and Sealing of Casing-Cement Ring-Strata Assemblies Under Sour Gas Production Conditions

During the exploitation of deep and ultra-deep sour gas-bearing gas wells in Sichuan, the casing-cement sheath-formation combination has been under high pressure and acidic environment for a long time. The stress variation law of the combination is complex, and the problem of sealing failure annulus pressure is serious. In this paper, the finite element software is used to establish a model of casing-cement sheath-formation combination with corrosion defects under high-pressure acid gas production conditions. The stress variation law of the combination under the change of casing internal pressure and formation pressure is clarified, and the sealing failure mechanism of casing-cement sheath-formation combination is revealed. The results show that corrosion is the key factor affecting the sealing failure caused by the stress change of the assembly, and the sealing failure of the assembly is related to the corrosion position. When the casing is subjected to uniform corrosion, as the degree of corrosion increases, the overall stress of the combination becomes larger, resulting in plastic strain between the cement sheath and the casing, and debonding between the plastic strain and the cement-well interface. The two factors together aggravate the continuous expansion of the microannulus at the cement-well interface. When the cement sheath is corroded, the cementation strength of the local cementing interface will be destroyed. With the expansion of the corrosion range, the stress of the combination is redistributed, and the stress distribution is gradually complicated. A certain degree of stress concentration is easy to occur at the corrosion position, resulting in plastic strain between the cement sheath and the casing, which in turn aggravates the failure risk of the cementing interface. In the design of gas well cement slurry containing acid gas, the corrosion resistance and the sealing performance of the cementing interface should be considered.