| Title | Correcting for variations in borehole diameter in fluid velocity profiles |
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| Authors | F. Purnomo, K. McLean |
| Year | 2024 |
| Conference | New Zealand Geothermal Workshop |
| Keywords | Well testing, geothermal wells, stage testing, completion testing, pressure-temperature-spinner (PTS) logging, caliper logging |
| Abstract | Pressure-temperature-spinner (PTS) data is the most common dataset collected in geothermal wells. It provides valuable information about the well and reservoir, such as feed zone characteristics, well conditions, and well injectivity or productivity. A combination of temperature and fluid velocity profiles (calculated from spinner response) are used to accurately identify the location of feed zones in the well, and how much each feed contributes under certainconditions. However, analysing the fluid velocity profile is often not straightforward when the borehole is heavily washed out. In this situation, the spinner data does not only reflect the actual feed zones but also the variation in borehole diameter. This interferes with feed zone interpretation, potentially providing misleading information about the well or reservoir. Currently, to differentiate between the effects of feed zones and changing borehole diameter, reservoir engineers would look at a caliper log, which provides a physical measure of well bore diameter (if available), and visually compare the shape of it to the spinner data. This is an imprecise exercise and can fail to definitively determine whether there is a feed zone present within a washout, for example. Another current method is the spinner ratio method (Grant & Bixley, 1995), which divides one fluid velocity profile by another from a different flow rate to give a ratio. The limitations of this are: the requirement to have two high quality fluid velocity profiles, complications in aligning the datasets, and the tendency for the ratio to be very noisy. This work therefore explores ways to explicitly remove the effects of changing diameter from the fluid velocity profile by means of applying a correction method based on caliper data. This has been recently enabled by an increase in open hole caliper data collection. The correction method was applied to field data from three wells in Wairākei geothermal field, with significant changes to feed zone interpretation as a result of the correction. |