An Efficient Gas Displacement Project Raleigh Field, Mississippi

Abstract

Abstract In the unitized Raleigh field, gas from separators, treaters and small gas reservoirs is being used for pressure maintenance of the larger oil pools. Sufficient gas has been available to maintain pressure within 90 per cent of original in two of the larger Lower Cretaceous Houston oil pools. These are the 12,600-ft sand and the 12,100-ft sand. This paper concerns only the reservoir behavior of the 12,600-ft sand, which is the most advanced pressure maintenance project in the Raleigh field. To date, liquids recovery from the 12,600-ft sand has been 52.4 per cent of the stock-tank oil, or 49 per cent of the total propanes and heavier originally in place in the productive zone. Current production rates are 5 per cent of the original oil in place per year. The high recovery shown is believed to be partly due to mass transfer occurring as reservoir vaporization of residual oil components into injected gas. Introduction The 12,600-ft sand is a small, deep, oil sand, whose 40-acre development pattern consists of three rows of wells outward from the single gas-injection well. The original bubble point was 3,236 psig. Before gas injection began, the reservoir pressure had dropped from 5,783 to less than 3,500 psig. Concurrently, well productivities declined. After gas injection began, productivity from wells adjoining the injection well rapidly improved. The nearest wells went to high GOR soon; but, with increasing reservoir pressure, the GOR of the more distant wells increased more gradually. Wells in the last row, most of which were deficient or non-commercial on original completion, improved to produce low-GOR oil at high rates. The oil recovery from the 12,600-ft sand appears to be much higher than normally expected from gas injection projects. It is the purpose of this paper to present the basic properties of this reservoir, describe the method of its operation, report its performance to date, and discuss the results of preliminary performance analysis. Description The Raleigh field in Smith County, Mississippi, lies about 50 miles southeast of Jackson. The field was discovered in Jan., 1957, by The California Co.'s Central Oil Co. 5 Unit I No. I in Section 33, T-2-N, R-7-E. This well was successfully completed in the Cretaceous Bailey sand at 226 BOPD. The 12,600-ft Hosston sand was put on production first in March, 1958. By Feb., 1960, development of the field was essentially complete on 40-acre spacing. To date, 34 wells and nine dry holes have been drilled at Raleigh. In Jan., 1965, there were 39 streams on production and three streams on gas injection in 32 wells. Each of the injection streams is one side of a dual completion. Recognizing serious production problems early, a meeting of operators' representatives was held in Nov., 1958. Immediate action was called for by the rapidly declining reservoir pressure and well productivities in the 12,600-ft sand. Details were rapidly worked out and, as an interim program, effective Jan. 1, 1960, a unit was formed to permit gas injection into the 12,600-ft sand. Injection began on Feb. 4, 1960. Through Jan., 1965, 19 wells had produced from the 12,600-ft sand at one time or another. During Jan., 1965, production averaged 607 BOPD from 10 streams with an average producing GOR of 10,365 cu ft/bbl, and the gas injection rate was 7,233 Mcf/D. Geologic Description The Raleigh field structure is a relatively small dome divided by two major faults into a graben, an east, and a west flank area. The Raleigh field unit discussed herein is confined to the east flank area, which is separated from the graben by a main fault with a displacement of 500 - ft at 600 dip. Hydrocarbon accumulations exist in many of the sand members of the Rodessa, Pine Island, Bailey and Hosston formations of Cretaceous age. The major oil sands are the 12,100-, 12,200- and the 12,600-ft Hosston sands. Fig. 1 shows the structure on the 12,600-ft sand. The 12,600-ft sand is made up of upper and lower members. The upper member consists of two thin, tight, lenticular sand bodies. These lenses do not appear in the injection well and, although they are open in several wells, it is believed that they are ineffective in the gas injection project. The lower member of the 12,600-ft sand consists of 812 acres of productive oil sand averaging 18 ft thick. Fig. 2 is an isopach of the sand. JPT P. 509ˆ