Laboratory Measurements on Changes in Carbonate Rock Properties due to CO2-Saturated Water Injection

Abstract

Abstract Carbon dioxide enhanced oil recovery (CO2-EOR) is gaining attention as a technique for reducing environmental impact and enhancing crude oil recovery. The injected CO2 reaches supercritical states at certain temperature and pressure conditions to be miscible with crude oil. Then, miscible CO2-oil mixture has lower viscosity and higher mobility than oil alone, leading to increased oil recovery rate. However, carbonate minerals are known to dissolve when CO2 is injected into reservoirs; therefore, it is important to evaluate how properties of rocks in a carbonate rock reservoir are affected by CO2-EOR. In this study, CO2-rich water was injected on carbonate rock samples mainly collected from Middle Eastern oil field and laboratory measurements including SEM image, liquid chromatography analysis, NMR T2 relaxation time and elastic wave velocity measurement, were conducted. The dissolution states of carbonate minerals were investigated by SEM images. Furthermore, liquid chromatography analysis was applied to CO2-rich water flooded through the samples to detect the amount of dissolved carbonate minerals. The porosity increase estimated from the chromatography analysis could be 0.6% or higher at 100 pore volumes (PV) injection. On the other hands, it was observed that samples became shorter as injection increased. The sample shortening may be caused by mechanical compaction associated with pressure loading test. The corresponding porosity reduction due to the sample shortening was estimated; magnitude of the average value of four sample is comparable to porosity increase due to chemical dissolution. Elastic wave velocity measurements showed P-wave velocity decreased in all the samples, as the amount of CO2-rich water increased; P-wave velocity increase at 100 PV was 2 - 10 %. S-wave velocity decrease was also observed to be 2 - 4% in the most of samples. Moreover, intensity reduction of small size pore was observed by NMR T2 relaxation time analysis. Thus, it was confirmed that effects of CO2-rich water injection on reservoir rock properties cannot be negligible.