Imbibition is very common, occurring in life, material, chemistry, and energy. It plays an important role in enhanced oil recovery (EOR). The development of many reservoirs is beneficial to the imbibition process, such as fractured reservoirs, conventional reservoirs developed by a water-injection mode of huff-n-puff in their later development, and unconventional reservoirs with abundant micro–nanopores developed by the fracturing technology. Here, we present a critical review of EOR through imbibition. First, the mechanisms of EOR through imbibition are reviewed, including the mechanical analysis of imbibition in a capillary, imbibition models for rocks, and the scaling law. Then, the governing factors of EOR by imbibition are summarized, including the properties of rocks and fluids and the effects of the temperature and pressure. Besides, the EOR by imbibition in the oil and gas development is discussed, including the roles of surfactants, nanofluids, salinity, shut-in time, and injection/production rates. Finally, conclusions and outlooks are presented. This review provides systematic and recent insights about EOR by imbibition and a direction for future research on this topic, which can help for a better understanding of EOR by imbibition.
Wetting hysteresis is an important factor affecting the production of oil reservoirs, especially in the nanopores of shale, and it has an important practical significance to study the influence of wetting hysteresis on fluid flow in shale oil reservoirs. The threshold pressure model and flow resistance model after the movement of the droplet in two-phase fluid systems were established on the basis of the static wetting hysteresis equation and dynamic contact angle equation, respectively. Combining the proposed models and experimental data in the literature, the threshold pressure and flow resistance with the varying flow velocity were obtained for different hydrocarbons in pores with different sizes. The results show that the threshold pressure and flow resistance both increase with the decrease of the pore size, and they cannot be ignored in nanoscale pores of shale oil reservoirs. Moreover, the flow resistance caused by the dynamic wetting hysteresis is greater than the threshold pressure caused by the static wetting hysteresis, and the flow resistance increases with the increase of the flow velocity. This work provides mathematical models for accurately characterizing the threshold pressure and flow resistance of two-phase fluid systems in nanopores, which will pave a way for the accurate numerical simulation of shale oil reservoirs.
Imbibition is one of the most common physical phenomena in nature, and it plays an important role in enhanced oil recovery, hydrology, and environmental engineering. For the tight reservoirs, the imbibition method has an obvious advantage in fracturing, shut-in, and huff-puff development. Although the current imbibition studies focus on oil recovery, and the inertial effect in imbibition is neglected and its mechanism is also unclear. In this paper, the inertial effect on spontaneous oil-water imbibition at micron-scale is studied by molecular kinetic theory (MKT). The frictional coefficient in the model is a fitted parameter to match the experimental data during the total imbibition process. Then, the simulation of the initial imbibition stage is conducted and the inertial effect on imbibition is identified by the difference between the model considering the inertial effect (CI) and the model neglecting the inertial effect (NI), or by the proportion of inertial force to the total resistance. Results show that (i) with an increase in the water phase viscosity, the inertial effect time shortens, maximum imbibition height and rate decrease, and thus the inertial effect on imbibition weakens; (ii) with an increase in the oil phase viscosity, the inertial effect time changes little, the maximum imbibition height and rate decrease slightly, namely, the inertial effect depends slightly on the oil phase. (iii) with an increase in the capillary wettability (hydrophilicity), the inertial effect time shortens, the maximum imbibition rate first increases and then decreases, and the inertial effect on imbibition weakens. This work sheds light on the inertial effect on oil-water imbibition by MKT, considering the effects of dynamic contact angle, water phase viscosity, oil phase viscosity, and wettabilities, which is helpful to understand the role of inertia in the oil-water or oil-fracturing fluid imbibition process.
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