Experimental measurements and characterization models of caprock breakthrough pressure for CO2 geological storage

Caprock plays a critical role in the long-term safety of CO2 geological storage, and breakthrough pressure serves as a key indicator for evaluating caprock sealing. The purpose of this review is to discuss the latest research progress in experimental testing and characterization models of caprock breakthrough pressure. First, this review provides a summary of the definitions and classifications of caprock sealing and breakthrough pressure. Comprehensive reviews of the measurement apparatuses, methods, influencing factors, characterization models, and caprock sealing thresholds related to breakthrough pressure are provided. In this article, we first review the measurement apparatuses, which include a static-state testing apparatus, triaxial-state testing apparatus, online computed tomography scanning apparatus, and micro/nanofluidic testing apparatus. Static-state and triaxial-state testing apparatuses are suitable for obtaining measurements of breakthrough pressure under in situ conditions. The step-by-step pressure method and residual pressure method are the most widely used measurement methods, but the results of the residual pressure method are 20% to 50% of those obtained by the step-by-step pressure method. We then found that the impact order of lithology on breakthrough pressure is gypsum or saltstone > mudstone or shale > limestone > argillaceous mudstone > muddy siltstone > igneous rock > sandstone, with a minimum threshold value of 2 MPa for caprock breakthrough pressure. For shale and gypsum, the breakthrough pressure of CO2 is 50% to 80% that of CH4 and 55% to 85% that of N2. The breakthrough pressure of rock saturated with water is 2.3 to 6.5 times that of rock saturated with oil and 8.2 to 31.1 times that of rock saturated with air. Moreover, we review classical theoretical models and experimental empirical models for characterizing breakthrough pressure. Empirical models are more accurate than theoretical models for characterizing the actual breakthrough pressure, especially models relating to breakthrough pressure and permeability, which have been widely applied. We finally conclude that the Tarim Basin, Junggar Basin, Ordos Basin, Songliao Basin, and central Sichuan Basin have high caprock sealing capacities. Future research trends include rapid and accurate measurements of breakthrough pressure, characterization and application of breakthrough pressure across multiple scales, and development of models and standards for evaluating caprock sealing capacity.