--> --> Water Quality Profiles From Well Logs: When They Don't Match Lab Analyses, Consider the Conductive Clays
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AAPG Pacific Section and Rocky Mountain Section Joint Meeting

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Previous HitWaterNext Hit Previous HitQualityNext Hit Profiles From Well Logs: When They Don't Match Lab Analyses, Consider the Conductive Clays

Abstract

In recent years, documenting Previous HitwaterNext Hit Previous HitqualityNext Hit in shallow aquifers has become a critical issue for oil and gas operations. Laboratory analyses on Previous HitwaterNext Hit samples are necessary for Previous HitwaterNext Hit Previous HitqualityNext Hit work, but they are expensive and often difficult to obtain where they are needed. Well logs are available in most areas, and the use of formation Previous HitwaterNext Hit salinity/resisitivity data is routine since it is required for obtaining saturation data from log analysis. Previous HitWaterNext Hit resistivity can easily be related to salinity as NaCl, and TDS for more complicated chemical profiles can be estimated with information on the ion species likely to be present in the Previous HitwaterNext Hit. Over the years, a number of simple methods have been proposed for estimating Previous HitwaterNext Hit resistivity (Rw) from logs, which are adequate for the high-TDS and dominantly NaCl Previous HitwaterNext Hit found in many oilfields. However, these methods can lead to inconsistent or misleading results when applied to low-TDS aquifers with waters whose chemistry is not primarily NaCl. Often the reason for this is the presence of clays which contribute an increasing fraction of the formation conductivity as Previous HitwaterNext Hit salinity declines. Extensive research on cores revealed a temperature- and salinity-dependent relationship between the cation exchange capacity of the clays and the salinity of the equilibrating brine solution (formation Previous HitwaterNext Hit). Petrophysical methods based on this research have been applied to derivation of hydrocarbon saturation in O&G reservoirs having a wide range of salinities and temperatures with great success. With proper methodology, the same principles can be applied to determination of Previous HitwaterNext Hit Previous HitqualityNext Hit in fresh to brackish aquifers. Where only Previous HitwaterNext Hit is present, Previous HitwaterNext Hit saturation models simplify since Sw= 1.0 and the saturation exponent is not needed. The remaining variables are the contributions to formation conductivity from Previous HitwaterNext Hit and clay. Of the well-known shaly-sand log analysis equations, the Waxman-Smits model is most applicable to fresh-Previous HitwaterNext Hit systems since the non-linear behavior of the clay conductivity in low-TDS brines is best documented for this model in published research. By properly accounting for the contribution of the clays to formation conductivity, the accuracy of Previous HitwaterNext Hit Previous HitqualityNext Hit profiles from logs can be significantly improved over those derived from SP logs or simple Archie-based calculations. The log-based Previous HitwaterNext Hit Previous HitqualityNext Hit profile can be further refined by accounting for the varying electrical activity of the non-NaCl ions usually present in low-TDS waters. The analytical model for Previous HitwaterNext Hit Previous HitqualityNext Hit should be calibrated to laboratory Previous HitwaterTop analyses, in concept analogous to calibrating reservoir petrophysical models to core data.