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Oklahoma Geological Survey, Norman, OK

Abstract: Coal Geology And Underground-Mine Degasification Applied To Horizontal Drilling For Coal-Bed Methane

In eastern Oklahoma's Arkoma Basin in 1998 at least three energy companies drilled coal-bed methane wells 700-2,000 feet deep that eventually penetrated 700-800 feet horizontally into the 4-6-feet-thick Hartshorne coal, but also drilled through shale, mudstone, and interlaminated shale and sandstone. Those well segments in shale and mudstone may collapse, leading to well abandonment. Obviously coal will not be penetrated if the bit drifts into strata overlying or underlying the coal bed. Coal may be missing if the bit intersects a normal fault, a thick non-coal parting, or a channel-fill sandstone.

Coal geology studies, including coal characterization before drilling begins, should help in lease selection and hold down costs. Also data should be tabulated and maps constructed showing net coal-bed thickness, cleat frequency and orientation, coalbed structural contours, faults and secondary coal fractures, cleat-filling minerals, coal-rank isocarbs, inherent moisture, vitrinite reflectance, and lithology of strata overlying or underlying the coal bed. Most of these items, in addition to the laws of gas movement, affect or control the permeability and porosity of the coal-bed methane reservoir and the flow of gas to the well.

Twenty-three horizontal, open-hole, experimental boreholes, 300-2,200 feet long, were drilled by a coal company into the 4-feet-thick Hartshorne coal in an underground mine in Oklahoma in the middle 1970s, removing great quantities of 97 percent methane gas without hydraulic fracturing.

Therefore, detailed geological evaluation combined with information from the history of horizontal drilling to drain gas from coal beds in underground mines, should be applied to maximize success in coal-bed methane drilling, production, and profit.

AAPG Search and Discovery Article #90921©1999 AAPG Mid-Continent Section Meeting, Wichita, Kansas