Simulation of Waterflood Behavior in a Reservoir Previously Invaded by Water
Authors: Weaver Richard H. 
Published: December 2007

A two-dimensional, three-phase simulation of a low-dip reservoir has been used successfully to depict waterflood performance. Two principal questions were considered in the simulation: What will be the ultimate effect of the gas invasion that occurred during the 19 years of primary recovery? How will oil recovery be affected by the wide variations in permeability? permeability?

Reservoir Description
The Borregos field is a complex of more than 90 oil, gas, and associated oil reservoirs located along the Vicksburg flexure in Kleberg County, Tex. One of the major oil columns is the associated R-5 (South) reservoir, which shares a gas cap with oil columns in other fields (Fig. 1). The R-5 (South) reservoir is characterized by clean sand near the top of the deposit and decreasing permeability along the structural strike from southwest to northeast. The reservoir is bounded on the west by a shale barrier and on the east by an undefined permeability barrier. A small, nonsealing fault crosses the reservoir in the north. The dip cross-section (Fig. 2) through the central portion of the reservoir illustrates the stringered portion of the reservoir illustrates the stringered nature of the reservoir. Measured permeabilities on cores vary from less than 1 md to more than 1,500 md, with the highest permeabilities in the upper stringer. The variation in sand quality from south to north is illustrated in Fig. 3, which shows the permeability distribution for three cored wells. permeability distribution for three cored wells.

Reservoir Performance
Discovered in 1949, the R-5 (South) reservoir was fully developed by 1954 when the five K.R.-Seeligson Ebanito wells were combined with 21 K.R.-Borregos wells. The reservoir rate, which varied between 1,000 and 2,000 B/D from 1953 to 1966, climbed to a maximum rate of about 9,500 B/D in late 1968, then dropped to about 3,000 B/D (Fig. 4). During early years reservoir GOR remained low because well allowables were transferred if GOR increased. Since 1967 wells have continued to be produced with increasing GOR's and the result has been an apparently abrupt change in GOR history. The reservoir pressure - which is influenced by production from other pressure - which is influenced by production from other fields and cycling operations as well as by withdrawals from the R-5 (South) - has dropped from 3,822 to 2,980 psi over this time. However, the reservoir still has a gradient above that of salt water. In 1968, a down-structure water injection program was started, with a maximum injection rate of 16,500 B/E) being reached by Jan., 1969. Because of the well control. the reservoir performance history, and the fact that the R-5 (South) is one of the larger oil columns in the Borregos field, a reservoir simulation of waterflood performance was undertaken.

Selection of Simulator Model
The selection of a model for simulating the behavior of the R-5 (South) was governed by the following considerations: 1. Waterflood had been started in late 1968 following 19 years of production by gas cap expansion. Thus, the presence of three mobile phases had to be considered and the effect that the gas saturation developed during primary depletion had upon ultimate residual saturations needed to be accounted for.
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