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Profiling commingled gas production
A suite of production logs can help to determine the fraction of total production contributed by different perforated intervals.
This well produces gas from four perforated intervals; however, the bottom two intervals are in such close proximity that they are considered as one perforation set. Therefore, the perforations are referred to as:
- The "top set" (A)
- The "middle set" (B)
- The "bottom set" (C)
The well produces 175 to 320 Mscf/D, with negligible water production. The objective of the production logs was to determine the fraction of total production contributed by each perforation set.
With the well flowing, fluid density, temperature, and continuous-spinner flowmeter profiles were recorded. With the well flowing, a diverting flowmeter log was also recorded with the flowmeter stationary below and above each perforation set.
Analyzing the results
Viewed upward from the bottom, the fluid-density profile (Fig. 2) responds to a column of water below the bottom perforation set. Then it decreases above each of the three perforation sets, indicating that each set produces gas. Above the top perforation set, the measured density still exceeds the density of the gas, showing that water is present in the wellbore throughout the survey interval. Even though the water production is negligible, the gas rate is insufficient to lift water out of the wellbore.
Below the bottom perforation set, the temperature profile (Fig. 2) coincides with the estimated geothermal temperature profile. At each perforation set, the temperature profile shows cooling, which occurs because of the expansion of the produced gas as it passes through the near-wellbore region and the perforated completion. These cooling effects corroborate the gas production at each set indicated by the fluid-density profile. Between perforation sets, the temperature profile warms toward geothermal and then cools as it approaches the perforation set above. The tangent lines with their slopes illustrate the relative rates of warming of the stream above each perforation set.
Although there are two phases present in the wellbore (the produced gas and the captive water), the diverting flowmeter can respond to the gas flow rate at each measurement depth. This flowmeter was calibrated in a flow loop by measuring its response to a variety of air flows as they were passed through a column of stationary water. A "percolation calibration" was generated by plotting the diverting flowmeter’s responses to the various air-flow rates. With this percolation calibration, the flowmeter’s downhole measurements, shown in the depth track, were interpreted to yield the profile in Table 1, which shows the fraction of total gas production from each perforation set.
An independent analysis of the production profile was made from the temperature log by a profiling technique described in the Appendix under injection-well profiling. At a fixed distance above each perforation set, a tangent line was constructed to the temperature profile at a depth on its recovery toward geothermal. For each tangent, a determination was made of the slope of the tangent and the difference between the profile temperature at the depth of tangency and the corresponding geothermal temperature. By appropriately combining these data, the fractional contributions of the three perforation sets were obtained, as shown by Table 2. The two analyses are in good agreement.
The fractional contributions of the three sets also were determined by a linear analysis of both the diverting-flowmeter log and the continuous-spinner-flowmeter log. With this approach, a perforation set’s fractional contribution to production is determined as the ratio of the change of the flowmeter’s response across the perforation set to the total change of response from below the bottom perforation set to above the top set. The linear analyses of the two flowmeter logs both erroneously indicate that only one-quarter of the total production is from the top set, as opposed to the correct contribution of nearly one-half. A linear analysis is appropriate when only one phase is present in the wellbore but not when there are two.
In this example, the production profiles based on the temperature log and a proper analysis of the diverting flowmeter log corroborate each other, showing the advantage of more than one log in the suite. Also, this example demonstrates that when reliable data are analyzed with an inappropriate method (the linear-flowmeter technique), appreciable errors can result.
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