Print this pageThree-Dimensional Modeling of a Shoreface-Shelf Parasequence Reservoir Analog, Part Two: Geologic Controls from Fluid Flow and Hydrocarbon Recovery
Wave-dominated, shoreface-shelf parasequences are generally modeled as simple, high net-to-gross (>70%), ‘layercake’ reservoirs, yet analysis of modern and ancient analogs demonstrates that these intervals contain a more complex physical stratigraphy. We investigate the impact of subtle stratigraphic heterogeneity caused by gently dipping clinoform surfaces on fluid flow and recovery during waterflooding, using a new, surface-based modeling approach to capture the complex facies architecture within a well exposed outcrop analogue. We demonstrate that heterogeneity associated with barrier-lined clinoforms can strongly influence sweep efficiency and hence oil recovery. Sweep efficiency is low when waterflooding is down-depositional-dip, because oil is bypassed in ‘dead ends’ at clinothem toes as water flows preferentially into high-quality sandstone facies in the upper part of the parasequence. Sweep efficiency is higher when waterflooding is up-depositional-dip, because the gravity-driven, downward flow of water ‘back sweeps’ poorer quality sandstone facies in the lower part of the parasequence. In both cases, injectors may offer limited pressure support to producers. Waterflooding along-depositional-strike results in poor sweep, because the gravity-driven downward flow of water into the lower part of the parasequence is significantly reduced, but good pressure support of producers. This yields highly variable fluid saturations but a uniform pressure gradient, which is consistent with pressure and fluid saturation data from the mature Rannoch Formation reservoir, Brent Field, UK North Sea. Simple layercake models of facies architecture fail to capture the wide range of flow behaviours described above, and over-predict recovery by up to 20% as a result.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009