Utilization of Waxman-Smits Equations for Determining Oil Saturation in a Low-Salinity, Shaly Sand Reservoir

Abstract

The Waxman-Smits equations have been applied in a shallow, shaly sand, low-salinity reservoir to determine if oil saturation can be reliableestimated. Known in-situ oil saturations were used as the standard ofcomparison. Results indicate that the Waxman-Smits equations can becombined with independently, determined parameters to produce reasonableestimates of oil in place for the reservoir studied. Introduction A significant portion of the reserves in California exist inshallow shaly sand reservoirs. Oil gravity in many ofthese reservoirs is very low. Therefore, significantvolumes of oil remain in the reservoirs following primarydepletion and become targets for secondary or tertiaryrecovery projects. Accurate estimates of oil in place arerequired for planning these relatively expensive projects. Determination of oil saturation in clayey reservoirshas long been a challenge because of the complexities ofthe clay conductance mechanism. Additional complexitiesin many shallow California fields are very low watersalinities and uncertainties in water resistivities causedby interzone mixing, water injection, etc. Because ofthese complexities, oil saturations derived from log datahistorically have been highly uncertain to the extent thatcores or other means have been relied upon for saturationestimates. A resistivity model for describing effects of dispersedclays in oil-bearing shaly sands has been described byWaxman and Smits and by Waxman and Thomas. The objectiveof this study was not to verify the Waxman-Smits equations;these equations have been confirmed experimentally for awide range of conditions. Instead, the objective of thisstudy was to apply the equations to determine whether oilsaturation could be reliably predicted in uncored wells inthese shallow, fresh-water, shaly sand reservoirs. A reservoir where oil saturation could be determinedby independent means was chosen for the study. Thisreservoir is considered to be a severe test of the equationsbecause of the wide variations in clay content andsaturations and the low-water salinity. Because of thesensitivity of the calculated saturations to variationsin the input parameters, determination of these parametersare discussed in detail. Determination of In-Situ Oil SaturationFrom Cores In-situ oil saturation normally cannot be determined fromlaboratory measurements because of flushing of the oil bythe coring fluid and blowdown effects. However, theKern River field in California is an exception. As a resultof very high oil viscosity at Kern River (about 3,000 cp), laboratory measurements of oil volume in plugs cut fromthe center of large-diameter (5 1/4-in.) cores can be used toestimate in-situ oil saturation. Using tritium tracers, invasion in cores in the KernRiver field has been shown repeatedly to be minimalwhen the coring program is properly designed and controlled.Concentration of the tritium in the mud is monitoredcontinuously while coring. Small center plugs are cut forconventional laboratory analysis and for measurement oftritium concentration in the extracted fluids. Theratio of the tritium concentration in the core plug, C, tothat in the coring fluid, Co, is indicative of mud filtrateinvasion. For the Kern River field, this ratio is normallyless than 5 percent. JPT P. 1204