Induction

An induction log is a wireline measurement of formation resistivity that uses electromagnetic induction rather than direct electrical contact to measure how well the formation conducts electricity. An alternating current in a transmitter coil induces circulating eddy currents in the formation; these eddy currents generate their own electromagnetic signal that is detected by receiver coils at a fixed distance from the transmitter. The strength of the detected signal is proportional to the electrical conductivity of the formation (the reciprocal of resistivity). Induction tools work best in low-resistivity formations and in wells drilled with non-conductive mud (oil-based or air), where direct-contact resistivity tools (laterologs) cannot function properly. In freshwater-mud or oil-based mud environments, the induction log is the preferred deep-reading resistivity measurement used in Archie water saturation calculations.

Key Takeaways

Induction tools are most accurate when the formation resistivity is low to moderate (1 to 100 ohm-metres). At very high resistivities (above 200 ohm-metres, common in gas sands with low water saturation), the induced eddy currents are very weak and the induction signal approaches the noise floor. For high-resistivity formations (tight gas, dry carbonates), laterolog-type tools (which inject current directly into the formation) provide more accurate measurements.
Modern array induction tools (such as Schlumberger's Array Induction Sonde, AIT, or Baker Hughes's HDIL) measure resistivity at multiple depths of investigation simultaneously, from about 10 centimetres (flushed zone) to 150 centimetres (uninvaded formation) from the borehole wall. By comparing resistivity at different depths of investigation, the log analyst can characterize the invasion profile and correct the measured resistivity toward the true undisturbed formation resistivity.
The skin effect is a signal distortion that occurs in induction logs at high formation conductivity (low resistivity) caused by absorption of the primary electromagnetic field by the conductive formation before it reaches the full depth of investigation. Modern induction tools apply skin-effect corrections algorithmically to compensate for this distortion. Without correction, the apparent resistivity in very conductive formations can be lower than the true resistivity by 10 to 30 percent.
Induction tools measure conductivity (in millisiemens per metre, mS/m) internally and convert to resistivity (ohm-metres) for display. One ohm-metre equals 1,000 mS/m reciprocal. Formation water with typical Alberta salinity of 30,000 mg/L total dissolved solids has a resistivity of about 0.2 ohm-metres at 25°C; fresh formation water (typical for many Cretaceous formations in central Alberta) has a resistivity of 1 to 5 ohm-metres at the same temperature.
In horizontal wells, induction tools mounted in LWD drill collars provide real-time formation resistivity used for geological steering. The tool's multiple depths of investigation can detect approaching shale boundaries above or below the horizontal wellbore before the bit reaches them, allowing the directional driller to adjust the trajectory to stay in the reservoir. This application of real-time induction measurements is sometimes called geo-steering or formation evaluation while drilling (FEWD).

How an Induction Log Works

Think of a metal detector at an airport. It generates a magnetic field that induces circulating currents in any nearby metal object. The metal object then generates its own magnetic field that the detector picks up. An induction log tool uses the same physics, but instead of looking for solid metal objects, it is looking for the electrical conductivity of the pore water in the formation surrounding the borehole.

The induction tool's transmitter coil carries an alternating current at a fixed frequency (typically 20 to 40 kHz). This current generates a varying magnetic field. Faraday's law of electromagnetic induction says that a varying magnetic field induces currents in any conductor in the field. The formation rock, saturated with brine, is a conductor. Circulating eddy currents flow in rings around the wellbore, in planes perpendicular to the borehole axis. These eddy currents are stronger where the formation is more conductive (lower resistivity).

The eddy currents generate their own magnetic field (in phase with the primary field if the medium is purely conductive, 90 degrees out of phase if it is purely resistive). The receiver coil detects this secondary field and measures its amplitude. The amplitude is proportional to the formation conductivity. The tool electronics convert this signal to an apparent resistivity reading and transmit it up the cable to the surface recording unit.

Fast Facts

The induction log was invented by Henri-Georges Doll of Schlumberger and first deployed commercially in 1949. Doll developed it specifically for formations drilled with oil-based mud (where no direct electrical contact with the formation was possible) and for freshwater mud environments (where the older laterolog tools were not appropriate). The original Induction Electric Log combined an induction coil for deep resistivity with a focused electrode device for shallow resistivity and a spontaneous potential (SP) measurement. Schlumberger's Dual Induction Laterolog (DIL), introduced in the 1960s, became the industry standard combination tool for most of the late 20th century. Modern array tools replaced the DIL in the 1990s, providing more information about invasion profiles and better correction algorithms.

Induction Versus Laterolog: Choosing the Right Tool

The two main families of resistivity tools are induction and laterolog (focused direct-current tools). They are complementary: each performs best in conditions where the other is less accurate.

Induction tools excel in: low-resistivity formations (up to about 100 ohm-metres), freshwater mud and oil-based mud environments, moderately deep invasion profiles, and airdrilled wells. They become less accurate above 200 ohm-metres because the signal-to-noise ratio drops as formation conductivity decreases.

Laterolog tools (Dual Laterolog, High-Resolution Laterolog Array) excel in: high-resistivity formations (100 to 10,000 ohm-metres), salty brine mud environments, and formations with thin beds where vertical resolution matters. They require electrical contact between the tool and the formation, so they cannot be used in oil-based or air mud. They perform poorly in freshwater mud because the conductive mud short-circuits the current.

In the Alberta Deep Basin (deep, tight gas formations with high resistivity), laterolog tools are preferred because induction tools give poor readings above 200 ohm-metres. In the shallow Mannville Group and Viking formations of central Alberta (moderate resistivity, freshwater mud), induction tools are the standard. For wells drilled with oil-based mud (most Duvernay and Montney horizontal wells), induction is the only option for resistivity measurement unless a special dual-physics tool (combining both measurement types in one run) is used.

Formation Water Resistivity and Induction Log Interpretation

The induction log gives a resistivity reading (Rt, the true undisturbed formation resistivity after invasion correction). To convert Rt to water saturation using the Archie equation, the formation water resistivity (Rw) must be known. Rw is measured from produced water samples (most accurate), calculated from the spontaneous potential log (if conductive mud is used), or estimated from water chemistry databases for the formation.

In the Viking Formation of central Alberta, the formation water is typically freshwater to slightly brackish, with Rw values of 0.5 to 3.0 ohm-metres at reservoir temperature. An induction log reading of 15 ohm-metres in a 20-percent-porosity Viking sand with Rw = 1.0 ohm-metre gives a water saturation of about 32 percent using Archie, indicating an oil pay zone. The same 15 ohm-metre reading in a Devonian carbonate with Rw = 0.05 ohm-metre (salty formation water) would give a water saturation of about 90 percent, indicating a water zone. The same resistivity reading means completely different things in different salinity environments.

The induction log is also called the induction resistivity log or electromagnetic induction log. The measurement unit, the ohm-metre, is sometimes abbreviated as ohm·m or Ω·m. Related terms include resistivity log (the general category of wireline measurements of formation electrical resistivity; the induction log is one type, used in freshwater mud and OBM; the laterolog is the other main type, used in saltwater mud and high-resistivity formations), laterolog (a focused direct-current resistivity tool that injects current laterally into the formation; complements the induction log; preferred in high-resistivity formations and salty mud environments), Archie equation (the empirical formula relating water saturation to formation resistivity, water resistivity, and porosity; the induction log provides the deep formation resistivity (Rt) used in the Archie water saturation calculation), invasion (the displacement of formation fluids near the wellbore by mud filtrate during drilling; the different depths of investigation of array induction tools characterize the invasion profile and allow correction to true formation resistivity), and logging while drilling (LWD, formation evaluation measurements made by sensors in the drill string in real time during drilling; induction-type resistivity measurements in LWD tools are used for geological steering of horizontal wells).

How a Wrong Tool Selection Added Two Logging Runs to a Duvernay Well in Alberta

An operator was logging a Duvernay horizontal well in the Fox Creek area of west-central Alberta. The well had been drilled with oil-based mud (OBM) throughout the horizontal section. The logging program specified a dual laterolog array tool as the primary resistivity measurement for the horizontal section.

When the wireline crew ran the tool downhole, the logs immediately showed anomalous readings: the laterolog was giving erratic, tool-noise-dominated resistivity values in the low range (0.5 to 2 ohm-metres) when the Duvernay silts are known to have true resistivities of 15 to 40 ohm-metres in this area. The wireline engineer identified the problem: the laterolog tool requires a current path from the tool through the formation and back via the borehole. With OBM (which is essentially non-conductive), no current path existed. The tool was effectively measuring the OBM resistivity, not the formation.

The laterolog was pulled and an induction tool was run instead. The induction log works with OBM because it uses electromagnetic induction rather than direct current contact. The induction log returned quality resistivity readings in the expected range, confirming oil-saturated Duvernay intervals. The incorrect tool selection required pulling the first tool, rigging up the second tool, and rerunning the logging suite, adding 6 hours of logging time and CAD 32,000 in additional wireline costs. The logging program had been prepared by a new engineer who had not checked the mud system compatibility with the tool selection. Mud system and tool compatibility review is now a required step in the logging program sign-off workflow at that company.