Reservoir Application of Mobility Control Foams in CO2 Floods
Authors: Heller J.P. 
Published: December 2007

Member SPE-AIME

Abstract
This paper discusses some of the important factors in the reservoir application of a "foam-like dispersion," an alternative to Water-Alternated-with-Gas (WAG), for retarding the formation of viscous fingers during CO2 floods. With a high volumetric concentration of dense CO2, the "foam" is able to approach the high microscopic displacement efficiency observed in a developed-miscibility, laboratory slim-tube test. At the same time, the mobility of this composite fluid is considerably less than that of pure CO2. With the lower mobility, of the same order of magnitude as the oil and excess water to be displaced, early breakthrough of displacing fluid and inefficient displacement can be avoided or at least reduced even in tertiary floods.
Economic considerations dictate that the injection rate must be maintained at a high level in any displacement program. Thus, there is often a trade-off between this rate and the degree of thickening of the CO2 The basic factors to he considered in this compromise, along with other practical matters relevant to reservoir use of CO2 practical matters relevant to reservoir use of CO2 foam, are discussed. Among these factors are the means of estimating the surfactant requirements from laboratory measurements of foam mobility and of the adsorption of surfactant on reservoir rock. A further question concerns the needed quantity of foamed CO2, or the "slug size" to be used in the displacement program. Methods of calculation for the above features of flood design are presented and discussed. In addition, the expected points of difference between such a "mobility controlled" and an "unprotected" CO2 flood are described. The methods are illustrated by reference to a hypothetical five-spot pattern displacement.
This paper is not intended to suggest that the discussions given are complete or in final form; they are offered as a basis for further discussion in the industry.

Introduction
From the pore-scale point of view, dense CO2 is an ideal displacement fluid for many crude oils, because it can generate with the oil a miscible transition zone able to achieve nearly complete displacement, at least in laboratory slim-tube experiments. Unfortunately even when pressure conditions for miscibility are met, this high microscopic sweep efficiency is not often approached in reservoir operations, due principally to the nonuniformity of the flow patterns.
For CO2 flooding this spatial variability of flow velocity is due not only to such constant geometric causes as permeability variations in the reservoir rock, and the flow pattern characteristic of the well distribution, but also to severe frontal instability. Because the CO2 has such low viscosity (about 0.04 cp at a typical density of 0.5 gm/cc, required for efficient displacement of a light oil), the displacement front is very susceptible to the growth of viscous fingers in which the injected CO2 bypasses the other reservoir fluids. In such -fingers- the CO2 moves faster than the oil and water exposed to the same pressure gradient by a factor equal to the mobility ratio. These fingers not only degrade the sweep efficiency, and cause early breakthrough and excess production of CO2, but also interfere, at least to production of CO2, but also interfere, at least to some extent, with the formation of the required Hutchinson-Braun type generated miscibility front. Such interference can occur by extending the domain 12 of mixing crude oil and CO2 that is unfortified with enough light hydrocarbons for direct miscibility, causing reprecipitation of "heavy ends" in formerly "cleaned out" regions of the rock.

Attempts to Improve Conformance - WAG
A commonly used procedure designed to remedy mobility control troubles is the -Water Alternated with Gas- (WAG) flood in which water is injected along with the solvent. This procedure was first developed more than two decades ago to alleviate the same problem when it was encountered in solvent and LPG floods.
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