Eight-Well Interference Test in the Anschutz Ranch East Field

Abstract

Summary In many new fields, it is necessary to plan for secondary recovery methods before the field is fully developed or extensive reservoir data have been obtained. Shortly after discovery of the Anschutz Ranch East field, the working-interest owners recognized that planning for an improved recovery program should begin as soon as possible. Knowledge regarding the degree of communication in the reservoir, the magnitude of any directional permeability trend, and the interwell reservoir properties would be needed to select a suitable reservoir management method. A large-scale interference test was chosen as a way to provide part of this information during the early stages of development. This paper describes how the test was planned and executed and includes some of the problems that were encountered during the test performance and interpretation. The results obtained from the test confirmed the geological interpretation of a directional formation flow capacity and indicated that the Nugget formation is at least partially anisotropic. Introduction The Anschutz Ranch East field, discovered in Dec. 1979, is one of the major recent hydrocarbon discoveries in the continental U.S. The field is about 20 miles [32 km] south of Evanston, WY, in the Overthrust Belt of Utah and Wyoming. Fig. 1 shows the location of this field and other fields in the area. The complexity of the field is shown on the structure map in Fig. 2 and by the northwest-southeast cross section, A-A', in Fig. 3. The field consists of two asymmetric anticlinal structures.1 The larger west structure is separated from the east structure by a vertical-to-overturned fold. The crest of the rollover probably contains some fracturing and hinge-line faults. The hydrocarbons are contained in the Nugget formation, which has a gross thickness of about 1,050 ft [320 m]. The Nugget is an eolian sand-dune deposit with a large amount of crossbedding. The reservoir rock properties vary vertically and horizontally. The vertical Nugget section can be divided into three layers of differing rock properties. These divisions are shown on the type log in Fig. 4. The three zones can be correlated across the entire reservoir. There is a considerable contrast in average zone properties, as shown in Table 1. The Nugget formation in this field is similar to other Nugget formations in the Overthrust Belt.2 The hydrocarbon fluid in the Nugget is a rich retrograde gas condensate. The initial reservoir pressure in the west structure was approximately 5,310 psia [36 611 kPa] at a datum of -5,324 ft [-1623 m] subsea, which was about 230 psia [1586 kPa] above the dewpoint pressure. Tabulated values of viscosity, µ, z-factor, z, and gas compressibility, cg, for the pressures encountered during the interference test are provided in Table 2. Ref. 3 discusses the west structure fluid properties further. Soon after discovery, it became apparent that a large portion of the hydrocarbons would condense and remain in the reservoir if the reservoir pressure were allowed to fall below the dewpoint. This would greatly reduce liquid recoveries unless some form of pressure-maintenance program was initiated. One important question that needed to be answered before such an expensive project was begun was the degree of interwell communication in this thick, crossbedded reservoir with complex geology. Moreover, little information was available relating to the effectiveness of communication in the reservoir. The completed wells were widely spaced, and very little production history was available. To answer this question, the working-interest owners agreed that a multiwell interference test should be conducted. The test objectives wereto establish whether communication existed between wells;to determine the magnitude of this communication;to determine whether an extensive fracture system existed that might adversely affect the injection program; andto obtain information concerning the interwell reservoir properties. Development History and Depletion Plan After the discovery of the field, the performance of Bountiful Livestock Well I was followed closely for about a year to determine whether a large hydrocarbon reservoir was present. Starting in 1981, development of the field was begun on 160-acre [647 500-m2] spacing to define the field limits and to obtain information about rock and fluid properties for use in depletion studies. When the interference test began in April 1982, there were eight wells completed, six wells in the process of completion, and seven wells being drilled in the west lobe.