Abstract
Coal deformation is one of the leading problems for carbon dioxide (CO2) sequestration in coal seams especially with respect to different-phase CO2 injection. In this paper, a series of core flooding tests were conducted under different confining stresses (8–20 MPa), injection pressures (1–15 MPa), and downstream pressures (0.1–10 MPa) at 50 °C temperature to investigate the effects of coal deformation induced by adsorption and effective stress on sub-critical, super-critical, and mixed-phase CO2 permeability. Due to the linear relationship between the mean flow rate and the pressure gradient, Darcy Law was applied on different-phase CO2 flow. Experimental results indicate that: (1) Under the same effective stress, sub-critical CO2 permeability > mixed-phase CO2 permeability > super-critical CO2 permeability. (2) For sub-critical CO2 flow, the initial volumetric strain is mainly attributed to adsorption-induced swelling. A temporary drop in permeability was observed. (3) For super-critical CO2 flow, when the injection pressure is over 10 MPa, effective-stress-generated deformation is dominant over the adsorption-induced strain and mainly contributes to the volumetric strain change. Thus, there is a linear increase of the volumetric strain with mean pore pressure and super-critical CO2 permeability increased with volumetric strain. (4) For mixed-phase CO2 flow, coupling effects of adsorption-induced swelling and effective stress on the volumetric strain were observed but effective stress made more of a contribution. CO2 permeability consistently increased with the volumetric strain. This paper reveals the swelling mechanism of different-phase CO2 injections and its effect on coal permeability.
Highlights
Since carbon dioxide (CO2 ) is one of the leading greenhouse gases, many researchers are trying to find the appropriate methods to capture and store anthropogenic CO2 emissions [1,2]
This paper reveals the swelling mechanism of different-phase CO2 injections and its effect on coal permeability
The permeability experiments for sub-critical, super-critical, and mixed-phase CO2 were performed in a drain condition to measure the flow rates under different effective stresses and the mean flow rates were investigated to estimate whether Darcy’s Law can be applied
Summary
Since carbon dioxide (CO2 ) is one of the leading greenhouse gases, many researchers are trying to find the appropriate methods to capture and store anthropogenic CO2 emissions [1,2]. CO2 sequestration in deep un-minable coal seams is currently identified as one of the promising solutions to reduce CO2 emissions due to its potential large-scale storage capacity [3,4]. According to Gale’s estimation [5], it will take less than $110 per ton of CO2 sequestration in coal seams if overall 148 Gt of CO2 could be stored in worldwide coal basins. The permeability of the coal matrix is very low due to the narrow flow path while cleats network will effectively improve the overall permeability. Large block samples contain more natural cleats and fractures, which can reflect the effect of cleats and fractures on the permeability. A literature investigation was conducted to study the effect of natural coal sample size on permeability (Table 1). It can be seen that the permeabilities of large coal samples are clearly larger than those of small samples by several orders of magnitude due to the contribution of more cleats and fractures in large coal samples
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