Challenges and issues for CO2 storage in deep coal seams

Global warming is one of the most critical environmental issues facing the world today, and according to scientific reports, ignoring this issue will lead to severe economic and environmental costs to society. The increased greenhouse gas concentration in the atmosphere is the major cause of global warming, and the effect of CO₂ is critical. It is therefore necessary to use a rational combination of all the possible CO₂ mitigation strategies, such as the use of renewable fuels, improved energy efficiency, carbon sequestration and increased conservation to achieve a safer CO₂ level in the atmosphere with the rapidly increasing emissions. CO₂ sequestration is considered here.

CO₂ sequestration involves the capture of CO₂ released from anthropogenic sources and its secure storage in deep underground locations, either off- (ocean sequestration) or on-shore (geo-sequestration). However, in terms of safety, reliability and cost, onshore sequestration is more effective than offshore sequestration. Of the various potential CO₂ storage sinks in deep underground (e.g. depleted oil and gas fields, saline aquifers, shale beds and coal mines), CO₂ storage in deep coal seams offers unique advantages over the others, and the potential for offsetting CO₂ sequestration costs by producing a valuable energy product such as methane (coal seam gas). For nearly 20 years, CO₂ has been injected into coal seams to enhance the recovery of methane in a process known as enhanced coal bed methane (ECBM) recovery.

However, adsorption of the injected CO₂ into the coal mass causes it to swell, causing significant alterations in its internal rock mass structure, resulting in major modifications to its flow and strength properties. This CO₂ adsorption-induced coal matrix swelling process is dependent on the properties of both the seam and the injected CO₂, and swelling is reduced with increasing temperature due to the reduced sorption capacity. Furthermore, swelling exhibits inverted-U shaped variation with coal maturity or rank, due to the corresponding variation of the coal mass properties such as moisture, carbon content and pore space. On the other hand, the extent of swelling is largely dependent on the pressure and the physical state of the injected CO₂, and super-critical CO₂ creates much greater swelling than gas or liquid CO₂ due to its higher chemical potential. In addition, high- pressure injection of CO₂ causes the swelling process to be enhanced due to the higher flow ability of the injected CO₂ under reduced effective stress conditions at increased injection pressures. However, potential coal seams for CO₂ sequestration are available at extremely deep locations and there is a high possibility of phase change from gas/liquid to the super-critical state in the underground environment owing to changes in field conditions. This confirms the likely occurrence of high swelling rates in deep coal seams with CO₂ injection.

88The effectiveness of the CO₂ storage process in any coal seam is greatly influenced by the flow ability of the injected CO₂ through the seam, which is dependent on a number of factors. These factors include i) coal matrix swelling that causes the internal coal seam pore space available for fluid/gas movement to be reduced, ii) CO₂ injecting pressure and seam depth, both of which change the effective stress on the coal seam and eventually, the seam’s pore space, and iii) seam temperature that creates thermal expansion in the seam and leads to reduced sorption capacity with kinetic energy increment in injected CO₂ molecules. Importantly, the injected CO₂ phase condition critically influences its flow behavior in the seam, and super-critical, the form of CO₂ expected in deep coal seams, has significantly lower permeability than gas/ liquid CO₂, which causes unpredictable CO₂ injectibility into deep coal seams. Reduction of seam permeability over time after CO₂ injection is a common issue faced by many field-scale CO₂ sequestration projects. For example, there was around 50% reduction in CO₂injectivity at the Allison unit CO₂ sequestration project in the San Juan basin, USA, during the first two years of injection.

Secure storage of injected CO₂ in the seam is critically important in CO₂ geo-sequestration in deep coal seams in terms of environmental safety and health. It is important to prevent the upward migration of CO₂, as this has potential to cause distress and on occasion death, and lateral migration into aquifers and surrounding more permeable geologic strata. These factors are mainly dependent on the coal mass strength properties. However, the fact that CO₂ adsorption-induced swelling creates a weakening effect in deep coal seams is well known. This strength reduction is greatly dependent on injecting CO₂ phase and pressure, it increases with increasing pressure, and super-critical CO₂ causes much greater strength reduction than gas/liquid CO₂. Furthermore, CO₂ adsorption into any coal seam mainly occurs through the walls of its natural cleat system that were formed during the coalification process. High rank coal seams, located deeper underground, are therefore likely to be subjected to a higher strength reduction with CO₂ injection, offering more numerous loci for CO₂ adsorption. However, it is generally accepted that the deeper the seam, the more secure the CO₂ storage. On the other hand, the likely super-critical nature of CO₂ in deep coal seams also causes greater strength reduction with CO₂ injection.

Existing government policies, public perceptions, and strict rules on coal mining affect the implementation of CO₂ sequestration in deep coal seams. For example, mines are generally required to have a maximum 3% CO₂ by volume in the mine air, and therefore injection of CO₂ into coal seams causes the mining environment to be polluted by the injected CO₂ and eventually, the seams become unsafe to mine forever. The current lack of knowledge related to coal seam CO₂ sequestration severely affects the implementation of this process in potential coal seams around the world. The complex heterogeneous nature of coal, which creates location-dependent coal seam properties, is the main reason for the limited knowledge of coal CO₂ sequestration.