Pressure Maintenance by Bottom-Water Injection in a Massive San Andres Dolomite Reservoir
Authors: Borgan R.L. , Frank J.R. , Talkington G.E. 
Published: December 2007

Abstract
The Waddell field is being pressure maintained by the injection of Hendrick reef salt water, oil a peripheral pattern, below the oil-water contact. The reservoir is a San Andres dolomite of Permian Age, occurring at approximately 3,450 ft. The original producing mechanism was dissolved-gas drive. Reservoir Pressure decline and increasing gas-oil ratios indicated the need for pressure maintenance to maximize the oil recovery. Performance to date, as influenced by water injection, has been very encouraging. During six years of pressure maintenance operations, the reservoir pressure has increased from 843 to 1,171 psig. The gas-oil ratio during this period has dropped from 1,100 to 456 scf/bbl. The dissolved gas-oil ratio, at a saturation pressure of 1,134 psig, is 484 scf/bbl. To date, 41.9 million bbl of water have been injected. The current fluid withdrawals are cutting 10 per cent water. Total ultimate recovery will be increased appreciably as a result of pressure maintenance operations. The actual performance to date is compared to an estimated performance to date under continued primary depletion.
Introduction
The Waddell field is located in the north-central portion of Crane County, Tex., as shown on Fig. 1, approximately 18 miles southwest of Odessa, Tex. The field was discovered in 1927. A number of wells, drilled from 1927 to 1929, encountered some oil in the Grayburg zone with minor oil shows from the San Andres. However, it was not established as a major commercial reservoir until the completion of Gulf's W.N. Waddell No. 9, which was completed for 449 BOPD as a flowing producer in 1935. This well is located on the extreme eastern flank of the field. The location of the W.N., Waddell No. 9 and the direction of step-out development resulted in the east side of the reservoir being developed first. At the present time, the limits of production have been reasonably well established.
The initial wells were drilled and completed open-hole with cable tools, with casing set near the top of the San Andres zone. Subsequent wells were drilled with rotary tools to the casing point, following which cable tools were used for drilling the pay section, and the wells completed open-hole. The most recent wells were taken to total depth with rotary tools, after which casing is set to total depth and perforated opposite the selected pay intervals. To date, approximately 385 wells have been completed in the main San Andres pay in the Waddell field, Production to date has been 32.79 million bbl of oil. The developed acreage at present is approximately 8,000 acres, with a well density of approximately one well per 20 acres. The Grayburg-San Andres interval is productive over a very extended area, which includes the Penwell, Jordan, Edwards, Waddell and Dune fields, among others. Development -has merged it into one large producing area. However, within this large area are smaller areas or reservoirs which perform relatively independently. It is the San Andres reservoir within the Waddell field in which we are presently interested.
JPT
P. 883ˆ