Environmental Engineering Reference
In-Depth Information
11
Formation Evaluation Through Casing
Tools and Techniques Available
A number of tools are available for formation evaluation through tubing and casing.
They include:
1. Measurements of formation resistivity through casing
2. Passive detection of natural gamma radiation from the formation
3. Neutron logging to detect the thermal-neutron flux returning from the formation
after neutron bombardment from a chemical source
4. Pulsed neutron logging
5. Elemental concentration logging
Resistivity Through Casing
Tools to measure formation resistivity through casing are now available. These tools
require that any tubing in the well be pulled prior to running the resistivity logging
tool into the hole. They are equipped with a number of electrode pads that are forced
against the inside of the casing once the tool reaches the bottom of the hole or the
depth from which the log is to be attempted. If good physical and electric contact
can be maintained between the pads and the casing then a reasonable measurement
of the formation resistivity behind the casing may be obtained.
The operating principal relies on the fact that while the majority of any current flow-
ing will be “short circuited” by the conductive casing itself a minute portion will in fact
“leak off” and be detectable by measurement of the minute voltage difference between
the tool and “ground” electrode located at surface. The principal is illustrated in Fig. 11.1 .
The logical application of resistivity through casing measurements is in old wells
where either no resistivity log is available or where a comparison can be made
between original and the current formation resistivities. Potential benefits include
detection of bypassed pay zones and/or mapping of fluid contact changes with time.
 
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