Residual-Oil-Induced Injectivity Impairment During PWRI

Abstract

Injection of water with oily particles happens during waterflooding by re-injection of produced water (PWRI). The injected oily particles are captured by porous rock causing decrease of permeability and consequent injectivity decline. Maintenance of the injection rate results in increase of pressure gradient near to well, snap-off the residual oil and additional formation damage due to released drops retention. We study effects of residual oil mobilization in well vicinity due to pressure gradient increase and, consequently, the increase of capillary number. The mobilized oil ganglia perform snap-off, move together with the injected water; they are captured by rock causing additional formation damage. The system of governing equations includes mass balances for oil, for particles and equation for desaturation curve. The analytical model derived includes expressions for suspended and deposited particle concentrations, for velocities of concentration fronts and for injectivity decline. The main result is an additional formation damage induced by residual oil. Comparison of the injectivity decline models with and without residual oil allows estimate the efficiency of the nearwell area treatment by solvent in order to remove residual oil and to reduce injectivity decline. Another important application of the model is determination of residual oil with PWRI from routine coreflood tests on relative phase permeabilities. If one injects produced water, the formation damage caused by the capture of injected particles and release of excessive residual oil fraction must be taken into account. Copyright 2007, Society of Petroleum Engineers.