Carbon dioxide sequestration in coal: Characterization of matrix deformation, sorption capacity and dynamic permeability at in-situ stress conditions.

摘要

Sequestration of anthropogenic carbon dioxide in geological formation is one of the climate change mitigation options. The successful application of this technology is dependent on reliable estimates of carbon dioxide storage capacity and insightful indication of the variability of geological storage. Injection into deep, unmineable coal formations is one option being investigated. Current basic CO2 sequestration work on powdered coal does not adequately capture the science of gas flow and storage processes at geologic sequestration conditions. To better assess the storage capacity and flow properties of CO2 and CH4 in coal, it is important to characterize the interplay of the various physical and chemical processes occurring during gas injection or production at simulated confining stress conditions representative of sequestration depths. In this thesis, the interactions of CO2 and CH4 with powder, non-powder unconfined, and non-powder confined bituminous coal were investigated under different stress conditions. The effects of stress on coal behavior at simulated sequestration conditions were determined. It includes the characterization of three-dimensional regional strain distribution induced by the application of stress, the sorption and desorption of CO 2 determined with X-ray computer tomography. Carbon dioxide and methane sorption capacity was evaluated using the volumetric method. Dynamic permeability of coal under different stress condition was determined using a transient-pulse approach. Results demonstrated that the deformations of a dry bituminous coal core upon stress application, or by CO2 sorption or desorption are highly heterogeneous and differ among the various lithotype bands of a bituminous coal. Coal swelling was observed, but the extent was attenuated by compression/compaction in adjacent lithotype bands. The CO2 and CH4 sorption capacity and sorption rates are reduced under stress, emphasizing that estimates of storage capacity and transport parameters based on powdered unconfined coal samples are misleading for sequestration capacity predictions. The application of 6.9 and 13.8 MPa of confining stress contributed to 39% and 64% of CO2 sorption capacity reductions respectively in comparison to powder coal. Similarly, 85% and 91% CH 4 uptake capacity reductions due to 6.9 and 13.8 MPa of confining stress in comparison to powder coal were recorded. Due to presumed methane's limited ability to dissolve in coal matrix compared to CO2, its sorption capacity reduction with applied stress was more pronounced. Coal permeability decreases with increasing confining stress. Average permeability was 0.001865 millidarcies when subjected to 6.9 MPa and decrease around 4 times to 0.000427 millidarcies when the confining stress was doubled. This decrease is likely due to the cleat and pore aperture reduction with increasing effective stress. Coal permeability also decreases over time even at constant effective stress. Permeability for both confining stress values decrease over time although at different rate: 26% reduction at 6.9 MPa and 47% at 13.8 MPa. This reduction can likely be attributed to the swelling, structure rearrangement or compression/compaction of certain lithotypes occurring at different time-scales when exposed to CO2.