SPWLA) India - Dispersion Characteristics of a Small ......SPWLA-INDIA 3rd Annual Logging Symposium,...

SPWLA-INDIA 3rd Annual Logging Symposium, Mumbai, India Nov 25-26, 2011

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DISPERSION CHARACTERISTICS OF A SMALL DIAMETER CROSS

DIPOLE SONIC LOGGING TOOL WITH APPLICATION IN

CHALLENGING WELLS

D. Eccles, T. Mayor and P. A. S. Elkington; Weatherford

ABSTRACT

Flexural wave dispersion characteristics are determined for a battery-memory capable small diameter cross-dipole

wireline tool. We investigate the sensitivity of the dispersion effect to borehole size and mud slowness, and show the

importance of taking account of the tool’s mechanical properties in the forward model - particularly when the tool

diameter is a substantial fraction of the well diameter. Results show reduced tool-effect in common hole sizes. The

results are also used to examine the size of dispersion corrections needed to calculate shear slowness in situations

where low frequencies from the broadband transmitter are coupled relatively weakly to the formation. Results are

verified using data from wells with diameters in the range 3 ¾ to 12 ¼ inches, and can be applied to data from wells

that are technically or commercially challenging for conventionally-sized tools (such as high dogleg severity wells,

slim wells, and wells for which logging out of drill pipe may be the only cost-effective evaluation option).

Extending the measurements and their associated analyses into reservoirs drilled with such wells provides critical

information for characterizing rock mechanical properties and anisotropy for use in well placement, drilling and

stimulation design.

INTRODUCTION

Compressional and fast / slow shear velocities from cross-dipole sonic logs add significant value in hydrocarbon

exploration, appraisal and production.

Applications include:

Well placement (from improved seismic calibration and enhanced AVO)

Wellbore stability (from formation elastic properties and stress field characterization)

Formation damage control (from formation mechanical properties)

Production optimization (from mapping permeability attributes, including natural fracture characteristics)

Completion optimization (sand control, perforation and fracture design)

Wireline cross-dipole tools have been available for many years but their size, physical strength and dependence

on wireline communication has been a limiting factor in challenging wells, particularly directional and horizontal

wells, or wells with high dogleg severity, slim wells, or wells drilled with a pressurised mud cap (1). In response to

these limitations a small-diameter memory-capable tool has been developed which reduces acquisition risk and

extends the application of cross-dipole data into wells that are technically or commercially challenging for

conventionally-sized tools.

The tool may be deployed with or without a wireline. Regardless of the deployment method, data is always

recorded to memory so that the tool can be combined with other high data-rate tools (such as the micro-resistivity

imager) without compromising logging speed or data density. The data is always recorded raw, which means it

always includes the four constituent single-sided waveforms per receiver station (these are lost in tools that combine

them into X and Y prior to transmission); consequently there is always the flexibility to re-process the data.

This data is used to calculate velocity (1/slowness) logs which are key components in generating seismic ties, in

the derivation of data used to populate mechanical earth models, and for fluids identification and related

petrophysical applications. In fast formations, refracted shear waves are generally present in monopole waveforms,

but shear velocity in slow (as well as fast) formations is commonly derived from flexural waves excited by dipole

sources mounted in a wireline tool. Borehole flexural waves are dispersive, however. In the low frequency limit

flexural waves travel at the formation shear velocity, but in the high frequency range they travel more slowly. Very often

the processed shear velocity needs to be corrected to the true shear velocity according to its dispersion characteristics.


In this paper we analyze the tool effect for a new generation of small diameter cross-dipole wireline tool, and

include results for holes as small as 3.8 inches (96mm) – for which we also have field data.

Description of the Memory Cross-Dipole Tool The new small diameter cross dipole tool (CXD) has a maximum external diameter of 2.25 inches (57mm); as such

it is substantially slimmer than prior generation wireline tools which are typically 3.5 inches (89mm) or more in

diameter.

The tool comprises three sections with a total tool length of 26 feet (7.9m). – A schematic of the tool is shown in

Fig-1.

Transmitter Section This comprises three transmitters - one monopole and two directional high-output wideband dipole transmitters

oriented at right angles to each other and designated X and Y as shown in Fig 2. The peak energy from each dipole

transmitter is close to 1.3 kHz. Energy from the dipole transmitters interacts with the wellbore to create flexural

waves in the X and Y directions that propagate at close to the formation shear velocity (and equal the shear velocity

in the low frequency limit); analysis of the flexural energy allows us to identify formation shear anisotropy from

differences between velocities in the fast and slow directions. The efficiency with which energy is transferred into

the formation (and subsequently transferred from the formation back into the borehole) depends on frequency and

environmental factors; the efficiency of this coupling was a key factor in the design of the tool.

Isolator The Isolator slows and attenuates energy that travels directly between the transmitters and receivers along the tool

body. In older designs this has been achieved with a slotted sleeve, but these are mechanically weak and present

significant risks when pushing the tools down high angle wells in pipe conveyed logging and during fishing

operations. Existing slot designs were examined for strength, attenuation characteristics and operational longevity,

and all were found to be unsatisfactory in the context of a small diameter tool. A completely new design was

developed based on a sprung-mass principle. This gives almost perfect acoustic isolation with no significant loss of

mechanical strength relative to a solid pressure tube of the same size.

Receiver Section The receiver section has an array of eight receiver stations, each comprising four gain-matched highly sensitive

piezoelectric hydrophones aligned with the dipole transmitters (Fig 2). These provide 64 dipole waveforms plus 32

additional waveforms associated with the monopole transmitter (96 waveforms for each depth sample). Each

receiver element is constructed to maximize acoustic coupling with the borehole fluid without the need for pressure

balancing – a major advantage from a tool maintenance perspective. The receiver electronics (amplifiers and high-

bandwidth A/D converters) are integrated into each receiver housing in order to maximize signal-to-noise and

minimize electronic cross-talk.

Memory Section The memory section has sufficient memory for 16,500ft (5,000m) of uncompressed waveform data, which it records

independently from the data sent via the wireline (when present). This delivers data assurance during wireline

operations and enables stand alone memory logging, i.e. conveyance of the tool without wireline.

Tool Orientation Tool orientation information is needed in order to compute the fast shear azimuth direction in order to characterize

the shear-splitting associated with anisotropic formations. This comes from a navigation sub run in combination

with the tool (or from the navigation pack integral to the micro-resistivity imager tool if that is present in the string).

CONVEYENCE IN CHALLENGING WELLS

Wireline logging systems traditionally use wireline for conveyance, providing power and for communicating to and

from the logging tools. It is used not only for conveyance under gravity (in the case of vertical or near vertical wells)

but also when conveyed via drill pipe or tractor in high angle or horizontal wells. In these cases, the size and weight

of conventionally-sized tools impose operational limits, and concerns about well control are an additional


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operational challenge. Eliminating the wireline in battery-memory operations overcomes many of the operational

limitations of conventional size logging tools. Conveyance options for the new tool include:

Wireline – The most widely used method of conveyance for logging tools on electric cables.

Well Shuttle - Well Shuttle (1), (4), (5), (6), (7) conveys tools inside the safety of a drill pipe garage. The Well

Shuttle conveyance sequence is shown in Fig 3.

Through Drill Pipe Logging - Through Drill Pipe Logging is a conveyance technique that can be used when well

restrictions, caused by sloughing formations, ledges, and other obstructions, make open-hole wireline conveyance

problematic or impossible. With this technique, open ended drill pipe is lowered into the well below the zone(s) of

restriction. Through drill pipe conveyance is shown in Fig 4.

Coiled Tubing - Coiled Tubing conveyance employs coiled tubing to convey small diameter logging tools into

difficult-to-access wells.

Slick-line - Small diameter logging tools can be deployed on Slick-line by attaching the tools directly to the end of

the line. Deployment of the tools must be in memory mode and can be run on any slick-line, regardless of provider.

Wireline Drop-off - Wireline Drop-off is a conveyance system that allows for open-hole data acquisition while

tripping. In this technique, small diameter logging tools in memory mode are conveyed down-hole by wireline

through the drill pipe conduit. The logging tools pass through the open ended drill pipe, and hang into the open-hole

on a no-go at the bottom of the drill string. The wireline drop-off system is activated by mechanical or electrical

means, and after release, the wireline is removed from the well. Drill pipe is tripped from the well and log data is

recovered from the memory sub at the surface upon tripping out of the hole. Wireline drop-off is shown in Fig 5.

Wireline Tractor - Wireline Tractor conveyance is a conveyance technique where logging tools are conveyed into

highly deviated or horizontal wells on wireline with the aid of tractors (mechanical devices powered with electricity

transmitted through the wireline from the surface).

Continuous Rod (Co-rod) - Co-Rod is a conveyance method where small diameter logging tools in memory mode,

are conveyed on the end of specially designed strings of continuous sucker rods. The Co-Rod is 1 1/8 inches in

diameter, and is capable of conveying logging tools into highly deviated and horizontal wells. Log data is acquired

as the Co-Rod is pulled from the well.

Drill Pipe Conveyed Logging – In this method the logging tools are attached to the end of drill pipe and pushed to

the bottom of the well. The logging tools can be operated in memory mode and also in real time with wireline

connected to a logging unit on the surface. With wireline applications, the wireline is introduced into the drill pipe

though a side door sub in the drill string when the logging tools enter the highly deviated or horizontal section of a

well. After pumping the wireline to the top of the logging tools, it is connected to the tool string using a wet connect

latch. When the logging and drill stings are at TD, data acquisition is ready to begin with the trip out of the well.

In addition to the conveyance benefits, the small-diameter cross-dipole sonic tool has other advantages:

It can be run through the tubing into existing cased or barefoot completion wells; saving the costs of pulling the tubing which is required when running conventional cross-dipole acoustic tools.

It can be run in small internal diameter cemented casing where conventional cross-dipole acoustic tools either cannot be run or risk getting stuck due to their size.

The tool can be fully combined with a small diameter micro-imager and other logging tools which complement the evaluation techniques and applications derived from the cross-dipole data.

ANISOTROPY ANALYSIS

Each receiver station comprises four receiver elements designated A, B, C and D (see Fig 2). Receiver elements A

and C are in line with the X source; elements B and D are in line with the Y source. Individually, the waveforms are

referred to as single-sided, and contain both flexural and Stoneley energy. Subtracting C from A and D from B

creates composite waveforms in which the Stoneley energy is nulled. Specifically, when the X source is fired,

subtracting C from A creates the in-line XX waveform, and subtracting D from B creates the cross-line XY

waveform. Similarly, when the Y source is fired, subtracting D from B creates the in-line YY waveform, and

subtracting C from A creates the cross-line YX waveform. In ideal conditions, the in-line composites contain only

strong flexural energy, and the cross-line composites contain only the residual energy associated with flexural

energy that has leaked into the cross-line direction.


Moveout analysis of XX and YY waveforms provides the slowness curves DTX and DTY respectively. These

correspond to shear slownesses in formations that couple efficiently to the low frequencies from the dipole

transmitters. Some formations couple more efficiently to higher frequencies and a dispersion correction may be

needed in these cases to derive the slowness values for the low frequency limit.

In isotropic formations DTX and DTY are the same, but differences may be observed in anisotropic formations

(depending on the relative orientation of tool and formation fast shear azimuth). The azimuth of the fast shear

(AZR1) can be observed directly if the tool rotates while logging an interval, but even when it is not rotating, an

analysis of the in-line and cross-line energies is used to derive the fast shear azimuth at any tool orientation. The

relative difference between maximum and minimum slowness values is the degree of anisotropy, expressed as a

percentage.

DISPERSION CALCULATIONS

Flexural mode dispersion has been examined using the method of Nolte (2) & Tang (3). The effect of the tool in the

borehole on slowness calculations can typically be ignored if we process only the low frequency (least dispersive)

part of the received waveforms. However, if the intention is to draw conclusions about the formation from the

difference between observed dispersion and a forward modelled dispersion characteristic, then it is necessary to

include the tool, and to have a realistic representation of its mechanical properties which includes the compressional

and shear velocities of the isolator – the most mechanically complex part of the tool. Direct measurement of these

properties is difficult, and so we determined them using the finite element method in the CAD domain with the

elastic properties modified to reflect the anisotropic behaviour (using the finite element model results in the

longitudinal direction, and the elastic properties of solid steel in the radial direction).

RESULTS AND OBSERVATIONS

Data has been analyzed here from wells with diameters from 3.8 to 12 inches. The smaller hole size is of particular

interest, as this cannot be logged with conventionally-sized dipole tools. The larger hole size is included because the

tool is run most commonly in holes in the 6 to 8 ½ inch bit size range. The formation shear slowness range studied

here is 110 to 230 μs/ft. In all cases shown here the CXD was run centralized in the well. In the figures the green

line represents the expected dispersion curve for a borehole in which the tool effect is excluded, the black line is the

result using our CAD derived mechanical properties for the dipole tool, and the orange line is computed assuming

the dipole tool behaves as a solid steel cylinder which is included for reference; the grey line is the result using our

CAD derived mechanical properties for the dipole tool using a standard logging tool size of 3.375 inches.

Fig 6 shows dispersion characteristics derived from XX. On top of this plot are the theoretical curves under

different circumstances and the peak value plotted at every frequency. Fig 6b shows the peak dispersion

characteristics derived from XX (red) and YY (blue) waveforms in a 12 inch borehole. In this case the formation is

moderately fast, and there is little difference between results with and without consideration of the tool effect. On

the other hand, if we include a tool modelled as a solid steel cylinder the dispersion characteristic departs

substantially from reality for frequencies above about 3 kHz. This is consistent with published findings (8). Note

that the dispersion curve reaches its low frequency asymptote only below about 1.5 kHz.

When the bit size reduces to 3.8 inches even the small CXD occupies 34% of the borehole volume (compared to

6% in a 9 inch well). Fig 7 shows a substantial difference between the dispersion model results (the borehole if

fresh-water filled in this example). The dispersion curve that ignores the tool separates from the actual values at

moderately low frequencies and remains below the observed dispersion data. Model results assuming a solid steel

tool are closer to the real data up to about 8 kHz before but depart significantly thereafter. The model data using the

CAD derived properties (black) closely follows the actual dispersion curves recorded. Note also that it becomes

increasingly difficult to excite the borehole above 10 kHz in this well.

Fig 8 shows a comparison of three borehole sizes (a) = 3.8 inches, (b) = 6 inches and (c) = 9 inches showing the

dispersion curves for no tool (green), small diameter tool (black) and larger tool (purple). This figure shows that the

larger tool’s dispersion is always greater than the 2.25 inch tool, and this effect is greatest in small diameter

boreholes. It is also observed that as the borehole gets smaller, the frequency range present in the data increases.

These data show that dispersion corrections are required if the formation is fast and the frequency content used in


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STC processing is above 2-3 kHz. Finally, Fig 9 shows the size of the dispersion effect on a real data set from a 3.8

inches well.

CONCLUSIONS

Borehole cross-dipole acoustic data has many applications in hydrocarbon exploration, appraisal and production.

Significant economic value is created from these applications. A memory-capable 2.25 inch diameter wireline-style

monopole/cross-dipole sonic tool has been developed that reduces acquisition risk and extends the application of

cross-dipole data into wells that are technically and/or commercially challenging for conventionally-sized tools.

These include directional and horizontal wells, or wells with high dogleg severity, slim wells, and/or wells drilled

with a pressurised mud cap.

The tool was evaluated in a broad range of borehole environments and formation properties, including boreholes

from 3.8 up to 12 inches in size where the theoretical dispersion characteristics compared favourably to the

dispersion observed in real data. In the smaller boreholes, the larger the effect the tool has on waveform dispersion,

but this effect is less than the larger (3 3/8 inch) tool with the same mechanical properties.

ACKNOWLEDGEMENTS

The authors are grateful to Weatherford for permission to publish this paper.

REFERENCES

1. Toralde, Julmar Shaun S. et al,:”Drillpipe Conveyed Logging Provides Solutions for Pressurized Mud Cap Drilling Operations,” paper SPE 132116, CPS/SPE International Oil & Gas Conference and Exhibition, Beijing,

China, 8–10 June 2010.

2. Nolte, D., Rao, R., and Huang, X. 1997. Dispersion analysis of split flexural waves. Borehole Acoustics and Logging Consortium, MIT.

3. Tang, X. M., and Cheng,C. H. 2004. Quantitative Borehole Acoustic Methods: Elsevier Science Publ. Co., Inc 4. Elkington, P.A.S., Spencer, M.C., and Spratt, D.L., J..:”An Openhole Memory-Logging System for High-Angle

Wells and Bad Hole Conditions”, paper SPE 77560, SPE ATCE, San Antonio, Texas, 29 September 2002.

5. Spencer, M.C., Ash, S.C. and Elkington, P.A.S. “A Pressure Activated Deployment System for Openhole Memory Logging Tools and its Application in Directional Wells,” 2004. SPE Paper 87170, IADC/SPE Drilling

Conference, Dallas, Texas, 2-4 March 2004.

6. Christie, R., Elkington, P. A. S., McIlreath, I., and Natras, T.: “Acquiring Microresistivity Borehole Images in Deviated and Horizontal Wells Using Shuttle-Deployed Memory Tools”. SPE Paper 116266 presented at the

2008 SPE Annual Technical Conference and Exhibition, Denver, Colorado, USA, 21–24 September 2008.

7. Kuchinski, R.,”Deployment Strategies to Reduce Risk in the Acquisition of Formation Evaluation Data”, paper SPE 125509, SPE/IADC Middle East Drilling Technology Conference & Exhibition, Manama, Bahrain, 26-28

October 2009.

8. Zhao, W., Kim, J., and Kim, Y. 2009. A comparative study of borehole size and tool effect on dispersion curves. Exploration Geophysics, 40, 154-162.


ABOUT THE AUTHORS

David Eccles is Application Development Engineer in Weatherford’s Geoscience Development group, East Leake,

UK. He holds a BSc in Ocean Science from the University of Wales Bangor, an MSc in Applied Geophysics from

the University of Birmingham, and a PhD in rock physics from University College London. He is a Chartered

Scientist in the Geological Society.

Terry Mayor is Deputy Director of Research at Weatherford’s Wireline Centre of Excellence at East Leake, UK.

Yibing Zheng is a Principal Geoscientist within Geoscience Development at Weatherford International, Houston.

He was an assistant professor in instrumentation engineering in Zhejiang University. After receiving a Ph. D. degree

from Yale University in 2002, he studied borehole acoustics and reservoir simulation as a postdoctoral fellow at the

Earth Resources Lab of Massachusetts Institute of Technology. He worked for Baker Hughes as a research scientist.

He joined Weatherford International in 2010. His current research interests include borehole acoustics, downhole

seismic processing, rock mechanics and numerical modeling. He is a member of SEG and SPWLA.

Peter Elkington is Chief Geoscientist in Weatherford’s Geoscience Development group. He has over 30 years of

service company experience in R&D, and in operational and commercial support. He holds Bachelor of Science

degrees in geology and mining engineering, and a PhD in petrophysics from the University of Nottingham.


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Fig 1: Tool schematic (not to scale).


Fig 2: Transmitter-receiver element orientation and designation.

RECEIVER

STATION

MONOPOLE

HOOP


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Fig 1. Well Shuttle conveyance sequence.


Fig 2. Through drill pipe conveyance logging sequence.

Fig 3. Wireline drop-off conveyance sequence.


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Fig 6: (a) Dispersion plot, XX waveforms in a slow formation with 12 inch diameter well and (b) peak

XX & YY dispersion points, with model results.

Frequency (kHz)

Slo

wness (

µs/ft)

S

low

ness (

µs/ft)

(a)

(b)

Frequency (kHz)


Fig 7: XX & YY dispersion, 3.8 inch well, with model results.

Slo

wness (

µs/ft)

Frequency (kHz)

Frequency (kHz)

Slo

wness (

µs/ft)


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Fig 8: Model dispersion curves in a 3.8 inch well (top), 6 inch well (centre) and 9 inch well with (bottom).

Slo

wness (

µs/ft)

S

low

ness (

µs/ft)

S

low

ness (

µs/ft)

No tool

2.25 inch tool

3.375 inch tool

Frequency (kHz)

(a)

(b)

(c)


Fig 9: Slowness results from a 3.8 inch diameter well.

SPWLA) India - Dispersion Characteristics of a Small ......SPWLA-INDIA 3rd Annual Logging Symposium,...

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