Multiple Salinity

Key Takeaways

• Waxman-Smits model is the most widely used theoretical framework for interpreting multiple salinity data in shaly sand reservoirs and provides the equation C0 = (1/F*) × (Cw + B × Qv), where C0 is the rock conductivity at full water saturation, F* is the intrinsic (clay-corrected) formation factor, Cw is the brine conductivity, Qv is the cation exchange capacity per unit pore volume (in equivalents per liter), and B is the equivalent conductance of the cations on the clay surfaces (in (S/m)/(eq/L)); the multiple salinity experiment determines F* and Qv from the linear regression of C0 versus Cw, with B being a temperature-dependent parameter calculated from the experimental temperature using published correlations; once F* and Qv are determined for the rock type, the in-situ water saturation is calculated using the Waxman-Smits equation Sw = ((F* × Cw) / (Sw^n × (Cw + B × Qv / Sw)))^(1/n), which reduces to the Archie equation when Qv = 0 (clean sand) but accounts for clay surface conductivity when Qv > 0; the model has been extensively validated against laboratory and field data and remains the standard for shaly sand petrophysical interpretation in major operating companies worldwide.
• Dual water model is an alternative to Waxman-Smits that explicitly separates the water in shaly sands into two components: free water (in the pore bodies, conducting electricity normally according to Archie behavior) and bound water (in the clay double layer, having different conductivity properties due to the high concentration of counterions in the diffuse layer); the dual water model uses the equation C0 = (1/F*) × (Vfw × Cw + Vbw × Cbw), where Vfw is the free water volume fraction and Vbw is the bound water volume fraction (both as fractions of total pore volume), Cw is the bulk free water conductivity, and Cbw is the bound water conductivity; the multiple salinity experiment provides the data needed to fit Vbw and Cbw for the specific rock type, with the bound water conductivity Cbw typically being 2 to 5 times higher than the free water conductivity due to counterion accumulation in the clay double layer; the dual water model often gives slightly different (but generally similar) saturation results compared to Waxman-Smits for the same multiple salinity data, with the choice between models often being a function of operator preference and historical practice.
• Cation exchange capacity (CEC) is the rock property quantified by multiple salinity analysis that represents the total number of negatively charged sites on clay mineral surfaces capable of exchanging cations with pore water — typically expressed as milliequivalents per 100 grams of rock (meq/100g) or as Qv in equivalents per liter of pore volume; clean quartz sandstones have CEC near zero, while shaly sandstones with 10 to 20 percent volumetric clay content have CEC values of 1 to 5 meq/100g, and shales with 60 to 80 percent volumetric clay content have CEC values of 10 to 30 meq/100g; specific clay minerals contribute different CEC values per unit weight: smectite (montmorillonite) is the highest at 80 to 150 meq/100g, illite is moderate at 10 to 40 meq/100g, kaolinite is low at 1 to 10 meq/100g, and chlorite is very low at less than 5 meq/100g; multiple salinity-derived CEC is the in-situ measurement that integrates the mineralogical contributions with the pore-scale geometry, providing the directly relevant parameter for petrophysical interpretation rather than requiring conversion from XRD-derived clay mineralogy.
• Salinity range and selection for the experimental flushes depends on the expected formation water salinity and the desired sensitivity to the clay conductivity term — typical multiple salinity experiments use 4 to 8 different brines spanning Cw values from 0.1 S/m (low salinity) to 20 S/m (saturated NaCl brine), with intermediate values selected to provide good resolution near the formation water salinity expected in the reservoir; the lowest salinity brines provide the highest sensitivity to clay conductivity (because at low Cw, the BQv term dominates the conductivity equation), while the highest salinity brines establish the formation factor F* (because at high Cw, the brine conductivity dominates and the rock conductivity becomes proportional to brine conductivity through F*); the experimental procedure between flushes requires careful pore volume verification (replacing 5 to 10 pore volumes of the previous brine to ensure complete equilibration with the new brine), pressure equilibration (the salinity changes can cause minor osmotic and stress effects that need to relax before measurement), and stable temperature control (typically 20 to 25°C laboratory temperature, with temperature corrections applied if the experiment is performed at elevated temperature for simulated reservoir conditions).
• Sample preparation for multiple salinity analysis requires preserved fresh-state cores or high-quality cleaned restored-state cores — multiple salinity is highly sensitive to the in-situ wettability and mineral surface state of the rock, so samples that have been damaged by aggressive cleaning (hot solvent extraction with toluene, methanol, or chloroform-methanol mixtures) may show different multiple salinity responses than the same rock at native conditions; modern best practice for shaly sand SCAL programs uses preserved-state samples taken from sealed cores that have been kept at native moisture content from the wellsite to the laboratory, or restored-state samples that have been cleaned with mild techniques (CO2-water flushing, low-temperature toluene wash) and then re-saturated with synthetic formation water to restore the in-situ ionic environment before testing; the entire multiple salinity protocol from sample preservation through final data interpretation typically takes 4 to 12 weeks per sample and costs $5,000 to $25,000 per sample depending on laboratory rates and the number of brine flushes — making it an expensive but essential element of shaly sand reservoir characterization programs.