Flooding for Tertiary Recovery After Successful Gas Injection for Secondary Recovery-Brookhaven, Mississippi
Authors: Meltzer B. David 
Published: December 2007

Gas injection processes in Brookhaven field have increased oil recovery from 36 million barrels to 64 million barrels. Water injection after the gas injection has already increased recovery by 3 1/2 million barrels and is expected to account eventually for more than 5 million barrels. Thanks largely to monitoring programs, the tertiary recovery has been profitable.

Introduction
The description, analysis, and performance of the Brookhaven, Miss., oil field under gas injection have been well and accurately documented in the literature. As a result, the success of the gas displacement processes in this reservoir with large permeability variation is generally well known in the permeability variation is generally well known in the industry.This paper describes the successful recovery of additional oil by supplementing and then replacing gas displacement with water displacement. Success in recovering the waterflood oil was assured by the monitoring programs, which eliminated or delayed expenditures until oil production was reasonably well assured.

Reservoir History and Description
The Brookhaven field, located in South Central Mississippi, was discovered in June 1943. Active development on 40-acre spacing began in 1945. Production is from the Basal Tuscaloosa, in the Upper Production is from the Basal Tuscaloosa, in the Upper Cretaceous, at an average producing depth of approximately 10,300 ft subsurface, or 9,800 ft subsea. The proved closure is approximately 400 ft. The Tuscaloosa is a sandstone reservoir of erratic deposition. A large permeability variation exists, but the zones of high permeability do not normally appear to be continuous between wells. Several north-south trending faults have combined with sand pinchouts to subdivide the field into isolated producing areas. Fig. 1 is a structure map on the top of the Basal Tuscaloosa. The area farthest west, defined as Block H, was discovered in 1968, fault separated from the remainder of the field. Block H production has been involved in neither the gas injection program nor the waterflood program, and consequently, has been omitted from the field totals given here.The Brookhaven reservoir fluid was a highly undersaturated oil, and the need for pressure maintenance was soon apparent. By June 1948, the field was unitized and gas injection was begun. The reservoir pressure initially was 4,537 psi at a subsea datum of pressure initially was 4,537 psi at a subsea datum of 9,800 ft, but the pressure in much of the field bad dropped below 2,500 psi when gas injection began. By 1961, sufficient gas had been injected to raise the reservoir pressure above 3,300 psi. Fig. 2 is a map indicating the maximum gas saturation reached in the gas-swept zone. The gas saturation was estimated through the use of Buckley-Leverett displacement calculations.In 1957 a program of alternately injecting slugs of produced water with the injected gas (WAG) was produced water with the injected gas (WAG) was instituted. Later, the program was revised in an attempt to return all of the produced water to the reservoir through certain previous gas injection wells.In 1965 a pilot waterflood program was begun in an economically depleted gas-swept portion of the field. Waterflooding has since been expanded to include most of the major areas of the field.
JPT
P. 783