In Situ Synchrotron X‐Ray Microtomography Observations of Fracture Network Evolution of Coal Due to Waterflooding

Abstract

Water adsorption on coal plays a significant role in the process of CO 2 sequestration and enhanced coalbed methane recovery. Direct evidence of how coal adsorbs water and swells, affecting permeability, is still limited. Here, we studied the impact of waterflooding on fracture networks in coal by means of in situ synchrotron X-ray microtomography combined with permeability measurements. We demonstrated that swelling-induced fracture closure was responsible for an order of magnitude permeability reduction after waterflooding for over 4 days. Permeability loss was found to be time dependent, following a logistic function, and about 80% permeability reduction happened in the first 24 hr. There were probably three driven forces for water uptake, including hydrodynamic forces in fractures and macropores, capillary forces in micropores, and diffusion from micropores into deeper coal matrix. Swelling of coal matrix narrowed down and even closed the fractures and as a result weakened fracture connectivity. Residual fractures were mainly mineral-supported fractures, which have strong resistance to swelling-induced stresses.Plain Language Summary CO 2 storage in coal and simultaneously enhanced coalbed methane recovery (CO 2 -ECBM) is a global-warming mitigation technique with dual benefit. Porosity and permeability of coal are critical for CO 2 -ECBM, which determine the CO 2 storage capacity and methane productivity. Although it is widely reported that coalbed water adsorption on coal causes coal matrix swelling and reduces coal permeability, there is a lack of understanding the evolution of coal fracture networks and explaining the resulting permeability changes. We thus directly characterized and quantified the fracture network changes due to waterflooding using in situ synchrotron X-ray microtomography combined with permeability measurements. We identified the direct evidence-fracture closure resulting from adsorption-induced coal swelling-responsible for the permeability reduction after waterflooding. The existing fractures were found to narrow down readily and even close, causing an order of magnitude permeability reduction after waterflooding for over 4 days. Swelling process was found to be time dependent, and approximately 80% permeability reduction occurred in the first 24 hr. We also found that some residual fractures supported by minerals, which have strong resistance to the swelling-induced stresses, still remained after waterflooding.

Influence of H2O on Adsorbed CH4 on Coal Displaced by CO2 Injection: Implication for CO2 Sequestration in Coal Seam with Enhanced CH4 Recovery (CO2-ECBM)

“…The aforementioned results conform to the current knowledge regarding surface chemical properties of various rank coals. , Particularly, the coal matrix underwent decarboxylation and dehydration reactions and gradually aromatized and graphitized with an elevated coalification degree, causing decomposition and transformation of the oxygenic functional group and enhanced graphitization of coal. Furthermore, the content of the oxygenic functional group of all moisture-equilibrated coals is less than that of dry coals due to H 2 O adsorption on the oxygenic functional group, the hydrophilic −COOH and CO in particular . The decreasing amplitudes of −COOH and CO obey the order of SCG > DQ > SH, consistent with the order of the content of the oxygenic functional group of these coals.…”

“…Generally, the H 2 O adsorption on coal is greatly dependent on both the oxygenic functional group and pore structure of coal . On one hand, the oxygenic functional groups, particularly −OH and −COOH with high binding energy, can interact with H 2 O via hydrogen bonds . Moreover, the van der Waals force and the electrostatic force also favor H 2 O adsorption on the oxygenic functional group.…”

“…Furthermore, the content of the oxygenic functional group of all moisture-equilibrated coals is less than that of dry coals due to H 2 O adsorption on the oxygenic functional group, the hydrophilic −COOH and CO in particular. 76 The decreasing amplitudes of −COOH and CO obey the order of SCG > DQ > SH, consistent with the order of the content of the oxygenic functional group of these coals. Therefore, the weakening effect of H 2 O on the oxygenic functional group of these coals is positively correlated with their content.…”

Energy Evolution and AE Failure Precursory Characteristics of Rocks with Different Rockburst Proneness

“…Many research studies have clarified that the mechanical behavior of rock and energy storage capability are essentially determined by mineral composition, micromorphology, and combination mode [18][19][20][21][22][23][24][25]. e mineral composition and micromorphology characteristics of two kinds of rocks are obtained by combining powder X-ray diffraction (XRD) and binocular transmission polarization microscopy.…”

Influence of Joint Angle on the Acoustic Emission Evolution Characteristics and Energy Dissipation Rule of Rock Mass

“…A large number of studies show that there is acoustic emission in the process of damage and fracture of rock mass, which can be used to effectively monitor tunnel rockburst (Shiotani et al 2001;Hirata et al 2007;Cai et al 2007;He et al 2010;Wang et al 2018Wang et al , 2020aShi et al 2020). At the same time, the rock deformation under load is a process of energy transformation and transfer, and its ultimate failure is also a state instability phenomenon driven by internal energy (Izquierdo and Chiang 2004;Xie et al 2004;Dong et al 2018;Zhang et al 2020;Xu et al 2020). Using energy principle to analyze the energy evolution law of rock in the process of deformation and failure can also provide new ideas for rock theory and engineering practice.…”