CO2 Mass Transfer and Oil Replacement Capacity in Fractured Shale Oil Reservoirs: From Laboratory to Field

Abstract

CO2-based fracturing is widely introduced to stimulate shale oil reservoirs for its multiple advantages. However, the range of CO2 entering the matrix around fractures and CO2-oil replacement capacity between matrix and fractures cannot be fully explained. To address this issue, a radial constant volume diffusion experiment on shale cores was designed in this study, and the pressure drop curve history was matched through numerical model to determine the composition effective diffusion coefficient. A field-scale numerical model was established, in which a series of certain grids were used to explicitly characterize fracture and quantify the prosess of CO2 mass transfer and oil replacement. Based on the field-scale numerical model, the process of shut-in, flow back, and oil production was simulated. The distribution of CO2 in fractured shale oil formation and its impact on crude oil during shut-in stage and flow back stage were investigated. This study concludes that CO2 gradually exchanges the oil in matrix into fractures and improve the fluidity of oil in matrix until the component concentrations of the whole reservoir reaches equilibrium during the shut-in process. Finally, about 30∼35 mole % of CO2 in fractures exchanges for oil in matrix. The range of CO2 entering the matrix around fractures is only 1.5 m, and oil in matrix beyond this distance will not be affected by CO2. During the process of flow back and production, the CO2 in fracture flows back quickly, but the CO2 in matrix is keeping dissolved in oil and will not be quickly produced. It is conclued that the longest possible shut-in time is conducive to making full use of the CO2-EOR mechanism in fractured shale oil reservoirs. However, due to the pursuit of economic value, a shut-in time of 10 days is the more suitable choice. This work can provide a better understanding of CO2 mass transfer mechanism in fractured shale oil reservoirs. It also provides a reference for the evaluation of the shut-in time and production management after CO2 fracturing.

Mechanism and Main Control Factors of CO2 Huff and Puff to Enhance Oil Recovery in Chang 7 Shale Oil Reservoir of Ordos Basin

“…In the production stage, the crude oil in the large pore was also driven out first, and the crude oil in the meso-small pore flowed into the large pore under the effect of dissolved gas driving and expansion, and finally be driven out. The crude oil in micropores was subjected to weak diffusion, and a trace amount of crude oil was also driven out [37,38]. As can be seen in Figure 7b, the T 2 spectrum of the cores showed a double-peaked feature, but the left peak was higher than the right one, indicating that the macropore was poorly developed.…”

Dissolving Nonionic Surfactants in CO2 to Improve Oil Recovery in Unconventional Reservoirs via Wettability Alteration

“…One strategy for producing oil from already-fractured shale wells involves injecting oil-miscible gases such as CO 2 , ethane, propane, nitrogen, or natural gas to extract the remaining oil. − Of all the gases tested, CO 2 has demonstrated several advantages. CO 2 increases oil extraction by a variety of mechanisms including diffusion, vaporization, oil swelling, oil viscosity reduction, pressure support, CO 2 –oil interfacial tension (IFT) reduction, solution gas drive, and relative permeability hysteresis .…”