Advances in Experimental Methods for Characterisation of Porous Solids



This thesis describes work on the development of new integrated methods for the characterisation of porous media. Porous media are of wide importance in a variety of applications including oil and gas production, tissue engineering, filtration and separation and ground water hydrology. Such porous media are characterised by the fraction of their volume occupied by pores (the porosity) and by parameters characterising the ease of flow and diffusion through the medium (the permeability and the diffusivity). However, the flow processes are very complex, reflecting the complex nature of the pore structure. The objective of the work described in this thesis was to develop and apply two new integrated pieces of apparatus which were aimed at elucidating several aspects of the complex flow processes. The first integrated apparatus was aimed at the study of gaseous transport and the second at the study of mercury penetration, flow and electrical conduction, in the pores of selected media. Thin-section imaging was also applied to obtain supplementary information. The integrated gaseous transport apparatus was designed not only to measure permeability (by the pressure rise technique), but also to study both steady state and (importantly) transient diffusion of oxygen in nitrogen in the pores of the selected media. The system was capable of operating with media with a wide range of permeabilities and yielded accurate values of the viscous permeability and the slip flow coefficient. In addition, experiments were carried out in the partially turbulent flow region. The diffusion measurements yielded information on the ratio (rD) of the diffusion coefficient in the media to that in free space; this ratio is also a specific property of the medium. Combining the steady state and transient diffusion measurements, it was possible to deduce the effective porosity and the pore length. The second integrated apparatus was for the study of porous media subjected to mercury penetration under pressure. This apparatus allowed conventional mercury porosimetry measurements to be performed (i.e. measurements of the pore volume occupied as a function of pore size penetrated) but, crucially, it also allowed measurements of the permeability to mercury and of the conductivity with mercury to be made simultaneously. The permeability to mercury approaches the gaseous value when complete saturation of the medium is achieved; however the manner in which mercury permeability varies with pore size gives a striking indication of the role of the various pores in the flow process. In the conductivity experiments, the ratio (rCp) of the conductivity of the medium penetrated with mercury at pressure p to the conductivity of pure mercury is determined as a function of p. At high penetration pressures, the value of rCp approaches that of rD reflecting the analogy between conduction and diffusion when the medium is saturated with mercury. However, the variation of rCp with pore size penetrated gives a remarkable indication of the significance of the pores in given size ranges in the diffusion process (analogous to the information yielded by the mercury permeability measurements relating to the overall permeability).