ABSTRACT

Determination of water-saturations in shaly sands using the Waxman-Smits approach is limited by the elusiveness of cation exchange capacity/Qv data. This parameter cannot be measured directly downhole at present, while inferences from logs are constrained by geological and drilling conditions. Laboratory techniques thus remain the most practical option. However, removal of samples from the reservoir introduces problems of compatibility between laboratory determined Qv and other log-derived properties. Sample destruction, experimental temperatures and pressures and the chemical environment in which Qv is measured are subjects of notable concern. The membrane potential method offers a reliable, repeatable, non-destructive approach to Qv determination (Burck & Steward, 1984). Thomas (1976) calculated the emf membrane potentials generated at 25 degC and atmospheric pressure across a liquid-liquid junction formed between NaCl brines of different salinity within shaly sand samples and related this to Qv. Using Thomas' theoretical approach, a computer code and chemical data base have been developed to permit calculation of the membrane potentials for NaCl brines up to 80%. The work has been extended to KCl and CaCl2 brines. These solutions are considered because they are encountered in formation water, in seawater and in drilling muds. Measurement of emf between brines of different salinities in shaly sand core plugs can now be used at variable temperature using NaCl, KCl and CaCl2 solutions. Hence, the experimental conditions for Qv estimation may be selected to concord more closely with logging conditions in the formation of interest. The facility to carry out tests at different temperatures provides a multi-point measurement system superior to that of the one-point determinations made in alternative laboratory techniques. Furthermore, sample comninution is not a pre-requisite of the membrane potential test, unlike traditional wet chemistry methods. Consequently, there is less risk of exposing clay exchange sites additional to those which would be effective in a whole sample or in situ in a formation. Qv results given in destructive wet chemistry tests are usually larger then those given by the nondestructive membrane potential method, particularly at high Qv. A second comparison with Qv obtained from Nuclear Magnetic Resonance (NMR) techniques on core plugs confirms the benefit of employing nondestructive methods.

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