Porosity Heterogeneity and Mineralization Roles During CO2 Geological Storage in Carbonate Saline Aquifers

Abstract The feasibility of subsurface geological storage in carbonate formations, which should significantly contribute to offsetting CO2 emissions, can be determined by subsurface geochemical reactions. Injecting CO2 into a carbonate storage site tends to alter the petrophysical properties due to mineral dissolutions and precipitations. This paper investigates the role of porosity heterogeneity and its alterations because of CO2 mineralization in carbonate saline aquifers. A three-dimensional carbonate aquifer model examined a plethora of porosity heterogeneities. The aquifer model was utilized to investigate numerous sensitivities, incorporating the porosity heterogeneities using the Dykstra-Parsons coefficient of variation and facies distributions. Dykstra-Parsons coefficients of 0.25 and 0.50 with multiple correlation lengths were applied for generating the porosity distributions. 1%, 5%, and 10% bulk mass were modified to represent the facies. The porosity heterogeneities were designed to determine their influence throughout the CO2 geological storage using a compositional reservoir simulator that captures CO2 plume migration as well as subsurface geochemical reactions. The rock-fluid and fluid-fluid subsurface geochemical reactions were simulated for 200 years of CO2 geological storage, in which the stored CO2 was represented by the free, dissolved, and trapped CO2 quantities in the aquifer. Additionally, the impacts of carbonate minerals precipitation and dissolution, the CO2 plume migration, and porosity alteration were thoroughly detailed in each scenario. The developed model demonstrated the role of porosity variation in a carbonate saline aquifer during CO2 geological storage. The findings suggest that the porosity heterogeneity had no significant impact on the long-term CO2 inventory. However, short-term deviations can be noticeable. Since the long-term CO2 inventory appears to be similar, the results yield similar CO2 plume extensions. Furthermore, the altered porosity due to dissolution occurred early in CO2 storage, regardless of the degree of porosity variations. It is noteworthy that most of the dissolution occurs around the wellbore region with minimum CO2 mineralization. The dolomitization impact, on the other hand, happens throughout the storage process at a steady rate. The various porosity heterogeneities lead to changes in the calcite dissolution without significant changes in the dolomite precipitation. The calcite dissolution shows a greater influence on the porosity alteration compared to that from dolomitization. Hence, the porosity alteration can also be noticeable at the beginning of the storage. Moreover, the pressure profile follows the same trend as calcite dissolution. For instance, the high dissolution rate yields smaller pressure buildup. The path of the CO2 plume remains similar because the flow and transport tend to be faster than the rock-fluid reactions. Even though multiple porosity distributions were investigated, the free, dissolved, and trapped CO2 quantities remain nearly the same in the long term. The study aims to explore the influence of the porosity heterogeneity in carbonate saline aquifers and alterations imposed by geochemical reactions. A myriad of porosity distributions were evaluated to determine the role of subsurface geochemical reactions, specifically the mineralization trapping mechanism, on the long-term storage process. Ultimately, the findings provide guidance on tackling porosity heterogeneity during CO2 geological storage in carbonate formations.