Coal Bed Methane Extraction and Use: A Complete Technical Overview

Rising energy costs and mounting environmental pressures have pushed industry and governments alike to look beyond conventional fossil fuel sources. Coal bed methane (CBM) – natural gas trapped within underground coal seams – has emerged as a well-established alternative energy resource, with extraction technology refined over more than two decades of commercial development. This article explains what CBM is, how it’s produced, and what’s involved in bringing it to market.

What Is Coal Bed Methane?

Coal bed methane is a natural gas, typically composed of more than 90% methane, that forms and is stored within coal seams during the coalification process. Unlike gas trapped in conventional reservoirs, CBM is held in place by water pressure and adsorbed onto the enormous internal surface area of the coal itself.

This internal surface area is so extensive that coal can store six to seven times more gas than a conventional natural gas reservoir of the same rock volume. Much of this coal – and therefore much of the methane – sits at relatively shallow depths, which keeps drilling straightforward and exploration costs low.

Coal can hold six to seven times more methane-rich gas than a conventional natural gas reservoir of equal volume, thanks to its large internal surface area.

As depth increases, however, rising pressure closes the natural fractures (called cleats) within the coal, reducing permeability and making it harder for gas to migrate and be recovered.

Is a Coal Seam Worth Developing?

Not every coal seam is a viable CBM resource. Several geological factors determine whether extraction will be commercially and technically successful.

Favourable Reserve Indicators

• A coal seam producing 1.5–2 cubic metres of gas per metric tonne of coal is generally considered to have favourable reserves.
• Extraction becomes economical at 1.5 cubic metres per tonne when the seam is more than 6 metres thick.
• CBM only exists where the coal seam is buried deep enough to maintain sufficient water pressure to hold gas in place, and where the groundwater has a high concentration of sodium bicarbonate.

Seam Thickness Requirements

The required coal thickness depends heavily on gas content, which is linked to the coal’s rank and maturity:

Interestingly, seam thickness isn’t always directly proportional to gas yield – some coal formations show high gas concentrations regardless of thickness, due to other underlying geological factors.

The Project Route: From Desk Study to Operation

Developing a CBM resource follows a structured project pathway:

1. Desk study to evaluate available geological data
2. Field data collection to determine factors affecting viability
3. Estimating methane quantities, using one of two methods:
4. Method 1: Bore to the top of the coal seam and extract a core sample. The methane recovered from the core provides a basis for estimating gas content per unit volume of coal.
5. Method 2: Estimate reserves through calculations based on existing regional knowledge of the coal.
6. System design, covering the gas extraction system and water treatment (if required)
7. Procurement of component parts
8. Construction and commissioning
9. Operation and maintenance

How Coal Bed Methane Is Produced and Extracted

CBM wells are typically completed open hole. Casing is set to the top of the target coal bed, and the underlying zone is under-reamed and cleaned out with a fresh water flush. A downhole submersible pump then lifts water up the tubing, allowing gas to separate from the water and rise up the annulus.

From there, gas and water from individual wells are piped to a metering facility that records production volumes. The gas then flows to a compressor station for pipeline shipment, while the water is routed to a central discharge point.

The Extraction Process in Detail

To extract CBM, a steel-cased hole is drilled into the coal seam, typically 100–1,500 metres below ground. As pressure within the seam declines – either naturally or through active water pumping – both gas and “produced water” rise to the surface through tubing.

Extraction works because CBM has very low solubility in water and readily separates out as pressure decreases. The goal isn’t to fully dewater the coal seam, but to reduce the water pressure (or head of water) down to just above the top of the seam – though in practice the water level can sometimes drop into the seam itself.

The objective is not to dewater the coal seam, but to decrease water pressure to just above the top of the seam – a subtle but critical distinction for well performance.

Production Profile: The Three Stages of a CBM Well

CBM wells follow a distinctive production cycle quite different from conventional gas wells:

1. Dewatering stage – water production initially exceeds gas production, but as pumping continues, water volume falls while methane volume rises.
2. Stable production stage – methane output reaches its maximum and water production stabilises.
3. Decline stage – methane production gradually falls until continued extraction is no longer economical.

This pattern is often described as a “negative decline,” since gas production actually increases early on as water is pumped off and methane begins to desorb from the coal.

For many coal beds, after several months of dewatering, average production settles at around 4,500 cubic metres of gas per day and 60 cubic metres of water per day. CBM wells generally produce at lower gas rates than conventional gas reservoirs, typically peaking near 8,500 Nm³ per day.

Key Geological Factors Affecting Gas Flow

Several intrinsic coal properties govern how much gas can ultimately be recovered:

Adsorption and the Langmuir Isotherm

Most of the gas within coal beds exists in adsorbed form rather than as free gas. The relationship between gas content and reservoir pressure follows a curve known as the Langmuir isotherm, described by two coal-specific properties: the Langmuir volume (maximum gas content at infinite pressure) and the Langmuir pressure (the pressure at which half that gas content exists within the coal).

Cleat Density and Maceral Composition

The commercial potential of a coal seam also depends on:

• Cleat density/intensity – the joints within coal that provide permeability; higher cleat density supports more profitable extraction.
• Maceral composition – a high vitrinite content is favourable for CBM extraction, while high inertinite content hampers it.

Production forecasting for CBM wells and fields typically requires specialised material balance tools or numerical simulators, since it involves modelling the coupling of gas diffusion through the coal matrix with flow through the fracture network.

Are Coal Seams Aquifers?

Yes – coal seams frequently function as aquifers. Water moves through the cleat system within the coal, and where this system is well developed with sufficient water volume, it can support an economically viable water supply. In regions such as the Powder River Basin, coal seams are the most regionally continuous geologic unit and often have aquifer characteristics equal to or better than sandstone, making them frequent targets for water-well completions.

Managing Coal Bed Methane Product Water

Extracting CBM inevitably means pumping large volumes of groundwater to release the pressure holding gas in place. Each well typically produces 20 to 80 litres of water per minute – at an average of 45 litres per minute, that’s roughly 65 cubic metres of water per day per well. Well spacing is commonly around one well per 30 hectares.

Water Quality Concerns

CBM product water often carries a moderately high salinity hazard and, frequently, a very high sodium hazard when assessed against irrigation suitability standards. Over time, salts can accumulate in the root zone of irrigated land, stunting plant growth as roots struggle to draw water from increasingly saline soil.

The sodium content poses additional risks:

• Soil crusting and reduced hydraulic conductivity
• Impaired water availability and aeration for crops
• Increased swelling and dispersion of clay particles when soils containing swelling clays are wetted

Disposal and Beneficial Use Options

Operators currently manage CBM product water through several routes:

• Stream discharge – generally discouraged for new wells, though regulated discharge may be permitted under certain flow conditions
• Impoundment – storing or infiltrating water in holding ponds, zero-discharge ponds, or infiltration ponds (most are unlined and some seepage can reach stream channels via subsurface flow)
• Land application – irrigating crop or rangeland, requiring careful management given water quality concerns
• Other uses – dust control and use by coal mining operations

Turnkey CBM Solutions

A complete CBM development programme can involve everything from initial geological assessment through to long-term operation. Full-service providers typically offer:

• Equipment delivering more than 95% availability for complete facilities, and 99% availability for gas pumping and processing equipment specifically
• Turnkey design, manufacture, and installation services, or component supply only
• Ongoing operation and maintenance services
• A single point of contact for the full commercial exploitation of a CBM resource

Key Takeaways

• CBM is a methane-rich gas (typically >90% methane) stored within coal seams, held in place by water pressure and adsorption onto coal’s large internal surface area.
• Favourable reserves generally require 1.5–2 m³ of gas per tonne of coal, with economical extraction typically needing seams over 6 metres thick.
• Extraction involves pumping water to reduce reservoir pressure, allowing gas to desorb and separate for collection – without fully dewatering the seam.
• Production follows a characteristic three-stage cycle: dewatering, stable production, and decline.
• Managing produced water responsibly – given its salinity and sodium hazard – is one of the central operational challenges of any CBM project.
• Full project delivery spans geological desk studies through to long-term operation and maintenance, and can be delivered on a turnkey or component-supply basis.

Get in Touch

Whether you’re evaluating a coal seam’s potential, planning a CBM extraction project, or need support managing produced water and gas processing infrastructure, our team can help guide your project from feasibility through to commissioning and ongoing operation. Contact us today to discuss how coal bed methane could fit into your energy strategy.