Bisti-Solvent Production During Three Years of Unit Operations

Abstract

Introduction The Central Bisti unit was formed July 1, 1959, and was the first LPG miscible-phase flood project in the state of New Mexico; also it is one of the few large-scale LPG floods to date. The project consists of 10 LPG injection wells located along the major axis of the reservoir, with producing wells forming five-spot and nine-spot 160- acre patterns. A slug of nearly 1 million bbl (5.4 per cent of hydrocarbon pore volume) of LPG was injected into the reservoir and driven away from the injection wells by dry gas. The volume of produced solvent was calculated, using the equations of Dean and Brinkley. As a result of these calculations, a total of 278,269 bbl of propane and 71,682 bbl of butanes had been produced to the end of Aug., 1962, representing the first 38 months of unit operations. Reservoir Characteristics The Bisti field is in the San Juan basin near the northwest corner of New Mexico. The field is approximately 32 miles long and varies from 0.5 to 3 miles in width, with the Central Bisti unit comprising some 5,000 acres in the west central portion. Production is encountered at a depth of about 4,900 ft from bar sands of the Lower Gallup sandstone of Upper Cretaceous age. The productive interval averages 130 ft in gross thickness and consists of alternating layers of sandstone, sandy shale, and shale with only about 20 per cent of the total section having permeability of more than I md. The most permeable section, identified as the First bench, is located at the top of the Lower Gallup section. This is the principal oil producing interval and is light- gray, medium-grained, clean, well-sorted sandstone with variable permeability. The lower sands, immediately below the First bench and referred to as Second and Third benches, are inferior in porosity, permeability and productivity. The lower sands are silty and fine-grained with irregular, dark-grey, micaceous shale inclusions and partings. The best development of porosity, permeability and thickness parallels the major axis of the long, narrow sand bar, with sand pinchout along the edges. This development forms a stratigraphic trap. Analysis of cores from 35 wells in the western portion of the field revealed an average hydrocarbon porosity (excluding water saturation) of 12.16 per cent for the First bench and 7.83 per cent for the Second bench. These values represent averages for wells located near the productive boundary as well as wells located within the fairway area. Hydrocarbon porosity of the fairway portion of the First bench averaged 14.7 per cent. Reservoir Fluid Two bottom-hole samples of reservoir oil were obtained during the early development of Bisti. These samples represent reservoir oil at British American Marye No. 1, NE NE Sec. 1–25N-13W, and Sunray DX Federal C-21 (Central Bisti unit No. 17) NE NE Sec. 8–25N-12W. The samples reflect similar physical properties. Bubble-point pressures of the Marye No. I and Federal C-21 samples were 1,155 and 1,260 psia at 145F respectively. Original reservoir pressure was estimated to be 1,612 psig at + 1,300 ft datum, the latter approximating the mid-point in the reservoir oil column. Structurally, the reservoir oil samples represent the mid-oil column at a position in the field located between the gas-cap area in Range 11-West and the structurally low pinchout areas. JPT P. 515^