Removing methane (degasification) from the Pittsbupgh coalbed in Northern West Virginia

W.M.Årritts ,W.N.Oundstone,and B.A.Iht

REMOVING METHANE (DEGASIFICATION) FROM THE PITTSBURGH COALBED IN NORTHERN WEST VIRGINIA

SUMMARY AND INTRODUCT ION

A jointinvestigation by the Bureau of Mines and the Christopher Coal Co. at the Humphrey No. 7 mine, operating in the Pittsburgh coalbed in northern West Virginia, was undertaken to gain more insight concerning the hazardous emission of methane gas in mining. This report describes the equipment, procedure, and results.

Horizontal bleeder holes drilledin the faces drained large quantities of methane, eliminating downtime of mining equipment. Also, larger quantities of methane were removed by forcing 14 to 25 g,.p.m. water at 25 to 200 p . s.i.5 pressure into the bleeder holes. In one infusion test a fluorescein dye was added to the infusion water; in later mining operations, the infusion water was traced for a distance of 1,500 feet . With vacuum pump applied to the horizontal hole the gas-flow rate increased, and the methane content decreased. Tests indicated that methane and water infused in bleeder holes traveled along the bedding planes of the Pittsburgh coalbed a tafasterrate than across the planes.

Methane associated with coalbeds is produced by the slow decomposition of carbonaceous material in the absence of ai.r and the presence of water. It is lighter than air and, depending on its concentration, forms an inflammable or explosive mixture with air. It is believed that methane is entrapped in and around the pores of the coal and adjacent strata - - often under great pressure. In many cases where the strata have been disturbed by geologic movements the gas has escaped from the coalbeds and is confined in pockets and void spaces in the overlying strata - usually under high pressures.

Gas emission during mining is neither regular nor continuous but varies with the rate of coal extraction and conditions existing within the strata. In general, more methane is liberated during rapid extraction of coal, possibly by opening new gas feeders into the working faces. In localities where methane existed under considerable pressure in crevices and voids in the strata, dangerous falls of roof and coal have occurred accompanied by large gas outbursts.

Methane emission into mine workings and subsequent ignition have caused many coal-mine explosions, resulting in loss of life and destruction of property. To minimize this hazard, elaborate ventilating systems are being employed todilute the methane at the working faces and remove the harmless mixture from the mine. In addition, permissible explosives and explosionproof mining equipment are being used to reduce further the possibility of igniting gas accumulations.

Before the advent of continuous mining, few American mines had gas emission problems that could not be solved by increasing the volume of ventilating air. In exceptionally gaseous mines, large coal areas were developed and allowed to drain for long periods before mining. The widespread use of continuous-mining machines has resulted in a more rapid rate of coal extraction from fewer working places, which, in turn, has resulted in an increased rate of methane liberation. Often, it is not possible to increase the volume and velocity of the ventilating air necessary to dilute the gas without creating another safety hazard - that of suspending dangerous concentrations of coal dust. In the severest cases, mining is stopped periodically so that the ventilating air can dilute the methane concentration. Thus, the inability to maintain a safe mine atmosphere can place a severe limitation on the highly productive machines.

A safe mine atmosphere could be maintained, however, by approaching the problem in another manner. Instead of allowing the methane to drain into the mine atmosphere during mining, it could be removed before mining, thus avoiding the necessity of large volumes of ventilating air .

Degasification, thee xtraction of methane from coal-bearing formations before and during mining, has been well received in the United Kingdom, Europe, and other major coal-producing countries. A survey of foreign liter - ature shows that the techniques of draining methane from coal-bearing formations has been thoroughly investigated and has proven to be both practical and economical. In a recent report,6 it was stated that 135 collieries removed more than 17 billion cubic feet of methane in 1958 for use as town gas, for automotive and gas-turbine fuel, and for steam generation. The various degasification techniques maybe classified generally as:

1. Cross-measure borehole method: Boreholes are drilled through overlying beds or into the floor strata from the working areas.

2. The Hirschbach method: Superjacent entriesare developed over the working areas.

3. Packed- cavity method: Drainage of methane i s controlled as mining progresses.

4 . Surface and underground borehole drainage.

5. Drainage from virgin coalbeds by collecting gas from pockets.

The cross-measure borehole method is the most widely used for removing methane from the coal formations. In this method, the gas is collected by free flow and vacuum pumping from boreholes and is piped to the surface, Analysis of the literature indicates that European degasification techniques would not be adaptable to existing natural conditions or mining methods used by the U.S. coal industry. Longwall mining is practiced extensively in Europe, while room-ando pillar mining predominates in the United States. In longwall mining the roof subsides as mining progresses, resulting abutment pressures at and ahead of the longwall fisce breaking and cracking the overlying strata. In room-and-pillar mining,, the roof is supported and generally remains undisturbed until the pillars are removed during retreat mining. The effect of the longwall-mining method on inethane liberation is that when the pressure on the longwall face is relieveld, much of the gas in the coalbed is liberated and is trapped in the overlyin,gstrata. In room-and-pillar mining the overburden pressure is not relieved and the gas enters the mine workings or remains in the coal pillars and immediate adjacent strata. Other differ - ences in mining conditions and practices such as the number beds worked simultaneously, interval between workable coalbeds, attitude of the beds, and the character of the adjacent strata have a marked effect on the success of degasification techniques.

Increased use of continuous-mining equipment and the encouraging results obtained by the degasification techniques being used in foreign countries prompted the Bureau of Mines to initiate laboratory and field studies to determine the fesibility of degasifying coalbeds before and during mining and to develop techniques applicable to U.S. mining conditions and practices. The laboratory phase of the work was to determine the sorption characteri stics of coal, while the field investigations were to study migration of methane in the coalbed, use of surface boreholes and long underground horizont aldrill holes in advance of mining, and water-infusion technique for degasification.

The cooperation and assistance of the late G. R. Spindler, director , and G. W. Campbell, oil and gas research engineer, School of Mines, West Virginia University.