Reservoir heterogeneities are known to influence performance of infill wells in water flood or miscible flood projects to the extent of determining their success or failure, but no satisfactory ways currently exist for their characterization, quantification or prediction. In 1950, Dykstra and Parsons1 presented empirical correlations for performance of water floods in certain California reservoirs, based on observed variation in permeability.
In general, procedures for most reservoir engineering predictions could be greatly simplified if heterogeneity could be characterized by one or more terms. However, it is painfully obvious that because of the multitude of reservoir descriptions encountered in any oil prone basin, a single parameter (or a small number of parameters) may not be adequate. This is because of different ways in which heterogeneity impacts the performance of vertical or horizontal infill wells placed to enhance production/ reserves in a variety of water flood/ miscible flood situations.
In recent years, a large number of infill wells were placed in various water flood and miscible flood projects in the Western Canadian Sedimentary Basin (WCSB). Performance of both horizontal and vertical infill wells varied widely in terms of initial oil rates and reserves. They also resulted in mixed economic successes in that many of these infill wells were unable to payout. In general, horizontal infill wells drained about twice the amount of oil as contemporaneous vertical infill wells placed in the same reservoirs. However, in spite of this, they may sometimes be less attractive on a risk-weighted basis, because of their higher cost.
It is postulated that reservoirs are divided into predominantly horizontal or vertical 'compartments' due to spatial variations in reservoir attributes such as permeability, thickness, environment of deposition (stratification, shaliness) and post-depositional changes (structure, fractures, digenesis). Oil contained in these compartments, though often not strictly isolated, is not easily contacted or displaced by injected water/ gas. Alternately, flow within the reservoir might be predominantly through certain pathways, some of which might involve cross-flow between various intervals. Thus horizontal wells will do a better job of draining oil than vertical wells in certain situations and, vice-versa. However, in reservoirs with certain kinds of heterogeneity, distribution of mobile water and/or gas in an ongoing water flood or miscible flood project might make horizontal wells more vulnerable to prematurely watering/ gassing out and hence, more risky.
This paper presents a few case studies of infill wells in water flood and miscible flood projects in WCSB. Comparative performance is presented for infill horizontal and vertical wells placed in these projects during the past fifteen years. Through a review of statistical data, certain observations are made regarding heterogeneity and its relation to performance of horizontal wells. Finally, inferences are generalized to obtain clues to situations where horizontal or vertical infill wells may be more appropriate.
In water flood or EOR recovery projects, recovery at any point in time is a product of displacement efficiency, conformance (vertical sweep efficiency) and areal sweep efficiency. Whereas conformance could be estimated using Dykstra-Parson's procedures1, sweep efficiency largely depends on areal heterogeneity in different intervals, besides factors such as mobility ratio, injected fluid throughput and flood pattern geometry2