In the long-term storage process of CO2 storage wells, the integrity of the cement is vulnerable to damage or even failure due to factors such as cementing quality, corrosion, stress change, etc., resulting in microcracks or leakage channel, resulting in the inability to guarantee the effectiveness of the storage. For this reason, this paper establishes a mechanical model of downhole casing-cement-formation combination in CCUS well, and studies the influence of wellbore pressure, temperature change, performance parameters of cement, etc. on the stress distribution of cement; The leakage characteristics of the combination are studied through the sealing integrity test of the combination, and the influence of different factors on the CO2 leakage rate is discussed in combination with the leakage model of the cement. The research results show that (1) the integrity failure of the cement mainly reflects the failure of the seal due to the leakage channel, which can not achieve effective sealing; (2) The leakage rate of CO2 is directly proportional to the permeability and cross-sectional area of the leakage channel, but the influence of the size of the cement on the leakage is limited; (3) The CO2 leakage rate increases significantly with the increase of the pressure and temperature of the storage layer, and both of them are nonlinear. (4) The leakage rate of CO2 is basically proportional to the crack width of cement. In theory, when the crack width of cement reaches 640 μm, the leakage rate of CO2 exceeds the safety value, and the leakage rate of CO2 increases exponentially with the increase of the crack opening of cement. The slight change of the crack opening will greatly increase the leakage rate of CO2. This paper reveals the potential leakage path and law of cement in CCUS well, and the research results can provide guidance for the effectiveness of long-term storage design of CCUS well.
Keywords CCUS well, sealing integrity of cement, casing- cement- formation combination, failure analysis of cement