Comments on 'The electrical conductivity of sandstone, limestone and granite' by A. Duba, A. J. Piwinskii, M. Santor and H. C. Weed
1979; Oxford University Press; Volume: 56; Issue: 3 Linguagem: Inglês
10.1111/j.1365-246x.1979.tb00180.x
ISSN1365-246X
Autores Tópico(s)Geochemistry and Geologic Mapping
ResumoThe main conclusions reached by Duba et al. (1978) in their recent paper are that ‘Archie’s Law is not valid for pore fluids of low conductivity but is valid for pore fluids of high conductivity’. Similar conclusions were in fact reached more than 20 years ago by workers such as Patnode & Wyllie (1950) and Hill & Milburn (1956) and it is unfortunate that reference to these works was not made in the paper by Duba et al. These earlier workers went on to conclude that Archie’s first law (i.e. R o / R , = F) is only strictly applicable in the case of pure sandstone or rock in which the matrix is perfectly insulating and within which all conduction occurs through a uniform electrolyte. In this equation Ro is the bulk resistivity of the rock, R , is the resistivity of the saturating electrolyte and F is a constant termed the formation factor. If the matrix does contribute to conduction in some way the ratio R o / R , will be a variable termed the apparent formation factor, Fa. In practice most rocks contain a small percentage of clay or other material which contributes to the electrical conduction. Because of this matrix conduction it is wrong to expect observed data to satisfy a single porosity-formation factor relationship such as given by Archie’s Second Law (i.e. RoIR,=aq5-m where q5 is the porosity and a and m are constant). Patnode & Wyllie (1950) suggested the following relationship by which a corrected or true formation factor may be obtained from conductivity measurements made on samples saturated with electrolyte of various conductivities, namely
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