Enhanced Coal Bed Methane Recovery: Using Injection of Nitrogen and Carbon Dioxide Mixture
Authors: Perera M. S. A. , Ranjith P. G. 
Published: July 2015

Conventionally, coal bed methane (CBM) is produced by pumping out naturally existing pore fluid (water). However, this takes extensive time, does not produce commercially viable amounts ofCBM, and is associated with many environmental hazards. Therefore, it is necessary to find new technologies to recoverCBMin a safer and more economical way. The process of injecting a gas or a mixture of gases into a coal seam to enhance methane recovery is calledenhanced coal bed methane (ECBM) recovery, andCO2‐ECBMandN2‐ECBMare the main techniques currently used. In theCO2‐ECBMprocess, methane is desorbed from the seam by injecting more reactiveCO2into the coal seam and, if performed properly, will also result in long‐term sequestration ofCO2. However,CO2‐adsorption‐induced swelling in coal causes reduced fracture pore space and gas flow through the coal seam; two disadvantages that have negatively affected fieldCO2‐ECBMprojects (cf. the Allison project in the San Juan Basin). In theN2‐ECBMprocess, injectingN2first displaces freeCH4from the seam, creating a zero methane partial pressure, which eventually causes the adsorbed phaseCH4to be released. The rapidN2breakthrough in producing methane is the major issue experienced in presentN2‐ECBMfield projects (see the Tiffany unit in the San Juan Basin).

Therefore, to find the optimum technique for theECBMprocess, the merits and demerits of the two processes need to be compared in relation to productivity, environmental impact, and economical aspects. In relation to productivity, although theN2‐ECBMprocess creates a quicker and higherCBMrecovery, it also involves earlierN2breakthroughs compared to theCO2‐ECBMprocess. Regarding the environmental impact, leakage ofCO2from the reservoir during theCO2‐ECBMprocess creates local hazards for humans, ecosystems, and groundwater, and global hazards such as climate change. Such hazards are minimal with theN2‐ECBMprocess because of the inert nature ofN2. However, theCO2‐ECBMprocess also assists in protecting the environment by contributing to the mitigation of the atmosphericCO2level by the geological sequestration ofCO2. If the economic aspect is considered, although theN2‐ECBMprocess involves higher processing cost, it is more economically viable because of the lower quantity ofN2required for the process, which is around 0.5 ft3ofN2to displace 1 ft3of methane from the seam, compared to 2–3 ft3ofCO2for theCO2‐ECBMprocess. However, the considerable contribution to the reduction of atmosphericCO2levels of theCO2‐ECBMprocess cannot be ignored. The injection of a mixture ofCO2andN2is believed to create a better production mechanism, and according to field projects (cf. the Fenn Big Valley basin in Alberta, Canada), theN2+CO2–ECBMprocess offers a higher production rate with early response, and sequestrates a similar amount ofCO2to theCO2‐ECBMprocess. Furthermore, the use of the mixture reduces problems associated withCO2injection‐induced coal swelling and early breakthrough withN2injection. However, finding the optimumN2+CO2gas mixture to recover a maximum amount of methane from a coal seam while sequestrating an optimum amount ofCO2is a challenge due to the rank dependency of coal.

To date, there is a lack ofECBMapplications worldwide because of geological, economic, and policy barriers. In relation to the geological barriers, noECBMproject will be economical if there is not a commercially viable amount of gas in the coal seam or the available gas is difficult to harvest because of the geological condition of the reservoir. The large capital cost associated with drilling, exploration, production, and field‐scale testing with limited return on investment is the main economic barrier and has resulted in less investment. Moreover, the current lack of penalties forCO2emissions and the strict environmental rules for safe coal mining have also had negative effects. Most importantly, theECBMtechnique is still in its infancy because of lack of knowledge of the process due to the complex hydro‐chemical–mechanical behavior of coal during the injection process.