Abstract

The Gavilan Mancos Pool, located in the San Juan Basin of New Mexico, produces from the Niobrara siltstone interval within the Mancos Shale. This type of reservoir in this area has previously been described as a fractured shale in which the matrix previously been described as a fractured shale in which the matrix has been assumed not to contribute to low porosity, and low permeability. This paper discusses the dual porosity character of permeability. This paper discusses the dual porosity character of the formation and shows the method in which the dual porosity nature of the reservoir was identified. The identification process reflects an integration of all available information including geologic description, core analysis results, pressure buildup analysis and a review of performance history. The presence of capillary pressure effects give rise to inverse rate sensitivity behavior in which the gas-oil ratio declines at higher withdrawal rates. Maintaining maximum withdrawal rates in this reservoir increases ultimate recovery by one-half percent of oil in place. which is an increase of approximately 4 percent of recoverable oil.

Introduction

The Gavilan Mancos Pool is located along the eastern edge of the San Juan Basin in Rio Arriba County, New Mexico (Figure 1 ). The field is concentrated in T25N, R2W, although it extends partially into T24N and T26N, RlW. The Pool boundaries are partially into T24N and T26N, RlW. The Pool boundaries are formed on the east by the intersection with the West Puerto Chiquito Pool and on the west with the West Lindrith and NW Ojlto and Bear Canyon Pools. Mancos reservoir quality deteriorates to the south but extends to the north where the Bear Canyon Unit is located (Figure 2). While multiple pay zones exist, production is predominantly from the Niobrara member of the production is predominantly from the Niobrara member of the Cretaceous, Mancos Shale at a depth of approximately 7,000 feet. The structural configuration of the pool is that of a low relief dome (dips generally less than 2 degrees) separated by a syncline from the steeply dipping monocline which exists on the eastern side of West Puerto Chiquito Field (dips of as much as 60 degrees). The Niobrara Member of the Mancos Shale consists of calcareous marine shale and thin, hard quartzitic siltstone beds which appear as the zones of higher resistivity on the electric logs (Figure 3). The Niobrara interval is subdivided into the "A", "B" and "C" zones. In the Gavilan Pool, production is derived primarily from the "A" and "B" zones, while in the offsetting West Puerto Chiquito Pool, production in the south part of the pool is from the "C" zone Pool, production in the south part of the pool is from the "C" zone with production in the north part from the "A" and "B".' in the western part of West Puerto Chiquito, production surveys have shown a varying contribution from all three zones. Previous authors' have attributed the source of oil production to be primarily, if not entirely, from fracture permeability, although in primarily, if not entirely, from fracture permeability, although in the West Puerto Chiquito Field, the interpretation is of a reservoir comprised of individual fracture blocks of low permeability joined and interconnected to a high-capacity fracture system.

The Gavilan Mancos Pool, which contains approximately 47,200 acres was discovered in June 1982, with the completion of the Gavilan 1 in Sec. 26, T25N, R2W for an initial rate of 62 BOPD. Additional development was slow until the second half of 1983, when an active drilling program began boosting production to a maximum rate of 6,300 BOPD in June 1986, from 48 wells. It was noted early in the study that wells in the western two tiers of sections of the West Puerto Chiquito Field exhibited a high degree of communication with wells in the Gavilan Mancos Pool. Consequently, future reference to the Gavilan Mancos Area refers to both the Gavilan Mancos Pool proper, as well as that portion of West Puerto Chiquito, which is in communication with Gavilan. The composite oil production and gas-oil ratio history of the common producing area is shown in Figure 4. In September 1986, production rates were restricted by the New Mexico Oil production rates were restricted by the New Mexico Oil Conservation Division (OCD), which limited the top allowable production rate to 400 BOPD and 240 Mcfd as compared to the production rate to 400 BOPD and 240 Mcfd as compared to the statewide allowable rate of 702 BOPD and 1404 Mcfd for a well on 320 acre spacing. These allowable restrictions were lifted for the period between July and October 1987 and then reimposed. period between July and October 1987 and then reimposed. Production from the offsetting West Puerto Chiquito Field is Production from the offsetting West Puerto Chiquito Field is generally by primarily depletion, although the steeply dipping portion of the pool, designated as the Canado Ojitos Unit, is portion of the pool, designated as the Canado Ojitos Unit, is producing by gas pressure maintenance. producing by gas pressure maintenance. Wells in Gavilan are completed throughout the Mancos interval, which approaches 600 feet in thickness. However, the highest concentration of perforations are located across the Niobrara "A", "B", and "C" zones. Production logs have shown these to be the source for more than 95 percent of the production in both Gavilan and West Puerto Chiquito. (Perforation selection is typically made by overlaying mud logs, induction logs, and Density-Neutron logs and choosing sections logging oil shows, high resistivity and/or showing fracture indications from various interpretive methods.)

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