Mechanisms And Monitoring Of CO₂ Leakage During EOR And Geological Storage Processes

CO₂ capture and storage（CCS）technique,as an effective method to reduce greenhouse gas emission,has been recognized worldwide,with a lot of CCS demonstration projects being currently conducted.One of the greatest concern in CCS is its reliability and safety,which means whether or not the storaged CO₂ will leak out of the reservoirs.For evaluating this,it is of great importance to clarify the trapping mechanisms of storage bodies,analyze potential leakage pathways,mechanisms and intensity.Moreover,effective measures are needed to monitor leakage processes.In this paper,the trapping and sequestration mechanisms of CO₂ in various geological structures are described,and the potential pathway for CO₂ leakage and the related control factors on leakage risks are analyzed and assessed based on case studies and review of the literature.Potential CO₂ leakage pathways include badly sealed injection wells or abandoned wells,fractures or fissures existing in storage bodies and reservoir caps.A fault tree analysis（FTA）model of CO₂ leakage in CO₂ storage is constructed by analyzing the trap mechanisms of CO₂ storage formations and potential leakage pathways,based on which major causes of CO₂ leakage are explored.The minimal cutset method（MCM）is applied to calculate the relative contributions of different causes to CO₂ leakage.Mathematical models are established for various CO₂ leakage pathways to investigate CO₂ leakage characteristics of different leakage pathways with typical parameters.Based on numerical simulation methods,a geological model of the target block is constructed to explore the impacts of CO₂ injection on formation pressure and gas saturation,which is beneficial to safety evaluation of CO₂2R storage.Meanwhile,conceptual models of potential leakage pathways are established to the influences of various CO₂ injection pressures and rates on CO₂ migration and distribution characteristics in the formation.Surface monitoring methods have been proposed based on CO₂ migration principles and distribution characteristics,which is applied in target blocks to optimize the monitoring scheme in areas with abnormal monitoring results.The monitoring results indicate that CO₂ contents distribution in soils is well consistent with the calculated results of the constructed mathematical model.This theory can be used for prevention and monitoring leakage of the storaged CO₂.