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SPWLA-INDIA 3rd Annual Logging Symposium, Mumbai, India Nov 25-26, 2011
1
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.
SPWLA-INDIA 3rd Annual Logging Symposium, Mumbai, India Nov 25-26, 2011
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
SPWLA-INDIA 3rd Annual Logging Symposium, Mumbai, India Nov 25-26, 2011
3
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.
SPWLA-INDIA 3rd Annual Logging Symposium, Mumbai, India Nov 25-26, 2011
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
SPWLA-INDIA 3rd Annual Logging Symposium, Mumbai, India Nov 25-26, 2011
5
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.
SPWLA-INDIA 3rd Annual Logging Symposium, Mumbai, India Nov 25-26, 2011
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.
SPWLA-INDIA 3rd Annual Logging Symposium, Mumbai, India Nov 25-26, 2011
7
Fig 1: Tool schematic (not to scale).
SPWLA-INDIA 3rd Annual Logging Symposium, Mumbai, India Nov 25-26, 2011
Fig 2: Transmitter-receiver element orientation and designation.
RECEIVER
STATION
MONOPOLE
HOOP
SPWLA-INDIA 3rd Annual Logging Symposium, Mumbai, India Nov 25-26, 2011
9
Fig 1. Well Shuttle conveyance sequence.
SPWLA-INDIA 3rd Annual Logging Symposium, Mumbai, India Nov 25-26, 2011
Fig 2. Through drill pipe conveyance logging sequence.
Fig 3. Wireline drop-off conveyance sequence.
SPWLA-INDIA 3rd Annual Logging Symposium, Mumbai, India Nov 25-26, 2011
11
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)
SPWLA-INDIA 3rd Annual Logging Symposium, Mumbai, India Nov 25-26, 2011
Fig 7: XX & YY dispersion, 3.8 inch well, with model results.
Slo
wness (
µs/ft)
Frequency (kHz)
Frequency (kHz)
Slo
wness (
µs/ft)
SPWLA-INDIA 3rd Annual Logging Symposium, Mumbai, India Nov 25-26, 2011
13
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)
SPWLA-INDIA 3rd Annual Logging Symposium, Mumbai, India Nov 25-26, 2011
Fig 9: Slowness results from a 3.8 inch diameter well.