The mechanical behaviour of coal with respect to CO2 sequestration in deep coal seams (original) (raw)
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Theory from fracture mechanics and thermodynamics coupled with the results of experimental studies provides evidence to suggest that the adsorption of carbon dioxide on coal causes a decrease in the coal strength. Coal weakening by the introduction of CO2 to a coal seam may induce fracturing, causing a permeability increase under in situ conditions. Such effects present significant implications for proposals regarding the storage of CO2 in coal seams. A uniaxial and triaxial laboratory study was carried out to explore the effects of the adsorption of CO2 on the compressive strength and permeability of southeast Australian brown coal. Comparison of the stress–strain response of air-saturated and CO2-saturated specimens revealed a compressive strength decrease in the order of 13% and an elastic modulus decrease of about 26% for the uniaxial testing, but no significant strength or elastic modulus decrease for the triaxial testing. The absence of an adsorptive effect on the mechanical behaviour of the triaxial specimens may have been due to an insufficient saturation period under simulated ground conditions, or due to mechanical variability in the brown coal test specimens, however, further testing is required to reveal the reason for the apparent negligible strength reduction with CO2 adsorption at the higher confinement. Carbon dioxide outflow measurements during the stress–strain process demonstrated an initial permeability decrease with pore closure, followed by a significant increase in specimen permeability with fracturing. Issues that require consideration in the application of these results to coal seam CO2 sequestration include: whether the expected regional and localised in situ stresses are sufficient to initiate fracturing with adsorptive weakening; how coal properties (e.g. rank, moisture content) are likely to affect the geomechanical influence of CO2 adsorption, and the expected magnitude of the proposed fracture related permeability increase.
Computers and Geotechnics, 2016
To reduce the adverse effects of global warming, geological sequestration has been suggested, which includes capturing and pumping anthropogenic CO 2 into deep underground formations, such as coalbeds. Despite the advantages of coalbed sequestration, its geomechanical aspects are not well studied. In particular, the coupling between geomechanical and reservoir/adsorption behaviours of coal seam has been largely neglected. This paper aims to address this shortcoming by developing a coupled chemo-poromechanical model that predicts the geomechanical performance of a coal seam in which CO 2 is being injected. Recent studies have shown that the interaction of CO 2 and coal results in changes in geomechanical properties of coal, namely elastic modulus and peak strength. In order to investigate the significance of these changes in the geomechanical response of the storage site, an analytical solution was found to predict the distribution of stress within an axisymmetric reservoir, considering the effect of sorptioninduced changes on mechanical behaviour. The analytical model was then coupled with a simplified dual porous reservoir simulation tool to study the effect of CO 2 injection on reservoir and geomechanical performance of the coal seam. The results of the simulation showed that the changes in geomechanical properties of coal can significantly influence the stress and strain distributions within the formation, and therefore, the permeability distribution. Also, it was found that for the example simulated in this paper the adsorption induced reduction in strength did not influence the extent of the mechanical failure zone of the reservoir.
Journal of Rock Mechanics and Geotechnical Engineering, 2016
To reduce the emissions of carbon dioxide (CO 2) into the atmosphere, it is proposed to inject anthropogenic CO 2 into deep geological formations. Deep un-mineable coalbeds are considered to be possible CO 2 repositories because coal is able to adsorb a large amount of CO 2 inside its microporous structure. However, the response of coalbeds is complex because of coupled flow and mechanical processes. Injection of CO 2 causes coal to swell, which leads to reductions in permeability and hence makes injection more difficult, and at the same time leads to changes in the mechanical properties which can affect the stress state in the coal and overlying strata. The mechanical properties of coal under storage conditions are of importance when assessing the integrity and safety of the storage scheme. On the other hand, the geomechanical response of coalbed will also influence the reservoir performance of coalbed. This paper provides an overview of processes associated with coalbed geosequestration of CO 2 while the importance of geomechanical characteristics of coalbeds is highlighted. The most recent findings about the interactions between gas transport and geomechanical characteristics of coal will be discussed and the essence will be delivered. The author suggests areas for future research efforts to further improve the understanding of enhanced coalbed methane (ECBM) and coalbed geosequestration of CO 2 .
Experimental investigations on the effect of CO 2 on mechanics of coal
To mitigate the adverse effects of greenhouse gases on global climate, several options have been proposed. One of the main options is to reduce gas emissions by the storage of CO 2 in deep underground formations. Coal seams are of interest owing to their naturally stored methane which can be produced during sequestration and thereby partially offset the costs. However, there are some concerns about the practicality of using coal seams for carbon dioxide storage due to insufficient understanding of the associated physical and chemical processes. In order to develop an efficient strategy for coal seam sequestration, variables that affect the transport and mechanical properties need to be investigated. In this paper the results of a series of experimental investigations are reported, which have been conducted to enhance our understanding of the effects of CO 2 adsorption on the mechanical properties of coal. The experimental results show that the adsorption of CO 2 changes the elastic modulus and the strength of coal samples, and that these effects are reversible. Simple models are described that have been developed to represent the adsorption-induced changes on the mechanical properties of coal. SEM images suggest that the coal microstructure may also change in the presence of CO 2 . The adsorption-induced changes in the elastic modulus, strength, and microstructure of coal are believed to be similar to the effects of plasticisers on polymers.
International Journal of Rock Mechanics and Mining Sciences, 2013
We report measurements of deformation, strength and permeability evolution during triaxial compression of initially intact coals. Permeability is continuously measured by the constant pressure differential method, together with axial and volumetric strains for both water (H 2 O) and strongly adsorbing carbon dioxide (CO 2) gas. Strength and Young's modulus increase with increasing confining stress and permeability is hysteretic in the initial reversible deformation regime. As deviatoric stress and strain increase, permeability first decreases as pre-existing cleats close, and then increases as new vertical dilatant microcracks are generated. Post-peak strength the permeability suddenly increases by 3-4 orders-of-magnitude. During loading, the inflection point where permeability begins to increase occurs earlier than the turning point of volumetric strain, which may be explained by the competing processes of axial crack opening and closure of oblique and transverse cracks. The generation of these vertical microcracks does not enhance gas migration in the horizontal direction but will accelerate the rate of gas desorption and weaken the coal. Based on this mechanistic observation, we propose a process-based model for bursting in underground coal seams. Horizontal and vertical stresses redistribute ahead of the mining-face immediately after the excavation and influence pore pressure, permeability, and desorption rate. Due to this redistribution, the zone closest to the mining-face may experience tensile failure. Interior to this zone a region may develop with gas overpressures induced by desorption and this may contribute to the occurrence of coal and gas outbursts. Beyond this, an overstressed zone may initiate shear failure driven by gas pressures if the desorption rate outstrips the rate of drainage. We discuss the implications of this on the instability of coal seams to CO 2 injection and the potential for induced fault slip.
Geological storage of carbon dioxide in the coal seams : from material to the reservoir
2012
CO2 emissions into the atmosphere are recognized to have a significant effect on global warming. Geological storage of CO2 is widely regarded as an essential approach to reduce the impact of such emissions on the environment. Moreover, injecting carbon dioxide in coal bed methane reservoirs facilitates the recovery of the methane naturally present, a process known as enhanced coal bed methane recovery (ECBM). But the swelling of the coal matrix induced by the preferential adsorption by coal of carbon dioxide over the methane in place leads to a closure of the cleat system (a set of small natural fractures) of the reservoir and therefore to a loss of injectivity. This PhD thesis is dedicated to a study of how this injectivity evolves in presence of fluids. We derive two poromechanical dual-porosity models for a coal bed reservoir saturated by a pure fluid. The resulting constitutive equations enable to better understand and model the link between the injectivity of a coal seam and th...
International Journal of Greenhouse Gas Control, 2011
Permeability is one of the most important parameters for CO 2 injection in coal to enhance coalbed methane recovery. Laboratory characterization of coal permeability provides useful information for in situ permeability behavior of coal seams when adsorbing gases such as CO 2 are injected. In this study, a series of experiments have been conducted for coal samples using both non-adsorbing and adsorbing gases at various confining stresses and pore pressures. Our observations have showed that even under controlled stress conditions, coal permeability decreases with respect to pore pressure during the injection of adsorbing gases. In order to find out the causes of permeability decrease for adsorbing gases, a nonadsorbing gas (helium) is used to determine the effective stress coefficient. In these experiments using helium, the impact of gas sorption can be neglected and any permeability reduction is considered as due to the variation in the effective stress, which is controlled by the effective stress coefficient. The results show that the effective stress coefficient is pore pressure dependent and less than unity for the coal samples studied. The permeability reduction from helium experiments is then used to calibrate the subsequent flow-through experiments using adsorbing gases, CH 4 and CO 2 . Through this calibration, the sole effect of sorption-induced strain on permeability change is obtained for these adsorbing gas flow-through experiments. In this paper, experimental results and analyses are reported including how the impact of effective stress coefficient is separated from that of the sorption-induced strain on the evolution of coal permeability. (Z. Pan). methane recovered as an energy source, while providing the additional benefit of reducing greenhouse gas emissions by storing the CO 2 underground .