Coal Swelling Research Articles

CH4, CO2, N2 diffusion in Bowen Basin (Australia) coal: relationship between sorption kinetics of coal core and crushed coal particles

To improve the general understanding of diffusive processes in coal and investigate if representative reservoir properties can be estimated from measurements on crushed coal particles, adsorption and desorption experiments on crushed samples and large, intact core samples are carried out. Diffusion properties are analysed for methane, carbon dioxide and nitrogen up to a pressure of 10 MPa at a temperature of 35 °C. The samples are a low volatile bituminous coal and a medium volatile bituminous coal from Australia's Bowen Basin.A bidisperse model (a simplified version of the Ruckenstein model for micro-pore dominated flow), characterised by a ‘fast’ and a ‘slow’ fraction, is required to describe the observed diffusion behaviour. Results from the modelling show that effective diffusivities of the core samples are typically an order of magnitude lower than for the crushed coals, and the ‘slow’ effective diffusivities are typically at least one order of magnitude lower than the corresponding ‘fast’ effective diffusivities. A pressure dependency of the effective diffusivities is noted but an explicit relationship with gas content could not be established. This may be due to the experimental set-up, which does not eliminate the effect of differential pressure.Calculation of the diffusion path length for the coal core reveals that the diffusion path length in coal may not be constant but decreases with increasing matrix gas content as a result of coal swelling. Furthermore, there is a distinct difference between the diffusion paths derived from the ‘fast’ and the ‘slow’ fractions; the ‘slow’ diffusion path of the coal core is 100s of microns shorter than the ‘fast’ diffusion path. The faster diffusion associated with the longer diffusion path is interpreted to relate to micro-porosity, while slower diffusion via the shorter diffusion path is interpreted to relate to macro-porosity.

A comparative study on the deformation of unconfined coal during the processes of methane desorption with successively decreasing outlet pressure and with constant outlet pressure

The shrinkage of coal induced by methane desorption during coalbed methane (CBM) production will change the porosity and permeability of reservoirs. It is of great significance to analyze the deformation characteristics of coal in the process of desorption with successively decreasing outlet pressure, which is conducive to the continuous production of CBM. In this study, five cylindrical samples in Shanxi Province, China were used to reflect the deformation and desorption characteristic in the process of methane desorption with decreasing outlet pressure (simulated the change of bottom hole pressure in the field CBM production) by independently designed instruments. And the results were compared with that in the process of desorption with constant outlet pressure (atmospheric pressure). The results show that the coal swells (shrinks) when methane is absorbed (desorbed). There is a linear relationship between the strain and adsorption/desorption volume. Moreover, the shape of desorption rate-time curves is similar to that of the Langmuir curves in two desorption processes. However, the cumulative desorption rate of desorption with successively decreasing outlet pressure is higher than that of desorption with constant outlet pressure due to the smaller attenuation effect on permeability, which leads to the shrinkage during desorption with successively decreasing outlet pressure is also higher. The shrinkage of the samples is always less than the swelling. There is a negative correlation between the strain differences and the desorption rate. The strain differences in the process of desorption with successively decreasing outlet pressure are lower than that of desorption with constant outlet pressure due to the higher desorption rate.

Laboratory characterization of coal P-wave velocity variation during adsorption of methane under tri-axial stress condition

Adsorption swelling and desorption shrinkage inside a coalbed affect coal porosity, permeability, stress state and thus the production of coalbed methane, degasification of high gassy coal seam and CO2 enhanced coalbed methane recovery. Since coal P-wave velocity is sensitive to porosity variation, using P-wave velocity to monitor the porosity/permeability behavior inside the coalbed will be helpful. To realize this idea, laboratory characterization of P-wave velocity variation with gas pressure is needed. In this paper, a tri-axial experimental system combined with an ultrasonic instrument are used to measure P-wave velocity and adsorption swelling of bituminous coal. Helium and methane are used to study the effect of effective stress and adsorption swelling on P-wave velocity, respectively. Experiment results show that both effective stress and adsorption swelling affect the P-wave velocity mainly by changing the porosity of coal sample. The calculation model derived from the Palmer and Mansoori permeability model, the Shi and Durucan permeability model and velocity-porosity model shows high consistency with measurement results. Effective stress effect and adsorption swelling effect is evaluated under constant confining pressure condition. Experiment results show that the dominant factor that affects the P-wave velocity varies with confining pressure and pore pressure. Adsorption swelling effect is dominant with a combination of high confining pressure and low pore pressure, and it will leads to a rebound of P-wave velocity curve if the confining pressure is high enough. Furthermore, this rebound matches the conclusion of P&M permeability model that permeability rebound only appears at low porosity.

Geochemistry effects of supercritical CO2 and H2O on the mesopore and macropore structures of high-rank coal from the Qinshui Basin, China

Abstracts The effects of supercritical CO2 (scCO2) on coal pores play a critical role in the geological storage of CO2 and enhanced coalbed methane recovery (CO2-ECBM). To investigate the effects of scCO2 - H2O on the mesopore - macropore structure of high-rank coals, a series of experimental simulations and analyses were performed. Coal samples from the Qinshui Basin, China were exposed to scCO2 and deionized water for 240 h at 45 °C and 10 MPa and at 80 °C and 20 MPa. The influences on the pore structure were analyzed by performing a mercury intrusion capillary pressure (MICP) measurement. Using a field emission scanning electron microscope (FE-SEM) and an advanced mineral identification control system (AMICS), the geochemical effects of scCO2-H2O on the same minerals and the same pore or fracture (nanometer to micrometer scale) before and after the reaction were investigated. Based on the changes of the minerals, pores and fractures, the types, targets, expressions, and scopes of the geochemical effects were explored. The experimental results show that after the injection of scCO2 into the coal samples, the pores and fractures were newly generated or expanded under the interactions of mineral dissolution-precipitation, clay hydration and swelling, coal swelling, and pyrite oxidation with gypsum precipitation. Moreover, the largest improvement of the pore volume was observed in pores larger than 30 μm or smaller than 100 nm, and higher temperatures and pressures were beneficial to the formation of large pores and fractures. The change in the specific surface area (SSA) was mainly caused by the change in pore volume. The corrosion of carbonate minerals showed the most significant contribution to the pore volume of full-scale macropores and mesopores, while most geochemical effects on other minerals had impacts on the mesopores and the macropores smaller than 1 μm. The coal swelling had the most significant effect on the connectivity of the pores in the coal reservoir.

Numerical Analysis of Improvements to CO2 Injectivity in Coal Seams Through Stimulated Fracture Connection to the Injection Well

This work presents a hybrid discrete fracture-dual porosity model of compressible fluid flow, adsorption and geomechanics during CO2 sequestration in coal seams. An application of the model considers the influence of hydraulic fractures on CO2 transport and the stress field of the coal. The low initial permeability of coal is compounded by the injectivity loss associated with adsorption-induced coal swelling, which is recognised as the major challenge limiting CO2 sequestration in coal seams. In this model, the natural fracture network and coal matrix are described by a dual porosity model, and a discrete fracture model with lower-dimensional interface elements explicitly represents any hydraulic fractures. The two models are coupled using the principle of superposition for fluid continuity with a local enrichment approximation for displacement discontinuity occurring at the surface of hydraulic fractures. The Galerkin finite element method is used to solve the coupled governing equations, with the model being verified against analytical solutions and validated against experimental data. The simulation results show that the presence of a hydraulic fracture influences the distribution of gas pressure and improves the gas flow rate, as expected. The stress field of a coal seam is disturbed by CO2 injection, especially the vertical stress, and the presence of a hydraulic fracture leads to a reduction in stress with permeability recovery starting earlier.

Dual porosity modelling of coal core flooding experiments with carbon dioxide

This paper presents the validation of a recently developed dual porosity numerical model and provides insights into coal core flooding experiments with N2 and CO2. Experimental data for anthracite coal from the South Wales Coalfield, UK, allows the coal-gas constitutive behaviour to be defined, leading to the validation of the model using gas flooding data for a mm long and mm diameter core. N2 and CO2 injection scenarios are considered with the coal initially saturated with CH4. It is demonstrated that the model can simulate the physical and chemical phenomena involved in multicomponent gas flow and storage in coal. Further analysis shows that N2 breakthrough in the effluent gas is controlled by dual porosity flow without significant influence of adsorption-desorption, whereas for CO2 this influence is greater. Coal swelling caused by CO2 is identified as the predominant factor, with the preferential displacement of adsorbed CH4 being limited by the time scale of flow across the core relative to the CH4 desorption kinetics. These insights are useful for future experiments concerning the influence of core size. The importance of using sorption data from intact coal rather than powdered coal is highlighted by comparing the numerical predictions and experimental measurements.

A theoretical model for coal swelling induced by gas adsorption in the full pressure range

The phenomenon of coal swelling caused by gas adsorption is well known. For Enhanced Coal Bed Methane Recovery and carbon storage, coal swelling induced by gases adsorption may cause significant reservoir permeability change. In this paper, based on the assumption that the surface energy change caused by adsorption is equal to the change in elastic energy of the coal matrix, a theoretical model is derived to describe coal swelling induced by gas adsorption in the full pressure range. The Langmuir constant, coal density, solid elastic modulus, and Poisson’s ratio are required in this model. These model parameters are easily obtained through laboratory testing. The developed model is verified by available experimental data. The results show that the presented model shows good agreement with the experimental observations of swelling. The model errors are within 14% for pure gas, and within 20% for mixed gas. It is shown that this model is able to describe coal swelling phenomena for full pressure range and different gas type including pure gas and mixed. In addition, it is also shown that the errors of the presented model and the Pan’s model are almost the same, but the presented model is solved more easily.

A study on the carbon dioxide injection into coal seam aiming at enhancing coal bed methane (ECBM) recovery

Coal seams, particularly deep unmineable coal reservoirs, are the most important geological desirable formations to store CO2 for mitigating the emissions of greenhouse gas. An advantage of this process is that a huge quantity of CO2 can be sequestrated and stored at relatively low pressure, which will reduce the amount of storage cost required for creating additional platform to store it. The study on CO2 storage in coal seam to enhance coal bed methane (ECBM) recovery has drawn a lot of attention for its worldwide suitability and acceptability and has been conducted since two decades in many coalmines. This article focuses on the coal seam properties related to CO2 adsorption/desorption, coal swelling/shrinkage, diffusion, porosity and permeability changes, thermodynamic/thermochemical process, flue gas injection, etc. Here, the performance analysis of both CO2 storage and ECBM recovery process in coal matrixes is investigated based on the numerical simulation. In this study, a one-dimensional mathematical model of defining mass balances is used to interpret the gas flow and the gas sorption and describe a geomechanical relationship for determining the porosity and the permeability alteration at the time of gas injection. Vital insights are inspected by considering the relevant gas flow dynamics during the displacement and the influences of coal swelling and shrinkage on the ECBM operation. In particular, pure CO2 causes more displacement that is more efficient in terms of total CH4 recovery, whereas the addition of N2 to the mixture assists to make quicker way of the initial methane recovery. However, this study will support future research aspirants working on the same topic by providing a clear conception and limitation about this study.

CO2 adsorption and swelling of coal under constrained conditions and their stage-change relationship

The adsorption of CO2 into coal will cause the volumetric swelling of coal body, which is a highly concerned issue for the CO2 enhanced coalbed methane recovery (CO2-ECBM). However, the research of adsorption and swelling of coal is not all-inclusive and their relationship is ambiguous. This paper conducted the experiments regarding the CO2 adsorption and corresponding swelling of coal influenced by multiple factors using the self-designed device. Results show that the adsorption capacity is promoted by the injection pressure and is restrained by confining pressure, temperature and water content, but the thermal-induced swelling strain and moisture-induced swelling strain can offset the swelling loss to some extent for the wet coal at a high temperature. There has a stage-change relationship between swelling strain and adsorption capacity of coal, and the adsorption swelling can be divided into three parts: the slow swelling stage, the rapid swelling stage and the stable swelling stage. Under the action of the external stress, the adsorption swelling develops towards to the interior of coal body first and then develops towards to the external when the swelling stress exceeds the external stress. Different extended directions of adsorption swelling are the main causes of the stage adsorption swelling. The study contributes to the deep understanding for the gas adsorption swelling mechanism and the attenuation-rebound of the coal reservoir permeability during the CO2-ECBM.

Deformation of Shale and Coal Organic Carbon Slit Micropores Induced by CO2-Enhanced Gas Recovery: A Monte Carlo Simulation Study

The swelling of shale and coal induced by the CO2-enhanced gas recovery (CO2-EGR) has proved to reduce the reservoir permeability and the CH4 production. In this work, we have studied the adsorptio...

Effects of ethanol-mixed supercritical CO<SUB align="right">2 fluid on CO<SUB align="right">2 adsorption on coals: implications for CO<SUB align="right">2 geologic storage in coal seams

The coal matrix swelling induced by CO2 decreases the permeability of the target coal seams, and it will trigger disadvantages to CO2 storage in deep coal reservoirs. In order to mitigate such swelling effect, the addition of ethanol into CO2 is proposed. The effects of ethanol-mixed supercritical CO2 on the adsorption equilibrium and kinetics behaviors of CO2 adsorption on coals are studied in this paper. The results indicate that the ethanol-mixed supercritical CO2 fluid could increase CO2 adsorption capacity of two coals with low metamorphic degree, but decrease that of two coals with high metamorphic degree. The addition of ethanol into CO2 fluid remarkably enhances the diffusion and adsorption rate of CO2 within the micropores and macropores of coals. The alterations of pore system of various rank coals could account for the changing law of CO2 adsorption equilibrium and kinetics behaviors on coals after interacting with ethanol-mixed supercritical CO2.

Structural and swelling study of Karharbari coal with various combinations of solvent

Swelling kinetics of Karharbari coal in various combinations of solvents such as T/PY, T/PNT, T/FLT, T/ANT, T/MNAP, T/NAP, T/FL, T/D were studied using the solvent swelling technique. The values of equilibrium swelling ratio Qe and diffusional exponent n were calculated to find the nature of diffusion while changing temperature and solvent combinations. Results conclude that with an increase in swelling temperature the nature of swelling shifts from relaxation diffusion controlled to the Fickian diffusion controlled. The activation energies with different solvents were also calculated to suggest the actual energy barrier for swelling of coal. Structural changes in coal due to penetration of solvents were also determined by using FTIR analysis of all combinations of solvents. The resultant energy barriers for swelling of coal were found to be hydrogen bond (H-bond). This work also includes the percentage extraction of H-bond obtained for all combinations of solvents. The results obtained firmly conclude that the solvent combination of tetralin and pyrene (T/PY) is the best combination of solvents for the pretreatment of coal for coal liquefaction. [Received: October 24, 2019; Accepted: July 30, 2020]

Solvent swelling behaviour of Shenmu-Fugu and Shengli coals

Swelling behaviors of Shenmu-Fugu bituminous coal (SFBC) and Shengli lignite (SLL) in various organic solvents were investigated at temperatures ranging from 0 °C to 55 °C. The results show that tetrahydrofuran (THF) has a relatively higher swelling ratio in comparison to other solvents, suggesting that THF likely disrupts coal-coal cross-links between macromolecular network structures in coal. SLL swelled more highly than SFBC in methanol (MA), indicating that MA interacts strongly with specific sites on SLL. Swelling ratios of SFBC in carbon disulfide (CDS), recycle liquefaction solvent (RLS) and initial liquefaction solvent (ILS) are appreciably higher than those of SLL. Coal swelling increases with raising temperature within the temperature range studied (0-55 °C), especially in polar solvent.

Use of organic polymers for improvement of coking potential of poor coking coal

ABSTRACT As the reserve of prime coking coal is low, there is a good amount of research going on worldwide to improve the coking potential of non-coking coal. Researchers have revealed the importance of hydrogen generation/transfer to improve the coking potential of coal. Based on this hypothesis, in this paper, the authors have tried to improve the coking potential of non-coking coal by adding different polymers. The polymers are derived from formaldehyde and phenol derivatives. Coking potential has been tested through conventional tests like crucible swelling number. Thermogravimetric mass spectroscopy (TG-MS) has been used to get an idea of plasticity and hydrogen generation in the plastic zone. High-temperature microscopy has also been used to evaluate the swelling of coal. Results indicate that the addition of polymers in a small percentage has an effect on hydrogen generation in the plastic zone, and thus, it can improve the coking potential of non-coking coal.

Cleat-scale modelling of the coal permeability evolution due to sorption-induced strain

Coal permeability is known to be affected by the sorption-induced strain. Indeed, coal swells with gas sorption and shrinks with desorption, which is likely to modify the cleat aperture and thus the permeability. Coal permeability evolution is crucially important for either Coalbed Methane production (CBM) or Carbon dioxide Capture and Storage (CCS).A model is developed at the scale of the fractures to take into account the hydro-mechanical couplings observed experimentally. It is implemented in the finite element code Lagamine. This numerical model is developed at the scale of the fractures and the matrix blocks. Fractures are modelled with interface elements specially adapted to manage sorption/desorption by taking into account the Langmuir's isotherm (Langmuir, 1918). In the matrix blocks, the sorption strain is assumed proportional to the adsorbed gas content. Depending on the boundary conditions, sorption strain affects the stress state and thus the fracture aperture. Contrary to macroscale models (Bertrand et al. 2017), this model does not require the use of shape factors since the geometry can be explicitly represented. The implementation of the model is validated by comparison with the analytical solution on a simple geometry. The model is finally used to simulate gas injection on a representative elementary volume (REV) made up few matrix blocks. This modelling highlights the interests of a numerical approach to compute the permeability evolution compared to a porosity-based model.

Influence of water soaking on swelling and microcharacteristics of coal

AbstractImproving the coal seam permeability is an important measure for increasing the coal bed methane (CBM) production and preventing gas disasters. Hydraulic technologies are effective ways of improving coal seam permeability. However, hydraulic technologies can also cause water to enter the coal seam, allowing the coal seam to soak for a long time. In this study, to obtain the influence of water soaking on the microscopic characteristics of coal, X‐ray diffraction (XRD), scanning electron microscopy (SEM), free swelling ratio tests, and low‐temperature nitrogen adsorption tests (LT‐NATs) were conducted. The mineral compositions of raw coal samples, the variation regularities of micromorphologies, and pore characteristics of the samples with different soaking times were obtained. The results showed that the coal samples contained about 8.5% clay minerals, of which 71% were illite/smectite mixed‐layer. Expansions of different sizes in the areas where the surface of the soaked coal samples contained clay minerals were observed, and the swelling was also observed macroscopically. The swelling not only led to an increase in the coal sample volume but also led to a decrease in pore volume. This change was magnified with the increase in soaking time (within 30 days). The cumulative pore volume of the samples soaked for 30 days was 0.00681 cm3/g. This was a reduction of 29.9% compared to the unsoaked samples. Moreover, the pore volumes show a logarithmic dependence on soaking time. This study provides evidence that the coal containing clay minerals will swell obviously when soaked in water, and the hydration swelling of clay minerals has a great influence on the swelling of coal. This swelling would lead to a decrease in the pore volume and the efficiency of CBM transport, thus affecting the effect of hydraulic measures.

Dynamic transport and reaction behaviour of high-pressure gases in high-rank coal

This paper presents the results obtained through continuous and simultaneous measurements of gas flow, temperature and coal deformation during high-pressure gas injection in high-rank coal samples, obtained from the South Wales coalfield (UK). The results demonstrate that CO2 flow rates experience an initial decline due to internal coal swelling, followed by the flow rate recovery and global coal swelling. As the flow of high-pressure CO2 induces measurable temperature drop within the sample related to the Joule-Thomson cooling, the changes induced by the variations in thermal state of the system are associated with abrupt shift in coal response to reactive gas flow. However, subsequent injections of He and N2 show that the changes induced by CO2 sorption on coal permeability to gases are irreversible. This work demonstrates the importance of considering the coupled reactive gas and heat transport, and consequent coal deformation mechanisms while assessing the storage potential of coal seams.

Effects of temperature and pressure on pore morphology of different rank coals: Implications for CO2 geological storage

The understanding of CO2 effects on coal pore morphology is important during CO2 geological storage (CGS). In this study, the CGS was simulated by conducting experiments on four different samples at temperatures and pressures of 32–80 ℃ and 9–20 MPa respectively. Mercury intrusion porosimetry (MIP) and low-pressure nitrogen adsorption (LP-N2GA) were employed to study pore size distributions, pore shapes, roughness and compressibility. The results show that the pore size distributions of high-rank coals are mainly influenced in the micropore range while low-rank in the macropore range. The pores in ScCO2 treated samples become more complicated indicated by two peaks in the MIP curves at 7.2 nm and 13.7 nm, while only one occurs in the untreated samples. The CGS process is able to open, reshape or enlarge existing pores, which improve pore connectivity and porosity. The fractal dimension shows that the surface and structure morphology becomes more complex after the treatment with the increasing temperatures and pressures, leading to increments in the main characteristics, specific surface area and total pore volume. These characteristics of the Sihe coal mine samples, treated under 62.5 ℃ and 15 MPa, increase from 1.3579 m2/g to 3.048 m2/g and 0.001076 cm3/g to 0.001338 cm3/g respectively. The compressibility changes indicate that coal samples are more easily compressed after the treatment, which might negatively affect CGS. Even though no clear tendencies exist among different coal ranks, the compressibility increases with increasing temperatures and pressures. Even though coal swelling accompanying CO2 adsorption may decrease the permeability of the coal as cleats and pores are squeezed, the results of our study do show that the CGS tends to increase the gas adsorption volume, specific surface areas and pore connectivity, which improve the storage capacities and benefit the enhanced coalbed methane recovery.

Synchrotron X-ray tomographic characterization of microstructural evolution in coal due to supercritical CO2 injection at in-situ conditions

Microstructural evolution in coal due to CO2 injection directly influences the gas transport and storage properties of coal during CO2 sequestration in deep coal seams. Using synchrotron X-ray tomography, we performed CT imaging under simulated reservoir conditions with a novel X-ray transparent core holder and investigated the time-dependent evolution of 3-D microstructures of an anthracite coal with 1.37% moisture as-received interacted with supercritical CO2 (ScCO2). Three sets of CT scans were taken: (1) coal was subjected to 10 MPa confining pressure before exposed to ScCO2, (2) coal was subjected to 10 MPa confining pressure and after 7 h of exposure to ScCO2, and (3) coal was subjected to 10 MPa confining pressure and after 53 h of exposure to ScCO2. When ScCO2 interacts with coal, complex physico-chemical reactions occur, changing coal microstructures in multiple ways and resulting in significant permeability changes. After 7 h ScCO2 injection, we directly observed the pre-existing microfracture closure, which was attributed to coal swelling. A significant permeability reduction was measured from 3.0 to 0.24 µD with up to 26 h CO2 injection. With increasing injection duration, interestingly, we observed the wormhole growth in coal due to hydrocarbon mobilization and mineral dissolution, which accordingly caused a permeability rebound to 1.2 µD after 53 h CO2 injection. Coal swelling occurred readily upon ScCO2 injection but hydrocarbon mobilization and mineral dissolution were delayed to affect the permeability probably due to the slower reaction kinetics. This study suggests that ScCO2 injection has the potential to improve the permeability and enhance the coalbed methane recovery due to the effects of hydrocarbon mobilization and mineral dissolution.

A model of cement-formation interface failure length in supercritical CO2-ECBM and storage injection well considering the coal swelling effect under high pressure

The swelling effect of high-pressure carbon dioxide (CO2) in coal seam is obvious. In the restrained deep formation, it is converted to stress acting on the wellbore and the caprock. The action stress is concentrated near the wellbore and poses a threat to the cement-formation interface. Due to interface failure to micro-annulus, wellbore integrity will be lost and this will have an impact on carbon dioxide-enhanced coalbed methane recovery (CO2-ECBM) and storage. In this paper, the pseudo-steady pressure distribution and steady pressure distribution of CO2 injection process were established after considering the change in permeability of coal seam injected with high-pressure supercritical CO2, and the vertical stress distribution model was derived. A one-dimensional radial numerical simulation formed by iterative method was established. A model for calculating the failure length at the cement-formation interface is obtained, and the shear stress and debonding length at the interface at various injection rates and times are calculated. The results show that the shear stress on the cement-formation interface has the maximum magnitude on the height of the interface between coal seam and caprock. The shear stress generated by coal swelling may break the fragile cement-formation interface into a narrow debonding interface. The injection rate has an influence on the interface failure length. For the same total injection amount, low injection rate is beneficial to protect the cement-formation interface integrity. This study provides a reference for the design of maximum injection speed for CO2-ECBM and storage to avoid leakage.