A Field Test of the In-Situ Combustion Process in a Near-Depleted Water Drive Reservoir
Authors: Casey Terence J. 
Published: December 2007

A Field Test of the In-Situ Combustion Process in a Near-Depleted Water Process in a Near-Depleted Water Drive Reservoir
The recovery of additional oil from a watered-out reservoir has been found to be economically feasible using in-situ combustion techniques. Employing a single-point injection scheme, one company has recovered more than half a million barrels of oil, with few operating difficulties, from the North Government Wells field in South Texas.

Introduction
Most of the early in-situ combustion projects were conducted in shallow reservoirs containing relatively viscous crudes. Generally, the primary oil production from these reservoirs was very low. When Mobil Oil began in-situ combustion operations in the North Government Wans field in Sept., 1962, most of the basic guidelines of that time for successful thermal recovery operations were severely challenged. If this recovery technique were to become a strong tool in South Texas fields, its ability to overcome many conditions prevalent in the area, considered adverse to the success of the process, had first to be proven. It was with this goal in mind that Mobil chose a very demanding reservoir for their initial in-situ combustion operation in Texas (Fig. 1). The Government Wells reservoir underlying Mobil's Duval Ranch Section 208–09 lease contains approximately 13,100 acre-ft of net oil sand. The combination of a moderately strong water drive and a 10-cp crude had resulted in a very efficient sweepage of the reservoir. In mid-1962, the WOR had risen to 45:1 and the productive economic limit of the lease was rapidly being approached. Despite the fact that the primary oil recovery had been 40 percent of the original oil in place, some 800 bbl of oil per acre-ft still remained in the reservoir to be per acre-ft still remained in the reservoir to be recovered. The presence of this 10 million bbl of residual oil provided sufficiently strong incentive to embark on operations for determining the limits to which this process could be extended. The experience that was process could be extended. The experience that was to be acquired from the project would be adapted for future use in similar reservoirs.

Geology
The Government Wells sand is of the Jackson group of Eocene age and occurs at an average depth of 2,320 ft. The gross thickness of the sand underlying Mobil's DCRC Section 208–09 lease is approximately 60 ft, of which, in some instances, as much as the upper 30 ft is considered to be oil productive. Because of the early development of the field and a lack of electric logs, it is difficult to determine the effective sand thickness. The sand is quite heavily stratified with thin shale and bard lime streaks. The Government Wells reservoir is limited to the north and east of the lease by an oil-water contact originally located at - 1,790 ft. The aquifer has been fairly active throughout the life of the field, with encroachment predominantly from the east-northeast. This water influx gave initial indications of the anisotropy of the formation, which was later confirmed by gas movement through tile reservoir. Production characteristics and differential reservoir pressures indicate that an anomaly or permeability barrier exists in a north-south direction permeability barrier exists in a north-south direction across the middle of the lease in the immediate area of the project.
JPT
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