Tight oil reservoirs generally have no natural productivity due to the influence of micro/nano pore structure. The horizontal well volume fracturing technique is widely used to achieve effective development of this type of reservoirs. The remaining oil in matrix pores is mainly produced by spontaneous imbibition in the actual soaking and flowback process. Due to the great limitation of observation scale, the traditional experimental testing methods cannot accurately characterize the distribution and production of remaining oil at nano-scale in tight oil reservoirs, the microscopic mechanism of spontaneous imbibition has not been clearly explained.

In order to resolve those issues, a two-dimensional geometric model is firstly established based on the raw images of typical tight rock sample obtained by a highresolution CT imaging device. A novel pore-scale mathematical model of spontaneous imbibition in tight porous media is developed from the Navier-Stokes equations, which is numerically solved by using the phase-field method. Compared with the analytical results of the single-pipe imbibition model, the reliability of the proposed method was fully validated. Based on the analysis of the spontaneous imbibition microscopic dynamic phenomena, the influences of wettability, oil-water interfacial tension, oil-water viscosity ratio and micro-fracture morphology on the remaining oil in micro/nano pores and spontaneous imbibition recovery are further explored.

Results show that, the results obtained by the phase-field modeling of spontaneous imbibition are in good agreement with the analytical solution of the single-pipe imbibition model, and the front evolution process of oil-water interface during spontaneous imbibition is accurately described. The pore-scale dynamic events of oil droplets, snap-off, cross flow and coalescence are well characterized. The oil/water interfacial tension has little effect on spontaneous imbibition recovery, but significantly affects the imbibition time. The smaller the oil/water interfacial tension is, the longer the imbibition time is. Wettability and oil-water viscosity ratio can exert a significant effect on imbibition recovery. The existence of micro-fractures increases the imbibition contact area, improves the fluid flow capacity, and significantly improves the degree of remaining oil production and imbibition recovery.

A novel methodology to study two-phase fluid flow during spontaneous imbibition in tight porous media is proposed using the phase-field method in this work, which can provide a theoretical basis for understanding the underlying fluid flow dynamics during fracturing, soaking and flowback process of tight oil reservoirs.

You can access this article if you purchase or spend a download.