A theoretical model for gas adsorption-induced coal swelling

Pan, Zhejun; Connell, Luke D.

doi:10.1016/j.coal.2006.04.006

Cited by 496 publications

(259 citation statements)

References 9 publications

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“…Above 1.1 mmol·g coal -1 , the slope gradually decreases until flattening at ~1.7 mmol·g coal -1 . This effect is recognized by other authors, and is attributed to superposition of adsorption-induced and poroelastic strains (Hol and Spiers, 2012;Pan and Connell, 2007;Vandamme et al, 2010), where at higher CO 2 pressure (here: above ~4 MPa) poroelastic compression of the matrix significantly affects the measured swelling strains. Correcting the data of Day et al (2008) for poroelastic effects (following Hol and Spiers, 2012), yields a roughly linear trend that extends down to ~0.2 mmol·g coal -1 (Fig.…”

mentioning

confidence: 77%

Sorption and changes in bulk modulus of coal — experimental evidence and governing mechanisms for CBM and ECBM applications

Hol

Gensterblum

Massarotto

2014

International Journal of Coal Geology

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Recent studies report that the stiffness of coal changes, when exposed to sorbing gases such as CH 4 and CO 2 . Although this effect might be potentially important for predicting the in situ mechanical behavior of coal seams during CH 4 production or CO 2 sequestration, very little understanding exists on the underlying mechanisms. In this paper, we report a single, long-duration mechanical test on a large, cube-shaped coal sample (Ruhr Basin, Germany). The sample was subjected to isostatic loading and unloading; first in the evacuated state (as received), and then each time after exposing it to pressurized, non-adsorbing He, or sorbing N 2 , CH 4 , and CO 2 . We find that exposure to sorbing gases led to swelling, which introduced a hysteresis in the stress-strain trajectory followed during mechanical loading. This hysteresis was almost absent in the evacuated state, and its magnitude depended strongly on the amount of adsorption-induced swelling exhibited by the sample. The apparent bulk modulus determined here for the CO 2 -equilibrated state was approximately 25% lower compared to the evacuated state. We qualitatively consider mechanisms that affect the elastic behavior of coal at the matrix and bulk scale, and argue that the observed effects are a combined effect of 1) changes in surface roughness at cleat interfaces and microfractures, and 2) a thermodynamic coupling between stress state, sorption capacity and swelling strain. The changes in stiffness relate to fluid-rock interaction effects, as well as to the presence of fractures, and hence not to the inherent elastic properties of the solid phase itself. We discuss the characteristics of each mechanism and the connection of the proposed mechanisms with formation-scale geomechanical effects of coal layers under conditions relevant to CBM/ECBM.

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mentioning

confidence: 77%

Sorption and changes in bulk modulus of coal — experimental evidence and governing mechanisms for CBM and ECBM applications

Hol

Gensterblum

Massarotto

2014

International Journal of Coal Geology

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“…The geometry of pores and porosity value are directly related to coal permeability (Reiss 1980;McKee et al 1987;Seidle and Huitt 1995). Gas adsorption/desorption is a well-known phenomenon that results in coal swelling/shrinkage and is regarded as a key factor for permeability behavior in coalbed methane recovery (Pan and Connell 2007;Pan et al 2010). Besides, coal permeability is also affected by other factors, such as different gases (Chen et al 2011;Siriwardane et al 2009), temperature (Hu et al 2010), and wettability (Han et al 2010).…”

mentioning

confidence: 99%

Coal permeability model on the effect of gas extraction within effective influence zone

Xie

Gao

et al. 2015

Geomech. Geophys. Geo-energ. Geo-resour.

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In situ gas extraction is the main measure adopted by mines to achieve mining safety standards. Coal permeability is significantly influenced by coal shrinkage during gas release. Based on effective stress and additional damage that resulted from gas extraction, a unique model was proposed to describe the spatial and temporal distribution of coal permeability within the effective influence zone. Calculation models for the conditions of one and two extracting boreholes were developed. A parametric study on both two models was also conducted, in which the effect of extraction time, extraction pressure, interval between two boreholes, and extraction pressure combination were qualitatively analyzed. The results demonstrated that with the increase of extraction time, gas extraction can result in less permeability reduction in the beginning but can enhance permeability. Coal permeability obviously increases when extraction time increases but gradually stabilizes, presenting ''high middle and low around'' surrounding the extracting borehole. Meanwhile, a decreasing relative amplification implies that permeability ratio does not increase linearly with the increase in extraction pressure. Both intervals between two extracting boreholes and different combinations of extraction pressure significantly influence the distribution of coal permeability. A comparison with some previous permeability studies was conducted to validate the proposed model, and the predicted evolution results matched the permeability testing data. In conclusion, the proposed model is consistent with the phenomenon in real engineering, and the model results can provide preliminary guidance for the design of extracting borehole distribution.

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“…After a small drawdown, the mechanically induced stress change dominates since the gas desorption-induced strain is small at high pressure [Levine, 1996; (Figures 7b-7e) show the state of stress after the same pressure drawdown with different reservoir formation properties; the dashed semicircles illustrate the initial state of stress for reference. Pan and Connell, 2007;Harpalani, 2013a, 2013b]. Therefore, Mohr's circle moves to the right, as illustrated by the thin solid line in Figure 8.…”

Section: Coal Formation Stability Analysismentioning

confidence: 89%

Evaluation of in situ stress changes with gas depletion of coalbed methane reservoirs

Liu

Harpalani

2014

JGR Solid Earth

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A sound knowledge of the stress path for coalbed methane (CBM) reservoirs is critical for a variety of applications, including dynamic formation stability evaluation, long-term gas production management, and carbon sequestration in coals. Although this problem has been extensively studied for traditional oil and gas reservoirs, it is somewhat unclear for CBM reservoirs. The difference between the stress paths followed in the two reservoir types is expected to be significant given the unique sorption-induced deformation phenomenon associated with gas production from coal. This results in an additional reservoir volumetric strain, which induces a rather "abnormal" loss of horizontal stress with depletion, leading to continuous changes in the subsurface formation stresses, both effective as well as total. It is suspected that stress changes within the reservoir triggers formation failure after significant depletion. This paper describes an experimental study, carried out to measure the horizontal stress under in situ depletion conditions. The results show that the horizontal stress decreases linearly with depletion under in situ conditions. The dynamic stress evolution is theoretically analyzed, based on modified poroelasticity associated with sorption-induced strain effect. Additionally, the failure tendency of the reservoir under in situ conditions is analyzed using the traditional Mohr-Coulomb failure criterion. The results indicate that depletion may lead to coal failure, particularly in deeper coalbeds and ones exhibiting large matrix shrinkage.

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A theoretical model for gas adsorption-induced coal swelling

Cited by 496 publications

References 9 publications

Sorption and changes in bulk modulus of coal — experimental evidence and governing mechanisms for CBM and ECBM applications

Sorption and changes in bulk modulus of coal — experimental evidence and governing mechanisms for CBM and ECBM applications

Coal permeability model on the effect of gas extraction within effective influence zone

Evaluation of in situ stress changes with gas depletion of coalbed methane reservoirs

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