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.
fax 01-972-952-9435. AbstractThis case history reviews the systematic well performance monitoring procedures developed and implemented for the Eldfisk and Ekofisk Fields in the Norwegian North Sea. Examples of production optimization projects implemented as a result of the performance monitoring system are presented. Changes in well performance due to acid washes, acid fracture restimulations, perforation additions and tubing changeouts are illustrated in the examples presented.
This paper presents the various techniques and measurements undertaken to determine both the magnitude and direction of horizontal stress anisotropy present in the Eldfisk Field in preparation for a planned waterflood. Results from field measurements of in situ stress obtained from a vertical pilot hole designed specifically for gathering stress data reveal a highly an isotropic state of stress, with a maximum horizontal in situ stress oriented approximately 47 from grid North. These measurements include stress orientations measured from an elastic strain recovery (ASR) tests on core, oriented acoustic log data and 2 micro-frac tests with pre- and post fracture imaging log runs. Micro-fracture tests yield values of 5218 psi to 5326 psi for the minimum horizontal stress and 6904 psi to 7154 psi for the maximum horizontal stress. This represents a relatively large stress anisotropy which, in turn, could result in a sizeable permeability anisotropy. A numerical simulation study was conducted to investigate the impact of horizontal permeability an isotropy and completion strategy on hydrocarbon recovery. The results of this sensitivity study show that the optimum completion strategy consists of suitably staggered perforation schemes in the injection and producing wells. P. 49
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