Measurement of Rock Properties in Coal for Coal-Bed Methane Production

GASH, BRUCE W., Amoco Production Company, Tulsa, OK

Production of methane from coal seams has resulted in increased interest in the laboratory measurement of rock properties (porosity, permeability, and gas-water relative permeability) in coal. Unlike most other porous media, fluid flow in coal occurs only in naturally occurring microfractures or cleats. Standard techniques for rock property measurements are stretched to the limit when applied to coal. Since drying alters the cleat system in coal, standard porosity measurements cannot be made. Permeability measurements are hampered by coal fines migration. Unsteady-state gas-water relative permeability measurements are limited by the dead volume of the core holder because coal porosity is so low (approximately 1%). In steady-state measurements, NaI is sorbed on the coal surfaces.

Other studies in the literature have not addressed these unique limitations on the measurement of rock properties in coal. For this work a semiautomated instrument for measurement of permeability and unsteady-state gas-water relative permeability in coal has been built and modifications have been made to the standard JBN relative permeability calculations which address the limitation of core holder dead volume. With this instrument, the unsteady-state technique is accurate and reproducible, and it has been used to measure representative gas-water relative permeability curves on coal cores from the San Juan and Warrior basins with brine permeabilities as low as 0.1 md. The relative permeability curves for the San Juan and Warrior basins have been successfully used in simulation studies without modification. A semiautomated tracer test has been developed to determine porosity in coal. The tracer test has been used to show that within experimental error unsteady-state and steady-state gas-water relative permeability measurements in coal produce the same results. The tracer test further indicates that all water in the cleat system of a coal core is displaceable by gas.

AAPG Search and Discovery Article #91008©1991 AAPG Mid-Continent Section Meeting, Kansas Geological Society, Wichita Kansas, September 22-24, 1991 (2009)