This paper deals with the development of an analytical tool to predict recovery under both waterflood and miscible flood environments. The recovery predictions are based on correlations in the literature derived as a function of mobility ratio. These correlations are applied on a pattern by pattern basis for the entire reservoir. Extensions are developed to handle the impacts on recovery resulting from the stratified multiple layer character of a reservoir in addition to the areal discontinuities.
Recovery prediction methods for five spot patterns have been extensively analyzed in the literature. In general. they tend to be complex and tedious to apply. E.L. Claridge (1) has developed a correlation to model laboratory miscible flood data Which simplifies these calculations. The enhancements proposed in this paper are extensions of the original formulation that deal with predictions of both miscible and waterflood performance while taking into account the geological complexities specific to each reservoir.
The sweepout correlation developed by Claridge is comprised of three components dealing with specific aspects of a flood. He found that breakthrough of the displacing phase is a function of mobility ratio and can be represented by the following:
Equation (1) (Available in full paper)
The second component of the correlation was actually developed by Koval (2), who proposed that for miscible floods the effects of fluid mixing needs to be considered. He postulated that this mixing behaviour resulted in an effective viscosity ratio represented by: Equation (3) (Available in full paper)
Koval included a term (H) to allow for the ability to account for reservoir hetrogeneities. In practise, it was found by the authors to be more practical to account for this factor by a different approach which will be discussed later in the paper.
Combining these two correlating equations with the laboratory data resulted in the following relationship between breakthrough recovery, effective viscosity ratio, poor volumes of injected fluid and ail recovery: Equation (4) (Available in full paper)
Equation (5) (Available in full paper)
Equation (6) (Available in full paper)
These equations are capable of determining recovery as a function of both mobility ratio and injected hydrocarbon pore volumes, but apply only to a single layer homogeneous reservoir.
The need exists then to extend this approach to account for the geological complexities of the reservoir specifically in the areas of permeability distribution and reservoir continuity. Also, these equations are developed for a single fluid displacement from initial conditions to final depletion. However, in determining performance under a miscible flood environment, modifications were required to account for a pre-existent secondary waterflood. These modifications are discussed in the next section.
In a complex reservoir situation, two parameters and greatly influence the recovery estimates. These are the permeability distribution and reservoir continuity. The permeability distribution effects are applied by considering the reservoir as being comprised of a number of layers, each with a specific permeability, porosity and thickness characteristic.