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Fluid capacitance logging
Fluid capacitance logging is used to distinguish the mix of water and hydrocarbons in the wellbore fluid.
The fluid-capacitance-logging tool includes an inside dielectric probe located on the tool’s axis. The probe is surrounded by an outside housing that is open to the wellbore fluid. Together, the probe, the housing, and the fluid constitute an electrical capacitor, the capacitance level of which depends on the particular fluid, or fluids, within the capacitor.
Circuitry within the tool is connected to the electrical capacitor, with the result that the circuitry generates an oscillating signal that varies inversely with the capacitance level. Water has the greatest capacitive effect, resulting in the lowest frequency. Gas has the least capacitive effect, resulting in the highest frequency. The frequency with oil is intermediate to those of water and gas. However, the oil frequency is much closer to the gas frequency than to the water frequency. Consequently, the tool distinguishes principally between water and hydrocarbons.
Preferably, the tool is calibrated at the surface in produced water from the well, establishing the trace for water. Normally, the recording system is adjusted so that the water trace is at the left edge of the track. Air customarily establishes the trace for gas. Normally, the recording system is adjusted so that the air trace is at the right edge of the track. If the well produces any oil, the tool can be calibrated in produced oil, establishing the trace for oil. Sometimes tap water is used to establish the water trace.
Obviously, the tool poses no hazard to personnel who are exposed to it at the surface. If the tool is dropped into the well and it must be left there, it is not necessary to cement it over, as with a nuclear tool. Furthermore, the recording sensitivity can be greatly increased above normal sensitivity because the tool produces a signal that is "clean" (free of statistical events), unlike a nuclear tool. At such an increased sensitivity, the tool can detect the slightest "whiff" of hydrocarbon that passes close enough to its sensor. With the sensitivity increased, the tool also can detect very small amounts of water dispersed in oil.
A small gas entry into water looks to the fluid-capacitance tool just about the same as a small oil entry. Whereas the small oil entry, because of its low density contrast with water, changes the fluid density only slightly, the small gas entry, because of its low density relative to water, changes the density log significantly. Thus, by a comparison of the two log types, an analyst can fathom the nature of a hydrocarbon entry.
From the prior discussion, it is obvious that the fluid-capacitance tool can distinguish between water and the two hydrocarbons, but it cannot distinguish one hydrocarbon from the other. Also, the tool has a very nonlinear response over the range from water to hydrocarbon. During use downhole, there can be a calibration drift because of filming of the housing or the dielectric probe, or both. If the drift is severe, the film possibly can be removed with the tool pulled into the tubing, where the velocity of flow may be high enough to remove the film.
In a production well, the tool should be logged down at a logging speed between 20 and 30 ft/min. Maintain a constant logging speed, and use the same speed for all passes. The log should begin at a location above the perforations and end at the deepest depth that can be reached. In a slugging or churning multiphase flow, the log may show variable behavior, even in intervals that are not perforated. In that case, another logging run is advisable to establish the degree of repeatability. If results are less than desirable, a stationary measurement can be time-averaged at each selected location. Usually, a log is run with the well shut-in after flow. If the well has been shut in before logging, a shut-in log can be recorded, but the well must flow for 2 or 3 hours before the first flowing log.
Fig. 1 pertains to a well producing 3,520 RB/D at 68% oil and 32% water. Notice the shut-in log (left trace); at the bottom, below the perforations, the water response is near the left edge of the track. At 8,250 ft, the log shows a water/oil interface in the wellbore. In the oil above the interface, the response appears near the right edge of the track. In gas, the response would be approximately 2,350 Hz.
Below the perforations, the flowing log shows a water response indicating stagnant water. Across the bottom perforations, the log shifts somewhat to the right, indicating some contribution to the oil production.
At 8,420 ft, there is a spike in the oil direction caused by perforations which jet oil directly at the tool’s sensor. Just above the spike, the log is somewhat farther in the oil direction than it is just below, identifying the additional oil in the wellbore.
Near the top of the upper perforations (8,400 ft), there is a major shift in the oil direction. Moreover, the log response persists from this location to the end of tubing. This means that the major contribution to the oil production is from the top part of the upper perforations. Above 8,350 ft, the elevated fluid velocity within the tubing results in the oil being more effective at sweeping the water out of the pipe’s cross section than it is in the casing. The reduced presence of water across the tubing cross section results in a shift of the log in the oil direction. The presence of the water production is indicated because the log never shifts as far right as the oil response identified by the shut-in log.
Note that in Fig. 1, the flowing trace in the tubing crosses the oil/water contact on the shut-in trace at approximately 62% of the total deflection from water to oil. If the tool’s response was completely linear in holdup, then the flowing trace would cross at 68% of the total deflection (i.e., at a point slightly closer to the oil frequency). Unfortunately, the "calibration" for these instruments depends upon the viscosity of the oil owing to the filming of oil on the electrode. The smaller the diameter of the electrode, the larger this effect. In gas/water flows, water tends to film the electrode instead, which biases the "calibration" toward water.
Noteworthy papers in OnePetro
Guo, H., Wu, X., Jin, Z. et al. 1993. The Design and Development of Microwave Holdup Meter and Application in Production Logging Interpretation of Multiphase Flows. Presented at the SPE Annual Technical Conference and Exhibition, Houston, Texas, 3-6 October 1993. SPE-26451-MS. http://dx.doi.org/10.2118/26451-MS
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