Seismicity induced by geological CO2 storage: A review

Geological storage is a valuable strategy for reducing CO2 emissions to the atmosphere, although seismicity induced by CO2 injection can be a serious hazard that also becomes an obstacle to the development of CO2 geological storage. The most important challenge in this field is fault systems that are difficult to detect and that have complex activation mechanisms, making the evaluation, prediction and control of CO2 injection induced seismicity extremely difficult. It is also challenging to determine the triggering mechanism of injection induced seismicity. To promote the solution of these problems, we first review the experience and lessons learned from recent induced seismicity monitored in CO2 geological storage (CGS) projects, and summarize the mechanisms that can be used to analyze CO2 injection induced seismicity, including critical pressure theory, Biot's incre-mental strain theory, rate-and state-dependent frictional theory and fracture potential theory. We then discuss the theory and modeling of thermo-hydro-mechanical-chemical (THMC) coupling in CGS-induced seismicity. Knowledge of THMC coupling is an efficient way to improve the prediction of fault activation and seismic ac-tivity and enables characterization of pore pressure perturbation, temperature changes, and stress and geochemical effects. Through THMC simulation, we can more accurately characterize the change process of stress field, analyze and speculate the triggering and spatio-temporal evolution of induced earthquakes. We also summarize the advantages and disadvantages of maximum magnitude prediction based on statistical and physical models. The statistical method is easy to use, but ignores the physical characteristics of the reservoir. Although physical method overcomes this deficiency, obtaining sufficient modeling input parameters is always an challenging work. Finally, we analyze the challenges involved in the seismicity forecasting, including the quantification of stress state, the identification and characterization of complex fault system, the seismic miti-gation injection scheme design, and reasonable seismic risk analysis model selection. This paper focuses on the scientific function of THMC coupling in the studies of CO2 injection induced seismicity, so as to provide reference and guidance for the researches on the mechanism analysis and forecasting of such induced seismicity.