Improved Oil Recovery from Heterogeneous Carbonate Reservoirs Associated with
Authors: Carlson Eric C. , Benson D. Joe , Groshong Richard H. , Mancini Ernest A. 
Published: December 2007

The Appleton Oil Field was discovered in 1983 in Escambia County, Alabama, eastern Gulf Coastal Plain and unitized in 1988. The original oil in place is estimated to be 3.8 million stock tank barrels. Production from the five productive wells in the field is from two distinct Upper Jurassic Smackover carbonate reservoir units, an upper grainstone/packstone shoal facies and a lower boundstone/bindstone reef facies located over three localized paleotopographic basement structures. To date, 2.6 million stock tank barrels of sweet oil with an API gravity of 51.7 degrees have been produced under primary recovery chiefly from the reef facies associated with the central and eastern paleohighs.

Total carbonate reservoir thickness ranges from 20 to 82 ft. Reservoir grade porosity in the lower less heterogeneous flow unit (algal reef facies) is a combination of primary shelter porosity overprinted by secondary dolomite intercrystalline and vuggy porosity and ranges from 9.5 to 25.3% (average 16.9%) with a permeability of 1.1 to 4106 md (average 356 md). Reservoir grade porosity in the upper heterogeneous flow unit (shoal facies) is a mixture of primary interparticulate and secondary grain moldic and intercrystalline dolomite porosity and ranges from 7.2 to 21.5% (average 14.2%) with a permeability of 0.4 to 618 md (average 56 md). Not only are the reservoir characteristics and quality better in the lower reef flow unit, but this interval has excellent vertical and lateral continuity and connectivity both across the oil-water contact and with the upper shoal flow unit. The shoal interval includes several tight carbonate zones which act to inhibit flow. Without additional infill drilling and strategically placed wells, undrained attic oil could remain over the localized pale highs.

The engineering properties of the fluids and the high connectivity of the reef facies above and below the oil-water contact results in a large volume of fluid (oil and water) being in communication with the reservoir over a substantial area (16 sq mi). Also, the high primary recovery efficiency of 68% in the field indicates that the encroaching water is displacing the oil uniformly from the bottom with the water level rising and the light oil remaining above the water. The oil in Appleton Field originally was undersaturated and the reservoir pressure has remained above the bubble point pressure of 3028 psia. The original reservoir pressure was 6270 psi a. Aside from the excellent hydraulic communication and strong bottom water drive, the rising water level serves to isolate the oil which is trapped in each of the three localized structural highs. Because of the structural elevation differences, a well draining oil from one paleohigh cannot effectively drain attic oil from the other paleohighs. Therefore, the most reasonable improved recovery method for this field is strategic infill drilling of each of the three localized paleohighs. Utilizing this technique, an additional 300,000 stock tank barrels of oil could be recovered from this field.

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