Deep Induction Log

The deep induction log (ILD) is the deepest-reading resistivity channel of a dual induction or array induction logging tool, employing a long transmitter-to-receiver coil array with additional focusing coils to measure formation resistivity at a radial depth of investigation of approximately 1.5 to 2.5 metres into the formation, thereby minimizing the influence of drilling fluid invasion and providing the true formation resistivity (Rt) required for hydrocarbon saturation calculation using Archie's equation.

Key Takeaways

The ILD (induction log deep) operates in low-salinity and fresh mud environments where the formation is more conductive than the mud; induction tools work by electromagnetic induction and do not require conductive mud contact with the formation wall, unlike electrode-type laterolog tools.
The dual induction tool (DIT) provides three resistivity curves: ILD (deep, approximately 1.5 to 2 m radial depth), ILM (medium, approximately 0.6 to 0.9 m), and SFL or SFLU (shallow focused log, approximately 0.3 m), enabling interpretation of invasion profiles and radial resistivity variation.
Phasor induction processing, developed by Schlumberger in the 1980s, corrects for skin effect at high formation conductivities, extending induction tool accuracy to saline formation waters where uncorrected readings can significantly underestimate true resistivity.
Array induction tools (AIT, HRAI) replace the single ILD channel with multiple receiver arrays, generating six or more radial depth-of-investigation curves from 0.3 to 2.5 m that enable continuous invasion profile inversion without assuming a step-profile invasion model.
In high-salinity mud environments (saline water-based muds, typical of Gulf Coast and offshore operations), the deep laterolog (LLD) is preferred over deep induction because conductive mud entering the borehole creates a parasitic signal that degrades induction tool accuracy.

Fast Facts

Induction logging was invented by Henri-Georges Doll at Schlumberger in 1949 to solve the problem of measuring resistivity in wells drilled with oil-based mud, where electrode-based tools fail due to lack of electrical contact with the formation. The operating frequency of most induction tools is 20 kHz. Formation conductivity (in mS/m) is the primary measured parameter; resistivity (in ohm-m) is the reciprocal. ILD vertical resolution is approximately 1.2 metres for the 6FF40 standard spacing tool.

Tip: Always examine the separation between ILD and ILM curves when interpreting a well. In a water zone, invasion of fresh mud into saline formation water increases resistivity near the borehole, so ILD reads lower (less invaded) and ILM reads higher (more invaded), a pattern called normal invasion or resistivity increase toward the borehole. In a hydrocarbon zone invaded by fresh mud, the opposite pattern (ILD higher than ILM) suggests oil or gas behind the invasion front, making ILD the reliable Rt estimate.

What Is the Deep Induction Log

Resistivity logging is the cornerstone of formation evaluation because hydrocarbon-bearing formations (oil, gas) are resistive (poor conductors of electricity) while water-bearing formations, especially those containing saline formation water, are conductive. The challenge is measuring the true, undisturbed formation resistivity (Rt) rather than the altered near-borehole resistivity resulting from drilling fluid invasion. Invasion occurs when the pressure differential between the drilling fluid column and the formation drives mud filtrate into the permeable rock, partially or fully displacing formation fluids from the region nearest the wellbore.

The deep induction log is specifically designed to look past this invaded zone. By using a long coil spacing (typically 40 inches for the ILD in the classic 6FF40 configuration) and focusing coils that cancel signals from the borehole and near-borehole region, the ILD preferentially measures the resistivity of the deeper, uninvaded formation. This makes it the primary input to Archie's equation for water saturation (Sw = sqrt(Rw / (phi^m x Rt))) and the preferred resistivity curve for most petrophysical interpretations in fresh and low-salinity mud environments.

How the Deep Induction Log Works

The induction tool induces eddy currents in the formation by transmitting a primary alternating magnetic field from the transmitter coil at 20 kHz. These eddy currents flow in ring-shaped paths coaxial with the tool in the conductive formation, and each ring of current generates its own secondary magnetic field. The receiver coil measures the secondary field, which is proportional to the formation conductivity at each radial and vertical distance. The geometric factor of each coil configuration determines the radial and vertical depth weighting.

The ILD uses a six-coil bucking array (the 6FF40 design: six coils with specific spacings and polarities) that maximizes the deep radial response and cancels borehole and near-formation contributions. Phasor processing (used in Schlumberger's DIT-E and successors) deconvolves the measured signal using the in-phase and quadrature components to apply skin effect correction, recovering accurate resistivity readings in saline, high-conductivity formations where the raw ILD would read too low. Array induction tools (AIT, Baker Hughes HRAI, Halliburton HDIL) replace the single-depth output with multi-frequency or multi-spacing measurements that are combined by inversion to generate continuous radial conductivity profiles, enabling quantitative invasion correction without assumptions about invasion geometry.

Deep Induction Log Across International Jurisdictions

In Canada and the WCSB, dual induction tools or array induction tools are standard for wells drilled with fresh water-based mud or oil-based mud. AER requires wireline log data submission for all exploratory and development wells; ILD/ILM/SFL or array induction curves are mandatory components of the submitted log package under AER Directive 065. In Montney and Duvernay horizontal wells, LWD array resistivity tools (Schlumberger ARC, Halliburton EcoScope, Baker Hughes OnTrak) provide real-time deep resistivity while drilling, used for geosteering and real-time petrophysical evaluation. The ILD equivalent in LWD tools is the deep propagation resistivity (typically referred to as the 2 MHz deep phase-shift resistivity or 2 MHz deep attenuation resistivity).

In the United States, the ILD has been the primary Rt measurement in continental US wells since the 1960s. BSEE requires well log data submission for all OCS wells; dual induction or array induction logs are standard deliverables. In the Permian Basin, where fresh water-based polymer muds are common in horizontal Wolfcamp and Bone Spring wells, array induction tools provide the invasion-corrected Rt needed for saturation modeling across highly variable invasion profiles in the heterogeneous carbonate-siliciclastic interbeds. In the Bakken and Eagle Ford, tight rock (less than 0.1 mD permeability) limits invasion so that ILD and ILM readings often coincide; any separation signals natural fractures or high-permeability streaks.

In Norway, array induction tools are used in wells drilled with low-salinity synthetic oil-based mud, where electrode tools would require correction for the highly resistive mud column. Sodir's DISKOS well log database archives ILD and array induction data from all Norwegian shelf wells. In deepwater North Sea exploration wells, array induction tools provide Rt measurements in low-permeability Paleocene and Eocene turbidite reservoirs where Archie-based water saturation determination requires accurate ILD-derived Rt to differentiate hydrocarbon charge from brine in high-porosity clean sands.

In the Middle East, the choice between induction and laterolog tools reflects local mud systems. Saudi Aramco and ADNOC frequently use potassium chloride water-based muds or oil-based muds in carbonate drilling; fresh KCl and OBM environments favor induction tools. The deep induction log is routinely run in the Arab D and Khuff formation wells for saturation determination in the prolific carbonate reservoirs. Ghawar field reservoir characterization by Aramco's exploration and production teams has relied extensively on dual induction logs archived from thousands of wells drilled since the 1950s, providing the resistivity foundation for volumetric oil-in-place estimates in the world's largest conventional oil field.

The deep induction log is formally designated ILD (induction log deep) and is sometimes called the deep induction resistivity or DI. Related terms include dual induction log, induction medium log (ILM), array induction tool (AIT), deep laterolog (LLD), Archie's equation, true formation resistivity (Rt), invasion profile, and water saturation. Phasor processing and array induction inversion are the signal processing methods used to correct deep induction readings for skin effect and invasion respectively.

FAQ

Why can't the deep induction log be used in salt-saturated mud?
Induction tools measure conductivity by inducing eddy currents; highly conductive drilling fluid (salt-saturated mud conductivity can exceed 10,000 mS/m) filling the borehole creates a dominant eddy current path in the borehole itself that overwhelms the formation signal. The borehole signal correction becomes large and unreliable, making the ILD reading inaccurate. In salty mud environments, electrode-type resistivity tools (deep laterolog, LLD) are preferred because they inject current directly into the formation and are less sensitive to borehole fluid conductivity when run with appropriate dual-laterolog focusing.

What is the difference between ILD and the 2 MHz deep resistivity from LWD?
The wireline ILD measures formation resistivity at 20 kHz after the borehole has been drilled and invaded. LWD propagation resistivity tools measure at 2 MHz (and sometimes 400 kHz) while drilling, before significant invasion has occurred. LWD deep resistivity often reads closer to Rt in fast-drilling operations because invasion time is minimal, but the 2 MHz frequency has a shallower radial depth of investigation than the 20 kHz wireline ILD for equivalent coil spacings. Integrated interpretation uses both LWD resistivity while drilling and wireline ILD after the drill string is pulled to characterize invasion dynamics and invasion-corrected Rt.

Why the Deep Induction Log Matters

The deep induction log is the primary input to hydrocarbon saturation calculations in the majority of the world's oil and gas wells drilled with fresh or oil-based muds. An error in ILD of just 20% can translate into a 15 to 25% error in calculated water saturation using Archie's equation, which in a commercial discovery can shift reserve estimates by hundreds of millions of barrels of oil equivalent. In developed fields, accurate ILD-based saturation logs integrated across thousands of wells underpin the reservoir models that guide infill drilling, waterflood management, and EOR investment decisions worth billions of dollars annually. The deep induction log's ability to see past the invaded zone is not a technical nicety but a requirement for reliable formation evaluation.