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: