Pressure Maintenance by In-Situ Combustion, West Heidelberg Unit, Jasper County, Mississippi

Abstract

Introduction The West Heidelberg Cotton Valley Sands Unit in Jasper County, MS (Fig. 1), is the location of what is probably the world's deepest in-situ combustion project. The Cotton Valley sands are of Jurassic age, occurring, at an average depth of 11,400 ft (3475 m). Fourteen sands are identifiable in the Cotton Valley, but only eight are productive. Of these eight, Sands 4 and 5 are the major oil reservoirs, and their performance under various drive mechanisms is the subject of this paper. Poor primary oil recovery from all the Cotton Valley sands caused studies to be conducted for a process that would substantially increase ultimate oil recovery. Waterflooding was considered but rejected because of unfavorable water/oil mobility characteristics and lifting costs that would be incurred by producing high-watercut wells from this depth. Favorable results of burn-tube tests and the presence of heavy oil on the flanks of the Sand 5 reservoir made the in-situ combustion process seem feasible, and a pilot air injection project was started in Dec. 1971. This pilot has since been expanded into a full-scale project. The development and performance of this project and the problems associated with it are discussed. Historical and Development Background The West Heidelberg field in Jasper County, MS. was discovered in Jan. 19 as a part of the Heidelberg field. The Cotton Valley sands discussed in this paper are of Jurassic age and range in depth from 10,900 to 11,800 ft (3322 w 3597 m). Fig. 2 is a type log showing the upper identifiable sands in the Cotton Valley formation. including the Sands 4 and 5 project. Structurally. the Cotton Valley lies on the flank of a piercement salt dome and dips about 8 degrees from east to west, as shown in Fig. 3. The eastern updip limit is the salt intrusion, and the western downdip limit is an essentially immobile tar or asphalt deposit. Only 8 of the 14 Cotton Valley sands have proved productive. Of these, Sands 4 and 5 are the major reservoirs. The Sand 4 and Sand 5 reservoirs have very similar characteristics, with average porosity of 14 %, permeability to air of 85 md, and an initial water saturation of 15%. Table 1 lists average reservoir rock and fluid data for Sands 4 and 5 as determined from logs, core, and fluid analyses. From data obtained when additional wells were drilled, the original oil in place (OOIP) is now estimated at 10 million bbl (1.590 × 10(6) m3) for Sand 4 and 8 million bbl (1.272 × 10(6) M3) for Sand 5. The initial reservoir pressure for both sands was about 5.100 psi (35 MPA) with a fluid bubblepoint pressure of 930 (6410 kPa) and a solution gas ratio above bubblepoint of 100 (18 m / m ). There is an oil gravity gradation from about 15 degrees API (966 kg/m ) near the tar to about 27 degrees API (893 kg/m ) at the top of the structure. Reservoir temperature is about 221 degrees F (105 degrees C), and reservoir oil viscosity at formation temperature was about 6 cp (0.006 Pa.S). During the primary oil-production phase, a total of 10 wells were drilled and completed in the Cotton Valley Sands 4 and 5. All wells were produced from more than one Cotton Valley sand member. Dual completions had 7-in. (178-mm) production casing, while single completions had either 4 1/2- or 5 1/2-in. (114- or 140-mm) casing. JPT P. 1877^