Dual Induction Tool
1.1 Principal applications
The dual induction tool is a multi-frequency dual-induction tool that provides the following major
measurements:
·Induction Deep (ID) and Induction Medium (IM)
·SFL (current and voltage)
·SP (analog)
·SPARC (digital).
1.2 Tool history
The Dual-Induction Tool, has two induction devices: one for obtaining a deep measurement (ILD)
and another for a medium measurement (ILM). In addition to the induction devices, a spherically
focused electrode device provides a shallow formation conductivity measurement when recorded
in conductive mud. A spontaneous potential curve (SP) is also measured.
1.3 Tool theory
An electromagnetic field of constant frequency (20 kHz) transmitted from the induction tool can
penetrate a formation. The induction tool generates a 20 kHz electromagnetic field around the
sonde by passing current at this frequency into a transmitter coil (or coils) inside the array. This
primary field propagates into the formation and generates a continuous current distribution in the
formation. This current distribution (called eddy currents in older literature) is approximately
coaxial with the borehole axis. The formation currents are phase-shifted 90 deg from the
transmitter current (at least in high-resistivity formations¾a consequence of Lenz' law). The
formation currents set up a secondary electromagnetic field, which is detected in the induction
receiver coils. The phase of the received voltage is shifted 90 deg from the formation currents,
making a total phase shift of 180 deg. The received voltage is proportional to the formation
conductivity. (It should be noted that in high conductivity formations, the phase shifts are greater
than 90 deg in each step. This additional phase shift is called skin effect.)
The distribution of currents in the formation determines the contribution each formation element
makes to the total received signal. The ILD and ILM arrays are focused arrays, which means that
the signals coming from the borehole and very close to the tool are attenuated. The ILD array is
focused so that its median depth of investigation is about 60 in. away from the tool. The ILM
array is shallower, providing a 30-in. median depth of investigation. Vertical focusing is
somewhat less successful, with ILD having a basic vertical resolution of 7 ft and ILM about 5 ft.
Both arrays have considerable inherent shoulder effect, which is reduced slightly by the
deconvolution algorithm. Shoulder effect must be suspected when the bed-shoulder ratio is over
10:1 and the shoulder resistivity is above 1 ohm-m. At bed-shoulder contrasts of 500:1, the bed
resistivity can be too low by a factor of 10. Shoulder effect can occur in beds as thick as 50 ft.
Charts are available for partial correction of shoulder effect in low to moderate contrasts.
Although focusing reduces borehole effect, it is not completely removed from the ILD. Wellsite
software is now available for real-time borehole corrections. When using this, it is very important
that the needed parameters are set (see Parameter initialization).
2 Operating technique
2.1 Recommended maximum logging speed
250 ft/min or 75 m/min
2.2 Response in known conditions
After borehole and bed thickness corrections, the relative position of the SFLU, ILM and ILD
curves depends on the values of Rmf and Rw, their respective saturations and the depth of
invasion (and therefore the time since drilling).
In impermeable zones such as shales drilled with fresh muds, SFLU will normally be equal to or
larger than ILM and ILD (provided that the hole is in gauge and the effects of overpressure, shale
hydration and anisotropy are ignored).
3 Data quality/environment
3.1 Operational limitations
Conditions where additional tools/devices are required
Thin Beds (<8ft)
Rt/Rm > 20:1
Rxo<Rt
Large Hole (Bit Size > 13 in)
Thin Laminations
Possible Annulus
Bad Hole (Rt/Rm > 100, large or irregular hole)
Borehole temperature
350°F
Borehole size
Maximum 20 in. (depending on mud conductivity)
Borehole fluid
Conductive mud is required for SFL and SP. Very conductive muds and large hole sizes, together,
may cause unreliable ILM and ILD readings. Caves may cause thorns effects on ILM and OLD.
3.2 Borehole corrections
The induction signals are affected by the borehole signal, whose amplitude is a function of hole
diameter (borehole geometrical factor) and mud conductivity
3.3 Bed thickness corrections
For DIL bed thickness corrections, log interpretation charts must be used
Array Induction Tool
1.1 Principal applications
The Array Induction Tool provides the following major measurements, each of them available
with a one (AO), two (AT), and four (AF) ft vertical resolution:
·Five basic log resistivity curves with a median radial depth of investigation of 10, 20, 30, 60, and
90 in.
·Model dependant Rt, Rxo, inner and outer diameters of invasion.
·Model independent resistivity and RWA images
·Model dependant water saturation (Sw) and hydrocarbon saturation (So) images
·Volume of mud filtrate estimation.
The tool is designed to accurately measure open hole formation conductivity as a function of well
depth and radial distance into the formation.
1.2 Tool history
The array induction tool have the capability to provide the customer with adaptive resistivity
imaging. There are many differences; however, only a few major ones will be discussed.
Previous induction tools achieve, at most, two fixed-focus depths of investigation, a deep and a
medium, by use of multiple coil arrays which are hardwired together. A shallow resistivity
measurement is generated by means of a non-induction type of sensor such as the SFL which uses
an electrode for a sensor. The array induction tool can be processed to emphasize various regions
of the formation, both vertically and radially. It can also produce measurements with matched
vertical resolution and shallow resistivity estimates in oil based mud, (electrode type
measurements do not work in oil based mud).
The concept of multiple focusing is enhanced by multiple arrays with the simplest possible (least
focused) configuration. The array induction tool contains 8 three-coil sondes having main
spacings which range from 6 in. to 6 ft. This information is combined at the surface in software
with varying weights to produce spatially focused responses. The result is logs which can have
higher resolution and reduced environmental effects compared to conventional induction tools.
1.3 Tool theory
An induction tool generates an electromagnetic field around the sonde by passing current into a
transmitter coil. This primary field propagates into the formation and generates a continuous
current distribution in the formation. This current distribution is approximately coaxial with the
borehole axis and phase shifted 90° with respect to the transmitter current. The formation currents
set up a secondary electromagnetic field which is detected as a voltage by receiver coils. This
voltage is proportional to the formation conductivity and it is 90° out of phase with respect to the
formation currents. As the transmitter/receiver distance increases, the contribution to the receiver
voltage will come mostly from currents which are deeper in the formation. In high conductivity
(low resistivity), the phase shifts are greater than 90° in each step. This additional phase shift is
called "skin effect."
The array induction tool sonde contains eight mutually balanced induction arrays, with spacings
ranging from several in. to several feet. A single transmitter operates simultaneously at three
frequencies, with adjacent frequency pairs being utilized by six of the arrays. In phase (R), and
quadrature (X) signal components are measured for each array and each frequency, thus providing
28 induction measurements acquired at 3 in. depth intervals.
As with previous induction tools, primary calibration is done with a set of test loops. Like the
phasor induction tool, the array induction tool tool is continuously auto-calibrated while logging.
The 28 array conductivity measurements are borehole corrected and combined using software;
functions are weighted in both the radial and depth directions to produce a set of five logs. The
logs have closely matched vertical resolution, and median depths of investigation of 10, 20, 30, 60
and 90 inches from the center of the borehole. Weights are chosen so that the deep reading are
insensitive to the borehole and the near wellbore region. Unlike previous tools, median depth of
investigation is constant through the range of resistivity normally presented.
The AO basic log set has a vertical resolution of 1 ft, that is 90% of the log conductivity response
through a 1 ft bed is contributed by that bed. Two alternative sets of logs are simultaneously
available:
·A 2 ft set (AT) can be used to emulate the best previously obtainable induction vertical response
(Very Enhanced Phasor DIT-E logs).
·A 4 ft set (AF) may be used to emulate older induction logs.
The 2 ft logs are less affected than the 1 ft logs by the perturbing effects of sudden changes in
borehole size or shallow invasion character. The 4 ft set is even more robust, and has extremely
small sensitivity to the borehole and near-borehole region. Since all logs in a set utilize a common
measurement principle, an overlay of the five log curves should be expected in impermeable
zones. Subject to the limits of resolution and accuracy, differences between log curves are
therefore interpretable. This contrasts with the traditional Dual Induction/SFL log set that presents
conductivity measurements together with a resistivity measurement. When logs based on different
measurement principles are compared, differences due to anisotropy and tool response can obscure
interpretation.
Response functions are used to radially deconvolve the set of matched vertical resolution logs to
produce a detailed description of the radial conductivity. This radial conductivity description can,
in turn, be transformed into a number of computed products including both color coded images
and discrete valued log curves. See Figure 3 for a comprehensive overview of the array induction
tool processing available.
2 Operating technique
2.1 Recommended maximum logging speed
60 ft/hr or 18 m/min
2.2 Tool positioning
·The array induction tool will give consistent borehole performance when run centralized. In
deviated wells (hole deviation greater than 10 to 15 degrees) the tool should be run with maximum
possible size of standoffs.
·In rugose holes with saline mud (Rt/Rm > 100), it is very important to run it centralized or with
maximum possible size of standoffs.
·If not centralized it is very important to keep a constant standoff so the use of multiple standoffs,
multiple centralizers, and knuckle joints is recommended.
·The use of two knuckle joints is recommended when the array induction tool is combined with
pad tools.
2.3 Response in known conditions
After depth matching, borehole correction and multi-channel signal processing the relative
positions (values) of the different curves (10 in., ..., 90 in.) depend on the value of Rmf and Rw,
the respective saturation and the depth of invasion.
In impermeable zones, such as shales, the curves will match each other (provided that the hole is
in gauge, and the effects of overpressure, shale hydration and anisotropy are ignored).
In front of permeable zones, and keeping in mind Rw and Rmf values, the relative position of the
curves, should show a coherent profile (could be distorted in the presence of annulus).
2.4 Repeatability
Table 7. Repeatability specifications
AT10, AT20, AT30, AT60, AT90
The larger of 2 % or 0.75 mmho.
SP, SPAR
2 mV after allowance for baseline shift (when Rt/Rm contrast is low).
3 Data quality/ environment
3.1 Operational limitations
Conditions where additional tools/devices are required
None
Borehole temperature
350°F
Borehole size
Minimum 4.75 in. for array induction tool
Maximum hole diameter not yet established. Large holes give large corrections and the loss of
shallow measurements.
4.2 Borehole corrections
Multi-array induction tools have new shallow-array measurements, so the influence of the
borehole is more readily observed. The redundancy of information allows the formulation of a
self-consistent borehole-correction scheme that can correct for the borehole signals, even in
difficult cases such as irregular shaped borehole with caves. In difficult conditions optimization
with variable standoff might be required.
The shallow arrays provide the means for accurately correcting the many array measurements and
to extend the range of reliability of the shallow logs to Rxo / Rm < 100, by using the adaptive
borehole corrections. The caliper signal coming from an external caliper or BS is used, and the
mud resistivity is obtained through the use of an adaptive method. An external caliper should be
used in preference to BS whenever possible. The caliper is selected by setting the parameter
GCSE to the appropriate value (e.g., CALI). Mud conductivity can be acquired with the Auxiliary
Measurement Sonde (AMS) or the Environmental Measurement Sonde (EMS). The measured
mud resistivity is then used to enhance log quality.
Figure 6 and Figure 7 provide a guide for which resistivity tools are appropriate for the derivation
of an accurate Rt or Rxo. While compromises have necessarily been made in order to present the
information in a simple form, the limiting factors such as signal-to-noise ratio, skin effect, sonde
error stability, borehole correction, power and gain have been considered. Note that there is a
substantial area of overlap, where the acquisition of the array induction tool and MicroSFL tools
can improve the definition of the invasion parameters.
Response difference between dual induction tool and array induction
tool for different environments.
Note the difference in resolution against the real values of resistivity
between Dual Induction Log showed in the first graph and the Array
Induction Log showed in the second graph in each page, specifically
when thin beds or washouts are present. Shoulder effect is highly reduced
in the Array Induction Log compared with Dual Induction.