Barrier Design and Evaluation Process to Achieve Cement Integrity in CCS Wells

Barrier design processes for oil and gas wells have historically demonstrated their benefit in risk reduction and cement design optimization when using Portland cement. Carbon Capture and Sequestration (CCS) wells have a different set of risks which must be considered. These risks are a consequence of mechanical, thermal, and hydraulic loads acting on the barrier which is also undergoing mechanical changes due to chemical alteration in the case of Portland cement. Such changes are a function of time, CO2 state, cement composition, temperature, and pressure. The specifics depend on the use case, for example a plug, an annular sealant, a monitoring well, or an injection well. The goal of any risk assessment is to determine whether the above-mentioned events will compromise the barrier. Given the chemical and mechanical changes occurring in a Portland cement barrier, there is a need to tailor existing barrier design processes to suit CCS wells and account for this alteration. The proposed design process comprises of four steps. The first step will assess the extent of the reaction between CO2 and the cement sheath under downhole conditions. The second step will determine the reaction driven changes to the mechanical properties of the cement sheath. Such changes may occur throughout the cement or only in a portion of cement depending on the location and injection plans. The third step will use the modified properties and determine the stresses and temperatures in the cement sheath under the action of temperature and pressure loads. The fourth step will determine the risk of mechanical failure by calculating operational loads and applying them to the altered cement. If the failure risk is too high, the cement composition can be iteratively tailored to determine an acceptable design. The first two steps are enabled by experimentally generating CO2 exposure data on multiple cement systems and exposure conditions. Data for alteration rates, physical changes, mechanical changes, etc. are gathered in order to build semi-empirical models that can predict the effect of CO2 based on the cement system design over time. The third step utilizes existing wellbore stress modelling software and incorporates the spatially varying mechanical property inputs determined by the alteration models. Stresses from the third step are compared with predicted failure properties (compressive and tensile strengths) to assess the risk of loss of cement sheath integrity. The proposed design evaluation process describes the chemical and mechanical events unique to CCS wells. Consequently, it will provide a vital tool to the industry for tailoring cement compositions and minimizing risk of loss in cement sheath integrity in a dynamic CO2 environment.