Ekofisk Waterflood Pilot

Abstract

Thomas, L.K.; SPE, Phillips Petroleum Co. Phillips Petroleum Co. Dixon, T.N.; SPE, Phillips Petroleum Co. Phillips Petroleum Co. Evans, C.E.; SPE, Phillips Petroleum Co. Phillips Petroleum Co. Vienot, M.E.; SPE, Phillips Petroleum Co. Phillips Petroleum Co. Copyright 1987 Society of Petroleum Engineers Summary. This paper describes the evaluation of a waterflood pilot in the highly fractured Maastrichtian reservoir of the Ekofisk field in the Norwegian sector of the North Sea. A four-well pilot consisting of one water injector and three producers was initiated in Spring 1981 and was concluded in mid-1984. A total of 21 × 106 bbl [3.3 × 106 m3] of water was injected, and water breakthrough occurred in two of the production wells. Simulation of waterflood performance in the pilot was conducted with a three-dimensional (3D), three-phase dual-porosity model. Initial and boundary conditions were taken from a full 3D single-porosity model of the reservoir. The pilot was conducted to determine the following information for the Maastrichtian: water-cut performance vs. time, water imbibition characteristics, and anisotropy. Results from this work have been incorporated into a full-field waterflood study. Reservoir description included the determination of fractured areas, matrix block sizes, water/oil capillary imbibition, matrix permeability and porosity, and effective permeability. These data were derived from porosity, and effective permeability. These data were derived from fracture core analysis, pressure transient tests, laboratory water/oil imbibition studies, repeat formation pressure test results, and open- and cased-hole logs. An excellent match of waterflood performance was obtained with the dual-porosity model. Of particular interest are the imbibition characteristics of the Maastrichtian in the Ekofisk field and the character of the water-cut performance of the producing wells following injector shutdowns and startups. Introduction The Ekofisk field was discovered in Nov. 1969 in Block 2/4 of the Norwegian sector of the North Sea. The field is a north/south-trending anticline located about 160 miles [257 km] from land in about 240 ft [73 m] of water. In July 1971, production began from four subsea wells. These were later abandoned in 1974 when production began through permanent facilities. Field production peaked in Oct. 1976 at about 350,000 STB/D [55 600 stock-tank m3/d] and currently averages 110,000 STB/D [17 500 stock-tank m3/d]. Original oil in place (OOIP) in Ekorisk is estimated to be 6.7 × 109 bbl [1.1 × 109 m3]. The reservoir consists of about 600 ft [180 m] of productive limestone that can be divided into the Ekofisk productive limestone that can be divided into the Ekofisk formation (Danian Age), approximately 400 ft [120 m] thick, 50 to 90 ft [15 to 30 m] of dense limestone and a 200-ft [60-m] -thick section of highly fractured Tor formation (Maastrichtian Age). The reservoir rock is naturally fractured, with fracture intensity increasing with depth. The reservoir was overpressured initially and contained an undersaturated oil at an initial pressure of 7,120 psig at 10,400 ft [50 MPa at 3170 m] subsea. The psig at 10,400 ft [50 MPa at 3170 m] subsea. The bubblepoint pressure was approximately 5,545 psig [38 MPa] at a reservoir temperature of 268 deg. F [131 deg. C]. Initial solution GOR at producing separator conditions was 1,530 scf/STB [276 std m3/stock-tank m3]. Table 1 presents a summary of the Ekofisk reservoir parameters. The field was developed with three production platforms. Produced gas in excess of sales gas has been platforms. Produced gas in excess of sales gas has been reinjected into the Danian formation in the crest of the field. Oil produced from the field is sent by pipeline to Teesside, England, and gas production is transported by pipeline to Emden, Germany. As of Jan. 1, 1984, a total pipeline to Emden, Germany. As of Jan. 1, 1984, a total of 690 × 106 bbl [110 × 106 m3] of stock-tank oil and 2,263 Bcf [64 × 109 m3] of gas have been produced. Gas reinjection totals 621 Bcf [17.6 × 109 m3]. Primary oil recovery with excess gas injection is forecast to be about 1.2 × 109 bbl [190 × 106 m3] or 18% of the OOIP. A Maastrichtian pilot waterflood was initiated in the Ekofisk field in April 1981 to evaluate the performance of water injection in this highly fractured formation. The four wells that make up the heart of the pilot are B-16, of water injection well, and B-19, B-22, and B-24, the three closest Maastrichtian-only producers. Both model and laboratory studies were undertaken to assist in the evaluation and interpretation of waterflood results. The model study of water injection into the Ekofisk Pilot, which is located in the Platform B area of the field, Pilot, which is located in the Platform B area of the field, was conducted with a dual-porosity model. An analysis of available data was made to determine fractured zones in the pilot area, and only those areas were assigned dual porosities. History for this study consists of the period from Jan. 1, 1978, to April 1984 and includes a total pilot water injection of 21 × 106 bbl [3.3 × 106 m3]. pilot water injection of 21 × 106 bbl [3.3 × 106 m3]. During the injection period, 107 STB [1.5 × 106 stock-tank m3] of oil and 38.2 Bcf [1.1 × 109 m3] of gas were produced from the three pilot producers. produced from the three pilot producers. Initial conditions for the study were taken from a 3D history match of the field. The area selected for inclusion in the study is about 1,100 acres [445 ha] and includes five edge wells-B08, B-14, B-18, B-21, and B-23- in addition to the primary pilot wells. JPT P. 221