The geological sequestration of carbon dioxide (CO2) to reduce greenhouse gas emissions into the atmosphere faces difficulties related to non-homogeneous underground conditions, poorly characterized interconnected geo-systems, and complex hydro-chemo-mechanical effects that involve the reservoir rock and cap-rock mineralogy, the saturating fluid, and the injected fluid. Given these uncertainties, extensive monitoring of CO2 injection projects is required. We developed a unique laboratory facility for the observation of subsurface CO2 leakage evolution. A thin transparent tank is filled with different sizes of glass-beads to form controlled layered stratigraphies; then the medium is saturated with water mixed with a universal pH indicator. The flow-controlled injection of CO2 is carefully controlled using pressure transducers with precise needle valve and, time-lapse photography permits capturing the evolution of gas invasion and diffusion. Results show the nature of CO2 gas migration in the near surface, the effect of fine-grained layers such as the cap-rock, water acidification near conduits and subsequent diffusion, the convection of carbonated water. In addition to this trial to understand salient characteristics on subsurface CO2 leakage, applicability of borehole based resistivity tomography is assessed. The measurement system for resistivity tomography is attached to the CO2 gas migration monitoring system. An adequate inversion scheme is proposed based on 3-D forward modeling. The array types are dipole-dipole and modified pole-dipole, which were specially designed for this lab-scale test. A mixture of in-line and cross surveys is employed. Produced resistivity images are compared with time-lapse digital images taken during CO2 gas leakage simulation. The visible feasibility check for the borehole resistivity tomography in detecting subsurface CO2 leakage is expected.
, J. C.