Medium Induction | Oil Authority

Medium induction in wireline logging refers to the resistivity measurement from an induction tool that investigates the formation at an intermediate depth — approximately 40 to 80 centimeters from the borehole wall — using a coil array specifically designed to focus induced electromagnetic current flow to this intermediate radial distance, providing a resistivity reading between the shallow investigation of the microresistivity tools (which measure the flushed zone) and the deep investigation of the deep induction curve (which samples the undisturbed formation beyond invasion); the medium induction curve (variously labeled ILM, IM, IL-medium, or R-medium depending on tool and manufacturer) is one of the standard components of induction log suites from Schlumberger, Halliburton, Baker Atlas, and their modern successors, used primarily to characterize the invasion profile and provide the intermediate depth measurement needed for resistivity invasion correction to determine true formation resistivity (Rt) for water saturation calculations.

Key Takeaways

Radial depth of investigation of the medium induction is controlled by the transmitter-receiver coil geometry and the focusing coil arrangement — the standard dual induction tool places the medium array coils at a shorter spacing than the deep array but longer than the shallow focused tool, producing a geometrical focus at the intermediate depth range of 40 to 80 cm; the geometric factor (Gj) describes the fractional contribution to the total apparent resistivity from concentric cylindrical shells at increasing radial distances from the borehole axis, and the medium array's geometric factor peaks between 40 and 60 cm from the axis while the deep array's factor peaks between 80 and 100 cm; both arrays still have significant sensitivity contributions from shallower and deeper zones outside their peak response range, so in very shallow invasion (less than 20 cm), both the medium and deep readings may be significantly influenced by the invaded zone, making invasion correction difficult from the three-curve suite alone.
Modern array induction tools (Schlumberger AIT, Halliburton HDIL, Baker Hughes MCIL) have superseded the classical dual induction tool by providing five or more simultaneous depths of investigation (typically at 10, 20, 30, 60, and 90 inches or 25, 50, 75, 150, and 225 cm), with the intermediate-depth arrays corresponding functionally to the classical medium induction and providing equivalent or better invasion characterization; the additional depth steps in array induction tools allow more sophisticated radial inversion processing that fits the full multi-depth resistivity profile to a three-parameter model (Rt, Rxo, di) or a more complex invasion profile model, yielding invasion-corrected Rt estimates with lower uncertainty than the two-measurement dual induction trio could provide from a single medium and deep reading.
Shoulder effect and thin-bed response of the medium induction creates apparent resistivity distortion at bed boundaries and in thin beds — in a 0.5-meter sand bed sandwiched between lower-resistivity shales, the medium induction reads an intermediate apparent resistivity between the sand and shale values because its 1.0-meter vertical resolution (full width at half maximum of the vertical response function) averages the two contrasting lithologies; the deep induction has an even coarser vertical response (approximately 1.2 to 1.5 meters), while the medium has slightly better vertical resolution, so the difference between medium and deep readings at a thin sand boundary is partly due to vertical resolution differences rather than purely to invasion; shallow shoulder effects at layer boundaries must be corrected (or recognized as a resolution artifact) before the medium-to-deep resistivity ratio is interpreted as an invasion indicator.
Borehole corrections for the medium induction account for the conductive wellbore fluid path (the mud in the borehole, particularly saline water-based mud) that couples inductively to the transmitter coils and contributes a spurious signal to the receiver; in large-diameter boreholes (greater than 12 inches), or when the mud salinity creates a very conductive mud resistivity (less than 0.1 ohm-m), the borehole correction for the medium induction can be significant (greater than 10% of the apparent reading), requiring application of standard borehole correction charts (Chart ID series in the Schlumberger chart book) before the corrected ILM is used in invasion analysis; the medium induction typically requires smaller borehole corrections than the deep induction because the medium array coils are positioned closer to the tool axis and are less geometrically sensitive to the large-borehole conductive annulus than the widely spaced deep array coils.
Resistivity ratio interpretation using the medium-to-deep (ILM/ILD) ratio provides a rapid visual indicator of the invasion pattern and invasion direction without requiring the full tornado chart correction — ratios greater than 1.0 (medium reads higher than deep) indicate that the invaded zone is more resistive than the uninvaded zone, consistent with fresh filtrate invasion into a saline formation water bearing or hydrocarbon-bearing interval; ratios less than 1.0 (medium reads lower than deep) indicate saline filtrate invasion into a zone where the undisturbed formation is more resistive, consistent with saline OBM filtrate invading a hydrocarbon-bearing zone or formation water more saline than the drilling mud; the magnitude of the ratio (how far above or below 1.0) is roughly proportional to the degree of invasion, with large ratios indicating shallow, significant invasion and near-unity ratios indicating either no invasion or very deep invasion beyond both tools' radial resolution.

Fast Facts

The medium induction log was introduced as part of the dual induction logging suite in 1962, when Schlumberger introduced the first commercial dual induction tool that simultaneously provided deep and medium depth induction measurements as complementary resistivity curves on the same log pass; the combination of deep and medium induction with a shallow resistivity measurement (laterolog-8 or spherically focused log) provided the three-depth resistivity profile that became the standard induction log suite for formation evaluation in fresh-to-moderately-saline mud environments through the 1990s. The transition to array induction tools in the 1990s and 2000s did not eliminate the medium induction concept but refined it — array tools provide equivalent measurement at multiple intermediate depths that together constrain the invasion profile much better than a single medium array measurement, while maintaining backward compatibility with the classical medium-induction interpretation workflows that remain in use for legacy log re-evaluation worldwide.

What Is the Medium Induction Log?

Formation resistivity changes with radial distance from the borehole because drilling fluid filtrate invades the permeable formation, displacing native fluids outward and creating a zone of altered resistivity near the wellbore. The shallowest zone — the flushed zone immediately adjacent to the borehole — contains mainly filtrate and is measured by microresistivity tools. The deepest zone — well beyond the invasion front — contains undisturbed native formation fluids and is measured by deep resistivity tools. Between these extremes, the transition zone contains a mixture of filtrate and formation fluids at a composition that varies with radial distance from the wellbore.

The medium induction log measures this intermediate zone. Its depth of investigation is calibrated to sample the formation at radial distances where the invasion profile's intermediate resistivity values are found — not the extreme shallow reading of the flushed zone, nor the far-field reading of the undisturbed formation, but the gradient in between. The information this intermediate measurement contains is what makes it valuable: it allows the three-curve induction log interpreter to characterize the shape of the invasion profile and extract the true undisturbed formation resistivity that would be needed even if the deep induction alone were available.

In practice, the medium induction log is most diagnostic in wells with moderate invasion depth — where the invasion front is within the medium tool's radial sensitivity range and both the medium and deep tools see distinguishably different resistivities. In wells with very shallow or very deep invasion, the three-curve diagnostic power is reduced, and additional measurements or modeling are needed for reliable invasion correction.

Medium Induction in Log Interpretation Workflows

Integrated petrophysical interpretation using the medium induction in combination with deep induction and shallow resistivity follows a sequential workflow: first, apply borehole size and mud weight corrections from standard charts; second, apply shoulder-bed corrections to remove the vertical resolution differences between the medium and deep curves at formation boundaries; third, use the corrected ILM/ILD ratio at each depth level with the tornado chart (invasion correction chart) to determine the invasion diameter and the correction factor Rt/ILD; fourth, apply the correction factor to the deep reading to obtain the invasion-corrected true resistivity Rt; fifth, use Rt in the Archie water saturation equation with the appropriate Rw, porosity, and cementation exponent values to calculate water saturation for each formation interval; the medium induction measurement is essential to step three and four, providing the degree-of-invasion information that makes the Rt determination possible without requiring additional resistivity data types.

Quality control of the medium induction reading requires verifying that it behaves physically reasonably relative to the deep induction — at depth intervals where the gamma ray and density-neutron logs indicate clearly clean, non-invaded rock (tight streaks with no fluid entry), ILM should approximately equal ILD with no separation; at permeable intervals where invasion is expected, ILM may diverge from ILD in a direction consistent with the expected mud filtrate versus formation water salinity contrast; sudden large separations between ILM and ILD in tight, low-porosity zones that cannot have significant invasion are artifacts of borehole irregularity, tool eccentering, or formation heterogeneity and should be flagged as potentially unreliable data in the petrophysical interpretation workflow.

Medium Induction Across International Jurisdictions

Canada (AER / WCSB): WCSB formation evaluation from the 1970s through 1990s established a large database of dual induction logs with ILM and ILD curves in conventional Devonian carbonate, Cretaceous sandstone, and Mississippian carbonate formations that continues to be used for infill well evaluation, pool production review, and EOR project planning; AER regulations require formation evaluation logs to be submitted to the WDMS for all wells, and the ILM curve is part of the standard log data package archived in the WDMS for wells logged with dual induction tools; reprocessing of legacy WCSB induction logs using modern invasion correction algorithms applied to the stored ILM and ILD digital data has improved the accuracy of Rt estimates and water saturation calculations for formation evaluation studies conducted decades after the original logging programs.

United States (API / BSEE): Gulf Coast and Mid-continent formation evaluation from the 1960s through 1990s produced an enormous database of dual induction logs with ILM and ILD curves that forms the historical record for resource assessment in Tertiary and Cretaceous clastic formations in these basins; state regulatory databases (Texas RRC TexNet, Louisiana LDNR, Mississippi MDEQ) and BOEM's GoM database archive these legacy logs, which are actively used in basin-scale formation evaluation studies, resource assessment, and CO2 EOR feasibility analysis for mature fields; API publication API RP 31G (Enhanced Recovery Log Analysis) and the SPE Applied Petroleum Reservoir Engineering textbook reference ILM/ILD invasion correction as a standard procedure in its formation evaluation workflow chapters.

Norway (Sodir / NORSOK): NCS Jurassic Brent Group and Statfjord Formation evaluation from early North Sea development wells (Statfjord discovery 1974, Brent discovery 1971) used dual induction tools with ILM and ILD curves as the primary resistivity suite in the conductive low-salinity mud systems used in early NCS drilling; Sodir's DISKOS database archives digital log data including ILM curves from NCS exploration and appraisal wells, providing publicly accessible formation evaluation data that supports basin-wide NCS reservoir characterization studies by operators, universities, and research institutions; the NCS legacy induction log database is particularly valuable for North Sea deep exploration plays where the invasion profile data from historical wells constrains the formation water salinity and Rt estimates used in modern exploration risk assessment.