A Study of the Mechanism of Oil Displacement Using Water and Polymer in Stratified Laboratory Core Systems

Summary Oil displacement experiments are reported which were performed in stratified rectilinear systems consisting of two lower permeability sandstone slabs, enclosing a central high permeability layer made of unconsolidated glass ballotini. The adjacent layers were in good flow communication, except for a very small region close to the outlet. This arrangement allows the crossflow mechanisms that occur when the viscosity of the displacing brine is increased by adding a water soluble polymer. These mechanisms cannot be assessed by performing experiments in one-dimensional cores or packs. Results of oil displacement experiments are presented for different mobility driving fluids, in which glycerol is used to viscosify the aqueous phase. carbon-I 4 and chlorine are used as radioactive tracers in the experiments. The effluent fluids from each layer were collected and analysed to produce data on the cumulative oil recovery, watercuts, flow rates in each layer, viscosities and concentrations of radioactive tracers. The results of these oil displacement experiments were modelled using computer simulation, and a very good match to the experiments was obtained. This simulation work confirms the flow mechanisms involved when water soluble polymers are used to increase the oil production from stratified reservoirs. Introduction In stratified reservoirs with high permeability contrasts between layers, early water breakthrough can occur during a waterflood, causing low vertical sweep efficiencies. The use of water soluble polymers to increase the water viscosity has been shown to be an effective way of improving the sweep efficiency and hence increasing the oil recovery. Earlier theoretical work has shown that the mechanism of increasing the recovery of oil in this way involves the crossflow of fluids between the various layers, as well as increasing the velocities in the lower permeability layers adjacent to the high viscosity polymer solution). A large number of papers have been published which investigate oil displacement using polymer solution in one-dimensional cores. Some experiments have also been performed in two-dimensional systems, but most of these used multiple radial or linear cores in parallel, with no contact between the cores except at the inlet. In the early literature, waterflooding experiments in communicating layered systems were reported, generally from the viewpoint of investigating scaling laws, or recovery mechanisms. More recently, polymer displacement experiments have been performed in stratified systems, with crossflow between layers. Whilst these have provided valuable information on the displacement processes, they have not provided sufficient data to allow a theoretical analysis of the flow mechanisms. In our laboratory, we are performing experiments which provide detailed information on the oil recovery, watercuts, flow rates and tracer concentrations produced from individual layers. The results of these experiments are being analysed using numerical simulation, in order to reproduce the flow mechanisms quantitatively. Earlier work performed at Winfrith used a heterogeneous core made from a sandstone cylinder with the centre removed and replaced with a pack of glass ballotini. This work has recently been extended using a rectilinear core assembly, and the results of these later studies are discussed in this paper. P. 823^