Waterflood performance in a fluvial reservoir is examined to assess which aspects of the reservoir are important to describe for accurate simulation of recovery performance. The study is based on a low net-to-gross fluvial reservoir and we consider the influence of sedimentary heterogeneities which are sub-seismic-scale in a full-field model. A detailed hierarchical 3D geological model is developed and flow properties are upscaled. Particular attention is given to representing the influence of cross-stratification. A screening study is then conducted to assess which heterogeneities and scales have the greatest influence on recovery predictions. By using experimental design the influence of a large number of factors is examined in an efficient manner. The results show that parameters describing channels and channel belts have the greatest influence. Somewhat surprisingly, the number of channels in a belt and their width and thickness has at least as much influence on sweep efficiency as net-to-gross and the stacking of belts. In addition to the influence of these large-scale heterogeneities, capillary effects at the laminae scale within cross-stratification have a significant effect on recovery. Early identification of important heterogeneities will allow for more efficient field exploitation. The strongest impact of this approach may be to focus data collection early in the life of a field on the most important types of heterogeneities to be expected for a given depositional and stratigraphic setting.

This paper investigates the impact of sedimentary heterogeneities on performance of waterflooding in shallow marine reservoirs. Reservoir simulation models are developed from detailed geological models based on a four-level sedimentary architectural hierarchy ranging from sedimentary structures (wave ripples, hummocky cross-stratification) up to parasequences. Single- and two-phase flow properties are upscaled to capture the effects of small scale heterogeneities on performance. The relative influence of 12 heterogeneity factors is determined using experimental design and analysis of variance. The analysis identified large scale features (parasequence thickness and stacking patterns) to be of primary importance for waterflood performance. These features control the relative proportion of fades belts in the reservoir, which correlates with recovery. Other important, though secondary, heterogeneities include interfingering of facies belts, and small scale bedding.

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The Pereriv Suite reservoir in the Azeri culmination of the ACG Oilfield is characterized by laterally continuous layers of variable net-to-gross (NTG) ratio deposited in a channel-dominated, fluvio-deltaic environment. The reservoir is being developed by down-dip water injection, with up-dip gas injection on the more steeply dipping central north flank. We use high-resolution models derived from outcrop analogue and subsurface data to demonstrate that four key sedimentological heterogeneities control recovery in both oil-water and gas-oil displacements: (1) local variations in NTG within low NTG (<55%) layers; (2) the degree of communication between low NTG layers and adjacent high NTG (>85%) layers; (3) sinuosity and (4) stacking pattern of channel-fill sandbodies in low NTG layers. The first three heterogeneities control sandbody connectivity; the fourth controls sweep efficiency in the connected sandbodies. Two further heterogeneities control recovery in gas-oil displacements in high NTG layers: (5) verticalto-horizontal permeability ratio of channel-fill sandbodies and (6) mud clast lags at channel bases. Models which omit these small-scale features predict that sedimentological heterogeneity has little impact on water-oil or gas-oil displacements in high NTG layers, but fail to capture the effect of heterogeneity on the gravity stability of the gas-oil displacement, which significantly impacts on recovery.