eccentricity, Oil & Gas Glossary

What Is Eccentricity in Well Logging?

Eccentricity in well logging is the off-centre displacement of a logging tool from the borehole axis, quantified as the radial distance from the tool's geometric centre to the borehole wall, which degrades measurements that require uniform sampling of the formation around the tool, particularly affecting induction resistivity, density, and neutron porosity logs in enlarged or non-circular wellbores.

Key Takeaways

Most wireline tools hang eccentred against the low side of the borehole due to gravity in deviated wells.
Density and neutron pad tools are designed to be pressed against the borehole wall (zero eccentricity) to maintain formation contact.
Induction and resistivity tools are designed to be centred; eccentricity increases the borehole signal and distorts the measurement.
In enlarged boreholes (washouts), all tools are eccentred by definition because the hole is larger than the centraliser.
NMR tools are particularly sensitive to eccentricity because their measurement volume is a thin cylindrical shell at a precise radial distance.

How Eccentricity Affects Wireline Measurements

Different logging tools respond to eccentricity in different ways depending on their measurement physics and geometry. Induction and deep resistivity tools are designed to be centred in the borehole so that the electromagnetic signals average uniformly around the circumference of the borehole. When an induction tool is eccentred, one side of the measurement coil array is closer to the formation and the other closer to the conductive drilling mud; the resulting measurement is biased toward the properties of the mud in the gap between the tool and the formation on the open side. In very conductive mud with a highly resistive formation, this eccentricity bias can cause the induction log to read a formation resistivity 20-40% lower than the true value.

Density and neutron pad tools are designed to be pressed against the formation by a bow-spring or pad system that eliminates standoff. When hole enlargement causes the pad to lose formation contact, the standoff gap fills with mud and the density reads the density of a mixture of mud and formation rather than formation alone, biasing bulk density toward mud density (typically 1.0-1.2 g/cm³ lower than formation density). This standoff effect on density is detected by the photoelectric correction (Pe short-long count rate ratio), which shows anomalous values when the pad is not in formation contact.

Eccentricity Effects Across International Jurisdictions

In Canada, eccentricity effects are a significant concern in WCSB horizontal wells where gravity causes the drill string and subsequently the logging tool to lie on the low side of the wellbore throughout the lateral section. AER well logs submitted in support of pool establishment under Directive 065 must have quality control documentation; density and neutron logs affected by standoff in the horizontal section are flagged in the log header and quality control report, and alternative porosity measurements (NMR, sonic) may be substituted where density is unreliable.

In the United States, eccentricity-related log degradation in Gulf of Mexico deepwater wells with enlarged boreholes from reactive shale swelling is a common log quality concern. BSEE reserve submissions using log-derived porosities from degraded density logs must document the correction methodology applied or justify using alternative measurements. In Norway, NCS well operations on the Halten Terrace frequently encounter borehole enlargement in the Åre Formation coals and organic shales; Sodir's well data quality standards require that eccentricity-affected log intervals be flagged in the header and that environmental corrections including standoff corrections be documented for all submitted density and neutron logs. In the Middle East, Saudi Aramco's Arab Formation wells encountering vugs and cavernous porosity may have irregular borehole shapes that create unpredictable tool eccentricity even in near-vertical wells, requiring repeat passes or alternative measurements to characterise vuggy porosity accurately.

Fast Facts

The NMR logging tool is uniquely sensitive to eccentricity because it measures proton signals from a thin cylindrical shell at a specific distance (typically 3-4 cm) from the tool body defined by the resonance condition of the static magnetic field. If the tool is eccentred by even 5-10 mm, the resonance shell intersects the borehole mud on one side rather than the formation, and the NMR signal includes mud hydrogen (which may have very different T2 relaxation times from formation fluids). Most modern NMR tools include centralising systems specifically designed to minimise eccentricity because even small displacement dramatically changes the measurement volume composition.

Eccentricity Correction Methods

Eccentricity corrections are applied for measurements where the tool position is known and the tool's response function in an eccentred geometry has been characterised. The caliper log provides borehole diameter; the tool position (whether centralised or pad-pressed) determines whether a correction is needed. For induction logs run without centralisers in large washout zones, eccentricity corrections use the tool's sensitivity function and the known borehole fluid resistivity to calculate the fraction of the signal attributable to borehole mud and subtract it from the total measurement. For density pad tools, the Pe correction signal provides a real-time quality indicator of pad contact; where Pe is anomalous, the density measurement is flagged and corrected or substituted. In complex borehole geometries where analytical corrections are insufficient, numerical forward modelling of the tool response in the specific eccentricity geometry provides more accurate environmental corrections.

Tip: In horizontal well logging, run a short-arm caliper alongside any pad-type density tool to confirm that the pad is maintaining formation contact throughout the lateral section. When the caliper indicates borehole diameter greater than the pad spring force range (typically 20-25% above bit size is the practical limit), the density log is unreliable regardless of the Pe correction applied. At those depths, use the sonic log or NMR tool for porosity rather than corrected density, and flag the density interval as quality-controlled out in your petrophysical interpretation workflow documentation.

Eccentricity in well logging is also referenced as:

Tool standoff — the specific radial gap between the tool body and the borehole wall when the tool is not in contact with the formation; standoff and eccentricity describe the same geometric condition from different reference perspectives
Decentralisation — used in some technical papers to describe the condition where a tool designed to be centralised is displaced from the borehole axis; contrasted with centralised tool positioning
Off-centre displacement — the descriptive geometric term used in quantitative eccentricity correction calculations where the exact radial displacement must be specified for the correction model

Frequently Asked Questions

How can eccentricity be minimised during wireline logging?

Eccentricity can be minimised through tool string design and borehole quality. Centralising bow springs or roller centralisers on induction and resistivity tools maintain the tool near the borehole axis in vertical and low-angle wells. In deviated wells above approximately 40 degrees, centralising tools are less effective because gravity pulls the tool string to the low side regardless of the spring force, and pad tools must be verified to be on the correct (low) side of the hole to sample formation rather than mud. Maintaining borehole gauge (avoiding washout) through mud weight and chemistry management is the most effective long-term approach, because in-gauge boreholes allow centralising tools to work as designed and allow pad tools to maintain continuous formation contact.

Does eccentricity affect all logging tools equally?

No. Different tools have very different sensitivities to eccentricity depending on their measurement physics and geometry. Gamma ray logs are relatively insensitive to eccentricity because natural gamma rays are emitted from the formation in all directions and the detector integrates the signal from the entire annular volume around the tool. SP logs are similarly relatively robust to eccentricity because the potential is a property of the formation-mud interface rather than a radially selective measurement. Induction and deep resistivity logs are moderately sensitive to eccentricity in conductive mud environments. Density pad logs and NMR tools are highly sensitive because they measure only the formation volume immediately adjacent to the tool face, and any standoff gap dramatically changes what is being measured.

Why Eccentricity Matters in Oil and Gas

Accurate porosity and resistivity measurements from wireline logs are the inputs to water saturation calculations that determine net pay and reserves in every conventional and many unconventional reservoir evaluations. Eccentricity-induced errors in these measurements propagate directly to errors in calculated water saturation and net pay thickness. In horizontal wells that now constitute the majority of new development wells in tight formations, eccentricity is not an occasional complication but a systematic characteristic of the logging environment throughout the lateral section. Operators who do not account for eccentricity effects in their horizontal well log interpretation routinely overestimate net pay in washout sections where density underestimates porosity, or underestimate water saturation in enlarged boreholes where induction resistivity is biased low. Understanding and correcting for eccentricity is therefore a non-optional element of rigorous petrophysical practice in the era of horizontal well drilling.

Eccentricity (Well Logging): Definition, Tool Standoff, and Measurement Effects