The controversy concerning stratigraphic architecture of channelized reservoirs and recovery by waterflooding

There is a subtle controversy in the petroleum industry regarding the relationship between reservoir architecture and recovery efficiency from petroleum reservoirs. Stratigraphers tend to believe that facies architecture and geometric shapes strongly govern recovery, whereas engineers stress factors such as permeability heterogeneity and anisotropy. To understand the relationship between architecture and recovery better, a suite of conceptual models of different channelized clastic reservoir architectures was constructed and pore-volume replacement waterflood simulations were performed on each. Three 11-member suites of models were constructed at net: gross values of 35%, 60% and 85%. The reservoir architecture features that were varied were channel width to thickness, thickness, sinuosity, stacking patterns, orientation and reservoir element type (point bar vs. channel). The three sets of models appear visually very different from a reservoir architecture standpoint. Permeability and porosity were simulated using geostatistical techniques but share the same conditioning population. The same water saturation, well count (110 acre spacing), fluid properties and relative permeability were used in each waterflood simulation. In this situation, in which the only significant variable was reservoir architecture, only a few percent spread in recovery was noted for each net: gross suite. Variation in recovery efficiency can be shown to be associated with reservoir connectivity, which is generally high for all models but shows some variation. Additional studies were made to address the influence of well count, mobility ratio, permeability heterogeneity and geostatistical seed number on recovery efficiency.

Two stratigraphic factors are shown to influence recovery efficiency: reservoir connectivity and permeability heterogeneity. Both factors influence the volumetric sweep efficiency of reservoirs. Models that appear to be visually different, but have similar connectivity, well count and permeability heterogeneity, have similar sweep efficiencies and, therefore, recovery efficiencies. The practical importance of this observation for development studies is that building and flow-simulating suites of models with different reservoir architectures may not help to characterize recovery uncertainty unless the models are constructed to emphasize differences in sweep efficiency.