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==Electrical properties of materials==

==Electrical properties of materials==

The application, interpretation, and understanding of electrical methods requires a familiarity with the relationship between soil and rock characteristics and the resistivities obtained from electrical data. The resistivity of subsurface rock formations is one of the physical properties determined through the process of logging that is performed on most oil and gas wells, utilizing instrumentation inserted into the wellbore (see Part 4 of this Manual). The concept of formation resistivity plays an important part in log analysis. Although there is a correlation between rock resistivities measured by well logs and those measured by electrical methods, the log is used to investigate properties only in the immediate vicinity of the wellbore while electrical methods yield information on bulk properties averaged over a considerable volume of material.

The application, interpretation, and understanding of electrical methods requires a familiarity with the relationship between soil and rock characteristics and the resistivities obtained from electrical data. The resistivity of subsurface rock formations is one of the physical properties determined through the process of logging that is performed on most oil and gas wells, utilizing instrumentation inserted into the wellbore (see [[Wireline methods]]). The concept of formation resistivity plays an important part in log analysis. Although there is a correlation between rock resistivities measured by well logs and those measured by electrical methods, the log is used to investigate properties only in the immediate vicinity of the wellbore while electrical methods yield information on bulk properties averaged over a considerable volume of material.

The resistivity of most soils and rocks (including virtually all of the rocks of interest to hydrocarbon exploration) at the frequencies utilized by electrical methods is controlled by the fluids contained within the rock<ref name=pt07r46>Parkhomenko, E. I., 1967, Electrical properties of rocks: New York, Plenum Press, 314 p.</ref> (see “Determination of Water Resistivity”). This is because the dry soil or rock matrix is a virtual insulator at DC and near DC frequencies. The pore fluid is in most cases water, with dissolved salts. The salinity is the primary factor in determining the resistivity of the pore fluid, with pore configuration also playing a part. Of lesser importance at oil reservoir depths is the temperature of the formation. Oil and/or gas, when present, occur over such limited formation thicknesses that their effects on bulk average resistivity is, in most cases, undetectable.

The resistivity of most soils and rocks (including virtually all of the rocks of interest to hydrocarbon exploration) at the frequencies utilized by electrical methods is controlled by the fluids contained within the rock<ref name=pt07r46>Parkhomenko, E. I., 1967, Electrical properties of rocks: New York, Plenum Press, 314 p.</ref> (see [[Determination of water resistivity]]). This is because the dry soil or rock matrix is a virtual insulator at DC and near DC frequencies. The pore fluid is in most cases water, with dissolved salts. The salinity is the primary factor in determining the resistivity of the pore fluid, with pore configuration also playing a part. Of lesser importance at oil reservoir depths is the temperature of the formation. Oil and/or gas, when present, occur over such limited formation thicknesses that their effects on bulk average resistivity is, in most cases, undetectable.

Faulting or fracturing of porous sedimentary formations in most instances has little effect on the bulk average resistivity since the additional fracture [[porosity]] changes the already high porosity by only a small percentage. However, in very tight rocks, such as igneous, metamorphic, and nonporous carbonate rocks, where intrinsic porosity is very low, the fluids in joints, cracks, and faulted zones may become the primary conducting paths (see “[[Porosity]]”).

Faulting or fracturing of porous sedimentary formations in most instances has little effect on the bulk average resistivity since the additional fracture [[porosity]] changes the already high porosity by only a small percentage. However, in very tight rocks, such as igneous, metamorphic, and nonporous carbonate rocks, where intrinsic porosity is very low, the fluids in joints, cracks, and faulted zones may become the primary conducting paths (see [[Porosity]]).

In summary, the factors affecting ''in situ'' average resistivity are the total porosity, including fault and fracture porosity, and the resistivity of the fluids present within the rock. The average resistivity can be considered constant over the frequency range of interest to most of the methods under consideration here.

In summary, the factors affecting ''in situ'' average resistivity are the total porosity, including fault and fracture porosity, and the resistivity of the fluids present within the rock. The average resistivity can be considered constant over the frequency range of interest to most of the methods under consideration here.