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 has been assumed not to contribute to low porosity and low 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.
