WEBER, K. J., Technical University Delft, The Hague, Netherlands

ABSTRACT: Major Influences of Clastic Reservoir Anatomy on Oil Recovery

Even in simple layer cake reservoirs, contrasting strata, pinch-outs, and impermeable intercalations commonly impair vertical sweep efficiency. However, extensive impermeable streaks may prevent coning of water or gas. For many reservoir configurations, certain features can be either favorable or unfavorable for oil recovery depending on accumulation conditions, mobility ratio, structural dip magnitude and orientation, and applied drainage and injection schemes. Although this complicates relating classification systems of recovery efficiency directly to geological aspects, clastic reservoirs commonly display predictable production behavior closely related to environment of deposition.

For example, fluid flow in deltaic "jigsaw puzzle" reservoirs (composed of sand bodies with contrasting permeabilities) is dominated by pathways of least resistivity. These occur in typical patterns related to paleogeographical trends. The threat of local cusping, coning, and breakthrough of water or gas dictates careful planning of locations, completions, and production levels of individual wells.

Extreme anisotropy is often associated with low net-to-gross "labyrinth" reservoirs deposited in coastal plain environments. Overall connectivity may be attained at net-to-gross levels of between 0.3 and 0.4, but major vertical and lateral discontinuities will exist on a well-to-well scale. Sweep efficiency is strongly influenced by sand body trends relative to dip orientation, and specific well patterns are required.

Reservoir simulation input should preserve reservoir architecture and correctly incorporate no-flow boundaries and intercalations. Probabilistic modeling systems using representative sand-body geometry databases can provide estimates of connectivity and simulation input at an early stage of field development. The crux of the matter is the prediction of production behavior for alternative development plans to reduce the negative influences of reservoir heterogeneity.

AAPG Search and Discovery Article #90990©1993 AAPG International Conference and Exhibition, The Hague, Netherlands, October 17-20, 1993.