Case Study: Observations of a Coal Bed Methane Extraction Pilot Program via Well Bores in Greene County, Pennsylvania
Authors: Reese R. , Reilly J. 
Published: December 2007

This study measured the effects of stimulation treatment on pre-mining methane removal and potential roof damage to longwall sections at the Cumberland Mine. Measurement criteria included the rate and extent of methane removal correlated with mine productivity and the extent of roof damage caused by the stimulation treatment. The study consisted of two vertical hydraulically stimulated wells and five horizontal wells.

Introduction

A pilot program was initiated at the Cyprus Amax Coal, Cyprus Cumberland Resources, Inc., Cumberland Mine to test the effectiveness of coal bed methane extraction. Two studies were completed in the 1970's on Pittsburgh coal mines in Greene County Pennsylvania prior to the introduction of long wall mining. The small stimulation treatments (29,000 gal of 75% quality foam and 14,000 lbm. of sand typical) used on the boreholes proved the feasibility of using hydraulic stimulation treatments on coal to stimulate production of methane. Subsequent minethroughs of the fractured areas showed that the areas surrounding a hydraulic stimulation treatment could be successfully mined with a continuous mining machine. The purpose to this study was to measure the effects of stimulation treatments and horizontal boreholes on pre-mining methane removal in the long wall sections at the Cumberland Mine.

Study Area and Geologic Situation

Cyprus Cumberland Resources is located in Southwestern Pennsylvania in the Northern Appalachian Coal Basin (Fig. 1, 2). Located on the eastern flank of the Bellevernon Anticline, the mine typically dips to the southeast at 1% to 2%. The average depth of cover for the mine is 750 ft.

The Pittsburgh coal, contiguous over parts of the states of Pennsylvania, Ohio and West Virginia is part of the Pennsylvanian system. It forms the base of the Monongahela Group (Fig. 3). The main coalbed thickness in the pilot study area ranged from 7.2 to 7.5 ft. The immediate 8-ft of roof (rocks overlying the main coal bed) consists of interbedded shales coals and clay stone, some of which are highly fractured or unconsolidated. It was extremely important to note the effect of any stimulation treatment on the roof due to the safety hazards a damaged roof could pose to mine personnel.

The coal contains a natural bi-directional fracture system called cleats. The predominant cleat, the face cleat, has an azimuthal direction of 290 degrees while the secondary cleat direction, the butt cleat, is normal to the face cleat. The face cleat is the longest cleat with the butt cleat literally terminating against the face cleat. Hydraulic stimulation in coal strives to force the face cleat apart and prop it open to allow a channel for gas flow. There was some question as to whether a stimulation treatment would be effective at Cumberland due to the heavy mineralization of the cleats. Secondary calcite up to one-quarter inch thick is frequently found in the face cleat along with secondary deposits of sheet pyrite.

Mine Construction, Considerations and Criteria

Cumberland Mine is a one-unit longwall mine with three ancillary development sections. Development sections mine the coal that forms the tunnels that access the longwall block of coal. Three parallel tunnels (entries) spaced 100 ft apart are developed on each side of the longwall panel parallel to each panel's long axis. During mining of the entries, methane is released from the freshly cut coal face and accumulates in the entry designated to remove methane laden air, the return entry. Methane in amounts greater than 1% of the air volume is not allowed by federal and state law (methane is explosive in concentrations of 5% to 15%.). When methane levels exceed 1%, mining must cease until the levels drop. P. 139^