Pre-mining Drainage of Coal Seams – U.S. Practices and Applicability to Worldwide Coal Mine Methane Drainage Projects

摘要

Hydraulic fracturing of coal seams to produce methane in advance of mining has been an establishedtechnique in the United States (U.S.) for over 30 years. Trial projects in the Black Warrior basin, initiatedin the 1970’s by the U.S. Bureau of Mines, demonstrated reduction of up to 70% of in-situ methane,over a ten year period, using vertically stimulated wells. Large scale commercial production of methanefrom coal seams started in the 1980’s and once further study had demonstrated the safety of theprocess, several U.S. coal mining companies began using the technique to remove methane from coalseams before mining. This has four main benefits: (1) methane content in the coal seams is significantlyreduced, resulting in lower methane concentrations in the mine during mining, which improves bothmine safety and productivity as well as reducing ventilation requirements; (2) a valuable resource thatwould otherwise escape to the atmosphere is captured and can be utilized for profit; (3) produced gas isvery high quality; and (4) emissions of methane - a powerful greenhouse gas (GHG) - from the mine aregreatly reduced.Several underground U.S. mines currently use vertical and horizontal wells to drain methane beforemining. Drilling vertical wells and hydraulically fracturing the coal seam has been the standardcompletion method, but developments in drilling technology over the last decade have allowed thecost-effective drilling of horizontal wells. These wells have multiple lateral boreholes, drilled in specificpatterns, allowing the drainage of a large area of coal from one well-site. Operators report recoveryrates of 80-90% of in-situ gas in a two to three year time period.Vertical, hydraulically fractured wells and horizontally drilled wells have been used in coalbed methane(CBM) projects outside the U.S., mainly in Australia and Canada. Recent projects in China and India haveyet to emulate U.S. production rates. Lower coal permeability values, more complex coal geology, a lackof service companies, regulatory issues and limited land availability increase technical risks and costswhen compared to U.S. operations.Most coal mines outside of the U.S. use in-mine drainage methods to degas their coals. In talks withUSEPA, several of these coal mining companies have expressed interest in pre-mining drainage as analternative degassing method. This paper will review the main pre-mining methane drainage techniquesemployed in the U.S. and will discuss the applicability of such technologies to methane drainageoverseas, with particular focus on China, India and Kazakhstan.