A Method For Direct Log Calculation Of Excess Conductivity Applied To Shaly Sand Log Interpretation Available to Purchase

Classical models for shaly sand log interpretation (the Waxman-Smits and Dual Water models) assume the formation behaves like a clean formation of the same porosity, pore geometry, and water saturation, except that the water appears to be more conductive than expected from its bulk salinity. The excess conductivity is due to exchange cations concentrated in a diffuse layer near the clay surfaces. To account for the excess conductivity, the classical models rely on core or log determinations of Qv, the charge concentration of the exchange cations per unit pore volume. For log interpretation, any limited Qv data are correlated with some appropriate log measurement (usually, log-derived porosity) in order to define the Qv profile over the entire section of interest. In other techniques, Qv is obtained theoretically from classical clay indicators, such as SP, gamma ray, and neutron logs. Although accounting for clay effect on the conductivity of shaly sands is theoretically complex, the classical shaly sand models are based on simple Archie equations, assuming two parallel conductors in the pore system, one the normal electrolyte formation water and the other the water with a diffuse layer of cations near the clay surfaces ("clay water"). The "effective" water conductivity is given by Cwe = Cw + Cx/SwT, where SwT is the total water saturation. Theoretically, the conductivity contribution of the clay water, Cx, can be found in logging data by comparing the observed conductivity in shaly water-bearing sand to the expected conductivity of clean sand with the same water salinity, porosity, and pore geometry. On this principle, a simple method for calculating Cx from log-derived porosity has been developed. The procedures of the method extend the application of logging data from water-bearing sand commonly used for deriving the Archie parameters, aRw and m, to include also the derivation of an algorithm relating Cx to log-derived porosity. Water saturation in hydrocarbon sand is calculated with the classical shaly sand equation, Ct = (1/F*)(CwSwTn* + CxSwTn*-1).