Abstract: The mechanisms of nonlinear seepage in low permeability reservoirs were analyzed based on the microseepage effect, and a mathematical model was constructed for these nonlinear seepages in low permeability reservoirs using the forcebalance equation and experimental data from mercury porosimetry experiments. This model was further used to analyze the characteristics of nonlinear seepage in low permeability reservoirs, and the concept of dynamic resistance gradients(DRGs) was introduced to enable the dynamic characterization of these nonlinear seepages. Based on the results of this study, the primary causes of nonlinear seepage are the additional resistances against low permeability seepage caused by the boundary layers, fluid yield stress, and surface forces between boundary layer fluids and bulk fluids. Nonlinear seepages are always present at all displacement pressure gradients, but the effects are more pronounced at low DPGs and weaker at high DPGs. It was also found that the characterization of nonlinear seepage became more realistic when the starting pressure gradient was replaced by the DRG. As the DPG increases, the dynamic displacement gradient initially undergoes an instantaneous decrease from a high initial value and then gradually increases. In addition, the ratio of the DRG to DPG initially increases with the DPG, but subsequently decreases with further increase in the DPG.

Key words: low permeability reservoir, nonlinear seepage, microscale effect, capillary bundle model, dynamic resistance gradient