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Quantitative knowledge of the salinity of deep formation waters is critical in assessing the degree of hydrocarbon saturation in sands, in predicting methane solubility, and in deducing pathways of fluid migration. Calculation of salinity from the spontaneous potential (SP) response of borehole logs using conventional algorithms can yield inaccurate values, particularly for geopressured waters. Thus, a theoretical reevaluation has been made of the relation between pore-water salinity in NaCl-dominated waters and SP response, taking into account pressure as a variable and recently-developed, improved, thermodynamic models for brines.
The following expression satisfactorily relates pore-water salinity, as molality of total dissolved NaCl, to the static spontaneous potential (SSP) over P-T-salinity conditions of sedimentary interest:
log m NaCl (pore water) = (SSP × F)/(2.303 RT × b × t) + log m NaCl (mud filtrate)
where m = molality, F = faraday constant, R = gas constant, and T = absolute temperature. The complex, non-ideal behavior of NaCl solutions can be described by a single, pressure-temperature dependent, concentration independent variable, b. The term t accounts for the differential mobility of Na+ and Cl- ions through sands and shales.
Use of the equation provides several important advantages over conventional techniques: (1) pore water salinity is given explicitly as a dependent variable, facilitating analysis of error; (2) the expression is simple and avoids use of electrical resistivities, which have no direct theoretical role in the relation between SP and salinity; (3) the improved thermodynamic base provides a more rigorous means of assessing the effects of grain size, mineralogy, and streaming potential on SP response.
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