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EFFECT OF FLUID INVASION ON COMPRESSIONAL SLOWNESS

FROM A MULTI-RECEIVER ACOUSTIC TOOL

Anil Tyagi, Ashish Kundu, Vikram Pandey and Deepak Voleti (Reliance Industries Ltd.)

Kanchan Prasad, Udit Guru, Bhaswati Das andArindam Pal (Schlumberger INM)

Copyright 2011, held jointly by the Society of Petrophysicists and Well Log Analysts (SPWLA) and the submitting authors.

This paper was prepared for presentation at the SPWLA-India 3rdAnnual Logging Symposium held in Mumbai, India, November 25-26, 2011.

ABSTRACT

Wellbore sonic velocity plays an important role in geoscientific studies; it links petrophysics to geophysics. It is

difficult to petrophysically characterize a formation on the basis of only velocity.However, Its the only measure-

ment that can link the formation to the seismic data. One of the most debated topics has been the effect of fluid

invasion on the velocity measurement. Companies that provide inversion services tend to include corrections for

fluid invasion in the workflow without any substantiate logic. The mismatch between modeled and acquired sonic

curves is attributed primarily to invasion correction, which otherwise is not required with the current-generation

sonic tools. The processor needs to adjust the sensitivity of the other input parameters, rather than applying theinvasion correction.

Thetransmitter-receiver geometry of the latest-generation sonic tool is capable of profiling the compressional and

the shear slowness as the fluid front moves radially away from the wellbore. Out of the three monopole transmitters,

two are placed close to the thirteen-receiver arraywhereas one monopole transmitter spaced far away from the

receiver arrays (Spacing from middle receiver to monopole far transmitter 14ft). Near transmitters (Monopole Upper

and Monopole Lower) investigates the near borehole zone whereas far transmitter (Monopole Far) investigatesthe

deeper zoneof the formation.

The computational parameters required to determine the amount of invasion effect are currently estimated empirical-

ly. The current study demonstrates the working principle of a new generation multi-receiver tool and its ability to

make measurements beyond the invaded zone. In this study an attempt has been made to see the effect of invasion in

OBM on the compressional slowness computed from the new generation multi-receiver acoustic tool. In the methodused, the compressional slowness is calculated using a combination of four receivers (receivers 1 to 4, receivers 4 to

8, and so on) for the monopole far transmitter.Thedataset is processed for obtaining compressional slowness from

the monopole far transmitter using the various combinations of its 13 receivers. This is done to observethe effect of

alteration on its nearest receiver (spaced at 11ft from the monopole fartransmitter) and on the farthestreceiver

(spaced at 17ft from the monopole far transmitter). All the compressional slowness values calculated for the same

monopole transmitters are then compared to see any change or variation in the computed slowness.

Monopole radial profiling assesses the extent of borehole damage or alteration,which can result from borehole stress

concentration, filtrate invasion, shale swelling, or drilling-induced fractures.The dispersion curves are also analyzed

to understand the radial profile and acoustic frequency response of the formation

No significant change in the measured transit times indicates that data from the new-generation sonic tool are quite

robust and are independent of the invasion diameter. The radial profile also suggests that there is no significant

formation damage, confirming the findings from the present method.

INTRODUCTION

Borehole acoustic measurements have been around for a long time. The reason for their longevity is because the

measurements have value for geoscientists in various workgroups - from Petrophysics, Geophysics, Geome-chanics,

and Reservoir teams. The influence of acoustic measurements spans the lifecycle of the well ranging from seismic to

production.

Wellbore sonic data is often considered as one of the most important tool when it comes to integrated reservoir

characterization. Sonic traveltime, porosity, and clay volume are the petrophysical properties which are directly

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correlatable with the seismic data. Becauseporosity and clay volume are derived products and are dependent on

many parameters, sonic traveltimeremains the only parameter thatcan be directly correlated to seismic data. Howev-

er, direct comparison of seismic velocities with wellbore sonic velocities is never advisable because the measure-

ments are made differently.

Velocity measured in wellbore is vertical velocity.

Velocity measured through VSP in wellbore is vertical with some horizontal component.Velocity measured thorugh surface seismic have both horizontal and vertical components.

With so much uncertainty and without knowing the horizontal and vertical velocities, it is often concluded that the

data mismatch occurs because one of the datasets always reads high or low or is bad data and requires corrections.

One of the most common corrections is the invasion correction (Chi et al., 2004; Vasquez et. al., 2005;Mallanet al.,

2009). Without understanding the sensitivity ofborehole acoustic measurements to insignificant amounts of inva-

sion, parameters in the rock physics analysis were adjusted to show that even insignificant invasion results in a

significant change in measured velocity.

The current study highlights the misconception arrivedin the past due to the lack of information available from the

old generation acoustic tools. Over several decades, the sonic toolhas evolved and been improved with the purpose

of determining accurate formation slowness values. Early tool designconsisting of a single transmitter and a single

receiver with very short spacing could measure only the compressional slowness and in a very limited environment.

Then, borehole compensated devices were constructed, but they could not probe deep enough into the formation toread the slowness beyond chemically or mechanically altered zones. By using two transmitters at opposite ends of

the tool and averaging the traveltime values from two sets of receivers (Fig.1), the spikes can be reduced and effects

due to tilt of tool and borehole washouts could be compensated.. This tool, called the borehole compensated sonic

log (BHCS), was used almost universallyfrom 1970 to 1990. The long-spacing sonic tool was developed in the

1970s.

Newer sonic logging systems called full wave sonic or array sonic logs provide access to more acoustic information

than provided by traditional tools. In the late 1970s, sonic waveforms were recorded digitally and processed to

acquire compressional, shear, and Stoneley wave travel time. Theiradvantage was to probe beyond any formation

alteration. Note that these were still monopole sources; so shear could not be obtained in slow formations. An array

processor wasrequired in the logging truck computer to extract this information in the real time, as the hole was-

logged. These measurements provide valuable information on rock types, gas zones, porosity, permeability, forma-

tion elastic properties, stress field around the borehole, and acoustic impedance.

During the 1980s, a digital tool became availablethat enabled sophisticated processing of digital waveform data

similar to that used in seismic processing. Later in the decade, sonic flexural logging was developed,which provided

the extraction of shear slowness in slow formations or in formations slower than the fluid in the wellbore. These

tools featured an arrangement of transmitters and receivers that enabledprocessing of the data to characterize the

acoustics in the main shear-propagation planes.

Dipole and array sonic processing software wasdesigned to find and analyze all propagating waves in the composite

waveform. The technique uses a digital semblance (coherence) method called slowness-time-coherence(STC) to

identify and align the multiple arrivals across the array and to determine travel times of all coherent components of

the waveforms (Fig.2).

The STC concept is very similar to the velocity analysis process used in seismic data processing. By positioning a

time window on the reference waveform at time Tand then defining a corridor through the array with amoveoutS

(Fig.3), the total or incoherent energy Eiis calculated as the sum of the squares of the samples within the corridor.

The coherent energyEcis then calculated as the arithmetic sum of the samples along the moveout squared. When the

signalsofall waveforms within the window are perfectly correlated, the coherent energy is equal to the total energy

multiplied bythe number of waveforms. The process locates the position of the maximum coherence at each peak

and outputs the values of slowness, time, and coherence in a list.

For a given tool configuration, we expect that the time of arrival of a wave will be approximately the product of the

slowness andthe spacing between the transmitter and the reference receiver. The slowness-time plane shown in Fig.4

is from a set of monopole waveforms. The three main peaks observed probably represent the compressional,

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shear,andStoneleywave arrivals. However, at this stage, the peaks only represent coherent arrivals in the waveforms

and are not identified. The process is repeated for each set of waveforms acquired by the tool at each depth sample.

Digital first-arrival detection can also be used to obtain compressional travel time, mimickingthe analog first-arrival

detection (also called first-motion detection or FMD) of older sonic logs.

Because the new-generation sonic tool has many receivers, waveforms from common depth points (CDPs) in the

wellbore can be stacked to improve signal-to-noise ratio, similar to CDP stacking of seismic data. On the basis of

this knowledge, an ambitious vision was conceived for a service to provide a new dimension to sonic datato

extract radial acoustic properties in addition to the information in the plane orthogonal to the wellbore axis provided

by theexisting technology. A tool was needed that would acquire monopole and dipole waveforms over a wide range

of spacing and covera wide frequency band. Simultaneously, associated processing workflows had to be developed

to integrate all of the data and derive a set of answers for both traditional and new users who could derive benefit

from the new 3D acoustic information.

TOOL THEORY

The latest-generation acoustic tools provide a true 3D representation of the formation surrounding the borehole by

scanning both orthogonally and radially. The technology acquires borehole-compensated monopole measurements

with long and short spacingsand cross-dipole measurements.In addition to making axial and azimuthal measure-

ments, the fully characterized tool radially measures the formation for both near-wellbore and far-field slowness.

The typical depths of investigation are 2 to 3 times the borehole diameter.

The wide frequency spectrumused by these tools captures data at a high signal-to-noise ratio, regardless of the

formation slowness, and eliminates the need for multiple logging passes. The tool features a 6-ft receiver array of 13

axial stations with 6-inch separation between stations (Fig. 5). Eight azimuthal receivers are located every 45 de-

grees around the tool,with 104 sensors for the whole receiver array.

Threemonopole transmitters of the same design provide much more pressure sensitivity than that provided by

previous technologies.Their tuning to compensate for monopole compressional excitation function ensures good

generation of the compressional wave data. The radial profiling accomplished with the help of these three monopole

transmitters makes it possible to detect formation damage.

Monopole Radial Pr ofili ng- The monopole radial profiling algorithm inverts data for radial variation of the com-

pressional slowness from monopole compressional headwave signals acquired at multiple TRspacings (Fig. 6). The

algorithm implements a ray-based iterative scheme developed to convert the estimated differential transit times

(DTTs) across the receiver array as a function of TR spacing Tc(TR)into aradial profile for the compressional

slowness Tc(r) (Fig. 7).

The model used in this algorithm assumes that a borehole of radius ris filled with a fluid of speed Vfsurrounded by

a cylindrically layered formation in whicheachlayer is characterized by a thickness Hiand a velocity Vi. A sequence

of Vivalues is assumed to monotonically increase with increasing radius of the cylindrical layers,whichneed not be

uniformly spaced.

Each of these layers is probed by certain TR spacing. Within the TR spacing considered, an assumption is made that

there is no axial variation of the compressional wave speed. Assuming that from the compressional measurement at

each tool depth position (or differential transit times, DTTs) can be estimated, with the associated transmitter-receiver spacing (TR). The velocity estimates may come from eachreceiver pair comprising two closely spaced

receiving elements, with associated TR spacing given by the distance between the transmitters to the midpoint of the

receiver pairs (Zerouget al., 2006).

To minimize borehole effects (shape, etc.), the DTT is computed for various pairs of receivers and transmitters. The

DTT is plotted as a function of TR spacing and a fit is obtained on the DTT estimate. The result of the fit is fed into

the inversion scheme.

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METHODOLOGY

During and after drilling, the near-wellbore formation is often altered by stress buildup and release, mud-filtrate

invasion, chemical reaction, and many other factors. These alterations cause the physical properties in the near-

wellbore region to be different from those of the virgin rock formation. Stress concentration around the wellbore

may cause near-wellbore damage and induce formation anisotropy on sonic velocities. Often the mistie between thesynthetic seismogram and seismic data has led us to relook into the rock model, where the mismatch between the

model and measured sonic curves are attributed to invasion effects on sonic data. Also, the seismic inversion process

tends to include invasion corrections as a part of the workflow without any substantiated logic. The sensitivity

variation of the other parameters is also alteredwith the invasion correction, which otherwise is not required when

usingthe latest-generation tools.

The extent of alteration depends largely on the type of mud used for drilling; water-base mud has a larger effect than

OBM. In this paper, we focus on the effects of OBM filtrate invasion on the sonic log measurements. The approach

includes a theoretical mathematical model followed by examples from two wells withdifferent reservoir characteris-

tics.

Mathematical Model

A single transmitter-receiver model (Fig. 8) was mathematically created to understand the invasion effects on the

sonic logmeasurements. The simulated results assume that the tool is not tilted and the hole is in gauge. For different

depths of invasion, the change in sonic slowness wascomputed for variousformation porosities. The separate charts

have been prepared using Wyllie et al (1956) and Raymer et al (1980) slowness computation methods. User can

estimate the depth of invasion from the multi-induction resistivity tool through 1D inversion and use that invasion

value with the modeled charts to determine the expected slowness value In the case of OBM, with increase in depth

of invasion, slowness increases for water-bearing reservoirs and decreases for gas reservoirs. The workflow used in

the mathematical model is shown in Fig. 9. The effect of mud invasion as it relates to porosity ( ) is shown in Figs.

10 and 11 for gas reservoirs.In unconsolidated gas reservoirs ( > 15%), the change in slowness is negligible (~1

us/ft);in consolidated reservoirs (

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In the second case, the compressional slowness is computed using all 13 receiver stations with three different mono-

pole transmitters namely, far,lower, and upper monopole (Fig. 13). The invasion effects will be largely seen in the

lower and upper monopole data, whereas with the far monopole transmitter, the invasion effects will benegated. The

effects will also largely depend on the depth of invasion and TR spacing. Moreover, the depth of invasion depends

on the type of formationgreater invasionin consolidated reservoirs and lesser invasionin unconsolidated reservoirs.These phenomenaare highlighted in the results of the two study wells, Well A and Well B.

WELL A

This well has both unconsolidated and consolidated gas-bearing reservoir sands;the mud filtrate invasion is greater

in the consolidated reservoir intervals. The tightness of the sand can be seen inthe sonic semblance plot where the

coherency of shear waves is plotted in the monopole processing frequency window. The tighter or the more invaded

zones show changes in compressional slowness as we go from first receiver to the thirteenth (last) receiver ( Fig. 14).

Inthe unconsolidated reservoir sands (where the invasion isless), the difference in slowness between different trans-

mitter combinations is negligible. This is the case when the slowness is derived from the farmonopole transmitter. In

the case of the lower and upper monopole transmitters, the differences in slowness values would be larger, as shown

in Fig. 15. In this figure, the radial profiling data also showsa difference in the compressional slowness in the range

of 0.5-7 us/ft. In the same well, for water-bearing sands in which the mud filtrate invasion is appreciable, the effecton slowness is minimal in the case of the far monopole transmitter data (Fig. 16).

WELL B

This well isinunconsolidated gas- and water-bearing sandsin whichthe depth of invasion is less and, hence, the effect

on slowness computation from the farmonopole is minimal and yieldsthe same value with any combination of

receivers (Figs. 17 and 18). In this well, the slowness computation is from the lower and upper monopole transmit-

ter in water-bearing sand that is closer to the receiver arrays. It shows lower compressional slowness values than

those from the far monopole transmitter, as expected in the case of OBM invading water-bearing sands ( Fig. 19).

CONCLUSIONS

A mathematical model built to determine the effect of oil-base mud filtrate invasion on compressional-wave slow-

nessshows that there is no significant change in the transit times irrespective of the type of formation drilled.

The compressional slowness computed from varioustransmitter-receiver combinations also shows negligible effect

of mud filtrate invasion.

The dispersion curves from radial profiling show negligible effect of formation damage or mud filtrate invasion on

compressional slowness as transmitter-receiver spacing is increased; hence there is minimal effect from fluid inva-

sion on slowness computationderived from far transmitter-receiver combinations.

The study suggests that the transit times provided by the latest-generation sonic tools are quite robust and are inde-

pendent of the invasion diameter in the case of oil-base drilling fluid. Hence, in general,misties between synthetic

and seismic data as well as the mismatch with the rock model reflect problems with the input parameters and the

model rather than problems in the measured sonic data.

ACKNOWLEDGMENT

We thank Reliance Industries Limited for permission to publish this work. Discussions with rock physicists within

the organization helped our efforts and aided our understanding. The authors would also like to thank Schlumberger

for providing the support required for processing of the data.

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REFERENCES

Alberty, M., 1994,The influence of borehole environment upon compressional sonic logs: Society of Professional

Well Log Analysts 35th Annual Logging Symposium, Houston, Texas, June 19-22.

Batzle, M., and Wang, Z., 1992, Seismic properties of pore fluids: Geophysics, 57, pp1396-1408.

Biot, M.A., 1956, Theory of propagation of elastic waves in a fluid saturated porous solid. II. Higher-frequencyrange:Journal of Acoustical Society of America, 28, pp168-178.

Chi,et. al. 2004, Invasion correction of acoustic logs in a gas reservoir: SEG International Exposition and 74th

Annual Meeting, Denver, Colorado, October 10-15.Chi, et. al. 2006,Assessment of mud-filtrate invasion effects on

borehole acoustic logs and radial profiling of formation elastic properties: SPE Reservoir Evaluation and Engineer-

ing, v. 9, no. 5, pp.553-564.

Domenico, S. N., 1976, Effect of brine gas mixture on velocity in an unconsolidated sand reservoir: Geophysics,

v.41, no. 5, pp.882-894.

Kimball, C.V., and Marzetta, T.L., 1986, Semblance processing of borehole acoustic array data: Geophysics, 49, no.

3, pp.274-281.

LaVigne, J., Barber, T., Bratton, T., 1997, Strange invasion profiles: what multiarray induction logs can tell us about

how oil-based mud affects the invasion process and well-bore stability: Paper B, Transaction of the Society of

Professional Well Log Analysts 38th Annual Logging Symposium, Houston, Texas, June 15-18.

Mallan, R.K.,Ma, J., Torres-Verdin, C., and Wang, G.L., 2009, Joint radial inversion of resistivity and sonic logs toestimate in-situ petrophysical and elastic properties of formations: paper SPE124624 presented at the SPE Annual

Technical Conference and Exhibition, New Orleans, Louisiana, October 4-7.

Pistre, V., Kinoshita, T., Endo, T., Schilling, K., Pabon, J., Sinha, B., Plona, T. Ikegami, T., and Johnson, D., 2005,

A modular wireline sonic tool for measurements of 3D (azimuthal, radial and axial) formation acoustic properties:

SPWLA 46th Annual Logging Symp., June 26-29.

Raymer, L.L., Hunt, E.R., and Gardner, J.S., 1980. An improved transit time to velocity transform, Trans. Soc. Prof,

Well log analyst, 21st Annual Logging Symposium, Paper P.

Sinha, B. K., 2004, Near-wellbore characterization using radial profiles of shear slowness:SEG Expanded Abstracts,

23, pp 326.

Zeough S, Valero H-P and Bose S, Monopole Radial profling of compressional slowness 76th SEG annual Inte r-

national Meeting, New Orleans 2006

Vasquez, G., Dillon, L., Varela, C., Justen, J., Neto, G., Nunes, C., Bacelar, C., 2005, Automatic invasion correction

of elastic logs or Let the tools speak by themselves: SEG Expanded Abstracts, 24, 1546-1549.Vasquez, G.F., L. Dillon, C. Varela, G. Neto, R. Velloso, and C. Nunes, 2004, Elastic log editing and alternative

invasion correction methods: The Leading Edge, Vol 23, No. 1, pp 20-25.

Walls, J., and Carr, M.B., 2001,The use of fluid substitution modeling for correction of mud filtrate invasion in

sandstone reservoirs: 71st Annual Meeting of Society of Economic Geophysicists, San Antonio, Texas, United

States, September 2001

Wyllie, M.R.J., Gardner, G.H.F., and Gregry, A.R., 1963, Study of Elastic wave attenuation in porous medium.

ABOUT THE AUTHORS

Dr. Anil Tyagicompleted his PhD in 2010 and his MTechdegree (Applied Geophysics) in 1981. He is Assistant

Vice President in Reliance Industries Limited (RIL) and is responsible for petrophysical interpretation. He has more

than 30 years of experience as a petrophysicistin carbonate and clastic environments.

He started his career with Indicos Computer Services in 1981 and subsequently joined ONGC in 1982 as Logging

Field Engineer. He has the rare distinction of working in all the possible streams of open hole, cased hole, produc-

tion logging operations, and interpretation. In 1997, when ONGC started its transformation from a functional- to an

asset-based system, he joined Neelam Asset. As Specialist Pool Manager of NeelamandHeera Asset, he led ateam of

geoscientificexperts for the integrated geological modeling, reservoir characterization, and simulation of numerous-

fields. In 2003, he joined RILto head the Petrophysics team and has beenassociated with most RIL discoveries. He

introduced the concept of resistivity anisotropy and brought the necessary technology to the Indian subcontinent.

The technology not only helped to identify the hydrocarbon-bearing intervals but also helped to properly evaluatein-

place reserves.

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He has also brought another new technology to the Indian market:controlled-source electromagnetic (CSEM)/ sea

bed logging. It has provided good results as an exploration tool for risk mitigation in the costly deepwaterregion.

Ashish Kunduobtained MTech degree in Geoexplorationfrom the Indian Institute of Technology,Bombay (2005)

and hisMSc degree in Applied Geology from Jadavpur University of Kolkata (2003). He is currently working asSeniorPetrophysicist in RIL (5 years). He has 7 years of industry experience; his interests include advanced log

processing and interpretation integrated withrockphysics and core studies.

Vikram Pandeyis currently Senior Petrophysicist in RIL(E&P Business).He holds an MTech degree from the

Indian Institute of Technology, Bombay. He has worked extensively in siliciclasticenvironments, mainly in lami-

nated shaly sands. He is associated with the operations and interpretation of Krishna basin. His expertise is in

deterministic and probabilistic methods of formation evaluation, core-log integration, and capillarypressure satura-

tion analysis.Heis presently working on advanced CMR interpretation, rock physics, and fluid substitution.

Deepak Voletiis currently a Senior Petrophysicist in RIL (E&P Business). He holds an MSc (MTech) degree from

Andhra University, AP. He has worked extensively in siliciclastic environments, mainly in laminated shaly sands.

He is associated with the operations and interpretation of Krishna Godavari basin. His expertise is in deterministic

and probabilistic methods of formation evaluation, core-log integration, and capillary pressure saturation analysis.Presently, he is working on advanced CMR interpretation, rock physics, sonic anisotropy, fluid substitution, and

AVO analysis.

Kanchan Prasad:has graduated from Indian Institute of Technology, Roorkee, India. She has approximately six

years of experience in well logging operations and log interpretation with Schlumberger. As Petrophysicist she has

worked in formation evaluation of Carbonate and Clastics types of reservoirs dealing with interpretations of key

advance technologies present in the oilfield industry to date. Currently she is working as lead petrophysicist in

Mumbai, India.Udit Guru: is currently petrophysics domain champion with Schlumberger India. In Schlumberger, he has been

involved in many high end and new technology establishment in his role as a domain champion in India, Egypt,

UAE and Indonesia. He is involved in many multi domain projects and has contributed for more than 29 years in Oil

and Gas industry. Prior to joining Schlumberger, he has also worked with Oil & Natural Gas Corporarion Ltd

(ONGC), a premier national oil company of India for 16 years. He has several papers on his work published to SPE,

SPWLA and equivalent forums. Mr. Udit Guru holds a Masters degree in Earth Science from Indian Institute of

Technology, Kharagpur.

Bhaswati Das:Obtained MTech degree in Applied Geology from the IIT Roorkee and her BSc degree in Geology

from the Jadavpur University of Kolkata in the year 2008 and 2005 respectively. She joined Schlumberger in 2008,

worked as a Petrophysicist in Consulting Services for few months. She moved into the Data Services at the end of

2008. She is currently a Petrophysicist supporting WL activities in India.

Arindam Pal: Obtained M.Tech degree in Geo-exploration and M.Sc in Applied geology from IIT Bombay in the

year 2002 and 2004 respectively. He joined Schlumberger India, Data & Consulting Services (DCS) in 2008 as a

Petrophysicist. He is currently working as a Reservoir Petrophysicist in Shale Gas project with consulting services.

He was worked with Reliance Industries Ltd, CBM project as a Geologist.

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Fig.1Borehole compensated sonic log (BHCS).

Fig.2 Slowness time coherence plot from array sonic tool.

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Fig.5Multi- receiver acoustic tool.

Fig. 4Slowness-time plane monopole example.Fig. 3 Slowness-time-coherence computation principle.

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T4 T4

R

T3

T3

T1

T T1

T2 T2

T-RS

PACING

DOI OF TOOL

INVAD

EDZ

ONE

T

OOL

DOI OF TOOL

T5

INVAD

EDZ

ONE

Fig.7Ray-based inversion in radialprofiling for compressional slowness.

Fig.8Mathematical model for single transmitter-receiver acoustic tool. DOI = depth of invasion.

Fig. 6Compressional radial profiling concept.

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Fig.9 Workflow used in the mathematical model.

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Fig.10Wyllie slowness computation with different porosities for different depths of invasion.

Fig.11Raymer slowness computation with different porosities for different depths of invasion.

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Fig.12Different combinations of receivers with far monopole transmitter.

Fig.13Different transmitter combinations with similar set of receivers.

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Fig.14STC processing (Well A) using different receiver combination sets in gas-bearing sands.

Fig.15 Slowness comparison of different transmitter and receiver combinations with monopole

radial profiling plots.

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Fig.16 STC processing (Well A) using different receiver combination sets in water-bearing sand.

Fig.17 STC processing (Well B) using different receiver combination sets in gas-bearing sands.

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Fig.18 STC processing (Well B) using different receiver combination sets in water-bearing sand.

Fig.19 Slowness comparison of different transmitter and receiver combinations with monopole radial

profiling plots.