Unearthing Subterranean Gas
An academic exploration of coalbed methane, from its geological origins to its extraction and environmental considerations.
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Overview Coalbed Methane?
Gas from Coal Seams
Coalbed methane (CBM), also known as coalbed gas or coal seam gas (CSG), represents a distinct form of natural gas directly extracted from coal beds. This resource has gained significant prominence in recent decades, particularly in nations such as the United States, Canada, and Australia, as a vital energy source.
"Sweet Gas" Composition
CBM is often referred to as "sweet gas" due to its notable absence of hydrogen sulfide. Unlike conventional natural gas, CBM typically contains minimal quantities of heavier hydrocarbons like propane or butane, and no natural-gas condensate. It may, however, include a small percentage of carbon dioxide.
Adsorption vs. Conventional Storage
A key characteristic differentiating CBM from conventional gas reservoirs (e.g., sandstone) is its storage mechanism. Methane in coal beds is primarily held within the solid matrix of the coal through a process called adsorption, existing in a near-liquid state that lines the internal pores of the coal matrix. The natural fractures within the coal, known as cleats, can also contain free gas or be saturated with water.
Origins: Thermal vs. Microbial
The genesis of coalbed methane can occur through two primary pathways. It is commonly formed via the thermal maturation of kerogen and other organic matter within the coal. Conversely, in coal seams that experience regular groundwater recharge, methane is frequently generated by in-situ microbial communities, highlighting a fascinating biological dimension to its formation.
History of CBM
From Safety Venting to Resource
The understanding of methane in coal beds originated from its presence as a significant safety hazard in underground coal mining, where it was known as "gassy" coal. Historically, boreholes were drilled from the surface into these seams to vent the methane, a critical safety measure to prevent explosions before mining operations commenced.
US Government Catalysis
The development of coalbed methane as a viable natural gas resource received substantial impetus from the U.S. federal government in the late 1970s. During a period when federal price controls were suppressing conventional natural gas drilling, the government sought to stimulate gas production by funding research into unconventional sources, including CBM. This initiative was further bolstered by exempting CBM from federal price controls and providing a federal tax credit, significantly encouraging its exploration and development.
Australian Commercialization
Commercial extraction of coal seam gas in Australia commenced in 1996. This pioneering effort took place in the Bowen Basin of Queensland, marking a pivotal moment in the global expansion of CBM as an energy commodity.
Reservoir Properties
Gas Composition & Porosity
The gas within coalbed methane reservoirs is predominantly methane, with minor constituents including ethane, nitrogen, and carbon dioxide. Coalbed methane reservoirs are characterized as dual-porosity systems. In this model, the porosity associated with natural fractures (cleats) governs the fluid flow, while the porosity of the coal matrix is responsible for gas storage. Cleat porosity typically ranges from 0.1% to 1%, whereas the overall reservoir porosity can be between 10% and 20%.
Adsorption Capacity
The adsorption capacity of coal quantifies the volume of gas adsorbed per unit mass of coal, commonly expressed in standard cubic feet (SCF) of gas per ton of coal. This capacity is intrinsically linked to the coal's rank and quality, generally ranging from 100 to 800 SCF/ton in most U.S. coal seams. The majority of gas in coal beds exists in this adsorbed state. Production initiates by pumping off water from the fracture spaces, which reduces reservoir pressure and subsequently enhances the desorption of gas from the coal matrix.
Fracture Permeability Dynamics
Fracture permeability serves as the primary conduit for gas flow; higher permeability directly correlates with increased gas production. Permeability in most U.S. coal seams typically falls within the range of 0.1–50 milliDarcys. A unique characteristic of CBM reservoirs is their stress-sensitive permeability, which changes with applied stress. Intriguingly, fracture permeability tends to increase with gas depletion, a phenomenon attributed to the shrinkage of the coal matrix as methane is released, thereby opening up fractures. However, at very low reservoir pressures, a loss of horizontal stress can induce in-situ coal failure, leading to a sudden decrease in fracture permeability.
Formation Thickness & Pressure
The thickness of a coal formation is not always directly proportional to the volume of gas produced. For instance, in the Cherokee Basin of Southeast Kansas, thin pay zones (1-2 feet) can yield excellent gas rates, while thicker formations may produce very little. This suggests that other geological factors play a significant role. As with all producing reservoirs, a substantial pressure difference between the well block and the sand face is crucial for maximizing production.
Extraction Process
Well Drilling & Production
The extraction of coalbed methane involves drilling a steel-encased well into the coal seam, typically at depths ranging from 100 to 1,500 meters (330 to 4,920 feet) below the surface. As the pressure within the coal seam naturally declines or is reduced by pumping water from the coalbed, both methane gas and "produced water" are brought to the surface through specialized tubing. The extracted gas is then directed to a compressor station for processing before being fed into natural gas pipelines.
Produced Water Management
The produced water, a byproduct of gas extraction, exhibits significant variability in quality depending on the specific geological formation. It often contains dissolved solids such as sodium bicarbonate and chloride, and potentially heavy metals and radionuclides. This water undergoes various management strategies, including reinjection into isolated geological formations, controlled release into streams, utilization for irrigation, provision for livestock, or disposal in evaporation ponds, often after treatment via methods like reverse osmosis.
Production Profiles & Langmuir Isotherm
CBM wells typically exhibit lower initial gas production rates compared to conventional reservoirs, often peaking around 300,000 cubic feet (8,500 m³) per day. A distinctive feature of CBM production profiles is a "negative decline," where the gas production rate initially increases as water is pumped off, allowing gas to desorb and flow more freely. The methane desorption process adheres to a Langmuir isotherm, a curve that describes the relationship between gas content and reservoir pressure. Key parameters of this isotherm are the Langmuir volume (maximum gas content at infinite pressure) and Langmuir pressure (pressure at which half the maximum gas exists), which are unique properties of the coal.
Optimizing Recovery
The potential of a coalbed as a CBM source is influenced by several critical coal properties. A high cleat density/intensity is essential, as cleats are the natural fractures that provide permeability to the coal seam. The maceral composition is also vital; a high vitrinite content is ideal for CBM extraction, while inertinite can hinder the process. Furthermore, the rank of coal, indicated by a vitrinite reflectance of 0.8–1.5%, has been correlated with higher coalbed productivity. Ensuring the methane composition of the extracted gas is above 92% is crucial for commercial marketability, often requiring the removal of non-flammable gases like nitrogen or carbon dioxide, or blending with higher-BTU gas to achieve "pipeline quality."
Environmental Impacts
Methane Emissions & Climate
Like all carbon-based fossil fuels, the combustion of coalbed methane releases carbon dioxide (CO₂) into the atmosphere, contributing to the greenhouse effect. A significant concern with CBM production is the leakage of fugitive methane into the atmosphere. Methane is a potent greenhouse gas, with an effect on global warming rated at 72 times that of CO₂ over a 20-year period, reducing to 25 times over 100 years. However, life-cycle analyses suggest that electricity generation from CBM, similar to conventional natural gas, results in less than half the greenhouse gas emissions compared to coal-fired power generation.
Infrastructure Footprint
The development of coalbed methane fields necessitates a considerable infrastructure footprint. This includes an extensive network of access roads, pipelines for gas transportation, and compressor stations to maintain gas flow. Over the operational lifespan of a field, wells may be spaced more closely to optimize the extraction of remaining methane, further increasing the surface impact.
Produced Water Challenges
The management of produced water remains a significant environmental consideration. While its quality varies, it can contain undesirable concentrations of dissolved salts, naturally occurring chemicals, heavy metals, and radionuclides. Incidents such as those in the Pilliga Scrub in Australia, where Eastern Star Gas was fined for discharging polluting water with high salt levels into Bohena Creek, highlight the risks. A NSW Legislative Council inquiry criticized the use of open holding ponds, recommending a ban on their use. Conversely, in regions like the Powder River Basin in Wyoming, US, CBM produced water often meets federal drinking water standards and is beneficially used for livestock, though its high sodium adsorption ratio can limit irrigation applications.
Groundwater Depletion
The withdrawal of water during CBM extraction can lead to the depression of aquifers over large areas, potentially altering regional groundwater flows. In Australia, the CBM industry estimates annual groundwater extraction between 126 billion and 280 billion liters, with the National Water Commission estimating it to be over 300 billion liters per year. The long-term effects of such large-scale water withdrawal on interconnected aquifer systems are a subject of ongoing environmental scrutiny.
Global CBM Regions
Australia
Australia's coal seam gas (CSG) resources are concentrated in the major coal basins of Queensland and New South Wales, with additional potential in South Australia. Commercial recovery began in 1996. As of 2014, CSG contributed approximately 10% of Australia's total gas production, with demonstrated reserves estimated at 33 trillion cubic feet (35,905 petajoules) in January 2014. Key basins include the Bowen Basin, Surat Basin, and Sydney Basin.
Canada
Canada possesses significant coalbed gas resources, with British Columbia estimated to hold around 90 trillion cubic feet (2.5 trillion cubic meters). Alberta, as of 2013, was the sole province with commercial CBM wells, boasting approximately 170 trillion cubic feet (4.8 trillion cubic meters) of economically recoverable CBM, and total reserves potentially reaching 500 trillion cubic feet (14 trillion cubic meters). A notable debate exists regarding whether CBM in Alberta should be classified as a renewable or non-renewable resource, impacting ownership rights, as bacterial action can continuously generate methane, though dewatering may disrupt this process.
United Kingdom
The United Kingdom's coal fields are estimated to contain 2,900 billion cubic meters of gas in place, though only a small fraction (potentially as little as 1%) may be economically recoverable. While some methane is extracted from coal mine venting for electricity generation, independent commercial CBM extraction began in 2008. IGas Energy's wells at Doe Green were the first commercial CBM operations separate from mine venting in the UK as of 2012. The table below illustrates CBM usage for electricity generation, coal extraction, and other industries in the UK (in GWh):
United States
In 2017, the United States produced 1.76 trillion cubic feet (TCF) of coalbed methane, accounting for 3.6% of the nation's total dry gas production. This figure represents a decrease from its peak production of 1.97 TCF in 2008. The majority of CBM production in the U.S. originates from the Rocky Mountain states, particularly Colorado, Wyoming, and New Mexico.
Kazakhstan
Kazakhstan is poised for significant development in its coalbed methane sector over the coming decades. Preliminary assessments indicate that the country's main coalfields may hold as much as 900 billion cubic meters of gas, representing approximately 85% of Kazakhstan's total gas reserves. This substantial potential positions CBM as a key future energy resource for the nation.
India
India marked a milestone in CBM commercialization on July 14, 2007, with Great Eastern Energy (GEECL) being the first company to have an approved field development plan. GEECL's completion of 23 vertical production wells made CBM commercially available, initially distributing 90% as Compressed Natural Gas (CNG) for vehicles. GEECL also established Southeast Asia's first CBM station in Asansol, West Bengal. Another major player, Essar Group's Essar Oil and Gas Exploration and Production Ltd., holds five CBM blocks, with Raniganj East currently operational and an estimated 10 trillion cubic feet (CBF) of CBM reserves across its portfolio.
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References
References
- Coal Gas, clarke-energy.com, retrieved 25.11.2011
- McKee, C. R., Bumb, A. C., & Koenig, R. A. (1 March 1988). Stress-Dependent Permeability and Porosity of Coal and Other Geologic Formations. Society of Petroleum Engineers. doi:10.2118/12858-PA
- Okotie, V. U., & Moore, R. L. (1 May 2011). Well-Production Challenges and Solutions in a Mature, Very-Low-Pressure Coalbed-Methane Reservoir. Society of Petroleum Engineers. doi:10.2118/137317-PA
- Ryan Driskell Tate, Bigger Than Oil or Gas? Sixing Up Coal Mine Methane Emissions, March 2022
- US EPA, [Evaluation of Impacts to Underground Sources of Drinking Water by Hydraulic Fracturing of Coalbed Methane Reservoirs, The Powder River Basin], June 2004, EPA 816-R-04-003 Attachment 5.
- John Squarek and Mike Dawson, Coalbed methane expands in Canada, Oil & Gas Journal, 24 July 2006, p.37-40.
- US Energy Information Administration, Coalbed methane production, accessed 9 October 2013.
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