Sonic Logging While Drilling—Shear Answers
Jeff Alford
Matt Blyth
Ed Tollefsen
Houston, Texas, USA
Engineers use acoustic data from sonic logging tools to drill more efficiently with
John Crowe
Chevron Cabinda Gulf Oil Company Ltd
Luanda, Angola
wave data in all formations. A new LWD acoustic tool measures shear wave data in
Julio Loreto
Sugar Land, Texas
Saeed Mohammed
Dhahran, Saudi Arabia
Vivian Pistre
Sagamihara, Japan
Adrian Rodriguez-Herrera
Bracknell, England
Oilfield Review Spring 2012: 24, no. 1.
Copyright © 2012 Schlumberger.
For help in preparation of this article, thanks to Raj Malpani,
Houston; and Utpal Ganguly, Sugar Land, Texas.
Mangrove, Petrel, SonicScope, Variable Density and
VISAGE are marks of Schlumberger.
4
greater safety margins and to optimize completions. LWD sonic tools introduced in
the mid-1990s delivered compressional wave data but were unable to provide shear
formations where this was previously impossible, and engineers are using this
information to drill with greater confidence, determine optimal directions for drilling
and identify rocks with better completion characteristics.
The downhole drilling environment creates
inhospitable conditions for logging-while-drilling
(LWD) tools. The drill bit grinds through layers of
rock as the rotating drillstring and BHA continually slam against the borehole wall, shocking sensitive electronic components. Drilling mud surges
through the drillpipe and exits through the bit,
sweeping the hole clean and returning cuttings to
the surface. Although LWD tools are designed to
endure these environments, LWD sonic tools are
further required to acquire data in a setting inundated with noise and vibration.
Sonic acquisition is challenging while drilling; however, service companies have worked to
develop LWD sonic tools because they provide
information that is not readily available from
other logging devices while drilling. Measurements derived from the propagation of sound
waves through porous media provide helpful
information about geologic and geophysical
Oilfield Review
Some Sound Basics
Acoustic logging tools measure the time it takes
for an audible pulse of sound to travel from a transmitter, through the mud, along the borehole, back
through the mud and then to an array of receivers
along the body of the tool. This measured time
equals the cumulative time of travel through the
various media that have been traversed.
The velocity of the sound wave measured
across the receiver array is the speed of sound
through the formations directly opposite the
receivers. Petrophysicists refer to this measurement as slowness—the inverse of velocity; it is
expressed as traveltime per unit length. This
measurement is also referred to as a delta t (Δ t)
measurement because it is the interval transit
time for the sound wave to travel through 1 m or
1 ft of formation.
Spring 2012
S-wave and
Rayleigh wave arrivals
Transmitter
pulse
Amplitude
properties. Petrophysicists have developed methods to use real-time acoustic measurements to
determine formation attributes that include pore
pressure and overburden gradients, lithology and
mechanical properties. Petrophysicists also use
sonic data for gas detection, fracture evaluation
and seismic calibration.
The first LWD sonic tools, introduced in the
mid-1990s, provided compressional wave measurements, along with shear wave data in some formations. These data were used for computing sonic
porosity, estimating pore pressure and correlating
downhole depth-based data with surface seismic
time-based data. Wireline sonic tools used different sources and, because they could process and
transmit data at higher rates, provided answers
that were beyond the capability of their early LWD
counterparts. These capabilities include measurements of high-quality compressional and shear
wave information to estimate geomechanical properties in soft formations and the ability to determine the orientation of rock properties in
anisotropic formations. A recently introduced
LWD sonic tool provides real-time compressional
and shear wave data in formations where this was
not possible with earlier tools.
This article reviews the use of sonic data in oil
and gas operations, with special emphasis on
LWD tools. A discussion of quadrupole sonic measurements is included, along with the process of
deriving mechanical properties from sonic data.
Case studies demonstrate how engineers have
been able to extract shear data in soft formations
using quadrupole sonic modes. These data, along
with compressional data, are then used to optimize drilling practices, monitor real-time pore
pressure while drilling, improve completions and
estimate geomechanical formation properties.
Stoneley wave
arrivals
Time
P-wave
arrivals
Mud wave
arrivals
> Acoustic waves. Sonic tools measure the time it takes for an acoustic
pulse of sound to travel from a transmitter to a receiver array. The sound
wave strikes the borehole, travels through the formation and then arrives
back at the tool where the receivers measure the amplitude of the signal
versus time. As the sound wave passes through rock, different types of
waves are generated. The first two arrivals are the compressional, or
P-waves, followed by shear, or S-waves. These two are the most important
for oilfield applications because they are used to compute porosity and
mechanical properties. Rayleigh, mud and Stoneley waves arrive later.
Sound waves propagate through a solid medium
in a variety of modes, such as compressional and
shear waves, and these modes have different velocities (above). In addition to these, other modes,
including Rayleigh, mud and Stoneley waves, can
be identified in the sonic signal.1
Many materials have been characterized by
their acoustic slowness (below). For instance, a
compressional sound wave travels through steel
at 187 μs/m [57 μs/ft]. Compressional waves
travel through zero-porosity sandstone at approximately 182 μs/m [55.5 μs/ft] and through
limestone at around 155 μs/m [47.3 μs/ft].
Compressional waves that pass through formation rocks containing water, oil or gas have longer
traveltimes than through rocks with no porosity.
Material
Steel
The change in traveltime is related to the volume
of fluid in the rock’s pore space, which is a function of the porosity. Sonic porosity measurements
were a key driver in the initial development of
acoustic logging tools.
Depending on the physical measurement
needed, the acoustic logging tool can be designed
with transmitters, or sources, to generate a particular type of pressure pulse. The most basic form,
and the type that is common across all forms of
acoustic tools, is the monopole source. Monopole
sources produce a radial pressure field, analogous
to the wave pattern produced by a pebble dropped
onto the surface of a pond but in three dimensions.
They are used primarily to obtain the compressional slowness of the formation.
Compressional
Slowness Time
Δt c, μs/m [μs/ft]
Shear
Slowness Time
Δt s, μs/m [μs/ft]
187 [57]
338 [103]
Sandstone
182 [55.5]
289 [88]
Limestone
155 [47.3]
290 [88.4]
143 [43.5]
236 [72]
Dolomite
200 to 300 [61 to 91]
varies
Freshwater
715 [218]
Not applicable
Brine
620 [189]
Not applicable
Shale
> Characteristic values for compressional wave slowness (Δtc) and shear
wave slowness (Δts).
1. Rayleigh waves, named for Lord Rayleigh, who predicted
their existence in 1885, are frequency-dependent
dispersive waves that travel along the surface of the
borehole. Rayleigh waves are used to evaluate velocity
variation with depth. Mud waves are arrivals from the
original sonic pulse that have traveled from the
transmitter through the mud column and are then
detected at the tool receivers. Stoneley waves, named for
Robert Stoneley, are surface waves that are associated
with the solid/fluid interface along a borehole wall. They
are used to estimate fracture density and permeability.
5
As part of the process to measure compressional slowness, a monopole source generates a
compressional wave in the borehole fluid surrounding the tool. The wave pattern expands
radially, traveling at the compressional slowness
of the fluid, until it encounters the borehole wall,
where some of the energy is reflected back and
some is refracted into the formation (below).
Snell’s law defines the relationship between
the angle of refraction and the ratio of sound
velocities in the fluid and the formation.2 The
energy that is critically refracted travels along
the borehole wall toward the receivers. The
refracted energy propagates through the formation as a compressional wave and travels faster
than the fluid wave because the formation is
stiffer than the fluid.
The critically refracted compressional wave
generates a head wave in the borehole that travels
at the formation compressional velocity.3 Following
Huygens principle, at each point along the borehole wall, the compressional wave acts as a new
source, transmitting waves back into the borehole.
Fast Formation
The compressional head wave eventually arrives at
the receiver array, allowing computation of the
compressional velocity of the formation.
When the compressional wave from a monopole source is refracted into the formation, some
compressional energy is converted to shear waves
that refract into the formation. Whereas compressional waves propagate through both the
fluid-filled borehole and the porous rock matrix,
shear waves are not supported by fluids and propagate through fluid-filled porous media by traveling from grain to grain through the rock matrix. If
the shear slowness in the formation is less than
the compressional slowness in the borehole
fluid—a situation known as a fast formation—
the refracted wave is critically refracted and generates a shear head wave in the borehole. This
head wave travels at the shear velocity of the formation and may be detected by the receiver
array. In this way, monopole acoustic tools can
provide shear velocities, but only in the case of
fast formations.
Slow Formation
Wellbore
Wellbore
Compressional wave
Compressional wave
Head waves
Shear wave
Head wave
Fluid wave
Fluid wave
Monopole
source
Monopole
source
Stoneley wave
P-wave
Stoneley wave
Transmitter-receiver distance
Transmitter-receiver distance
P-wave S-wave
Traveltime
Traveltime
> Sonic waves from monopole sources. Monopole sonic tools generate a pulse of energy that strikes
the formation and then travels along the borehole as a compressional head wave. In hard, or fast,
formations (top left), the compressional wave, or P-wave, generates shear waves, or S-waves, that
arrive later in time than P-waves (bottom left). Soft, or slow, formations (top right) sustain shear
waves, but they are refracted into the formation and may not arrive at the receivers (bottom right).
Current tools have multiple receivers, and the sonic signal arrives later as the transmitter-receiver
distance increases. Although the signal amplitude diminishes with distance between transmitter and
receiver, data can be time shifted and stacked to improve coherence and signal-to-noise ratio.
Stoneley waves (green) arrive later in time than the P- and S-waves.
6
If the shear slowness in the formation is
greater than the compressional slowness in the
borehole fluid—a condition known as a slow formation—the compressional wave will still refract
upon reaching the borehole, but the angle of
refraction is such that critical refraction never
occurs, and no head wave is produced in the borehole. Therefore, no shear head wave is detected
at the receivers, and the shear velocity cannot be
determined. This is a fundamental limitation of
monopole sources for acoustic logging.
The ability to measure shear slowness with a
monopole source thus depends on both borehole
fluid and formation properties. Borehole fluid
slowness values vary from around 620 μs/m
[189 μs/ft] for water-base muds to 787 μs/m
[240 μs/ft] or slower for synthetic oil-base muds.
Slow formations are common at shallow well
depths because of a lack of compaction by overburden pressure. For the same reason, slow formations are common in deepwater drilling
environments. Shear data, which are crucial for
determining wellbore strength and stability in
slow formations, cannot be extracted from data
acquired with tools that employ only monopole
sources. In wellbore sections where these data
are often most needed, they are unavailable.
Limitations of monopole sources in measuring shear wave data in slow formations led to the
development of dipole logging technology.4 Tools
with dipole sources generate a flexural wave that
is analogous to shaking the borehole (next page).
Flexural waves are dispersive—their speed varies with frequency—and at low frequencies, they
travel at the velocity of shear waves. Tools with
dipole sources have the ability to deliver shear
slowness measurements regardless of the mud
slowness; therefore, they are useful for obtaining
slowness measurements in slow formations.
The dipole source is also directional in nature,
and by using directional receiver arrays and two
such sources separated by 90°, engineers are
able to derive oriented shear data from around
the wellbore. This cross-dipole measurement provides information such as maximum and minimum stress azimuths, radial velocity profiles with
distance away from the borehole wall and the
orientation of anisotropic shear data.
Wireline acoustic logging tools that combined
a monopole source for compressional and shear
data in fast formations and cross-dipole sources
for oriented shear data in slow formations were
introduced in the 1980s. Service companies continue to use tools of this type, although current
wireline tools with these sources can deliver a
Oilfield Review
Borehole
Radial Displacement
Radiation Patterns
Compressional wave
Nondeformed
cross section
Radial
displacement
Monopole mode
Cross Section
90
180
270
360
Azimuth
Flexural wave 1
Flexural wave 1
Flexural wave 2
Flexural wave 2
Radial
displacement
Dipole mode
90
180
270
360
Azimuth
Quadrupole wave
Radial
displacement
Quadrupole mode
90
180
270
360
Azimuth
> Acoustic sources. Three types of acoustic sources are used in well logging: monopole (top), dipole (center) and quadrupole (bottom).
Monopole sources generate sound waves that radiate from the tool and travel through the formation as compressional waves. Dipole
sources generate directional flexural waves. Cross-dipole sources emit two flexural waves that are oriented 90° apart. Quadrupole sources
generate complex waveforms that are frequency dependent. At very low frequencies, they travel at velocities that approximate the velocity
of shear waves. The blue stars represent the approximate location along the borehole of the wave represented in the cross section.
wider range of measurements for petroleum
applications than the earlier tools could.5
A third acoustic source, which was recently
introduced for oilfield applications, generates
quadrupole waves. At very low frequencies, these
waves travel through the formation at a speed
comparable to that of shear waves. As with dipole
shear data, the quadrupole data converge asymptotically to the shear wave velocity.6 Although
somewhat similar to dipole waves, they exhibit a
different propagation pattern, which is more difficult to conceptualize. Another term applied to
them—screw waves—presents an image of how
they travel along the borehole. At present, service companies use quadrupole sources in LWD
tools only.
The Rise of LWD Acoustic Tools
Wireline acoustic tools deliver high-quality measurements in a relatively low-noise environment,
but they have shortcomings. The lag between
drilling and logging, along with conveyance methods needed to deploy wireline tools, presents
complications. Delivering tools to TD in extendedreach horizontal wells can also be complicated
and time consuming, although a number of conveyance techniques have evolved over the years.7
Furthermore, wireline sonic tools should be centralized, and tool weight can make this problematic in high-angle and horizontal wellbores. In
addition, shutting down drilling operations while
logging dramatically increases the incremental
cost of the logging operation, particularly in
deepwater drilling operations where rig spread
rates—the total daily operating cost—routinely
reach US$ 1 million.
For many applications, including pore pressure prediction and wellbore stability analysis,
the ability to acquire data during the drilling process, and use the data as soon as possible, significantly increases the value of the data. Wireline
measurements are obtained days or even weeks
after a formation has been drilled, and therefore
may be useful only for problem review or for planning future wells.
Acoustic data are also affected by borehole
conditions and challenges—such as mud filtrate
invasion and rugosity—that may introduce measurement errors, the severity of which tends to
increase with time after an interval has been
drilled. Additionally, in settings involving damaged
2. Dutch mathematician Willebrord Snellius is credited with
formulating the laws of refraction of waves. Snell’s law
states that the ratio of the sines of the angles of
incidence, i, and refraction, R, is equivalent to the ratio of
phase velocities, V, in the two media. In this case, the
media are the mud, m, and the formation, ƒ . The
relationship can be written as follows:
V
sin i
= m .
sin R
Vƒ
Critical refraction occurs when the angle of refraction is
greater than or equal to 90°, meaning the wave travels
along the borehole wall.
3. Named for Dutch scientist Christiaan Huygens, the
Huygens principle states that every point of a wavefront
may be considered the source of secondary wavelets
that spread out in all directions with a speed equal to the
speed of propagation of the waves.
4. For more on cross-dipole sonic tools: Brie A, Endo T,
Hoyle D, Codazzi D, Esmersoy C, Hsu K, Denoo S,
Mueller MC, Plona T, Shenoy R and Sinha B: “New
Directions in Sonic Logging,” Oilfield Review 10, no. 1
(Spring 1998): 40–55.
5. For more on advances in sonic logging: Arroyo Franco JL,
Mercado Ortiz MA, De GS, Renlie L and Williams S:
“Sonic Investigations In and Around the Borehole,”
Oilfield Review 18, no. 1 (Spring 2006): 14–33.
6. A dispersion plot of shear slowness from dipole data
versus the frequency of the acoustic wave will converge
asymptotically on the formation shear slowness.
7. For more on logging tool conveyance methods:
Billingham M, El-Toukhy AM, Hashem MK, Hassaan M,
Lorente M, Sheiretov T and Loth M: “Conveyance—
Down and Out in the Oil Field,” Oilfield Review 23, no. 2
(Summer 2011): 18–31.
Spring 2012
7
Wireline Dipole
LWD Dipole
Wellbore
Wellbore
Tool
Tool flexural
response
Tool
Formation flexural
response
Formation shear
slowness
Slowness
Slowness
Weak interference
Strong interference
Shear asymptote
LWD dipole sonic
tool response
Frequency
Frequency
> Dipole sources in wireline and LWD tools. Flexural waves from dipole sources are dispersive. A
wireline tool (left) in the borehole is designed so that the flexural signal (blue line) passing through the
body of the tool does not interfere with the formation flexural slowness data (red). Slowness data
plotted versus frequency on a dispersion plot will approach the formation shear slowness value at the
asymptote (horizontal dashed line). To withstand the rigors associated with drilling, LWD sonic tools
(right) are built into a stiff drill collar. The flexural wave (green) that propagates through an LWD tool
interferes with and distorts the measurement (heavy dashed black line) such that it does not follow
the shear slowness asymptote of the formation flexural response (red). For this reason, service
companies have adopted quadrupole sources rather than dipole sources for LWD sonic tools.
or unstable boreholes, wireline tools may not be
able to reach TD, or operators may choose to forgo
logging operations out of concern for tool sticking.
These concerns led, in part, to the development of
LWD acoustic tools.
The LWD sonic tools introduced in the
mid-1990s used monopole sources and measured
formation compressional slowness.8 These measurements were made available in real time by
sending the acoustic data, along with other LWD
measurements, to the surface using mud pulse
telemetry systems.
Engineers could monitor pore pressure
trends and compute sonic porosity from compressional data, and geophysicists could relate
depth-derived borehole events with time-based
surface seismic events. Using pore pressure
trends measured while drilling, engineers can
avoid hazards such as drilling into overpressured
zones and can optimize drilling mud density. For
advanced processing, such as extracting shear
data in fast formations, full waveforms for each
transmitter firing were available, but were
stored in memory and retrieved when the tools
returned to surface.
Over the years, LWD sonic tools have evolved
through several stages, primarily focusing on
enhancing reliability and consistency of monopole-derived answers and increasing the amount
of data available in real time. One hurdle to the
development of LWD sonic tools was accounting
for the energy from the transmitter that arrived
at the receiver array after passing through the
body of the tool. For integrity during drilling and
because they must be as strong as the rest of the
drillstring, LWD tools are built into steel drill
8. Degrange J-M, Hawthorn A, Nakajima H, Fujihara T and
Mochida M: “Sonic While Drilling: Multipole Acoustic
Tools for Multiple Answers,” paper IADC/SPE 128162,
presented at the IADC/SPE Drilling Conference and
Exhibition, New Orleans, February 2–4, 2010.
9. For a detailed explanation of quadrupole modeling and
processing: Scheibner D, Yoneshima S, Zhang Z,
Izuhara W, Wada Y, Wu P, Pampuri F and Pelorosso M:
“Slow Formation Shear from an LWD Tool: Quadrupole
Inversion with a Gulf of Mexico Example,” Transactions
of the SPWLA 51st Annual Logging Symposium, Perth,
Western Australia, Australia, June 19–23, 2010, paper T.
10. Scheibner et al, reference 9.
11. The SonicScope tool can also generate cross-dipole
flexural waves but they are not currently used.
12. Bulk density is usually provided by a density porosity
measurement.
13. Named for 17th century British physicist Robert Hooke,
this law states that the strain within an elastic material
is proportional to the applied stress. For anisotropic
media, the law can be expressed as a second-rank
stiffness tensor.
14. Zoback MD: Reservoir Geomechanics. New York City:
Cambridge University Press, 2007.
8
collars. Sound waves propagate easily through
these collars and their arrival at the receivers
overwhelms the signals from the formation.
Eliminating collar arrivals was a considerable
problem for early generation tools.
Slotted tool housings and materials designed
to attenuate tool arrivals for wireline sonic tools
are not an option for LWD tools, so engineers had
to develop other methods to limit the energy
coupled directly from the collar. Early generation
LWD sonic tools featured heavily grooved collars,
which were successful in limiting the effects of
tool arrivals on the measured data. This design,
however, resulted in a collar that was more flexible than the rest of the BHA, which increased the
tool’s susceptibility to shock, vibration, tool tilt
between receivers and eccentering.
One of the most crucial shortcomings that
engineers sought to address was the inability to
obtain shear data in all formations, which monopole sources could not do. Engineers first
attempted to replicate the physics upon which
wireline tools are based. Experimenting with
dipole sources, they discovered that at precisely
the frequency range needed to acquire shear
information in most formations, there was interference between the dipole collar flexure signal
and the formation signal (above left). Therefore,
instead of dipole measurements, Schlumberger
and other service companies adopted a quadrupole technique for LWD sonic tools.9
As with dipole waves, quadrupole waves are
dispersive, meaning their velocity depends on
frequency. At low frequencies, the velocity
approaches an asymptote equal to the shear
velocity of the formation. Processing and an
inversion technique extract shear slowness values from the measured quadrupole dispersion
data. However, because the low-frequency components of the quadrupole signal attenuate
quickly, the quadrupole dispersion profile does
not reach the asymptote of formation shear speed
as clearly as the dispersion data from flexural
waves created by dipole sources.
The more dispersive profile of quadrupole
data may result in a wave velocity that falls below
the actual formation shear speed. Quadrupole
data are affected by formation properties, borehole conditions, drilling mud properties, tool
characteristics and the tool’s presence and position in the wellbore. It is crucial that engineers
understand these effects, which can be tool specific, to extract shear slowness from quadrupole
data. In addition, the processing of quadrupole
data is more complex than the processing of
dipole data.10
Oilfield Review
Engineers have performed extensive modeling
and testing to confirm the validity of quadrupole
source technology and of the processing technique
used to extract shear data in slow formations.
Because of these efforts, quadrupole sources are
the common mode used by service companies for
extracting shear data in slow formations using
LWD tools, although the methods of extracting the
answers differ from company to company.
Quadrupole LWD sonic tools offer answers
that were not available from monopole tools.
However, they do not yet fully replace the capabilities of cross-dipole wireline tools because
quadrupole sources are not directional. But this
newly acquired ability to deliver shear data for
fast and slow formations in real time greatly
increases the value of LWD sonic tools.
The Scope of LWD Tool Design
To address the need for a quadrupole LWD tool,
Schlumberger developed the SonicScope multipole sonic while drilling tool. The SonicScope
tool has a wide spectrum of applications because
it can acquire data in several modes. Although
the answers depend on the type of data acquired
and how it is processed, drillers, geophysicists,
geologists, petrophysicists, reservoir engineers
and completion engineers can all use the information it provides.
The SonicScope tool acquires monopole and
quadrupole measurements using a powerful broadband transmitter that excites the borehole in both
modes over a frequency range from 1 to 20 kHz.11
There are 48 receiver sensors with 10-cm [4-in.]
spacing mounted on the outside of the tool in protected grooves positioned 90° apart (above right).
The receivers are arranged in four arrays that provide 12 axial and 4 azimuthal measurements. Each
array contains 12 digitizers, one for each sensor.
The transmitter-to-receiver spacing is optimized
to maximize the signal-to-noise ratio. The tool’s
1-GB memory capacity enables the recording of all
modes even with data recording rates of up to once
per second. The current version of the tool has a
43/4-in. diameter; larger tools, with diameters of 63/4,
81/4 and 9 in., are in development.
Generally, the tool is programmed in the field
to record high-frequency monopole measurements for compressional slowness and shear
slowness in fast formations, low-frequency monopole data for Stoneley waves and quadrupole data
for shear acquisition in slow formations. For the
quadrupole mode, data are acquired in a frequency range down to 2 kHz. From dispersion
analysis, which uses an inversion algorithm to
Spring 2012
Wideband multipole transmitter
48 wideband digital receivers
> SonicScope LWD tool. Built into a stiff drill collar that is about 9 m [30 ft] in length, the SonicScope
tool has a wideband multipole transmitter and can be programmed to acquire data in several modes.
The 48 receivers located on the outside of the tool are 4 in. apart and provide high-resolution data at
high spatial density.
best fit modeled responses, engineers can extract
shear slowness values lower than 2,000 μs/m
[600 μs/ft]. The SonicScope tool is fully combinable with other MWD and LWD tools. When combined with other measurements, such as density
data, the acoustic data offer solutions for applications that include seismic correlation, pore pressure determination, log interpretation in complex
lithologies and geomechanical rock properties.
Using the Data
In situ geomechanical properties cannot be measured directly; however, they can be computed
using compressional and shear slowness values
in combination with the bulk density of the
rock.12 For the case of isotropy, in which material properties are the same in every direction,
geomechanics specialists apply Hooke’s law of
elasticity to derive simple equations that use
log-derived measurements to calculate several
elastic moduli (right).13 The compressional
modulus, M (also referred to as the P-wave or
longitudinal modulus), is computed from compressional wave data. Similarly, the shear modulus, G, a measure of a material’s ability to
withstand shearing, is computed from the shear
wave data. Once these two values are determined, the bulk modulus, K; Young’s modulus, E;
and Poisson’s ratio, ν, can be calculated.
The bulk modulus is the ratio of average normal stress to volumetric strain and is the extent
to which a material can withstand isotropic
compressive loading before failure. Young’s
modulus relates strain to stress in one direction
and is a measure of the stiffness of a material.
Stiffer rocks have higher Young’s modulus values and are easier to fracture than rocks with
lower values. Poisson’s ratio, which is the ratio
of transverse strain to axial strain, is related to
closure stress; rocks with higher Poisson’s ratio
values are more difficult to fracture and prop
open than those with lower values.14 Targeting
intervals for hydraulic fracturing that have
higher Young’s modulus values and lower
Poisson’s ratio values may improve stimulation
performance and well productivity.
M=
a ρb
(Δtc ) 2
K =M –
.
4G .
3
ν=
G =
E =
a ρb
(Δt s ) 2
.
9KG .
3K + G
3K – 2G .
6K + 2G
> Hooke’s law and isotropic elastic moduli. For
the case of isotropic rocks, engineers use three
log-derived measurements to come up with five
mechanical properties. The compressional
modulus, M, is computed from the compressional
slowness time (Δtc) and bulk density, ρb. The
shear modulus, G, is calculated from the shear
slowness time (Δts) and bulk density. The a in
both equations is a unit conversion constant. In
turn, these two moduli are used to compute the
bulk modulus, K, Young’s modulus, E, and
Poisson’s ratio, ν.
9
However, the simple equations relating logderived measurements to mechanical rock properties are not valid when elastic anisotropy is
encountered.15 The general formulation relating
stress to strain as described by Hooke’s law is
represented by a fourth-order stiffness tensor
that has 81 elastic constants and summations.
Although symmetry reduces the number of con-
stants to 21, deriving the relationships used to
determine mechanical properties in an anisotropic formation is a formidable task that is beyond
the scope of this article.
When acoustic data are available, engineers
use these data to compute pore pressure, derive
elastic properties and correlate downhole data
with surface seismic results. Drilling engineers
Depth, ft
Attenuation Resistivity
0.2
Gamma Ray
0
gAPI
ohm.m
2,000
Phase Shift Resistivity
150 0.2
ohm.m
2,000 150
LWD Sonic Slowness
μs/ft
50
X2,000
Compaction
trend
X3,000
X4,000
X5,000
X6,000
X7,000
X8,000
> Watching for trends. Real-time LWD gamma ray data (Track 1) indicate the
well is penetrating shale in the upper half of this section. As long as the bit
remains in a shale section, there is little potential for encountering
overpressure and taking a kick; however, should the bit enter a permeable
zone, there is a risk of influx of formation fluids. The driller would typically
choose to manage overpressure by increasing mud weight, but if the
shallower formations are not strong enough to sustain mud weights
sufficient to control an overpressured condition, casing must be set.
Because changes in lithology or fluid can mask changes in the pressure
regime, resistivity (Track 2) may not always indicate overpressured
conditions. The increase in sonic slowness (Track 3) at around X5,000 ft
indicates a potential overpressured condition (red shading). If real-time
shear data are available from the LWD sonic tool, engineers can compute
the strength of shallower formations and determine the thresholds for mud
weight maximum values.
10
use pore pressure to facilitate drilling and
increase safety margins. Using mechanical properties derived from sonic data, they can optimize
drilling programs and validate their ability to follow a given well profile while maintaining wellbore stability. Completion engineers use these
same data to design stimulation programs.
Geophysicists refine seismic data acquired at the
surface using information derived from downhole
sonic data.
Real-time data from LWD sonic tools have two
main applications for pore pressure determination: identifying overpressured formations and
selecting mud density (left). For drilling engineers, overpressured zones present hazards that
can range from mildly annoying to catastrophic.
Optimizing mud weights to maintain borehole
stability and drill safely may result in considerable cost savings.16
During lithification, sediments are compacted by overburden pressure and fluids are
expelled. The effects of compaction can be
observed in sonic slowness data as a steady
decrease in the compressional slowness. This is
most obvious in shale intervals. Conversely, when
fluids cannot escape, the formation retains fluids
and becomes overpressured. Higher fluid content
results in higher compressional slowness values.
Drilling through overpressured shale zones
usually does not pose a hazard because these
zones have inherently low permeability; however,
should the bit encounter a porous layer that is
overpressured, the hydrostatic pressure in the
wellbore may be insufficient to contain the pore
pressure. The result may be a rapid influx of reservoir fluids, or a kick. In extreme cases, the well
may blow out.
Engineers can also use mechanical properties
computed from acoustic data to construct a 1D
mechanical earth model using programs such as
the single-well geomechanics module in Petrel
seismic-to-simulation software (next page, top).
The models can be adjusted while drilling using
real-time data from LWD sonic tools. Such models
allow drillers to maintain a drilling mud density,
or mud weight, that strikes a balance between
the hydrostatic pressure in the wellbore and any
anticipated increase in reservoir pore pressure.
There is a point, however, at which raising
the mud weight can cause weaker rocks to fail.
Pore pressure prediction programs can determine the maximum mud density that can be
maintained before the formation breaks down.
When the maximum mud weight threshold is
reached, casing is run to isolate weaker formations. A mistake of a few meters can result in an
expensive extra casing run or create hazardous
Oilfield Review
15. For information on application of sonic data in
formations with elastic anisotropy: Armstrong P,
Ireson D, Chmela B, Dodds K, Esmersoy C, Miller D,
Hornby B, Sayers C, Schoenberg M, Leaney S and
Lynn H: “The Promise of Elastic Anisotropy,”
Oilfield Review 6, no. 4 (October 1994): 36–47.
16. Brie et al, reference 4.
17. King GE: “Thirty Years of Gas Shale Fracturing: What
Have We Learned?” paper SPE 133456, presented at the
SPE Annual Technical Conference and Exhibition,
Florence, Italy, September 19–22, 2010.
Spring 2012
Effective stress, psi
800 600 400 200
0
–200 –400
> Integrating sonic data. By including sonic data in reservoir models, such as this Petrel example,
operators can design wellbore profiles that are compatible with the mechanical properties of the
formation. Geoscientists compute mechanical properties from surface seismic data, and the LWD
sonic data are used to update models in near real time. For instance, advanced computations deliver
stress profiles that vary in a complex manner around the wellbore projection, and are graphically
displayed along a near-wellbore grid (shown encircling the wellbore). These displays allow engineers
to better understand the borehole geomechanical status and adjust well plans to safely reach a target
(lower green area). The magenta background to the left represents Young’s modulus, an elastic
parameter used to define the stress state, determined from seismic inversion. These types of
information can be updated with downhole sonic data as the well is drilled. Sonic data can also tie
time-based surface seismic data, such as the cross section displayed on the right, to specific depth
references downhole.
Mud weight safety margin
drilling conditions. Mechanical properties of the
formations must be known in order to determine
the mud density limits.
Once the mechanical properties are computed from compressional and shear slowness
data, geomechanical modeling programs can provide solutions to drilling and completion questions. Examples of modeling programs are
VISAGE reservoir geomechanics modeling software and Mangrove reservoir-centric stimulation
design software. VISAGE software is a full-scale
reservoir-modeling program that engineers use to
predict behavior during drilling, injection and
production. Using finite-difference methods, the
software calculates detailed 3D and 4D models
that can display patterns of pressure, stress,
strain, porosity and permeability at specific
points or across an entire reservoir (below right).
Fracture stimulations in conventional reservoirs
can be modeled along with expected production.
Mangrove software was developed for use with
unconventional reservoirs.
An example of how geomechanical data are
used in the development of unconventional
resource plays is in identifying targets with
better characteristics for multistage fracture
stimulation. Spacing and location of perforation
clusters are crucial elements in stimulation
design of these reservoirs.17 A manual approach
of identifying intervals with completion-quality
rock can be tedious. However, current industry
practice of designing stimulation jobs with
evenly spaced perforation clusters regardless of
variations in rock properties can result in suboptimal recovery.
Other key challenges in completion design
involve modeling the complex fracture networks
that are frequently observed in unconventional
reservoirs and evaluating their impact on production. Accounting for heterogeneity in completion
design can help engineers enhance well productivity, especially by identifying changes in geomechanical properties—paticularly those that can
be derived from sonic data. The absence of a single integrated solution to incorporate rock heterogeneity has been an impediment to optimizing
fracture stimulation designs.
Low
High
Narrow operational window
Negative operational window
> Drilling through operational windows. After populating 3D and 4D models with mechanical
properties, engineers can perform field-scale stress simulations to determine magnitude and
orientation of stresses (cyan crosses). Areas to be avoided within the reservoir can be identified,
such as those shown in red in the background. Narrow operational windows, which may correspond
to many factors, including maximum mud weight, regions of high fluid loss and mechanical
instability, are displayed in 3D, allowing engineers to choose a well path that maximizes safety and
efficiency. Drillers may decide to set casing above or below these zones, or proceed with caution,
aware of the increased risks. An acceptable path can be located between areas with narrow
operating windows (purple). The vertical cross section also provides detailed information about the
effects of a nearby salt dome on the stress field. The mud weight safe operating margins, computed
from seismic and sonic data, actually increase from top to bottom, which is the opposite of
conditions in most reservoirs. The corresponding color changes go from blue (low safety margin) to
green to yellow to orange (high safety margin).
11
Geometrically placed perforation clusters
Rock quality
Good CQ and good RQ
Good CQ and bad RQ
Bad CQ and good RQ
Bad CQ and bad RQ
Rock quality
Stress
Stress
Low
High
Selectively placed perforation clusters
Rock quality
Stress
> Logging data for fracture design. In unconventional reservoirs, such as gas shales, operators
frequently use geometry (top) rather than geology and geomechanics to determine fracture staging
and perforation cluster locations. LWD acoustic data, such as those from the SonicScope tool, can
identify rocks with low stress, which offer better completion quality (CQ), and petrophysical analysis
can identify intervals with better reservoir quality (RQ). The Mangrove program generates a composite
quality score combining CQ and RQ to rank the rock along the wellbore, recommends preferred
locations for perforation clusters and groups similar rocks in treatment stages (bottom). The stress is
presented beneath the well projection and ranges from low (red) to high (blue). The same number of
perforation clusters are used in both examples—colored ovals represent perforation clusters in each
stage—but in the recommended results they are concentrated in better quality rock (blue, green and
yellow), and poor quality rock (red) is avoided. Operators following this engineering approach for
completion design have seen substantial improvement in production. (Adapted from Cipolla et al,
reference 18.)
To address unconventional hydraulic fracture
design and to help optimize fracture stimulations, Schlumberger engineers developed
Mangrove stimulation modeling software
(above).18 The software incorporates seismic,
geologic, geomechanical and microseismic data
along with reservoir simulations to model fracture propagation and geometry. The software
includes two different fracture simulators that
are designed for complex hydraulic fracture modeling. They are linked to reservoir models for
optimizing fracture design and production forecasting. Reservoir and completion quality are
quantified from these multidomain reservoir data
so that completion engineers can optimize stage
placement and perforation programs.
Operators recognize the benefits of using acoustic log data for well and completion design.
Acquisition of data in extended-reach horizontal
wellbores has been problematic with wireline tools
because it is difficult to convey them to TD and it is
hard to keep the tools centered in the wellbore.
LWD sonic tools, designed to acquire data in these
types of environments, provide real-time formation
mechanical properties that may improve drilling
decisions and stimulation programs.
18. Cipolla C, Weng X, Onda H, Nadaraja T, Ganguly U and
Malpani R: “New Algorithms and Integrated Workflow
for Tight Gas and Shale Completions,” paper SPE 146872,
presented at the SPE Annual Technical Conference and
Exhibition, Denver, October 30–November 2, 2011.
19. Mohammed S, Crowe J, Belaud D, Yamamoto H,
Degrange J-M, Pistre V and Prabawa H: “Latest
Generation Logging While Drilling Sonic Tool: Multipole
Acoustics Measurements in Horizontal Wells from
Offshore West South Africa,” Transactions of the
SPWLA 52nd Annual Logging Symposium, Colorado
Springs, Colorado, USA, May 14–18, 2011, paper CC.
12
Horizontal Application
Chevron Cabinda Gulf Oil Company Ltd uses
acoustic data to optimize drilling and completions in a Lower Congo basin field offshore
Angola.19 Shear data are required for computing
mechanical properties, which are then used in
well design to ensure wellbore stability. Engineers
planned to acquire SonicScope data from two
separate 6-in. horizontal boreholes, drilled
sequentially, to confirm that high-quality shear
data could be extracted while drilling. Along with
the SonicScope tool, the LWD logging program
included azimuthal density, neutron porosity and
resistivity tools.
The reservoir consists of unconsolidated thinbedded sands. To maximize exposure to the thin
layers, lateral wellbores are drilled with sinusoidal trajectories. Interval A was drilled to a measured depth of 4,570 ft [1,390 m] and then,
without pulling out of the hole, the sidetrack
Interval B was drilled to 4,240 ft [1,290 m]. The
deviation ranged from 78° to 93° in Interval A
and from 80° to 97° in Interval B.
The primary focus for the study was to compare the compressional and shear measurements obtained using monopole sources with
measurements extracted from quadrupole data.
The engineers programmed the tool to obtain
high-frequency monopole, low-frequency monopole and low-frequency quadrupole waveform
data, which were acquired while running in the
hole and while drilling in open hole. Highfrequency monopole data were also acquired
while in casing. Compressional slowness data
were transmitted to surface in real time, and logging engineers transmitted the information to
geoscientists at the onshore office. Data were
also stored in tool memory for further processing
after TD was reached in Interval B and the tools
could be retrieved from the well.
Monopole data provided good compressional
measurements; however, shear slowness data
from the monopole source were frequently missing from both intervals (next page). Processing of
the quadrupole waveform data yielded continuous shear slowness data of good quality across
the majority of both intervals. The shear slowness
values from the quadrupole data in Interval A
ranged from 132 to 310 μs/ft [433 to 1,020 μs/m],
and in Interval B the range was 145 to 264 μs/ft
[476 to 866 μs/m]. With the monopole data, no
shear slowness values greater than 243 μs/ft
[797 μs/m] were observed. With the quadrupole
source, Chevron Cabinda Gulf Oil was able to
quantify shear slownesses in zones that were too
slow for the monopole source.
Oilfield Review
2-MHz
Resistivity
Data
10-in.
Phase Shift
ROP
200 ft/h
0.2 ohm.m 200 0
0
22-in.
Phase Shift
Gamma Ray
0
gAPI 150
5
in.
34-in.
Phase Shift
10
5
in.
10 Measured
Washout
depth, ft
24-in.
Attenuation
0.2 ohm.m 200 0
45
%
2.95
Monopole Projection
–15
40
PEF
0.5
Monopole
Vp /Vs Ratio
g/cm3
Neutron Porosity
0.5
0.2 ohm.m 200 0
Bit Size
1.95
Quadrupole
Poisson’s Ratio
0.2 ohm.m 200 0
Caliper
Bulk Density
Monopole
Poisson’s Ratio
20
0
Density Correction
5 –1
g/cm3
Monopole
Δtc
Quadrupole
Vp /Vs Ratio
5 240
μs/ft
μs/ft
340
Monopole Δtc
1
40
μs/ft
Monopole Δts
40
μs/ft
Quadrupole Projection
340 40
340 1
Quadrupole Δts
40 40
μs/ft
μs/ft
440
Quadrupole Inversion Quality
–4
Quadrupole Δts
340 40
μs/ft
Waveform
Variable Density Log
440 0
μs
5,000
XX,000
XX,100
> Good quality shear data from the SonicScope LWD tool. Chevron Cabinda engineers use mechanical properties computed from acoustic data for well
design and to optimize drilling practices in a Lower Congo basin field offshore Angola. In this example, several LWD logs were run in a horizontal well and
provided rate of penetration (ROP), gamma ray and caliper data (Track 1) along with resistivity (Track 2) and porosity data (Track 4). The SonicScope tool
was included in the suite to evaluate its ability to provide shear data in soft formations. Extracting shear slowness from monopole data is difficult in the
unconsolidated formations that are typical of the field. Track 5 presents the coherence projections for the monopole compressional data (black curve on
left) and monopole shear data (black curve on right). In several places across the logged interval, such as the gap in the middle of this interval, the
monopole shear data are incomplete. Quadrupole shear data acquired with the SonicScope tool are continuous (Track 5, red curve). The coherence
(Track 6) of the quadrupole data provides high confidence in the measurement quality. There is also a difference between the two shear slownesses
measured by the different methods. This difference is associated with acoustic anisotropy in this horizontal well. Where monopole shear data are available,
Vp /Vs ratios from the two datasets are shown (Track 3, green and dashed magenta lines). Monopole Poisson’s ratio (Track 3, purple) is compared with
quadrupole Poisson’s ratio (Track 3, dashed red) and these data also exhibit some differences across the interval. A Variable Density log (Track 7) is used to
check the quality of the received sonic data. (Adapted from Mohammed et al, reference 19.)
Spring 2012
13
Variable Density Log
High-Frequency Monopole Waveforms
0
μs
3,000
Casing Arrival Window Start
Casing
0
Bit Size
5
in.
μs
3,000
Casing Arrival Window Stop
10
0
μs
3,000
Casing shoe
> Cement bond logging with an LWD sonic tool. Data from the SonicScope
tool can be presented in a format similar to that of wireline cement bond logs
(CBLs) to evaluate cement behind casing. The measurements are qualitative
rather than quantitative. The Variable Density log is a presentation of the
acoustic waveform at a receiver, in which the amplitude is presented in
shades of a gray scale. Because cement bonded to the outside of the casing
attenuates the signals that would normally be present, the Variable Density
log is a useful indicator of the presence of cement behind pipe. In this
interval, the depth of end of casing is shown (red line). The absence of
waveform arrivals in the casing window (dashed yellow line to dashed blue
line) indicates good bonding of the cement behind the pipe. The patterns to
the right of expected casing arrivals come from the formation, which signify
bonding of cement to the formation. (Adapted from Mohammed et al,
reference 19.)
The absence of shear data in softer formations would have made it impossible to compute
mechanical properties in these zones. Because
measurements with the quadrupole source delivered shear slowness in slow formations intersected by both intervals, engineers are able to
incorporate mechanical property data in future
well designs.
In addition to compressional and shear slowness, the SonicScope tool provided cement bond
logging (CBL) information in the 7-in. casing
(above). From high-frequency monopole data, log
analysts identified the top of cement (TOC) and
estimated the cement quality. A Variable Density
log, similar to wireline CBL logs, was also generated.
14
The interpretation based on the LWD sonic data is
only qualitative, but is often sufficient to verify that
the pipe is adequately cemented in place.
The Lower Congo basin reservoir described in
this case study consisted of unconsolidated
sands, which can pose drilling challenges. The
ability to extract usable-quality acoustic shear
data from LWD sonic quadrupole measurements
in these unconsolidated sands enabled engineers
to derive geomechanical properties for planning
future extended-reach horizontal wells. These
data were used for several purposes, including
developing safer drilling programs, optimizing
drilling, managing mud properties and understanding limiting factors for future wells.
Sweet Spots in Real Time
In addition to improving well design and optimizing drilling with increased safety, sonic data help
engineers make and validate real-time well
placement decisions. Recently, engineers used
data from the SonicScope tool to identify sweet
spots in a horizontal well.20 Two drilling runs were
made in the well, one of more than 1,500 ft
[460 m] and a second of 886 ft [270 m].21 The
LWD assembly did not include density or porosity
data. Identification of sweet spots was based
solely on changes in the ratio of compressional
and shear velocities (Vp /Vs).
For this reservoir, a correlation had been
observed between drilling rate of penetration
(ROP) and production; zones with higher ROPs
exhibited better hydrocarbon production.
Drilling rates can, however, be influenced by factors that are unrelated to reservoir quality, such
as bit type and condition. On the other hand,
stable Vp /Vs ratios had also been associated with
better quality rock, and they reflect reservoir
properties. Log analysts identified seven separate zones within the drilled interval based on
Vp /Vs ratios. Zone 1 represents the interval containing the landing point. Zone 2 is the interval
over which angle was built to penetrate the reservoir. Changes in formation lithology and variable
formation properties were identified from
Vp /Vs ratios in zones 4 and 6. Zones 3, 5 and 7 have
steady Vp /Vs ratios and correspond to 10%
increases in ROP compared with the average
ROP for the drilled section (next page).
From sonic data, engineers confirmed that
three intervals offered the best quality rock for
completion. The driller was also able to guide the
well back to better quality intervals after inadvertently exiting the preferred zones. The results of
this study demonstrate the value of real-time
sonic data to quantify rock quality.
Sound Future
Engineers recognize the importance of using
mechanical property data in optimizing drilling
programs and designing effective stimulation
jobs. Identifying and responding to seemingly
small variations in properties can mean the difference between disastrous results and a well
drilled with few complications. Small variations
in mechanical properties can be exploited to
improve commercial viability of drilling prospects where fracture stimulation is indicated.
20. Sweet spots refer to target locations or areas within a
play or a reservoir that represent the best production or
potential production. Geoscientists and engineers
attempt to map sweet spots to allow wellbores to be
placed in the most productive zones of the reservoir.
21. Degrange et al, reference 8.
Oilfield Review
Zones 1 and 2
Zone 3
Zone 4
Zone 5
Zone 6
Zone 7
100
ROP, ft/h
80
60
40
20
0
140
Δt c, Δt s, μs/ft
120
100
80
60
40
Δt c recorded mode
Δt c real time
Δt s recorded mode
Δt s real time
Vp /Vs ratio
2.000
1.875
1.750
1.625
1.500
Poisson’s ratio
0.500
0.375
0.250
0.125
0
Zone 3
Zone 4
Zone 5
Zone 6
Zone 7
True vertical depth, ft
Zones 1 and 2
0
500
Original well plan
1,000
1,500
2,000
2,500
Horizontal departure, ft
Revised trajectory
> Sweet spot drilling. ROP has been identified by the operator of this well as a sign of good completion-quality rock. However, ROP is influenced
by factors other than reservoir quality. The ROP data (green curve) are not conclusive and have considerable variability. Stable Vp /Vs ratios are
also an indicator of completion quality and can be computed from sonic compressional data (top, blue and red curves) and shear data (purple and
green curves) acquired in real time or recovered from downhole memory. Engineers identified seven different zones (yellow and green shading)
across the interval based on LWD Vp /Vs data (red curve). Poisson’s ratio (blue curve) is an indicator of rock stiffness. The cross section (bottom)
shows the location of each zone of the wellbore relative to the sweet spot (between light blue lines). Zone 1 is the heel of the horizontal section
where the well was kicked off, and Zone 2 is where angle was being built to enter the reservoir. Zones 4 and 6 were drilled out of zone for short
intervals. Zones 3, 5 and 7 have stable Vp /Vs ratios around 1.625, were drilled in zone and were identified as good targets for fracture stimulation.
(Adapted from Degrange et al, reference 8.)
New LWD sonic tools and techniques allow access
to these data in real time.
Integration of acoustic data in drilling, completion and evaluation workflows is a key to the
future of LWD sonic operations. Service companies have demonstrated conclusively that these
data can be extracted and that the information is
Spring 2012
relevant to drilling and completion operations.
Presenting the data in a form that decision makers can use to visualize the downhole environment is crucial.
The area around the bit is noisy and wracked
by sound and vibrations while drilling. However,
engineers have designed LWD acoustic tools that
overcome these conditions and answer fundamental questions about the rocks being penetrated by the bit. These tools are saying
something important about the reservoir and
the rocks, and geoscientists are listening. —TS
15