Subsurface CCUS Monitoring & Surveillance: Insights on CO2 Resistivity Measurements

Abstract

The topic of carbon dioxide (CO2) enhanced oil recovery (EOR) has received increased attention since five decades for two main applications: Improvement of oil recovery in reservoirs, using miscible CO2 injection as tertiary oil recovery technique. Carbon dioxide can be injected into the reservoir using different injection strategies: injected by itself, simultaneously injected with water or as water-alternating-gas (WAG) mode. CO2 can be found in different phase states: liquid, gas or supercritical, depending on the reservoir conditions (pressure/temperature). CO2 sequestration (geologic storage) as a way to reduce CO2 emission. The realization of the CO2 sequestration into reservoir requires that long term stability of the reservoir seal is ensured. It is considered that measuring electric resistivity is useful to monitor CO2 migration in the reservoir. Resistivity shows a high sensitivity to fluids saturation in reservoirs. Therefore, it is considered that deep electromagnetic technologies (e.g., crosswell EM) can also be useful as a surveillance method in case of CO2 injection. The variation of resistivity due to CO2 injection into a carbonate reservoir is not thoroughly studied, especially in a mixed salinity environment. Thus, the study presented in this paper provides a better understanding of the resisitivity responses in a mixed-salinity carbonate cores during drainange, imbibtion and CO2 injection processes, which may aid in CO2 montiroing. The study addresses the following objectives: Conduct the flooding tests on one carbonate core plug, varying the brine salinity. Conduct the CO2 injection at reservoir conditions. Measure resistivity of cores at different injection rates of CO2 into a carbonate plug, already partially saturated with brine and oil. Monitor a change in the overall resistivity of the rock while CO2 is being injected at different rates and also at a constant rate. Investigate the frequency effect on resistivity response while injecting fluids.