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Electrical Properties of Reservoir Rocks
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Electrical Properties
Of Rock-Fluid System
Well LogSP Resistivity
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)
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
Definitions
Electrical Properties of Rocks
Electrical Properties of Rocks
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)
RESISTIVITY – DEFINITION OF THE
OHM-METER
From Halliburton (EL 1007)
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
RESISTIVITY OF EARTH MATERIALSR
esis
tivit
y
(1) Rock
(2) Gas
(3) Oil
(4) Fresh Water
(5) Salt Water
Co
nd
uctiv
ity
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
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.
• 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
– 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
Rt
Ro
Rw
Cube of water
having resistivity,
Rw
Non-shaly rock, 100% saturated
with water having resistivity,
Rw
Rock containing pores saturated
with water and hydrocarbons
Inc
rea
sin
g
Resis
tivit
y
(1) Rock
(2) Gas
(3) Oil
(4) Fresh Water
(5) Salt Water
Inc
rea
sin
g
Co
nd
uc
tivity
= 100%
Sw = 100%
= 20%
Sw = 100%
= 20%
Sw = 20%
Formation
Factor
Equation
Saturation
Equation
Example: