In recent time, global warming of the earth is on the rise due to entrapment of greenhouse gases in the atmosphere; and is viewed as the most important environmental challenge with negative consequences threatening both human, animals and plants, hence requires urgent attentions to mitigate the rise. Available data suggest that anthropogenic emission of gases due to oil and gas exploration activities are the major contributors of greenhouse gases and are empirically represented in the following percentage: 〖CO〗_2 (63.5 %), 〖CH〗_4 (19.2 %), N_2 O (5.7 %) while the balance is other sources such as chlorofluorocarbon (CFC) and other human related activities.
In order to reduce or eliminate environmental hazards caused by greenhouse gases that are always entrapped in space resulting to intense heat of the earth, there is a need for the sequestration of 〖CO〗_2 that is produced in oil and gas exploration and production activities for either geological storage, sequestered or injection into a coal bed methane reservoir to enhance gas production. Coal seam gas is one of the unconventional resources that is commercially present in coals across the world especially Australia, China, US and other countries in large quantity and primarily produced by dewatering process. To exploit and produce methane from coal; it involves dewatering process to partially reduce the reservoir pressure in order to release adsorbed methane and the process is technically known as primary recover mechanism.
In contrast to the conventional resources homogenous reservoirs with single porosity network; coal seam is fractured naturally with dual porosity coal matrix (microspores) as the main gas storage system whilst the cleat (macrospores) functions as flow path for adsorbed methane to undergo desorption due to reduction in reservoir pressure(depressurization techniques)by dewatering processes. This associated recovery quantity is about 50% of the gas- in- place. However, the recovery of the remaining gas- in-place that could not be produced due to the limitations of primary recovery mechanism requires an alternative process that can improve recovery and boost production of these earth resources.
Invariably, injection of flue gases, nitrogen (N2) and carbon dioxide(CO2) as second gas into coal formation or coal bed methane reservoir maintain the overall reservoir pressure while partially reducing the pressure of the coal bed methane to free(methane) gas. The coal has high affinity for both 〖CH〗_4 and Carbon di oxide (〖CO〗_2 ). Zhu et al. (2002) concluded that CO2 injection can increase recovery by30% of coal bed methane (CBM) can by relative to the natural depletion. Yi et al. (2006) briefly described the molecular dynamics that takes place within high coal matrix (meso, micro and macro) as an integrated block – like fractured network that functions as a pathway for counter – diffusivity and absorption mechanism of molecules initiated by injected 〖CO〗_2, hence this incoming, flowing and diffusive molecule of 〖CO〗_2 is anticipated to activate and displace methane molecules on the matrix. The process associated with this mechanism of 〖CO〗_2- ECBM process is illustrated with the diagram below.
A cartoon diagram suggesting the mechanism in micro-scale during injection of CO2 and displacement of CH4 suggested that there are various factors that can affect can affect successful CO2 injection such as geological parameters (depth, nature and location of seam) as well as their interaction with other processes such as desorption and adsorption; matrix and grain compression; swelling and shrinkage of coal resulting in permeability alteration.
Furthermore, permeability reduction occurs at initial stage due to the induced strain caused by 〖CO〗_2 absorption and increment in injection caused by push back phenomenon leading to rebound permeability. A flow chart depicting processes of adsorption and desorption of gasses and consequent swelling/shrinkage of coal with alteration in during ECBM recovery via CO2 sequestration” suggested that the flow of gas during injection including the breakthrough time as well as effluent concentration and recovery is affected by the gas composition.
Apart from the limitation imposed by gas composition there are challenges that are yet to be thoroughly investigated such as population data into reservoir simulator for effective prediction of gas recovery due to heterogeneous nature of coal formation.