Abstract
Huff-n-puff by water has been conducted to enhance oil recovery after hydraulic fracturing in tight/shale oil reservoirs. However, the mechanisms and capacity are still unclear, which significantly limits the application of this technique. In order to figure out the mechanisms, the whole process of pressurizing, high-pressure soaking, and depressurizing was firstly discussed, and a mechanistic model was established. Subsequently, the simulation model was verified and employed to investigate the significances of high-pressure soaking, the contributions of different mechanisms, and the sensitivity analysis in different scenarios. The results show that high-pressure soaking plays an essential role in oil production by both imbibition and elasticity after hydraulic fracturing. The contribution of imbibition increases as the increase in bottom hole pressure (BHP), interfacial tension, and specific surface area, but slightly decreases as the oil viscosity increases. In addition, it first decreases and then slightly increases with the increase in matrix permeability. The optimal soaking time is linear with the increases of both oil viscosity and BHP and logarithmically declines with the increase in matrix permeability and specific surface area. Moreover, it shows a rising tendency as the interficial tension (IFT) increases. Overall, a general model was achieved to calculate the optimal soaking time.
Highlights
Tight/shale oil resource extensively distributes around the world
In order to clearly understand the significances of high-pressure soaking, the contributions of different mechanisms, the optimal soaking times, and the huff-n-puff performances, an ideal model based on the average values in J Oilfield is employed to conduct simulations
The maximum bottom hole pressure (BHP) is set as 70 MPa for injecting water, and the BHP is set as 15 MPa for production
Summary
Tight/shale oil resource extensively distributes around the world. It is regarded as a promising resource to provide fossil energy in the future. Water Oil Fluid flow direction seen from the above analysis, both imbibition and elastic energy have non-negligible contributions to oil production, so the capillary force, specific gravity, and compressibility of rock and fluids should be included. Where Swc is the connate water saturation; Sor is the residual oil saturation; a = 1 − Sor − Swc and b = Swc. Generally, some of tight oil reservoirs are water-wet, so imbibition can happen even if there is no surfactant in the fracturing fluid. In some mixed-wet or oil-wet reservoirs, surfactant is usually added into water or the fracturing fluid to promote imbibition by changing the wettability (Begum et al 2017; Alvarez et al 2017; Meng et al 2018; Huang et al 2020). Because a and b are the functions of residual oil and connate water saturation, the impact of surfactant on relative permeability is included
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