Utilization of Waxman-Smits Equations for Determining Oil Saturation in a Low-Salinity, Shaly Sand Reservoir
- E.A. Koerperich (Shell Development Co.)
- Document ID
- Society of Petroleum Engineers
- Journal of Petroleum Technology
- Publication Date
- October 1975
- Document Type
- Journal Paper
- 1,204 - 1,208
- 1975. Society of Petroleum Engineers
- 4 in the last 30 days
- 574 since 2007
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The Waxman-Smits equations have been applied in a shallow, shaly sand, low-salinity reservoir to determine if oil saturation can be reliable estimated. Known in-situ oil saturations were used as the standard of comparison. Results indicate that the Waxman-Smits equations can be combined with independently, determined parameters to produce reasonable estimates of oil in place for the reservoir studied.
A significant portion of the reserves in California exist in shallow shaly sand reservoirs. Oil gravity in many of these reservoirs is very low. Therefore, significant volumes of oil remain in the reservoirs following primary depletion and become targets for secondary or tertiary recovery projects. Accurate estimates of oil in place are required for planning these relatively expensive projects.
Determination of oil saturation in clayey reservoirs has long been a challenge because of the complexities of the clay conductance mechanism. Additional complexities in many shallow California fields are very low water salinities and uncertainties in water resistivities caused by interzone mixing, water injection, etc. Because of these complexities, oil saturations derived from log data historically have been highly uncertain to the extent that cores or other means have been relied upon for saturation estimates.
A resistivity model for describing effects of dispersed clays in oil-bearing shaly sands has been described by Waxman and Smits and by Waxman and Thomas. The objective of this study was not to verify the Waxman-Smits equations; these equations have been confirmed experimentally for a wide range of conditions. Instead, the objective of this study was to apply the equations to determine whether oil saturation could be reliably predicted in uncored wells in these shallow, fresh-water, shaly sand reservoirs.
A reservoir where oil saturation could be determined by independent means was chosen for the study. This reservoir is considered to be a severe test of the equations because of the wide variations in clay content and saturations and the low-water salinity. Because of the sensitivity of the calculated saturations to variations in the input parameters, determination of these parameters are discussed in detail.
Determination of In-Situ Oil Saturation From Cores
In-situ oil saturation normally cannot be determined from laboratory measurements because of flushing of the oil by the coring fluid and blowdown effects. However, the Kern River field in California is an exception. As a result of very high oil viscosity at Kern River (about 3,000 cp), laboratory measurements of oil volume in plugs cut from the center of large-diameter (5 1/4-in.) cores can be used to estimate in-situ oil saturation.
Using tritium tracers, invasion in cores in the Kern River field has been shown repeatedly to be minimal when the coring program is properly designed and controlled. Concentration of the tritium in the mud is monitored continuously while coring. Small center plugs are cut for conventional laboratory analysis and for measurement of tritium concentration in the extracted fluids. The ratio of the tritium concentration in the core plug, C, to that in the coring fluid, Co, is indicative of mud filtrate invasion. For the Kern River field, this ratio is normally less than 5 percent.
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