Appraising Carbon Geological-Storage Potential in Saline Aquifers Using Pressure-Transient Analysis

Abstract

Pressure transient analysis (PTA), as a powerful technique for CO2 injection data analysis, plays an essential role in assessing the CO2 storage performance of saline aquifers. However, the appearance of the two-phase CO2/brine flow together with pressure-dependent properties introduces significant nonlinearity into the governing equation and makes the use of PTA erroneous. Accordingly, we presented a two-phase approach to estimate the storage capacity and spatial distribution of CO2 plumes by analyzing injection data. The proposed method consists of an analytical two-phase flow model for CO2 injection in saline aquifers and a straight-line analysis approach for estimating the CO2 storage capacity. The PTA model incorporates the two-phase flow of CO2-brine, and the boundary condition of moving CO2 injection front by employing a modified equation of the distance of investigation. To account for the pressure-dependent reservoirs properties, we established new definitions of pseudo-pressure and pseudo-time and reached the analytical solutions. In addition, approximate solutions in straight-line form are obtained to appraise the CO2 storage potential through an iterative workflow. We assessed the precision of our proposed method by conducting tests with synthetic data. This data was generated from four numerical cases related to CO2 storage in saline aquifers, simulated under constant injection rate and bottom-hole pressure conditions. We presented a two-phase diagnostic plot to identify transient and boundary-dominated flow regimes. A two-phase specialty plot is proposed to calculate essential reservoir parameters and the CO2 storage capacity. The numerical validation confirms the accuracy of the proposed method with an extremely low relative error in estimating CO2 storage capacity. The good match demonstrates the superiority of the proposed method in the quick evaluation of storage potential over the numerical simulation. Additionally, the method offers early detection of leakage through the prediction of the CO2–brine front. Instead of assuming constant CO2 properties in two-phase PTA, this paper incorporates pressure-dependent fluid properties into a straight-line analysis approach by defining new pseudo-properties. Unlike the prior single-phase model without an injection front, the moving boundary condition of CO2 injection front is considered in the superposition pseudo-time to provide a more accurate estimation of CO2 storage capacity. This study shows that the use of PTA along with a simple CO2 injection model is simple yet rigorous for CO2 storage projects.