4 Oilfield Review

Sonic Logging While Drilling—Shear Answers

Engineers use acoustic data from sonic logging tools to drill more efficiently with

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

wave data in all formations. A new LWD acoustic tool measures shear wave data in

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.

Jeff AlfordMatt BlythEd TollefsenHouston, Texas, USA

John Crowe Chevron Cabinda Gulf Oil Company LtdLuanda, Angola

Julio Loreto Sugar Land, Texas

Saeed MohammedDhahran, Saudi Arabia

Vivian PistreSagamihara, Japan

Adrian Rodriguez-HerreraBracknell, 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.

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 continu-ally slam against the borehole wall, shocking sen-sitive 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 inun-dated with noise and vibration.

Sonic acquisition is challenging while drill-ing; 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. Measure-ments derived from the propagation of sound waves through porous media provide helpful information about geologic and geophysical

Spring 2012 55

properties. Petrophysicists have developed meth-ods 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 measure-ments, along with shear wave data in some forma-tions. 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 differ-ent 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 measure-ments of high-quality compressional and shear wave information to estimate geomechanical prop-erties in soft formations and the ability to deter-mine 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 mea-surements 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 opti-mize drilling practices, monitor real-time pore pressure while drilling, improve completions and estimate geomechanical formation properties.

Some Sound BasicsAcoustic logging tools measure the time it takes for an audible pulse of sound to travel from a trans-mitter, 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 measure-ment 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.

Sound waves propagate through a solid medium in a variety of modes, such as compressional and shear waves, and these modes have different veloc-ities (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 approx-imately 182 μs/m [55.5 μs/ft] and through limestone at around 155 μs/m [47.3 μs/ft].Compressional waves that pass through forma-tion rocks containing water, oil or gas have longer traveltimes than through rocks with no porosity.

The change in traveltime is related to the volume of fluid in the rock’s pore space, which is a func-tion 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 par-ticular 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 compres-sional slowness of the formation.

> 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.

S-wave andRayleigh wave arrivals

Ampl

itude

Stoneley wavearrivals

P-wavearrivals

Time

Mud wavearrivals

Transmitterpulse

> Characteristic values for compressional wave slowness (Δ tc) and shear wave slowness (Δ ts).

Steel

MaterialCompressionalSlowness TimeΔtc, μs/m [μs/ft]

Shear Slowness TimeΔts, μs/m [μs/ft]

Sandstone

Limestone

Dolomite

Shale

Freshwater

Brine 620 [189]

715 [218]

200 to 300 [61 to 91]

143 [43.5]

155 [47.3]

182 [55.5]

187 [57]

Not applicable

Not applicable

varies

236 [72]

290 [88.4]

289 [88]

338 [103]

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.

6 Oilfield Review

As part of the process to measure compres-sional slowness, a monopole source generates a compressional wave in the borehole fluid sur-rounding 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 forma-tion 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 bore-hole wall, the compressional wave acts as a new source, transmitting waves back into the borehole.

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 mono-pole source is refracted into the formation, some compressional energy is converted to shear waves that refract into the formation. Whereas com-pressional waves propagate through both the fluid-filled borehole and the porous rock matrix, shear waves are not supported by fluids and prop-agate through fluid-filled porous media by travel-ing 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 gen-erates a shear head wave in the borehole. This head wave travels at the shear velocity of the for-mation 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.

If the shear slowness in the formation is greater than the compressional slowness in the borehole fluid—a condition known as a slow for-mation—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 bore-hole. 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 over-burden pressure. For the same reason, slow for-mations 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 measur-ing 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 var-ies 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 pro-vides information such as maximum and mini-mum 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 con-tinue to use tools of this type, although current wireline tools with these sources can deliver a

> 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.

P-wave P-waveS-wave

Fast Formation Slow Formation

Stoneley wave Stoneley wave

Compressional wave

Compressional waveShear wave

Fluid wave

Monopolesource

Monopolesource

Fluid wave

Head waves

Wellbore Wellbore

Head wave

Tran

smitt

er-re

ceiv

er d

ista

nce

Tran

smitt

er-re

ceiv

er d

ista

nce

Traveltime Traveltime

Spring 2012 7

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 asymp-totically to the shear wave velocity.6 Although somewhat similar to dipole waves, they exhibit a different propagation pattern, which is more dif-ficult to conceptualize. Another term applied to them—screw waves—presents an image of how they travel along the borehole. At present, ser-vice companies use quadrupole sources in LWD tools only.

The Rise of LWD Acoustic ToolsWireline acoustic tools deliver high-quality mea-surements in a relatively low-noise environment, but they have shortcomings. The lag between drilling and logging, along with conveyance meth-ods needed to deploy wireline tools, presents complications. Delivering tools to TD in extended-reach horizontal wells can also be complicated and time consuming, although a number of con-veyance techniques have evolved over the years.7

Furthermore, wireline sonic tools should be cen-tralized, and tool weight can make this problem-atic 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 pres-sure prediction and wellbore stability analysis, the ability to acquire data during the drilling pro-cess, and use the data as soon as possible, signifi-cantly 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 plan-ning future wells.

Acoustic data are also affected by borehole conditions and challenges—such as mud filtrate invasion and rugosity—that may introduce mea-surement 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:

Critical refraction occurs when the angle of refraction is greater than or equal to 90°, meaning the wave travels along the borehole wall.

> 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. Dipolesources 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.

Monopole mode

Quadrupole mode

Dipole mode

90Azimuth

Flexural wave 1

Flexural wave 2

Nondeformedcross section

Radi

aldi

spla

cem

ent

180 270 360

90Azimuth

Radi

aldi

spla

cem

ent

180 270 360

90

AzimuthRa

dial

disp

lace

men

t

180 270 360

Compressional wave

Borehole Cross Section Radial Displacement Radiation Patterns

Flexural wave 1 Flexural wave 2

Quadrupole wave

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.

sin i = .sin R

VmVƒ

8 Oilfield Review

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 mea-surements 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 com-pressional 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 mono-pole-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

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 flexi-ble 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 mono-pole 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 inter-ference between the dipole collar flexure signal and the formation signal (above left). Therefore, instead of dipole measurements, Schlumberger and other service companies adopted a quadru-pole 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 val-ues from the measured quadrupole dispersion data. However, because the low-frequency com-ponents 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, bore-hole conditions, drilling mud properties, tool characteristics and the tool’s presence and posi-tion in the wellbore. It is crucial that engineers understand these effects, which can be tool spe-cific, to extract shear slowness from quadrupole data. In addition, the processing of quadrupole data is more complex than the processing of dipole data.10

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.

> 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.

Formation shearslowness

Shear asymptote

Weak interference

Tool flexuralresponse

Wellbore

Formation flexuralresponse

Frequency

Slow

ness

Slow

ness Strong interference

LWD dipole sonictool response

Frequency

LWD DipoleWireline Dipole

Tool Tool

Wellbore

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.

Spring 2012 9

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.

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 capa-bilities 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 DesignTo address the need for a quadrupole LWD tool, Schlumberger developed the SonicScope multi-pole 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 infor-mation it provides.

The SonicScope tool acquires monopole and quadrupole measurements using a powerful broad-band 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 pro-tected grooves positioned 90° apart (above right).The receivers are arranged in four arrays that pro-vide 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 measure-ments for compressional slowness and shear slowness in fast formations, low-frequency mono-pole data for Stoneley waves and quadrupole data for shear acquisition in slow formations. For the quadrupole mode, data are acquired in a fre-quency range down to 2 kHz. From dispersion analysis, which uses an inversion algorithm to

best fit modeled responses, engineers can extract shear slowness values lower than 2,000 μs/m[600 μs/ft]. The SonicScope tool is fully combin-able with other MWD and LWD tools. When com-bined with other measurements, such as density data, the acoustic data offer solutions for applica-tions that include seismic correlation, pore pres-sure determination, log interpretation in complex lithologies and geomechanical rock properties.

Using the DataIn situ geomechanical properties cannot be mea-sured 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 mate-rial 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 com-pressional wave data. Similarly, the shear modu-lus, G, a measure of a material’s ability to withstand shearing, is computed from the shear wave data. Once these two values are deter-mined, the bulk modulus, K; Young’s modulus, E;and Poisson’s ratio, ν, can be calculated.

The bulk modulus is the ratio of average nor-mal 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 val-ues 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.

> 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.

K = 4G3

M –

=6K + 2G3K – 2G

aρbM = ..

. .

.

(Δtc) 2

E

ν

= 9KG3K + G

aρb

(Δts) 2G =

10 Oilfield Review

However, the simple equations relating log-derived measurements to mechanical rock prop-erties 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 anisotro-pic 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

use pore pressure to facilitate drilling and increase safety margins. Using mechanical prop-erties derived from sonic data, they can optimize drilling programs and validate their ability to fol-low a given well profile while maintaining well-bore 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 determina-tion: identifying overpressured formations and selecting mud density (left). For drilling engi-neers, 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 consider-able cost savings.16

During lithification, sediments are com-pacted 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 res-ervoir 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 deter-mine 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 forma-tions. A mistake of a few meters can result in an expensive extra casing run or create hazardous

>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 ftindicates 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.

X2,000

LWD Sonic SlownessPhase Shift ResistivityGamma Ray

Attenuation Resistivity

150150 0.2 2,000

2,000

0 ohm.m

0.2 ohm.m

gAPI μs/ft 50Dept

h, ft

X3,000

X4,000

X5,000

X6,000

X7,000

X8,000

Compactiontrend

Spring 2012 11

drilling conditions. Mechanical properties of the formations must be known in order to determine the mud density limits.

Once the mechanical properties are com-puted from compressional and shear slowness data, geomechanical modeling programs can pro-vide solutions to drilling and completion ques-tions. Examples of modeling programs are VISAGE reservoir geomechanics modeling soft-ware 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 sub-optimal 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 produc-tion. Accounting for heterogeneity in completion design can help engineers enhance well produc-tivity, especially by identifying changes in geome-chanical properties—paticularly those that can be derived from sonic data. The absence of a sin-gle integrated solution to incorporate rock het-erogeneity has been an impediment to optimizing fracture stimulation designs.

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.

> 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.

800 600 400 200 0 –200 –400Effective stress, psi

> 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).

Narrow operational window Negative operational window

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12 Oilfield Review

To address unconventional hydraulic fracture design and to help optimize fracture stimula-tions, Schlumberger engineers developed Mangrove stimulation modeling software (above).18 The software incorporates seismic, geologic, geomechanical and microseismic data along with reservoir simulations to model frac-ture propagation and geometry. The software includes two different fracture simulators that are designed for complex hydraulic fracture mod-eling. They are linked to reservoir models for optimizing fracture design and production fore-casting. 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 acous-tic 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.

Horizontal ApplicationChevron Cabinda Gulf Oil Company Ltd uses acoustic data to optimize drilling and comple-tions 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 thin-bedded sands. To maximize exposure to the thin layers, lateral wellbores are drilled with sinusoi-dal trajectories. Interval A was drilled to a mea-sured 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 Aand from 80° to 97° in Interval B.

The primary focus for the study was to com-pare the compressional and shear measure-ments obtained using monopole sources with measurements extracted from quadrupole data. The engineers programmed the tool to obtain high-frequency monopole, low-frequency mono-pole and low-frequency quadrupole waveform data, which were acquired while running in the hole and while drilling in open hole. High-frequency monopole data were also acquired while in casing. Compressional slowness data were transmitted to surface in real time, and log-ging 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 miss-ing from both intervals (next page). Processing of the quadrupole waveform data yielded continu-ous 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.

> 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.)

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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.

Spring 2012 13

> 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.)

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14 Oilfield Review

The absence of shear data in softer forma-tions would have made it impossible to compute mechanical properties in these zones. Because measurements with the quadrupole source deliv-ered shear slowness in slow formations inter-sected by both intervals, engineers are able to incorporate mechanical property data in future well designs.

In addition to compressional and shear slow-ness, 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.

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 under-standing limiting factors for future wells.

Sweet Spots in Real TimeIn addition to improving well design and optimiz-ing 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 fac-tors 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 sepa-rate zones within the drilled interval based on Vp/Vs ratios. Zone 1 represents the interval con-taining the landing point. Zone 2 is the interval over which angle was built to penetrate the reser-voir. 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 inadver-tently exiting the preferred zones. The results of this study demonstrate the value of real-time sonic data to quantify rock quality.

Sound FutureEngineers 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 dif-ference between disastrous results and a well drilled with few complications. Small variations in mechanical properties can be exploited to improve commercial viability of drilling pros-pects where fracture stimulation is indicated.

> 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.)

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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.

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New LWD sonic tools and techniques allow access to these data in real time.

Integration of acoustic data in drilling, com-pletion and evaluation workflows is a key to the future of LWD sonic operations. Service compa-nies have demonstrated conclusively that these data can be extracted and that the information is

relevant to drilling and completion operations. Presenting the data in a form that decision mak-ers can use to visualize the downhole environ-ment 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 funda-mental questions about the rocks being pene-trated by the bit. These tools are saying something important about the reservoir and the rocks, and geoscientists are listening. —TS

> 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.)

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