Handbook for Methane Control in Mining - AMMSA

842. In-mine inclined or vertical boreholes: Short vertical or long inclined boreholes have beendrilled from an existing mine (or roadways expressively driven for this purpose) to intersectother coal seams in the gas emission space, allowing for the seams to be degassed prior to mining.Again, success depends on high permeability. A far better way to degas these coal seamslying in close proximity to each other is to use vertical frac wells.3. Vertical frac wells: Vertical frac wells are ideally suited to highly gassy, deep, lowpermeabilitycoal seams where it takes several years prior to mining to adequately degas the coal.These wells are drilled from the surface on a grid pattern over the entire property or only onlongwall panels to intersect the coal seam to be mined in the future.Vertical wells drilled into the coal seam seldom produce measurable amounts of gas withouthydraulic stimulation. High-pressure water (or other fluids) with sand are pumped into the coalseam to create fractures (Figure 6–6). The fluid (water) is then pumped out, but the sandremains, keeping the fractures open for gas to escape to the well bore. Under ideal conditions,if the vertical frac wells are drilled more than 5–10 years in advance of mining, 60%–70% of themethane in the coal seam can be removed prior to mining.Vertical frac wells have been very successful in the Appalachian and San Juan Coal Basins of theUnited States. They have also been attempted in the United Kingdom, Germany, Poland, China,and Australia, but met with only limited success. Major reasons for the lack of success abroadare (1) cost and (2) lack of sufficient permeability, which are further explained below.1. The cost of drilling and hydrofracing a well in Europe and Australia is typically threetimes the cost in the United States. The cost of permitting and site preparation is alsohigher. In many countries, the drilling and hydrofracing equipment are not convenientlyavailable.Figure 6–6.—Premining methane drainage from surface.