Petrol geology

1. Electrical Resistivity

2. Resistivity Applications
 Determine whether hydrocarbons are present.
 Identify at what depths hydrocarbons are present.
 Determine the quantity of hydrocarbon material that
exists (if porosity is known) through Archie's equation.
 Determine mud resistivity to help define the invasion
profile.

3. Material Resistivities
Material Resistivity in ohm-m2/m
Silver 1.59 x 10-8
Bismuth - Tin 400 x 10-8
Dielectrics > 1010
Marble 5 x 108
Quartz 3 x 1014
Petroleum 2 x 1014

4.  The resistivity of a substance is a measure of its ability to impede the flow of electrical current.
 Resistivity is the key to hydrocarbon saturation determination.
 Porosity gives the volume of fluids but does not indicate which fluid is occupying that pore space.
Current can only pass through the water in the formation, hence the resistivity depends on:
 Resistivity of the formation water.
 Amount of water present.
 Pore structure.
Resistivity Theory

5. Resistivity Model
Mud Resistivities
 The first resistivities encountered are those of the mud, mud
filtrate and mud cake.
 The surface measurements to obtain these values are often
erroneous.
Key points:
• The samples must be identical to the mud used in the logging
interval.
• Check answers using the Chart Book formulae.
Rmf < Rm < Rmc
• Identify the sample source (measured or charts).

6. Salinities chart
This chart is used to compute salinities from
resistivities of solution e.g. mud, and vice versa.
It is also used to find the resistivities at a given
temperature.

7. RESISTIVITY
R = r . L /AR = Resistance (ohms)
r = Resistivity ( property of the material, Ohm.m
/m )
L = Length (m)
A = Cross sectional Area (m )
Resistance decreases with increasing Cross-Sectional Area
 Resistance increases with increasing Length

8. RESISTIVITY
• Resistivity of the salt water is
low ( highly conductive )
 Resistivity of the Oil is high
 ( Poor conductor )

9. RESISTIVITY
• Dry, nonmetallic minerals (rock matrix)
have a very high resistivity
All dry, nonmetallic minerals (rock matrix)
have a very high resistivity.
Table 2 gives you an idea of the resistivity
measurements that you can expect for
some common formation types.

10. RESISTIVITY
• The resistivity of a rock will depend on the conductivity of the fluids
saturating the porous volume
Hydrocarbons (oil or gas) have a
very high resistivity.
Formation water has a
resistivity value that changes
depending on the salinity and
temperature of the water.

11. Common formation Resistivities
Formation type Resistivity Range (ohm-m)
Soft Formations - shaley sands 0.2 to 50
Hard Formation - carbonates 100 to 1000
Evaporites - salt and anhydrite > 1000

13. Formation Resistivity Measurements
 Ro is the resistivity a formation whose pore space contains only water
 Rw is the resistivity of the water saturating the rock
 F is a constant for the given rock under examination and is called Formation
Resistivity Factor, or more commonly Formation Factor
 Formation factor can be related to formation porosity by the general
formula
Where
m = cementation factor