Invasion

Key Takeaways

• The invasion profile consists of three distinct zones that each have different fluid compositions and therefore different electrical resistivities measured by resistivity logging tools: the flushed zone (Rxo zone) is the innermost annulus where the drilling fluid filtrate has displaced nearly all original pore fluid, with the remaining saturation consisting almost entirely of filtrate and irreducible water plus any residual hydrocarbon saturation that the filtrate could not displace; the invaded zone (or mixing zone) is the transition region between the flushed zone and the uninvaded reservoir where both filtrate and original formation fluid coexist in varying proportions; and the uninvaded formation (Rt zone) beyond the invasion front where the original hydrocarbon and water saturations are undisturbed; the resistivity log reads a composite response of all three zones simultaneously, with shallow-reading tools (microresistivity or microspherically focused logs that read a few inches into the formation) dominated by the flushed zone response, medium-reading tools (8-inch to 40-inch arrays) averaging the invaded and transition zones, and deep-reading tools (induction logs or laterolog tools with deep investigation) providing a response weighted toward the uninvaded Rt zone; in high-permeability formations, the invasion front may extend dozens of feet from the wellbore, making even the deep-reading tools respond partially to the invaded zone rather than the true uninvaded formation.
• Water-based mud invasion into an oil-bearing formation creates an annulus of increased water saturation around the wellbore (because the oil is displaced by water-based filtrate), which increases the apparent conductivity (reduces the apparent resistivity) of the near-wellbore zone relative to the uninvaded oil-bearing formation: this invasion effect causes shallow resistivity tools to read lower than deep resistivity tools in oil-bearing formations invaded by water-based mud, a pattern called "positive invasion" (shallow reads lower, deep reads higher) that is the standard diagnostic for productive oil or gas sands invaded by water-base mud; the ratio of the deep to the shallow resistivity (Rt/Rxo) is used to diagnose the invasion profile and to estimate the hydrocarbon saturation using the Tornado chart or other invasion correction charts that determine the true formation resistivity Rt from the combination of shallow, medium, and deep resistivity measurements; the invasion correction removes the bias introduced by the contrast between the filtrate and the original formation fluid, allowing the true water saturation to be calculated from Rt using the Archie equation or other saturation models; in gas-bearing formations invaded by water-based mud, the gas (which has much higher resistivity than water) is displaced by the conductive filtrate, creating a dramatic reduction in near-wellbore resistivity relative to the uninvaded gas zone and a particularly strong positive invasion pattern that is a sensitive indicator of gas in the formation.
• Oil-based mud (OBM) invasion creates a different and more complex invasion effect than water-based mud invasion, because the OBM filtrate is a non-conductive hydrocarbon that when it invades a water-bearing formation displaces the conductive formation water and increases the near-wellbore resistivity, creating a "negative invasion" pattern (shallow reads higher than deep) that would normally be interpreted as the signature of a hydrocarbon-bearing formation but is actually a formation damage artifact from the OBM filtrate invasion: the diagnosis of OBM invasion in a water-bearing formation (negative invasion profile in what should be a water zone) is critical because it prevents the petrophysicist from incorrectly interpreting the high near-wellbore resistivity as oil saturation and incorrectly classifying a water zone as pay; in an OBM-drilled oil-bearing formation, the filtrate (oil-based) mixes with the reservoir oil, creating relatively little contrast between the invaded and uninvaded zones, and the invasion pattern is less diagnostic than in water-based mud drilling; OBM invasion also interferes with the measurement of formation water resistivity (Rw, needed for the Archie equation) because the OBM filtrate contaminates formation water samples collected by the MDT or RFT, requiring special measurement techniques (pumpout of the filtrate-contaminated zone before sampling, or the use of optical sensors to detect when clean formation water is flowing into the sample chamber rather than filtrate).
• Invasion depth estimation from multiple resistivity measurements is performed by log analysis software using iterative forward modeling and inversion of the resistivity tool responses: given measurements from multiple resistivity tools with different depths of investigation, the inversion computes the best-fit invasion model (typically a piston invasion model with a sharp step from Rxo to Rt at the invasion radius, or a more gradual transition zone model) that reproduces the observed tool responses simultaneously; the invasion radius from the inversion provides information about the formation permeability and the mud filtration efficiency — deep invasion (tens of feet into the formation) indicates high permeability or a poor mudcake with inadequate filtration control, while shallow invasion (a few inches) indicates low permeability or an excellent mudcake that quickly builds to limit further filtration; the invasion depth also affects the time required for the formation to return to its original saturation after casing is run (the natural imbibition that displaces the filtrate back toward the wellbore can take weeks to months in tight formations), which affects the interpretation of time-lapse resistivity measurements taken at different times after drilling; in tight formations with permeability below 0.1 millidarcy, invasion is so limited that the distinction between flushed zone and uninvaded zone may be imperceptible at the scale of the resistivity tool's investigation, and the resistivity logs essentially read the undisturbed formation resistivity without invasion correction being necessary.
• Formation damage from invasion occurs when the drilling fluid filtrate is chemically incompatible with the reservoir minerals or reservoir fluids, causing clay swelling (montmorillonite clay in the formation swells when contacted by fresh or low-salinity filtrate, reducing permeability), fines migration (clay particles mobilized by the filtrate flow plug pore throats in the near-wellbore zone), emulsion formation (if filtrate and reservoir oil combine in a stable emulsion that blocks pore throats), or scale precipitation (if the filtrate mixing with formation water creates oversaturation with respect to mineral scales such as calcium carbonate or barium sulfate); these invasion-related formation damage mechanisms reduce the permeability of the flushed zone relative to the uninvaded zone, which affects both the production performance of the well (the skin factor from invasion damage increases the pressure loss in the near-wellbore zone beyond what the Darcy flow equations predict for undamaged rock) and the log interpretation (because the permeability reduction slows the equilibration of the saturation profile and may lead the petrophysicist to underestimate the invasion depth); the selection of drilling fluid formulation (salinity, density, filtration control additives) to minimize invasion-related formation damage is an important element of the well design process, particularly for horizontal wells drilled through the producing interval where the long exposure length and large wellbore-formation contact area creates a larger invasion zone than in vertical wells, and where the invasion damage has a proportionally larger impact on well productivity.