Gooddy, Daren Clive; (2003) Controls on the distribution of dense non-aqueous phase liquids in the matrix of permo-triassic sandstones. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The Permo-Triassic sandstones are geographically and lithologically diverse exhibiting large variations in porosity, pore-throat size, permeability (k) and mineralogy that influence flow and transport of contaminants in the rock matrix. This thesis investigates how the movement of a dense non-aqueous phase liquid (DNAPL), tetrachloroethene, through the saturated zone of the Permo-Triassic sandstone is affected by the physical and chemical properties of the matrix. Specifically, the thesis aims to establish which parameters need to be determined to improve understanding of DNAPL penetration into the saturated zone of Permo-Triassic sandstones.A series of standard physical property measurements have been made on a large number of Permo-Triassic sandstones from across the UK. A weak acid extraction has been performed on the samples to determine the bound chemical species in the matrix. In addition, two new laboratory methods have been developed to measure: DNAPL entry pressures using a centrifuge to produce pressure-saturation curves; and liquid relative permeabilities with a DNAPL residual saturation. A percolation based model has been developed to describe data from pore-throat size measurements.Experimentally determined entry pressures are lower than those predicted by the Washburn equation assuming a pore structure equivalent to a 'bundle of capillaries'. The differences between observed and modelled pore entry pressure can be explained in part by changes in the wettability of the system relating to varying amounts of calcite and dolomite present in the matrix. The results of the percolation model suggests the presence of much larger pores than characterised by standard mercury injection tests. This may also explain in part why observed DNAPL entry pressures are lower than expected on the basis of capillary theory. Experiments show permeabilities decreases by 30-90% when a DNAPL is present. The reduction in permeability positively correlates with the hydraulic diameter. DNAPL present in a core at a given pressure as determined from the pressure-saturation curve also positively correlates with the hydraulic diameter. Knowledge of the sample porosity and permeability should enable good estimates of residual DNAPL and relative permeability in Permo-Triassic sandstones. Key advances are the acquisition of baseline data on representative DNAPL-sandstone systems and the development and application of methodologies from the hydrocarbons sector to contaminant hydrogeology.

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