DEVELOPMENT OF A HYDRO-MECHANICALLY COUPLED APPROACH TO ESTIMATE POTENTIAL LEAKAGE IN CARBON STORAGE WELLS

In the last few decades, the Underground Gas Storage (UGS) operation has been proven to be a sustainable solution for mitigating greenhouse gas emissions toward the European union’s aim of being climate neutral by 2050. One of the main gases for storage operations is carbon dioxide, which accounts for 82% of greenhouse gases in the atmosphere. Despite the fact that CO_2 has a lower global warming potential than other greenhouse gases, due to the amount of the emissions into the atmosphere, it is the most important greenhouse gas to be considered for efficient climate control. Depleted hydrocarbon reservoirs are suitable candidates for such purpose, and the integrity of wellbores which are pathways for injecting CO_2 into underground storage sites is a major concern for ensuring an efficient storage operation. In case of failure in the wellbore integrity, different kinds of leakage paths can appear and lead to the outflow of the captured gas. Due to the change of the pressure and temperature inside the wellbore system or shrinkage of the cement behind the wellbore casing, a clearance can be created between casing and cement or between cement and rock formation which is called microannulus. Microannulus behaves like a vertical fracture with a relatively high permeability and provides a perfect discharge way for the material. Carbon dioxide leakage poses a hazard depending on its rate of discharge. Therefore, a model of CO_2 leakage is an important part of a risk assessment framework. As the main leakage pathway within the wellbore, characterization, and estimation of the outflow amount in microannuli is essential in UGS projects. For this reason, several experimental, numerical and analytical studies have been conducted to characterize microannuli and evaluate their discharge rates. In all carbon capturing and storage (CCS) projects, a caprock layer above the reservoir with a very small value of permeability plays a crucial role to trap the captured material. Consequently, there is a need for fairly high levels of integrity for both the caprock layer and well systems traversing this layer in such operations. Due to this reason, microannulus structures along the caprock layer have been taken into account in this research for modeling and flow calculations. In this study, a finite element model is utilized for calculating the initial mechanical size of the microannulus in the intended platform under different stress conditions throughout the entire life-cycle of the CCS well. Accounting for the effects of buoyancy, viscous, inertial, and capillary forces, we have modeled the two-phase flow of water and CO_2 inside the microannulus. Using the Python programming language, a calculator package is developed based on an analytical solution. The package calculates the fluid pressure inside the microannulus along the pathway and updates the mechanical size of the annular area due to the extra fluid pressure inside the microannulus according to the mechanical parameters of the system. The fluid properties are updating along the microannulus based on the corresponding thermodynamic conditions. The mechanical sizes are then corrected for the hydraulic apertures in the calculator using an empirical correlation. As a result, the permeability of the system can be calculated using the hydraulic size distribution inside the microannulus and cubic law for the estimation of the leakage rate.