Microspherical Log

Key Takeaways

• Spherical focusing is the electrode geometry principle that gives the microspherical log its name and its ability to provide accurate Rxo measurements in formations with varying mudcake thickness — the MSFL electrode pad contains a central button electrode (A0) surrounded by concentric focusing rings (A1 and A2) that are supplied with bucking current held at the same potential as A0 by a servo-amplifier feedback circuit; this spherically focused geometry channels the measurement current from A0 into a hemispherical volume of formation directly ahead of the pad, preventing the current from traveling preferentially through the low-resistance mudcake (which would cause the device to read mudcake resistivity rather than formation resistivity); the spherical focusing provides a shallow radial depth of investigation of approximately 3 inches (75 mm) into the formation, which means the MSFL reads formation resistivity in the zone that has been invaded by mud filtrate and from which the original hydrocarbons have been flushed; for fresh mud filtrate (Rmf greater than Rw), the flushed zone contains mostly filtrate with residual oil saturation Sor, giving a higher resistivity signal that allows estimation of residual hydrocarbon saturation when compared against the virgin zone deep resistivity Rt.
• Mudcake correction is required for accurate Rxo determination from the MSFL because the pad presses against the mudcake surface rather than directly against the formation rock, and the mudcake (which may be 3 to 10 mm thick in water-base mud systems with moderate overbalance) acts as an additional resistive barrier in series with the formation resistivity measured by the pad; the standard mudcake correction uses the MSFL apparent resistivity reading, the caliper-derived borehole size, the mud resistivity Rm, and a chart (Schlumberger Chart Rmce-1 or equivalent service company chart) to calculate the corrected Rxo value; for thin mudcakes (less than 4 mm) and medium-resistivity formations (Rxo between 2 and 100 ohm-m), the mudcake correction is typically less than 10 to 15 percent and is applied automatically by logging acquisition software; for thick mudcakes (greater than 8 mm) in high-overbalance mud systems (overbalance greater than 500 psi) or for very resistive formations (Rxo greater than 200 ohm-m), the mudcake correction can exceed 30 percent and the corrected Rxo may have significant uncertainty that limits the precision of flushed-zone saturation calculations.
• Tornado chart invasion correction uses the MSFL-derived Rxo in combination with the medium induction (ILM) and deep induction (ILD) readings to correct both resistivity measurements for the presence of the invaded zone and solve for the true formation resistivity Rt — the tornado chart (named for its shape, with curves spreading from a central point like a tornado outline) is a graphical or computational solution to the three-zone invasion model (mud filtrate zone at Rxo, transition zone, and virgin zone at Rt) with a specific invasion diameter; the MSFL ratio (MSFL/ILD) is one axis of the tornado chart, the ILM/ILD ratio is the other axis, and reading the intersection of these two ratios on the chart gives the Rt/ILD correction factor and the invasion diameter; without the MSFL input to the tornado chart, only a two-curve invasion correction is possible (which assumes a fixed Rxo/Rt ratio) and is substantially less accurate for variable invasion profiles encountered in laminated or thinly bedded formations; the MSFL's contribution to invasion correction is the reason it is run as a standard add-on sensor to virtually every resistivity logging run — the Rxo data it provides enables a more accurate Rt and therefore a more accurate water saturation throughout the logged interval.
• Residual oil saturation estimation from MSFL and deep resistivity comparison is a key application in secondary and tertiary recovery planning — in a formation that has been invaded by mud filtrate (which displaces original hydrocarbons from the pore space near the borehole), the flushed zone saturation Sxo measured from the MSFL Rxo using the Archie equation (Sxo = sqrt(F × Rmf / Rxo) where F is the formation factor) represents the water saturation after flushing, and the moveable oil fraction is approximated as 1 - Sxo - Sor where Sor (residual oil saturation, the hydrocarbon that cannot be displaced by water invasion) is estimated from the difference between virgin zone water saturation Sw and Sxo; the moveable oil index (MOI = 1 - Sw/Sxo) provides a quick-look indicator of how much of the total oil in place can be produced by conventional water flooding — a MOI close to 1 indicates highly moveable oil (Sw approximately equals Sxo, meaning flushing has completely swept the zone just as efficiently as water flooding will in the reservoir), while a MOI close to 0 indicates poorly moveable oil (high residual saturation or poor formation permeability to two-phase flow).
• MSFL limitations in rugose and caved boreholes arise because the pad cannot make full contact with a highly irregular borehole wall, resulting in gaps between the pad face and the formation that are filled with drilling mud and cause the measurement to read anomalously low resistivity (close to mud resistivity) rather than formation resistivity; formation evaluation in caved or washed-out boreholes (often evident on caliper logs as borehole diameters well above bit size) should treat MSFL data in those intervals with caution and crosscheck against the microlog (which shows separation between micro-normal and micro-inverse curves when mudcake is present, confirming formation contact) or the shallow laterolog (SFLU) from the dual laterolog tool, which has a deeper investigation that is less sensitive to poor pad contact; borehole rugosity in hard rock formations (carbonates, tight sands) rather than mudcake buildup in soft formations is the primary source of MSFL data quality problems in carbonate plays, and intervals with caliper readings greater than bit size plus 1 inch should be flagged as potentially unreliable for Rxo-based saturation calculations.