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 pay zones of mixed wettability (slightly more oil wet). On the average, after flooding cores with 2 ~ 2.5 PV (pore volumes) of water, the water cut practically reaches 100% and the average recovery is about 40% OOIP (original oil in place). However, results from pressure coring in the field show that sometimes the recovery of high permeability streaks of the pay zone can reach 80% ~ 90% OOIP, much higher than that obtained in the lab. Since Daqing is a very heterogeneous reservoir, the pore volumes of water that pass through high permeability streaks of the formation is much higher than the average. In order to see the increase in recovery and decrease in residual oil saturation of cores after water flooding by multi-pore volumes to check if the end point of the relative permeability curves that are drawn is the real residual oil saturation, multi pore volumes (some more than 30 thousand PV) water flooding on natural cores was performed in the lab. The results show that for mixed wettability (slightly more oil wet) cores of Daqing Oil Field’s pay zones, when using multi PV of water to flood the cores, and when the effluent water cut is very high (practically 100%), oil can still slowly flow in the oil film on the pore rock surface and accumulate to form mobile oil (many cores can obtain a final recovery of more than 90% OOIP, some as high as 99%), which means that the oil has not reached “real residual oil” saturation, it is only because the oil cut of the effluent fluid of the core is extremely low, therefore it is difficult to observe the oil in the produced fluid. This paper shows many relational curves of recovery versus multi PV of water injected; it also shows the changes in pore structure of the cores. These changes are similar to those seen in pressure coring in the field. The reasons why such high recovery can be obtained and the large difference in recovery of different cores is explained.