Antisqueeze

Key Takeaways

• The antisqueeze effect is a focused-current measurement artifact unique to laterolog-type resistivity tools: Laterolog tools (including the dual laterolog, LLD/LLS, and array laterolog tools like the HDIL or HALS) operate by injecting current from a central electrode and using guard electrodes above and below to force that current to flow horizontally into the formation in a thin disc. The underlying assumption is that the injected current disc stays within the target formation and the measured voltage divided by the measured current gives the formation resistivity. When a high-resistivity thin bed is bounded by low-resistivity beds, however, the current disc cannot maintain confinement within the resistive target: electrons preferentially migrate along the path of least resistance into the flanking conductive beds, so the apparent current path samples a mixture of the high-R target and the low-R flanking beds. The resulting apparent resistivity falls between the true Rt of the target and the true Rt of the adjacent beds, but because the weighting favours current flow through low-R paths, the apparent reading is pulled toward the average of the system and ends up higher than the true Rt of the low-R adjacent beds but also (in the antisqueeze case) effectively higher than the true Rt of the target as measured in a thick-bed reference environment. This is a systematic, not random, bias that must be corrected by software-based inversion or borehole correction charts before the resistivity data are used in petrophysical evaluation.
• Correction methods for the antisqueeze effect include chart-based borehole corrections, software inversion, and multi-array resistivity analysis: The traditional correction method uses the Schlumberger or Halliburton correction charts (published in log interpretation chartbooks) that relate the ratio of apparent-to-true resistivity as a function of bed thickness, contrast ratio (R_target / R_adjacent), and borehole diameter. These charts were developed from forward modelling of the tool response in layered formation geometries and require the interpreter to first estimate the true Rt of the adjacent beds and the bed thickness from the gamma ray or density log before applying the correction factor to the raw laterolog reading. Modern array laterolog tools (5 to 7 radially spaced receivers) provide multiple depths of investigation in the same logging pass, and software inversion products (such as Baker Hughes TrueVertical or Halliburton AIT processing) use all array measurements simultaneously to solve for a layered resistivity model that best fits the observed multi-spacing data, including the antisqueeze geometry. For thin beds below 0.3 m (common in laminated carbonate sections), even array laterolog inversion has limited resolution and the petrophysicist must apply additional statistical net-pay corrections to account for sub-resolution heterogeneity.
• Failing to correct for the antisqueeze effect can result in false pay identification or overestimation of hydrocarbon column height: When a tight, cemented, water-saturated limestone stringer (true Rt = 200 ohm-m due to calcite cement, not hydrocarbon fill) sits between water-wet dolomite beds (Rt = 8 ohm-m), the antisqueeze effect can inflate the apparent laterolog reading of the limestone stringer to 350 to 500 ohm-m. Applying the Archie equation naively with this apparent Rt and assuming a porosity of 8 percent (from the density log), the calculated water saturation would be approximately 25 to 35 percent, which falls in the range that many petrophysicists would flag as movable oil. Cored intervals frequently reveal that the tight cemented limestone has no producible hydrocarbons and is simply a cementation zone where diagenetic calcite has filled the pore space, raising the resistivity of a water-wet rock to values normally associated with oil saturation. The antisqueeze misinterpretation is particularly dangerous in carbonate sections where tight limestone stringers alternate with porous dolomite at sub-metre scale and core may not be taken through every thin-bed cycle.
• The degree of antisqueeze bias is proportional to the resistivity contrast between the target and adjacent beds: Quantitative assessment of the expected antisqueeze magnitude for a given geological setting requires knowledge of the resistivity contrast ratio Rc = R_target / R_adjacent. At Rc = 5 (e.g., 50 ohm-m tight limestone in 10 ohm-m dolomite), the antisqueeze correction to the apparent laterolog reading is relatively modest (5 to 15 percent overcorrection in the apparent Rt depending on bed thickness), within the noise of typical petrophysical interpretation. At Rc = 50 (500 ohm-m limestone in 10 ohm-m dolomite), the correction can be 30 to 60 percent, which is large enough to shift the Sw calculation from the oil zone threshold to the water zone threshold. At Rc = 200 (2,000 ohm-m very tight cemented limestone in 10 ohm-m dolomite), the antisqueeze bias can make the apparent Rt two to three times the true Rt, completely masking the tight cemented nature of the interval and creating a convincing false pay signal that persists even after basic borehole environmental correction. Thin-bed correction charts are calibrated for contrast ratios up to about Rc = 100; above that, the uncertainty in the corrected value exceeds the value itself and the petrophysicist must rely on core data to ground-truth the interpretation.
• Induction-type resistivity tools do not exhibit the antisqueeze effect and provide a useful cross-check in thin-bed sections: Unlike laterologs, induction tools (array induction, high-definition induction, propagation resistivity) measure formation resistivity by electromagnetic induction, in which the transmitter coil generates an alternating magnetic field that induces eddy currents in the formation and the receiver coil detects the resulting secondary field. In conductive formations (Rt below about 50 ohm-m), induction tools have better vertical resolution and are less susceptible to thin-bed artifacts than laterologs. However, in resistive formations (Rt above 100 ohm-m), induction tools lose sensitivity and suffer from their own type of thin-bed bias (induction logs in highly resistive formations tend to read too low, which is the opposite sense from the antisqueeze effect). Comparing the LLD and LLS readings on the dual laterolog with the deep induction (ILD) reading from an induction tool run in the same well (as sometimes happens with tool relogging programmes) allows the petrophysicist to bracket the true Rt: if the laterolog reads significantly higher than the induction in a thin high-R bed, the antisqueeze effect is the probable cause. This comparison is a standard quality-check in Nisku carbonate petrophysics in Alberta.