Surrounding Bed

Key Takeaways

• Vertical resolution and the surrounding bed influence are directly linked by the design of each logging tool: the vertical resolution of a logging measurement is defined as the minimum bed thickness at which the tool's measurement at the center of the bed reaches 90 percent (or another specified fraction) of the true bed value, with surrounding beds contributing the remaining 10 percent (or specified fraction) of the measurement; gamma ray tools (which measure natural radioactivity using a Geiger counter or scintillation detector in a 6 to 12 inch detector housing) have a vertical resolution of approximately 1 foot (0.3 meter) for wireline logs and 2 to 3 feet (0.6 to 0.9 meters) for LWD tools (which average the measurement over the distance traveled during the integration time); standard resistivity tools (including dual laterolog, dual induction, and array induction tools) have vertical resolutions ranging from 2 feet (0.6 meters) for the shallow focused measurement to 8 feet (2.4 meters) or more for the deep-reading measurements; density tools (borehole compensated density using 60-cm source-to-detector spacing) have a vertical resolution of approximately 1 foot (0.3 meter); understanding these resolution limits is essential for correctly interpreting log-derived properties in formations with bed thicknesses near or below these limits, where the measurement is significantly influenced by the surrounding beds rather than being representative of the target bed alone.
• Resistivity surrounding bed correction is particularly important for formations with significant resistivity contrast between the target bed and the enclosing beds: a thin (less than 10 feet) resistive sand surrounded by conductive shale will have its deep resistivity measurement suppressed below the true sand resistivity because the tool's volume of investigation overlaps the conductive shale on both sides; conversely, a thin conductive (water-bearing) sand surrounded by resistive carbonates will have its true resistivity boosted by the surrounding carbonate contribution, potentially masking the water zone and causing a false hydrocarbon indication; the surrounding bed correction charts (published by tool manufacturers including Schlumberger Chart Book, Halliburton Log Interpretation Charts, and Baker Hughes Reference Manual) provide correction factors as a function of bed thickness, contrast between target bed and shoulder bed resistivities, and the specific tool design (DLL deep/shallow laterolog, ILD/ILM dual induction); modern software-based surrounding bed corrections (using inversion processing of full array resistivity or array induction data, the approach incorporated in tools such as Schlumberger's AIT Array Induction Tool and Halliburton's HDIL High-Definition Induction Log) can deliver bed-by-bed resistivity profiles with vertical resolution of 1 to 2 feet from logs that have 8-foot nominal resolution, by inverting the full waveform data from multiple receiver spacings to recover the true formation resistivity profile.
• Thin-bed petrophysics in laminated reservoirs (where the individual sand laminae may be 1 to 30 cm thick interbedded with shale laminae at the same thickness scale) requires special evaluation techniques because standard wireline tools cannot resolve the individual sand and shale laminae; instead, the tools measure the bulk (averaged) properties of the laminated sequence, which are intermediate between the sand properties and the shale properties and depend on the lamina thickness distribution and the sand-shale volume fraction (net-to-gross, NTG); Thomas-Stieber analysis uses the density-neutron crossplot and the density-resistivity crossplot to estimate the sand fraction and the shale distribution (laminar versus dispersed versus structural) in the unresolved laminated sequence; high-resolution image logs (FMI, OBMI, XRMI micro-resistivity image tools) that have vertical resolution of 2 to 5 mm can resolve individual sand and shale laminae in thin-bed sequences, providing a direct measurement of the NTG and the lamina thickness distribution that the surrounding-bed-affected bulk logs cannot resolve; the image log NTG is used to upscale the bulk log properties to the individual sand (or shale) properties by removing the surrounding bed averaging effect algebraically, enabling accurate Sw and permeability calculations for the sand laminae alone.
• Nuclear magnetic resonance (NMR) tool response in thin beds is affected by surrounding beds through a different mechanism than resistivity or density tools: the NMR measurement (which records the relaxation of hydrogen proton spin signals after a radio frequency pulse) integrates the signal from the formation volume within the sensitive volume of the tool (a thin shell at a radius of approximately 10 cm from the tool face), and the vertical integration length of that sensitive volume determines the NMR tool's vertical resolution (typically 1 to 2 feet for standard wireline NMR tools); in thin-bed sequences, the NMR T2 distribution measured in the center of a thin sand bed includes contributions from the surrounding shale through the vertical integration, which add a fast-decaying T2 component (from clay-bound water in the shale) to the T2 distribution that would otherwise show only the free fluid T2 of the sand; this shale contribution from the surrounding beds causes the NMR-derived clay-bound water volume in the sand to be overestimated, the NMR-derived total porosity (phi_NMR = integral of T2 spectrum) to be inflated beyond the true sand porosity, and the free fluid index (FFI = integral of T2 above the T2 cutoff) to underestimate the movable fluid saturation in the sand, all of which can lead to pessimistic reserve estimates and incorrect pay identification if the surrounding bed NMR correction is not applied.
• Surrounding bed effects in horizontal wells are geometrically different from those in vertical wells because the wellbore intersects the bed boundaries at a low angle (near-parallel to the beds rather than perpendicular to them in vertical wells), causing the tool to see the surrounding beds over a much longer apparent depth interval than in a vertical well drilled perpendicular to the beds; in a horizontal well drilled through a 1-meter sand bed dipping at 2 degrees from horizontal, the apparent bed thickness seen by the well (measured along the wellbore axis) is approximately 1 meter / sin(2 degrees) = approximately 29 meters, so the logging tool passes through 29 meters of wellbore length within the sand before encountering the bounding shale; the long apparent thickness of the sand in the horizontal well means that the deep resistivity measurement reaches the true sand resistivity for most of the sand interval, with surrounding bed effects limited to the transition zones near the bed boundaries; in contrast, for a thin (0.3 meter true thickness) sand bed in the same geometry, the apparent thickness is only 8.6 meters and the surrounding bed effect is significant throughout the sand interval, requiring surrounding bed corrections similar to those used in vertical wells to recover the true sand resistivity.