Effects on Heterogeneity on Waterflooding in a San Andres-Grayburg Carbonate Reservoir, West Texas

Graham E. Fogg

Simulation of waterflooding in Section 15 of the Dune field, a San Andres-Grayburg reservoir in west Texas, illustrates the effect of heterogeneity on sweep efficiency. Two cross-sectional models with vertical and horizontal grid spacings of 1.2 and 30.48 m represent detailed permeability patterns that were mapped using log and core data from wells spaced approximately 395 m apart. Permeability patterns between wells were estimated by interpolation. A 2-year simulation of model A, in which permeability is laterally discontinuous, shows a rapid decline in oil production and a corresponding rise in the producing ratio of water to oil, which agrees with the production data. Computed saturations show the cause to be (1) rapid initial production of oil in high-permeability (&g ; 10 md) grainstone facies and (2) increasing cycling of injected water through grainstone facies connecting the injection and production wells. Aided by gravity, water injected at perforations near the top of the 30-m thick reservoir model flows across strata and exits at perforations in a production well near the bottom of the model. Thus, substantial vertical fluid migration occurs in spite of the pronounced horizontal stratification of high and low permeability. Oil near the top of the higher permeability layers tends to be bypassed by the waterflood. In simulation of model A, 27% of the movable oil in the cross section was produced. In contrast, the more continuous permeability pattern of model B supports much more uniform flooding, which leads to production of 46% of the movable oi over the simulation period.

Simulations show that eliminating perforations in high-permeability thief zones does little to improve sweep efficiency unless those zones are separated from adjacent perforations by thick, low-permeability strata. If between-well permeability values in the model are uniformly low (0.1-10 md) and laterally continuous, results show that addition of an infill injection well increases production by a factor of only 1.2. If, however, the infill well intersects just two discontinuous high-permeability stringers, simulated production rises by a factor of 1.6. Better geologic prediction and/or stochasic simulation of geologic facies is needed to evaluate the probability that infill wells would intersect yet undetected zones of good permeability between wells.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.