Conventional CBM recovery calls for the operator to pump water from coalseams to release most of the pressure than helps methane attach to the cleats in the coal. The more cleats and the looser the coal, the better the permeability. Fracturing often can help both the speed of recovery and the size of reserves, but in general, according to the International Energy Agency’s Greenhouse Gas Program findings, conventional recovery produces about half the methane adsorbed onto the coal.

Depending on the coal quality, enhanced gas recovery (EGR) techniques could increase that recovery by 20 to 30%.

The two most advanced studies on EGR for CBM have taken place in the San Juan Basin in the United States, one in southwestern Colorado and the other in northwestern New Mexico. In the case of CO2, operators would use only deeper, unminable coal beds. It would do no good to sequester CO2 in coal if mining later released the greenhouse gas.

Tiffany
Amoco Production Co., now part of BP, conducted the Tiffany project in the 1990s on its CBM properties in southern Colorado. Company engineers reasoned that nitrogen injection would improve recovery from the coal since nitrogen has twice as great an affinity for San Juan Basin coals as methane. In addition, the sweep of the nitrogen front through the formation would push the released methane to the production wells.

According to an Advanced Resources International study of EGR, if the nitrogen flushes the methane, it can reach a saturation of almost 100% on the coal.

This is not an inexpensive technique. Even though air is mostly nitrogen, a nitrogen generator must create nitrogen with high enough quality for an effective sweep, and nitrogen produced with the methane must be separated, treated and reinjected.

Amoco started the project after the group of wells had produced conventionally for 9 years. It drilled 10 new directional wells in January 1998 and in December of that year converted two producing wells to injectors. It injected 22 MMcf/d to 28 MMcf/d of nitrogen into the 12 wells.

Before the injection, the project wells produced 5 MMcf/d of gas from 34 wells. The speed of the nitrogen action was immediately evident as well production peaked at 29 MMcf/d of gas, a five-fold increase, in March 1999.

Now, the company is studying the potential of adding CO2 injection to the already-generated nitrogen.

Allison
In its Allison project in New Mexico, Burlington Northern, now part of ConocoPhillips, injected CO2 and shut in five producing wells for 6 months to let the CO2 “soak” and give the CO2 and methane a chance to change places.

The company reasoned CO2 has four times the affinity for coal that methane has, and it should displace the methane from the coal.

After 6 months, the company opened the production wells and saw an immediate increase in water production. The company also found a 0.6% concentration of CO2 after injection compared with 0.4% before injection.

The company injected 4.7 Bcf of CO2 into the partially dewatered coals and got a 1.5 Bcf incremental increase in methane production. About 4.2 Bcf of the CO2 remained adsorbed on the coal.

Economics
Advanced Resources International came up with a calculation for base methane recovery and recovery with nitrogen and CO2 individually in a five-spot recovery pattern.

Base recovery with no enhanced mechanisms totaled 1.171 Bcf of gas. Recovery with nitrogen rose to 2.933 Bcf, and recovery with CO2 was in between at 2.147 Bcf. In other words, incremental recovery with nitrogen more than doubled recovery from the pattern, and the CO2 injection nearly doubled the production.

The US Environmental Protection Agency (EPA) offered a look at the economics of a 100-well CO2 injection project, assuming a wellhead price of US $2/Mcf for the CBM and assuming the cost of drilling the production wells and the infrastructure already had been sunk in the conventional production phase. It estimated an undiscounted cash flow of $1.l6 to $1.36/Mcf of gas at a 40% recovery. A new project with new production wells and infrastructure would lower cash flow by $0.13 to $0.20/Mcf, leaving the operator a profit between $0.96 and $1.23/Mcf, undiscounted.

The agency pointed out this was a conceptual project and not a true-to-life case study.

Combination
Given the qualities of both CO2 and nitrogen in improving CBM production and given the potential of coal to adsorb four times as much CO2 as methane in a world looking for ways to get rid of greenhouse gases, a combination of the two technologies was a natural extension.
Advanced Resources International, and later the Alberta Research Council in Canada, speculated on the value of combining the technologies using flue gas from coal-fired power plants, which contains both gases.

The CO2 remains adsorbed to the coal, and most of the nitrogen returns to the surface in the methane stream.

Utility companies also try to build power plants near the power source, in this case coal. That opens another attractive alternative. An operator could collect flue gas from a coal-fired power plant, use the gas to enhance recovery from the coal beds and use the produced methane for cleaner industrial utility power.

International
The Alberta Research Council’s work with enhanced recovery led it to join Sproule International and the Computer Modelling Group in a joint CBM project sponsored by the Canadian International Development Agency and the Chinese Ministry of Commerce.
A one-well test in the south Qinshui Basin in Shanxi Province in northern China showed the coal was stable and permeable enough to adsorb CO2 and increase methane production.
In related activity, the Asia Canada Energy Inc. subsidiary of Pacific Asia China Energy Inc. and the Alberta Research Council started a cooperative research study to evaluate EGR potential in the company’s coalbed methane concession in Guizhou Province in China.

That program will be conducted by the Canadian Consortium on Enhanced Coalbed Methane Recovery, led by the Alberta Research Council with a sizeable group of cooperating companies. The Chinese partner is China United Coalbed Methane Co.

The study will evaluate the ability of coals in the area to adsorb CO2 and to improve methane recovery for the unmineable coal beds, and it’s part of a larger coal seam study for EGR and sequestration in China led by China United Coalbed Methane Co.
Other projects are under consideration or operation in Poland, the United Kingdom and Australia.

Questions remain
A CBM enhanced recovery conference in Boulder, Colo., revealed that operators had no problem with the mechanics of injecting CO2, nitrogen or flue gas into coal seams.
The big questions centered on the economics of gathering, processing and treating the gases to get them ready for injection.