By Sid Phatak

Induction logging is a process of logging the resistivity of the formation by inducing current loops through the surrounding formation. Induction logging is the only resistivity testing system that works in oil-based mud (where oil is the continuous phase) or air-filled holes.

Logging systems used before the introduction of induction logging were dependent on the presence of an electrically conductive fluid in the borehole to transmit electric current to the formation. This was acceptable for water-based drilling fluids as water-based muds conduct electricity. However, some wells are drilled with nonconductive fluids, such as oil-based muds, air, and gas. Under such conditions, it is impossible to obtain a satisfactory electrical log using conventional electric logging tools. Induction logs are recommended when:

· The hole to be logged is filled with fresh water or

· The hole to be logged is filled with oil-base mud

· The hole to be logged was air drilled

· The Rmf/Rw ratio is greater than 3

· The Rt is less than 150 ohm-m and bed thickness is greater than 30 feet.

Where, Rmf is the resistivity of mud filtrate
Rw is the resistivity of formation water &
Rt is the resistivity of the formation

In the simplest device, an alternating current of medium frequency (10’s of kHz) is passed through a transmitter coil, which induces an alternating magnetic field in the formation. This field creates current loops in the formation. The loops produce their own magnetic field, which induce a current when they cross the receiver coil. This signal is proportional to the conductivity of the formation, with contributions from different regions of the formation summing approximately in conductivity. As a result, the induction log is most accurate at high conductivities and with resistive invasion. However, below about 1 ohm-m skin effect becomes important.

Induction logging tools are very versatile as they can be combined with the other sensors, thereby allowing multiple logs to be recorded simultaneously. The figure below shows a typical Induction-Sonic tool string combination with a possible Gamma ray extension for recording simultaneous sonic & resistivity logs.


There are different types of induction log techniques available. Some of the common ones are:

· Rt Scanner Triaxial Induction Service: This calculates the vertical and horizontal resistivity from direct Rt Scanner triaxial induction measurements while solving for formation dip.

· Array Induction Imager Tool: This measures openhole formation conductivity accurately by using a series of depths of investigation.

· Azimuthal Resistivity Imager: This makes directional deep measurements around the borehole with high vertical resolutions.

· High-Resolution Laterolog Array: This technique accurately identifies the invasion profile with optimized array spacing to estimate reserves using true formation resistivity.

· Phasor-Induction SFL: This is a conventional dual induction-SFL array for recording resistivity data at three depths of investigation. It’s especially useful for measuring thin-bed resistivity values.