LOG
HEADING:
A well log is a permanent record of the geophysical information
measured at the time the log was performed.
The header information should
also be included in the same computer file as the log data!
If information is not available or not
applicable it should be noted on the heading!
The following information should be
included:
Background well information: Owner of well and address.
Location of well; date; logging contractor and address; Logging operator;
drilling contractor and address; client and address; observer and address;
elevation of top casing and distance above ground; and drilling history,
methods etc.
Borehole conditions: Casing description; description of log depth datum;
elevation of log depth datum; type of drilling fluid; resistivity and
temperature of borehole fluid; depth of origin of borehole fluid samples; fluid
level; time since last circulation; bottom hole temperature; and problems and
unusual conditions.
Equipment data and logging
parameters: Description of probe reference
point; model and manufacturer of logging tools; logging company tool number;
date and type of last calibration; date, type, and response of field
standardization; top and bottom of logged interval; logging speed and direction;
vertical depth error after logging; time constant or the time interval of
digital samples; identification of the disk containing digitized logs; and
equipment problems.
Specific information for nuclear
logging probes: source description, initial
source strength and date determined; source to detector or receiver spacing;
detector description; and data filtering or enhancement parameters.
Specific information for acoustic and
electric logging probes: Source or
transmitter description and signal output; source or transmitter to detector or
receiver spacing; detector or receiver description; and data filtering or
enhancement parameters.
(Reference: ASTM)
API LOG
GRID:
Well
logs are graphic representations of tool response with reference to depth. On occasion a log is made at a fixed depth
(station) with reference to time. As a
log Analyst, it is important to properly interpret the graphically indicated
numeric response from the log.
The API
Log Grid is the standard format used for recording Well Logging measurements.
Other
formats that may be acceptable can be seen at http://www.rockware.com
.
TRACKS:
The log
is organized into three tracks in the following format:
From
left to right;
A
single track (on the left) (track 1), a depth track, and two tracks called
track 2 and track 3. Each track is 2.5
inches wide. The depth track is .75
inches wide. Each track is divided or
scaled.
COLUMNS:
Tracks
appear as columns. Each track may contain one or more curves representing the
logged data at a given depth.
DEPTH
SCALES:
The
Depth scale is established in terms of inches of log per 100 feet of well.
Standard scales are:
1 inch per 100 feet - called
1 inch.
2 inches per 100 feet - called
2 inch.
5 inches per 100 feet - called
5 inch.
GRID
SCALES:
The
divisions within a track are referred to as the grid scale.
Three
types of grid scales are available: Linear, Logarithmic, and Split Grid.
Data that is linear in nature is recorded on a linear
scale. For example, porosity on a scale from 0 to 40 percent across 10 linear
divisions provides 4 percent porosity per division.
Data
that needs a larger dynamic scale, for example resistivity, may be indicated on
a logarithmic scale. A typical logarithmic scale may be two-cycle meaning the
scale is from 1 to 10 to 100 ohm-meters or it might be
on a four cycle scale .2 to 2.0 to 20.0 to 200 to 2000 ohm-meters depending on
the desired ranges.
LOG ASCII FILES:
Digital
logging systems record logs in a text file using a format called log ASCII,
abbreviated LAS. These text files use a .las extension. You may learn more
about LAS format at the Canadian Well Logging Society web site www.cwls.org
.
Most
oil producing states in the United States have websites that contain LAS files
on wells drilled in that state.
Later
in this webinar, log interpretation includes import to Microsoft Excel
spreadsheets. Charting is performed on LAS files.