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.
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•Two anthracite coal samples are tested by injecting high-pressure CO2, He, N2.•Changes in flow rates, deformation and temperature across the samples are measured.•CO2 permeability is one to two orders of magnitude lower than He permeability.•Under the high flow rate of CO2, a temperature drop up to 6.8 °C is observed.•CO2 sorption induced changes are irreversible and detrimental on permeability.
Termički aktivne podzemne konstrukcije predstavljaju nisko-energetsku i održivu tehnologiju čija je primjena prioritet za mnoge države. Izmjena topline između podzemnih konstrukcija i okolnog tla ima ...važnu ulogu u ukupnoj energetskoj učinkovitosti zgrada te doprinosi naprezanjima u konstruktivnim elementima i okolnom tlu. U ovom je radu prikazan pregled nedavnih terenskih, laboratorijskih i numeričkih znanstvenih istraživanja te izvedenih projekata i dobivenih saznanja na području energetskih pilota gdje su prikazani glavni aspekti energetskih pilota i utjecaji njihova rada na okolno tlo. Na temelju navedenih spoznaja, u ovom se radu navode smjernice za buduća istraživanja u ovom području.
This thesis presents a comprehensive investigation into the underlying coupled processes in coal in response to high pressure gas injection. This is achieved by i) developing a new high pressure gas ...experimental facility and conducting a series of experimental tests, and ii) developing and applying a theoretical and numerical model. A novel experimental facility was designed, which offers stable and continuous high-pressure injection of gases in fractured rocks, for detailed study of the reactive transport processes. It consists of the gas supply and backpressure control system. Using the newly developed experimental facility, the response of coal subject to subcritical and supercritical gas injection under stable and variable temperature conditions was studied. The experimental investigation consisted of a series of tests: i) sorption capacity and kinetics tests, ii) uniaxial compressive tests, iii) sieve analysis tests, iv) flow and deformation tests. Thirty anthracite coal samples from different depths (i.e. 150 m and 550 m) and locations from the South Wales coalfield were characterised and tested. The capabilities of the theoretical and numerical modelling platform of thermal, hydraulic, chemical and mechanical processes were advanced. A new theoretical approach was adopted which successfully incorporates reactive gas transport coupled with coal deformation. The development of constitutive relationships describing the sorption induced elastic isotropic swelling of coal and changes in permeability was considered in detail. Numerical solutions of the governing flow and deformation equations were achieved by employing the finite element method for spatial discretisation and the finite difference method for temporal discretisation. The new model was verified for its accuracy via a series of benchmark tests and validated using high-resolution experimental data. The results of the experimental study showed that the sorption capacity and kinetics are sample-size dependent, particularly for deeper coal. Higher and faster sorption of CO2 obtained on powdered samples compared to intact samples indicated that sorption processes are governed by fracture interconnectivity and accessibility of pores. Sorption of CO2 was found to significantly reduce the brittleness, uniaxial compressive strength and elastic modulus of anthracite coals. The results of the post-failure sieve analysis showed that CO2 saturated samples disintegrated on smaller particles than non-saturated samples indicating that sorption induced swelling weakens the coal structure by enhancing the existing and inducing new fractures. During CO2 flow through coal under constant stress, samples experienced swelling resulting in initial reduction followed by recovery of measured flow rates. CO2 sorption induced changes were found to be non-reversible. The results of high CO2 flow through coal showed that CO2 reduced the temperature of the system, associated with Joule- Thomson cooling, enhancing the coal swelling and opposite to expected, increasing the flow rates. Overall, the high-resolution data-set obtained is a significant contribution to the scientific community and is able to provide a means of validation for future models. The results of the verification and validation exercises demonstrated the capability of the developed model to simulate coupled processes involved in gas transport in coal. A series of numerical simulations were conducted to investigate the permeability evolution and CO2 breakthrough in coal subject to supercritical CO2 injection using the developed model. Different scenarios were considered, involving a range of values of the elastic modulus and the parameter defining the coal swelling. The results of the advanced numerical simulations showed that the effect of CO2 sorption induced swelling on permeability reduces with a decrease in coal stiffness suggesting that CO2 sorption induced reduction of elastic modulus would have a positive effect on the ability of coal to conduct CO2. In this work, confidence in the feasibility of CO2 storage in anthracite coals was improved by enhancing the knowledge of high pressure gas-coal interactions through both experimental and numerical investigations. Moreover, it is claimed that newly developed model enables predictions of coupled processes involved in carbon sequestration in coal.
This thesis presents a comprehensive investigation into the underlying coupled processes in coal in response to high pressure gas injection. This is achieved by i) developing a new high pressure gas ...experimental facility and conducting a series of experimental tests, and ii) developing and applying a theoretical and numerical model. A novel experimental facility was designed, which offers stable and continuous high-pressure injection of gases in fractured rocks, for detailed study of the reactive transport processes. It consists of the gas supply and backpressure control system. Using the newly developed experimental facility, the response of coal subject to subcritical and supercritical gas injection under stable and variable temperature conditions was studied. The experimental investigation consisted of a series of tests: i) sorption capacity and kinetics tests, ii) uniaxial compressive tests, iii) sieve analysis tests, iv) flow and deformation tests. Thirty anthracite coal samples from different depths (i.e. 150 m and 550 m) and locations from the South Wales coalfield were characterised and tested. The capabilities of the theoretical and numerical modelling platform of thermal, hydraulic, chemical and mechanical processes were advanced. A new theoretical approach was adopted which successfully incorporates reactive gas transport coupled with coal deformation. The development of constitutive relationships describing the sorption induced elastic isotropic swelling of coal and changes in permeability was considered in detail. Numerical solutions of the governing flow and deformation equations were achieved by employing the finite element method for spatial discretisation and the finite difference method for temporal discretisation. The new model was verified for its accuracy via a series of benchmark tests and validated using high-resolution experimental data. The results of the experimental study showed that the sorption capacity and kinetics are sample-size dependent, particularly for deeper coal. Higher and faster sorption of CO2 obtained on powdered samples compared to intact samples indicated that sorption processes are governed by fracture interconnectivity and accessibility of pores. Sorption of CO2 was found to significantly reduce the brittleness, uniaxial compressive strength and elastic modulus of anthracite coals. The results of the post-failure sieve analysis showed that CO2 saturated samples disintegrated on smaller particles than non-saturated samples indicating that sorption induced swelling weakens the coal structure by enhancing the existing and inducing new fractures. During CO2 flow through coal under constant stress, samples experienced swelling resulting in initial reduction followed by recovery of measured flow rates. CO2 sorption induced changes were found to be non-reversible. The results of high CO2 flow through coal showed that CO2 reduced the temperature of the system, associated with Joule- Thomson cooling, enhancing the coal swelling and opposite to expected, increasing the flow rates. Overall, the high-resolution data-set obtained is a significant contribution to the scientific community and is able to provide a means of validation for future models. The results of the verification and validation exercises demonstrated the capability of the developed model to simulate coupled processes involved in gas transport in coal. A series of numerical simulations were conducted to investigate the permeability evolution and CO2 breakthrough in coal subject to supercritical CO2 injection using the developed model. Different scenarios were considered, involving a range of values of the elastic modulus and the parameter defining the coal swelling. The results of the advanced numerical simulations showed that the effect of CO2 sorption induced swelling on permeability reduces with a decrease in coal stiffness suggesting that CO2 sorption induced reduction of elastic modulus would have a positive effect on the ability of coal to conduct CO2. In this work, confidence in the feasibility of CO2 storage in anthracite coals was improved by enhancing the knowledge of high pressure gas-coal interactions through both experimental and numerical investigations. Moreover, it is claimed that newly developed model enables predictions of coupled processes involved in carbon sequestration in coal.
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