Field and Laboratory Experience in Stimulating Ekofisk Area North Sea Chalk Reservoirs

Abstract

This paper describes field and laboratory work carried out recently to better understand stimulation behavior in Ekofisk Area North Sea chalks. Laboratory work has been done to study chalk hardness, chalk mechanical properties, and the development and stability of propped and acid fracture conductivity. The laboratory work, combined with a review of available stimulation techniques for these formations, shows that stimulations in most of these reservoirs can be optimized with a pseudo limited entry acid fracture technique. Cased hole logging, pressure transient tests, and other field data have been utilized to optimize the design of these stimulations. The best treatments have resulted when several widely spaced clusters of perforations are treated in stages using ball sealers for diversion. Introduction The Greater Ekofisk Area (GEA) chalk fields are located in the Norwegian Sector of the North Sea (Figure 1), in 250 feet of water 180 miles Southwest of Stavanger,, Norway and comprise four oil fields (Edda, Ekofisk, Eldfisk, and Tor) and two gas condensate reservoirs (Albuskjell and West Ekofisk). The discovery well in the area was drilled on the Ekofisk structure in 1969. The Ekofisk field was the first giant oil field discovered in Western Europe and the first to produce significantly from a North Sea chalk formation. Cumulative production through 1987 from the six fields was 1.43 BSTB of oil and 5.5 TSCF of gas. Ultimate recovery is estimated to be 2.5 BSTB and 8.6 TSCF. Since the Ekofisk discovery, several studies related to North sea chalk completions have been published. The most extensive study was a five year EEC sponsored project conducted by Shell. A North Sea chalk research group sponsored a similar study by Elf. In addition to these essentially theoretical studies, Amoco has published their work on the Valhall and Hod fields, which are located slightly south of the Ekofisk fields. In general, these studies have indicated that these formations are difficult stimulation targets for any of the following reasons:The massive, uniform nature of the chalk sections result in short, radial fracturegrowth.The low hardness and ductile behavior of the chalk results in pore collapse and creep causing excessive proppant embedment.The homogeneous nature of the chalk results in evenly etched fracture faces during acid fracture stimulation.The low hardness and low yield strength of the chalk can lead to healing of natural fractures and failure of any acid-etched grooves.The chalks lose mechanical strength when contacted by foreign fluids, causing stimulation with water based fluids to actually damage, and not stimulate, the formation. Very little actual field data related to North Sea chalk stimulation has been presented in the literature. This paper reports some of the field and laboratory experience which Phillips has had with the Ekofisk Area chalks.

Intelligent Completions and Un-Cemented Liners Combine to Provide a Fully Completed Solution with Zonal Isolation in Norway

“…The wells were then stimulated with acid by bullheading alternating stages of pad and acid with ball sealers for diversion. These cluster perforation designs typically showed high initial productivity followed by sharp production declines (Snow and Hough 1988). Production logging results indicated that many of the perforation clusters were not effectively stimulated, with some clusters, typically at the heel of the well, receiving far more acid than required and others receiving little or no acid.…”

Lessons Learned from 27 Years' Experience of Stimulation Vessel Design and Operation - a Case Study

“…The most active has always been the southern Norwegian and Danish sectors, focused on the Valhall, Ekofisk, Dan and Tor chalks (Mancillas et al, 1976, Snow and Hough, 1988, Phillips Petroleum Co., 1995and Martin, 1996. These formations vary widely in depth, temperature, permeability, net height and mechanical properties, producing a corresponding variation in treatment design.…”