ABSTRACT

The Anschutz Ranch East field is the largest hydrocarbon accumulation discovered to date in the Western Overthrust Belt. Analyses of reservoir fluid samples from the discovery well indicated that the fluid is a rich gas condensate with liquid dropout under reservoir conditions as high as 40 percent of the hydrocarbon pore volume. The proximity of the fluid dew point pressure to the original reservoir pressure dictated that a plan-of-depletion be established very early in the life of the field to avoid reservoir damage and the potential loss of significant liquid reserves. This requirement was further amplified by the fact that some of the depletion alternatives involved major front-end monetary commitments and long lead times for implementation.

Using preliminary reservoir data together with compositional simulators, various depletion alternatives ranging from primary depletion to full pressure maintenance were evaluated. Variables examined in the study included such factors as type of injection fluid and injection-production well configuration. Full pressure maintenance using nitrogen preceded by a hydrocarbon-nitrogen gas buffer in an inverted nine-spot pattern was selected as the most attractive depletion method. This plan was approved by the working interest owners and state regulatory agencies and is currently being implemented.

INTRODUCTION

Anschutz Ranch East is located in the Western Overthrust Belt on the Wyoming-Utah border as shown in Figure 1. Discovered in December 1979 with the drilling of Amoco's Bountiful Livestock No. 1 (unit well W16-14), the field is one of the most significant finds in the United States in recent years. A year after completion, the discovery well was still producing at its initial potential rate of 1,054 BCPD (168 m³/d) and 4 053 MscfD (114.8×10³ m³/d). The reservoir pressure decline of less than one-half of one percent (25 psi, 172 kPa) associated with the well's cumulative production of 381 Mbbl (60.6×10³ m³) of condensate and 1.69 Bscf (47.9×10⁶ m³) of gas indicated a reservoir of significant size. Subsequent data has led to estimated original in-place hydrocarbons in excess of 800 million barrels (127.2×10⁶ m³) of oil equivalent.

Preliminary PVT analyses showed that the reservoir fluid is a rich gas condensate with a dew point pressure only 150-300 psi (1 034 - 2 068 kPa) below the original reservoir pressure. It was obvious that a detailed evaluation of this giant reservoir would be required in order to optimize a plan-of-depletion such that both recovery and profitability could be maximized. The following discussion describes the method by which such a plan-of-depletion was developed.

The approach presented is not limited in applicability to Anschutz Ranch East or other fields of comparable size. In fact, the concepts applied in this study nay be used to evaluate all rich gas condensate reservoirs where various depletion alternatives are available.

RESERVOIR CHARACTERISTICS

Geology and Rock Properties

Anschutz Ranch East is comprised of two hydrocarbon accumulations in tightly folded and overturned anticlinal structures.¹ Figures 2 and 3 show the current structural interpretation in plan and cross-sectional views, respectively. The reservoir characteristics and plan-of-depletion study discussed in this paper pertain to the larger and more developed west structure. The optimum depletion plan for the smaller and less developed eastern feature is still being investigated.

Geology and Rock Properties

Anschutz Ranch East is comprised of two hydrocarbon accumulations in tightly folded and overturned anticlinal structures.¹ Figures 2 and 3 show the current structural interpretation in plan and cross-sectional views, respectively. The reservoir characteristics and plan-of-depletion study discussed in this paper pertain to the larger and more developed west structure. The optimum depletion plan for the smaller and less developed eastern feature is still being investigated.