Coal Beds

 

Another method for carbon dioxide sequestration is storage in coal beds.  Coal beds are present globally, and are an option for sequestration on almost every continent.  This method of sequestration is particularly effective because it both stores CO₂ and improves recovery of methane.

 

What is a coal bed?

 

source Exposed Coal Seams

Orthogonal fractures known as cleats occur naturally in coal beds.  Cleats make the bed somwhat permeable. Coal is not conventionally porous, but it does contain mircopores (<20 Å) and mesopores (20-500 Å), within which methane can be found [1] .

 

Methane (CH₄) is present as a by-product of coal formation, either adsorbed to the surface of the coal or dispersed throughout its pores [2] .  Methane is valuable as natural gas, but recovery using the conventional method, reservoir pressure depletion, is only 50% efficient [3] .  Currently, new technology is being developed for enhanced coal-bed methane (ECBM) production involving injection of carbon dioxide. In the presence of CO₂, methane desorption occurs and carbon dioxide adsorbs to the coal in its place. CO₂ adsorbed to coal is in a stable state, and will be sequestered as long as the coal remains unmined. 

 

Sequestration in unmineable coal beds is an attractive alternative for many different countries.  The Netherlands and Belium have expressed interest [4] , and other potential sites include Poland, China, India, and Australia [5] .  This method is already being used with success in New Mexico and Canada.  In the United States alone, there are currently 6 trillion tons of coal available, 90% of which is unmineable due to bed thickness, structural integrity, and depth [6] .  Worldwide, there is the possibility to sequester more than 150 Gt CO₂ [7] .

 

There are several criteria for the coal beds to make them suitable for CO₂ sequestration:

·         homogeneous structure: isolated vertically from the surrounding strata and horizontally continuous

·         minimal faults and folds.

·         moderate permeability (1-5 mDarcy)

·         300-1500m deep

·         concentrated coal deposits with thick beds

·         gas-saturated

·         low ash content

·         unmineable coal [8]

 

The specific mechanisms involved in the replacement of methane with carbon dioxide are still being investigated.  Theoretically, two CO₂ molecules should replace every one CH₄ molecule, but in practice the ratio seems to be over three to one [9] .

 

 

Advantages to Coal Bed Sequestration

 

Besides the fact that ECBM serves a dual purpose-recovering natural gas while sequestering a greenhouse gas, there are a many advantages to CO₂ sequestration in coal beds over other methods.   Between 5 and 15 Gt CO₂ can be sequestered at a profit, and about 60 Gt can be sequestered at a reasonable price (under $50/t CO₂) [10] .  In addition, unmineable coal beds are often near CO₂ high emissions locations [11] , lowering transportation costs. 

 

 

An Attractive Alternative

 

Instead of injecting pure CO₂, which can be costly and somewhat inefficient, flue gas offers an attractive alternative. Flue gas is a mixture of CO₂ and N₂.  It costs less to obtain and can enhance methane production better than pure CO₂.  The drawbacks are that less carbon dioxide will be sequestered and that an extra step of separating nitrogen from recovered methane will be required.  Further research must be done to determine the ideal mixture of CO₂ and N₂ [12] _.

 

 

Field Tests

 

The Allison Unit ECBM project in the San Juan Basin of New Mexico has been sponsored by Burlington Resources since 1996.  As of 2000, the project had injected 1.33 x 10⁸ m³ of CO₂ from McElmo Dome into coal beds using four CO₂ injection wells and nine methane production wells.  A minimal amount of carbon dioxide broke through and came up with the methane.  At the San Juan Tiffany Unit in Colorado, 2.3 x 10⁵ m³/day CO₂ is planned to be injected [13] .

 

The Alberta Research Council of Alberta, Canada is currently working to use CO₂-ECBM to recover methane from Canada's low permeability coal reservoirs [14] .  The project was divided into three phases: I) proof of concept study (feasibility), II) design and implementation of a micro-pilot test, and III) design and implementation of a full scale project.  Phase I was completed without problems in July 1997, and Phase II was successfully completed in April 1999. The testers were able to accurately measure data from injecting carbon dioxide and recovering methane from a single well and evaluate the data to estimate reservoir properties and sorption behavior. From this small-scale pilot test, the researchers estimated the behavior for the full-scale project. The full-scale project is currently underway at Fenn-Big Valley, Canada [15] .