An experimental investigation of flow and reaction processes during gas storage and displacement in coal

An advanced laboratory facility has been designed, developed and commissioned which offers an extensive capability for detailed study of various aspects of geoenergy problems in fractured rocks. It comprises i) a high pressure manometric sorption apparatus, ii) a high pressure triaxial core flooding system and iii) an ancillary system including pure and mixed gas supply and analysing units. The manometric sorption apparatus is capable of measuring adsorption/desorption isotherms of various gas species on powdered and intact samples. The triaxial core flooding system is capable of measuring the gas flow properties and deformation behaviour of coal samples, up to 0.1m diameter and 0.2m length. Deep underground conditions in terms of pore pressure and confining pressure can be replicated using the high pressure triaxial cell for depths up to 2000m. The laboratory facility has been designed and developed to produce high resolution data for a broad range of gas injection pressures (up to 20MPa) and temperature values (up to 338K). Appropriate pressure transducers and flow meters were selected and have been incorporated into the system following a series of detailed and thorough analyses performed to define and optimise the specifications of the measurement devices. Anthracite coal samples from the South Wales coalfield (6-ft seam measure) have been characterised and tested. Equilibrium and kinetic phenomena of the adsorption and desorption of different gases, i.e. nitrogen (N2), methane (CH4) and carbon dioxide (CO2), at injection pressures up to 7MPa have been studied. A series of core flooding experiments have been carried out on samples of 0.07m diameter and 0.12m length, at gas injection pressures up to 5.5MPa and confining pressures up to 6MPa. The absolute and relative permeability of the samples, to different gases and the permeability evolution with changes in the gas pressure and confining stress condition have been studied. The fate of adsorbed CO2 was studied via a sequential series of N2 and CH4 flooding experiments. The storage and displacement of N2 and CO2 in a sample saturated with CH4 at 5MPa pressure was investigated via another series of flooding tests. During the injection of the gases, the composition of the outflow gas was analysed. Modelling work has been carried out to further investigate the experimental results and processes involved in gas transport and reactions. The numerical model used, includes a theoretical approach for modelling the permeability evolution in coal. The results of the gas adsorption tests indicated a higher adsorption capacity to CO2 compared to CH4 and N2, i.e. 1.3 and 2.5 times higher, respectively. Also, different hysteresis behaviours were observed during the adsorption and desorption measurements, for the different gases studied. An improved understanding of the controlling mechanisms of gas adsorption rate and the kinetics of the processes has thus been achieved. From the results of the core flooding experiments, it was found that the permeability evolution of the coal sample to CO2, due to an increase in gas pressure, exhibited a different pattern compared to the other gases. A considerable reduction above a certain gas pressure value was observed. This was found to be related to coal matrix swelling induced by CO2 adsorption. The results of following N2 and CH4 flooding experiments showed a partial restoration of the initial permeability of the coal sample, indicating the stability of the adsorbed CO2 in the coal matrix during the period of analysis. The results of N2 and CO2 storage and displacement in coal showed that CO2 injection into coal was more efficient in terms of total CH4 recovery, gas displacement ratio, breakthrough time and amount of the gas storage than achieved through N2 displacements. The effect of swelling on the coal permeability however was found to be considerable. The application of the experimental results in the adopted theoretical model led to the identification of the major mechanisms controlling the behaviour of coal during gas displacement, together with the influential factors on flow behaviour. The results also highlighted coupled physico-chemical effects during carbon dioxide sequestration in coal. It is claimed that the work presented in this thesis has provided a new and comprehensive set of high resolution data. Various aspects related to high pressure flow and reaction of various gas species in coal have been studied. A detailed set of benchmarks have been produced that can be used for the development and validation of theoretical models. New insights into several phenomena related to carbon sequestration in coal are thus claimed to have been achieved