USE OF VERTICAL BOREHOLES FOR ASSISTING 

VENTILATION OF LONGWALL GOB AREAS 

C. H. Elder 

Bureau of Mines Methane Control Program 

Technical Progress Report - 13 

May 1969 

U. S . DEPARTMENT OF THE INTERIOR 
CONTENTS 

Page 

Abstract. ................................................................ Introduction ............................................................. Geology .................................................................. Procedure. ............................................................... Vertical borehole completion ........................................ Monitoring gas production ........................................... Evaluation......... ...................................................... 

ILLUSTRATIONS 

1. Ventilation borehole completion plan, experimental hole drilled 

in 2 left D west section, Mine 33, Cambria Division, Bethlehem 

Mines Corporation ................................................ 2. Location of ventilation borehole in longwall panel and progress 

mining ........................................................... 

TABLE 

1. Gas production from the vertical drainage borehole................. 
USE OF VERTICAL BOREHOLES FOR ASSISTING 

VENTILATION OF LONGWALL GOB AREAS 

An experimental degasification program using a vertical borehole and 

vacuum pump to drain gas from the gob area of a longwall panel was successful 

at Bethlehem Mines Corporation, Cambria Division, No. 33 coal mine. Sixty-one 

million cubic feet of methane has been exhausted during a 9-month period. 

Daily production time in the panel increased as a result of lower methane levels in the returns. Removal of large quantities of methane in this way and 

the exclusion of it from the ventilating system will provide reduced cost in 

mine ventilation and provide a safer environment for the mine. 

INTRODUCTION 1 

A comprehensive methane control research program is being conducted by I 

the Bureau of Mines to establish the geological structural controls that govern the migration and retention of methane in coalbeds; determine the physical 

properties of methane and of the coal and other strata with which methane is 

associated; devise methods by which methane may be removed from, or controlled 

in, coalbeds prior to and during coal extraction; develop mechanical equipment 

and instrumentation needed to control methane; and conduct field trials of the 

methane control techniques developed. This is one of a series of reports on 

specific aspects of the program. 

Cooperative studies, conducted either as part of a formal agreement or 

informally arranged by letter, offer opportunities for expediting investigation that would be unduly delayed or prohibitively costly if done independently. This work is typical of those cooperative investigations where benefits accrue to industry immediately. 

An experiment in degasifying a longwall coal mining panel and the gob 

area resulting therefrom was implemented at the Bethlehem Mines Corporation, 

Cambria Division, No. 33 mine in Cambria County, Pa. The prime objectives 

were to reduce gas emissions from gob areas resulting from longwall mining, 

and to gain knowledge of the nature of gas transmission and emission 

IGeologist, Pittsburgh Mining Research Center, Bureau of Mines, Pittsburgh, 

Pa. 
characteristics in the Kittanning coals and related rock strata. As the longwall system became more perfected and shift tonnage continued to increase, it 

became apparent that mining would have to be limited to two 8-hour shifts 

because the third shift would be used to ventilate the area to keep the gas 

content in the returns at a safe concentration. As the longwall approached 

the vertical borehole it was necessary, on occasion, to stop mining during a 

shift to ventilate and reduce the gas to a proper level in the returns. 

GEOLOGY 

The Cambria 32 and 33 mines, mining Lower Kittanning (B) coal, sit on the 

crest and down the east limb of the Ebensburg anticline into the Wilmore syncline to the east. The structures have a general northeast-southwest trend 

and substantial concentrations of methane are encountered in mining. 

The Allegheny Formation of the Pennsylvanian Age, which forms the immediate overlying strata, consists of carbonaceous shales, argillaceous carbonaceous sandstones, coals, and some thin argillaceous limestones. It is known 

that methane is usually present in the Upper Freeport (E), Lower Freeport (D), 

Upper Kittanning (C'), and Middle Kittanning (C) coalbeds and in the contiguous carbonaceous shales and sandstones. 

Slow emission of methane from the two vertical drill holes and the nature 

of samples collected and studied during drilling of the holes indicate formations of very low permeability and high porosity. A great number of shales 

with up to 1,000 feet of overburden have an average measured porosity of 

approximately 30 percent. Shales are known to have porosities ranging from 24 

to 38 percent. 

In the course of longwall mining fracturing over the face greatly 

increased the permeability of the rock strata, providing an easy escapeway for 

the methane in the overburden into the ventilation returns of the mine. 

PROCEDURE 

Vertical Borehole Com~letion 

The drainage hole was drilled 594 feet deep through the B coal with an 

8-3/4-inch-diameter rotary air drill. It was reamed to 12 inches in diameter 

567 feet deep. Eight-inch casing with the bottom 48 feet slot-perforated was 

cemented into the hole, leaving 29 feet of uncased hole and the 48 feet of 

ungrouted slotted casing. The hole was located 675 feet in advance of the 

face (figs. 1 and 2) . 

The 77 feet of ungrouted hole provided free drainage of methane from the 

exposed formations in the borehole, and from fractures intercepted by the borehole when the overburden was collapsed. Because of the low permeability of 

the unfractured formation, a natural flow of only 400 cubic feet of gas per 

day (cfd) was measured on completion of the hole. 
Formation tops and casing details ore 

shown at depth below surface 

I ,Top of 8"diam 

14' diam X 13' long 

A 

4 -Ring 11.5" diam w/ flange 

516' 3 t I 

Ring 11.5"diam w/flange , &la' ,; / 

3' UI 4 

5 I . -Top of slotted pipe / 

/ 

; I , -- Slots in pipe ore 12" long / 

, ' 2" wide and on 5' intervals 

; I I 

/ I 

; I I I 

, 24' 

/ 

567' ',; 5' p Bottom of slotted pipe 

I / 

/ and I%" coal / 

, i 

No scale 

FIGURE 1. - Ventilation Borehole Completion Plan, Experimental Hole Drilled in 2 Left D 

West Section, Mine 33, Cambria Division, Bethlehem Mines Corporation. 

During the drilling of a nearby borehole, gas was observed bleeding from 

the cuttings while drilling the carbonaceous shales and sandstones overlying 

the coalbed. Gas was encountered in the borehole from the Lower Freeport (D) 

coalbed down through the B coalbed. 

Monitoring Gas Production 

An explosion-proof vacuum pump with a capacity of 170 cubic feet per minute (cfm) was installed on the borehole. Pumping was intermittent with negligible methane flow, until the longwall face intersected this drainage hole, 

and then a sustained flow was established. Flame arrestors and protective 

check valves were installed for safety. The borehole was monitored regularly 
FIGURE 2. - Location of Ventilation Borehole in Longwall Panel 

and Progress Mining. 

deveoped a atu

ral flow of 

215,000 cfd of 

methane, indicating that the lowvolume pump was 

actually inhibiting the flow. 

A larger 

vacuum pump, 700 

cfm, was installed 

and an average 

methane product ion 

of 335,000 cfd was 

evacuated during 

the next 4 months. 

The exhaust averaged 34 percent 

me thane. 

2Reference to specific brands is made for identification only and does not 

imply endorsement by the Bureau of Mines. 
TABLE 1. - Gas production from the vertical drainage borehole 

vacuum 

June 18, 1968 

Dec. 26, 1968 Dec. 30, 

19681 - 

July 23, 1968 

NOV. 7, 1968 

July 20, 1968 170 cfm 

vacuum 

Nov. 6, 1968 

Nov. 14, 1968 

Dec. 30, 1968 

700 cfm 

vacuum 

Pump . 

- 

Feb. 25, 1969 

Mar. 13, 1969 

Cubic 

feet of 

me thane, 

average 

dai ly 

production 

172,000 

Zubic feet 

of 

methane, 

total 

production 

Feb. 25, 1969 

Natural 

flow 

Methane 

in 

exhaust, 

percent 

1,100 cf 

Mar. 13, 1969 

Mar. 17, 1969 

- Open hole, 

8 - inch 

casing. 

1,100 cf 

1,100 cf 

- Open hole, 

8- inch 

casing. 

320,000 

- Open hole, 

8- inch 

casing. 

Cubic 

feet of 

methane, 

total 

product - t ion 

Methane, 

percent 

- 

Cubic 

feet of 

methane : 

average 

daily 

product ion -- 

215,000 

140,000 

57,000 

(24-hou I 

basis) 

-- 

Com

Intermittent 

Hole pumped t

cury vacuum. 

Pumping 24 ho

with sustain

Installing la

Pump. 

Pumping 24 ho

Installing bl

Pumping 24 ho

Longwall pane

Nov. 23, 19

Longwall pane

tinuous high 

Drainage hole 

Intermittent 

natural flo

shut-in per

Intermittent 

Pumping 24 h
A 5-horsepower, 1,100-cfm blower was installed on the borehole. This 

blower has evacuated 220,000 to 320,000 cfd of methane. The exhaust was 24 to 

30 percent methane. 

The borehole during its 9-month production period has exhausted 61 million cubic feet of methane from the gob area of the completed longwall panel, 

thus eliminating from the ventilation returns that amount of methane. 

EVALUATION 

Drainage of the gob through vertical boreholes has resulted in a substantial reduction in methane content of the ventilation air. Monitoring of the 

returns off the longwall panel showed a reduction of approximately 1 percent 

methane in the main air returns ventilating the panel. 

This reduction has made possible a mining period to be three 8-hour 

shifts per day. Thelongermining period could increase production by several 

hundred tons through longer use of high-cost, rapid mining equipment. 

An additional 35 to 40 million cubic feet of air would be necessary to 

dilute the volume of methane exhausted each day through one vertical borehole. 

The use of the vertical borehole yields savings on fan power costs and, if 

enough holes are utilized, the need for large additional ventilation shafts 

will be reduced substantially. 

The vertical borehole has proved so beneficial that six additional holes 

are being drilled for use in draining methane from the gob areas of all active 

longwall panels. A second vertical borehole, which was completed with an additional 30 feet of overburden, has been exposed to open flow. A more powerful 

blower at the hole has produced, during 2 months of service, an average daily 

production of 1 million cubic feet of methane. The exhaust averaged 88 percent methane.