Wireline Temperature Data Processing
ODP logging
contractor: LDEO-BRG
Well name: 767B
Leg: 124
Location: Celebes Basin (Celebes Sea)
Latitude: 4° 47.49' N
Longitude: 123° 30.30' E
Logging date: November, 1988
Bottom felt: 4916 mbrf
Total penetration: 739 mbsf
Total core recovered: 585 m (79 %)
Water Depth: 4905.3 mbsl
Temperature
Tool Used: LDEO-TLT
Depth versus time recording available: NO
Logging
Runs
Logging string 1: DIT/LSS/NGT passes 1 and 2)
No information could be found about use of wireline heave compensator 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 = BD *(Pressure-MudlineP)/(Pmax- MudlineP)
where
Depth = mbsf
BD = bottom depth (mbsf)
Pressure = bars
Mudline P = bars
Pmax = pressure at bottom depth (bars)
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 767B:
Logging Run: DIT/LSS/NGT
(pass 1)
Mudline P=505.5 bars at 563 sec
Pmax=537 bars at 300 mbsf
Depth calculated from pressure at
mudline and bottom of hole (300 mbsf)
Depth =
300 * (Pressure - 505.5)/(537 - 505.5)
Logging Run: DIT/LSS/NGT
(pass 2)
Mudline P=501 bars at 373 sec
Both temperature and pressure measurements were severely disturbed by the use of the Side Entry sub in conjunction with intense pumping. The maximum depth is unknown because of wireline failure.
Depth can be estimated from
relationship derived from pass 1 and from apparent pressure at seafloor after
smoothing the pressure with a 15-sample window in order to correct for
pumping and tool jerking..
Depth = 300 * (Pressure -
501)/(537 - 501)
Information about the temperature logging operations can be found in the Site Chapter (Operations, Downhole Measurements, and Heat Flow sections), ODP IR volume 124.
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia