Impact of Parameters\u27 Time Variation on Waterflooding Reservoir Performance

Currently, most oil reservoirs are in high water cut stage due to long term waterflooding, especially for those water-drive naturally fractured formations. Long term waterflooding changes the microscopic pore structure, rock skeleton, and clay minerals of these reservoirs through complex physical and chemical reactions during the process, and causes reservoir and fluid parameters changing over time. Our experimental data show that rock permeability, wettability, and oil viscosity are time-variant parameters during waterflooding, and they are closely related to the continuous water flow through the porous media, which is important to characterize water-drive reservoirs and understand fluid flow in them.In this paper, experimental statistic data from cores of Shengli oilfield, China were utilized. Firstly, the relationship between permeability/wettability/oil viscosity and the ratio of flow-through water volume to pore volume were presented. Secondly, the ratio of grid flow-through water volume to pore volume, denoted as Rwpv, was used to describe the parameter time-variation mechanism. Finally, a numerical simulation software package was developed by introducing Rwpv into the traditional black oil model to characterize the time-variation mechanism in water-drive reservoirs. The new simulator was verified through comparing with commercial reservoir simulator ECLIPSE and experimental data. Simulation results show that the time-variation of permeability and oil viscosity decreases the ultimate recovery compared to the cases without considering time-variation effects; and wettability time-variation makes the reservoir rock more water-wet, which leads to higher ultimate recovery. This research found that both time-variation effects affect the ultimate oil recovery, but in different ways; and time-variation impact becomes increasingly significant as the waterflooding process becomes longer. Both the experimental results and numerical simulations significantly improve the interpretation of the parameters\u27 time variation mechanisms and effect on waterflooding