ODP Technical Note 10


D. Use of ODP Temperature Tools for Discrete Temperature "Logs"

The previous sections in this chapter concerned the deployment of the ODP temperature recorders in their normal mode, to determine a single equilibrium sediment temperature per tool lowering during the process of coring a sediment hole. It is also possible to use these tools to obtain readings of temperatures at multiple depths during a single lowering, either in open boreholes or in sections of sediment that are not cored. For such stations, the procedures for tool preparation and data processing are basically the same as discussed above, although somewhat longer recording periods may be required. Only the methods of deployment are different, as are discussed here.

To obtain such discrete temperature "logs", either the APC tool or the WSTP can be used, although the APC tool is often preferable for reasons discussed below. During these stations, the tool is lowered to the bit, and the pipe is moved in stages so that the temperature sensor is held for about 10 minutes at each of the measurement depths. The tool can be lowered to the bit either on the coring line or by freefall, but if it is run on the coring line, the range of measurement depths is limited to about 20 m because it will not be possible to add or remove pipe over the coring line. Twenty meters corresponds to the approximate length of drill pipe that can be drawn up in the derrick before the station and then lowered into the hole with the coring line once the tool is run to the bit at the beginning of the station. Thus, deployment by freefall is necessary if it is desired to measure temperatures over a depth range that is greater than about 20 m.

1. Logging sediment temperatures without coring
If the temperature profile is to be measured in sediments, the tool must be latched into the bit. The pipe is then pushed and washed through the sediments to just above each of the measurement depths; the pumps are then shut off and the tool is pushed into place and held stationary for 10-15 minutes using the drill-string heave compensator as described in section (B). It is preferable to use the APC tool for this kind of station because (1) less sediment must be displaced in penetrating with the annular APC cutting shoe than with the larger-profile WSTP, and (2) the APC can be free-fall deployed more safely. As illustrated by the example in Figure III-5 (from Bryan, Juteau, Adamson, et al., 1988, at a low heatflow site), the resulting temperature record will clearly show the effects of fluid circulation and frictional heating as the probe is moved between measurement points, but good data can still be collected at the measurement depths if the probe can be held stationary.

2. Logging open-hole temperatures
Upon returning to a reentry hole in which the borehole fluids have reequilibrated with the geothermal gradient, it is usually critical to log borehole temperatures and sample fluids before any other operations disturb the borehole. Ideally, a continuous temperature log should be obtained using a lightweight tool on the logging cable, as was done when Leg 111 revisited Hole 504B (Becker, Sakai, Merrill, et al., 1988). However, this will not be allowed if the hole is not expected to be completely free of bridges or obstructions. In that case, true logging will not be permitted until the hole has been cleaned out by running the pipe and bit to hole bottom, but this cleanout pipe trip will disturb the thermal equilibrium of the borehole fluids.

Nevertheless, a temperature log and fluid samples can still be obtained by deploying a WSTP to the bit as this cleanout trip is made. To minimize disturbances to the borehole fluids, the pipe should be run very slowly and carefully into the hole, avoiding circulation and rotation unless absolutely necessary to clear an obstruction. Pauses can be made during this cleanout pipe trip at nearly any depth, to measure temperatures, sample fluids, or retrieve and re-deploy the tool. It is even possible to configure the WSTP and APC tool together, and run them simultaneously to assure recovery of equilibrium temperature data in the event of a tool failure. Note that a strict temperature limitation of 125°C applies to this kind of WSTP deployment, as it will not be desirable to circulate around the electronics to keep them cool in the event higher temperatures are encountered. Figure III-6 shows the data obtained with the APC tool during this kind of station when Leg 92 reentered Hole 504B (Leinen, Rea, et al., 1986); the WSTP was run simultaneously to sample fluids and record backup temperature data, which are not shown.

In addition to the ODP temperature tools described in detail above, there are two specialty tools presently maintained or being developed by the ODP logging contractor, the Borehole Research Group of LDEO. These tools are the temperature-pressure memory tool (TLT), that can be run as part of a conventional logging string, and a high-temperature logging tool run as a separate tool deployment. These tools are different from the ODP-TAMU tools described above, in that the ODP-TAMU tools are run on the coring line, while the LDEO tools are run on a logging line, either as part of a conventional logging run, or on a dedicated logging run.

The TLT is a self-contained instrument for use at temperatures up to 125°C and pressures up to 10,000 psi. The TLT measures 3.25" in diameter and 36" in length. A personal computer is used to initiate data acquisition before logging and to retrieve the temperature data after logging, via a modem in the tool. TLT data acquisition is activated downhole by pre-selecting a trigger pressure. The TLT will then measure the time gap between measurements, for later data merging. The TLT can be attached to the bottom end of most conventional (Schlumberger) logging strings and provides temperature information with essentially no associated "time-cost." The TLT records one channel of pressure data, and two channels of temperature data, one from an extremely accurate, slow-response thermistor, and another from a less accurate, fast-response thermistor. After the TLT is recovered and the data are uploaded to a shipboard computer, the user must merge the TLT temperature records with a time record of wireline depth, or transform the pressure channel into a depth record. The latter procedure is tricky, since the fluid in the borehole may contain some component of logging mud or cuttings, and its density is unknown.

The second LDEO temperature instrument (now under development following field testing) is a high-temperature, slim diameter (60 mm) logging tool capable of recording temperatures in excess of 300°C. Because this is a wireline tool, however, its temperature range is limited to that of the high-temperature wireline. The high-temperature wire presently onJOIDES Resolution is rated to 260 °C. In addition, cable heads are notorious flaky at high temperatures; several cable head designs are now being explored. The high temperature tool is 3 m long and weighs 50 kg. The temperature sensors are accurately calibrated thermistors housed in a 5-mm-diameter tube at the lower end of the tool. Temperatures are determined from the resistance of two thermistors that can be monitored individually or in series for maximum sensitivity across the entire logged interval. The control unit uses a programmable precision voltmeter and a custom interface panel monitored with a personal computer. The computer records depth, two resistance values, and the calculated temperature approximately every 10 cm. Data are output to standard ASCII files.

To Drill-string Packers, Section A.
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