Enhanced oil recovery formulations for liquid-rich shale reservoirs

This study aims to evaluate in situ CO2 generation and its enhanced oil recovery (EOR) potential in liquid-rich low-permeability shale reservoirs. The process involves injecting the CO2-generating compound, aqueous solutions of urea, into the tight formation where it hydrolyzes to generate adequate amounts of CO2 under reservoir conditions. Additionally, the synergistic effects of coupling urea with a thermally stable anionic surfactant to further improve oil recovery performance from low-permeability shale formations was studied.The experimental procedures were designed to simulate the huff-n-puff operation for oil recovery. Imbibition tests were carried out with oil-saturated Woodford outcrop shale cores for 3 and 14 days soaking periods. To assess recovery performance and mechanisms, tests were conducted with four different formulations: brine only, urea in brine, thermostable anionic surfactant in brine, and a blend of urea and surfactant in brine. Surfactant stability at test temperature was investigated. Furthermore, interfacial tension (IFT) and wettability alteration tests were conducted to understand their effect on total recovery.Results revealed that the selected EOR formulations are stable at reservoir conditions and compatible with the crude oil sample. There is a distinct difference in the oil recoveries for the 3-days and 14-days soaking periods in the case of the ternary brine/urea/surfactant mixture. This might indicate that the oil recovery processes in the shale cores are not only controlled by multiple mechanisms, such as wettability alteration, IFT reduction, fluid imbibition and CO2 diffusion, but are also time-dependent. Oil recovery in the case of the ternary mixture was 4 % and 18 % of the original oil in place (OOIP) after soaking periods of 3 days and 14 days, respectively. Combining urea with surfactant in the formulation showed a favorable synergistic effect in releasing oil from the shale core samples, leading to higher oil recovery after a 14-day soaking period. The oil recovered in the case of 14-days soaking time for the brine only, binary brine/urea, binary brine/surfactant, and ternary urea/surfactant/brine mixture was 7 %, 9 %, 5 %, and 18 % of the OOIP, respectively. Furthermore, both IFT reduction and wettability alteration play critical roles in improving oil recovery.Prior studies have primarily addressed the application of in situ CO2 generation for EOR (ICE) in high-permeability sandstone and carbonate reservoirs. The findings of this work suggest that ICE can also be effective in low-permeability liquid-rich shale reservoirs, thus expanding the potential application of this EOR technique.