Enhanced Coal Bed Methane Recovery: Using Injection of Nitrogen and Carbon Dioxide Mixture

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 of CBM, and is associated with many environmental hazards. Therefore, it is necessary to find new technologies to recover CBM in 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 called enhanced coal bed methane (ECBM) recovery, and CO2-ECBM and N2-ECBM are the main techniques currently used. In the CO2-ECBM process, methane is desorbed from the seam by injecting more reactive CO2 into the coal seam and, if performed properly, will also result in long-term sequestration of CO2. 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 field CO2-ECBM projects (cf. the Allison project in the San Juan Basin). In the N2-ECBM process, injecting N2 first displaces free CH4 from the seam, creating a zero methane partial pressure, which eventually causes the adsorbed phase CH4 to be released. The rapid N2 breakthrough in producing methane is the major issue experienced in present N2-ECBM field projects (see the Tiffany unit in the San Juan Basin). Therefore, to find the optimum technique for the ECBM process, 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 the N2-ECBM process creates a quicker and higher CBM recovery, it also involves earlier N2 breakthroughs compared to the CO2-ECBM process. Regarding the environmental impact, leakage of CO2 from the reservoir during the CO2-ECBM process creates local hazards for humans, ecosystems, and groundwater, and global hazards such as climate change. Such hazards are minimal with the N2-ECBM process because of the inert nature of N2. However, the CO2-ECBM process also assists in protecting the environment by contributing to the mitigation of the atmospheric CO2 level by the geological sequestration of CO2. If the economic aspect is considered, although the N2-ECBM process involves higher processing cost, it is more economically viable because of the lower quantity of N2 required for the process, which is around 0.5 ft3 of N2 to displace 1 ft3 of methane from the seam, compared to 2–3 ft3 of CO2 for the CO2-ECBM process. However, the considerable contribution to the reduction of atmospheric CO2 levels of the CO2-ECBM process cannot be ignored. The injection of a mixture of CO2 and N2 is believed to create a better production mechanism, and according to field projects (cf. the Fenn Big Valley basin in Alberta, Canada), the N2+CO2–ECBM process offers a higher production rate with early response, and sequestrates a similar amount of CO2 to the CO2-ECBM process. Furthermore, the use of the mixture reduces problems associated with CO2 injection-induced coal swelling and early breakthrough with N2 injection. However, finding the optimum N2+CO2 gas mixture to recover a maximum amount of methane from a coal seam while sequestrating an optimum amount of CO2 is a challenge due to the rank dependency of coal. To date, there is a lack of ECBM applications worldwide because of geological, economic, and policy barriers. In relation to the geological barriers, no ECBM project 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 for CO2 emissions and the strict environmental rules for safe coal mining have also had negative effects. Most importantly, the ECBM technique 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. Keywords: CBM recovery; ECBM techniques; CO2-ECBM process; N2-ECBM process; N2 + CO2-ECBM process