Challenges and issues for CO2 storage in deep coal seams

A. S. Ranathunga, P. G. Ranjith, M. S.A. Perera

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Abstract

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.

Original languageEnglish
Title of host publicationRock Mechanics and Engineering Volume 4
Subtitle of host publicationExcavation, Support and Monitoring
EditorsXia-Ting Feng
Place of PublicationWiltshire, Great Britain
PublisherCRC Press
Chapter3
Pages87-119
Number of pages33
ISBN (Electronic)9781317481911
ISBN (Print)9781138027626
DOIs
Publication statusPublished - 18 May 2017

Publication series

NameRock Mechanics and Engineering
PublisherCRC Press
Volume4

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