Induction Electrical Survey

Key Takeaways

• The geometric factor concept governs the depth of investigation of induction arrays and explains why different coil configurations provide different radial profiles into the formation: each infinitesimal annular shell of formation surrounding the borehole contributes to the total receiver signal in proportion to its geometric factor (which depends on the shape and relative orientation of the transmitter and receiver coils and the radius and thickness of the shell), with shells at different radii contributing differently to the received signal; simple two-coil induction tools have a broad geometric factor response function centered at moderate investigation depths, while multi-coil (focusing) arrays use additional coil pairs with opposite winding directions to shape the radial response and create a tool that reads primarily at a specific depth (the designed investigation depth) while minimizing the contribution from the borehole and shallow invasion zone; modern array induction tools (with 6 to 8 coil pairs at multiple spacings) can simultaneously provide 5 or 6 different depth-of-investigation curves from shallow (5 inches) to deep (90 inches), enabling full radial profiling of the invasion zone that replaces the traditional ILD-ILm-SFL combination with a continuously resolved radial resistivity profile.
• Skin effect in induction logging creates a systematic error in the resistivity measurement at high conductivities (low resistivities below approximately 1 ohm-m) because the electromagnetic field is attenuated (skin depth reduces) as it penetrates a highly conductive formation, causing the eddy currents in the far formation to be weaker than the skin-effect-free geometric factor prediction would suggest: the skin effect makes the induction tool read too low a conductivity (too high a resistivity) in high-conductivity formations, and the skin effect correction (a multiplicative factor applied to the raw conductivity reading) restores the correct formation conductivity for accurate resistivity determination; the skin depth (the distance into a conductive medium at which the electromagnetic field amplitude has decreased to 37 percent of its surface value) in a 0.5 ohm-m formation at 20 kHz is approximately 25 centimeters, meaning that the electromagnetic field is significantly attenuated before reaching the designed investigation radius of the induction tool, causing a substantial skin effect error that must be corrected for accurate resistivity in saline formations and high-porosity water zones; the skin effect correction is built into all modern induction tool signal processing and is applied automatically before the corrected conductivity is presented on the log.
• Tool selection between induction and laterolog (focused current) tools is governed by the resistivity contrast between the borehole fluid and the formation: induction tools perform best in air, oil, or fresh-water muds (where the borehole is highly resistive and the formation is less resistive than the borehole), because the induction measurement does not depend on current flowing through the borehole fluid; laterolog tools (which use focused electrode arrays to direct current through the formation) perform best in salt-saturated muds (where the borehole is highly conductive and the formation is more resistive than the borehole), because the focused current can pass through the conductive borehole and be focused into the formation by the guard electrodes; in wells drilled with KCl polymer mud (intermediate resistivity borehole fluid), either tool can be used with adequate borehole corrections, and the choice may depend on availability, service company preference, or specific formation resistivity range; the criterion for tool selection is the ratio of mud resistivity (Rm) to formation resistivity (Rt): if Rm/Rt is greater than 1 (mud more resistive than formation), use induction; if Rm/Rt is less than 0.1 (mud much more conductive than formation), use laterolog; if Rm/Rt is between 0.1 and 1, either tool may be acceptable depending on the specific formation and borehole conditions.
• Array induction tools have largely replaced the classic dual-induction log (ILD/ILm) since the 1990s, providing superior radial resolution through simultaneous multi-frequency or multi-spacing measurements that enable true radial resistivity profiling and better separation of Rxo (invaded zone resistivity) and Rt (true formation resistivity): the Schlumberger Array Induction Tool (AIT), Halliburton High-Definition Induction Logging (HDIL), and Baker Hughes High-Resolution Array Induction (HRAI) all use 6 to 8 receiving coil arrays at different spacings (6 to 90 inches) processed simultaneously to generate resistivity curves at defined radial depths that can be inverted for the invasion profile (Rm plus Rxo plus Rt plus invasion diameter); the invasion profile information from the array induction tool allows the petrophysicist to calculate Sw (water saturation from Rt) and Sxo (flushed zone saturation from Rxo) independently, and to use the movable hydrocarbon saturation (Sxo minus Sw) as an indicator of producible oil in the invaded zone; high-resolution array induction tools with 1-foot vertical resolution have also improved the detection and characterization of thin resistive beds (tight streaks, carbonates, or hydrocarbon-saturated thin sands) that were blurred by the poorer vertical resolution of the classic ILD tool.
• Induction log interpretation in shaly sands requires corrections for the conductivity contribution of clay-bound water (which conducts electricity at the interlayer water of clay minerals rather than in free pore water), with the Waxman-Smits or dual-water models providing more accurate resistivity-to-saturation relationships in clay-rich formations than the simple Archie equation: the Archie equation (Sw equals (a times Rw divided by (porosity to the power m times Rt)) to the power 1/n) was derived for clean (clay-free) formations where all pore water conductivity is from dissolved salts in the free pore water; in shaly sands, the clay contributes an additional conductance (the exchange cation conductance, CEC-based) that makes the formation more conductive than the Archie equation predicts from the free water salinity alone, causing the Archie equation to overestimate water saturation in clay-rich intervals; the dual-water model partitions the total water in the formation into free (mobile) water in the large pores and bound (immobile) water in the clay interlayers, each with its own resistivity, and calculates the total conductance of the formation from the sum of both contributions, providing a more accurate Sw estimate in shaly sands that the induction tool measures resistivity in.