Electrical Propertiesof

Reservoir Rocks

DETERMINING FLUID SATURATIONS

1. Conventional core analysis

2. Capillary pressure measurements

3. Well log analysis

Methods of Determining Saturations

Well LogSP Resistivity

OPENHOLE LOG EVALUATION

From NExT, 1999

Most abundant data for formation evaluation anddetermination of fluid saturations

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)

RESISTIVITYResistivityResistivity 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 THEOHM-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 MATERIALSR

esis

tivi

ty

(1) Rock

(2) Gas

(3) Oil

(4) Fresh Water

(5) Salt Water

Co

nd

uctivity

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

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

• 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

– 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

Rt

Ro

Rw

Resistivity

Cube of waterhaving resistivity,

Rw

Non-shaly rock, 100% saturatedwith water having resistivity,

Rw

Rock containing pores saturatedwith water and hydrocarbons

Incr

easi

ng

Res

isti

vity

(1) Rock

(2) Gas

(3) Oil

(4) Fresh Water

(5) Salt Water

Increa

sing

Co

nd

uctivity

= 100%Sw = 100%

= 20%Sw = 100%

= 20%Sw = 20%

Formation Factor Equation

Saturation Equation