1. Electrical Propertiesof
Reservoir Rocks

2. DETERMINING FLUID SATURATIONS
Methods of Determining Saturations
Conventional core analysis
Capillary pressure measurements
Well log analysis
The remainder of the section will focus on conventional core analysis. Determination of fluid saturations from capillary measurements and log analysis will be discussed in later sections

3. OPENHOLE LOG EVALUATION
Most abundant data for formation evaluation and
determination of fluid saturations
Well Log SP
Resistivity
This figure depicts the basic setup of the logging process. A wireline truck with a spool of logging cable is setup so that the sonde (measuring equipment) can be lowered into the wellbore. The logging tools measure different properties, such as spontaneous potential and formation resistivity, as the sonde is brought to the surface. The information is processed by a computer in the logging vehicle, and is interpreted by an engineer or geologist.
From NExT, 1999

4. 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)

5. RESISTIVITY Resistivity
Resistivity is an intensive rock/fluid property, and is a measure of (the inverse of) the electrical flow capacity of the rock
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

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

7. RESISTIVITY 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)

8. 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

9. RESISTIVITY OF EARTH MATERIALS
(1) Rock
(2) Gas
(3) Oil
(4) Fresh Water
(5) Salt Water
Resistivity
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)
From J. Jensen, PETE 321 Lecture Notes

10. 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 also
Modified from J. Jensen, PETE 321 Lecture Notes

11. 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
From J. Jensen, PETE 321 Lecture Notes

12. 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
From J. Jensen, PETE 321 Lecture Notes

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