Conventional CBM production
Coal seams often have large amounts of methane adsorbed onto the coal. The methane comes from first biological activity and then thermal conversion of organic matter in the coals as the seam is buried. The amount of gas that can be held by the coal increases with pressure. The seam can be made to release this gas by depressuring, which is achieved by pumping off the ground water.The gas released from the coal matrix makes its way into natural fractures in the coal (cleats) which provide pathways for the gas to migrate down the pressure gradient to the well where the water is being pumped off.
Coal bed methane works best in coals that are under enough pressure to contain significant adsorbed gas ( greater than 300m ) but not so deep that the pressure of the overburden has closed the cleats and taken away the permeability ( less than 1,000m ).
CO2 and coal
In the same way as methane is adsorbed onto the surfaces of micropores in the coal, so is CO2. Coal has an even greater affinity for CO2 than for CH4, i.e. at a given pressure, coal will absorb more CO2 than CH4. If an equal mixture of CO2 and CH4 is present in the cleats, the matrix will release CH4 into the cleats and absorb CO2. Injected CO2 becomes bound to the coal and methane that was held by the coal is released as a free gas into the cleat system.
As long as pressure is maintained on the coal, the CO2 will be held in the coal. The natural state of a deep coal seam is water saturated under the hydrostatic head corresponding to its depth of burial (approximately 0.43 psi/ ft). If left to return to the natural state after CO2 injection, the coal seam will retain the CO2 indefinitely. Mining would, however, release the CO2 and that CO2 would pose a mining hazard.
CO2 enhanced CBM
Injecting CO2 into a coal seam creates a zone around the injector in which is saturated with CO2. Methane that was contained in the coal of this zone is now displaced away from the well to create a halo of increased methane concentration in the coal matrix and perhaps free gas in the cleats if the coal was already saturated with methane. The CO2 flood front advances away from the injector and is drawn toward any producer wells where the pressure in the seam has been reduced by pumping out water.
Methane production at any well in the halo of increased methane concentration benefits from a number of effects: enhanced methane desorption from the coal matrix, increased gas concentrations in the cleat system (increasing the relative permeability to gas) and the drive provided by the pressure gradient from the injector to producer well.
When the CO2 flood front reaches the producer well, a mixture of CH4 and CO2 is produced. Normally the producer would be shut in at this stage since the cost of removing CO2 from sales gas is high. The well pattern in any CO2 ECBM project would therefore be designed to maximise CH4 production before CO2 breakthrough.
Challenges to economic CO2 ECBM production
As far as we know, there are no commercial ECBM projects. A number of field pilot projects have been conducted, but none have continued into an economic project, probably for the following reasons:
- The intensity of CO2 usage. Much more CO2 is used than extra methane obtained. The ratio ranges from 2 to 6 (volume basis) depending much on the initial saturation of the coal and the well pattern. If CO2 must be purchased, the price of the methane sold must be at least that same multiple of the CO2 price, even before the cost of the injector wells and flow lines is taken into account.
- CO2 injection reduces the permeability of the coal. On adsorption of the CO2, the coal matrix swells and the cleats are closed. Since the cleats are the pathways for fluid transmission through the coal, the permeability of the coal is reduced. If the initial permeability was low, the injectivity might become too low for economic injection i.e. the pressure needed for CO2 injection becomes too high and the number of injection wells required for a given volume of CO2 ruins project economics. A number of field pilot experiments have, however, demonstrated that the increase in permeability with pressure tends to compensate for the reduction due to matrix swelling. Acceptable injection rates (e.g. 1.0 mmscfpd) seem to be achievable in coals with modest initial permeabilities (5 - 10 md). Since producer wells are generally shut in on CO2 breakthrough, the permeability effect is not important there.
Waiting for Carbon Credits
Since the biggest obstacle to CO2 ECBM is the intensity of CO2 usage, low cost, zero cost or even negative cost CO2 delivered to the coal field will make a huge difference to the economics. There will always be a certain cost in compressing and transporting the CO2 by pipeline to the CBM site, but if carbon credits can pay these costs, the delivered CO2 could be free to the CBM project operator. To date, no geological storage project has received any money through the Kyoto mechanisms (CDM and JI). The best developed CBM industry is to be found in the USA where there is no financial incentive to sequester CO2. The technology has been proven by field pilot projects, but must wait for the arrival of really cheap CO2 through offsets for sequestration.