Abstract

A program was undertaken by Terra Tek, Inc. of Salt Lake City, Utah, in cooperation with the Tiffany Gas Company of Farmington, New Mexico, to evaluate the recovery of hydrocarbon gas from the Fruitland Formation coal within the San Juan Basin in Southwest Colorado. The target coal was distributed through six seams at depths between 3062 and 3112 feet; and net coal thickness was 25 feet.

Well tests were performed to determine total permeability for the fully saturated reservoir, permeability for the fully saturated reservoir, skin factor and initial reservoir pressure. Core tests on coal samples determined the original adsorbed gas content, the adsorption isotherms, porosity, capillary pressure, and relative porosity, capillary pressure, and relative permeabilities. These parameters were used as input permeabilities. These parameters were used as input data to a computer code to history match the water/gas production for both the underreamed and stimulated well. Predicted productions were used in economic analysis. After the well was underreamed, gas production was uneconomic. Following hydraulic fracture stimulation, economic gas flow was achieved.

Introduction

Coalbed gas is generated during the coalification process. Methane is the primary component and generally comprises 85 to 99 percent of the gas by volume. Its calorific value is between 850 and 1050 BTU/scf. Between 398 and 850 tcf of such hydrocarbon gas is estimated to exist in coalbeds within the contiguous United States. Based on coal resource distribution in the deeper structural basins, between 78 and 186 tcf, lie in deeply buried coalbeds at depths in excess of 3000 feet. There is limited experience in the recovery of gas from deeply buried coals and, consequently, no public information is available on production economics. The Gas Research Institute (GRI) sponsored a project to generate data on a deeply buried coal reservoir to:

  1. Establish reservoir characteristics,

  2. Evaluate completion methods, and

  3. Determine production economics.

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