Micro-scale fracturing mechanisms in coal induced by adsorption of supercritical CO2

doi:10.1016/j.coal.2017.04.002

International Journal of Coal Geology

Volume 175, 15 April 2017, Pages 40-50

https://doi.org/10.1016/j.coal.2017.04.002 Get rights and content

Highlights

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A new method is proposed for predicting micro-mechanical coal behaviour
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Mechanistic evidence in terms of how swelling stress fractures the unswelling (mineral) phase in coal is provided
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Microscale rock mechanical behaviour and internal swelling stress fields in coal were successfully predicted

Abstract

Coal bed methane production can be assisted by CO₂ injection. However, CO₂ adsorption in the coal matrix leads to a dramatic reduction in permeability and an associated change in microstructure caused by coal matrix swelling. Furthermore, it has been recently observed that the induced swelling stress fractures the unswelling (mineral) phase in laboratory investigations. However, the failure mechanisms are still not understood, and the way internal swelling stresses are generated is not clear. Thus, in this paper, we propose a new method which combines X-ray microtomography imaging, nanoindentation testing and DEM modelling with which we can predict the rock mechanical performance at micro scale. Indeed we successfully simulated such swelling processes inside a coal sample, including a simulation of the fracture mechanism of the mineral phase, and a quantification of the in-situ von Mises stresses generated by swelling. We conclude that our proposed method is an efficient way for analysis and prediction of coal microfracturing and the associated microscale rock mechanical behavior.

Introduction

Coal bed methane (CBM) is an unconventional energy resource, which exists in coal mines and deep unmineable coal seams (Hamawand et al., 2013). Recently, due to the decline in conventional energy resources coupled with a globally increasing energy demand (Lior, 2008), CBM has gained increasing popularity (Connell et al., 2011, Pillalamarry et al., 2011, Hamilton et al., 2015, Vishal et al., 2015). Furthermore, CBM can be enhanced (enhanced coal bed methane, ECBM), e.g. through CO₂ injection, which efficiently displaces CH₄ from the coal matrix (White et al., 2005, Saghafi, 2010). However, CO₂ injection dramatically reduces the coal seam's permeability (Mazumder et al., 2006, Siriwardane et al., 2009, Anggara et al., 2016), which largely limits application of this technology. Mechanistically, cleats (the main flow conduits in coal) close due to coal matrix swelling induced by CO₂ adsorption (Shi and Durucan, 2005, Wu et al., 2011, Zhang et al., 2016a, Liu and Rutqvist, 2010, Espinoza et al., 2014) and it has recently been discovered that the swelling stress in the coal matrix can fracture the unswelling phase (i.e. inorganic mineral), (Zhang et al., 2016a). However, the detailed failure mechanisms and swelling stress quantification are still poorly understood due to only limited theoretical understanding of the micro-scale rock mechanical performance. It is thus of vital importance to further understand these mechanical changes in the coal so that advanced ECBM techniques can be developed.

The mechanical properties of small areas (up to nanoscale) on a material's surface can now be obtained by nanoindentation measurements; such method has for instance been applied to natural rock samples including sandstone, limestone, shale and coal (Zhu et al., 2009, Bobko et al., 2011, Lebedev et al., 2014, Manjunath and Nair, 2015, Vialle and Lebedev, 2015, Liu et al., 2016). Thus nanoindentation gives us a way to identify the mechanical properties of heterogeneous coal (note that coal consists of the organic coal base matrix, inorganic minerals and pores). These mechanical properties are essential input data into numerical models, which can predict the mechanical behavior of the whole (heterogeneous) material. Earlier studies considered the coal matrix as a homogenous elastic continuum (e.g. Izadi et al., 2011), which obviously cannot capture the clearly heterogeneous character of the coal, and thus can only provide rather biased predictions. To overcome this serious limitation we use discrete element method (DEM) modelling (cp. Cundall and Strack, 1979, Wang et al., 2014, Zhang et al., 2016e, Bai et al., 2016), where each material – coal matrix, mineral and void are assigned their respective true and individual mechanical properties, and combine this with high resolution x-ray micro-computed tomography (microCT) imaging, which can provide the detailed 3D morphology of the coal (Zhang et al., 2016b, Zhang et al., 2016c, Zhang et al., 2016d, Jing et al., 2016, Mostaghimi et al., 2017). Thus, in this paper, using this approach, we were able to quantify the swelling stresses generated by supercritical CO₂ injection into coal, and to identify the failure mechanisms occurring in the un-swelling phase.

Section snippets

Experimental work

A small cylindrical coal plug (5 mm diameter and 10 mm length) was cut from a heterogeneous subbituminous medium rank coal block obtained from a coal seam at ~ 650 m to 700 m depth (buried at Permian period) from Pingdingshan coal mine, China; the generalized stratigraphic column is shown in Fig. 1. The coal had a 54% (± 2%) carbon content and a 36% (± 1%) volatile matter content (measured by Chinese Standard GB/T 212-2008 and DL/T 1030-2006; Xu et al., 2016, Zhang et al., 2016d), additional

The stress-strain method

Initially we estimated the swelling stress via the traditional stress-strain method using the volume fractions measured (i.e. strains measured) on the microCT images, Table 2 (note that the volume strain ε = (the volume difference before and after CO₂ flooding)/(original volume). Negative values represent compression, while positive values represent expansion.

What is more, in an elastic 3D coordinate system, ε has following relation with Young's modulus (E), stress (σ), and Poisson's ratio (υ),

Discrete element method (DEM) simulation

The Discrete Element Method (DEM) has become a powerful numerical tool for analyzing the dynamic mechanical behavior of complex objects (Cundall and Strack, 1979, Scholtès and Donzé, 2012). Precisely, DEM models objects as an assembly of interacting particles, and the key advantage of DEM is that specific attributes (features, bonds, contacts, frictions and boundary conditions) can be assigned to each particle (and thus each material) simultaneously. We used the popular DEM built-in software –

Nanoindentation testing

The IBIS nanoindentation system and Berkovich nano-indenter were chosen for the nanoindentation tests, Fig. 7. A cuboid coal sample (l × w × h = 5 mm × 5 mm × 2 mm) was cut and carefully polished, and mounted on the objective stage. Subsequently the penetration depth (h) – loading/unloading force (P) curves were measured for each test point. Specifically 625 data points on a symmetric 25 × 25 grid (240 μm × 240 μm spacing) were measured. The maxim loading force was set to 4 mN (which is smaller than the one used

Results and discussion

The DEM simulations successfully predicted the change in coal microstructure caused by coal matrix swelling, which again was induced by scCO₂ injection (see Fig. 8, Fig. 9). Clearly cracks appeared in the mineral phase when the coal matrix volume increased by 1%, consistent with the experimental microCT observations (cp. Fig. 3). Most failures in the calcite mineral phase were identified as tensile failures (red colored cracks in Fig. 8, Fig. 9) these appeared during the coal matrix swelling.

Conclusions

CO₂ can be injected into coal seams to enhance methane production (White et al., 2005, Saghafi, 2010); however, the resulting coal matrix swelling effect leads to coal cleat closure and a dramatic permeability reduction (Karacan, 2003, Zhang et al., 2016f); furthermore, it has been recently observed that the unswelling phase fractured due to the induced swelling stresses (Zhang et al., 2016b). However, how precisely such swelling stresses are generated and the associated failure mechanisms in

Acknowledgements

The measurements were performed using the microCT system courtesy of the National Geosequestration Laboratory (NGL) of Australia, funding for the facility was provided by the Australian Government. This work was also supported by resources provided by the Pawsey Supercomputing Centre with funding from the Australian Government and the Government of Western Australia.

References (57)

F. Anggara et al.
The correlation between coal swelling and permeability during CO₂ sequestration: a case study using Kushiro low rank coals
Int. J. Coal Geol.
(2016)
Q.-S. Bai et al.
Discrete element modeling of progressive failure in a wide coal roadway from water-rich roofs
Int. J. Coal Geol.
(2016)
L.D. Connell et al.
History matching of enhanced coal bed methane laboratory core flood tests
Int. J. Coal Geol.
(2011)
D.N. Espinoza et al.
Measurement and modeling of adsorptive–poromechanical properties of bituminous coal cores exposed to CO₂: adsorption, swelling strains, swelling stresses and impact on fracture permeability
Int. J. Coal Geol.
(2014)
I. Hamawand et al.
Coal seam gas and associated water: a review paper
Renew. Sust. Energ. Rev.
(2013)
S.K. Hamilton et al.
Conceptual exploration targeting for microbially enhanced coal bed methane (MECoM) in the Walloon Subgroup, eastern Surat Basin, Australia
Int. J. Coal Geol.
(2015)
S. Hashemi et al.
Investigation of borehole stability in poorly cemented granular formations by discrete element method
J. Pet. Sci. Eng.
(2014)
G. Izadi et al.
Permeability evolution of fluid-infiltrated coal containing discrete fractures
Int. J. Coal Geol.
(2011)
M. Jiang et al.
DEM simulations of methane hydrate exploitation by thermal recovery and depressurization methods
Comput. Geotech.
(2016)
Y. Jing et al.
Coal cleat reconstruction using micro-computed tomography imaging
Fuel
(2016)

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Micro-scale fracturing mechanisms in coal induced by adsorption of supercritical CO2

Highlights

Abstract

Introduction

Section snippets

Experimental work

The stress-strain method

Discrete element method (DEM) simulation

Nanoindentation testing

Results and discussion

Conclusions

Acknowledgements

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Micro-scale fracturing mechanisms in coal induced by adsorption of supercritical CO₂