WELLOG RESISTIVITY
Revised:
© 2007-2008 WELLOG
All Rights Reserved
Our resources of water, energy and metals are largely possible due
to this method. Resistivity is one of the most frequently measured geophysical properties.
The importance of resistivity in exploration of three vital resources is shown
in the following paragraphs.
WHAT IS RESISTIVITY?
Resistivity is a measurement of the formation and fluid or mineral
conductivity. Resistivity and conductivity are reciprocals of each other.
To learn more about the concept of resistivity visit http://www.wellog.com/webinar/interp_p1_p6.htm
For an illustration: Resistivity model.
OIL and GAS EXPLORATION:
Resistivity surveys are conducted on the surface of the earth to
create a 3 dimensional profile of the formations below the surface. Certain
structures indicate areas having a high probability for trapping oil and gas
and forming a reservoir. Other geophysical methods may be used to follow up on
favorable targets.
OIL and GAS WELL LOGGING:
In the oil and gas industry, resistivity is measured in newly
drilled wells. The measurement of formation resistivity is used to define the
water, oil, and gas contained within the pore space of the formations traversed
by the well. Calculations are possible
using measured physical and electrical properties of the formations to
determine percent of water saturation (Sw).
Sw
= (F x Rw/Rt)1/2
View a typical Resistivity log
section.
MINERAL EXPLORATION:
Mineral deposits are normally located in regions that meet certain
criterion as defined in certain geologic models. When exploration begins, one
of the most important exploration methods used is the resistivity survey. A
resistivity survey can be used to define conductive mineral deposits that exist
far below the surface of the earth. Many mineral deposits have been located
that were completely covered by alluvial deposits.
Mineral ore resistivities contrast with barren rock resistivities
as follows:
Resistivities of Rocks: (in ohm-meters)
Limestone (marble) >
10, 000, 000, 000, 000
Quartz: >100,
000, 000, 000
Sandstone: 120
– 400
Clay 1
– 120
Resistivities of Ores:
Pyrrhotite: .00001
- .001
Chalcopyrite: .0001
- .1
Pyrite: .0001
- .1
Magnetite: .01
– 1
Zinc blende >
10,000
MINERAL WELL LOGGING:
Follow-up exploration activities usually include core drilling to
sample the mineral deposit. Sampling provides a direct method of collecting
data that gives preliminary economic value to a mineral deposit. Resistivity
logging has been shown to provide additional data about potential ore bodies
that can be helpful in further determination of economic value.
WATER EXPLORATION:
Resistivity surveys are conducted in exploration for water. Water
saturated sands and gravels offer a resistivity contrast to near surface
unsaturated sands and gravels. Vertical Electrical sounding (VES) can be used in this application.
Unconsolidated unsaturated sand has a resistivity in the range of
120 to 400 ohm-meters.
Unconsolidated freshwater saturated sand has a resistivity of 80 –
120 ohm-meters.
WATER WELL LOGGING:
Water quantity and quality may be determined using resistivity
logging methods.
Water quality in terms of total
dissolved solids (TDS) is determined by interpretation of relative high
resistivity compared to low resistivity water bearing zones.
For example:
Unconsolidated freshwater saturated sand has a resistivity of 80 –
120 ohm-meters.
Unconsolidated saltwater saturated sand has a resistivity of 2 –
20 ohm-meters.
SURFACE RESISTIVITY METHODS:
Surface resistivity measurements can be made of a large grid to
obtain a resistivity profile or pseudosection.
Resistivity measurements can be made in one location using an increasing
electrode spread – referred to as sounding as mentioned earlier.
Surface measurements of resistivity are conducted using equipment
that generates a commutated current. The current is applied to the ground
through stainless steel electrodes that are driven into the surface to make
electrical contact. Current is measured
at the transmitter and voltages are measured at selected receiving locations
using non-polarizing electrodes having known spacing. Electrode configurations
include Schlumberger, Wenner, pole-dipole and dipole-dipole.
Dipole-dipole is most often used because it requires less time to setup, move
and re-setup the electrodes.
Data is collected and used to create a resistivity contour map or
2D and 3D pseudosections.
Induced Polarization (IP)
is often obtained with surface resistivity as well as Spontaneous Potential (SP).
CALCULATORS:
Perform resistivity calculations using free WELLOG calculators:
Dipole-Dipole Wenner Schlumberger Pole-Dipole
BOREHOLE RESISTIVITY:
Specialized logging tools referred to as Elog logging sondes containing electrodes
are lowered into a water based mud filled borehole to measure formation
resistivity. In applications that require focused resistivity measurement i.e.
saline borehole fluids, a focused
resistivity tool is used. Other resistivity measurements may be conducted using
electromagnetic induction
methods. Induction methods operate in both fluid filled and non fluid filled
boreholes. Increasing use of oil based mud requires the use of an induction resistivity tool.
Other specialized resistivity tools measure the effect of fluids in the
borehole wall. Visit our web page
on borehole
resistivity. Most logging companies offer a simultaneous SP measurement.
Borehole Resistivity Imaging:
Resistivity imaging of a borehole can be performed using tools
with many pad mounted resistivity contacts. Formation dip and strike can be
determined when additional measurements of hole inclination and magnetic field
are incorporated.
MUD RESISTIVITY:
Containers called “mud cups” are frequently used in the laboratory
and in the field to measure a sample of the drilling mud. The mud cup has
electrodes in it that are used to measure resistivity of drilling mud (Rm and Rmf) and formation water.
Formation water resistivity is referred to as Rw
(water resistivity). Resistivity of mud cake and mud filtrate can also be
accomplished with similar instruments. A pressurized filter press is used to
force a drilling mud sample through a paper or similar filter to obtain mud
cake (for Rmc) and mud filtrate (for Rmf) measurement.
Each of the resistivities contribute to
the more complex solution of total formation resistivity (Rt). Determination of
resistivity of each of the fluids and muds can
provide valuable information about borehole fluid invasion into the formation
surrounding the borehole. The extent of formation fluid invasion and fluid
flushing is important for reasons related to permeability of the
formations surrounding the borehole.
For more information on the subject of resistivity contact info@wellog.com.
