Published in the December 2005 American Longwall Magazine

Domestic and international energy demands today mean United States coal companies are working to increase supply by opening new coal tracts in deeper and gassier reserves. Many of the larger US coal companies have also reinvented themselves as “energy companies” and several have focused on recovery of the “total resource” – coal and coalbed methane (CBM).

More than one dozen longwall mines in the US employ in-seam methane drainage to shield longwall gate developments and/or reduce the in-situ gas content of longwall reserves in advance of mining.

In addition to reducing the methane make in a mine’s ventilation system and improving mine safety, mine operators can recover high quality methane with this system of methane drainage.

Several longwall mines that employ this technique gather the drained gas and transport it to the surface for commercial use. These mines realize a significant cost benefit due to current pricing and the future demand for natural gas. They also gain an environmental benefit as methane is a deleterious greenhouse gas estimated to be 21 times more potent than carbon dioxide in trapping atmospheric heat over a hundred-year period.

Directionally drilled boreholes are maintained in the mining seam through the evaluation of drilling parameters, the use of downhole directional drilling technology, and state of the art measurement while drilling borehole survey equipment.

REI Drilling operates a fleet of seven directional drilling units and provides longhole directional drilling services to the coal and energy sector.

REI’s drilling units are unique in both physical dimension and capacity (maximum drilling distance), and are matched to each application (exploration for old workings, geologic exploration, de-watering and methane drainage), from surface or underground.

REI’s high capacity drilling units can accurately steer and place boreholes in excess of 5000ft in length in-seam, depending on conditions.

Shielding longwall developments

In coal seams that exhibit a high permeability to gas, long in-seam directionally drilled boreholes can immediately benefit mining developments that advance longwall gate entries into gassy virgin reserves.

Reducing methane emissions at the cutting face and along newly exposed coal surfaces minimizes gas related delays and improves mining productivity. Longwall mines exploiting the Pocahontas, Blue Creek/Mary Lee, Pittsburgh, and San Juan coal seams in the US, and the Los Olmos Coals in northern Mexico, realize the immediate impacts of shielding boreholes even when such holes are initiated outby the active section and drilled past the section, just in advance of mining.

Figure 1 illustrates the impact of drilling a 1640ft in-seam borehole immediately in advance of a mining section (initiated from outby) at the Minerales Monclova SA de CV’s Mine I longwall operations in the state of Coahuila in northern Mexico. Because of high cleat and natural fracture permeability, the borehole rapidly reduced methane emissions into the advancing section and enabled the mine to increase advance rates by 78% in two months.

Ideally, operators should place in-seam boreholes as far in advance of mining as possible to increase drainage time and reduce in-situ gas contents. This is particularly important for mines exploiting gassy, lower permeability coals, and those interested in maximizing gas recovery for commercial purposes.

Reducing gas contents of longwall reserves

Using numerical modeling, REI considers the reservoir characteristics of the coal seam, regional coalbed methane activity, and the duration of time available to drain gas in advance of mining, to derive the borehole spacing necessary to achieve specific gas content reductions of virgin coal reserves.

For example, numerical analyses performed for in-seam boreholes drilled into very gassy, low permeability longwall reserves (cross-panel boreholes) from gate entries, show that a borehole spacing of 50ft is required to reduce in-situ gas content by an average of 20% after 180 days. For gassy, high permeability coals, this in-situ gas content reduction rate can be achieved with in-seam boreholes spaced several hundred feet apart.

Mines with developments that bound large gassy virgin reserves that will be exploited in the future can take advantage of this existing infrastructure and use it to deploy long in-seam directional boreholes to reduce gas contents prior to mining, and commercialize the recovered gas.

Operations that bound reserves from which CBM was produced by vertical frac wells can further reduce gas contents and produce CBM by in-fill drilling from underground using ultra-long horizontal in-seam boreholes as shown on Figure 2. REI has successfully navigated in-seam boreholes in excess of 5000ft among vertical frac wells.

Mines that contemplate future shafts may be able to justify early development via blind boring techniques to provide access to drill long radial in-seam boreholes to degasify the mining and adjacent seams. The shaft would serve as a platform for horizontal drilling and a conduit for gas collection ranges.

Dual purpose directionally drilled boreholes

From underground mining horizons, REI can drill through interburden to place in-seam boreholes in overlying or underlying coal seams for the benefit of mining as well as to increase gas recovery for commercial purposes. Boreholes placed in an overlying coal seam in advance of mining can serve two purposes.

If the target coal seam is gassy and contributes to gob gas when undermined, in-seam drainage in advance of undermining will reduce the gas content of this coal seam and decrease its contribution to the gob gas make when undermined. Depending on the elevation of the coal seam relative to the mined seam, the overlying borehole can serve a secondary purpose as a gob gas borehole.

Horizontal gob boreholes placed in overlying coal seams between 50 and 120ft above the mined seam can remain intact in the fracture zone (can be lined). When placed adjacent to low ventilation pressure entries and in tension zones along gate roads, horizontal gob boreholes will successfully control gob gas when placed under vacuum. Figure 3 illustrates the dual purpose principle: reduce the gas content of gob gas contributing source seams before they are mined, and then use these boreholes to reduce gob gas emissions into workings upon mining.

Conclusions

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