Improved Procedures for Collection and Interpretation of Coal-Gas Reservoir Gas Content and Natural Fracture Geometry Data

Abstract

The validity of quantitative coal-gas reserve analyses depends upon the accuracy of formation evaluation data used as input parameters for reservoir simulation of fluid productivity. Coal-gas reservoir deliverability is strongly influenced by gas-in-place resources, natural fracture (cleat) permeability, and gas desorption and diffusion characteristics. Estimates of these parameters are predicted upon knowledge of coal core-gas content and cleat geometry. Ironically many accepted procedures for collecting and interpreting these data are still incomplete, and therefore coal-gas reserve analyses commonly are erroneous. Improved procedures for collecting and interpreting gas content and cleat geometry data are highlighted using eight Gas Research Institute Fruitland Formation (Upper Cretaceous) wells in the San Juan basin (Colorado and New Mexico). Fruitland gas content values, in places, have been underestimated by 30-85% (786 SCF ton vs. 122 to 557 SCF/ton, dry, mineral-matter-free basis), and therefore the corresponding gas-in-place resource estimates are commensurably conservative. Gas desorption measurements must be performed at reservoir temperature: dry, mineral-matter-free normalizations and canister headspace corrections must be performed correctly; and gas composition must be periodically quantified to correctly estimate the actual methane reserves and sorption time. Collection of cleat geometry data on whole cores is routinely biased because only the most apparent cleat systemsmore » are analyzed. Detailed macroscopic and X-ray radiograph data on cleat geometry for slabbed polished blocks should be collected as a function of bed thickness, lithology, and interconnectivity to further define core-scale cleat permeability and diffusion coefficient distributions.« less