The capacitance tool is designed for fluid identification. It works by reading the dielectric properties of fluid that contact its probe. The tool is calibrated from the extremes of water to air. Air and natural gas have very similar dielectric properties. Motor oil is about a 75 percent reading on the scale, and most hydro carbon fluids give approximately the same indication. Ideally you would like to have produced water and oil from the well you are running for calibration.
3.7:3.0:1.0 represents the ratios of fluid dielectrics for water - oil - gas. In actual usage, it is a simple matter to calibrate the tool by dipping it in pure water, then oil (produced from the well or from other sources), and finally in open air. Alternatively, the tool may be calibrate in-situ (in the well) as long as there are free phases of water, oil and gas present at some point in the well. Most producing wells have water if there is a stagnant area below the first fluid entry. In-situ calibration also corrects for any temperature effects on fluid dielectric measurement.
The capacitance data are enhanced by calibrating the tool signal with laboratory data. This calibration allows the tool data to be normalized to percentage water (water cut). From water cut and spinner or radioactive tracer derived flow measurements, it is possible to calculate fluid entry composition, i.e., rates of oil and water versus depth. Go here to see a brochure about calibrated capacitance tools.
The actual capacitance tool is round, with an insulated probe configured within an open window which allows inflow of wellbore fluids. The tool measures the frequency response of a capacitor in a circuit as a function of the fluid dielectric between the coaxial plates of a capacitor. Space age coating materials an special mechanical design techniques are used to eliminate surface adhesion and to improve fluid flow around the probe assembly. All Cardinal capacitance tools are designed and tested in-house, and represent a significant step forward in tool design and function.
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