Wireline Temperature Data Processing
ODP logging contractor: LDEO-BRG
Well name: 766A
Leg: 123
Location: Exmouth Plateau (tropical SE Indian)
Latitude: 19° 55.925' S
Longitude: 110° 27.243' E
Logging date: October, 1988
Bottom felt: 4008 mbrf (depth used onboard by Schlumberger engineer to shift the data to sea floor)
Total penetration: 527.2 mbsf
Total core recovered: 347.5 m (66 %)
Water Depth: 3997.5 mbsl
Temperature Tool Used: LDEO-TLT
Depth versus time recording available: NO
Logging Runs
Logging string 1: DIT/SDT/NGT
Logging string 2: LDT/CNTG/GPIT/NGT (upper section thorough pipe)
Wireline heave compensator was used to counter ship heave.
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 takes place at the mudline on each logging run. 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 766A:
Logging Run: DIT/SDT/NGT
Mudline (?) P=402 bars at 520 sec
Pmax=460 bars at 2663 sec
Logging Run: LDT/CNTG/GPIT/NGT (upper section thorough pipe)
Mudline (?) P=400 bars at 79 sec
Pmax = 461 bars at 961 sec
Averaged mudline pressure: 401 bars
x=WD/401=9.969
Averaged Pmax: 460.5 bars
x=(WD+450)/460.5=9.658
or
x=(WD+460)/460.5=9.680
or
x=(WD+462)/460.5=9.684
Pressure conversion factor calculated from average of pressure at mudline and bottom of hole
Depth=Pressure * 9.867-3997.5
Information about the temperature logging operations can be found in the Site Chapter (Operations, Downhole Measurements, and Heat Flow sections), ODP IR volume 123.