Stress-dependent fracture porosity and permeability of fractured coal: An in-situ X-ray tomography study

Zhang, Guanglei; Ranjith, P.G.; Liang, Weiguo; Haque, Asadul; Perera, M.S.A.; Li, Dongyin

doi:10.1016/j.coal.2019.103279

Cited by 60 publications

(28 citation statements)

References 49 publications

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“…Overburden stress has the most significant effect on permeability change. Stress effects are straightforward: As confining pressure increases, coal matrix feels an increase in effective stress (the difference between confining pressure and pore pressure), which narrows fracture aperture and lowers the permeability of the sample (Zhang, Ranjith, Liang, et al, 2019). Inversely, as pore pressure goes up, effective stress decreases and thus permeability increases.…”

Section: Resultsmentioning

confidence: 99%

“…Two ends of the sample were ground until flat and subsequently placed in an oven at 105 °C for 24 hr to remove the moisture inside before experiments. Geological conditions and physical properties of coal were detailed elsewhere (Zhang, Ranjith, Liang, et al, 2019). SEM‐EDS maps (Figure S1 in the supporting information) indicate that quartz (SiO 2 ) and kaolinite (Al 2 (Si 2 O 5 )(OH) 4 are the main mineral compositions in the coal.…”

Section: Methodsmentioning

confidence: 99%

“…High‐pressure syringe pumps are used to apply independent confining, axial, and injection pressures on the core sample. Detailed information about the core holder can be found in Zhang, Ranjith, Liang, et al (2019) and Zhang, Ranjith, Wu, et al (2019). The sample was loaded into the core holder where it was prestressed for a period of 24 hr at 5 MPa confining and axial pressures, allowing a full deformation of the rubber (Viton) sleeve.…”

Section: Methodsmentioning

confidence: 99%

“…Synchrotron source is much brighter compared to conventional micro‐CT, providing a powerful tool for investigating dynamic process with both high spatial and time resolution. The setup of the CT scanning system of IMBL was described in previous publications (Zhang, Ranjith, Liang, et al, 2019; Zhang, Ranjith, Wu, et al, 2019). The sample‐to‐detector distance was set large to 5 m, providing phase‐contrast imaging to enhance the edges of different phases in CT images (Mayo et al, 2018; Nesterets et al, 2015).…”

Section: Methodsmentioning

confidence: 99%

“…Enhanced coalbed methane (ECBM) recovery accompanying CO 2 injection is an attractive recovery option with an added benefit of sequestering CO 2 (White et al, 2005). Cleats are naturally occurring fractures in coal seams, accounting for most of the permeability and much of the porosity of coalbed gas reservoir (Laubach et al, 1998; Zhang, Ranjith, Liang, et al (2019)). Porosity and permeability of the fractures play significant roles on the success of CO 2 ‐ECBM projects, as they determine the CO 2 injectivity, storage capacity, and methane productivity (Pan & Connell, 2012; Zhang, Ranjith, Wu, et al, 2019).…”

Section: Introductionmentioning

confidence: 99%

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

Zhang

Ranjith

et al. 2020

Geophysical Research Letters

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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.

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Section: Resultsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 3 more Smart Citations

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

Zhang

Ranjith

et al. 2020

Geophysical Research Letters

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In situ loading of a pore network model for quantitative characterization and visualization of gas seepage in coal rocks

Jiao,

Chen,

Zhang

et al. 2024

Deep Underground Science and Engineering

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The flow characteristics of coalbed methane (CBM) are influenced by the coal rock fracture network, which serves as the primary gas transport channel. This has a significant effect on the permeability performance of coal reservoirs. In any case, the traditional techniques of coal rock fracture observation are unable to precisely define the flow of CBM. In this study, coal samples were subjected to an in situ loading scanning test in order to create a pore network model (PNM) and determine the pore and fracture dynamic evolution law of the samples in the loading path. On this basis, the structural characteristic parameters of the samples were extracted from the PNM and the impact on the permeability performance of CBM was assessed. The findings demonstrate that the coal samples' internal porosity increases by 2.039% under uniaxial loading, the average throat pore radius increases by 205.5 to 36.1 μm, and the loading has an impact on the distribution and morphology of the pores in the coal rock. The PNM was loaded into the finite element program COMSOL for seepage modeling, and the M3 stage showed isolated pore connectivity to produce microscopic fissures, which could serve as seepage channels. In order to confirm the viability of the PNM and COMSOL docking technology, the streamline distribution law of pressure and velocity fields during the coal sample loading process was examined. The absolute permeability of the coal samples was also obtained in order for comparison with the measured results. The macroscopic CBM flow mechanism in complex low‐permeability coal rocks can be revealed through three‐dimensional reconstruction of the microscopic fracture structure and seepage simulation. This study lays the groundwork for the fine description and evaluation of coal reservoirs as well as the precise prediction of gas production in CBM wells.

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An experimental investigation of the nonlinear gas flow and stress‐dependent permeability of shale fractures

Shen

Zhou

et al. 2020

Energy Science & Engineering

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The fluid flow characteristics in shale fractures are of great significance for shale gas reservoir evaluation and exploitation. In this study, artificial tension fractures in shale were used to simulate the hydraulic fractures formed by fracturing, and a gas flow test under different pressure gradients was conducted. The nonlinear gas flow and stress‐dependent permeability characteristics were analyzed. The experimental results show the following: (a) CO2 flow in shale fractures exhibits strong nonlinearity. Forchheimer's law, which considers gas compressibility, satisfactorily describes the nonlinear relationship between the flow rate and the pressure gradients in shale fractures. (b) The permeability sensitivity of shale fractures under stress is very strong, and the exponential relationship better describes the pressure dependency of the permeability for the tested shale samples. The permeability of the shale fractures is similar when measured parallel or perpendicular to bedding. Furthermore, the pressure dependence of fractures in shale obeys the Walsh permeability model. (c) As the effective stress increases, the nonlinear flow behavior appears earlier. Based on the Reynolds number and the nonlinear coefficient, a friction factor model is proposed. (d) The normalized transmissivity exhibits a strong correlation with the Reynolds number. CO2 flow through shale fractures is generally dominated by transitional flow. The critical Reynolds number ranges from 1.8 to 102.88 and decreases with increasing effective stress.

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Stress-dependent fracture porosity and permeability of fractured coal: An in-situ X-ray tomography study

Cited by 60 publications

References 49 publications

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

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

In situ loading of a pore network model for quantitative characterization and visualization of gas seepage in coal rocks

An experimental investigation of the nonlinear gas flow and stress‐dependent permeability of shale fractures

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