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ABSTRACT: Modeling Reservoir Heterogeneity to Determine Current Oil Saturation Distribution in a Mature Field

Ronald C. Collings, Diana Morton-Thompson

The Rangely field is a 900-well giant oil field located in northwest Colorado. The field is at the facies transition between the clean, eolian Weber Sandstone derived from the north and the tight, arkosic sandstones and shales of the Marcon Formation derived from the south. The Weber Sandstone produces from six discrete stratigraphic horizons. The character of these depositional packages varies, both vertically and laterally, across the field. This heterogeneity significantly affects field performance.

The Rangely Weber Sand Unit has produced 730 million STB of oil by primary and secondary recovery from an OOIP of over 1500 million STB of oil. A tertiary recovery project was initiated in 1986 using carbon dioxide as a solvent. Currently, over one-third of the 30,000 STB/day oil rate is attributed to solvent injection.

A team of development geologists and reservoir engineers has been working to ensure the continued success of the CO2 project by refining the geologic model. The porosity, permeability, and original oil saturation distributions have been redefined using modern data and also to account for reservoir heterogeneity. From this data, the current distribution of recoverable oil was determined through the use of a hybrid simulation technique that combines geostatistical interpolation with finite difference and stream-tube displacement simulators. Knowing the distribution of secondary and tertiary recoverable oil will allow aggressive reservoir management to improve recovery efficiency and project profitability through infill drilling, pattern realignment, and prioritized allocation of CO2 resources.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990