Electrical Properties

1
Electrical Properties of Reservoir Rocks
2
DETERMINING FLUID SATURATIONS
Methods of Determining Saturations

1. Conventional core analysis
2. Capillary pressure measurements
3. Well log analysis

3
OPENHOLE LOG EVALUATION
Most abundant data for formation evaluation
and determination of fluid saturations
From NExT, 1999
4
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)

5
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

6
RESISTIVITY DEFINITION OF THE OHM-METER
From Halliburton (EL 1007)
7
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)

8
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

9
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
(Sw1) with formation water (ohm-m) Rt True
formation resistivity (ohm-m) Rw Formation
water resistivity (ohm-m)
From J. Jensen, PETE 321 Lecture Notes
10
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
11
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
12
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
13
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