# Electrical Properties of Reservoir Rocks - PowerPoint PPT Presentation

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Electrical Properties of Reservoir Rocks. DETERMINING FLUID SATURATIONS. Conventional core analysis Capillary pressure measurements Well log analysis. Methods of Determining Saturations. Well Log. SP. Resistivity. OPENHOLE LOG EVALUATION. Most abundant data for formation evaluation and

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Electrical Properties of Reservoir Rocks

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Electrical Properties

of

Reservoir Rocks

### DETERMINING FLUID SATURATIONS

• Conventional core analysis

• Capillary pressure measurements

• Well log analysis

Methods of Determining Saturations

Well Log

SP

Resistivity

### OPENHOLE LOG EVALUATION

Most abundant data for formation evaluation and

determination of fluid saturations

From NExT, 1999

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

• 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

RESISTIVITY – DEFINITION OF THE

OHM-METER

From Halliburton (EL 1007)

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)

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 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

### 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

### 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

### 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

(1) Rock

(2) Gas

(3) Oil

(4) Fresh Water

(5) Salt Water

Increasing

Resistivity

Increasing

Conductivity

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%