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Pathegama Gamage Ranjith - One of the best experts on this subject based on the ideXlab platform.
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effect of coal maturity on co2 based hydraulic fracturing process in coal seam gas reservoirs
Fuel, 2019Co-Authors: K. H. S. M. Sampath, M S A Perera, Tharaka Dilanka Rathnaweera, Pathegama Gamage Ranjith, Stephan K Matthai, Derek Elsworth, G. ZhangAbstract:Abstract Hydraulic fracturing of deep coal seams is challenging due to both the complex processes involved in fracturing and the typically poorly defined characteristics such as natural Cleat System, mineral-maceral distribution and strength parameters of the subsurface. This study evaluates the effectiveness of fracturing using liquid CO2 as the propellant through observations of break-down pressures and the form of the induced fracture network in various ranked coals. Coal ranks are defined through a rigorous proximate analysis to determine the moisture, volatile matter, ash and fixed carbon contents of each coal type fractured. Fracturing experiments were conducted on 38 mm × 76 mm core samples, under fixed stress, temperature conditions (i.e. σ 3 = 6 MPa, σ 1 = 8 MPa and T = 25 °C). Break-down pressures are observed to increase with increasing coal maturity. Increasing rank or maturity identifies that the coal has been subjected to progressively higher pressures and temperatures, has gained proportionately higher strength and thus exhibits a higher break-down pressure. No direct relationship is observed between volatile matter content and either strength or break-down pressure. The colocation of acoustic emission (AE) hypocenters and mineral grain boundaries delineated by micro-CT imaging indicate preferred pathways for the propagation of fractures induced by liquid CO2. Stiffness contrasts between mineral phases result in stress concentrations and localized weakness at grain-grain boundaries. The complex mineral distribution in coal accentuates such heterogeneity of weakness and may be the key feature promoting the evolution of a well distributed rather than localized fracture network. For low rank coal, hydraulic fracturing is least effective, as the fracturing process does not create a significant fracture network to enhance the permeability. This may result, since low rank coals are intrinsically weak due to their low carbon content and high moisture content allowing extensive fracturing to develop at only very low break-down pressures – minimizing damage. These observations emphasize the sensitivity of break-down pressures and the resulting complexity of fracturing to pressurization rates and coal rank – inferring important controls on these parameters for the safe and effective use, when fracturing with CO2 as the propellant.
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CO2 interaction induced mechanical characteristics alterations in coal: A review
International Journal of Coal Geology, 2019Co-Authors: K. H. S. M. Sampath, Pathegama Gamage Ranjith, Mandadige Samintha Anne Perera, Stephan K MatthaiAbstract:Abstract Coal geo-mechanical alterations caused by CO2 interaction should be critically evaluated to enhance the efficiency of CO2-ECBM and CO2 sequestration processes, while mitigating associated hazards. We provide a detailed overview on the CO2 interaction induced mechanical alterations in coal, by comprehensively reviewing the influential parameters and causative factors. Although studies provide persuasive evidence to conclude the general fact that CO2 interaction causes severe mechanical degradation in coal, the degree of strength reduction is influenced by numerous parameters: 1) stress state - elevated in-situ stresses diminish the CO2-induced coal strength reduction, due to reduced CO2 adsorption potential and the hindered swelling effect, 2) CO2 adsorption pressure and phase - magnitude of strength reduction increases with increasing CO2 adsorption pressure and CO2 phase transition into supercritical state causes a noticeable strength reduction, because of the higher adsorption and polymerization capacities of supercritical CO2, 3) CO2 interaction time - due to rapid CO2 adsorption process, significant mechanical degradation occurs in the short-term, whereas additional CO2 exposure continuous to reduce the strength, but at a slower rate, 4) coal seam characteristics - high-rank coal with a well-formed Cleat System exhibits greater strength reductions, compared to low-rank coal, because coal Cleat System facilitates easy invasion of CO2 and acts as a locus for CO2 adsorption, expanding the coal matrix along the Cleat walls. Moreover, we highlight that there are multiple factors that may contribute to the overall mechanical strength alteration: 1) CO2 adsorption induced surface energy reduction, 2) CO2-induced plasticization of coal polymer-structure, 3) formation of micro-cracks due to shrinkage, free- and differential-swelling, and 4) dissolution of Cleat and pore filling minerals, causing bond energy reduction, influencing grain-to-grain contact and forming secondary porosity Systems in coal. However, we infer that the mechanical degradation depends on the heterogeneity and anisotropy of the coal reservoir. Therefore, the degradation should be specifically assessed by considering the targeted reservoir characteristics, prior to the implementation of CO2-ECBM and CO2 sequestration field projects.
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Experimental investigation of the influence of CO2 and water adsorption on mechanics of coal under confining pressure
International Journal of Coal Geology, 2019Co-Authors: Xiaogang Zhang, Pathegama Gamage Ranjith, A.s. RanathungaAbstract:Abstract Adsorption of CO2 into the coal matrix causes significant alterations in coal's mechanical properties and this process may be further complicated by water+CO2 saturation. The aim of this study is therefore to investigate the hydro-mechanical property alterations of coal with various fluid saturations under in-situ stress conditions. A series of tri-axial strength tests coupled with acoustic emission (AE) analysis was conducted on high-rank coal subjected to CO2, water and water+CO2 saturations. Based on the experimental results, both the strength and Young's modulus (E) of coal reduced significantly after CO2 saturation, especially for supercritical CO2-saturated coal samples, as 18.73% reduction in strength and 18.12% reduction in E were observed compared to the dry samples. A Langmuir-type curve well fits the changes in strength and E with CO2 saturation pressure. Adsorption of water causes minor deteriorations in coal's mechanical properties, but this reduction intensifies considerably with the introduction of CO2, due to the enhanced dissolution of mineral matter and organic compounds in the CO2-acidified solvent environment. With the application of in-situ stress, alterations in the mechanical properties of coal due to fluid adsorption decrease significantly compared to the results of the corresponding uniaxial tests, because of the reduction in fluid saturation and the mechanical strengthening effect in a high stress environment. High-rank coal is expected to experience greater mechanical property deteriorations than low-rank coal due to its highly-developed Cleat System. In addition, the AE signals captured in this study further explain the alterations in coal mechanical properties of coal due to fluid saturation.
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Characterization of coal porosity and permeability evolution by demineralisation using image processing techniques: A micro-computed tomography study
Journal of Natural Gas Science and Engineering, 2018Co-Authors: Guanglei Zhang, Pathegama Gamage Ranjith, Asadul Haque, Mandadige Samintha Anne Perera, Xavier Choi, K. H. S. M. SampathAbstract:Abstract The permeability of coal is the key parameter both in primary and enhanced coalbed methane recovery. The natural Cleat System in coal serves as the primary pathway for gas flow in coal seams though mineralisation in Cleats and is known to significantly reduce coal permeability. This paper reports on a numerical simulation of the pore network evolution of coal subject to Cleat demineralisation. A high-resolution micro-computed tomography scanner was used to characterize the micro-structures of three anthracite coal samples. The mineral phases available in the coal samples were selectively removed to different extents (20%, 40%, 60%, 80% and 100%) and merged into the pore space using image processing techniques. In this way, the coal demineralisation process could be simulated and its impact on porosity and permeability studied. Comprehensive pore structure characterizations, including porosity, connectivity and tortuosity, were then conducted on the reconstructed pore network using Avizo software. Pore network models were also extracted to investigate changes in the pore and throat attributes. The lattice Boltzmann method was adopted to identify the absolute permeability changes with Cleat demineralisation. The results reveal that demineralisation can increase coal porosity and permeability up to a percolation threshold. Although porosity was enhanced prior to the percolation threshold, the coal permeability was not enhanced due to poor pore connectivity. The permeability changed rapidly close to the percolation threshold, depending on the degree of demineralisation, and an exponential relation was observed between permeability and the amount of demineralisation. According to the observations, complete removal of the mineral phase can significantly increase the connected porosity while reducing the pore tortuosity, resulting in several orders of magnitude increase in coal permeability. This study shows that Cleat demineralisation is an effective permeability enhancement technique for coalbed methane recovery, if very high demineralisation can be achieved.
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Effect of Coal Rank on CO2 Adsorption Induced Coal Matrix Swelling with Different CO2 Properties and Reservoir Depths
Energy & Fuels, 2017Co-Authors: A.s. Ranathunga, Tharaka Dilanka Rathnaweera, Pathegama Gamage Ranjith, Mandadige Samintha Anne Perera, Xiaogang ZhangAbstract:Although the greater adsorption potential of carbon dioxide (CO2) in coal is an appealing fact in relation to the long-term safe storage of CO2 in coal seams, the resulting coal structure modification, particularly through coal matrix swelling, adds several uncertainties to the process. To date, many studies have been initiated, particularly on the effects of injecting CO2 and reservoir properties on this swelling process and the associated reservoir permeability depletion. These influences are largely dependent on the maturity of the coal mass and its structure, including the Cleat System. However, minor attention has been given to date to the effect of coal rank on CO2 adsorption-induced coal matrix swelling and was therefore investigated in the present study. The volumetric strain of the Australian brown coal samples for both CO2 and N2 under various confinements (triaxial) and injections was measured at 35 °C constant temperature to investigate the influence of CO2 properties and reservoir depth on CO...
Mandadige Samintha Anne Perera - One of the best experts on this subject based on the ideXlab platform.
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CO2 interaction induced mechanical characteristics alterations in coal: A review
International Journal of Coal Geology, 2019Co-Authors: K. H. S. M. Sampath, Pathegama Gamage Ranjith, Mandadige Samintha Anne Perera, Stephan K MatthaiAbstract:Abstract Coal geo-mechanical alterations caused by CO2 interaction should be critically evaluated to enhance the efficiency of CO2-ECBM and CO2 sequestration processes, while mitigating associated hazards. We provide a detailed overview on the CO2 interaction induced mechanical alterations in coal, by comprehensively reviewing the influential parameters and causative factors. Although studies provide persuasive evidence to conclude the general fact that CO2 interaction causes severe mechanical degradation in coal, the degree of strength reduction is influenced by numerous parameters: 1) stress state - elevated in-situ stresses diminish the CO2-induced coal strength reduction, due to reduced CO2 adsorption potential and the hindered swelling effect, 2) CO2 adsorption pressure and phase - magnitude of strength reduction increases with increasing CO2 adsorption pressure and CO2 phase transition into supercritical state causes a noticeable strength reduction, because of the higher adsorption and polymerization capacities of supercritical CO2, 3) CO2 interaction time - due to rapid CO2 adsorption process, significant mechanical degradation occurs in the short-term, whereas additional CO2 exposure continuous to reduce the strength, but at a slower rate, 4) coal seam characteristics - high-rank coal with a well-formed Cleat System exhibits greater strength reductions, compared to low-rank coal, because coal Cleat System facilitates easy invasion of CO2 and acts as a locus for CO2 adsorption, expanding the coal matrix along the Cleat walls. Moreover, we highlight that there are multiple factors that may contribute to the overall mechanical strength alteration: 1) CO2 adsorption induced surface energy reduction, 2) CO2-induced plasticization of coal polymer-structure, 3) formation of micro-cracks due to shrinkage, free- and differential-swelling, and 4) dissolution of Cleat and pore filling minerals, causing bond energy reduction, influencing grain-to-grain contact and forming secondary porosity Systems in coal. However, we infer that the mechanical degradation depends on the heterogeneity and anisotropy of the coal reservoir. Therefore, the degradation should be specifically assessed by considering the targeted reservoir characteristics, prior to the implementation of CO2-ECBM and CO2 sequestration field projects.
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Characterization of coal porosity and permeability evolution by demineralisation using image processing techniques: A micro-computed tomography study
Journal of Natural Gas Science and Engineering, 2018Co-Authors: Guanglei Zhang, Pathegama Gamage Ranjith, Asadul Haque, Mandadige Samintha Anne Perera, Xavier Choi, K. H. S. M. SampathAbstract:Abstract The permeability of coal is the key parameter both in primary and enhanced coalbed methane recovery. The natural Cleat System in coal serves as the primary pathway for gas flow in coal seams though mineralisation in Cleats and is known to significantly reduce coal permeability. This paper reports on a numerical simulation of the pore network evolution of coal subject to Cleat demineralisation. A high-resolution micro-computed tomography scanner was used to characterize the micro-structures of three anthracite coal samples. The mineral phases available in the coal samples were selectively removed to different extents (20%, 40%, 60%, 80% and 100%) and merged into the pore space using image processing techniques. In this way, the coal demineralisation process could be simulated and its impact on porosity and permeability studied. Comprehensive pore structure characterizations, including porosity, connectivity and tortuosity, were then conducted on the reconstructed pore network using Avizo software. Pore network models were also extracted to investigate changes in the pore and throat attributes. The lattice Boltzmann method was adopted to identify the absolute permeability changes with Cleat demineralisation. The results reveal that demineralisation can increase coal porosity and permeability up to a percolation threshold. Although porosity was enhanced prior to the percolation threshold, the coal permeability was not enhanced due to poor pore connectivity. The permeability changed rapidly close to the percolation threshold, depending on the degree of demineralisation, and an exponential relation was observed between permeability and the amount of demineralisation. According to the observations, complete removal of the mineral phase can significantly increase the connected porosity while reducing the pore tortuosity, resulting in several orders of magnitude increase in coal permeability. This study shows that Cleat demineralisation is an effective permeability enhancement technique for coalbed methane recovery, if very high demineralisation can be achieved.
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Effect of Coal Rank on CO2 Adsorption Induced Coal Matrix Swelling with Different CO2 Properties and Reservoir Depths
Energy & Fuels, 2017Co-Authors: A.s. Ranathunga, Tharaka Dilanka Rathnaweera, Pathegama Gamage Ranjith, Mandadige Samintha Anne Perera, Xiaogang ZhangAbstract:Although the greater adsorption potential of carbon dioxide (CO2) in coal is an appealing fact in relation to the long-term safe storage of CO2 in coal seams, the resulting coal structure modification, particularly through coal matrix swelling, adds several uncertainties to the process. To date, many studies have been initiated, particularly on the effects of injecting CO2 and reservoir properties on this swelling process and the associated reservoir permeability depletion. These influences are largely dependent on the maturity of the coal mass and its structure, including the Cleat System. However, minor attention has been given to date to the effect of coal rank on CO2 adsorption-induced coal matrix swelling and was therefore investigated in the present study. The volumetric strain of the Australian brown coal samples for both CO2 and N2 under various confinements (triaxial) and injections was measured at 35 °C constant temperature to investigate the influence of CO2 properties and reservoir depth on CO...
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Effects of gaseous and super-critical carbon dioxide saturation on the mechanical properties of bituminous coal from the Southern Sydney Basin
Applied Energy, 2013Co-Authors: Mandadige Samintha Anne Perera, Pathegama Gamage Ranjith, Daniel R. VieteAbstract:A study was initiated to investigate the effects of gaseous and super-critical carbon dioxide (CO2) adsorption on bituminous coal strength. Uniaxial compressive strength (UCS) experiments were conducted on bituminous coal samples from the southern Sydney Basin saturated with gaseous CO2, super-critical CO2 and N2 at various pressures, and a temperature 33°C. According to the results, gaseous CO2 adsorption causes the UCS and Young’s modulus of the bituminous coal to be reduced by up to 53% and 36%, respectively. Super-critical CO2 adsorption causes more significant modifications to the mechanical properties of the bituminous coal, resulting in 40% greater UCS strength reduction and 100% greater Young’s modulus reduction compared to gaseous CO2 adsorption. The greater influence of super-critical CO2 on the UCS of the bituminous coal is thought to be related to the greater adsorptive potential and coal swelling produced for super-critical CO2. The more significant influence of super-critical CO2 on the Young’s modulus of the bituminous coal is thought to relate to the greater dissolution (and thus coal plasticization) potential of the super-critical CO2. N2 saturation was not observed to have any significant effect on the mechanical properties of the bituminous coal. Acoustic emission data collected during testing support of the notion that the coal mass natural Cleat System largely contributes to the susceptibility of coal to mechanical weakening by CO2 adsorption. The results show that the mechanical influence of CO2 adsorption on coal is highly dependent on the phase state of the CO2.
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Effects of Cleat performance on strength reduction of coal in CO2 sequestration
Energy, 2012Co-Authors: Pathegama Gamage Ranjith, Mandadige Samintha Anne PereraAbstract:The natural Cleat System in coal is highly important in the CO2 sequestration process as injected CO2 first moves through the Cleat System, which eventually changes the coal's permeability and strength. The main objective of this study is to investigate the effects of Cleat density and direction on the strength reduction of coal with CO2 adsorption. A series of strength experiments was conducted on non-CO2-saturated and CO2-saturated (1, 2, 3 and 16 MPa) coal samples with two different Cleat densities (low-rank lignite and high-rank bituminous) and two different Cleat angles (around 20° and 70° to the loading directions). According to the experimental results, CO2 saturation, at up to 3 MPa saturation pressure, causes up to 4.5 times higher strength reduction in bituminous coal (43%) compared to lignite (9.6%). The compressive strength reduction percentage in coal shows a linearly increasing trend with CO2 saturation pressure (1–3 MPa), where the slope is significantly higher for bituminous coal (15.3) compared to lignite (3.2). When the Cleat direction reduces from 70 to 20°, the CO2 adsorption (at 16 MPa) induced UCS strength reduction in bituminous coal reduces by around 20%. It is interesting to report that Cleat density and direction do not exhibit a significant influence on elastic modulus reduction in coal compared to the strength reduction.
M S A Perera - One of the best experts on this subject based on the ideXlab platform.
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effect of coal maturity on co2 based hydraulic fracturing process in coal seam gas reservoirs
Fuel, 2019Co-Authors: K. H. S. M. Sampath, M S A Perera, Tharaka Dilanka Rathnaweera, Pathegama Gamage Ranjith, Stephan K Matthai, Derek Elsworth, G. ZhangAbstract:Abstract Hydraulic fracturing of deep coal seams is challenging due to both the complex processes involved in fracturing and the typically poorly defined characteristics such as natural Cleat System, mineral-maceral distribution and strength parameters of the subsurface. This study evaluates the effectiveness of fracturing using liquid CO2 as the propellant through observations of break-down pressures and the form of the induced fracture network in various ranked coals. Coal ranks are defined through a rigorous proximate analysis to determine the moisture, volatile matter, ash and fixed carbon contents of each coal type fractured. Fracturing experiments were conducted on 38 mm × 76 mm core samples, under fixed stress, temperature conditions (i.e. σ 3 = 6 MPa, σ 1 = 8 MPa and T = 25 °C). Break-down pressures are observed to increase with increasing coal maturity. Increasing rank or maturity identifies that the coal has been subjected to progressively higher pressures and temperatures, has gained proportionately higher strength and thus exhibits a higher break-down pressure. No direct relationship is observed between volatile matter content and either strength or break-down pressure. The colocation of acoustic emission (AE) hypocenters and mineral grain boundaries delineated by micro-CT imaging indicate preferred pathways for the propagation of fractures induced by liquid CO2. Stiffness contrasts between mineral phases result in stress concentrations and localized weakness at grain-grain boundaries. The complex mineral distribution in coal accentuates such heterogeneity of weakness and may be the key feature promoting the evolution of a well distributed rather than localized fracture network. For low rank coal, hydraulic fracturing is least effective, as the fracturing process does not create a significant fracture network to enhance the permeability. This may result, since low rank coals are intrinsically weak due to their low carbon content and high moisture content allowing extensive fracturing to develop at only very low break-down pressures – minimizing damage. These observations emphasize the sensitivity of break-down pressures and the resulting complexity of fracturing to pressurization rates and coal rank – inferring important controls on these parameters for the safe and effective use, when fracturing with CO2 as the propellant.
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influence of co2 adsorption on the strength and elastic modulus of low rank australian coal under confining pressure
International Journal of Coal Geology, 2016Co-Authors: A.s. Ranathunga, M S A Perera, Pathegama Gamage RanjithAbstract:Abstract Precise knowledge of changes in the CO2 adsorption-induced mechanical properties of deep coal seams is necessary for the safe and successful implementation of carbon dioxide-enhanced coal bed methane (CO2-ECBM) recovery. To date, little attention has been paid to the geo-mechanical property alterations in coal seams during CO2 adsorption under in-situ stress conditions. The aim of this study is therefore to discover how coal seam integrity varies with the introduction of CO2 under in-situ stress conditions, by conducting a series of tri-axial strength tests of Australian brown coal samples. The effect of CO2 exposure time on coal's mechanical properties was also investigated under super-critical CO2 saturation (10 MPa) conditions, in order to determine the effect of long-term CO2 injection on coal seams' mechanical properties, as CO2-ECBM is a long-term process. According to the test results, irrespective of coal rank, the strength reduction in coal with CO2 injection under field conditions is significantly less than would be expected based on simple laboratory testing such as uniaxial tests. However, in any stress environment, high rank coals are subjected to greater strength and stiffness reductions with CO2 adsorption than low rank coals, due to their well-developed Cleat System, and the injection of super-critical CO2 induces greater mechanical property alterations in coal than sub-critical CO2 injection. These strength and elastic modulus changes in coal with CO2 adsorption can be presented using a simple Langmuir-type equation, regardless of rank. Furthermore, the observation of the effect of long-term CO2 saturation on coal's mechanical properties revealed that, although CO2 adsorption-induced mechanical property alterations in coal are mostly completed with the first interaction with CO2, further structural re-arrangement may occur at a slower rate over time.
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super critical co2 saturation induced mechanical property alterations in low rank coal an experimental study
Journal of Supercritical Fluids, 2016Co-Authors: A.s. Ranathunga, M S A Perera, Pathegama Gamage RanjithAbstract:Abstract The adsorption of carbon dioxide (CO 2 ) into the coal matrix during CO 2 -enhanced methane recovery causes significant alterations to the coal mass chemical and physical structures, causing modifications to coal's mechanical properties. Hence, the main objective of this study is to investigate the effects of sequestrated CO 2 phase condition on coal strength. A series of unconfined compressive strength tests was conducted on Australian brown coal samples, saturated under various CO 2 pressures (2–10 MPa) at 35 °C using an advanced acoustic emission (AE) System and optical 3D deformation analysis. According to the results, super-critical CO 2 has the ability to cause a greater reduction of strength (by 46%) and enhancement of elasticity properties (by 20%) in brown coal compared to sub-critical CO 2 , because super-critical CO 2 has greater adsorptive potential, which eventually creates greater coal matrix swelling. According to the AE and deformation analysis, the coal mass natural Cleat System contributes significantly to the observed CO 2 adsorption-induced changes in mechanical properties.
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coal Cleat permeability for gas movement under triaxial non zero lateral strain condition a theoretical and experimental study
Fuel, 2013Co-Authors: M S A Perera, Pathegama Gamage Ranjith, S K ChoiAbstract:As the permeability of coal seams is mainly determined by the network of natural fractures known as the Cleat System, estimation of Cleat permeability is of utmost importance for the carbon dioxide sequestration process in deep coal seams. The main objective of this study is to develop a new mathematical model for predicting Cleat permeability under non-zero lateral strain conditions such as the conditions encountered in laboratory triaxial experiments. By applying the theory of elasticity to the constitutive behaviour of fractured rocks, a theoretical relationship between permeability and gas injecting pressure, confining pressure, axial load and gas adsorption in triaxial tests is developed. The new model was then verified using experimentally-determined permeability data of two coal samples. Results indicate that the new model can fairly accurately predict the combined effects of effective stress and coal matrix swelling on Cleat permeability for both CO2 and N2 injections at various injection pressures. The model also provides quite accurate prediction of the effect of confining pressure on Cleat permeability for both CO2 and N2 injections. The model includes parameters for fractured rock properties, namely Poisson’s ratio and Young’s modulus. The model can be applied to predict Cleat permeability, regardless of Cleat size. When the accuracy of the new model is compared with the existing Gilman and Beckie [5] model, with increasing injecting pressure both models show similar increments of N2 permeability and different reductions for CO2 permeability. This is due to the zero lateral strain assumption of the existing model, which is not applicable to the swelling process under triaxial test condition. The new model is more accurate for the prediction of CO2 Cleat permeability under triaxial test condition.
A.s. Ranathunga - One of the best experts on this subject based on the ideXlab platform.
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Experimental investigation of the influence of CO2 and water adsorption on mechanics of coal under confining pressure
International Journal of Coal Geology, 2019Co-Authors: Xiaogang Zhang, Pathegama Gamage Ranjith, A.s. RanathungaAbstract:Abstract Adsorption of CO2 into the coal matrix causes significant alterations in coal's mechanical properties and this process may be further complicated by water+CO2 saturation. The aim of this study is therefore to investigate the hydro-mechanical property alterations of coal with various fluid saturations under in-situ stress conditions. A series of tri-axial strength tests coupled with acoustic emission (AE) analysis was conducted on high-rank coal subjected to CO2, water and water+CO2 saturations. Based on the experimental results, both the strength and Young's modulus (E) of coal reduced significantly after CO2 saturation, especially for supercritical CO2-saturated coal samples, as 18.73% reduction in strength and 18.12% reduction in E were observed compared to the dry samples. A Langmuir-type curve well fits the changes in strength and E with CO2 saturation pressure. Adsorption of water causes minor deteriorations in coal's mechanical properties, but this reduction intensifies considerably with the introduction of CO2, due to the enhanced dissolution of mineral matter and organic compounds in the CO2-acidified solvent environment. With the application of in-situ stress, alterations in the mechanical properties of coal due to fluid adsorption decrease significantly compared to the results of the corresponding uniaxial tests, because of the reduction in fluid saturation and the mechanical strengthening effect in a high stress environment. High-rank coal is expected to experience greater mechanical property deteriorations than low-rank coal due to its highly-developed Cleat System. In addition, the AE signals captured in this study further explain the alterations in coal mechanical properties of coal due to fluid saturation.
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Effect of Coal Rank on CO2 Adsorption Induced Coal Matrix Swelling with Different CO2 Properties and Reservoir Depths
Energy & Fuels, 2017Co-Authors: A.s. Ranathunga, Tharaka Dilanka Rathnaweera, Pathegama Gamage Ranjith, Mandadige Samintha Anne Perera, Xiaogang ZhangAbstract:Although the greater adsorption potential of carbon dioxide (CO2) in coal is an appealing fact in relation to the long-term safe storage of CO2 in coal seams, the resulting coal structure modification, particularly through coal matrix swelling, adds several uncertainties to the process. To date, many studies have been initiated, particularly on the effects of injecting CO2 and reservoir properties on this swelling process and the associated reservoir permeability depletion. These influences are largely dependent on the maturity of the coal mass and its structure, including the Cleat System. However, minor attention has been given to date to the effect of coal rank on CO2 adsorption-induced coal matrix swelling and was therefore investigated in the present study. The volumetric strain of the Australian brown coal samples for both CO2 and N2 under various confinements (triaxial) and injections was measured at 35 °C constant temperature to investigate the influence of CO2 properties and reservoir depth on CO...
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influence of co2 adsorption on the strength and elastic modulus of low rank australian coal under confining pressure
International Journal of Coal Geology, 2016Co-Authors: A.s. Ranathunga, M S A Perera, Pathegama Gamage RanjithAbstract:Abstract Precise knowledge of changes in the CO2 adsorption-induced mechanical properties of deep coal seams is necessary for the safe and successful implementation of carbon dioxide-enhanced coal bed methane (CO2-ECBM) recovery. To date, little attention has been paid to the geo-mechanical property alterations in coal seams during CO2 adsorption under in-situ stress conditions. The aim of this study is therefore to discover how coal seam integrity varies with the introduction of CO2 under in-situ stress conditions, by conducting a series of tri-axial strength tests of Australian brown coal samples. The effect of CO2 exposure time on coal's mechanical properties was also investigated under super-critical CO2 saturation (10 MPa) conditions, in order to determine the effect of long-term CO2 injection on coal seams' mechanical properties, as CO2-ECBM is a long-term process. According to the test results, irrespective of coal rank, the strength reduction in coal with CO2 injection under field conditions is significantly less than would be expected based on simple laboratory testing such as uniaxial tests. However, in any stress environment, high rank coals are subjected to greater strength and stiffness reductions with CO2 adsorption than low rank coals, due to their well-developed Cleat System, and the injection of super-critical CO2 induces greater mechanical property alterations in coal than sub-critical CO2 injection. These strength and elastic modulus changes in coal with CO2 adsorption can be presented using a simple Langmuir-type equation, regardless of rank. Furthermore, the observation of the effect of long-term CO2 saturation on coal's mechanical properties revealed that, although CO2 adsorption-induced mechanical property alterations in coal are mostly completed with the first interaction with CO2, further structural re-arrangement may occur at a slower rate over time.
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super critical co2 saturation induced mechanical property alterations in low rank coal an experimental study
Journal of Supercritical Fluids, 2016Co-Authors: A.s. Ranathunga, M S A Perera, Pathegama Gamage RanjithAbstract:Abstract The adsorption of carbon dioxide (CO 2 ) into the coal matrix during CO 2 -enhanced methane recovery causes significant alterations to the coal mass chemical and physical structures, causing modifications to coal's mechanical properties. Hence, the main objective of this study is to investigate the effects of sequestrated CO 2 phase condition on coal strength. A series of unconfined compressive strength tests was conducted on Australian brown coal samples, saturated under various CO 2 pressures (2–10 MPa) at 35 °C using an advanced acoustic emission (AE) System and optical 3D deformation analysis. According to the results, super-critical CO 2 has the ability to cause a greater reduction of strength (by 46%) and enhancement of elasticity properties (by 20%) in brown coal compared to sub-critical CO 2 , because super-critical CO 2 has greater adsorptive potential, which eventually creates greater coal matrix swelling. According to the AE and deformation analysis, the coal mass natural Cleat System contributes significantly to the observed CO 2 adsorption-induced changes in mechanical properties.
K. H. S. M. Sampath - One of the best experts on this subject based on the ideXlab platform.
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effect of coal maturity on co2 based hydraulic fracturing process in coal seam gas reservoirs
Fuel, 2019Co-Authors: K. H. S. M. Sampath, M S A Perera, Tharaka Dilanka Rathnaweera, Pathegama Gamage Ranjith, Stephan K Matthai, Derek Elsworth, G. ZhangAbstract:Abstract Hydraulic fracturing of deep coal seams is challenging due to both the complex processes involved in fracturing and the typically poorly defined characteristics such as natural Cleat System, mineral-maceral distribution and strength parameters of the subsurface. This study evaluates the effectiveness of fracturing using liquid CO2 as the propellant through observations of break-down pressures and the form of the induced fracture network in various ranked coals. Coal ranks are defined through a rigorous proximate analysis to determine the moisture, volatile matter, ash and fixed carbon contents of each coal type fractured. Fracturing experiments were conducted on 38 mm × 76 mm core samples, under fixed stress, temperature conditions (i.e. σ 3 = 6 MPa, σ 1 = 8 MPa and T = 25 °C). Break-down pressures are observed to increase with increasing coal maturity. Increasing rank or maturity identifies that the coal has been subjected to progressively higher pressures and temperatures, has gained proportionately higher strength and thus exhibits a higher break-down pressure. No direct relationship is observed between volatile matter content and either strength or break-down pressure. The colocation of acoustic emission (AE) hypocenters and mineral grain boundaries delineated by micro-CT imaging indicate preferred pathways for the propagation of fractures induced by liquid CO2. Stiffness contrasts between mineral phases result in stress concentrations and localized weakness at grain-grain boundaries. The complex mineral distribution in coal accentuates such heterogeneity of weakness and may be the key feature promoting the evolution of a well distributed rather than localized fracture network. For low rank coal, hydraulic fracturing is least effective, as the fracturing process does not create a significant fracture network to enhance the permeability. This may result, since low rank coals are intrinsically weak due to their low carbon content and high moisture content allowing extensive fracturing to develop at only very low break-down pressures – minimizing damage. These observations emphasize the sensitivity of break-down pressures and the resulting complexity of fracturing to pressurization rates and coal rank – inferring important controls on these parameters for the safe and effective use, when fracturing with CO2 as the propellant.
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CO2 interaction induced mechanical characteristics alterations in coal: A review
International Journal of Coal Geology, 2019Co-Authors: K. H. S. M. Sampath, Pathegama Gamage Ranjith, Mandadige Samintha Anne Perera, Stephan K MatthaiAbstract:Abstract Coal geo-mechanical alterations caused by CO2 interaction should be critically evaluated to enhance the efficiency of CO2-ECBM and CO2 sequestration processes, while mitigating associated hazards. We provide a detailed overview on the CO2 interaction induced mechanical alterations in coal, by comprehensively reviewing the influential parameters and causative factors. Although studies provide persuasive evidence to conclude the general fact that CO2 interaction causes severe mechanical degradation in coal, the degree of strength reduction is influenced by numerous parameters: 1) stress state - elevated in-situ stresses diminish the CO2-induced coal strength reduction, due to reduced CO2 adsorption potential and the hindered swelling effect, 2) CO2 adsorption pressure and phase - magnitude of strength reduction increases with increasing CO2 adsorption pressure and CO2 phase transition into supercritical state causes a noticeable strength reduction, because of the higher adsorption and polymerization capacities of supercritical CO2, 3) CO2 interaction time - due to rapid CO2 adsorption process, significant mechanical degradation occurs in the short-term, whereas additional CO2 exposure continuous to reduce the strength, but at a slower rate, 4) coal seam characteristics - high-rank coal with a well-formed Cleat System exhibits greater strength reductions, compared to low-rank coal, because coal Cleat System facilitates easy invasion of CO2 and acts as a locus for CO2 adsorption, expanding the coal matrix along the Cleat walls. Moreover, we highlight that there are multiple factors that may contribute to the overall mechanical strength alteration: 1) CO2 adsorption induced surface energy reduction, 2) CO2-induced plasticization of coal polymer-structure, 3) formation of micro-cracks due to shrinkage, free- and differential-swelling, and 4) dissolution of Cleat and pore filling minerals, causing bond energy reduction, influencing grain-to-grain contact and forming secondary porosity Systems in coal. However, we infer that the mechanical degradation depends on the heterogeneity and anisotropy of the coal reservoir. Therefore, the degradation should be specifically assessed by considering the targeted reservoir characteristics, prior to the implementation of CO2-ECBM and CO2 sequestration field projects.
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Characterization of coal porosity and permeability evolution by demineralisation using image processing techniques: A micro-computed tomography study
Journal of Natural Gas Science and Engineering, 2018Co-Authors: Guanglei Zhang, Pathegama Gamage Ranjith, Asadul Haque, Mandadige Samintha Anne Perera, Xavier Choi, K. H. S. M. SampathAbstract:Abstract The permeability of coal is the key parameter both in primary and enhanced coalbed methane recovery. The natural Cleat System in coal serves as the primary pathway for gas flow in coal seams though mineralisation in Cleats and is known to significantly reduce coal permeability. This paper reports on a numerical simulation of the pore network evolution of coal subject to Cleat demineralisation. A high-resolution micro-computed tomography scanner was used to characterize the micro-structures of three anthracite coal samples. The mineral phases available in the coal samples were selectively removed to different extents (20%, 40%, 60%, 80% and 100%) and merged into the pore space using image processing techniques. In this way, the coal demineralisation process could be simulated and its impact on porosity and permeability studied. Comprehensive pore structure characterizations, including porosity, connectivity and tortuosity, were then conducted on the reconstructed pore network using Avizo software. Pore network models were also extracted to investigate changes in the pore and throat attributes. The lattice Boltzmann method was adopted to identify the absolute permeability changes with Cleat demineralisation. The results reveal that demineralisation can increase coal porosity and permeability up to a percolation threshold. Although porosity was enhanced prior to the percolation threshold, the coal permeability was not enhanced due to poor pore connectivity. The permeability changed rapidly close to the percolation threshold, depending on the degree of demineralisation, and an exponential relation was observed between permeability and the amount of demineralisation. According to the observations, complete removal of the mineral phase can significantly increase the connected porosity while reducing the pore tortuosity, resulting in several orders of magnitude increase in coal permeability. This study shows that Cleat demineralisation is an effective permeability enhancement technique for coalbed methane recovery, if very high demineralisation can be achieved.