Direct determination of the gas-water contact (GWC) or the oil-water contact (OWC) in new wells from geophysical logs, core analysis data and/or whole core inspection is often difficult. Rapidly changing reservoir quality in the interval of interest or gradually changing saturation profiles in the vicinity of the contact can make the determination of the GWC or OWC difficult. This paper presents a model for the accurate placement of the GWC or OWC. The input data are water saturations at various well depths as interpreted from induction logs and the average residual oil saturation. When available, permeability and porosity values from core analysis may improve the result.

Using a power-law equation or a Langmuir isotherm-type equation to represent the capillary pressure-saturation relationship, the GWC or OWC is expressed as a function of water saturation and well depth. A nonlinear optimization technique is then used to determine the GWC or OWC.

The applicability of this model is demonstrated with a field example. The calculated OWC values from all variations of the model fall within 4 feet of the actual OWC value. This model can only be applied in an oil or gas well where an equilibrium capillary curve and a hydrocarbon-water contact occur.

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