Diameter of Invasion: Definition, Mud Filtrate Invasion, and Resistivity Log Correction

What Is the Diameter of Invasion?

The diameter of invasion (di) is the radial extent of drilling fluid filtrate penetration into a permeable formation around the borehole, measured from the centre of the borehole to the outer boundary of the invaded zone where formation water is substantially replaced by mud filtrate, and used as a key parameter in resistivity log environmental corrections, saturation calculations, and quantitative log analysis workflows.

How Invasion Diameter Develops During Drilling

When a permeable formation is penetrated by the drill bit, the hydrostatic pressure of the mud column in the wellbore is typically maintained above formation pore pressure to prevent inflow. This overbalance pressure drives mud filtrate (the liquid phase of the drilling fluid after mud cake forms on the borehole wall) radially into the formation. The filtrate displaces the original formation fluids — connate water or hydrocarbons — outward from the borehole wall. The radial extent to which filtrate penetrates before the process reaches equilibrium (when mud cake hydraulic resistance limits further filtrate loss) defines the diameter of invasion.

Several factors control how large the invasion diameter grows. Formation permeability is the primary factor: high-permeability formations accept more filtrate volume per unit time, leading to larger invasion diameters, while tight formations may show di values close to the borehole diameter because very little filtrate enters. Mud filtrate loss rate (measured by API fluid loss test) controls the volume available for invasion. The time the formation is exposed to the mud column before logging (exposure time) determines how long invasion progresses; formations logged shortly after drilling may have smaller di than formations logged days later as invasion continues. Finally, the wettability of the formation affects how efficiently filtrate displaces oil or water from the pore system.

Diameter of Invasion Applications Across International Jurisdictions

In Canada, invasion diameter estimation is a routine part of petrophysical analysis for WCSB reservoir evaluation, where the presence of freshwater filtrate invasion in saltwater-bearing formations creates a characteristic low-resistivity invaded zone that must be corrected before true formation resistivity can be calculated for water saturation. AER pool evaluation submissions routinely include tornado chart corrections for invasion effects when multi-depth resistivity tools are available; this is documented in the petrophysical methods section of the submission. Cardium and Viking oil wells that are logged with freshwater mud show characteristic invasion signatures where the shallow-reading induction reads lower resistivity than the deep induction in oil zones — the freshwater filtrate is more conductive than the native oil-saturated formation.

In the United States, invasion diameter corrections are standard in Gulf of Mexico log analysis for turbidite sand reservoirs where large invasion diameters from high-permeability sands complicate resistivity-based saturation calculations. BSEE formation evaluation requirements for OCS wells include the use of environmental corrections appropriate to tool type; invasion corrections satisfy this requirement for wells where multi-depth resistivity data is available. In Norway, Equinor's Brent Group sandstone reservoirs on the NCS commonly show invasion diameters of 40-80 inches due to the high permeability (100-3,000 mD) of producing sands, requiring deep-reading triaxial induction tools to read beyond the invaded zone for accurate Rt determination. In the Middle East, Arab Formation carbonate reservoirs invaded with oil-based mud filtrate show resistivity profiles that require careful invasion diameter analysis to distinguish OBM filtrate from native formation hydrocarbons.

Estimating Invasion Diameter from Multi-Depth Resistivity Logs

The primary method for estimating invasion diameter in a logged well uses the ratio between shallow-reading and deep-reading resistivity tools. When the invaded zone resistivity (Rxo) differs from the true formation resistivity (Rt) — which it does whenever mud filtrate salinity differs from formation water salinity — the shallow and deep resistivity tools read different values. By comparing multiple resistivity readings with different depths of investigation (for example, a microspherically focused log at 10-15 cm, a shallow induction at 30-60 cm, and a deep induction at 60-120+ cm), it is possible to model the radial resistivity profile and determine the di that best matches all three readings simultaneously. This inversion process is implemented in commercial petrophysical software using iterative forward modelling against a three-parameter model: Rxo, Rt, and di.

Diameter of Invasion Synonyms and Related Terminology

Diameter of invasion is also referenced as:

• di — the standard abbreviation used in petrophysical formulae and on log displays; appears as a parameter in resistivity tool response equations and invasion correction charts
• Invasion front — the boundary at diameter di where filtrate saturation drops abruptly to near zero; in an idealised step-profile invasion model, the invasion front is a sharp radial boundary
• Depth of invasion — an alternative phrasing used in some petrophysical textbooks; technically "diameter" is more precise because the measurement is a radial distance from borehole centre, but "depth of invasion" is used colloquially in some operator documentation

Related terms: invasion, flushed zone, resistivity, tornado chart, Rxo

Frequently Asked Questions

How does oil-based mud affect the interpretation of invasion diameter?

In oil-based mud wells, the mud filtrate is non-conductive oil rather than the conductive brine filtrate of water-based mud. When OBM filtrate invades a water-bearing formation, it replaces conductive formation water with non-conductive oil filtrate, increasing the resistivity of the invaded zone above the true formation resistivity — the opposite of freshwater WBM invasion. This can make water-bearing intervals appear as potential hydrocarbon zones on resistivity logs if invasion is not recognised. Conversely, in oil-bearing formations, OBM filtrate is similar in conductivity to native oil and creates minimal resistivity contrast between invaded and uninvaded zones, making invasion difficult to detect from resistivity log separation alone. In OBM wells, the microresistivity tools may not show the classic positive separation that indicates permeability in WBM wells, and invasion diameter must be estimated by other means or assumed based on formation permeability.

What is the relationship between invasion diameter and mud filtrate volume?

In a simple cylindrical geometry, the volume of mud filtrate per unit length of borehole that has invaded the formation equals the annular volume between the borehole wall and the invasion front: V = π/4 × (di² - dh²) × φ × (1-Swi), where di is the invasion diameter, dh is the borehole diameter, φ is the porosity, and Swi is the irreducible water saturation of the displaced formation. This relationship allows the cumulative filtrate loss volume to be estimated from the invasion diameter derived from resistivity logs, and conversely, the expected invasion diameter to be estimated from the API fluid loss measurement and exposure time before logging. For quality control, the estimated di from the resistivity profile should be consistent with the filtrate volume calculated from the mud log filtrate loss data.

Why Diameter of Invasion Matters in Oil and Gas

Accurate formation water saturation — the fundamental petrophysical measurement that determines whether a formation is productive or wet — depends entirely on using the true formation resistivity Rt, not the invaded zone resistivity Rxo, in the Archie equation. When invasion is significant and uncorrected, Rt can be over- or under-estimated by factors of 2-10, leading to corresponding errors in water saturation and hydrocarbon volume calculations. Over a large reservoir, a systematic invasion-correction error that understates Rt by a factor of 2 translates into a corresponding overstatement of Sw and understatement of oil saturation that could misclassify a commercial discovery as non-economic or vice versa. Understanding and correcting for invasion diameter is therefore not a technical formality but a fundamental requirement for reliable hydrocarbon in-place estimation that underpins every major investment decision made from wireline log data.