Waterflooding by Linear Displacement in Little Creek Field, Mississippi

Abstract

Abstract Since April, 1962, Shell Oil Co. has operated a peripheral line-drive waterflood of the 10,750-ft lower Tuscaloosa (Cretaceous) Denkman sand in the Little Creek field. Located in southwestern Mississippi in the lower Tuscaloosa trend, the field was discovered by Shell in Jan., 1958. Among the largest in the trend, the accumulation is controlled stratigraphically and occurs in an extensive series of alluvial-point bar deposits across a nose that dips gently to the south. The crude oil is 39 degrees API gravity and was highly undersaturated at initial conditions. Early production behavior indicated a depletion drive with slight water influx; primary recovery was estimated to be 25 million bbl, or 24.5 percent of stock-tank oil originally in place. As a result of a favorable mobility ratio and remarkably uniform rock properties, the volumetric sweep efficiency in the waterflood has exceeded 90 percent. Despite the high connate water saturation (56 percent), the displacement efficiency by water injection has been quite efficient, contrary to traditional concepts. Based on observed field flood-out performance, ultimate recovery is calculated to be 46 million bbl. As anticipated, the production rate began declining during 1964 due to a continuing reduction in the number of producing wells as flood fronts advanced across the field. Cumulative recovery as of Jan., 1968, was 44.8 million bbl; waterflood operations should be complete by 1970. Introduction Most waterfloods described in reservoir engineering literature involve pattern operations, the most common of which is the five-spot pattern. Only occasionally are linear drives described, even though there probably are increasing numbers being put into operation. Increasing use of linear floods may reflect the accelerating trend towards early application of fluid-injection programs when linear drives are more efficient, as was the case at Little Creek. However, the Little Creek operation is unusual in several respects. It is one of a few successful waterfloods conducted in sands with connate water saturations approaching 60 percent, and it is among the deepest successful waterfloods being operated anywhere. Development The Little Creek field was discovered in Jan.,1958, in the lower Tuscaloosa trend in southwestern Mississippi (Fig. 1). The discovery well, Shell-Lemann No. 1, was drilled on a seismic closure about 2 miles southeast of lower Tuscaloosa production at Sweetwater. The well was completed in the Denkman sand at 10,752 ft, and on initial potential it flowed 588 BOPD of 39 degrees API crude on a 15/64-in. choke; psi was 730 and GOR was 442:1. Development proceeded rapidly on 40-acre spacing, and by the end of 1958 the field was producing approximately 9,100 B/D from 56 wells. Almost all this production came from the northern half of the field. The southern part was discovered in Nov., 1958, at a location almost 2 miles from previously established production. Subsequent drilling established an apparent connection about half a mile wide between the two areas. Development of the field was practically complete by 1961 with the drilling of 155 producers and 35 dry holes. Some early wells were completed open hole but most are completed with casing cemented through the productive interval and perforated for production. Geology The lower Tuscaloosa structure in this area is a northsouth nose with maximum dips of about 2 degrees on the flanks. There is no discernible faulting at the producing horizon. Productive limits of the field are shown in Fig. 2, which is contoured on a marker just above the main productive zone. A common water level at 10,415 ft subsea was logged at both the northwest and southeast ends of the field. There is a gross oil column of nearly 100 ft (Fig, 2). Since there is less than 40 ft of closure, it is apparent that the accumulation is controlled stratigraphically, being limited by a sand pinch-out across the crest of the nose.