The Influence of Multi-Pore Volume Water Flooding on Pore Structure and Recovery of Lacustrine Deposit Mixed Wettability Cores

摘要

Daqing Oil Field is a reservoir with lacustrine deposit payrnzones of mixed wettability (slightly more oil wet). On thernaverage, after flooding cores with 2 ~ 2.5 PV (pore volumes)rnof water, the water cut practically reaches 100% and thernaverage recovery is about 40% OOIP (original oil in place).rnHowever, results from pressure coring in the field show thatrnsometimes the recovery of high permeability streaks of the payrnzone can reach 80% ~ 90% OOIP, much higher than thatrnobtained in the lab.rnSince Daqing is a very heterogeneous reservoir, the porernvolumes of water that pass through high permeability streaksrnof the formation is much higher than the average. In order tornsee the increase in recovery and decrease in residual oilrnsaturation of cores after water flooding by multi-pore volumesrnto check if the end point of the relative permeability curvesrnthat are drawn is the real residual oil saturation, multi porernvolumes (some more than 30 thousand PV) water flooding onrnnatural cores was performed in the lab. The results show thatrnfor mixed wettability (slightly more oil wet) cores of DaqingrnOil Field’s pay zones, when using multi PV of water to floodrnthe cores, and when the effluent water cut is very highrn(practically 100%), oil can still slowly flow in the oil film onrnthe pore rock surface and accumulate to form mobile oilrn(many cores can obtain a final recovery of more than 90%rnOOIP, some as high as 99%), which means that the oil has notrnreached “real residual oil” saturation, it is only because the oilrncut of the effluent fluid of the core is extremely low, thereforernit is difficult to observe the oil in the produced fluid. Thisrnpaper shows many relational curves of recovery versus multirnPV of water injected; it also shows the changes in porernstructure of the cores. These changes are similar to those seenrnin pressure coring in the field. The reasons why such highrnrecovery can be obtained and the large difference in recoveryrnof different cores is explained.