Subsea Well Development and Producing Experience in the Ekofisk Field

Abstract

Jobin, T.J., SPE-AIME, Phillips Petroleum Co. Norway Phillips Petroleum Co. Norway Hoch, R.S., SPE-AIME, Phillips Petroleum Co. Norway Phillips Petroleum Co. Norway Johnson, D.A., SPE-AIME, Phillips Petroleum Co. Norway Phillips Petroleum Co. Norway APRIL 1978 This paper describes subsea well development and producing experience in the Ekofisk field of the North Sea. This field was developed first with four subsea oil wells and temporary production facilities, located on a converted jack-up drilling rig. Successes and problems associated with the drilling, completion, and operation of the subsea wells are emphasized. Introduction The Ekofisk field, discovered in the Norwegian sector of the North Sea by the Phillips Norway Group in late 1969, was the first major oil discovery in the North Sea. The field is located 180 miles offshore, near latitude 56 deg. 30's north in 230 ft of water. The discovery well and three delineation wells confirmed the existence of a major oil field. A temporary test production program called Phase 1 was implemented in late 1970, and four exploratory wells on the structure were re-entered and completed as subsea producer. These subsea wells were connected to production facilities installed on a converted jack-up production facilities installed on a converted jack-up drilling rig and oil was loaded into tankers using single-buoy moorings (SBM). Phase 1 production began in June 1971, 18 months after the field discovery, and continued through 1974. Phase 1 production totaled 28 million bbl of oil. When Phase 1 production totaled 28 million bbl of oil. When permanent drilling and production platforms became permanent drilling and production platforms became operational in 1974, the four subsea wells were plugged and abandoned. Field Data The Ekofisk reservoir is found in the Danian and Cretaceous limestones. The anticlinal structure of the field results from a deep-seated salt dome and is 8 miles long and 4 miles wide. The top of the pay is 10,000 ft subsea in a typical well. The Danian and Cretaceous pays are highly fractured throughout the structure. The reservoir is undersaturated and abnormally pressured. Crude-oil gravity is 37 deg. API and the average GOR pressured. Crude-oil gravity is 37 deg. API and the average GOR is 1,500 scf/STB. The oil composition includes a trace of H2S and a little more than 1 percent carbon dioxide. Water production from the field is negligible. Well productivity was 10,000 BOPD from the subsea wells and productivity was 10,000 BOPD from the subsea wells and has been tested in excess of 20,000 BOPD in several of the platform wells. This rate is limited primarily by fluid friction within the tubing string and flowlines. At the beginning of Phase 1, reservoir data were limited to log, core, and drillstem-production test data from the four exploratory wells drilled on the structure. Phase 1 Concept Phase 1 Concept Field development planning began for the Ekofisk field in 1970, before the completion and testing of the four exploratory wells on the structure. Preliminary planning required the installation of three steel-jacketed drilling and production platforms, plus auxiliary structures to be used production platforms, plus auxiliary structures to be used for processing, housing, and oil storage. The design, construction, and drilling phases of the development program were estimated to take at least 3 years. program were estimated to take at least 3 years. Phase 1 development plans were formulated to maximize use of the 3-year time lag between discovery and anticipated initial production from permanent facilities. JPT P. 513