This document discusses electrical resistivity of rock-fluid systems as measured by well logs. It defines resistivity as the ability of a fluid-saturated rock to impede electric current flow. Resistivity is determined by passing a measured current through a rock sample and measuring the required voltage. Resistivity depends on factors like porosity, pore geometry, lithology, cementation, and clay content. The resistivity of various earth materials is provided, from most resistive (rock) to least (salt water). Resistivity is used to determine water saturation and fluid types in formations.

1. Electrical Properties
Of Rock-Fluid System
Well Log
SP Resistivity

2. RHOMBIC PACKING OF SPHERES
•Electrical properties of reservoirs vary strongly with porosity and
characteristics of the fluids in the pore space; usually, basic properties
are determined assuming:
• “clean” reservoir rock (non-shaly)
• Sw = 1.00 (water saturated rock)

3. RESISTIVITY
The Electrical resistivity (reciprocal of conductivity) of a
fluid-saturated rock is its ability to impede the flow of
electric current through that rock. It is an intensive
rock/fluid property.
•Resistance is extensive and for linear, 1-D electrical
flow,
r=(R ·L)/A
•r electrical resistance, 
•R electrical resistivity, ·m
•L length of electrical flow path, m
•A cross-sectional area perpendicular to electrical
flow path, m2

4. Definitions

6. Electrical Properties of Rocks

7. Electrical Properties of Rocks

8. RESISTIVITY
Resistivity is determined by measuring the voltage
required to pass a measured amount of electrical current
through the rock.
• For a rock cube with sides of 1 m, if electrical
potential of 1 V (Volt) is required for current flow of 1 A
(Ampere), then resistivity is 1 ·m (Ohm ·meter)

9. RESISTIVITY – DEFINITION OF THE
OHM-METER
From Halliburton (EL 1007)

10. RESISTIVITY
Resistivity
•Electrical resistance () is inverse of Electrical
conductance (S, Siemens)
•2  = (1/2) S
•Resistivity is the inverse of conductivity
•2 ·m = (1/2) S/m

11. RESISTIVITY OF EARTH MATERIALS
Resistivity
(1) Rock
(2) Gas
(3) Oil
(4) Fresh Water
(5) Salt Water
Conductivity
Common Notations
Ro = Resistivity of non-shaly rock saturated (Sw=1) with formation
water (ohm-m)
Rt = True formation resistivity (ohm-m)
Rw = Formation water resistivity (ohm-m)

12. ELECTRICITY AND EARTH
MATERIALS
• Primarily, conduction is by ions in water
– Na+ and Cl- are very common
– Other monovalent ions K+ and OH-
– Bivalent ions, too: Ca++, Mg++
• But, clays can conduct electricity.

13. Electricity and
Earth Materials
• Water resistivity
controlled by
– Ion
concentrations
– Type of ions
– Temperature
• Chart GEN-4 to
convert to NaCl
equiv
• Chart GEN-5 for
temp/resist for
NaCl

14. Factors Affecting Electrical Resistivity
– Resistivity of water
– Porosity of the formation,
– Pore geometry - tortuosity
– Lithology of the formation
– Degree of cementation, and
– Type and amount of clay in the rock

15. Rt
Ro
Cube of water
having resistivity,
Rw
Rw
Non-shaly rock, 100% saturated
with water having resistivity,
Rw
Rock containing pores saturated
with water and hydrocarbons
Increasing
Resistivity
(1) Rock
(2) Gas
(3) Oil
(4) Fresh Water
(5) Salt Water
Increasing
Conductivity
= 100%
Sw = 100%
= 20%
Sw = 100%
= 20%
Sw = 20%
Formation
Factor
Equation
Saturation
Equation

16. Example: