Role of Structural Trapping in Preventing CO2 Leakage Through Faulted Caprock in Illinois Basin
Authors: Bondarenko N. , Kim H. , Makhnenko R. , Podladchikov Y. 
Published: September 2023

ABSTRACT
Preliminary analysis of the risks associated with on-going geologic carbon storage projects has to be conducted in conditions of limited knowledge about the participating rock formations to assess the feasibility of the potential injection site. Different sources of geomechanical data (e.g., geophysical well logging and laboratory testing) are utilized to initialize numerical models for the preliminary assessment. Both data sources are associated with intrinsic limitations, increasing the uncertainty in the model prediction. In this work, one of the potential CO2 injection sites in the Illinois Basin is considered for the potential leakage through the local fault system in the vicinity of the injection well, with a special attention given to the comparison of the well-log and laboratory geomechanical data. The conducted analysis shows that while the use of geophysical well log is beneficial to make a preliminary assessment, it cannot capture complex sensitivity of material properties to state of stress, temperature, and pore fluid composition. An additional challenge during the preliminary assessment is the presence of formations which properties are unknown or highly uncertain (e.g., properties of the damage zone surrounding the small-scale fault system). It is demonstrated that the numerical model could be significantly simplified for the initial assessment, and consideration of the worst-case scenario can help to overcome the uncertainties in material properties. Preliminary risk assessment suggests that the risk of leakage through the adjacent fault system in the considered case is moderate due to the structural trapping of the injected CO2.

INTRODUCTION
Over the last few years, increased attention to climate change mitigation resulted in the active development of carbon capture and storage (CCS) facilities. Based on the global review of CCS status, 27 commercial facilities worldwide (with a total of 36.6 Mt of CO2 annual capacity) are operational and about 100 projects with 113 Mtpa capacity are going through different stages of construction and operation (Global CCS Institute, 2021). Even though these numbers correspond to less than 1% of the annual emission volume of CO2, which is close to 40 Gtpa (IPCC, 2022), the current stage of development is essential in establishing safe and effective approaches for further industrial-scale projects. A successful project is aimed at storing CO2 in the subsurface without risks of leakage and induced seismic response since both factors might damage the infrastructure and significantly affect the public perception of geologic carbon storage. The multistage pre-injection assessment of the risks through characterization of the involved formations and intensive monitoring program should guarantee safe operation. However, some of the essential information might not be available during the preliminary examination of risks. One of the possible approaches is to assume the worst-case scenario for the unknown or highly uncertain parameters. Low or moderate risks predicted by the simplified model based on the worst-case scenario indicate that assessment during further stages is unlikely to predict significantly higher risks.