An Influence of Thermally-Induced Micro-Cracking under Cooling Treatments: Mechanical Characteristics of Australian Granite

The aim of this study is to characterise the changes in mechanical properties and to provide a comprehensive micro-structural analysis of Harcourt granite over different pre-heating temperatures under two cooling treatments (1) rapid and (2) slow cooling. A series of uniaxial compression tests was conducted to evaluate the mechanical properties of granite specimens subjected to pre-heating to temperatures ranging from 25–1000◦C under both cooling conditions. An acoustic emission (AE) system was incorporated to identify the fracture propagation stress thresholds. Furthermore, the effect of loading and unloading behaviour on the elastic properties of Harcourt granite was evaluated at two locations prior to failure: (1) crack initiation and (2) crack damage. Scanning electron microscopy (SEM) analyses were conducted on heat-treated thin rock slices to observe the crack/fracture patterns and to quantify the extent of micro-cracking during intense heating followed by cooling. The results revealed that the thermal field induced in the Harcourt granite pore structure during heating up to 100◦C followed by cooling causes cracks to close, resulting in increased mechanical characteristics, in particular, material stiffness and strength. Thereafter, a decline in mechanical properties occurs with the increase of pre-heating temperatures from 100◦C to 800◦C. However, the thermal deterioration under rapid cooling is much higher than that under slow cooling, because rapid cooling appears to produce a significant amount of micro-cracking due to the irreversible thermal shock induced. Multiple stages of loading and unloading prior to failure degrade the elastic properties of Harcourt granite due to the damage accumulated through the coalescence of micro-cracks induced during compression loading. However, this degradation is insignificant for pre-heating temperatures over 400◦C, since the specimens are already damaged due to excessive thermal deterioration. Moreover, unloading after crack initiation tends to cause insignificant irreversible strains, whereas significant permanent strains occur during unloading after crack damage, and this appears to increase with the increase of pre-heating temperature over 400

An influence of thermally-induced micro-cracking under cooling treatments: mechanical characteristics of Australian granite

The aim of this study is to characterise the changes in mechanical properties and to provide a comprehensive micro-structural analysis of Harcourt granite over different pre-heating temperatures under two cooling treatments (1) rapid and (2) slow cooling. A series of uniaxial compression tests was conducted to evaluate the mechanical properties of granite specimens subjected to pre-heating to temperatures ranging from 25–1000◦C under both cooling conditions. An acoustic emission (AE) system was incorporated to identify the fracture propagation stress thresholds. Furthermore, the effect of loading and unloading behaviour on the elastic properties of Harcourt granite was evaluated at two locations prior to failure: (1) crack initiation and (2) crack damage. Scanning electron microscopy (SEM) analyses were conducted on heat-treated thin rock slices to observe the crack/fracture patterns and to quantify the extent of micro-cracking during intense heating followed by cooling. The results revealed that the thermal field induced in the Harcourt granite pore structure during heating up to 100◦C followed by cooling causes cracks to close, resulting in increased mechanical characteristics, in particular, material stiffness and strength. Thereafter, a decline in mechanical properties occurs with the increase of pre-heating temperatures from 100◦C to 800◦C. However, the thermal deterioration under rapid cooling is much higher than that under slow cooling, because rapid cooling appears to produce a significant amount of micro-cracking due to the irreversible thermal shock induced. Multiple stages of loading and unloading prior to failure degrade the elastic properties of Harcourt granite due to the damage accumulated through the coalescence of micro-cracks induced during compression loading. However, this degradation is insignificant for pre-heating temperatures over 400◦C, since the specimens are already damaged due to excessive thermal deterioration. Moreover, unloading after crack initiation tends to cause insignificant irreversible strains, whereas significant permanent strains occur during unloading after crack damage, and this appears to increase with the increase of pre-heating temperature over 400◦C. © 2018 by the authors

Effect of Coal Rank on Various Fluid Saturations Creating Mechanical Property Alterations Using Australian Coals

During CO2 sequestration in deep coal seams, the coal mass may be subjected to various fluid (CO2, N2, etc.) saturations. Therefore, in order to maintain the long-term integrity of the process, it is necessary to identify the mechanical responses of preferable coal seams for various fluid saturations. To date, many studies have focused on the CO2 saturation effect on coal mass strength and less consideration has been given to the influence of other saturation mediums. Hence, this study aims to investigate coal’s mechanical responses to water and N2 saturations compared to CO2 saturation and to determine the effect of coal-rank. A series of unconfined compressive strength (UCS) tests was conducted on Australian brown and black coal samples saturated with water and N2 at various saturation pressures. An advanced acoustic emission (AE) system was utilized to identify the changes in crack propagation behaviors under each condition. According to the results, both CO2 and water act similarly with coal by enhancing the ductile properties of the coal mass and this mechanical weakening is greater for high-rank coal. Conversely, N2 saturation slightly enhances coal strength and delays crack propagation in coal and this strength enhancement can be improved by increasing the N2 saturation pressure

A Comprehensive Overview of CO2 Flow Behaviour in Deep Coal Seams

Although enhanced coal bed methane recovery (ECBM) and CO2 sequestration are effective approaches for achieving lower and safer CO2 levels in the atmosphere, the effectiveness of CO2 storage is greatly influenced by the flow ability of the injected CO2 through the coal seam. A precious understanding of CO2 flow behaviour is necessary due to various complexities generated in coal seams upon CO2 injection. This paper aims to provide a comprehensive overview on the CO2 flow behaviour in deep coal seams, specifically addressing the permeability alterations associated with different in situ conditions. The low permeability nature of natural coal seams has a significant impact on the CO2 sequestration process. One of the major causative factors for this low permeability nature is the high effective stresses applying on them, which reduces the pore space available for fluid movement with giving negative impact on the flow capability. Further, deep coal seams are often water saturated where, the moisture behave as barriers for fluid movement and thus reduce the seam permeability. Although the high temperatures existing at deep seams cause thermal expansion in the coal matrix, reducing their permeability, extremely high temperatures may create thermal cracks, resulting permeability enhancements. Deep coal seams preferable for CO2 sequestration generally are high-rank coal, as they have been subjected to greater pressure and temperature variations over a long period of time, which confirm the low permeability nature of such seams. The resulting extremely low CO2 permeability nature creates serious issues in large-scale CO2 sequestration/ECBM projects, as critically high injection pressures are required to achieve sufficient CO2 injection into the coal seam. The situation becomes worse when CO2 is injected into such coal seams, because CO2 movement in the coal seam creates a significant influence on the natural permeability of the seams through CO2 adsorption-induced swelling and hydrocarbon mobilisation. With regard to the temperature, the combined effects of the generation of thermal cracks, thermal expansion, adsorption behaviour alterations and the associated phase transition must be considered before coming to a final conclusion. A reduction in coal’s CO2 permeability with increasing CO2 pressure may occur due to swelling and slip-flow effects, both of which are influenced by the phase transition in CO2 from sub- to super-critical in deep seams. To date, many models have been proposed to simulate CO2 movement in coal considering various factors, including porosity, effective stress, and swelling/shrinkage. These models have been extremely useful to predict CO2 injectability into coal seams prior to field projects and have therefore assisted in implementing number of successful CO2 sequestration/ECBM projects

A Comprehensive Overview of CO2 Flow Behaviour in Deep Coal Seams

Although enhanced coal bed methane recovery (ECBM) and CO2 sequestration are effective approaches for achieving lower and safer CO2 levels in the atmosphere, the effectiveness of CO2 storage is greatly influenced by the flow ability of the injected CO2 through the coal seam. A precious understanding of CO2 flow behaviour is necessary due to various complexities generated in coal seams upon CO2 injection. This paper aims to provide a comprehensive overview on the CO2 flow behaviour in deep coal seams, specifically addressing the permeability alterations associated with different in situ conditions. The low permeability nature of natural coal seams has a significant impact on the CO2 sequestration process. One of the major causative factors for this low permeability nature is the high effective stresses applying on them, which reduces the pore space available for fluid movement with giving negative impact on the flow capability. Further, deep coal seams are often water saturated where, the moisture behave as barriers for fluid movement and thus reduce the seam permeability. Although the high temperatures existing at deep seams cause thermal expansion in the coal matrix, reducing their permeability, extremely high temperatures may create thermal cracks, resulting permeability enhancements. Deep coal seams preferable for CO2 sequestration generally are high-rank coal, as they have been subjected to greater pressure and temperature variations over a long period of time, which confirm the low permeability nature of such seams. The resulting extremely low CO2 permeability nature creates serious issues in large-scale CO2 sequestration/ECBM projects, as critically high injection pressures are required to achieve sufficient CO2 injection into the coal seam. The situation becomes worse when CO2 is injected into such coal seams, because CO2 movement in the coal seam creates a significant influence on the natural permeability of the seams through CO2 adsorption-induced swelling and hydrocarbon mobilisation. With regard to the temperature, the combined effects of the generation of thermal cracks, thermal expansion, adsorption behaviour alterations and the associated phase transition must be considered before coming to a final conclusion. A reduction in coal’s CO2 permeability with increasing CO2 pressure may occur due to swelling and slip-flow effects, both of which are influenced by the phase transition in CO2 from sub- to super-critical in deep seams. To date, many models have been proposed to simulate CO2 movement in coal considering various factors, including porosity, effective stress, and swelling/shrinkage. These models have been extremely useful to predict CO2 injectability into coal seams prior to field projects and have therefore assisted in implementing number of successful CO2 sequestration/ECBM projects

Investigation of the effect of carbon dioxide sequestration on the hydro - mechanical properties of coal

The process of carbon dioxide (CO2) sequestration in deep coal seam causes both coal seam permeability and strength to be significantly reduced due to CO2 adsorption-induced coal matrix swelling. In addition, in deep coal seams CO2 exists in its super-critical state, which has quite different chemical and physical properties compared to sub-critical CO2. However, to date, there has been a lack of understanding regarding the effect of super-critical CO2 injection on coal flow and strength. The main objective of this study is to understand the effects of sub-critical and super-critical CO2 injections on coal flow and strength properties through experimental, numerical, theoretical and analytical investigations. A high pressure triaxial set-up was first developed to conduct permeability tests under high injecting and confining pressures, axial load and temperature conditions. The developed set-up was then used to conduct permeability tests for naturally fractured black coal samples taken from the Appin coal mine of the Bulli coal seam, Southern Sydney basin to identify the effects of sub-critical and super-critical CO2 injections on coal permeability. According to the experimental results, the amount of swelling due to CO2 adsorption depends on the CO2 phase state and confining and injecting pressures, and super-critical CO2 adsorption creates approximately double the swelling effect compared to sub-critical CO2. In addition, super-critical CO2 exhibits somewhat lower permeability values compared to sub-critical CO2, and this permeability reduction increases with increasing injecting pressure. Interestingly, N2 has the potential to reverse the CO2 induced swelling areas to some extent. If the temperature effect on permeability is considered, temperature has a positive effect on CO2 permeability. The CO2 permeability increment with increasing temperature increases with increasing CO2 pressure, and the effect of temperature on coal permeability is negligible at low CO2 pressures (<9 MPa). UCS strength tests were then conducted for both high rank (black) and low rank (lignite) coals under different saturation conditions; sub-critical and super-critical CO2, N2 and moisture. According to the test results, the UCS strength and Young’s modulus of both types of coals are reduced due to CO2 saturation and N2 saturation does not have much influence on coal strength. The reductions of UCS strength and Young’s modulus in black coal due to super-critical CO2 adsorption are higher than the sub-critical CO2 adsorption by about 40% and 100 %, respectively. Furthermore, with increasing saturation pressure, the reductions of coal UCS strength and Young’s modulus due to super-critical CO2 adsorption become lower. However, the effect of super-critical saturation on the strength of low rank coal could not be investigated due to its low density and strength values. Both laboratory and field-scale model developments were considered in the numerical modelling approach to coal CO2 sequestration. In the case of the laboratory-scale model development, CO2 movement in coal under triaxial test conditions was successfully modelled using the COMET 3 field scale simulator. According to the field-scale models developed using the COMET 3 and COMSOL Multiphysics simulators, CO2 storage capacity in coal increases with increasing injecting pressure and temperature and decreasing bed moisture content. Moreover, pre-estimation of the distance between the wells (injecting and production) is important for the injection of optimum amounts of CO2. On the other hand, CO2 injection causes the coal seam cap rock to be significantly deformed in an upward direction and the amount of deformation is greatly dependent on the injection pressure. In relation to theoretical and analytical approaches to coal CO2 sequestration, a theoretical equation for coal cleat permeability under non-zero lateral strain, triaxial test conditions was developed using basic geotechnical engineering fundamentals and an empirical relationship for gas adsorption capacity in coal as a function of all the effective parameters was developed using basic statistics.Awards: Winner of the Mollie Holman Doctoral Medal for Excellence, Faculty of Engineering, 2012

Comparison of CO2 Flow Behavior through Intact Siltstone Sample under Tri-Axial Steady-State and Transient Flow Conditions

With its low viscosity properties, CO2 has much greater penetration capacity into micro-fractures, and therefore has more potential to create expanded and effective fractures in shales during the hydraulic fracturing process. However, the feasibility of this technique is dependent on the accurate prediction of formation flow characteristics, given the high leak-off of CO2 at deep depths. The aim of this study is therefore to understand the flow behavior of CO2 in deep shale plays. A series of tri-axial permeability tests was conducted under both steady-state and transient conditions. The test results show much lower permeability values for liquid CO2 than gaseous CO2, and the permeability under transient conditions is much lower than that under steady-state conditions, due to the combined effects of the reduced slip-flow effect under low pressures and the temperature variation influence under steady-state conditions. Under steady-state conditions, unstable flow behavior occurred at higher injection pressure (&ge;9 MPa) possibly due to the fine mineral particle migration and the deposition of small drikold particles, which indicates the serious error in permeability calculation under steady-state conditions. Importantly, a greater than 1 effective stress coefficient (&chi;) for permeability in tested siltstone was observed, confirming the greater sensitivity of CO2 to pore pressure than confining pressure

Effect of coal rank on various fluid saturations creating mechanical property alterations using Australian coals

During CO2 sequestration in deep coal seams, the coal mass may be subjected to various fluid (CO2, N2, etc.) saturations. Therefore, in order to maintain the long-term integrity of the process, it is necessary to identify the mechanical responses of preferable coal seams for various fluid saturations. To date, many studies have focused on the CO2 saturation effect on coal mass strength and less consideration has been given to the influence of other saturation mediums. Hence, this study aims to investigate coal’s mechanical responses to water and N2 saturations compared to CO2 saturation and to determine the effect of coal-rank. A series of unconfined compressive strength (UCS) tests was conducted on Australian brown and black coal samples saturated with water and N2 at various saturation pressures. An advanced acoustic emission (AE) system was utilized to identify the changes in crack propagation behaviors under each condition. According to the results, both CO2 and water act similarly with coal by enhancing the ductile properties of the coal mass and this mechanical weakening is greater for high-rank coal. Conversely, N2 saturation slightly enhances coal strength and delays crack propagation in coal and this strength enhancement can be improved by increasing the N2 saturation pressure

Characteristics of clay-abundant shale formations:Use of CO₂ for production enhancement

Clay-abundant shale formations are quite common worldwide shale plays. This particular type of shale play has unique physico-chemical characteristics and therefore responds uniquely to the gas storage and production process. Clay minerals have huge surface areas due to prevailing laminated structures, and the deficiency in positive charges in the combination of tetrahedral and octahedral sheets in clay minerals produces strong cation exchange capacities (CECs), all of which factors create huge gas storage capacity in clay-abundant shale formations. However, the existence of large amounts of tiny clay particles separates the contacts between quartz particles, weakening the shale formation and enhancing its ductile properties. Furthermore, clay minerals’ strong affinity for water causes clay-abundant shale formations to have large water contents and therefore reduced gas storage capacities. Clay-water interactions also create significant swelling in shale formations. All of these facts reduce the productivity of these formations. The critical influences of clay mineral-water interaction on the productivity of this particular type of shale plays indicates the inappropriateness of using traditional types of water-based fracturing fluids for production enhancement. Non-water-based fracturing fluids are therefore preferred, and CO2 is preferable due to its many unique favourable characteristics, including its minor swelling effect, its ability to create long and narrow fractures at low breakdown pressures due to its ultralow viscosity, its contribution to the mitigation of the greenhouse gas effect, rapid clean-up and easy residual water removal capability. The aim of this paper is to obtain comprehensive knowledge of utilizing appropriate production enhancement techniques in clay-abundant shale formations based on a thorough literature review

Mechanism, Cause, and Control of Water, Solutes, and Gas Migration Triggered by Mining Activities

Although the growth in global coal consumption has been sharply slowed with the falls in China offset to a greater extent by the increasing demand in India and other emerging Asian countries, coal still remains the largest source of energy for the world with a share of almost 30% by 2040 [1]. Mining industry plays an important role in extracting underground resources, including coal [2]. However, a large number of disastrous mine accidents, such as flood, water inrush, tunnel collapse, gas outburst, and gas explosion, have been reported due to water and gas migration caused by the mining activities, posing a threat to the environment and also to the health and safety of field workers [3–5]. According to incomplete statistics, mining-induced accidents kill over thousands of workers around the world every year, especially in developing countries such as China and India. Water inrush and gas explosion accidents are the major causes for the reported mine accidents [6, 7]