6FF40
The 6FF40 is a specific induction logging tool array designation used by Schlumberger in the era of dual induction logging. The code describes the physical configuration of the tool: 6 coils in a focused-field arrangement, with a 40-inch (102-centimetre) spacing between the primary transmitter coil and the main receiver coil. This 40-inch spacing gives the 6FF40 a depth of investigation of approximately 1.5 metres (60 inches) into the formation, making it the deep-reading induction measurement in the Dual Induction Laterolog (DIL) tool suite. The 6FF40 was the standard deep-resistivity measurement in freshwater mud and oil-based mud wells from the late 1960s through the late 1980s, when array induction tools with multiple simultaneous depths of investigation replaced it. In historical well files from Alberta, British Columbia, and other Schlumberger-served basins during this period, Rt (true formation resistivity) used in water saturation calculations was typically taken from the 6FF40 channel on the DIL log.
Key Takeaways
- The 6FF40 designation breaks down as: 6 = six coils total in the array, FF = focused field (a specific coil geometry designed to minimize the contribution of the borehole and invaded zone to the measurement), 40 = 40-inch transmitter-receiver spacing. The focused coil arrangement bucks out (cancels) signals from the near-field borehole region and emphasizes the signal from the formation at the designed depth of investigation. Without this focusing, the borehole mud and the invaded zone (where drilling fluid has displaced formation fluids) would contribute significantly to the measured conductivity, making the apparent resistivity different from the true undisturbed formation resistivity.
- In the Dual Induction Laterolog (DIL) tool, the 6FF40 provided the deep induction measurement (ILd), while a shallower induction array (typically the 6FF28, with 28-inch spacing) provided the medium induction measurement (ILm), and a Laterolog-8 (LL8) provided a shallow focused-electrode measurement of the invaded zone (Rxo). By comparing ILd, ILm, and LL8, the log analyst could estimate the invasion profile and correct the ILd reading toward true Rt using tornado charts (graphical invasion correction plots derived from forward modelling).
- Skin effect is a signal distortion that degrades the accuracy of the 6FF40 at high formation conductivity (low resistivity). At formation conductivities above about 1,000 millisiemens per metre (resistivity below 1 ohm-metre), the primary electromagnetic field is partially absorbed before it reaches the full depth of investigation, causing the apparent resistivity to read lower than the true value. Schlumberger applied Doll's skin-effect correction algorithm to the 6FF40 output, but in very conductive formations the corrected reading could still be off by 10 to 20 percent. This limitation was one of the drivers for developing the more sophisticated array induction tools in the 1990s.
- In Alberta and British Columbia well records from the 1970s and 1980s, the 6FF40 channel is labeled ILd (Induction Log deep) or simply ILD on the log header. Well files from major Schlumberger runs in the Cardium, Viking, Belly River, and Devonian carbonate reservoirs in central Alberta during this period all used the 6FF40 as the primary deep resistivity measurement. Re-evaluating these old wells using modern petrophysics still starts with reading the ILd channel, which is the 6FF40 measurement even if never labeled as such on the log.
- The 6FF40 was superseded commercially by Schlumberger's Array Induction Sonde (AIT), introduced in the early 1990s. The AIT uses eight receiver coils at different spacings to simultaneously measure resistivity at five depths of investigation (10, 20, 30, 60, and 90 inches) plus a borehole correction coil. It delivers continuous invasion-corrected Rt without tornado chart look-ups and performs better in high-salinity muds and high-resistivity formations than the 6FF40. Competing array induction tools include Baker Hughes's HDIL (High-Definition Induction Log) and Halliburton's HRAI.
What the 6FF40 Measured and Why It Mattered
The goal of any resistivity log is to measure the electrical resistivity of the undisturbed formation — the rock as it sits in the ground before the wellbore disturbed it. This is called Rt (true formation resistivity). Rt is what goes into the Archie equation to calculate water saturation. Getting Rt right is therefore critical for determining whether a formation is oil-bearing or water-bearing.
The problem is that drilling fluid invades the formation. As the drill bit cuts through rock, hydrostatic pressure from the drilling mud column pushes mud filtrate into the permeable formation. This displaced formation fluid (brine or oil) by filtrate creates an invaded zone around the wellbore where the fluid content is different from the undisturbed formation. A resistivity tool reading near the wellbore measures the invaded zone (Rxo), not Rt. A tool reading deeper into the formation measures a mixture of invaded and uninvaded formation.
The 6FF40 was designed to read deep enough (1.5 metres) that in moderate invasion conditions, most of its signal came from the undisturbed formation. In practice, for shallow invasion profiles (where the filtrate has only penetrated 30 to 60 centimetres), the 6FF40 was close to Rt without correction. For deep invasion (where filtrate has penetrated 1 metre or more), the 6FF40 needed to be corrected using the invasion profile estimated from the comparison with the shallower ILm reading.
Fast Facts
Henri-Georges Doll of Schlumberger designed the original focused induction array in the late 1940s. The 6FF40 designation became the dominant configuration through internal Schlumberger research in the 1950s and was commercialized as part of the Dual Induction-Laterolog system in the mid-1960s. By 1970 the DIL with the 6FF40 deep array was the global standard for resistivity logging in freshwater and oil-based mud environments, and it remained so for two decades. Schlumberger's 1979 log interpretation chartbook contains the 6FF40 tornado charts used by a generation of petrophysicists. The tool's transition to the AIT system was essentially complete by the mid-1990s, but many Alberta wells in current production were logged with the 6FF40, and understanding its characteristics is necessary for re-evaluating those wells.
Reading 6FF40 Data in Old Well Files
When a petrophysicist encounters a well log from the 1970s or 1980s in the Alberta Energy Regulator (AER) well file system, the log header will show the Schlumberger tool designation. A DIL-LL8 run will have the ILd channel (deep induction, from the 6FF40), ILm channel (medium induction, from the shallower array), and the LL8 channel (Laterolog-8, shallow focused electrode).
To get Rt from ILd, the analyst first checks whether the well was drilled with freshwater mud or oil-based mud. In freshwater mud, the invaded zone has lower resistivity than the formation (filtrate dilutes the saline formation water), and ILd will read too low in invaded formations. In oil-based mud, no water invasion occurs, so the 6FF40 reads the formation directly without an invaded zone problem.
If invasion correction is needed, the ratio ILd/ILm gives a measure of invasion depth, which is entered into the 6FF40 tornado chart to read off the corrected Rt/ILd ratio. Multiplying ILd by this ratio gives the corrected Rt. In many cases for Viking or Cardium sandstones with moderate invasion, the correction is less than 15 percent and can be ignored for screening purposes. For Devonian carbonate targets with deep invasion from long drilling time, the correction can be 40 to 60 percent and must be applied.
Synonyms and Related Terminology
The 6FF40 measurement is labeled ILd or ILD (Induction Log deep) on well logs. Related terms include induction (the electromagnetic induction method of measuring formation resistivity; the 6FF40 is the most widely used specific induction array configuration in historical well logs; the physical principle involves inducing eddy currents in the formation with a transmitter coil and detecting them with receiver coils), dual induction (a logging tool that runs two induction arrays at different depths of investigation simultaneously; the classic Schlumberger DIL combined the 6FF40 deep array with a shallower medium induction array and a Laterolog-8 for invasion analysis), invasion (the displacement of formation fluids by drilling mud filtrate in the zone around the wellbore; invasion causes the 6FF40 ILd reading to differ from the true formation resistivity Rt, requiring correction using the tornado chart comparison between deep and medium induction readings), resistivity log (the general category of wireline measurements of formation electrical resistivity; the 6FF40 was the dominant deep-reading resistivity measurement for freshwater and oil-based mud wells from the 1960s through the 1980s), and Archie equation (the empirical relationship between water saturation, porosity, resistivity, and fluid properties; the 6FF40 ILd reading provides the deep formation resistivity Rt that is the key measured input to the Archie water saturation calculation).
How Ignoring a 6FF40 Skin-Effect Error Misclassified a Belly River Sandstone in Alberta
A company was reviewing old well files in the Hanna area of central Alberta as part of a regional Belly River Formation evaluation. One well, drilled in 1974 with freshwater mud, showed an ILd reading of 0.9 ohm-metres in a 6-metre interval that the original log analyst had classified as water-bearing (the Archie calculation at ILd = 0.9 ohm-metres gave Sw = 85 percent using Rw = 0.3 ohm-metres and phi = 18 percent).
The reviewing petrophysicist noted that the ILm (medium induction) reading in the same interval was 0.75 ohm-metres, lower than the ILd. In a normally invaded freshwater mud well, ILm should read lower than ILd only when the formation is more resistive than the invaded zone (hydrocarbon effect). In this case the ILm being lower than ILd suggested either no invasion (unusual) or a conductance anomaly in the medium array.
She recognized the symptom: the Belly River in this area is a high-clay, brackish-water formation with Rw around 0.15 ohm-metres and formation conductivities in the 3,000 to 5,000 mS/m range. At these conductivities, the 6FF40 skin-effect correction is significant. Applying the Schlumberger skin-effect correction to the ILd reading changed the corrected deep resistivity from 0.9 to 1.4 ohm-metres. Re-running Archie with Rt = 1.4 ohm-metres gave Sw = 68 percent, a transitional zone rather than a water zone. The interval was in a Belly River pool that had been producing nearby for 15 years with initial water cuts of 60 to 70 percent that declined to 30 percent over time.
A nearby infill well was drilled, completed in the same Belly River interval, and produced at 4.2 cubic metres of oil per day with a 55 percent initial water cut that declined to 35 percent. The original well had been incorrectly classified as water because the 6FF40 skin-effect correction had not been applied to a high-conductivity formation in 1974. The missed well had left an oil volume equivalent to roughly 12,000 barrels behind for 30 years.