Resistive Invasion

Key Takeaways

• Resistive invasion diagnosis from the multiple resistivity tool response uses the pattern of shallow, medium, and deep resistivity readings to identify whether invasion is resistive, conductive, or negligible, and to estimate the invasion profile for application of the invasion correction charts (Tornado charts) that correct the deep tool reading to the true formation resistivity: in a resistively invaded formation, the shallow laterolog (LLS) or micro-spherically focused log (MSFL) reads higher than the medium laterolog or medium induction log, which reads higher than the deep laterolog (LLD) or deep induction log (ILD), producing the characteristic "decreasing with depth of investigation" pattern that is diagnostic of resistive invasion (Rxo greater than Ri greater than Rt, where Ri is the transition zone resistivity between the flushed and uninvaded zones); in a conductively invaded formation, the pattern is reversed (shallow tool reads lower than medium, which reads lower than deep), confirming that the flushed zone is more conductive than the uninvaded formation; the separation between the three-curve set (LLS, LLM or MRIL medium, LLD or ILD deep) is used with the Tornado chart to simultaneously determine both the true formation resistivity Rt and the invasion diameter (the depth of invasion from the wellbore wall to the invasion front) by finding the Rt and di combination that correctly predicts all three observed tool readings given the tool response functions for each depth of investigation.
• Resistive invasion in oil-bearing formations requires careful interpretation because the resistivity pattern of resistive invasion (higher shallow resistivity, lower deep resistivity) is similar to the resistivity pattern expected at the transition from an oil-bearing invaded zone to the water-bearing uninvaded zone (where the oil-wet flushed zone has high resistivity and the water-bearing formation below the oil-water contact has lower resistivity), creating the potential for misidentification of a water-bearing formation with resistive invasion as an oil-bearing formation without invasion: the diagnostic distinction between resistive invasion in a water-bearing formation and genuine hydrocarbon presence requires comparing the invasion pattern (which predicts a specific invasion-depth-dependent set of responses for the three tools that follows the Tornado chart relationships) with the hydrocarbon response (which predicts Rxo lower than Rt, because oil in the uninvaded zone is more resistive than the fresh filtrate that has displaced some of the oil in the flushed zone), with the key observation that resistive invasion of a water-bearing sand produces Rxo greater than Rt while a hydrocarbon-bearing formation produces Rxo less than Rt when fresh mud is used and a significant fraction of the original oil remains in the flushed zone as residual oil saturation; the movable oil saturation indicator (the Rmf/Rw ratio combined with Rxo/Rt) quantifies both the presence of invasion and the original versus residual hydrocarbon saturation, allowing a more definitive determination of whether the high resistivity contrast reflects invasion of a water-bearing formation or genuine oil presence.
• Resistive invasion depth and profile estimation from the multi-tool resistivity data is complicated by the non-uniqueness of the invasion inversion problem, where different combinations of invasion diameter and transition zone width can produce similar apparent resistivity readings in the three tools, requiring that the invasion interpretation be constrained by additional information including caliper log, mudcake thickness, and the known mud filtrate and formation water resistivities from the log header and water analysis: a sharp-front invasion model (assuming an abrupt transition from flushed zone to uninvaded zone with no gradual transition) is the simplest invasion model and is used with the standard Tornado charts, but may be inadequate in formations with variable permeability where the invasion front advances at different rates in different layers, creating a diffuse invasion profile (the annulus model or step-profile model) that requires more complex inversion methods; the annulus invasion model (where a zone of intermediate resistivity exists between the flushed zone and the uninvaded zone due to the mixing of displaced formation water with advancing filtrate) produces a resistivity maximum at an intermediate depth between the borehole wall and the invasion front, a pattern that can cause the medium-depth resistivity tool to read higher than either the shallow or deep tool, which is the opposite of the simple resistive invasion pattern and requires the annulus invasion correction rather than the standard Tornado chart approach.
• Resistive invasion implications for reservoir productivity evaluation use the contrast between flushed zone and uninvaded zone resistivity to estimate the movable hydrocarbon saturation, which is the fraction of the original oil in place that can potentially be produced by primary recovery mechanisms: the movable oil calculation from resistivity invasion analysis compares Sxo (the water saturation of the flushed zone, calculated from Rxo using the Archie equation with Rmf in place of Rw) with Sw (the water saturation of the uninvaded formation, calculated from Rt using the Archie equation with Rw); the difference Sxo minus Sw represents the fraction of the pore space whose original oil has been mobilized and at least partially displaced by mud filtrate, providing an estimate of the displacement efficiency of fresh water in the reservoir and a lower bound on the movable oil saturation (with the true movable oil being at least as high as the filtrate-displaced fraction shown by the invasion analysis); in formations with high Sxo and low Sw (resistive invasion pattern with high contrast between shallow and deep tool readings), the movable oil estimate is large and the reservoir has good producibility; in formations with similar Sxo and Sw despite resistive invasion (both high), the reservoir may contain only residual oil that cannot be produced by primary recovery, with the similarity in the two saturations despite the filtrate invasion indicating that the original oil was already at or near residual saturation when the well was drilled.
• Resistive invasion in naturally fractured formations presents a special interpretation challenge because the fractures provide preferential flow pathways for mud filtrate invasion that can produce an invasion profile that is much deeper in the fractured intervals than in adjacent tight matrix intervals, creating a depth-of-investigation-dependent resistivity response that reflects both the invasion geometry and the fracture permeability rather than the simple matrix invasion model assumed in the standard Tornado chart approach: in a naturally fractured carbonate or tight sandstone, the fractures can transmit mud filtrate to radial distances of several meters from the wellbore within hours of drilling through the formation, while the adjacent tight matrix may be invaded only a few centimeters in the same time period, creating a bimodal invasion profile with deeply invaded fractures surrounded by shallowly invaded matrix; the deep laterolog may still read a high apparent resistivity in this situation (because the fracture porosity is a small fraction of the total porosity and the resistivity contribution of the fracture-transmitted filtrate is averaged with the more resistive tight matrix), while the shallow laterolog reads a lower apparent resistivity (because the shallow measurement volume contains a higher fracture density near the borehole where the fractures are more open and less healed than at greater depth); the apparent reversal of the expected resistive invasion pattern (where the deep tool reads higher than the shallow tool in the fractured interval) can be misinterpreted as conductive invasion or hydrocarbon presence if the fracture-controlled invasion geometry is not recognized from the image log or the borehole caliper data.