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Unformatted text preview: sures. It is not the purpose of this paper to describe how DTS temperature measurements are obtained, this
has been described in many previous papers8,9. However, it is worth pointing out that the system can obtain a temperature
curve sampled every meter along the entire length of the slickline in time intervals as small as one minute.
A conventional slickline unit is employed, and fitted with the optical fiber drum (Fig 2). The fiber itself is a 125 micron
diameter polyimide fiber, located in a 0.033 inch diameter tube, surrounded by carbon weaves inside a 1/8th inch diameter
Inconel 825 tube (Fig 3). The slickline is 18,500 ft long, H2S corrosion resistant with a working load of 1,000 lb and a
maximum temperature rating of 120 Deg C. The slickline working load is sufficient to run pressure gauges or even a memory
production log, but not sufficient for jarring operations. Logging Procedure
A conventional arrangement of casing and production tubing above the reservoir interval is shown in Fig 4a. Velocity string
gas wells have production tubing set just above the lowest producing interval so flow from the higher reservoir intervals
travels down the annulus to the bottom of the well and then back up the production tubing, picking up any fluid from the
sump in the process (Fig 4b). It is not possible to run a production log with this arrangement and it is also not possible to
interpret a temperature log where there is counter flow with heat exchange between the tubing and annulus. The solution
employed in this paper is to remove the counter flow by switching the well to annular flow for 12 to 24 hours and using a
slickline fiber optic distributed temperature monitoring system to record the flowing temperature profile (Fig 4c).
During the logging operation, the well is first monitored under normal flowing conditions for a few hours to ensure a
thermally stable flow measurement is obtained. The well is then shut-in for approximately 12 hours, allowing the well to
build up towards reservoir pressure and warm back toward the geothermal gradient. There are two important reasons for
recording the shut-in temperature data:
Geothermal Definition. Most shut-in periods are relatively short and it is not feasible to wait for the well to return to a true
geothermal gradient. However, shut-in DTS data will determine the correct geothermal gradient in the following two ways:
• There will be depths in the well where the flowing temperature curve intercepts the unknown geothermal. As shut-in
progresses, the temperature at these depths will remain constant and indicate the geothermal temperature at that depth,
referred to as “pivot” point.
After shut-in, the surrounding rock returns to the geothermal temperature in a defined manner that can be extrapolated to
infinite Horner time (i.e. equivalent to P* in build-up analysis). However, it should be noted that this approach is not
valid for the producing reservoir intervals or between reservoir intervals where cross-flow oc...
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- Spring '08