An investigation of coupled processes in coal in response to high pressure gas injection

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

A review on performance of energy piles and effects on surrounding ground

Thermo-active ground structures represent low-energy and sustainable technology which is a clear priority for many countries. Heat transfer between such structures and the surrounding soil is understood to play an important role both in the overall thermal performance of buildings and in the evolution of stresses in structural elements and the surrounding soil. This paper presents an overview of recent research efforts and developments in relation to energy piles. General aspects on the performance of energy piles and their impact on the surrounding ground are presented based on previous field, laboratory and numerical investigations as well as existing case studies. Based on the current knowledge, further research opportunities are identified and highlighted

High-pressure CO2 excess sorption measurements on powdered and core samples of high-rank coals from different depths and locations of the South Wales Coalfield

The experimental analysis aimed at investigating the high-pressure (sub- and super-critical) CO2 sorption behaviour on two high-rank coals of different sizes is presented in this paper. Coals from the same seam (9ft seam), but from depths of 150 m (BD coal) and 550 m (AB coal) and different locations of the South Wales (UK) coalfield, known to be strongly affected by tectonically developed fracture systems, are employed for that purpose. Hence, the sorption behaviour of powdered (0.25-0.85 mm, 2.36-4.0 mm) and core samples obtained from locations associated with the deformation related changes is analysed in this paper to assess the CO2 storage potential of such coals. The results show that the coals exhibit maximum adsorption capacities up to 1.93 mol/kg (BD coal) and 1.82 mol/kg (AB coal). No dependence of the CO2 maximum sorption capacity with respect to the sample size for the BD coal is observed, while for the AB coal the maximum sorption capacity is reduced by more than half between the powdered and core samples. The CO2 sorption rates on BD coal decrease by a factor of more than 9 from 0.25-0.85 mm to 2.36-4.0 mm and then remain relatively constant with further increase in sample size. The opposite is observed for the AB coal where sorption rates decrease with increasing sample size, i.e. reducing by a factor of more than 100 between the 0.25-0.85 mm and core samples. The differences in behaviour are interpreted through the structure each coal exhibits associated with the burial depths and sampling locations as well as through the minor variations in ash contents. This study demonstrates that anthracite coals, having experienced sufficient deformation resulting in changes in fracture frequency, can adsorb significant amounts of CO2 offering great prospect to be considered as a CO2 sequestration option

Effects of thermo-osmosis on hydraulic behaviour of saturated clays

Despite a body of research carried out on thermally coupled processes in soils, understanding of thermo-osmosis phenomenon in clays and its effects on hydromechanical behavior is incomplete. This paper presents an investigation on the effects of thermo-osmosis on hydraulic behavior of saturated clays. A theoretical formulation for hydraulic behavior was developed, incorporating an explicit description of thermo-osmosis effects on coupled hydromechanical behavior. The extended formulation was implemented within a coupled numerical model for thermal, hydraulic, chemical, and mechanical behavior of soils. The model was tested and applied to simulate a soil heating experiment. It is shown that the inclusion of thermo-osmosis in the coupled thermohydraulic simulation of the case study provides a better agreement with the experimental data compared with the case in which only thermal expansion of the soil constituents was considered. A series of numerical simulations are also presented, studying the pore-water pressure development in saturated clay induced by a heating source. It is shown that pore-water pressure evolution can be considerably affected by thermo-osmosis. Under the conditions of the problem considered, it was found that thermo-osmosis changed the pore-water pressure regime in the vicinity of the heater when the value of thermo-osmotic conductivity was larger than 10−12 m2·K−1·s−1. New insights into the hydraulic response of the ground and the pore-pressure evolution due to thermo-osmosis are provided in this paper

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

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

Effects of subcritical and supercritical CO2 sorption on deformation and failure of high-rank coals

This paper presents the results of an extensive experimental analysis aimed at establishing the effects of subcritical and supercritical CO2 sorption on deformation and failure of coals. Two high-rank anthracitic coals from the South Wales coalfield, obtained from different locations and depths of 150 m and 550 m, are employed for that purpose. The investigations include i) determination of unconfined compressive strengths and elastic moduli of the cores both non-saturated and saturated with CO2 at 2.1 MPa, 4.3 MPa and 8.5 MPa, ii) assessing the dependence of the parameters obtained on CO2 pressure, iii) analysing the effect of CO2 saturation on failure patterns of the samples tested and iv) determination of the particle size distribution after the failure of the samples. Based on the results of twenty coal specimens tested, it is demonstrated that CO2 sorption reduces the uniaxial compressive strengths and elastic moduli by between 29% and 83% for the range of pressures studied. The reductions observed increase gradually up to 4.3 MPa and then reach a plateau. By accommodating the effect of effective stress on compressive strength values, it is shown that chemical weakening of high rank coals is mostly associated with sorption of subcritical CO2, with negligible impact of supercritical CO2 on further parameter reduction. Inspection of failure patterns during uniaxial compression suggests that non-saturated coal specimens fail through axial splitting with rapid crack propagation and high outburst of coal pieces while the failure of cores subjected to CO2 injection occurs through multiple fractures with negligible material outburst. The post-failure analysis demonstrates that CO2 treated samples disintegrate on smaller particles than non-saturated specimens, as up to 5.6 more CO2 saturated coal pieces passed through the sieves considered in this study than non-saturated pieces. It is claimed that this study presents novel insights into the geomechanical response of high rank anthracitic coals to high pressure CO2 injection

A review on performance of energy piles and effects on surrounding ground

Thermo-active ground structures represent low-energy and sustainable technology which is a clear priority for many countries. Heat transfer between such structures and the surrounding soil is understood to play an important role both in the overall thermal performance of buildings and in the evolution of stresses in structural elements and the surrounding soil. This paper presents an overview of recent research efforts and developments in relation to energy piles. General aspects on the performance of energy piles and their impact on the surrounding ground are presented based on previous field, laboratory and numerical investigations as well as existing case studies. Based on the current knowledge, further research opportunities are identified and highlighted

Thermo-osmosis in saturated shale

An investigation of thermo-osmosis in a saturated and chemically active rock such as shale is presented. The model presented is based on an existing coupled thermal, hydraulic and mechanical model (THM) for unsaturated soils. A simplified form of a coupled THM model for saturated porous medium is developed which considers the effect of solid-fluid thermal expansion and the effects of thermal osmosis in hydraulic behaviour. An example problem dealing with pore water pressure development in saturated shale rock surrounding the bentonite buffer installed around high-level nuclear waste canister emitting heat is presented. It is demonstrated that the heat input from the waste canister can affect the pore water pressure evolution in the shale. In particular, under the conditions considered, it is shown that thermally driven liquid water flow due to thermal osmosis contributes to the convective transport of dissolved species