conductance, Oil & Gas Glossary

What Is Conductance?

Conductance (also called electrical conductance or formation conductance) is the measure of how easily an electrical current passes through a material, defined as the reciprocal of electrical resistance and expressed in siemens (S) or millisiemens per meter (mS/m). In well logging, conductance is the primary physical variable that induction logging tools measure directly, because the alternating magnetic fields generated by transmitter coils induce eddy currents in formation fluids whose amplitude is proportional to the formation's electrical conductivity, making conductance the natural output variable for any induction-based measurement system.

Key Takeaways

Conductance is the reciprocal of resistance: C = 1/R, with units of siemens (S); in well logging, formation conductivity is expressed in millisiemens per meter (mS/m) or its equivalent milliohm per meter.
Induction tools measure formation conductance directly because transmitter coils induce eddy currents whose received voltage is proportional to conductivity, not resistivity, making conductance the natural raw output signal.
The conversion between conductivity and resistivity is exact: a formation with conductivity of 1 mS/m has a resistivity of 1,000 ohm-m; a conductivity of 1,000 mS/m corresponds to a resistivity of 1 ohm-m.
Saline formation water (high NaCl concentration) is a strong conductor; resistivity logs show low values (1 to 5 ohm-m) in brine-saturated sands and high values (20 to 1,000 ohm-m or more) in hydrocarbon-bearing or tight formations.
Petrophysicists apply Archie's law to convert measured formation resistivity (derived from conductance) to water saturation; the accuracy of Sw estimates depends on knowing formation water resistivity (Rw) and Archie cementation exponent (m).

How Induction Tools Measure Formation Conductance

An induction logging tool suspends a transmitter coil and one or more receiver coils on a mandrel lowered into the borehole on a wireline. The transmitter coil carries an alternating current at a fixed frequency, typically 20 kHz for deep-reading arrays, which generates an oscillating primary magnetic field. This primary field induces circular eddy currents in the formation around the borehole, analogous to transformer coupling. The eddy currents create a secondary magnetic field that is sensed by the receiver coils. For a homogeneous formation, the amplitude of the received secondary signal is directly proportional to the formation's electrical conductivity, which is why the raw tool output is a conductance signal measured in millisiemens per meter rather than in ohm-m.

Modern array induction tools (AIT, HDIL, or similar commercial variants) deploy multiple transmitter-receiver spacings ranging from 10 inches to 90 inches. Each spacing interrogates a different radial depth into the formation, from the flushed zone near the borehole to the undisturbed virgin formation. The raw conductivity signals from each array are processed by a multi-array inversion algorithm that separates borehole, invaded-zone, and deep-formation contributions, producing a set of radially focused resistivity curves at depths of investigation of approximately 10, 20, 30, 60, and 90 inches. The petrophysicist then reads the deep resistivity curve (Rt) for water saturation calculations while the shallow curves reveal the invasion profile and mudcake characteristics.

The conversion from the conductance-based raw signal to the resistivity log that appears on a standard log display is a simple arithmetic inversion: resistivity (ohm-m) equals 1,000 divided by conductivity (mS/m). This conversion is performed in the acquisition software before log delivery, so the field log displays resistivity in ohm-m on a logarithmic scale (typically 0.2 to 2,000 ohm-m). Petrophysicists working with raw induction data, however, sometimes prefer to work in conductivity units because the Archie equation and shaly-sand models can be expressed in terms of conductance, and because conductance averages linearly across parallel conductive layers while resistivity does not, making conductance the more physically intuitive variable for laminated sand-shale sequences.

Fast Facts: Conductance

SI unit: Siemens (S); logging unit is millisiemens per meter (mS/m)
Reciprocal relationship: Resistivity (ohm-m) = 1,000 / Conductivity (mS/m)
Tool type: Induction logging arrays (AIT, HDIL, MCIL); also laterolog tools measure resistivity directly via focused current injection
Brine-saturated sand: Conductivity typically 200 to 1,000 mS/m (resistivity 1 to 5 ohm-m)
Hydrocarbon-bearing sand: Conductivity typically 1 to 50 mS/m (resistivity 20 to 1,000 ohm-m)
Tight carbonate (dry): Conductivity near 0 mS/m (resistivity exceeding 1,000 ohm-m)
Archie application: Sw = (a x Rw / (phi^m x Rt))^(1/n); conductivity units used in some shaly-sand models
Induction vs. laterolog: Induction preferred in fresh mud or air-filled boreholes; laterolog preferred in saline mud or highly resistive formations

Petrophysics Tip:

When comparing resistivity values across wells drilled with different mud systems, always check whether the tool was an induction array or a laterolog. Induction tools read high in very resistive formations (above 200 ohm-m) because the secondary eddy-current signal becomes vanishingly small and the signal-to-noise ratio degrades, causing the conductance-to-resistivity conversion to overestimate resistivity. In tight carbonate or clean gas reservoirs above 100 ohm-m, a high-resolution laterolog or microresistivity tool gives a more reliable Rt for saturation calculations than a standard induction array.

Conductance is also referred to as:

Electrical conductivity — the volume-normalized form, expressed in S/m or mS/m, describing a material property independent of geometry (conductance describes a specific circuit element; conductivity describes the material)
Formation conductivity — used in petrophysics to refer specifically to the bulk electrical conductivity of a rock-fluid system at in-situ conditions
Induction signal — shorthand used in logging operations for the raw conductance-proportional voltage output from the induction tool receiver coils before processing
Admittance — the AC circuit analog of conductance, incorporating both resistive and reactive components; rarely used in oilfield logging but appears in electromagnetic modeling literature

Frequently Asked Questions About Conductance

Why do induction logs display resistivity if the tool measures conductance?

Induction tools measure conductance because the physics of electromagnetic induction makes the raw received signal directly proportional to formation conductivity. However, petrophysicists historically worked with resistivity because early electrode logs (laterologs and normal/lateral devices) measured resistance directly, and the industry standard for Archie's equation is written in resistivity terms. To maintain consistency with existing petrophysical workflows and cross-plot libraries, the logging service companies convert the induction conductance measurement to resistivity (by dividing 1,000 by the conductivity value in mS/m) before delivering the log. The underlying conductance data is retained internally and is available on request for special applications such as thin-bed inversion or electromagnetic modeling.

When is conductance a more useful variable than resistivity for log analysis?

Conductance (conductivity) averages linearly in parallel conductive layers, whereas resistivity averages harmonically. This distinction is critical in laminated sand-shale sequences, where thin conductive shale laminae dominate the bulk conductivity measurement and cause the log to read much lower resistivity than the clean sand component alone. Working in conductivity space allows straightforward volumetric mixing of sand and shale conductivities to estimate the net-sand resistivity. The Thomas-Stieber model and many modern shaly-sand conductivity models (Waxman-Smits, Dual-Water) are formulated in conductivity terms precisely because conductance additivity makes the mathematics tractable. In homogeneous clean formations, both units give equivalent information and the choice is a matter of convention.

What is the practical range of formation conductance values encountered in oilfield logging?

Formation conductance in logging spans roughly six orders of magnitude. Highly saline formation brines (150,000 ppm NaCl equivalent) in shallow Gulf Coast sands can drive bulk formation conductivity to 1,000 mS/m or more (resistivity below 1 ohm-m). At the opposite extreme, tight low-porosity carbonates or salt formations can exhibit conductivities below 0.01 mS/m (resistivity exceeding 100,000 ohm-m), which pushes induction tools beyond their reliable operating range. In practice, the most useful petrophysical discrimination occurs between water-saturated sands at 200 to 500 mS/m and hydrocarbon-bearing sands at 5 to 50 mS/m, a 10-fold to 100-fold contrast that makes resistivity (and its inverse, conductance) one of the most diagnostic variables in well log interpretation.

Why Conductance Matters in Oil and Gas

Conductance is the foundational electrical property that enables petrophysicists to distinguish hydrocarbon-bearing formations from brine-saturated rock using wireline and LWD logs. Every formation evaluation decision, from net pay identification to reserve estimation, ultimately depends on an accurate measurement of formation conductance and its correct conversion to a resistivity value for input to Archie's equation or equivalent saturation models. Without reliable conductance measurements, operators cannot determine water saturation, cannot estimate hydrocarbon pore volume, and cannot design completions to perforate only productive intervals. The continuous improvement of induction array tools, from single-frequency single-spacing devices to modern broadband multi-array systems, has been driven by the need to measure formation conductance with greater accuracy across wider resistivity ranges, thinner beds, and more complex invasion profiles than earlier tools could resolve.

Conductance: Electrical Measurement in Well Log Interpretation