Experimental Research on Inorganic Versus Organic Cross‐Linked Gel Systems for Mitigating CO2 Leakage in Moderate‐to‐High‐Temperature Reservoirs for Long‐Term CO2 Sequestration

This study presents a comparative analysis of inorganic and organic cross‐linked gel systems for mitigating CO2 leakage during geological storage operations. The polymer–cross‐linker fluid system (FS) was synthesized by cross‐linking sulfonated hydrolyzed polyacrylamide (SHPAM) with either an inorganic cross‐linker, chromium acetate (CrAc), or organic cross‐linkers, including polyethyleneimine (PEI), hydroquinone (HQ), and hexamethylene (HM). The SHPAM/CrAc system was classified as an inorganic cross‐linked FS, whereas SHPAM/PEI and SHPAM/HQ/HM were categorized as organic cross‐linked FS. To evaluate gelation time and long‐term thermal stability, conventional bottle testing and rheological analysis were conducted at various aging temperatures (90–130°C), representative of CO2 storage reservoir conditions. Rheological characterization was performed using both continuous and oscillatory shear approaches to examine the viscous and viscoelastic properties of the FS under a pressurized super‐critical CO2 environment (1450.0 psi [10.0 MPa]). Lastly, core‐flooding experiments conducted at 110°C under sub‐critical and super‐critical CO2 conditions to further validated the sealing efficacy of these gel systems. Experimental results demonstrated that, regardless of the cross‐linking agents, increasing polymer concentration and aging temperature led to a significant reduction in gelation time. The key inferences obtained from the conventional bottle testing method and rheological evaluation indicate that an inorganic gel system (SHPAM/CrAc) exhibited thermal stability up to an aging temperature of 90°C, whereas an organic gel system (SHPAM/PEI) maintained thermal stability up to an aging temperature of 110°C. In contrast, the SHPAM/HQ/HM organic gel system demonstrated thermal stability across all tested aging temperatures (90–130°C). The high temperature rheological investigation using two different methods under high‐pressure CO2 conditions showed good agreement and indicated long‐term thermal stability ranking as follows: SHPAM/HQ/HM > SHPAM/PEI > SHPAM/CrAc. Additionally, core‐flood tests revealed that, at the simulated reservoir temperatures of 110°C, the inorganic gel system (i.e., SHPAM/CrAc) was unable to effectively prevent CO2 leakage due to a low plugging efficiency of less than 10.0% in both sub‐critical and super‐critical CO2 conditions. On the other hand, under both sub‐critical and super‐critical CO2 conditions, the SHPAM/PEI organic gel system showed a moderate plugging efficiency in the range of 44.2%–74.5%, whereas the SHPAM/HQ/HM organic gel system achieved a permeability reduction of >98.0%. These studies demonstrated that polymer gel systems have potential for control of leakage in geological carbon sequestration contingent upon the appropriate selection of gel formulations tailored to specific geochemical and pressure–temperature (P–T) conditions and the ability to direct the gels to the regions where leakage is occurring.