Based on examination of the distribution of differential oil-in-place (OIP) in a reservoir, a method is presented to identify the effect of geological heterogeneities on nonuniform oil production. A well differential oil-in-place (DOIP) is defined by the equation: DOIP = volumetric OIP at a given spacing − material balance equation OIP. This method is based on the theory that positive differential OIP areas could indicate the presence of geological heterogeneities like high clay content, sealing faults, or different types of flow barriers, whereas negative differential OIP areas may imply presence of highly permeable clean sands, nonsealing faults, fractures, etc. By using the fairly complete geological and engineering data from 54 wells in the four sections enclosing the Department of Energy Tertiary Incentive Project (TIP) area of Bell Creek (MT) field, a differential OIP map was prepared. The map indicated several areas with significantly positive and negative differential OIP anomalies.

An overlay of the DOIP map with maps of structural anomaly, clay content, porosity, permeability, Dykstra-Parsons coefficient, and log-derived heterogeneity index indicated that low-permeability valley-fill sediments with a high degree of clay content and several sealing faults coincided with areas having a significantly positive differential OIP. High-permeability barrier sandstones adjoined by numerous small, conductive faults (throws around 10 ~ 20 ft) with excellent drainage efficiency were responsible for a significantly negative differential OIP. DOIP contour patterns indicated that optimum injection sweep efficiency could be obtained by locating the injectors parallel to a cluster of differential oil-in-place contour lines covering the area to be flooded.

The differential OIP was strongly dependent on geological heterogeneities and was an effective index for measuring the role of different heterogeneities on oil production in a reservoir. Main applications of the method are to determine: (1) remaining oil reserves, (2) geological heterogeneities, (3) well drainage area and efficiency, (4) optimal well spacing, and (5) best injection and production well sites for waterflooding, infill drilling, EOR, and horizontal well drilling programs. Results of the method can be used independently or integrated with other information for optimal reservoir production planning.

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