Well Logging Techniques: Gamma Ray, Caliper, and Resistivity

Well Logging in Petroleum Fields

Well logs are used to measure the physical and chemical properties of rocks, especially sedimentary rocks in petroleum fields.

Gamma Ray Logs

Gamma ray logs measure natural radioactivity in formations. This measurement is used for identifying lithologies and correlating zones.

Uses of Gamma Ray Logs:

• Identify lithologies
• Correlate between formations
• Calculate volume of shale

Caliper Logs

A caliper log is a well logging tool that measures the size and shape of a borehole along its depth. It can indicate cave or shale swelling in the borehole.

Uses of Caliper Logs:

1. Contributory information for lithological assessment.
2. Indicator of good permeability and porosity zones (reservoir rock) due to development of mudcake in association with gamma ray log.
3. Calculation of mudcake thickness.
4. Measurement of borehole volume.
5. Measurement of required cement volume.

Resistance and Resistivity

Resistance: The opposition of electrical current flow. Resistance is an absolute quantity measured in ohms.

Resistivity: The resistance of electrical current flow through a given volume. Resistivity is measured volumetrically as ohms per cubic unit of volume. In log interpretation, the cubic meter is used.

Invasion

Invasion occurs within the permeable layers by the clay slurry, due to the different pressure between the drilling fluid and the layers.

Where the pressure generated from the drilling fluid is higher than the pressure of the layers, the result of the invasion of the cake is deposited on this well, causing a decrease in the diameter of the well. The invasion depends on:

1. Weight of clay
2. The pressure of mud column into the well
3. The amount of loss mud or loss of mud water

SP (Spontaneous Potential) Log

SP Log Use:

• Detect permeable beds
• Detect boundaries of permeable beds
• Determine the formation water resistivity (Rw)
• Determine the volume of shale
• Detection of strategic traps