Pittsburgh, PA: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication No. 2008-114 (IC 9502), 2008 Mar; :1-83
Although longwall mining productivity can far exceed that of room-and-pillar mining, the total methane emissions per extracted volume associated with longwall sections are generally higher than those for continuous miner or pillar removal sections. Increased face advance rates, increased productivities, increased panel sizes, and more extensive gate road developments have challenged existing designs for controlling methane on longwalls. Methane control research by the National Institute for Occupational Safety and Health (NIOSH) recently examined a number of practices designed to maintain concentrations in mine air within statutory limits and consistently below the lower explosive limit. These included a reservoir modeling approach to predict methane inflows in gate road entries. The outputs suggested that emission rates in the gate roads decreased with the use of shielding boreholes and increased degasification time. Also, mining perpendicular to the face cleats liberated more gas into the mine workings, emissions were almost a linear function of Langmuir pressure and volume, and emissions were inversely related to sorption time constant. Subsequent simulations predicted changes in methane drainage using in-seam boreholes. The results showed that longer degasification times resulted in lower face emission rates. Premining degasification produced more methane than that produced during panel extraction, a fact attributed to the already decreased methane content of the coalbed. This work concluded that longer premining degasification periods would be more advantageous to the operator. The industry trend toward increasing longwall face width can produce increased methane emissions from the face due to a higher volume of cut coal on the face conveyor. Two methods were used to estimate face methane levels on longer faces. In the first method, segmented methane data were extrapolated for greater face widths. The second method estimated emissions contributions from the shearer, face conveyor, panel belt, longwall face, and ribs, and summed these for wider panels. Permeability changes in the gob behind the longwall shields were estimated using a NIOSH-developed numerical model. The results showed that permeability was highest near the edges and corners of the gob. Gob gas ventholes (GGVs) represent a very effective means for controlling methane gas on longwall faces. NIOSH work showed that (1) larger-diameter GGVs produced more methane, but at the expense of increased dilution due to the presence of mine ventilation air in the boreholes; (2) GGVs should not be sunk into the caved zone of a longwall panel since more ventilation airflow will be sent to the boreholes; and (3) increased slotted casing lengths improved GGV production. NIOSH conducted an extensive borehole monitoring experiment (BME) to assess the impacts of longwall mining on development of the coalbed reservoir. Three boreholes were drilled to different stratigraphic horizons in advance of undermining by a longwall and instrumented to measure pre- and postmining in situ permeabilities and gas pressures. The data showed that mining-induced disturbances occurred 20-46 m (80-150 ft) ahead of the retreating longwall face, causing a corresponding increase in formation permeability. In this report, several practical guidelines are recommended for controlling longwall coalbed methane. All predictions are based on determinations made for the Pittsburgh Coalbed in southwestern Pennsylvania. 1. It is recommended to use shielding degasification boreholes to decrease emission rates by at least 25% for development entries. Drill these boreholes as close as practically possible (~27 m (90 ft)) to the entries and operate them for at least 6 months to achieve a 25%-50% decrease in emission rates (Section 1). 2. Equations for predicting methane emissions rates into gate road developments are presented in Table 3 (Section 1). These relationships assume a supercritical longwall panel developed in the Pittsburgh Coalbed. 3. For a longwall panel width greater than 305 m (1,000 ft), a trilateral borehole configuration is recommended for effective draining of methane. Short across-panel holes at close spacing can also be effective in degassing a panel. The required number of holes will depend on seam anisotropic reservoir conditions, but at least 12 holes for a 3,400-m (11,000-ft) long panel are recommended to achieve the same degasification as the trilateral configuration (Section 2). If less than 12 months are available for premining gas drainage, it is recommended that degasification be continued until the borehole is approached by mining. This approach maximizes the quantity of removed methane and reduces methane emission rates (Section 2). To avoid shearer coal production delays, it is recommended that continuous GGV production be assured while GGVs are within about 500 ft of the working face. In many mines, the quantity of coalbed methane removed by a GGV is potentially 75% of the volume of gas emissions on the longwall face. A similar finding was observed by prior NIOSH research. If an operating GGV ceases producing gas, the gas that was being removed will enter the ventilation system (Section 3). Assuming a well-caved gob, increasing the longwall face length by X% will increase the rate of methane emissions by, at most, two-thirds of X% (Section 4). The length of the slotted casing section of a GGV will strongly influence its level of gas production. To effectively design the slotted casing section of a GGV, it is recommended to: a) Review the local geology to identify the location of gas-bearing units; and b) Set the top of the slotted section at the highest gas-producing stratigraphic horizon (Section 6). Completing a GGV into the caved zone is counterproductive and increases the likelihood of intermittent production from increased-width, supercritical panels. Therefore, the completion depth of GGVs should be at least 14 m (45 ft) above the top of coal for longwall panels, particularly in the Northern Appalachian Basin (Sections 6 and 7). Emphasize continuous GGV production since it will potentially produce 40%-50% more coalbed gas than GGVs operating intermittently (Section 7). Increasing the longwall panel width increases the quantity of methane present because of the increased fractured reservoir volume. However, this increase does not enhance the performance of GGVs. As panel width increases, the effectiveness of GGVs completed near the tailgate margin will not extend as close to the headgate side. Drilling GGVs on the headgate side or near the panel centerline can produce coalbed gas from this portion of the panel (Sections 5 and 7). In favorable cases, GGVs drilled on the headgate side can be effective. Completion depths must isolate the borehole from communication with the ventilation network. These findings are based on supercritical panel designs (Section 7). Mining-induced fracturing was observed to occur 24-46 m (80-150 ft) ahead of the mine face. Boreholes and exhausters should be installed before this occurs (Section 8). Data were reviewed for GGV configurations completed from 7 to 32 m (24 to 106 ft) above the mined coalbed for supercritical panels in the Northern Appalachian Basin. It is recommended that GGVs be completed toward the top of this interval and be designed to include the Sewickley Coalbed. Permeability increases following undermining were dramatic, with increases of about 100-500 times the premining values and instantaneous increases of up to about 1,000 times these values. These measurements did not differ significantly despite differences in borehole configurations. Fracture permeability pathways remain high to the mined coalbed toward the top of the described interval, yet the likelihood of drawing ventilation air into the borehole is minimized (Section 8).