Eneabba Gas Limited - Energy Generation for Regional Infrastructure in Western Australia
Coal Seam Methane
1. What is Coal Seam Methane (CSM)?
Under the mining nomenclature, CSM is methane gas that is mined from coal seams. This gas is found in most coal deposits / oil shale and is present in many parts of the world. CSM is created during coalification – the process in which coal is formed by biological process combined with the geological process of pressure and heat on buried plant material; the moisture content of the plant biomass is progressively removed and the material remaining is solidified into coal over millions of years. Pressure from overlaying rock and water within the coal cleats (natural fissures) keeps the CSM adsorbed within the coal.
CSM does not exist in a free state of accumulation "pool" and cannot be recovered from "a naturally occurring discrete accumulation of hydrocarbons"; rather on formation of coal it is adsorbed onto cleats within the coal seam and CSM is mined by altering the pressure regime within the coal seam – the 'dewater' process reduces the hydrostatic pressure within the coal structure to a point where CSM is desorbed from the interstitial cleats of the coal structure.
In most occurrences, CSM consists of primarily methane, with components of carbon dioxide (CO2) and nitrogen (N2) and in many cases can be delivered directly to the consumer in this form (once these trace components have been removed) thereby significantly reducing processing time and resultant cost.
2. Eneabba Gas Limited will conduct its CSM Project in four phases:
- Phase 1 – Exploration
- Phase 2 – Pilot Assessment
- Phase 3 – Development
- Phase 4 – Production
3. The most important characteristics of CSM are:
- Coal is a source rock and a reservoir rock –
The composition of gas as well as the gas content, diffusivity, permeability, and gas storage capacity of the coal is dependent on the depositional environment and burial history of the coal.
- The gas storage mechanism of coal –
Whereas most of the gas in conventional reservoirs is in a free state within the pore structure of the rock, most CSM in coal reservoirs is adsorbed onto the internal fissures of the coal. The reservoir pressure (hydrostatic) must be reduced to a sufficiently low level to achieve high gas production, as large amounts of CSM can be stored at low pressures in coal reservoirs.
- The unique mechanical properties of coal –
Coal is relatively compressible compared to the rock in conventional reservoirs; thus, the permeability of coal is more stress-dependent than most reservoir rocks.
- The fracture system of coal reservoirs –
Coal reservoirs typically contains large-scale natural fractures, distinctively in the form of regularly spaced, naturally occurring fractures (several per inch) called face cleats and butt cleats.
- Reduction of hydrostatic pressure before CSM is produced –
Water must be produced continuously from coal seams to reduce reservoir pressure, which causes CSM to be desorbed from the coal cleats and thus is CSM produced (see figure).
CSM Production Profile - Click to Enlarge
4. Fundamentals of CSM Production:
- Factors that Control Production in Coal Reservoirs –
It is established that gas is stored in an adsorbed state on coal, thus for a given reservoir pressure much more gas is stored in a coal seam than in a comparable sandstone reservoir. Many coal seams are water saturated, and water provides the reservoir pressure that holds the gas in the adsorbed state. Production of CSM is controlled by desorption of gas from the coal matrix, diffusion to the cleat system, and flow through fractures.
- Relationship Between Gas Content and Sorption Isotherm –
Another mechanism that controls production is the relationship of gas content to sorption isotherm. The sorption isotherm defines relationship of pressure to the capacity of a given coal to hold gas at a constant temperature. Gas content is a measurement of the actual gas contained in a given coal reservoir. The production of a CSM well can be estimated from accurate measurements of gas content and the isotherm.
- Maintaining Low Backpressure –
The ultimate production of CSM depends on gas content and reservoir pressure. CSM production will not initiate until the reservoir pressure falls below the point where the gas content of the coal is in equilibrium with the isotherm. As most coal reservoirs are aquifers, production of water (dewater) is the primary mechanism of (hydrostatic) pressure reduction. If the gas content of the coal reservoir is below the isotherm, then the reservoir will produce only water initially. Following this single phase flow period, bubble flow initiates when the reservoir pressure reaches saturation point on the isotherm. Eventually, two phase flow of gas and water occurs while the pressure is further reduced in the reservoir. Therefore, it is important to produce CSM wells at the lowest practical pressure.
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