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64 Oilfield Review
Triaxial InductionA New Anglefor an Old Measurement
Barbara Anderson
Consultant
Cambridge, Massachusetts, USA
Tom Barber
Rob Leveridge
Sugar Land, Texas, USA
Rabi Bastia
Kamlesh Raj Saxena
Anil Kumar TyagiReliance Industries Limited
Mumbai, India
Jean-Baptiste Clavaud
Chevron Energy Technology Company
Houston, Texas
Brian Coffin
HighMount Exploration & Production LLC
Houston, Texas
Madhumita Das
Utkal University
Bhubaneswar, Orissa, India
Ron Hayden
Houston, Texas
Theodore Klimentos
Mumbai, India
Chanh Cao Minh
Luanda, Angola
Stephen Williams
StatoilHydro
Stavanger, Norway
For help in preparation of this article, thanks to Frank Shray,Lagos, Nigeria; and Badarinadh Vissapragada, Stavanger.
AIT (Array Induction Imager Tool), ECS (Elemental CaptureSpectroscopy Sonde), ELANPlus, FMI (Fullbore FormationMicroImager), MR Scanner, OBMI (Oil-Base MicroImager),OBMI2 (Integrated Dual Oil-Base MicroImagers) andRt Scanner are marks of Schlumberger.
Excel is a mark of Microsoft Corporation.
Westcott is a mark of Acme United Corporation.
A new induction resistivity tool provides 3D information about formations far from the
wellbore. It improves the accuracy of resistivity measurements in deviated wells and
in dipping beds, and can measure formation dip magnitude and direction without
having to make contact with the wellbore. The tools highly accurate triaxial
resistivity measurement means fewer missed opportunities and better understanding
of the reservoir.
Triaxial induction resistivity is rejuvenating an
old measurement. Formation resistivity, the
fundamental property log analysts use to evaluate
oil and gas wells, was the first measurement
acquired with wireline logging tools. As the
equipment to provide resistivity measurements
evolved, induction resistivity logging became the
standard measurement technique for acquiring
formation resistivity. However, the accuracy of
tool response at high resistivities and in deviated
wells or dipping reservoirs was limited by thephysics of the measurement. A new tool
overcomes many of the limitations of previous
induction logging techniques. This 3D triaxial
induction measurement enables petrophysicists
to better understand and evaluate the types of
reservoirs where, before the new technology,
hydrocarbons could have easily been
underestimated or overlooked.
The resistivity story began a century ago,
when Conrad Schlumberger developed a
technique for measuring the resistivity of the
subsurface layers of the Earth. His experiments
demonstrated a practical application with
commercial possibilities. The concept was
promising enough that he formed a business
venture to put the technique into practice.1 On
September 5, 1927, with equipment designed and
built by Henri-Georges Doll, the first electrical
logging experiment, a measurement of formation
resistivity, was conducted in a well in the
Pechelbronn oil region, Frances only large oil
field (next page, bottom).2
The fledgling oil and gas industry adopted
this electrode-based resistivity measurement,
and, with modifications, used it to identify
hydrocarbon deposits. Porous, permeable zoneswith high resistivity indicated the potential for
oil or gas; low resistivity suggested the presence
of salt water. Then, in the 1940s, Doll introduced
the principles of induction resistivity logging to
the industry.3 This technique acquired formation
resistivity in wells without a conductive path,
notably in oil-base mud, overcoming a major
limitation of electrode-based measurements.
The process of measuring formation
resistivity is not as simple as taking a direct
reading from a tool or a measurement from
Point A to Point B; however, in the past half-
century, great strides have been made in
accurately measuring this critical parameter.
Because induction logging tools provide
1. Gruner Schlumberger A: The Schlumberger Adventure.New York City: Arco Publishing, Inc., 1982.
2. Oristaglio M and Dorozynski A: A Sixth Sense: The Lifeand Science of Henri-Georges Doll Oilfield Pioneer andInventor. Parsippany, New Jersey, USA: The HammerCompany, 2007.
3. Doll HG: Introduction to Induction Logging andApplication to Logging of Wells Drilled with Oil-BasedMuds, Petroleum Transactions, AIME1, no. 6(June 1949): 148162.
4. For more on induction tool response: Gianzero S andAnderson B: A New Look at Skin Effect, The LogAnalyst 23, no. 1 (JanuaryFebruary 1982): 2034.
7/24/2019 05 Triaxial Induction
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Summer 2008 65
apparent formation resistivity by taking a
measurement from a large volume of material
beyond the borehole, all the components within
that sensed region influence the final reading.Some of these interactions can negatively impact
the quality and accuracy of the measured
resistivity value.4 This is especially true when the
layers are not perpendicular to the axis of the
tool, as is the case with dipping beds and
deviated wells. Because of the effects of adjacent
conductive layers, the resistivity measured by
induction logging tools in dipping beds may beconsiderably lower than the true resistivity,
resulting in an underestimate of the hydrocarbon
in place. Heterogeneity between the subsurface
strata, and even within individual layers, also
affects tool response.
To account for these and other effects, log
analysts first used manual corrections and later
developed computer-based, forward-modeling
and inversion techniques to more closelyapproximate the true formation resistivity
However, they could not resolve all the
unknownsparticularly formation dip. Despite
these unresolved errors in the measurement, the
Rh
Rv
Rh
Rv
Z
X
z
x
y
Y
Transmitter
Receiver
> The first resistivity log. The first carottage lectrique(electrical coring) from a well in Frances Pechelbronn oil field was recorded on September 5,1927. The equipment to provide this resistivity log was based on tools used for surface mapping. The log is scaled in ohm.m, as are modern resistivitylogs. The high-resistivity interval correlated with a known oil sand in a nearby well, validating the use of log data to evaluate wells.
High resistivity
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industry has successfully discovered much of the
worlds hydrocarbon resources using induction
logging tools. Unfortunately, some reservoirs
have been overlooked or underestimated
because of the measurement limitations.
Another difficult formation property for
induction tools to contend with is electrical
anisotropyvariations in properties that change
with the direction of the measurement.5
Anisotropy is prevalent in shales as well as in the
parallel bedding planes of laminated sand-shale
sequences. When the beds are thinner than the
vertical resolution of the induction logging tool,
the measurement becomes a weighted average of
the properties of the individual layers,
dominated by the elements with the lowest
resistivities. This phenomenon may mask the
presence of hydrocarbons.
The effects of anisotropy on the induction
resistivity measurement have been known since
the 1950s, but until recently there has been no
way to resolve the horizontal and vertical
components.6 By taking a 3D measurementin
essence a tensor rather than a scalar approach
these types of ambiguities and errors can be fully
resolved. However, sensors with the ability to
measure induction resistivity in three dimensions
in tensor form had been beyond the limits of
existing hardware. Similarly, the processing
required to model and invert the measurement
was extremely time-consuming, even when using
supercomputers or distributed networks.7
Many of the limitations inherent in induction
logging have now been overcome with the
Rt Scanner triaxial induction service. Currently
available computational-processing power has
been combined with a new tool design to create
a step change in the evolution of induction
logging. This new tool is solving problems and
providing the industry with answers to questions
that have plagued log analysts and geologistsfrom the beginning of well logging.
Three primary applications of triaxial induc-
tion tools are accurate resistivity measurements
in dipping formations, identification and
quantification of laminated pay intervals and a
new structural dip measurement that requires no
pad contact. This article describes how these
measurements are made and demonstrates their
applications. Also included are case studies from
Africa, India and North America.
Induction Resistivity Basics
A two-coil array demonstrates the physics of atraditional uniaxial induction resistivity measure-
ment. Alternating current excites a transmitter
coil, which then creates an alternating-
electromagnetic field in the formation (left).8
This field causes eddy currents to flow in a
circular path around the tool. The ground loops of
current are perpendicular to the axis of the tool
and concentric with the borehole. They are at
least 90 out of phase with the transmitter
current, and their magnitude and phase depend
on the formations conductivity.
The current flowing in the ground loop
generates its own electromagnetic field, which
then induces an alternating voltage in the
receiver coil. The received voltage is at least 90
out of phase with the ground loop and more than
180 out of phase with the transmitter current.
Induction resistivity from the formation is derived
from this voltage, referred to as the R-signal.
Direct coupling of the tools primary transmitter
66 Oilfield Review
> The concept of induction resistivity. The basic physics of the inductionresistivity measurement is represented by a two-coil array. A continuousdistribution of currents, generated by the alternating-electromagnetic fieldof the transmitter (T), flows in the formation beyond the borehole. Theseground loops of current generate electromagnetic fields that are sensed bythe receiver coil (R). A phase-sensitive detector circuit, developed originallyfor land-mine detection during World War II, separates the formation signal(R-signal) from the directly coupled signal coming from the transmitter(X-signal). The R-signal is converted to conductivity, which is then convertedto resistivity. (Adapted with permission from Doll, reference 3.)
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Summer 2008 67
field in the receiver coil, the X-signal, combines
with the formation R-signal; however, the directly
coupled signal is out of phase with the
contribution from the formation. This phase
difference, detected using phase-sensitive
circuitry, permits the rejection of the X-signal and
measurement of the R-signal.
Conversion of the R-signal voltage to
conductivity was first accomplished by equations
based on the Biot-Savart law, which assumes the
major contribution of a single ground loop will
have a maximum value at the midpoint of the
transmitter and receiver coils.9 Schlumberger
mathematicians later developed equations
based on the complete solution for Maxwells
equationsthat provided more accurate measure-
ments.10 This solution can be visualized using a
simplified version of Maxwells equationsthe
Born approximationwhich is an accepted
method of determining the source and location
of the formation signal. For the two-coil axial
array, the response is essentially a toroid shape
surrounding the tool and perpendicular to its axis,with maximum values near the midpoint of the
transmitter and receiver (right).11
In vertical wells with thick homogeneous
horizontal beds, standard resistivity logging
tools, such as the AIT Array Induction Imager
Tool, work reasonably well. These uniaxial tools
measure apparent resistivity, Ra, in a horizontal
plane, which is equivalent to horizontally
measured resistivity, Rh. Resistivity measured in
a vertical plane, Rv, cannot be measured with
uniaxial induction tools in a vertical well.
Because the ground loops of induction tools
intersect a huge volume of the formation, theymay traverse a path that includes several different
layers with varying electrical properties.
Anisotropy results in a resistivity measurement
that changes based on the direction of the
measurement. This limitation in the measurement
was one of the factors that led to the development
of the Rt Scanner tool.
The Impetus for Triaxial Measurements
Although the concepts underlying triaxial
induction measurements first appeared in the
literature in the mid 1960s, the tools to make this
measurement were not developed. There were
three main reasons for the delay: a triaxial tool
could not be built with the existing technology,
the data processing required was beyond the
capability available at the time, and the tools
response to conductive fluids in the borehole
could be much larger than the signal from
the formation.
Interest in triaxial induction was renewed
chiefly because of the recognized limitations ofuniaxial resistivity measurements in two areas:
anisotropic reservoirs and bedding planes that
are not perpendicular to the axis of the tool.12
Although both of these limitations were
identified in the 1950s, there was then no direct
method of measuring anisotropy with an
induction logging tool, and the solution tonegative effects of real or relative dipping bed
on induction resistivity was not trivial.13 A
technology advanced, measurement under
standing, processing power and tool design al
played key roles in solving for these effects
5. For more on anisotropy: Anderson B, Bryant I, Lling M,Spies B and Helbig K: Oilfield Anisotropy: Its Origins andElectrical Characteristics, Oilfield Review6, no. 4(October 1994): 4856.
Tittman J: Formation Anisotropy: Reckoning with ItsEffects, Oilfield Review2, no. 1 (January 1990): 1623.
6. Kunz KS and Gianzero S: Some Effects of FormationAnisotropy on Resistivity Measurements in Boreholes,Geophysics23, no. 4 (October 1958): 770794.
Moran JH and Gianzero S: Effects of FormationAnisotropy on Resistivity-Logging Measurements,Geophysics44, no. 7 (July 1979): 12661286.
7. Anderson B, Druskin V, Habashy T, Lee P, Lling M,Barber T, Grove G, Lovell J, Rosthal R, Tabanou J,Kennedy D and Shen L: New Dimensions in ModelingResistivity, Oilfield Review9, no. 1 (Spring 1997): 4056.
8. For a detailed explanation of induction theory: Moran JHand Kunz KS: Basic Theory of Induction Logging andApplication to Study of Two-Coil Sondes, Geophysics27,no. 6, part I (December 1962): 829858.
9. The Biot-Savart law describes the magnetic fieldgenerated by an electric current.
10. Maxwells equations, named for physicist James ClerkMaxwell, are a set of four partial differential equationsthat explain the fundamentals of electric and magneticfield relationships.
11. Habashy T and Anderson B: Reconciling Differences inDepth of Investigation Between 2-MHz Phase Shift andAttenuation Resistivity Measurements, Transactions ofthe SPWLA 32nd Annual Logging Symposium, Midland,Texas, June 1619, 1991, paper E.
12. Moran and Gianzero, reference 6.
13. For the theoretical solution to Maxwells equations asapplied to induction logging: Moran and Kunz,reference 8.
Anderson B, Safinya KA and Habashy T: Effects ofDipping Beds on the Response of Induction Tools,paper SPE 15488, presented at the SPE AnnualTechnical Conference and Exhibition, New Orleans,October 58, 1986.
> Born approximation for a uniaxial induction logging tool. The sensed regionfor uniaxial induction tools is a toroid shape (red), perpendicular to the tool.The maxima are located approximately at the midpoint between the transmitter(T) and receiver (R). This rendering shows the Born approximation of the fullsolution to Maxwells equations. The shape is valid for thick beds andhomogeneous, isotropic formations. This region sampled by the uniaxialinduction tool corresponds to only one of the nine modes measured by thetriaxial Rt Scanner tool.
T
R
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ultimately resulting in the development of a
triaxial induction tool (below left).
Developing such a tool involved understanding
the effects of the borehole on the measurement. 14
There is a great sensitivity to eccentricity in the
borehole: the more conductive the mud, the
greater the effect. The sensitivity results in the
formation signal being overwhelmed by the
borehole signal. This situation, the effects of
which can be two orders of magnitude greater for
triaxial tools than for uniaxial induction tools,
would have been an insurmountable obstacle
without intensive computer modeling.
Iterative modeling allowed various triaxial
tool designs to betested without having to build
and test physical tools. Final tool design included
a sleeve with electrodes connected to a
conductive copper mandrel. This configuration
returned the borehole currents through the tool,
reducing the large signals caused by the
transverse eccentricity to a level equivalent to
that of the AIT tool. The correction for borehole
effects could then be handled in a manner
similar to that used for the AIT measurement.15
After engineers solved for borehole effects,
tool response to various geometrical scenarios
was investigated. For most of their history,
induction measurements have had to contend
with geometry, both in the borehole and in the
formation. Geometry was regarded by inter-
preters as a major nuisance or, at best, something
to be coped with.16 However, after the AIT tools
response was modeled, tool designers discovered
that the formation-geometry effects are the
strongest contributor to the induction signal.
When properly resolved and modeled, geometry
now provided a key to accurate measurement of
formation resistivity. In addition, dipping beds
those that are not perpendicular to the axis of
the logging toolcould be properly measured.
Dipping beds are the result of geological
tilting of formations, deviation of the wellbore
trajectory from vertical, or combinations of both.
Fast analytical codes, developed in the 1980s,
estimate resistivity in dipping beds using data
from uniaxial induction tools, but the processing
68 Oilfield Review
> Rt Scanner triaxial induction service. The RtScanner tool comprises a triaxial transmitter,three short-spacing axial receivers for boreholecorrections and six triaxial receivers. Electrodeson the tool and the Rm sensor in the bottom nose,which measures the mud resistivity, are alsoused for borehole corrections. An internal metalmandrel (not visible in the drawing) provides aconductive path for borehole currents to returnthrough the electrodes on the exterior of the tool.
Electronics housing
Triaxial transmitter
Three short uniaxialreceivers for boreholecorrection
Six triaxial receivers
Metal mandrel
Sleeve with shortelectrodes
Rmsensor
Triaxial transmitter
Triaxial receiver
Axial receiver
Electrode> Three-dimensional arrays. The Rt Scanner service produces a nine-elementarray for each transmitter and receiver pair. Traditional induction measurements
are made by passing current through coils that are wrapped around the axisof the tool, also called the z-axis (blue), which induces current to flow in theformation concentrically around the tool. Triaxial induction tools also includecoils that are wrapped around the x-axis (red) and y-axis (green), whichcreate currents that flow in planes along the tools x- and y-axes. The x, y andz components of the transmitter couple with the x, y and z receivers. Forvertical wells with horizontal beds, only the xx, yy and zz couplings respond tothe conductivity () of the formation. In deviated wells or wells with dippingbeds, all nine components ofthe array are needed to fully resolve theresistivity measurement. The multiple triaxial transmitter and receiver pairsgenerate 234 conductivity measurements for each depth frame.
Tz
Rz
Tx
Rx
Ry
Ty
xx xy xz
yx yy yz
zx zy zz
=
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Summer 2008 69
relies on inputs from other sources.17
Unfortunately, the uniaxial measurement may
become unreliable or provide nonunique
solutions when external data sources are used.All these issues posed problems for uniaxial
induction tools. In most cases, there was not
enough information to fully correct the data.
Triaxial induction tools, however, make the
necessary measurements to resolve the ambi-
guities and properly measure the resistivity of
anisotropic reservoirs, correct for nonuniform
filtrate invasion, correct for the effects of
dipping beds and deal with geometrical effects
on the measurement.18
Triaxial Resistivity Theory
Previous induction logging tools, such as those
from the AIT family, measure horizontal
resistivity (uniaxially). The Rt Scanner tool
measures in three dimensions (triaxially).
Although the physics of measurement are
similar, triaxial tools are much more complex
(previous page, bottom right).
The Rt Scanner tool consists of a collocated
triaxial transmitter array, three short axial
receivers and three collocated triaxial receiver
arrays. The triaxial transmitter coil generates
three directional magnetic moments in the x, y
and z directions. Each triaxial receiver array hasa directly coupled term and two terms cross-
coupled with the transmitter coils in the other
directions. This arrangement provides nine
terms in a 3x3 voltage tensor array for any given
measurement. All nine couplings are measured
simultaneously. An advanced inversion
technique extracts resistivity anisotropy, bed-
boundary positions and relative dip from the
tensor voltage matrix. The receiver arrays are
located at different spacings to provide multiple
depths of investigation.
The Born approximation for the triaxial
induction tools response provides a graphical
representation for the solution of the equations
representing the sensed region (above). The
uniaxial induction tools response was shown
earlier to have a single toroid shape; the triaxial
tool delivers nine responses superimposed on
each other. The zz term from the Rt Scanner tool
is essentially the same response as that
measured by the uniaxial induction tool.
Collocation of the coils is an important
feature of the Rt Scanner tool: when the
transmitter or receivers are not at the same
position, the spacings for the cross-terms will bedifferent from those of the direct terms. Because
the entire ensemble of measurements is made
within a single depth frame, no measurement
14. Rosthal R, Barber T, Bonner S, Chen K-C, Davydycheva SHazen G, Homan D, Kibbe C, Minerbo G, Schlein R,Villegas L, Wang H and Zhou F: Field Test Results of anExperimental Fully-Triaxial Induction Tool, Transactionsof the SPWLA 17th Annual Logging Symposium,Galveston, Texas, June 2225, 2003, paper QQ.
15. For details on Rt Scanner design and modeling:Barber T, Anderson B, Abubakar A, Broussard T,Chen K-C, Davydycheva S, Druskin V, Habashy T,Homan D, Minerbo G, Rosthal R, Schlein R and Wang H:Determining Formation Resistivity Anisotropy in thePresence of Invasion, paper SPE 90526, presented at
the SPE Annual Technical Conference and Exhibition,Houston, September 2629, 2004.
16. Moran and Gianzero, reference 6.
17. Barber TD, Broussard T, Minerbo G, Sijercic Z andMurgatroyd D: Interpretation of Multiarray Logs inInvaded Formations at High Relative Dip Angles, TheLog Analyst 40, no. 3 (MayJune 1999): 202217.
18. During the drilling process, fluids from the drilling mudleave the wellbore and enter permeable formations. Themud filtrate alters the electrical characteristics of theformation around the wellbore. The depth of filtrate inva-sion, and its associated geometry, may be unpredictable
> Born approximation for a triaxial induction tensor voltage array. The Born response function for a triaxial induction tool ismuch more complex than that for a uniaxial induction tool. There are nine elements, one for each component of the tensorvoltage array. Each transmitter-receiver pair has positive (red) and negative (blue) responses. The surfaces represent theregions where 90% of the signal measured by the receiver coil originates. Each of the nine components is superimposed atthe measure point of the tool. The xx, yy and zz elements are derived from the direct coupling of a triaxial transmitter and itsassociated triaxial receiver. The other six elements represent cross-coil responses. The zz response (bottom right) is theonly one measured by the simpler uniaxial induction tool.
50
50
0z-axis
y-axis
xx
x-axis
10050
050
100 10050
050
100
50
50
0z-axis
y-axis
yx
x-axis
10050
050
100 10050
050
100
50
50
0z-axis
y-axis
zx
x-axis
10050
050
100 10050
050
100
50
50
0z-axis
y-axis
xy
x-axis
10050
050
100 10050
050
100
50
50
0z-axis
y-axis
yy
x-axis
10050
050
100 10050
050
100
50
50
0z-axis
y-axis
zy
x-axis
10050
050
100 10050
050
100
50
50
0z-axis
y-axis
xz
x-axis
10050
050
100 10050
050
100
50
50
0z-axis
y-axis
yz
x-axis
10050
050
100 10050
050
100
50
50
0z-axis
y-axis
zz
x-axis
10050
050
100 10050
050
100
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have to be depth-shifted to form the measure-
ment tensors. When all nine components are at
the same spacing and location, the matrix can be
mathematically rotated to solve for relative
formation dip. A change from one coordinate
system to another is also greatly simplified
because it involves a simple transformation, and
all measurements are made along the same
coordinate system as well as at the same depth.
Collocation is especially important when bedding
planes are not perpendicular to the relative
position of the tool.
Power in the Processing
Collocated orthogonal transmitter and receiver
pairs made the triaxial resistivity measurement
feasible, but advancement in processing power
was the enablerthat spurred the development of
the tool. Even in the late 1990s, triaxial induction
was referred to as a theoretical concept, prima-
rily because the computing power needed to
model and develop fast processing codes was not
readily available.19 Moores law, the observation
that computing power doubles every two years, is
evidenced in the progression that has occurred
with induction resistivity logging.
The first induction resistivity tools converted
conductivity measured downhole to an analog
voltage that was measured at the surface. The log
analyst read the resistivity from the logs and
applied corrections from charts to account for
the effects of adjacent beds and filtrate invasion,
generally ignoring borehole effects. Borehole
correction charts were then developed based on
geometrical-factor curves obtained from labora-
tory measurements made in plastic pipes
immersed in waters of varying salinity.20 In the
mid 1980s, these empirically derived charts were
reproduced using computer modeling.
70 Oilfield Review
02,500 2,000
1 10 100
1,5001,000 500
Conductivity, mS/m
Resistivity, ohm.m
Conductivity, mS/mConductivity, mS/m
0 500 1,000 1,500 2,000 2,500 2,000 1,500 1,000 500 0 500 1,000 1,500 2,000 2,500 2,000 1,500 1,000 500 0 500 1,000 1,500 2,000
10 xxxyxzyxyyyzzxzyzzhv
20
30
40
Depth,
ft
50
60
70
80
0
10
20
30
40
Depth,
ft
50
60
70
80
RhRv
Rh(inverted)
Rv(inverted)
80 ft
50 ft
40 ft
30 ft
20 ft
0 ft
Rh= 1.9 ohm.m
Rv= 11.0 ohm.m
Rh= 1 ohm.m
Rv= 2 ohm.m
Rh
= Rv= 50 ohm.m
Rh
= Rv= 0.5 ohm.m
Rh
= Rv= 1 ohm.m
> Modeling the triaxial induction response. A 1D horizontally layered,transversely isotropic (TI) model was used to validate the triaxial inductionresponse to known conditions (bottom right). The five layers used in themodel consist of two low-resistivity homogeneous layers, a high-resistivityhomogeneous layer, and two anisotropic layers with high- and low-contrast beds. The first measurement is conducted with a vertical tool inhorizontal beds (top left). The zz (blue) and yy (green) components react tothe resistivity of the beds, but the xx and all cross-components are zero.Prior to inversion, none of the curves indicates the correct horizontal (pinkdash) and vertical (black dash) conductivity. Next, the model well is
deviated 75 () and the tool position is rotated 30 () from the high sideof the wellbore. All nine components become active (center) and nonereads the same as the vertical model. The zz (blue) componentcorresponds to a uniaxial induction measurement, and although it is similarto the curve in the vertical response model, the curves shape andamplitude have changed. The data are then rotated mathematically ( topright) to zero the yx and yz (green dash) cross-coil contributions. Theangle of rotation required to zero these components corresponds to therelative dip of the beds. Finally, the data are inverted, correcting for bedthickness and deviation, and converted from conductivity to resistivity(bottom left). In the three lower layers, which are homogeneous, Rv(blue)and Rh(red) are equal and match the input resistivity. In the laminatedlayers, the curves separate as a result of anisotropy.
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Summer 2008 71
The manual process of correcting induction
log data was carried out sequentially: apply
borehole corrections, correct for shoulder-bed
effects and correct for invasion. With the advent of
data recorders, log data could be processed using
computers. Codes were developed to perform 1D
corrections automatically, first at mainframe-
equipped computing centers and then as
processing power continued to grow, at the
wellsite using computer-equipped logging units.
Advances in computer technology rendered
the manual corrections obsolete, but there was a
problem in the methodology. The codes weredeveloped assuming horizontal, homogeneous
beds, and corrections were applied with the
same linear approach used by log analysts.
However, the ground loops produced by induction
tools intersect and interact with all the media
they come into contact with in a complex,
nonlinear fashion.21 The sequential approach,
used for decades, was found to be inadequate.
This situation was improved when fast 2D
asymmetric forward-modeling codes were
developed in the mid 1980s. They revealed just
how inaccurate sequential chartbook corrections
were for determining the true resistivity, Rt,
especially in thin beds invaded by mud filtrate.
Development of the AIT tool was a result of
lessons learned from those models. Since then,
various techniques have been applied to obtain
Rt, including iterative forward modeling and
inversion.22 Models have been developed that
include 1D corrections as well as corrections for
invasion and nonhorizontal bedding (2D) and
nonlinear invasion in tilted reservoirs (3D). Only
recently has advanced computer-processing
power enabled inversion codes that fully correct
the induction measurement. These codes allow
simulations to be run in hours instead of weeks.
If Moores law holds true, hours for processing
induction measurements will eventually be
reduced to seconds.
Induction resistivity data, acquired with a
triaxial tool, could now be processed in a
reasonable time frame. All the pieces of the
puzzle were available; the next step was to put
the triaxial toolto the test.
Testing the Code
To test the validity of the acquisition and
inversion algorithm for triaxial induction data, a
1D horizontally layered, transversely isotropic
(TI) model was constructed (previous page).
Five layers simulated a complex reservoir
comprising two low-resistivity sands, a high-
resistivity sand, an anisotropic low-resistivity
shale and a laminated sand-shale sequence.
This simulated reservoir included features
that present limitations for uniaxial resistivity
tools. The testing proved that a triaxial
resistivity measurement overcomes these
limitations and provides accurate resistivity in
challenging environments.
The outputs of the processing are true
resistivity corrected for dip in the nonlaminated
layers and a shale-affected resistivity in
laminated layers. Rv is provided from the
processing, although it is equivalent to Rh in the
isotropic intervals.
For the two laminated layers, Rv and Rh are
not equal, and the curves have separation based
on the degree of anisotropy. Neither Rh nor Rprovides the true resistivity of the modeled
reservoir in the case of laminated sections, but
techniques have been developed to provide the
resistivity of the sand layers.
True Resistivity
The true resistivity of a formation, Rt, is a
characteristic of an undisturbed, or virgin
region. Much study and research have been
carried out in the name of acquiring this elusivemeasurement. The measurement of induction
resistivity in a virgin zone is predicated on some
degree of homogeneity, consistent perpendicula
beds and isotropic reservoirs. In nature, this is
rarely the case.
The concept of vertical and horizonta
resistivities evolved early in the development o
electrical logging. Measured apparent resistivity
Ra, of stacked rock layers differs with changes in
the measurement direction. If the measurement
is made parallel to the layers, the result is
similar to measuring resistors in parallelthe
lowest resistances dominate (above). For a
parallel resistor circuit, more current flow
through the smaller resistors, and each resistor
19. Anderson BI: Modeling and Inversion Methods for theInterpretation of Resistivity Logging Tool Response. DelftThe Netherlands: Delft University Press, 2001.
20. Moran and Kunz, reference 8.
21. Anderson, reference 19.
22. Howard AQ: A New Invasion Model for Resistivity LogInterpretation, The Log Analyst33, no. 2 (MarchApril 1992): 96110.
> Direction matters. Under the right conditions, the deep-induction response to a homogeneous, isotropic bed ( left) is the same as that to an anisotropic,laminated bed (center). This occurs when beds are thinner than the vertical resolution of the measurement. For the 90-in. deep-induction array, thevertical resolution is 1 to 4 ft [0.3 to 1.2 m]. Horizontal resistivity (Rh) measurements are analogous to parallel resistor circuits, so the resistivity value of thelaminated bed is primarily influenced by the layer with the lowest resistivity, Rshale. With standard induction tools, hydrocarbon-bearing sand layers caneasily be overlooked. Vertical resistivity (Rv) is analogous to a series resistor circuit (right), and its value is dominated by the layer with the highestresistivity. A large difference between Rvand Rh indicates anisotropy.
1,800
Depth
ft
Computed Deep Induction
ohm.m0.2 2,000
1,810
1,820
1,830
1,840
1,800
Depth
ft
1,810
1,820
1,830
1,840
Computed Deep Induction
Model RtProfile Model R
tProfile Model R
h-R
vProfile
Rh
Rv
ohm.m0.2 2,000
Horizontal Resistivity, Rh
Vertical Resistivity, Rv
ohm.m0.2 2,000
Rsand
Rshale
Rsand
Rshale
Rshale
Rsand
Rsand
7/24/2019 05 Triaxial Induction
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divides the current according to the reciprocalof its resistance.
When the measurement is made across the
stack, the measured resistance is similar to
measuring resistors in series. In an electrical
series circuit, the resistance values are added
together. Higher resistance, which is the case for
the layers containing hydrocarbon, is dominant.
The concept that the measured resistance
depends on the direction in which it is made is
referred to as electrical anisotropy. Since well
logging began in vertical wells with stacks of
more or less horizontal layers, the resistivity
parallel to the layers was called the horizontal
resistivity, Rh, and the resistivity measured
across the layers was called the vertical
resistivity, or Rv. In an isotropic, thick sand Rh =
Ra = Rv. If, however, the thickness of the bedding
layers is less than the tools vertical resolution,
the Rh measurement is analogous to the parallel
electrical circuit.
Most of the technology for determiningformation resistivity measured the horizontal
component, giving rise to difficulties in
evaluating thin layers comprising shale and
hydrocarbon-bearing sands. For a uniaxial
induction measurement the formation currents
flow in horizontal loops, and the resulting
sensitivity is to the horizontal resistivity. For
most laminated reservoirs,Rh Rv. Based on the
parallel circuit analogy, Ra will be similar in
value to that of the layer with the lower
resistivity, usually the shale. Therein lies the
problem with interpreting induction resistivity in
laminated reservoirs: the dominant nature of the
less-resistive layers masks the more-resistive
layers that may have hydrocarbon potential. The
result is that pay zones may be overlooked or
underestimated.23 The Rv/Rh ratio is a useful
measurement for determining the level of
anisotropy, and when the ratio is higher than 5,
it alerts the log analyst to look for potential
laminated-pay reservoirs.
For a laminated sand-shale sequence, theportion of the reservoir that is of interest is the
sand. Although Rv does not provide the actual
resistivity of the hydrocarbon-bearing sand layer,
Rsand, it can be combined with other
measurements to derive it. The shale effects
must be removed from the volumetric
measurement to obtain the resistivity of the sand
layers (above). Calculating Rsand from Rh and Rv
requires a secondary source to determine the
volume of shale before its effects can be
eliminated. Shale volume can be obtained from
several sources, including the ECS Elemental
Capture Spectroscopy sonde. Once determined,
Rsand can be used to calculate water saturation,
Sw, using Archies equation. The full derivation of
the formula for Rsand and Sw in the presence of
anisotropy can be found in the literature.24
72 Oilfield Review
> Hidden saturation. Rhand Rvare outputs from the Rt Scanner tool. The resistivity of the sand layers can beresolved from these measurements in combination with fractional volumes of sand and shale. For this example,the conventional induction tool would have measuredRh= 2.3 ohm.m. Rv from the triaxial induction measurementis 12.8 ohm.m. The volume fractions, Fshaleand Fsand, could come from an ECS Elemental Capture Spectroscopytool. Because shales often exhibit anisotropy without the presence of sand laminations, two different shalevalues are used in this example: vertical Rshale-v is 2 ohm.m and horizontal Rshale-h is 1 ohm.m. These values shouldbe determined within an anisotropic shale interval. This method gives an Rv/Rh ratio in the shale of 2, comparedwith the 5.6 ratio of the entire sand-shale sequence. Solving the equations (right) for Rsandyields a value of 20 ohm.m.The 2.3 ohm.m measured by a conventional induction tool would considerably underestimate the hydrocarbon volume.
Rsand
Rsand
Rsand
Rshale-h
Rsand
Rshale-h
Rshale-h
Rshale-v
Rshale-v
Rshale-v
Rsand
R
sand
Rshaleh
= 1 ohm.m
Rshalev
= 2 ohm.m
Rv= 12.8 ohm.m
Rh= 2.3 ohm.m
1
Rh
= +Fsand
Rsand
Fshale
Rshale-h
Rv
= +x xFsand
Rsand
Fshale
= 40%
Fsand
= 60%
Rsand
= 20 ohm.m
Fshale
Rshale-v
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basin, off the east coast of India, is a deepwater
example of a thin sand-shale turbidite sequence
(above). Reliance Industries experienced initial
success in the area, but evaluating the reservoir
potential in the presence of anisotropy made in
situ hydrocarbon volume difficult to quantify.
Thin beds, by definition, are reservoir layers
that are thinner than the vertical resolution of
the tool. The thicknesses of the sand-shale-silt
sequences of the Krishna-Godavari basin were in
the millimeter range, well below the minimum
1-ft [0.3-m] resolution available from induction
tools, and even less than the 1.2-in. [3-cm]
vertical resolution of porosity devices. Logs
acquired using conventional tools did not provide
enough information to evaluate the anisotropic
zones (above right). Theinterval above X,X65 m,
where cleaner, productive sandstone sections
end, has resistivity values of 1 to 2 ohm.m. With
such low resistivity, hydrocarbon production
would not be expected.
74 Oilfield Review
> Krishna-Godavari basin off the east coast ofIndia. The KG-1 well is located in the KG-DWN-98/3 block. The laminations in this core example(above) are about a millimeter [0.04 in.] thick,typical of the turbidite sequences found in theKrishna-Godavari basin. The minimum verticalresolution for induction tools is 0.3 m. Evaluationand calculation of recoverable hydrocarbon aredifficult because of the low-resistivity, anisotropicnature of the reservoir.
INDIA
PAKISTAN
AFGHANISTAN C H I N A
SRI LANKA
KG-DWN-98/3
> Underestimated reserves. Typical of logs run in the field, the ELANPlus analysis calculateshydrocarbon (Track 5, red) in the sands (Track 6, yellow), but the volumes are low, considering thenet footage. Above X,X65 m the water saturation and hydrocarbon volumes indicate little oil or gaswould be produced. But, this zone is known to be a laminated sand-shale turbidite sequence. Atriaxial induction tool can help determine the degree of anisotropy and the hydrocarbon potential.
X,X45
Depth
m
Sigma
Resistivity
0.2 ohm.m 1000 cu 50
0 gAPI 150
6 in. 16
Sw
EffectivePorosity
X,X50
X,X55
X,X60
X,X65
X,X70
X,X75
X,X80
90-in. Array
Gamma Ray
Caliper
0.2 ohm.m 100
60-in. Array
0.2 ohm.m 100
30-in. Array
60 % 0
Neutron Porosity
60 % 0
Crossplot Porosity
1.65 g/cm3 2.65
Bulk Density
0.2 ohm.m 100
20-in. Array
0.2 ohm.m 100
10-in. Array
Crossover Hydro-carbon
Montmorillonite
Bound Water
Quartz
Gas
Water
100100
50 0%
%%
Lithology
00
Anisotropiczone
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Summer 2008 75
For its KG-1 well, Reliance acquired high-
resolution log suites and OBMI Oil-Base
MicroImager data (below). The OBMI images
revealed thin laminations, corroborated by the
core. A synthetic resistivity log was generated
from the high-resolution OBMI data, which
indicated anisotropy. The AIT resistivity
measurement was 1 to 2 ohm.m. The Rt Scanner
tool was added to the logging program because of
the low AIT resistivity measurements in the
laminated reservoir.
The log data from the Rt Scanner too
indicated a high degree of anisotropy in the
reservoir and provided an accurate measuremen
of sand resistivity. Several promising zones
denoted by an Rv/Rh ratio greater than 5, were
identified as areas for further evaluation. In the
> Logs and core from the KG-1 well. The core at right shows fine laminations, which can be seen on the OBMI image (Track 4). All fiveAIT curves (Track 2) overlay, but the spiky nature of the reconstructed resistivity from the OBMI data (green) indicates laminations. Thisis because the OBMI tool has better vertical resolution. Curves from the density-neutron tools (Track 3) are separated over most of theinterval, indicating high shale content. There are a few places where the density and neutron cross (yellow shading), indicating thepossibility of light oil or gas, but these zones are less than a meter [3 ft] thick. Low resistivity measurements from the AIT tool and littlesand content would result in a pessimistic evaluation of hydrocarbon production in this interval.
in. m
Bit Size Depth
6 16
in.
Caliper
6 16
cu
Formation Sigma
0 50
%
Neutron Porosity
60 0
g/cm3
Bulk Density
OBMI Image
Conductive Resistive
0 360240120
1.65 2.65
gAPI
Gamma Ray
0 150
ohm.m
OBMI Data
Resistivity
0.2 200
ohm.m
90-in. Array
0.2 200
ohm.m
60-in. Array
0.2 200
ohm.m
30-in. Array
0.2 200
ohm.m
20-in. Array
0.2 200
ohm.m
10-in. Array
0.2 200
73
74
75
76
77
78
79
Crossover
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KG-1 well, zones where the Rv/Rh ratio is below 5
lack laminations. Corroboration by core data
validated the Rt Scanner measurement (above).
The ELANPlus advanced multimineral log
analysis identified approximately 8 m [26.2 ft] of
quality reservoir using conventional inter-
pretation techniques. After the triaxial induction
data over the complete logging interval were
incorporated into the analysis, the net-pay
thickness, using 7% porosity and 80% water
saturation for cutoffs, was increased by 35%.
Calculated reserves values were 55.5% higher
than those previously obtained using traditional
logs and petrophysical evaluation programs
(next page).
76 Oilfield Review
> Anisotropy using Rv/Rh ratio. The Rt Scanner service provides an Rv/Rh ratio (Track 1, black) that is above 5 inseveral intervals (red arrow). These zones correspond to laminations in the core (left). In intervals where the Rv/Rhratio is low (black arrow), the core has few or no laminations ( right). Throughout this section, Rh (Track 3, blue)rarely measures above 2 ohm.m, although the Rv (red) and Rsand (black) curves are measuring much higher. The
density-neutron logs (Track 4) indicate hydrocarbon (red shading) below 100 m but do not provide much help inevaluating the reservoir above 100 m. Although the Rhvalues suggest little productive potential, the higher values ofRsand indicate hydrocarbon.
Density-Neutron
%
Neutron Porosity
1.65g/cm3
Bulk Density
2.65
60 0
%
Crossplot Porosity
60 0
Thin beds are
visible in core.
From Rt Scanner
tool, the Rv/Rhratio = 9. This
zone has high
electrical
anisotropy.
No thin beds
are visible in
the core.
The Rv/Rhratio
is low. This zone
has negligible
electrical
anisotropy.
80
90
100
110
120
m
Depth
0
Rv/RhRatio
20
8 in.
Bit Size
18
0 gAPI
Gamma Ray
100
8 in.
Caliper
18
Bad Hole
0 .2 ohm. m
Rsand
200
0 .2 ohm. m
Rv
200
0 .2 ohm. m
Rh
200
0.2 ohm.m
90-in. Array
200
0.2 ohm.m
60-in. Array
200
0.2 ohm.m
30-in. Array
200
0.2 ohm.m
20-in. Array
200
0.2 ohm.m
10-in. Array
Resistivity
200
7/24/2019 05 Triaxial Induction
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Summer 2008 77
> Incorporating Rt Scanner data. The AIT curves (Track 2) are approximately 1 ohm.m with a few 2-ohm.m sections. Rh(Track 3, blue) is equivalent to the AIT 90-in. curve. Rv (red) measures above 10 ohm.m in several intervals. Rsand (black),calculated from the Rt Scanner outputs, is used as an input for water saturation, Sw. Water saturation from the Rt Scanneroutputs (Track 5, red) is lower than the Sw from AIT data (blue). This finding indicates that more hydrocarbon is in thereservoir than originally computed.
0
Rv/RhRatio
m
Depth
30
40
50
60
70
20 0.2 ohm.m
90-in. Array
200
8 in.
Bit Size
18
8 in.
Caliper
18
Bad Hole Density-
Neutron
Montmorillonite
Bound Water
Quartz
Gas
Water
0.2 ohm.m
60-in. Array
200
0.2 ohm.m
30-in. Array
200
0.2 ohm.m
Rsand
200
0.2 ohm.m
Rv
200 60 %
Neutron Porosity
0
60 %
Crossplot Porosity
0
1.65 g/cm3
Bulk Density
2.65
100 %
AIT Sw
0 100 %
Lithology
0
100 %
Rt Scanner Sw
0
0.2 ohm.m
Rh
200
0.2 ohm.m
20-in. Array
200
0.2 ohm.m
10-in. Array
Resistivity
200
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Resolving Anisotropy in West Africa
Interpretation of electrically anisotropic reser-
voirs has been difficult with traditional
petrophysical analysis techniques. Klein et al
were the first to propose a framework for using
graphical crossplots to evaluate these reservoirs.27
The technique was further adapted to incorporate
data from additional logging tools, including
nuclear magnetic resonance (NMR) and triaxial
induction resistivity.28 The original Klein plots
assume a layering of isotropic, macro- and
microporous material, and layering of coarse-
grain and fine-grain sandsa condition that does
not commonly occur in laminated sand-shale
sequences surrounded by anisotropic shales.
Compaction, which typically increases with depth,
has been shown empirically to increase the level
of shale anisotropy (right).
To account for the more-realistic scenario of
anisotropic shales, a modified Klein plot has
been developed that graphically solves for Rv and
Rhwhile adjusting for shale anisotropy.29 Because
anisotropic shales can create false expectationsof low-resistivity pay if not accounted for
properly, NMR data are also used to differentiate
laminated shales from sand-shale sequences.
NMR tools measure free-fluid volume, or porosity,
in the reservoir. Shales usually have high fluid
volumes, but the fluid is bound to the clays that
make up the shales. By incorporating the NMR
porosity, which ignores the fluids in the shales,
log analysts can identify laminated sand-shale
sequences with hydrocarbon potential while
eliminating laminated shale sequences from
the analysis.
The modified Klein plots are similar to
density-neutron crossplots, and an anisotropic
shale point can be graphically determined from
them (below). Because of their characteristic
shape, these modified crossplots are referred to
as butterfly plots. From them, log analysts
graphically choose parameters, perform quality
checks and assess the potential for production
from laminated reservoirs.
Logs from an offshore West Africa well
demonstrate the modified Klein plot technique.30
The addition of NMR data further enhanced the
evaluation. The operator elected to run the
Rt Scanner tool, MR Scanner expert magnetic
78 Oilfield Review
> Klein plots. The traditional Klein plot ( left) does not take shale anisotropy into account. The modified butterfly plot (center) includes shale anisotropy andcan be partitioned into pay and nonpay regions, pivoting at the shale point. The crossplot Rv and Rh data fall into specific regions that can be analyzedquickly (right). The water point (blue circle) indicates 100% water saturation. The shale point indicates 100% shale.
101
Rh, ohm.m
Rv,
ohm.m
101
101
100
101
102
103
100
102
103
101
Rh, ohm.m
Rv,
ohm.m
101
101
100
101
102
103
100
102
103
101
Rh, ohm.m
Rv,
ohm.m
101
101
100
101
102
103
100
102
103
No shale anisotropyWater With shale anisotropy Water
Nonpay
Shale Pay
Water
Fshale Fshale
Rshale-v= 1
Rshale-h= 1
Shale
Rshale-v= 10
Rshale-h= 1
Shale
Rsand Rsand
> Anisotropy in sands and shales. As compaction (red) increasesthetypical case with deeper depositional environmentsthe clay porositydecreases and the shale Rv/Rh ratio increases. Triaxial induction tools alonecannot distinguish between compaction-induced shale anisotropy and thatmeasured in a laminated sand-shale sequence. And, while the NMR tool isbeneficial in identifying zones with movable fluids and differentiatinganisotropic shales from laminated sand-shale sequences, the volume of sandand shale must be determined from other sources, such as the ECS tool.
0
2
4
6
8
1
3
5
7
9
Rv
/R
h
0 10 20 30
Porosity, %
40 50 60
Com
paction
7/24/2019 05 Triaxial Induction
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Summer 2008 79
resonance service, and density-neutron and OBMI
tools. In one zone, the triaxial induction
measurement resulted in an 80% increase in net-
to-gross pay calculation and increased the calcu-
lated net hydrocarbon interval by 15 ft [5 m]
from 23 to 38 ft [7 to 11.6 m] compared with
calculations using conventional logs and traditional
petrophysical techniques (above).
The butterfly plots identified the shale point
and distinguished the anisotropic shales from
anisotropic sand-shale-silt sequences. Based on
their Rv/Rh ratio, nonproductive shale intervals
exhibited anisotropy that was similar to that of
the sand-shale laminated sequences. This case
study demonstrates how NMR data can be used
with triaxial induction data to differentiate
nonproductive shales from potentially productive
sand laminations.
Another West Africa example featured two
very different shale types, and modified Klein
plots differentiated reservoir-quality rock from
shales. Two hydrocarbon-productive interval
27. Klein JD, Martin PR and Allen DF: The Petrophysics ofElectrically Anisotropic Reservoirs, The Log Analyst38,no. 3 (MayJune 2007): 2536.
28. Fanini ON, Kriegshuser BF, Mollison RA, Schn JHand Yu L: Enhanced, Low-Resistivity Pay, ReservoirExploration and Delineation with the Latest
Multicomponent Induction Technology Integrated withNMR, Nuclear, and Borehole Image Measurements,paper SPE 69447, presented at the SPE Latin Americanand Caribbean Petroleum Engineering Conference,Buenos Aires, March 2528, 2001.
29. For more on the use of modified Klein plots: Cao Minh C,Clavaud J-B, Sundararaman P, Froment S, Caroli E,Billon O, Davis G and Fairbairn R: Graphical Analysis ofLaminated Sand-Shale Formations in the Presence ofAnisotropic Shales, World Oil228, no. 9 (September2007): 3744.
30. Cao Minh C, Joao I, Clavaud J-B and Sundararaman P:Formation Evaluation in Thin Sand/Shale Laminations,paper SPE 109848, presented at the SPE AnnualTechnical Conference and Exhibition, Anaheim,California, USA, November 1114, 2007.
This paper is one of a three-part series. See also:
Cao Minh C and Sundararaman P: NMR Petrophysicsin Thin Sand/Shale Laminations, paper SPE 102435,presented at the SPE Annual Technical Conference andExhibition, San Antonio, Texas, September 2427, 2006.
Cao Minh C, Clavaud JB, Sundararaman P, Froment S,Caroli E, Billon O, Davis G and Fairbairn R: GraphicalAnalysis of Laminated Sand-Shale Formations in thePresence of Anisotropic Shales, Transactions of theSPWLA 21st Annual Logging Symposium, Austin, Texas,June 36, 2007, paper MM.
> Modified Klein plot in action. The crossplot of Rv and Rh values is shown in the butterfly plot (right). The log analyst selects thedata points that fall in the hydrocarbon region (magenta), in water-productive regions (blue) and at the shale point (green). Thecolor-coding along the resistivity track (Track 3) of the ELANPlus log corresponds to the data points manually selected by the loganalyst. Points that are not selected (black) are not presented. The water saturation values change (Track 5, yellow shading) whenRsand(red) is used rather than the uniaxial resistivity, Rh (black). The interval above 700 m has significant anisotropy (Track 4, green)but little hydrocarbon. One of the advantages of the modified Klein plots is the ability to quickly identify these nonproductive zones.
101
Rh,ohm.m
Rv,
ohm.m
101
101
100
101
102
103
100
102
103
Fshale0 0.5 1.0
Neutron Density Rh, Rv,Rsand,Rsh Anisotropy
500
Depth,
m
600
700
800
900
1,000
1,100
1,200
1,300
40 30 20 10 100
0 5 10 15
Water Saturation
100 50 0101
102
SwRsandSwRh
Rshale-v= 3.27Rshale-h= 0.51
Shale
Fshale
Rsand
7/24/2019 05 Triaxial Induction
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were separated by a nonproductive shale section,
but a zone with similar characteristics had
production potential (below). Triaxial induction
data were instrumental in properly evaluating
the well. In the upper interval, the sand count
increased by 54% and the net-to-gross ratio by
70% compared with values obtained with
conventional techniques. In the lower interval,
the increase was not as pronounced because the
sands were not as heavily laminated. Still, the
net-to-gross ratio was approximately 20% greater
after incorporating the triaxial induction data
(next page, top left). The nonproductive
anisotropic shale was identified and eliminated
from further analysis. The MR Scanner tool
provided an independent verification of net
footage of hydrocarbon.
80 Oilfield Review
> Variable shale anisotropy. These examples are from intervals with twodifferent shale types that were logged with Rt Scanner, density-neutron,OBMI and MR Scanner tools. The NMR tool and the density-neutron toolswere used as sand-shale indicators (Track 1). Anisotropy is present, asindicated by the separation between Rv and Rh (Track 3) and the Rv/Rh ratiocurve (Track 4, green shading). Rh ranges from 1 to 2 ohm.m, whereas Rsand(Track 7, red) is consistently greater than 10 ohm.m in the upper interval.Because higher resistivity corresponds to greater hydrocarbon volume,
the calculated hydrocarbon (HC) volume (Track 9) is greater when calculatedusing Rsand (red) than uniaxial induction resistivity (black). In the upper log,the anisotropy values (Track 4, green) from X,680 to X,720 look similar tothose from Y,760 to Y,820 in the lower log. Although there is high anisotropy inboth intervals, it is the result of anisotropic shales in the lower log, nothydrocarbon. The butterfly plots quickly isolate and identify thesenonproductive zones from the pay zone (magenta) as shown on theELANPlus plots.
PhisandPhisandNMR Rv ,Rh Anisotropy
OBMIGR
T2Fsand
FsandNMRRt Scanner Rsand
NMR Rsand NMR Fluids HC Volume
PayZones
X,700
X,740
Depth,
m
Depth,
m
X,660
X,620
0.5 10 0.4 0.2 0 0 0 0 0 0 0 0.2 0.4 0 0.2 0.410 1000.5 11 0 1 00 1 ,0 005 10 1510 100
40m
Shale
Cutoff
Sand
Oil
OBM
Water
NMR Fluids
0 0.2 0.4
Oil
OBM
Water
PhisandPhisandNMR NeutronDensity
NeutronDensity
Rv ,Rh Anisotropy
OBMIGR
T2Fsand
FsandNMR
Rt Scanner RsandNMR Rsand HC Volume
PayZones
PayZones
Y,850
Y,900
Y,800
Y,750
0.5 10 0.4 0.2 0 0 0 0 0 0 0 0.2 0.410 1000.5 11 0 1 00 1 ,0 005 10 1510 100
10m
Shale
Cutoff
Sand
Rt ScannerData
AIT DataNMR Data
Rt ScannerDataAIT Data
NMR Data
101
Rh,ohm.m
Rv,
ohm.m
101
101
100
101
102
103
100
102
103
101
Rh,ohm.m
Rv,
ohm.m
101
101
100
101
102
103
100
102
103
Fshale
Rsand
Rshale-v= 1.24Rshale-h= 0.52
Shale
Fshale
Rshale-v= 2.54Rshale-h= 0.58
Shale
Rsand
7/24/2019 05 Triaxial Induction
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Summer 2008 81
In the final analysis, hydrocarbon net footage
and net-to-gross ratio were more accurately
quantified from data derived from the
Rt Scanner tool and information from the
MR Scanner service. Compared with traditional
AIT induction results, there were significant
gains in calculated reserves. Modified Klein plots
were also shown to be a powerful quicklook tool
for the log analyst.
Induction DipmeterThe final two case studies demonstrate the utility
of dipmeter data derived from the Rt Scanner
service. Using induction measurements to
provide formation dip is not newthe concept
was first patented in the 1960sbut there had
been no practical application. Triaxial induction
tools provide dipmeter data as a natural by-
product of their standard data processing.
Traditional dipmeter tools are equipped with
several pads that measure small resistivity
changes occurring along the borehole wall.
Software programs correlate similar readings
from adjacent sensors and pads to compute the
dip magnitude and direction of the formation
bedding planes. Data from the sensors on the
pads produce an electrical image of the wellbore
from which structural dip, stratigraphic features
and fractures can be visualized and manually
identified using software applications.Dipmeter tools have a vertical resolution less
than 0.5 in. [1.3 cm], whereas a triaxial induction
tool has a vertical resolution measured in feet.
Although fine details cannot be resolved with the
accuracy of the FMI Fullbore Formation
MicroImager or OBMI and OBMI2 tools, the
Rt Scanner service can provide structural dip.
Dipmeter imaging tools require a conductive
mud system to acquire readings, which are then
converted into images. Because the electrica
insulating properties of oil-base-mud drilling
systems create difficulty in acquiring data
engineers developed solutions, such as the OBM
and the OBMI2 tools, to overcome the problem
Pad contact with the formation is critical
especially when tools are used in oil-base muds.
Hole conditions, such as washouts and
rugosity, make pad contact difficult and degrade
the quality of the measurement. This is true in
both oil-base and water-base muds. Tools logging
in deviated wells can experience floating pads
caused by the weight of the tool collapsing the
caliper arms and preventing the pad from
contacting the borehole wall. In addition
irregular tool motion negatively affects the
quality of the images.
The Rt Scanner tool is insensitive to borehole
conditions such as rugosity and washouts, and i
can log up orwith a modified caliperdown
By contrast, because of the need to push thepads against the borehole wall, dipmeter tools
almost always log in an upward direction. The
exception is drillpipe-conveyed FMI tools run in
horizontal wells.
Conventional dipmeter tools take thei
measurements at a very shallow depth o
investigation, which is the region most affected
by the drilling process (below). A triaxia
> Padless dipmeter. The triaxial induction measurement senses a very large volume (left). The conventional dipmeter tool (right) provides a high-resolutioimage but sees a small electrical diameter. It must also make contact with the borehole wall to acquire usable data.
Dip
Azimuth
Electricaldiamete
r90in
.
Rh
Rv
Rh
Rv
Dip
Azimuth
Interval143 m (top) NMR ToolRt Scanner ToolAIT Tool
Summary of Results
Hydrocarbon (HC), m
Net to gross (NTG)
Net change, HC/NTG
8.2
0.26
12.6
0.44
54%/70%
12.5
Interval163 m (bottom) NMR ToolRt Scanner ToolAIT Tool
Hydrocarbon, m
Net to gross
Net change, HC/NTG
18.0
0.47
20.6
0.57
14%/21%
21.3
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induction tool surveys the region beyond the
near-wellbore and is less affected by the drilling-
induced damage. Induction-derived dipmeter
data are also available from multiple arrays. The
ability to compare dips from different depths of
investigation is useful for quality control,
although variations in the dips may result
from distortions in the bedding planes away from
the wellbore.31
Because the Rt Scanner tool requires no
conductive fluid to acquire data, structural dipcan be obtained in wells where it was difficult or
impossible in the past. Induction-derived
dipmeter data do not replace information from
conventional dipmeter imaging tools, but
complement their measurement, as for example,
when bad borehole conditions degrade the data
acquired with pad contact devices.
The workflow for generating dip information
is part of the data inversion and correction
process. Bed boundaries are defined using
borehole-compensated raw data that have been
corrected for tool rotation. As a first-order
approximation to define bed boundaries, a
second derivative technique produces a squared
log from the induction array (above). The
squared log has sharper boundary edges than
conventional smoothed curves, and the sharp
transition points are used to determine where to
output dip information.
Next, the rotated, borehole-corrected curve
from a single array is output with an initial
estimation of conductivity, bed dip and borehole
azimuth. Typically a 20-ft [6.1-m] window is
inverted, but this depends on how rapidly the dip
is changing. Rv, Rh and bed boundaries are
refined with this inversion step. The software
again solves for dip and azimuth for the best fit
over the entire window. The program then moves
one-half the window length and inverts with a
generous overlap of the previous interval toeliminate edge effects. This process continues
over the entire logged interval. The result is
borehole-corrected, dip-corrected resistivity
along with structural dip and borehole azimuth,
which are presented using conventional tadpoles
and azimuth plots.
Dipmeter in Air and Water
In the USA, an Rt Scanner tool provided
formation dip and direction in an air-drilled
prospect well. Air is used instead of drilling fluid
in formations that react with the drilling mud or
in hard-rock areas where conventional drilling
techniques are less effective. Because there is no
liquid in the wellbore, conventional dipmeter
tools do not workincluding the OBMI tool.
For the well in question, two intervals with
very different characteristics are shown (next
page). The zone from X,X00 to X,X50 ft has
consistent 15 dip oriented to the south-
southeast with little variation. Although difficult
to see, there are three independent measure-
ments from three depths of investigation
presented. Throughout the interval, the tadpoles
from all three measurements overlay, indicating
agreement among the different datasets.
In a deeper interval, the data show very high-
angle formation dips, which corroborated the
geologists interpretation and expectations. Such
high-angle dipsapproaching 70might be
considered questionable were it not for core data
from nearby wells showing similar charac-teristics. An unconformity can clearly be
identified on the log at Y,Y40 ft. Also, despite
considerable hole rugosity in the Y,Y00 to Y,Y50
interval, the dipmeter data are available; a pad
contact tool may have been affected by the
condition of the borehole.
In a second example, the operator, drilling
with water-base mud, ran the Rt Scanner tool in a
deepwater Gulf of Mexico exploration well. The
FMI tool was run for comparison. The well was
deviated 60, and the true formation dip,
corrected for well deviation, was approximately
30. A comparison of the data derived from FMI
measurements and data from the Rt Scanner tool
82 Oilfield Review
31. Amer A and Cao Minh C: Integrating Multi-Depths ofInvestigation Dip Data for Improved Structural Analysis,Offshore West Africa, presented at the OffshoreAsia Conference and Exhibition, Kuala Lumpur,January 1618, 2007.
> Steps in the process, induction to dipmeter. Dipmeter information from the triaxial induction tool is an automatic output of the processing used for dipcorrection and calculating Rv (red) and Rh (blue). In block intervals, the raw data (Track 1) are corrected for borehole effects and then inverted. Bedboundaries are identified from square logs (black curve), which are the result of a second derivative technique, output to show the bed boundaries. The dipis calculated where resistivity changes are apparent. Homogeneous, isotropic intervals produce no dips because there are no step changes of resistivityin the interval. After each section is fully processed, succeeding intervals are computed with a 25% overlap to eliminate bed-boundary effects.
300
200
100
Depth
0500 0 0 10 100 1,000500
R-signal, mS/m Resistivity, ohm.m
1,000 1,500 500 0 0 10 100 1,000500
R-signal, mS/m Resistivity, ohm.m
1,000 1,500 0 10 100 1,000
Resistivity, ohm.m
25% overlap
xx
xy
xz
yx
yy
yz
zx
zy
zz
Square log
xx
xy
xz
yx
yy
yz
zx
zy
zz
Square log
Rh
Rv
Rh
Rv
Rh
Rv
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shows excellent agreement (above). A low-resistivity laminated pay section, present in this
well, could easily be overlooked using
conventional methods. Incorporating the triaxial
resistivity data in the logging suite identified the
potentially productive zones.
Future Developments
Although many enhancements have been added to
induction logging tools since the first commercial
tool was introduced more than 50 years ago, the
basic theory of the measurement has changed
little. Advancements in computer simulations and
modeling have greatly improved the industrys
understanding of the measurement. The triaxial
induction measurement of the Rt Scanner toolbrings new information to the petrophysicist, such
as dip-corrected resistivity, laminated-reservoir
properties and induction-derived dipmeter data,
as discussed in this article.
This advanced technology has opened new
possibilities and presented new needs to the
industry. Development of fast inversion routines
applied at the wellsite would provide more
accurate resistivity measurements for calcu-
lating water saturation in real time. This
additional information would improve the ability
to make informed decisions, such as in
identifying optimum locations for measuring
pressure and taking fluid samples. Also,
laminated sand-shale sequences that may have
potential as hydrocarbon reservoirs could be
identified more quickly and reliably.
Potential application has been shown for
incorporating seismic data with induction
measurements.32Although the concept is promis-
ing, it remains unclear whether multiple deep
imaging of formations can be extended to resolveseismic structures from surface-acquired data.
Commercial processing of triaxial data is
currently limited to 1D inversion and includes
the assumption that invasion does not impact the
measurement. By using 2D and 3D inversion, the
invasion effects can be determined, including the
dip of the invasion.33 This is a nontrivial task;
currently it takes a week to process 100 ft [30.5 m]
of data on a high-end PC compared with half a
minute for 1D inversion. Commercial imple-
mentation will require time and innovation
both in the processing software and in hard-
ware configurations.
Resistivity is the oldest wireline logging
measurement, but interest has been renewed in
this technology because of the triaxial induction
tool. This advance presents exciting possibilities
for petrophysical evaluation and the potential to
locate and produce previously bypassed pay. TS
> Gulf of Mexico example. This high-angle Gulf of Mexico well had 30 dip and thinly laminated sands (Track 9). The induction-derived dipmeter data(Track 8, green) show excellent agreement with the FMI data (red) in both direction and magnitude of dip. This zone includes a low-resistivity pay intervalfrom X,820 to Y,000. The conventional resistivity data used to compute water saturation indicate little hydrocarbon content (Track 6, green). Using thetriaxial induction data to compute water saturation (Track 7, green) yields considerably more oil volume.
X,750
Depth
ft
Shale
Lithology
X,800
X,850
X,900
X,950
Y,000
Y,050
Y,100
Fsand
Gamma Ray
gAPI
ft3/ft3 1.51.5
Bound Water
% 050 deg
Rt Scanner Dip
QualityFMI Image
900
Bound Water
% 050
Bulk Density
g/cm3 2.651.65
Neutron Porosity
% 060
Sand Laminated Sw
Clay-Bound Water Clay-Bound Water
ELANPlus SwRh
ohm.m 2000.2
Rv
ohm.m 2000.2
90-in. Array
ohm.m 2000.2 Water
% 050
Water
% 050
Total Porosity
% 050
Total Porosity
AIT Saturation Rt Scanner
Saturation
% 050
Quality
deg
FMI Dip
Quality
900
Quality
32. Amer and Cao Minh, reference 31.
33. Abubakar A, Habashy TM, Druskin V, Davydycheva S,Wang H, Barber T and Knizhnerman L: A Three-Dimensional Parametric Inversion of Multi-ComponentMulti-Spacing Induction Logging Data, ExtendedAbstracts, SEG International Exposition and 74th AnnualMeeting, Denver (October 1015, 2004): 616619.