Acyclic pore model and its applications in imbibition efficiency calculation

Abstract The classical pore structure conceptual model, such as a bundle of capillaries and lattice model is challenged for applicability in unconventional reservoirs, due to the micro/nano pore scale in unconventional reservoirs, with most of them below 100nm. Meanwhile, it is important to obtain an accurate evaluation of imbibition efficiency in unconventional reservoirs since imbibition is the main method for sweeping oil in matrix. This paper improves the acyclic pore model which is more practical for unconventional reservoirs. An imbibition efficiency calculation method is proposed, the effects of bifurcation number, pore throat radii corresponding to a mercury saturation of 50% (r50) and wettability is studied. The mechanism about how the pore structure affects imbibition performance is revealed. Results show that the increment of bifurcation number can decelerate the imbibition time in the branch level with smaller diameter, enlarges the imbibition accessible pore size as well. As r50 and hydrophilic saturation increase, wetting fluid volume imbibed and driving force for imbibition is also enhanced, which results in an improvement of imbibition efficiency.