1. Introduction to Resistivity Logging

2. Outline Definitions and Uses Resistivity in Rocks Archie’s Equations
Invasion and Invasion Models
Measuring Resistivity (Rt and Rxo)
Basic ES Principles
Focused Devices
Micro Resistivity Devices

3. Resistivity Uses Locate Hydrocarbons Identify Water Type Correlation
Identify Lithology of Formations

4. Resistivity
The opposition to flow of electrical current offered by a material of unit length and unit cross sectional area.

5. Measuring Resistivity
V - Voltage in Volts R - Resistance in Ohms
I - Current in Amperes A - Area in Square Meters
L - Length in Meters
V = I  R
R = r  L / A
V = r  L / A  I
r = V/ I  A / L
r = V / I  k
( k = A / L )

6. Archie Porosity
Oil
eff
Water
Matrix
SW = 60%
Eff. = 30%

7. Water Wet Porosity - Ro
SW = 100%
EFF = 30%

8. Oil And Water
SW = 60%
EFF = 30%

9. Salt Water (Resistivity Rw)
Cube filled with
Salt Water (Resistivity Rw)
Rt = Rw
Porosity () = 100%

10. Rock Matrix Resistivity
Rock Cube, Rt = 
Porosity () = 0%  

11. Hydrocarbon Resistivity
Oil Cube, Rt = 
Porosity () = 100%  

12. Fresh-Water Filled Porosity 
Archie Cube, Rt =
Porosity () = 30%  

13. Salt-Water Filled Porosity
Archie Cube, Rt > Rw
Porosity () = 30%
Rt = Ro
Resistvity  Rw
Resistvity  1/ 

14. Archie Cube, Rt >> Rw
Salt Water & Oil 
Archie Cube, Rt >> Rw
Porosity () = 30%
Resistvity  1/SW 

15. Resistivity Which of these “Archie Blocks” has the largest resistivity
Rt = Ro
Rt > Ro
Rt = Ro
 = 10%
Sw = 100%
 = 30%
Sw = 66%
 = 20%
Sw = 100%

16. Resistivity Which of these “Archie Blocks” has the largest resistivity
> =
Rt = Ro
Rt > Ro
Rt = Ro
 = 10%
Sw = 100%
 = 30%
Sw = 66%
 = 20%
Sw = 100%

17. True Resistivity a Rw Rt = S F F a S n m m n w
- Relates Porosity to Resistivity
m is related to the degree and type
of cementation n S w
- n usually equal to 2

18. Resistivity Factors The Formation Resistivity is effected by:
Volume of Water - (Porosity & Sw)
Resistivity of Water - (Rw)
Current path through Water - “m” in Porosity equation

19. Resistivity Which of these “Archie Blocks” has the largest resistivity
> =
Rt = Ro
Rt > Ro
Rt = Ro
 = 20%
Sw = 100%
BVW = .02
 = 10%
Sw = 100%
BVW = .01
 = 30%
Sw = 66%
BVW = .0199

20. Archie Porosity EFF Oil Water Matrix SW = 60% EFF = 30%
BVW = Sw x  = 18%

21. Idealized Log Set A B C D E

22. Invasion

23. Mud Filtrate Invasion Porosity = 30 % Porosity = 10 % Porosity = 20 %
mud cake

24. Zones About the Well-Bore
Rmc Rs
Adjacent Bed
Rxo Rt
Invaded
Zone
Rmf Rw h
Virgin
Zone
Sxo Sw
tmc - mud cake thickness
Adjacent Bed Rm Di

25. Depth of Investigation
Shale
No permeability,
No Invasion
Resistivity
Virgin 90 10 60
Depth of Investigation

26. Depth of Investigation
Low Invasion
Water Zone
Resistivity
Virgin 90 10 60
Depth of Investigation

27. Low Invasion - Oil Low Invasion (Step) - Oil Resistivity Virgin 9010 60
Depth of Investigation

28. Depth of Investigation
Transition Zone - Oil
Resistivity
Virgin
Flushed 90 10 60
Depth of Investigation

29. Transition Zone - Water
Resistivity
Flushed
Virgin 90 10 60
Depth of Investigation

30. Depth of Investigation
Transition Zone - Oil
Annulus Profile,
Deep Invasion,
Oil
Resistivity
Flushed
Ann.
Virgin 90 10 60
Depth of Investigation

31. Basic Log Example
S P
An example of a basic Triple Combo Log showing SP, GR, Resistivity, neutron-porosity and density-porosity readings.
Other curves that would normally be presented are: Cable Tension, Head Tension, Hole Caliper, Computed Data.
Is there any Hydrocarbons in this well??? 24

32. Electrical Survey MeasurementsB
Unfocused System
Equipotential
Spheres B N
Normal
Device
Lateral
Device V
Ra = I k
The basic electrical measurements were made with electrodes placed on a “bridle”.
With a constant current flow, the voltage of equal-potential spheres which occurs at electrodes could be measured. This is used to calculate resistivity.
Various electrode patterns were used to give readings at different depths into the rocks.
Since this was an un-focused arrangement, the “k” constant is theoretically calculated based on an iso-tropic, homogeneous medium. In practice, “k” was a variable based on the resistivities encountered. 25

33. Non-focused vs Focused Systems
Non-focused electrical systems do not work good in wells drilled in high resistivity formations with salty muds. The current paths will concentrate in the mud column and surrounding low resistivity formations, resulting in low and incorrect values for Rt in the zone of interest.
Focused devices have voltage controlled focusing electrodes to force the measure currents into a finite path deeper into the formations, resulting in a better Rt determination. 26

34. Dual Laterolog LLD Ao Measure Current Electrode
M1 & M2 Monitoring Electrodes
A1 Bucking Current Electrode
A2 Bucking Current Electrode
LLS
Ao Measure Current Electrode
M1 & M2 Monitoring Electrodes
A1 Bucking Current Emitting Electrode
A2 Bucking Current Return Electrode
Used as the primary logging tool when the Rt/Rm ratio is high.
This occurs in wells drilled with Salt muds and formations of high resistivity (greater than ohms meters).
Two electrode arrangements are used to get readings from different depths of investigation. 27

35. Spherical Focused Log k (Vb - Vc) Rsfl = Io Io Return to Armor
Bucking Current
Monitoring Electrodes
Buckimg Current
Return Electrode
Monitoring Electrode
Current Electrode
Rsfl =
k (Vb - Vc) Io
This device is the currently accepted focused electrical measurement in wells drilled with fresh muds.
It does a good job of reading Rxo if the bed thickness is greater than 18 inches and the invasion diameter is greater than 30 inches. 28

36. Proximity Log - MicrologAo Mo A1 M2 M1
Microlog
The MSFL is designed to read the flushed zone resistivity and will help in defining thin permeable beds that are drilled with all types of water-based mud (fresh or salty).
The Microlog is an older un-focused device that works well in zones drilled with fresh muds. It is also used for thin, permeable bed definition. 29

37. Psuedo-Geometric Factors
Diameter of Invasion
Ra = J Rxo + (1-J) Rt
Depth of Investigation for logging tools are defined as the distance that gives a geometric factor of .5
This means that one-half of the signal is being received from the region between the borehole wall and that depth.
Example: The MSFL is receiving 50% of it’s signal from a region within 10 inches of the borehole wall. 30

38. Micro-Cylindrically Focused Log - MCFL
M - Monitor Electrode
N - Auxiliary Voltage Electrode
A0 and A1 - Guard Electrodes
The MSFL is designed to read the flushed zone resistivity and will help in defining thin permeable beds that are drilled with all types of water-based mud (fresh or salty).
The Microlog is an older un-focused device that works well in zones drilled with fresh muds. It is also used for thin, permeable bed definition. 29