caliper logs.pptx
TRANSCRIPT
7/29/2019 CALIPER LOGS.pptx
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CALIPER LOGS
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Introduction
• The Caliper Log is a tool for measuring thediameter and shape of a borehole.
• It uses a tool which has 2, 4, or more
extendable arms.• The arms can move in and out as the tool is
withdrawn from the borehole, and the
movement is converted into an electricalsignal.
• Caliper log is shown in track 1 of the masterlog together with the bit size for reference.
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• Borehole diameters larger and smaller than thebit size are possible.
• Many boreholes can attain an oval shape afterdrilling.
• This is due to the effect of the pressures in the
crust being different in different directions as aresult of tectonic forces.
• In oval holes, the two arm caliper will lock intothe long axis of the oval cross section, giving
larger values of borehole diameter thanexpected.
• In this case tools with more arms are required.
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• In the 4 arm (dual caliper) tool, the two opposite
pairs work together to give the borehole
diameter in two perpendicular directions.• An example of a 4 arm caliper tool is the
Borehole Geometry Tool (BGT).
•
This has 4 arms that can be opened to 30 inches(40 inches as a special modification), and give
two independent perpendicular caliper readings.
• The tool also calculates and integrates the
volume of the borehole and includes sensors that
measure the direction (azimuth) and dip of the
borehole, which is useful in plotting the
trajectory of the borehole.
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• In the multi-arm tools, up to 30 arms are
arranged around the tool allowing thedetailed shape of the borehole to be
measured.
•
Some of the other tools have sensors attachedto pads that are pressed against the borehole
wall.
• The pressing device is also a form of caliper,
and so caliper information can sometimes also
be obtained from these tools.
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Log Presentation
• The caliper logs are plotted in track 1 with the
drilling bit size for comparison, or as a
differential caliper reading, where the reading
represents the caliper value minus the drill bit
diameter.
• The scale is generally given in inches, which is
standard for measuring bit sizes.
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• The 4 arm (or dual caliper) tools are presented
in a range of formats.
• Note that data from the caliper pairs areshown together, and that they are different
indicating an oval hole.
• Also the tool rotates in the hole (the pad 1azimuth P1AZI changes).
• The hole azimuth is reasonably constant at
HAZI»180°, and the dip is almost vertical (DEVIis about 0°).
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• The ticks represent borehole volume.
• This information is useful to estimate the
amount of drilling mud in the borehole, and to
estimate the amount of cement required to
case the hole.
• There are engineering approximation formulas
to calculate both of these from caliper data.
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Simple Caliper Interpretation
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Uses of the Caliper Log
The commoner uses of the caliper log are as follows:
• Contributory information for lithological
assessment.
• Indicator of good permeability and porosity zones
(reservoir rock) due to development of mudcake in
association with gamma ray log.
• Calculation of mudcake thickness, hmc
= (dbit
–
dh)/2, where h stands for the hole, in inches.
• Measurement of borehole volume.
• Measurement of required cement volume.
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• Indication of hole quality for the assessment of the likely quality of other logs whose data qualityis degraded by holes that are out of gauge. Otherlog data can often be corrected for bad hole
conditions using the caliper readings, but thelarger the correction, the less reliable the finaldata will be.
• Selection of consolidated formations for wireline
pressure tests, recovery of fluid samples, forpacker seating for well testing purposes, and fordetermining casing setting depths.
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