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Electrical Propertiesof
Reservoir Rocks
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DETERMINING FLUID SATURATIONS
1. Conventional core analysis
2. Capillary pressure measurements
3. Well log analysis
Methods of Determining Saturations
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Well LogSP Resistivity
OPENHOLE LOG EVALUATION
From NExT, 1999
Most abundant data for formation evaluation anddetermination of fluid saturations
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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)
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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
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RESISTIVITY – DEFINITION OF THEOHM-METER
From Halliburton (EL 1007)
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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)
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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
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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
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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
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• 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
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– 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
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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