Analysis of the Influence of Defective Cement Ring on Casing Stress During CO2 Geological Storage

In the CCUS operation site, CO2 geological buried Wells are mostly worn out or abandoned old oil and gas reservoir Wells, which have undergone well testing, production, stimulation (fracturing, acidification) and other operations. The temperature and pressure in the wellbore have undergone great changes, and the wellbore cement ring has defects due to elastic mismatch. In this paper, from the perspective of missing ring opening, missing thickness, missing location, casing eccentricity, etc., the finite element numerical simulation method is used to establish a casing - cement ring with defects - formation model, and analyze the stress change rule of casing at the missing part of cement ring under the influence of temperature and non-uniform ground stress during CO2 injection. The research shows that the higher the ratio of cement ring diameter to thickness, the greater the casing stress, the casing is still in a safe state; The circumferential missing Angle of cement ring has great influence on the casing stress. When the missing Angle is 60°, the casing has reached the maximum stress state and has been damaged. The azimuth Angle of cement ring is closely related to the ground stress, and the casing has been damaged at 0-60°. The eccentricity of cement ring has no obvious effect on the stress rush of casing. Therefore, it is necessary to control reasonable CO2 injection parameters for the wellbore with cement ring defects, so as to improve the safety of the casing during CO2 storage and ensure the integrity of the wellbore. 1. INTRODUCTION With the rapid development of social economy, greenhouse gases generated by human activities have been discharged into the atmosphere beyond the standard, resulting in an increase in global temperature year by year (HööK, et al., 2023; Qureshi, et al., 2016; Peter, et al., 2018). According to the International Energy Agency's 2021 report, global carbon emissions will continue to grow in the future, predicting that global CO2 emissions will increase by 500 million tons by 2030, 600 million tons by 2040, and 700 million tons by 2050. The problem of climate change and environmental pollution caused by excessive CO2 emissions has attracted great attention from the international community. In order to cope with this problem, countries have actively explored CO2 emission reduction technologies, among which CCUS technology is one of the effective ways. CO2 storage technology is the core component of CCUS technology, aiming to store CO2 captured from industry or other emission sources in geological environment. Achieve long-term isolation of CO2 and atmospheric environment. The target reservoirs for CO2 geological storage mainly include deep salt water, depleted oil and gas reservoirs, unconventional oil and gas Wells and CO2 geothermal enhanced reservoirs, etc. The depleted oil and gas reservoirs are widely distributed and have good closed circles, which can not only efficiently bury CO2 to reduce its leakage risk, but also have good economic and social benefits. Therefore, CO2 injection into abandoned reservoirs is one of the important ways to achieve efficient CO2 storage.