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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.

Previous projects on CO2 storage in coal often reported the challenges associated with coal swelling and swelling induced loss of gas injectivity. Since coal seams are typically thin, commonly used vertical wells only intersect a target reservoir over a small contact area, placing constraints on CO2 injectivity in addition to those resulting from coal swelling. This leaves the storage reservoirs largely under-utilized and, therefore, questions the viability of this technology. To address the challenges/limitations of the current practice, a novel in-situ CO2 injection test is planned using horizontal injection wells in Mikolow, Poland. This paper presents the pre-operational simulation studies conducted to assist the design and operation of the in-situ test. An existing dual-porosity model that is built on a coupled thermo-hydro-chemical-mechanical (THCM) modelling framework is employed in this study. Sensitivity of the model parameters and validity of the model are tested. Several simulation scenarios are developed in reference to the selected test site for various horizontal well configurations and gas injection conditions. From the results and analyses, it is evident that by varying the coal-CO2 contact area via the length of the horizontal injection well, as well as the operating conditions including fixed pressure, and fixed rate injection scenarios, the targeted amount (between 1 to 10 tonnes) of CO2 can be injected into the seam without significant loss of permeability or injectivity, yielding sustained gas injection. Moreover, the spread of CO2 is predicted to be contained within the model domain suggesting no significant concern of spread exceeding the test area. Minister of Science and Higher Education entitled "PMW" in the years 2020-2023; (agreement No. 5144/FBWiS/2020/2). The ROCCS project has received funding from the Research Fund for Coal and Steel under Grant Agreement No. 899336.

Coupled thermo‐hydro‐mechanical‐chemical modelling has attracted attention in past decades due to many contemporary geotechnical engineering applications (e.g., waste disposal, carbon capture and storage). However, molecular‐scale interactions within geomaterials (e.g., swelling and dissolution/precipitation) have a significant influence on the mechanical behaviour, yet are rarely incorporated into existing Thermal‐Hydro‐Mechanical‐Chemical (THMC) frameworks. This paper presents a new coupled hydro‐mechanical‐chemical constitutive model to bridge molecular‐scale interactions with macro‐physical deformation by combining the swelling and dissolution/precipitation through an extension of the new mixture‐coupling theory. Entropy analysis of the geomaterial system provides dissipation energy, and Helmholtz free energy gives the relationship between solids and fluids. Numerical simulation is used to compare with the selected recognized models, which demonstrates that the swelling and dissolution/precipitation processes may have a significant influence on the mechanical deformation of the geomaterials.

This study presents a coupled dual porosity thermal-hydraulic-mechanical (THM) model of non-isothermal gas flow during CO2 sequestration in coal seams. Thermal behaviour is part of the disturbed physical and chemical condition of a coal seam caused by CO2 injection, and must be understood for accurate prediction of CO2 flow and storage. A new porosity-permeability model is included for consideration of the fracture-matrix compartment interaction. The new model is verified against an analytical solution and validated against experimental measurements, before being used to analyse coupled THM effects during CO2 sequestration in coal. A simulation of CO2 injection at a fixed rate shows the development of a cooling region within the coal seam due to the Joule-Thomson effect, with the temperature in the vicinity of the well declining sharply before recovering slowly. The temperature disturbance further from the well is more gradual by comparison. Under the simulation conditions studied, CO2 injection increases coal matrix porosity and decreases the porosity and permeability of the natural fracture network, especially in the vicinity of the injection well, due to adsorption-induced coal swelling. Compared with the effects of gas pressure and temperature, the matrix-fracture compartment interaction plays an important role in changes of porosity and permeability. Considering the temperature disturbance caused by CO2 injection under the set of representative conditions studied, the coupled model can provide an insight into the associated effects on CO2 flow and storage during its sequestration in coal seams. The first author gratefully acknowledges the financial support provided by the Welsh European Funding Office (WEFO), through the FLEXIS project. The financial support from the China Scholarship Council for the PhD studentship of the second author is also gratefully acknowledged.

This paper presents the development of a discrete fracture model of fully coupled compressible fluid flow, adsorption and geomechanics to investigate the dynamic behaviour of fractures in coal. The model is applied in the study of geological carbon dioxide sequestration and differs from the dual porosity model developed in our previous work, with fractures now represented explicitly using lower‐dimensional interface elements. The model consists of the fracture‐matrix fluid transport model, the matrix deformation model and the stress‐strain model for fracture deformation. A sequential implicit numerical method based on Galerkin finite element is employed to numerically solve the coupled governing equations, and verification is completed using published solutions as benchmarks. To explore the dynamic behaviour of fractures for understanding the process of carbon sequestration in coal, the model is used to investigate the effects of gas injection pressure and composition, adsorption and matrix permeability on the dynamic behaviour of fractures. The numerical results indicate that injecting nonadsorbing gas causes a monotonic increase in fracture aperture; however, the evolution of fracture aperture due to gas adsorption is complex due to the swelling‐induced transition from local swelling to macro swelling. The change of fracture aperture is mainly controlled by the normal stress acting on the fracture surface. The fracture aperture initially increases for smaller matrix permeability and then declines after reaching a maximum value. When the local swelling becomes global, fracture aperture starts to rebound. However, when the matrix permeability is larger, the fracture aperture decreases before recovering to a higher value and remaining constant. Gas mixtures containing more carbon dioxide lead to larger closure of fracture aperture compared with those containing more nitrogen.

This paper presents the inclusion of explicit dual poroelastic mechanical behaviour as part of an existing dual porosity numerical model of multiphase, multicomponent chemical-gas transport. The dual poroelastic framework employed considers the pore structure changes occurring as a result of high pressure carbon dioxide injection into coal, particularly the adsorption-induced coal swelling that has been found to limit injectivity in field trials of carbon sequestration in coalbeds around the world. To address this issue, the surface stress of the fluid-solid interface is introduced into the constitutive relation for dual porosity effective stress in order to investigate the coal deformation and porosity changes related to adsorption behaviour. A new porosity model is presented, in which the impacts of gas flow and coal deformation are incorporated, and an interaction coefficient is proposed to explain the effect of fracture-matrix interactions on the porosity evolution. The model is verified and validated in this work against relevant analytical solutions and experimental results, and applied to study the gas flow behaviour and structural changes of coal. The results show that carbon dioxide injection not only causes coal swelling but also has the potential to change the internal pore structure of coal. The variation of fracture porosity is not monotonic as a competing result of effective stress and internal fracture-matrix interactions. However, the matrix porosity is found to increase during carbon dioxide injection, which seems to be a key contributor to the swelling phenomenon.

© 2018, Canadian Science Publishing. All rights reserved. This paper presents the development of a dual porosity numerical model of multiphase, multicomponent chemical-gas transport using a coupled thermal, hydraulic, chemical, and mechanical formulation. Appropriate relationships are used to describe the transport properties of nonideal, reactive gas mixtures at high pressure, enabling the study of geoenergy applications such as geological carbon sequestration. Theoretical descriptions of the key transport processes are based on a dual porosity approach considering the fracture network and porous matrix as distinct continua over the domain. Flow between the pore regions is handled using mass exchange terms and the model includes equilibrium and kinetically controlled chemical reactions. A numerical solution is obtained with a finite element and finite difference approach and verification of the model is pursued to build confidence in the accuracy of the implementation of the dual porosity governing equations. In the course of these tests, the time-splitting approach used to couple the transport, mass exchange, and chemical reaction modules is shown to have been successfully applied. It is claimed that the modelling platform developed provides an advanced tool for the study of high-pressure gas transport, storage, and displacement for geoenergy applications involving multiphase, multicomponent chemical-gas transport in dual porosity media, such as geological carbon sequestration. Welsh European Funding Office

Modelling of coupled thermal (T), hydro (H), mechanical (M) and chemical (C) processes in geomaterials has attracted attention in the past decades due to many significant contemporary engineering applications such as nuclear waste disposal, carbon capture and storage etc. However, in very-low permeability membrane geomaterials, the couplings between chemical osmosis and thermal osmosis and their consequent influence on temperature, water transport and mechanical deformation remain as a long-lasting challenge due to the gap between geomechanics and geochemistry. This paper extends Mixture Coupling Theory by bridging the chemical-thermal field based on non-equilibrium thermodynamics, and develops a new constitutive THMC fully-coupled model incorporating the interactions between chemical and thermal osmosis. Classic Darcy's law has been fundamentally extended with osmosis as the major driving force of the diffusion process. A simple numerical simulation used for the demonstration purpose has illustrated that the couplings between chemical and thermal osmosis will significantly change the water flow directions, consequently influencing the saturation variation and mechanical deformation.

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.

Deep coal reserves represent a valuable resource, both in terms of their potential for the sequestration of anthropogenic carbon dioxide and for energy extraction through underground coal gasification (UCG). This paper looks at the current field and research status of these technologies and, in line with on-going work at Cardiff University’s Geoenvironmental Research Centre (GRC), focuses special attention on the role of numerical modelling in advancing the research agenda. In response to poor carbon dioxide injectivity experienced in carbon sequestration trials and order-of-magnitude permeability losses caused by sorption induced coal swelling, the direction of work at the GRC is to improve the current understanding of the mechanical response of coal using a coupled thermal, hydraulic, chemical, mechanical modelling framework. Hence, the current focus of theoretical developments in this area are summarized. In relation to UCG, on-going developments to a comprehensive numerical model are discussed. The ultimate aim is to identify and quantify coupled mechanisms controlling syngas composition and production rate, heat and mass transport, geochemical reactions, geomechanical responses, and cavity growth to subsequently improve the state-of-the-art towards minimizing geoenvironmental risks.

This paper presents a numerical investigation of non-isothermal effects during high pressure carbon dioxide injection in deep coalbeds. Whilst coalbeds provide storage security owing to the large carbon dioxide adsorption capacity of coal, high pressure gas injection may disturb the temperature of the coalbed and influence flow behaviour. A numerical model of coupled heat transfer and gas flow incorporating the Joule-Thomson cooling effect is presented in this paper to study the temperature change induced by carbon dioxide injection. The numerical model is firstly compared with existing results in the literature for verification, and then applied to simulate changes in temperature and pressure of a coalbed during gas flow. Results of predicted temperature and pressure under different injection temperatures show that a zone of cooling occurs when gas injection under the same and low temperature than that of coalbeds, leading to a further drop of pressure in this zone. Although injection under higher temperature can suppress cooling effect, the time for pressure to reach steady state increases.

This paper examines the assumption of pseudo-steady state inter-porosity mass exchange in dual porosity models of fractured rock. Models of this type rely on the assumption that a pseudo-steady pore pressure distribution prevails in the porous matrix at all times, thereby neglecting transient pressure gradients. The rate of inter-porosity mass exchange is then conveniently expressed as a linear function of the difference between the average pore pressures in the fracture and matrix domains. Whilst providing a relatively simple description of mass exchange, the accuracy of this approach has been debated and it is strictly only valid once the pressure front due to changing conditions in the fracture network reaches the centre of the matrix. The aim of this paper is to compare the pseudo-steady state model of mass exchange with an explicit model of diffusive flow into a rock matrix with parallel-plate geometry. Since the mass exchange coefficient is sometimes described as a function of matrix block geometry and effective diffusivity, an attempt is made to adopt this approach before curve fitting is used. The results indicate that the adopted function underestimates the mass exchange rate compared to the benchmark, although the pseudo-steady state model can provide close agreement if curve fitting is used. It is concluded that the assumption of pseudo-steady state mass exchange may be valid only for cases where calibration of the linear coefficient is possible. Constitutive relationships describing the coefficient should be approached with care, with the possible exception of those considering some level of transiency.

© 2019 Elsevier Ltd Combustion of coal for energy generation has been a significant contributor to increased concentrations of atmospheric carbon dioxide. It is of interest to evaluate the potential of former coalfields for mitigating these increases by carbon sequestration and to compare different options to achieving this end. Here, carbon sequestration in residual coal seams and through reclamation of spoil tips is compared, and their carbon dioxide storage potential in the South Wales Coalfield estimated. Coal seam sequestration estimates come from an established methodology and consider the total unmined coal resource below 500 m deep with potential for carbon sequestration. The most likely effective deep seam storage capacity is 104.9 Mt carbon dioxide, taking account of reservoir conditions and engineering factors. Whilst many spoil tips in South Wales have been reclaimed, the focus has not been on carbon sequestration potential. Estimates of minesoil restoration sequestration capacity were based on a survey of restored minesoil and vegetation carbon stocks, mainly on sites 20–30 years after restoration; data from this survey were then extrapolated to the coalfield as a whole. Minesoil storage is estimated at 1.5 or 2.5 Mt (+2.2 Mt in tree biomass) carbon dioxide based on average grassland or woodland measurements, respectively; modelled data predicted equilibrium values of 2.9 and 2.6 Mt carbon dioxide respectively in grassland or woodland minesoils. If all sites achieved close to the maximum capacity in their land use class, minesoil storage capacity would increase to 2.1 or 3.9 Mt carbon dioxide, respectively. Combining the best woodland minesoil and standing biomass values, sequestration capacity increases to 7.2 Mt carbon dioxide. The wider social, economic, environmental and regulatory constraints to achieving this sequestration for each approach are discussed. Coal seam sequestration has a much higher capacity but sequestration in mine sites is less costly and has fewer regulatory constraints. Findings indicate a significant combined potential for carbon sequestration in the South Wales Coalfield and highlight challenges in achieving this potential. On a global scale, ex-coalfield sequestration could contribute to broader efforts to mitigate emissions. Welsh European Funding Office (WEFO) from the European Regional Development Fund, through the FLEXIS project

This paper presents a preliminary evaluation of the carbon dioxide (CO2) storage capacity of the unmined coal resources in the South Wales Coalfield, UK. Although a significant amount of the remaining coal may be mineable through traditional techniques, the prospects for opening new mines appear poor. Also, many of the South Wales coal seams are lying unused since they are too deep to be mined economically using conventional methods. There is instead a growing worldwide interest in the potential for releasing the energy value of such coal reserves through alternative technologies – for example through carbon dioxide sequestration with enhanced coal bed methane recovery. In this study, geographical information systems and three-dimensional interpolation are used to obtain the total unmined coal resource below 500 m deep, where the candidate seams for carbon dioxide sequestration are found. The ‘proved’, ‘probable’ and ‘possible’ carbon dioxide storage capacities of the South Wales Coalfield are then obtained using an established methodology. Input parameters are based on statistical distributions, considering a combination of laboratory coal characterisation results and literature review. The results are a proved capacity of 70·1 Mt carbon dioxide, a probable capacity of 104·9 Mt carbon dioxide and a possible capacity of 152·0 Mt carbon dioxide.