Waterflood Prediction Methods Compared to Pilot Performance in Carbonate Reservoirs
Authors: Abernathy Bobby F. 
Published: December 2007

The waterflood performance of three carbonate reservoirs was predicted by a calculation procedure employing the Craig, et al., concept, as developed from laboratory model flow tests, modified to include a layering technique similar to that proposed by Stiles. The predicted performance was compared with actual performance and with predictions made utilizing the Stiles method, by the Craig, et al, method treating the reservoir as a single layer, and an additional comparison was made in one reservoir with the Band method proposed by Hendrickson (after Habermann). The Craig-Stiles method was found to closely approximate the water-cut vs recovery performance of these three carbonate (dolomite) reservoirs, and is generally superior to the Stiles method. It also appears to be somewhat better than the Band method in the instance considered herein.

Introduction
Prediction of the performance of water floods is subject to many qualifications. A number of prediction methods ranging from strictly empirical estimates to complex mathematical calculations and electrical model simulators have been proposed. Data comparing actual performance with calculated results for carbonate reservoirs have been submitted on only one or two calculation techniques; consequently, the practicing reservoir engineer has a minimum of actual case histories to assist his selection of the proper technique.As a great number of water floods are being placed in operation in West Texas carbonate reservoirs, this paper should assist the engineers responsible for predicting their performance. It is also hoped that this paper will stimulate interest in presenting additional comparative data on other calculation techniques.In this paper, actual performance of pilot water floods in three carbonate reservoirs is compared to performance calculated by two or more prediction methods.

Calculation Methods

Craig-Stiles Method
The basis of this calculation method was first offered by Craig, et al. These calculations incorporate the concept that areal sweep after breakthrough (defined as pattern sweep by some authors) increases with water throughput. Laboratory studies' have shown that it is possible for areal sweep to reach 100 per cent in some cases. The calculation method also incorporates the frontal advance concept as advanced by Welge, and the concept of stratification as advanced by Stiles.In simple terms, the basis of the Craig calculation can be expressed as follows:
In order to incorporate the effect of stratification, the excellent layering concept offered by Stiles' was utilized. This method assumes that permeability values taken from core analysis represent discrete layers within the reservoirs. The net effect of the layering concept is to apply a form of vertical sweep efficiency. In this adaptation of the Stiles method, the layers can have different thicknesses, permeabilities, relative permeabilities, oil, water and gas saturations, and porosities. The layering technique offered by Miller and Lents constitutes a useful refinement over the Stiles technique if sufficient core analysis data are available. In this method the permeability associated with each layer is obtained by averaging the permeabilities of cores taken at all wells in the same field, at the same relative position in the formation. This method probably gives a better value of vertical coverage since irregularities peculiar to individual wells or core samples may be balanced out. Once the layers have been established, the calculations would proceed in the same manner as with the Stiles layering.
JPT
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