The mechanism of the two-dimensional NMR response in depleted oil reservoirs during supercritical CO2 injection

Depleted oil reservoirs are an important site for geological CO2 sequestration. In depleted oil reservoirs, water and hydrocarbons may overlap in the transverse relaxation time (T2) distribution, and two-dimensional (2D) nuclear magnetic resonance (NMR) has a great advantage for pore fluid evaluation. However, the mechanism of the 2D NMR response has not been well explained for CO2-saturated rocks, which is not conducive to the development of an NMR-based method for CO2-flooded enhancing oil recovery potential evaluation and CO2 sequestration assessment and safety monitoring. In this study, we first simulate the 2D NMR diffusion-T2 responses (D-T2) of carbonate rocks, sand packs, and tight sandstones during supercritical CO2 injection using digital core technology. Based on the digital core models from micro-CT, we use mathematical morphology to construct the spatial distributions of the three-phase fluid of immiscible supercritical CO2, water, and oil at different saturations. We modify the random-walk method to simulate D-T2 acquisitions, based on the constructed digital core models, and analyze the corresponding D-T2 spectra of three types of rocks saturated with CO2. The results show that the T2 lifetime of water in the D-T2 spectra gradually shortens, the oil peak remains constant as the CO2 saturation increases and the oil and water saturations gradually decrease. The water peak in the D-T2 spectra gradually deviates from the water diffusion coefficient line as the water saturation decreases, indicating that the restricted diffusion of water is enhanced. One can determine water and oil contents from the D-T2 spectra of CO2-, water- and oil-saturated rocks. One can also calculate the CO2 content in the rock if the porosity of the rock is known. These simulations provide a theoretical basis for explaining the D-T2 response of CO2-saturated rock, which can be useful for evaluating CO2-flooded enhancing oil recovery potential in oil reservoirs and assessing CO2 sequestration and safety monitoring in depleted oil reservoirs.