Mining Topic: Methane Drainage and Migration

What is the health and safety problem?

Methane is an odorless, tasteless, colorless, inflammable, lighter-than-air gas formed by the decomposition of organic and other carbonaceous materials. Methane's low density (approximately 50% of the density of air) causes it to concentrate in the higher parts of underground mine environments. If ventilation is insufficient to properly mix the air with the mine air, methane levels between 5% and 15% can form an explosive mixture which, if ignited, can have catastrophic consequences. In this range, methane can be ignited easily, resulting in a violent explosion that may result in further explosions in the presence of combustible coal dust.

Drainage of methane from coal seams and from overlying formations is an effective technique to alleviate methane emission problems in coal mines. However, in order for methane drainage to be successful and produce the expected results, methane drainage systems must be designed according to specific geologic conditions, such as gassiness of the coal seam and overlying strata, specific gas emissions of the mine, and coal seam thickness and continuity.

What is the extent of the problem?

Gas drainage practices should be designed based on mine-specific conditions and needs using engineering control approaches. Currently, most gas drainage applications in individual underground coal mines are based on trial and error and by copying the practices used by other mines located in the same coal basin. However, most US coal mines using longwalls must utilize methane drainage to meet federal statutory limits for methane concentrations in air and cannot rely on ventilation systems alone.   

How is the NIOSH Mining program addressing this problem?

The former United States Bureau of Mines was very active in development of gas drainage boreholes for over two decades, mostly through field tests and observations in coal mines. The publication Methane Control for Underground Coal Mines is a significant source of material on this earliest work. Important early work included gas content testing development as an aid to ventilation design and drilling and gas extraction methods. A very important output of this early work is the dissemination and transference of the gas extraction technology and its conversion from tool of research to one of industry.

Building on this work, the NIOSH Mining Program is addressing methane drainage and control problems by developing technologies to identify potential migration pathways and controlling them through methane drainage research. These research studies include field experiments, numerical simulations, and intelligent and statistical modeling approaches with consideration to site-specific details. These technologies help to detect and reduce methane gas levels. Challenges are being further addressed through the development of practical Methane Control and Prediction software to assist in methane emission prediction and drainage modeling.

Methane control technology has continued to advance following this earlier work. Methane content testing begun by the USBM in the 1970's progressed to higher resolution methods. Although variants of the direct method for gas content measurement are still in use worldwide, NIOSH forwarded the modified direct method (MDM) for gas content measurement in 1990's for higher resolution determinations. This technology and many other developments are summarized in Measuring the gas content of coal: A review. Technological advances in methane control by NIOSH and others were aimed at improved coal mine safety but played important roles in the development of the commercial coalbed methane industry and the shale gas industry.

Through interaction with cooperating mine operators, research utilizing both field experiments, a range of analytical tools and simulation techniques, NIOSH has improved methane control methods and transferred these findings to the mining industry, regulators, and collaborating researchers.

What are the significant findings?

Findings of NIOSH's research on methane emission and degasification show that methane emissions from a mined coal seam and overlying strata can be significantly decreased by using in-seam and gob gas venthole drainage techniques. However, emission predictions and degasification design decisions must be based on operational and mine-design data, geologic and structural properties of the mining environment, and the size and in-place gas content of the gas emission zone. Much of the focus of NIOSH research has been on longwall coal mines due to the large volumes of coalbed methane emitted from longwall panels and due to the gas liberation and migration pathways produced by full extraction mining and induced fracturing. Gas extraction methods are designed to  utilize this strata behavior. Based on these findings, NIOSH research has recommended changes to methane drainage borehole designs and multi-borehole layouts to optimize gas extraction. NIOSH research has also produced prediction methods for extracted quantities of coalbed methane from boreholes and for increased face emissions quantities as longwall faces increase in length, a consistent trend in the industry.

Since the fractured overburden behind longwall faces is so important as a source of coalbed methane emissions and transport, this has been a focus of NIOSH research. In the U.S., there are seals in place isolating large areas of methane rich, mined out panels from active workings. Federal statutory requirements for seals changed in response to a mine disaster in 2006. NIOSH investigated leakage rates through the new seal configurations.

What are the next steps?

The depth and breadth of research studies find their best value when they are validated and accepted by the industry. The use of developed techniques and recommendations, including validation and acceptance of NIOSH's Methane Control and Prediction software, are important milestones. The education and training of mine operators and engineers is critically important in the utilization and design of methane drainage systems. The loss of methane control experience during employee turnover and the lean operational staffing for modern US coal mines makes technical knowledge of paramount importance. The mining of deeper coal reserves and the changing desirability of coal products from different basins are trends that will continue to challenge our knowledge basis, producing the need for continuing NIOSH research to produce a safe environment for underground mine workers.

See the NIOSH Mining Tools You Can Use page for software, guides, training materials or other items related to this topic.

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