title: Towards a better understanding of tetrachloroethene entry pressure in the matrix of Permo-Triassic sandstones.
abstract: The pressure required for a chlorinated solvent to enter a geological medium can be calculated given knowledge of the characteristic pore size of the medium and the interfacial tension (IFT) and contact angle of the solvent-water-rock system. Using a centrifuge-based method, capillary pressure-saturation curves have been determined for 30 water-saturated samples of Permo-Triassic sandstones for the solvent tetrachloroethene (PCE). These curves have been successfully fitted using the van Genuchten function to determine PCE entry pressure for each of the sandstone samples. A plot of PCE entry pressures against average pore diameter shows a linear relationship in log-log space; however, observed values for PCE entry pressure are significantly lower than would be expected theoretically for a sandstone-PCE-water system. This may be explained either by a decrease in the IFT or an increase in the contact angle. The IFT may decrease during contact with sandstones due to hysteresis effects during imbibition and drainage of fluids, but this is unlikely to be sufficient to account for the low entry pressures observed. Therefore, it is inferred that the low observed PCE entry pressures are due to higher than expected PCE contact angles, and that the average pore-throat surface of the sandstones is more solvent wetting than would be expected. A weak acid extraction indicates the presence of calcite and dolomite in the sandstone cores, and a correlation is observed between carbonate content per unit porosity and a reduction in PCE entry pressure. It is suggested that these mineral phases are responsible for observed wettability changes and a conceptual model is proposed. One consequence of the lower observed entry pressures is that solvents are likely to penetrate deeper into the matrix of water-saturated sandstones than previously expected.
authors.affiliation: British Geological Survey, Maclean Building, Wallingford, Oxfordshire OX10 8BB, UK. dcg@bgs.ac.uk
authors.forename: Daren C
authors.initials: DC
authors.lastname: Gooddy
authors.forename: John P
authors.initials: JP
authors.lastname: Bloomfield
authors.forename: Gavin
authors.initials: G
authors.lastname: Harrold
authors.forename: Steven A
authors.initials: SA
authors.lastname: Leharne
mesh_terms: D002119:Calcium Carbonate / Q000737:chemistry*; D005618:Fresh Water / Q000737:chemistry*; D008274:Magnesium / Q000737:chemistry*; D016062:Porosity; D012822:Silicon Dioxide / Q000737:chemistry*; D013499:Surface Properties; D013750:Tetrachloroethylene / Q000737:chemistry*; D006113:United Kingdom; D014874:Water Pollutants, Chemical; D014881:Water Supply
chemical_list: D014874:Water Pollutants, Chemical; D012822:Silicon Dioxide; D002119:Calcium Carbonate; D008274:Magnesium; D013750:Tetrachloroethylene; C028042:calcium magnesium carbonate