General Introduction to resistivety log in well loging

General Introduction
Objective of the Log
Difference Between Resistance and Resistivity
Factors Affecting Resistivity
Borehole Environment
Principle of Operation
Types of Resistivity Logs
Electrode Logs
Types of Electrode Logs
Induction Logs
Principle of Induction Logs
AIT-H Modern Tool
AIT-H Tool Specifications
Log Presentation
Log Interpretation
Relation to Archie’s Equation
Applications of Resistivity Logs
Challenges and Possible Errors
Calibration
Conclusion

1. General Introduction
The resistivity log is one of the most important electrical well logging tools.
Its purpose is to identify the presence of oil, gas, or water by measuring the rock’s
resistance to electrical current flow.
The principle is based on the fact that salty water is a good conductor, while oil
and gas are poor conductors.
Objective of the Log
Detect hydrocarbon-bearing formations.
Determine their depth accurately.
Estimate water saturation (Sw) using Archie’s equation.
Calculate the Original Oil in Place (OOIP).
Evaluate the reservoir and correlate between wells.
Difference Between Resistance and Resistivity
Resistance: A property of a physical object depending on its length and cross-
sectional area.
Resistivity: An intrinsic property of the material itself, independent of its
dimensions.

2. Mathematical relation:
ρ = (V / I) × (A / L)
The higher the resistivity, the more likely
hydrocarbons are present.
Factors Affecting Resistivity
Type of fluid in the pores (water, oil, gas).
Salinity level.
Rock type (sandstone, carbonate, shale).
Temperature and pressure.
Presence of fractures or thin beds.

3. Borehole Environment
During drilling, mud filtrate enters the
formation pores, creating:
1. Flushed Zone (Rxo): Filled with mud
filtrate.
2. Transition Zone (Ri): Contains a
mixture of mud filtrate and formation
fluids.
3. Uninvaded Zone (Rt): Contains the
original formation fluids (oil or gas).
Principle of Operation
An electrical current is sent from one electrode to another inside the borehole.
The potential difference is measured to determine the formation resistivity.
The greater the spacing between the electrodes, the deeper the depth of
investigation.

4. Types of Resistivity Logs
Resistivity logs are divided into
two main types:
1. Electrode Logs.
2. Induction Logs.
The choice depends on the type
of drilling mud used.
Electrode Logs
Used in water-based muds.
They measure resistivity directly by passing electric current through electrodes
into the formation.
Types of Electrode Logs
1.Short Normal (16 inch):
Shallow depth of investigation.
Measures invaded zone (Ri).
Used for lithological correlation.

5. 2. Long Normal (64 inch):
Deeper investigation.
Measures true formation resistivity (Rt).
3. Lateral Log:
Used in thick beds.
Provides high accuracy for Rt measurement.
Induction Logs
Used in oil-based or low-conductivity muds.
They operate by generating an alternating magnetic field that induces an
electrical current in the formation.
The induced current is measured to determine conductivity, which is then
converted to resistivity.
Principle of Induction Logs
The transmitter coil generates a magnetic field.
This field induces eddy currents in the
formation.
These currents generate a secondary magnetic
field detected by the receiver coil.
The signal is converted into a resistivity value.

6. AIT-H Modern Tool
A modern resistivity tool developed by Schlumberger.
It provides five investigation depths (10, 20, 30, 60,
and 90 inches).
It has high vertical resolution (1–4 ft).
It also measures Rm and SP, with automatic
calibration during logging.
AIT-H Tool Specifications
Length: 16 ft.
Diameter: 3.8 in.
Pressure limit: up to 10,000 psi.
Operating frequency: 26.325 kHz.
Equipped with environmental sensors for in-situ correction.

7. Log Presentation
Displayed in Track 2 of the log.
Typical range: 0.2 to 2000 ohm·m.
Values are plotted on a logarithmic scale.
Clear contrast appears between shallow and deep readings.
Log Interpretation
Similar readings (shallow & deep): indicate shale or water-bearing formations.
Divergent readings: indicate hydrocarbon-bearing sandstone.
The invasion profile helps identify the fluid distribution (water, oil, gas).
Relation to Archie’s Equation
Resistivity values are used to calculate water saturation (Sw):
Sw = (a / (φ^m)) × (Rw / Rt)^(1/n)
where Rt is the true formation resistivity.

8. Applications of Resistivity Logs
Identifying productive zones.
Evaluating reservoir quality.
Differentiating carbonate and sandstone formations.
Determining fluid contacts.
Correlating nearby wells.
Challenges and Possible Errors
Mud salinity affecting readings.
Deep invasion reduces Rt accuracy.
Thin beds cause resolution issues.
Calibration or mechanical errors.

9. Calibration
Calibration is performed every three months or after 20 logging runs.
A plastic tank with known standards is used.
Electronic checks are also made at the site to ensure accuracy.
Conclusion
Resistivity logs are essential tools in well logging.
They are used to identify fluids, estimate water saturation, and evaluate reservoir
properties.
Selecting the appropriate tool depends on mud type, and accurate readings are
vital for reliable interpretation.