Measurements of electrical resistivity, NMR pore size and distribution, and x-ray CT-scan for performance evaluation of CO2 injection in carbonate rocks: A pilot study

Injection of carbon dioxide (CO2) in hydrocarbon rock formation is one of the proven technologies for both enhanced oil recovery and carbon sequestration in geological storage. Accurate knowledge of electrical properties of reservoir rock injected with CO2 is required for successful design and evaluation of a CO2 sequestration project. This depends on the interaction of the injected CO2 with oil and interstitial water contained in the pore spaces, as well as the interaction with the rock grain material, where, heterogeneity also plays a major role in CO2 storage. In this work, a laboratory study was conducted on rock samples extracted from a Jurassic age carbonate reservoir that is being considered for CO2 injection, for the dual purpose of enhanced oil recovery (EOR) and geological sequestration of CO2. Continuous online electrical resistivity was measured, at 90 °C and 3000 psi (confining stress), for three carbonate core samples during different core flooding schemes, including a saturation aging period of about four weeks. Nuclear magnetic resonance (NMR), relaxation time and spatial distribution, showed a significant change in the distribution of the smaller pores in the three samples after CO2 injection. The changes in the pores and rock matrix were also captured by scanning the core samples, after CO2 injection, in slices of 1 mm depth, with x-ray computerized tomography (CT scan). Results also showed that the 4-pole resistivity was stable over a range of frequency from (100-1000) Hz, while the 2-pole resistivity is only stable at frequency >800 Hz. We also estimated the saturation of water during the different displacement schemes and calculated oil recovery from both water-EOR and CO2-EOR. The results obtained in this work are used to evaluate feasibility of using electrical resistivity tomography (ERT) logs to monitor and evaluate the performance of CO2 injection in a carbonate rock formation.