Electrical resistivity monitoring of CO2 injection at the Mont Terri underground laboratory, Switzerland

Monitoring CO${}_2$ injection in the subsurface using geophysical methods, particularly in caprocks or hard rock, presents unique challenges. These challenges arise due to the lower porosity and permeability in hard rock settings, which result in limited and complex pathways for fluid movement. The Mont Terri underground rock laboratory in Switzerland provides the opportunity to evaluate different geophysical measurements in boreholes to monitor a CO${}_2$ injection near a known fault zone. The motivation of the experiment presented in this study, along with its continuation in the Carbon Sequestration series D and E (CS-D and CS-E) is to assess the integrity of a fault zone within a caprock-like formation (such as Opalinus Clay) during long-term leakage experiment. Time-lapse electrical resistivity tomography (ERT) measurements were conducted during the steady-state injection period. In this work we present an ERT study using synthetic data to predict the effects of both the conductive fault zone and the injected fluid mixture at a single time step, while also analyzing the temporal evolution of the synthetic study. The synthetic results show a similar apparent resistivity distribution to that observed in the field data. The analysis then progressed to real field data, where various electrode configurations were tested, requiring meticulous assessment of data quality during processing. This study highlights the importance of using appropriate error estimation techniques, such as a reciprocal error model, to characterize the spatial and temporal behavior of measurement errors across different configurations. Three-dimensional time-lapse inversion results play a crucial role in deciphering the fluid interaction between the injected CO${}_2$, the properties of the host rock, and the presence of the main fault zone within the experiment. Our findings indicate that the fracture network within the host rock is intricate, exhibiting changes in resistivity during injection around the main fault zone. These insights not only complement other findings within the CS-D and CS-E experiments, but also showcase the utility of ERT measurements in CO${}_2$ monitoring within other hard rock settings.