Effect Of Shale Barriers On The Performance Of Bottom-Water Drive Reservoirs
Authors: Truitt N.E. 
Published: December 2007

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Abstract
The theoretical performance of a bottom-water-drive reservoir containing an idealized, horizontal shale harrier has been determined for the case of steady-state flow of a single, incompressible fluid. The sweep efficiency and production history of such a reservoir are dependent primarily on the radial extent of the barrier; vertical position in the reservoir is relatively unimportant, unless the barrier is close to the top of the pay.
In order to represent the problem in only two dimensions, we have considered a single, horizontal barrier, radially symmetrical about the well axis and intersecting the well axis. For reservoirs developed on close dimensionless spacing (10 acres per well in a reservoir with 100 feet of pay), such a barrier will result in little additional oil at the economic-limit water-cut. However, more oil is produced at low to medium water-cuts than if the barrier were not present, resulting in lower lifting and disposal costs of produced water. For reservoirs developed on wide spacing (say, 80 acres per well in a reservoir with 100 feet of pay), a barrier of sufficient radial extent can result in considerably higher sweep efficiencies at the economic limit. Therefore, bottom-water-drive reservoirs characterized by many irregular, discontinuous shale streaks can theoretically be developed on wider spacings than reservoirs without shale streaks.
Water encroachment into such a reservoir is irregular, with a water "tongue" typically entering the well above the barrier while part of the well section below the "tongue" is still producing oil. A barrier not intersecting the well axis can result in reduced sweep efficiency at the economic-limit water-cut.

Introduction
Several theoretical studies have been made on the performance of bottom-water-drive reservoirs, with each study necessarily containing many simplifying assumptions concerning the reservoir and reservoir fluids. The first analysis was presented by Muskat, who computed the production histories and sweep efficiencies for a few cases of well spacing and (fractional) well penetration into the pay assuming the "single-fluid" case, in which gravity effects and mobility ratio were neglected. An approximate treatment of the more complicated problem including gravity and mobility was made by Hutchinson and Kemp.