Microscopic mechanism of water flooding in tight reservoirs

Based on cores from tight oil reservoirs in Ordos Basin, water flooding experiments with both low and high displacement pressures were carried out. Combined with NMR, quantitative analysis approaches for produced oil under different microscopic effects were established for quantitative research of the microscopic mechanism of water flooding in tight reservoirs. The research indicated that under low displacement pressure, oil recovery mechanisms of hydrophilic cores mainly include displacement, imbibition, and denudation, and those of wetting cores mainly include displacement and imbibition. After increasing the displacement pressure, both hydrophilic and neutral wetting cores have a certain increase in oil recovery. The common point is that both oil controlled by small throats and remaining oil droplets controlled by traps in large pores have been activated, while the discrepancy is that the oil film of the neutral wetting core boundary layer becomes thinner, which improves oil recovery ratio, and these mechanisms have less effect on hydrophilic cores. The amount of oil produced by each type of oil recovery mode was quantitatively analyzed. Percentages of produced oil in hydrophilic cores by flooding and imbibition and denudation are 15% and 12%, respectively, which are the main oil recovery mechanisms; percentages of produced oil in neutral wetting cores by displacement and imbibition are 25% and 2%, respectively, with displacement as the main oil recovery mechanism. After increasing the displacement pressure, oil produced by hydrophilic and neutral wetting cores increased by 6% and 9%, respectively, indicating that with increasing the displacement pressure, a part of the boundary layer of oil could be produced in neutral wetting cores.