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ABSTRACT: An Integrated Study of a Fractured, Low-Permeability Gas Sand

Gas production from fractured, low permeability reservoirs is a significant component of our natural resources. Many of these reservoirs are far beyond initial development and have undergone previous infill drilling programs. Of primary concern today is how to optimize the well spacing. This relies on an understanding of the controlling feature; i.e., fractures and/or reservoir pay quality. This work attempts to discern the contributions between the two possibilities.

The target reservoir of this study is the well-recognized and prolific Dakota Formation in the San Juan Basin, New Mexico. The Dakota Sandstone consists of a complex set of fluvial and nearshore marine sandstones found at an average depth of 7000 ft. and is approximately 300 ft. in gross thickness. The members in order of increasing depth are the Twowells, Paguate, and Cubero.

The approach was to perform detailed reservoir characterization and then use simulation to model the reservoir behavior. Integration of core, log, production and pressure data provided the basis of the analysis. To distinguish the feature controlling production, both pay quality and fracture trends were determined from an exhaustive data collection. Development of distributions for shale volume, porosity, and water saturation resulted in cutoff values specific to the Dakota and improved the determination of net pay thickness. Fracture identification was either direct (core) or inferred (logs) and then correlated to provide fracture trends. Two examples from the Dakota study illustrate a fracturedominated area trending northeast/southwest, and a thickness-dominated region trending northwest/ southeast.

AAPG Search and Discovery Article #90915©2000 AAPG Rocky Mountain Section, Albuquerque, New Mexico