Wireline Temperature Data Processing
ODP logging
contractor: LDEO-BRG
Hole: 800A
Leg: 129
Location: Pigafetta Basin (tropical NW Pacific)
Latitude: 21° 55.38' N
Longitude: 152° 19.37' E
Logging date: December, 1989
Bottom felt: 5697 mbrf (used for depth shift to sea floor)
Total penetration: 544.4 mbsf
Total core recovered: 150.7 m (27 %)
Water Depth: 5686 mbsl
Temperature
Tool Used: LDEO-TLT
Depth versus time recording available: NO
Logging
Runs
Logging string 1: DIT/SDT/HLDT/NGT
Logging string 2: ACT/GST/NGT
Wireline heave compensator malfunctioned during the logging runs; seas, however, were calm.
Two different TLT tools were used.
The LDEO-TLT tool is a self-contained, high precision, low-temperature logging tool that is attached to the bottom of the Schlumberger tool strings. The tool provides two temperature measurements (in degree Celsius, recorded by a fast-response and a slow-response thermistor. The fast-response thermistor, though low in accuracy, is able to detect sudden, small temperature excursions caused by fluid flow between the formation and the borehole. The highly accurate, slow-response thermistor can be used to estimate heat flow. Pressure and the two temperature measurements are recorded as a function of time: conversion to depth can be based on the pressure reading (Legs 123-157) or, preferably, on simultaneous recording (by Schlumberger) of depth and time (Legs 159-181).
A linear relationship of pressure versus depth has been calculated from the pressure reading at the mudline and at the total logging depth for each logging run. The pressure at the mudline corresponds to the pressure recorded by the tool during the calibration stop (about 5 minutes), which usually takes place at the mudline on each logging run. In hole 800A, both TLT tools were stopped at 28 m above the mudline and held stationary for 3 minutes to obtain a calibration point. The pressure readings are then converted to depth using a pressure/depth conversion that is linearly interpolated between the values determined at the mudline and total logging depth.
Depth = Pressure * x - WD
where
Depth = mbsf
Pressure = bars
x = pressure conversion coefficient (m/bars)
WD = mbsl
This procedure does not fully account for the vagaries of the pressure readings that result in lots of ups and downs in the generated depth channel. Further problems arise because of pumping during logging operations, which affects the pressure, especially when the Side Entry Sub is used. Also, whenever heavy pills of mud are used, the pressure-depth calculation is affected, resulting in a non-linear effect that is difficult to account for. If the pressure conversion coefficient is recalculated for the mudline, the resultant total depth is often wrong.
The following processing has been performed at Hole 800A:
Logging Run: DIT/SDT/HLDT/NGT
Mudline P=573-574 bars (upgoing)
x=WD-28m/573.5=9.866 (see
processing section above)
Pmax=608 bars
Pressure conversion factor
calculated from pressure at mudline.
Depth=Pressure * 9.866-5686
Logging Run: ACT/GST/NGT
Mudline P=582.5 bars at 275 sec
(downgoing) and 580 bars (upgoing)
x=WD-28/581.25=9.734 (see
processing section above)
Pmax = 619 bars
x=(WD+315)/619=9.695
Pressure conversion factor
calculated from average of pressure at mudline and bottom of logged interval
Depth=Pressure * 9.714 - 5686
There is an offset of about 10 bars in the pressures observed at the mudline and bottom of the hole in the two runs.
Information about the temperature logging operations can be found in the Site Chapter (Operations, Downhole Measurements, and Heat Flow sections), ODP IR volume.
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia