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Factors and Impacts for CO2 Storage in Coal Bed Seams

Co2ecbm
Schematic of a CO2ECBM pilot project depicting the injection and production wells. Source: Advanced Resources International

Unmineable coal seams—seams too deep or too thin to be mined economically—are one of the main types of geological formations targeted for the sequestration of captured CO2. Using coal seams for CO2 storage also enables the enhanced recovery of coalbed methane—a result analogous to the enhanced oil recovery enabled by CO2 injection into oil reservoirs.

Such CO2 enhanced coalbed methane production and sequestration (CO2ECBM) is attractive for several reasons. Research indicates that at least two to three molecules of CO2 can adsorb onto the coal for each molecule of methane released, and there are regionally large unmineable coal seams capable of accepting large volumes of CO2 close to power plants, thus eliminating the need for long transport pipelines.

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Preferential adsorption of CO2 over CH4 will displace the CH4 from the coal matrix. Source: Stanford GCEP

Estimates put the maximum US capacity for CO2ECBM at 90 billion metric tons CO2, 40 billion metric tons of which are in Alaska.

However, according to a team of researchers at the Department of Energy’s (DOE) National Energy Technology laboratory (NETL), successful CO2ECBM depends on a number of factors that are still poorly understood. These include reservoir integrity, volume, porosity, permeability and pressure; the affects of the swelling and shrinkage of coal when injected with high-pressure CO2; and the environmental affects of the production.

Two papers published by the NETL team in the current issue of International Journal of Environment and Pollution explore the potential affects of CO2 sequestration on coal seams and the environment.

The NETL researchers analyzed data from nearly 2,000 coal samples from 250 coal beds across 17 states. The researchers found a moderate correlation (0.7) between depth and methane content for high-volatile coal ranks. Low-volatile rank coals average the highest methane content, 12.74 m3/ton (450 ft3/ton), subbituminous rank coals the lowest <0.71 m3/ton (<25 ft3/ton).

To replicate geological conditions, NETL has built a Geological Sequestration Core Flow Laboratory (GSCFL). The GSCFL accommodates core samples of coal and host rocks, such as sandstone, at a variety of representative overburden or confining pressures while monitoring pore-fluid chemistry, and controlling temperatures for long-term experimentation. Permeability changes can also be monitored continuously during flow.

Preliminary results obtained from Pittsburgh No. 8 coal indicate that the permeability decreases (from micro-darcies to nano-darcies or extremely low flow properties) with increasing CO2 pressure, with an increase in strain associated with the triaxial confining pressures restricting the ability of the coal to swell. The already existing low pore volume of the coal is decreased, reducing the flow of CO2, measured as permeability.

This is a potential problem that will have to be overcome if coal seam sequestration is to be widely used.

...Many more experiments are required to improve estimates on CO2 capacity and injectivity and any associated host rock petrology. For example, measurements with super-critical CO2 will need to be performed to replicate representative CO2 conditions relative to conditions at depths below 396.3 m (1,300 ft), at depths greater than 762 m (2,500 ft) and at the respective temperature of each depth.

Development or collection of these new types of data and coal characteristics are critical to any linked carbon sequestration-CBM recovery efforts. The effects of the restrictive swelling of the coal with a substantial increase in strain in the coal need to be further investigated because these conditions would appear to adversely impact CO2 permeability through the interconnected pores of the coal, and most importantly, diffusivity of CO2 through the coal matrix.

The research team also investigated some of the possible environmental side effects of sequestering CO2 in coal seams.

Nearly all coal seams contain some water, and the water pressure in the seam is normally greater than the methane pressure. To extract the gas, producers have to reduce the water pressure to a point equal to the gas pressure. At that point, the methane gas desorbs from the coal matrix, and gas and water then flow to the production well. The conventional production of coalbed methane thus generates enormous amounts of produced water.

Generally the produced water is brackish or briney, and contains substantial amounts of Na+, Cl, HCO3 and other dissolved solids and organics. However, trace element concentrations in the product water are generally low and the water is generally of better quality than waters produced from conventional oil and gas wells. The method for disposing of the product water depends on the characteristics of the water.

To begin assessing the affect of the addition of CO2 into the CBM process, the NETL team tested a high volatility bituminous coal with produced water and gaseous carbon dioxide at 40° C and 50 times atmospheric pressure. Using microscopes and X ray diffraction to analyze the coal after the reaction was complete, they found that the addition of the CO2 resulted in more mineral dissolution of the coal, with some trace metals originally trapped in the coal being released by the process.

...results from the present exploratory study show that the trace elements silver, arsenic, molybdenum, lead, antimony, selenium, titanium, thallium, vanadium and iodine remained below our detection limits in all samples. Further experiments with more sensitive analytical work are required to clarify mobilization potential for these elements.

Significant concentration increases were observed for the trace elements  beryllium, cadmium, mercury and zinc, and probably also for cobalt and manganese, but beryllium and mercury remained below drinking water standards. Concentrations of boron, barium and copper actually decreased as a result of reaction with CO2.

The degree of contamination of the product water with trace elements will determine what disposal methods might be used.

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Comments

Aussie

What happens if the CO2 mixes with the CH4?
What if the CH4 (21 times worse greenhouse effect) leaks out somewhere else?
How much net energy surplus does this leave?
Can the briny water be used for anything?

On a positive note at least this research is being made public.

Engineer-Poet

CH4 already leaks from coal seams; recovering it is better than releasing it.

I pondered the possibility of using brines as media for growth of halophilic algae (eliminating the problem of competition and predation by creating an environment where only the desired strains can grow), but without information about the brines (esp. toxic metals and other contaminants) it's impossible to do more than guess at feasibility.

K

To my mind nothing is more counterintuitive than jamming CO2 underground to get rid of it. And the idea that it might also produce a secondary benefit seems almost insulting.

Yet the studies and statistics about the capacity and costs of sequestration are impressive. I hope everyone keeps an open mind until more facts arrive; a scheme which seems like lunacy may actually prove worthwhile.

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