Abstract

Oil recovery from depleted light oil reservoirs (after a waterflood, with overone-half of the oil still in place) requires a large increase in the drivingfluid capillary number and a decrease in its mobility. These often conflictingrequirements are approached in a number of oil recovery methods, three of which- micellar, surfactant and caustic flooding techniques - are discussed in thispaper, in the light of the authors' laboratory experiments. Under optimalconditions, a 5% pore volume slug of micellar solution can recover as much as90% of the residual oil left after a waterflood. Surfactant floods are aboutone-half as effective, and caustic floods tend to be still less effective. Theproblem of scale-up to field conditions has to be faced also.

The paper considers the three processes and the governing capillary number and mobility ratio variations. Related processes arementioned also. Each process involves complex interactions leading to changingdisplacement regimes.

Experimental data obtained on consolidated sandstone cores is discussed, presented in a form where the essential features of the three processes can becompared.

Two Alberta crude oils were tested in such cores under similar conditions. Oilrecovery, chemical consumption and process efficiency were evaluated for eachcase.

Introduction

In most light oil reservoirs, over one-half of the oil originally in placeremains unrecovered even after a waterflood. Some of this oil may be recoveredby an appropriate enhanced oil recovery (EOR) method, such as chemicalflooding, miscible displacement, or carbon dioxide flooding. In rarecircumstances, even thermal methods may be applicable, although they are bettersuited for heavy oils. Chemical methods have great promise for the future inthe context of light oils. This paper looks at two such methods.

Chemical EOR methods have been the subject of many laboratory studies and fieldtests and include such methods as polymer, surfactant, micellar, emulsion, andalkaline looding techniques and their combinations. Although many field testshave been carried out1, chemical floods have not performed as wellin the field as in the laboratory, partly because the experiments are usuallyunscaled. Scaling criteria for chemical floods have been obtained2, but are difficult to satisfy; consequently, laboratory results. such as oilrecovery vs. pore volumes injected, are not directly applicable to fieldsituations. The extent of reliability of laboratory data for field use dependson the process under consideration. Our experience shows that laboratoryresults for micellar flooding are more indicative of field performance thanthose for caustic or surfactant floods, where the injected chemicals are lostto the rock and fluids in many ways, and are the principal cause of the lack ofsuccess in the field.

Displacement Efficiency

The microscopic efficiency of oil displacement within the pores of a rockdepends on mobility ratio and the capillary number which vary with time at anypoint during a chemical flood. Other factors may be present also, such as phasetransitions.

Mobility ratio, M, is usually defined as the mobility ?ing (=k/ µ, where k is effective permeability and µ is viscosity) of the displacing fluiddivided by the mobility ?ed of the displaced fluid (assumed to beoil in this discussion).

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