WELLOG

WELLOG RESISTIVITY

Revised: 01-12-2011

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 these 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. Apparent Resistivity of a formation can pinpoint zones having oil versus zones having water.

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 below the surface of the earth. Many mineral deposits have been located that were completely covered by alluvial deposits. Resistivity can be used to measure depth to bedrock in placer gold mining. Paleo-channels can be located using resistivity methods.

Mineral ore resistivities contrast with barren rock resistivities as follows:

Examples of 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

Galena: .01 – 300

Zinc blende > 10,000

MINERAL BOREHOLE LOGGING:

Follow-up exploration activities usually include core drilling to sample the mineral deposit. Core sampling provides a direct method of collecting data that gives preliminary economic value to a mineral deposit. Borehole 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 Vertical Electrical 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. Knowledge of the resistivity of the mud cake (Rmc), resistivity of the mud (Rm) and resistivity of the mud filtrate add to the accuracy of true resistivity (Rt).