Impact indicators for caprock integrity and induced seismicity in CO2 geosequestration: insights from uncertainty analyses

The geological sequestration (geosequestration) of carbon dioxide (CO2) is a mitigation method for reducing greenhouse gas emission into the atmosphere. The security and safety of CO2 geosequestration are strongly dependent on the mechanical stability of the caprock overlying the reservoir. Underground injection of CO2 increases the pore pressure and thus decreases the effective stress. It may lead to caprock failure, as well as the subsequent leakage of sequestered CO2. In particular, geothermal exploitation and the underground disposal of hazardous liquid wastes have demonstrated a risk of induced seismicity. We performed an uncertainty analysis using a novel response surface methodology and a two-step statistical experimental design, evaluated the statistical significance of operator choices and subsurface uncertainties to caprock integrity, and quantified the moment magnitude of the induced seismicity. Furthermore, the optimal combination (i.e., the worst-case scenario) with the desired properties was forecast. A series of numerical experiments was well designed, and 130 combinations were statistically determined. Based on the results from the analysis of variance for the response surface quadratic model, the impact indicators were presented in histograms according to their significances to the Coulomb failure stress and moment magnitude of the induced seismicity. Lastly, the values of the selected independent impact indicators were predicted to obtain optimal compositions for object function of both Coulomb failure stress and moment magnitude, and the desired properties were being picked out. The optimal combinations had desirability values of 1.000, demonstrating the fitness of the selected statistical models in analyzing the experimental data.