Electrical Properties of Reservoir Rocks

1. Electrical Properties
of
Reservoir Rocks

2. DETERMINING FLUID
SATURATIONS
1. Conventional core analysis
2. Capillary pressure
measurements
3. Well log analysis
Methods of Determining Saturations

3. Well Log
SP Resistivity
OPENHOLE LOG EVALUATION
From NExT, 1999
Most abundant data for formation evaluation and
determination of fluid saturations

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, Ω
•R electrical resistivity, Ω·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 Ω·m
(Ohm ·meter)

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

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

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. • 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
Electricity and
Earth Materials
From J. Jensen, PETE 321 Lecture Notes

12. – 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
Factors Affecting Electrical Resistivity
From J. Jensen, PETE 321 Lecture Notes

13. Rt
Ro
Rw
Resistivity
Cube of water
having resistivity,
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
φ= 100%
Sw = 100%
φ= 20%
Sw = 100%
φ= 20%
Sw = 20%
Formation
Factor
Equation
Saturation
Equation