Experimental and Numerical Study on the Phase Behavior and Distribution Characteristics of Oil-CO2-Water Three-Phase Fluid During Pre-CO2 Energized Fracturing in Shale Reservoirs

Abstract Analogous to the technique CO2 huff-n-puff, it has been determined that the preliminary injection of a predetermined volume of supercritical CO2, serving as a pre-fracturing fluid, holds significant promise in augmenting EOR and facilitate carbon storage from shale oil reservoirs when applied prior to hydraulic fracturing procedures. However, regardless of whether it's pre-CO2 energized fracturing or post-hydraulic fracturing CO2 huff-n-puff, the coexistence of oil-CO2-water in shale reservoirs leads to complex phase behavior and flow characteristics. To this end, this study firstly designs and conducts three-phasic experimental tests involving shale oil, CO2 and water. This is achieved by varying the order of CO2 and water injections into the shale oil to simulate both pre-CO2 energized fracturing and post-hydraulic fracturing CO2 huff-n-puff. Subsequently, based on the results of PVT experimental, further established a phase behavior calculation model of oil-CO2-water and used it to construct a numerical simulation model that takes into account the stress sensitivity of the SRV transformation zone, formulated the injection and production parameters of pre-CO2 energized fracturing and explored the fluid distribution mechanism at different stages. The equilibrium experiments and numerical simulations indicate that due to the dissolution of CO2 in water, the presence of the aqueous phase reduces the solubility of CO2 in oil, thereby diminishing the interaction intensity between CO2 and crude oil. The saturation pressure under the coexistence of oil-CO2-water three-phase behavior is lower than that of the oil-CO2 system. Furthermore, this phenomenon becomes more pronounced with the increase in water saturation. For pre-CO2 energized fracturing, compared with post-hydraulic fracturing CO2 huff-n-puff, the degree of dissolution of CO2 in water can be significantly reduced, which is conducive to the dissolution of CO2 in oil. Therefore, pre-injection of CO2 can maximize the use of CO2 and reduce unnecessary waste. Pre-CO2 energized fracturing can make CO2 spread more widely, effectively promote contact with oil to improve oil physical properties, and inhibit CO2 flowback ratio during the drainage process, which is conducive to carbon storage. Compared to the conventional CO2 huff-n-puff technique, pre-CO2 energized fracturing in horizontal wells can markedly ameliorate crude oil recovery. This research enhances our understanding of the oil-CO2-water phase behavior and fluid distribution with pre-CO2 energized fracturing in shale reservoirs, potentially offering insight for efficient shale oil reservoir development.