next generation lwd sonic tool - home, schlumberger

The 14th Formation Evaluation Symposium of Japan, September 29-30, 2008

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Next Generation LWD Sonic Tool

Toshihiro Kinoshita, Takeshi Endo, Hiroshi Nakajima, Hiroaki Yamamoto, Alain Dumont, and Andy Hawthorn

Schlumberger Oilfield Service

This paper was selected for presentation by the JFES program committee following the review of abstract submitted by author(s). ABSTRACT

LWD technology has progressed rapidly in recent years to address the need for saving rig time, making real-time decisions for drilling efficiency and risk managements, and accurate geosteering. Real-time LWD sonic measurements provide timely analysis for borehole stability problems, drilling optimization, and assisting with pore-pressure prediction and seismic well ties. The sonic measurement can provide data to accurately determine well placement on the seismic sections and can create direct ties between downhole and seismic measurements.

A new LWD sonic tool is under development. Requirements of the new tool are the robust measurements of compressional and shear slownesses in fast and slow formations, thus enabling advanced sonic applications. The tool can acquire multipole modes (monopole and quadrupole) with 48 sensors (12 axial and 4 azimuthal positions). It has a large acoustic aperture with a short inter-receiver spacing that enhances the slowness processing in terms of quality and resolution. The tool has a large downhole computing capability, enabling more complex processing and leading to broader applications in real-time.

Field test data in different formation and borehole conditions show the tool can acquire high-quality waveforms in a wide frequency band, providing reliable compressional and shear slownesses equivalent to the latest wireline sonic measurement. Additionally the tool has the capability to acquire low-frequency Stoneley data, thus enabling applications such as formation damage indicators and fracture evaluation.

High-quality waveform data combined with advanced processing will provide the robust compressional and shear slowness logs and further expand the applications of LWD Sonic measurements. INTRODUCTION

Sonic measurements have evolved significantly from rather simple application for correlating surface seismic sections in 1950s. Since then sonic applications have expanded to various domains; geophysics, petrophysics, geomechanics and reservoir characterization. For

example, in geomechanics analysis, sonic data can provide information for pore pressure, rock strength, formation alteration, stress direction and magnitudes. In petrophysics analysis, sonic data can be used to evaluate formation lithology and to identify the fluid in the pore. As the sonic measurements provide useful information to understand the rocks and fluids of a reservoir and of surrounding formations, sonic logging is one of the principal measurements to evaluate the presence of hydrocarbons in the reservoir and to enable an efficient and safe production of oil.

Nevertheless, the significant evolution has been made primarily on wireline sonic logging and progression in LWD sonic logging has been rather gradual. Meanwhile, expectations on real-time LWD sonic measurements have been built up in recent years. Real-time data from LWD services help us to compute mud weight windows and casing shoe points and will contribute to wellbore stability analysis. Well placement can be optimized with the real-time data for maximum production of oil, and the timely and informed decision reduces both risk and cost simultaneously.

We have started to develop a next generation LWD sonic tool to address the above requirements. The new tool is capable of multipole modal acquisition, which allows us to determine shear in a slow formation, and to deliver advanced answer products.

This paper describes the advances in the new tool design and resulting data quality of the LWD sonic logging. Some examples from field tests in US and North Sea will be presented.

TOOL DESIGN AND CHARACTERIZATION

As sound waves are propagated along a steel tool housing very efficiently, tool waves are a significant issue when designing sonic logging tools, and this is particularly the case for LWD tools. Looking back at the history of wireline sonic logging the tool designers have made continual efforts to minimize this unwanted effect by isolating the transmitters from the receivers via a tortuous path of machined slots or grooves in the steel sonde. An LWD sonic tool is a part of the drill string, and thus the major structural part of the tool is a rigid drill collar (thick steel pipe). Minimizing the tool propagation is one of the keys to obtain a high quality while drilling acoustic measurement.


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Figure 1: Example of power spectra of collar propagation measured at the bottom receiver with (solid curve) and without (dotted curve) attenuator grooves. The formation P- and S-waves to be measured are in the frequency band of the spectrum trough.

In order to attenuate the propagation along the collar, multiple grooves of different sizes are machined inside the LWD tool collar. We have conducted intensive finite-difference acoustic modeling to optimize the pattern of the grooves. Figure 1 shows an example of the energy of collar propagation recorded at the first receiver with and without the attenuator grooves. With the aid of iterative computer simulations, we have decided the frequency band in which formation P- and S- waves are measured, the transmitter-receiver geometry, and the optimal groove pattern for the T-R spacing. Because a single set of transmitter and receivers are used for multipole measurements, the T-R spacing groove pattern needs to be optimal for both monopole and quadrupole modes.

The presence of an LWD sonic tool in a borehole potentially introduces significant bias to the measurements of Stoneley, flexural, and quadrupole waves. As the bias varies as a complex function of borehole parameters such as formation, mud, and borehole size, the tool effects need to be well understood and predicted. Because we have frequently experienced that only a trivial aspect of the tool could affect the tool response for sonic measurements, we tried to incorporate as much details of the tool design as possible into the model. Through a series of iterations in conjunction with the hardware developments, the finite-difference model was tuned so that the tool-borehole-formation response can be reproduced in the reference environments of the calibration facility. As a result, tool effects can be predicted accurately in all isotropic homogeneous formations.

Figure 2: Slowness-frequency dispersions of borehole quadrupole mode measured in a test well (blue dots) and computed with finite-difference modeling (light red dots). The dashed curve indicates the theoretical dispersion curve without the tool.

Figure 2 shows an example of the effect of the LWD tool presence on the quadrupole slowness dispersions of the finite-difference modeling and the experiment in a test well. The size of the dots represents the spectral amplitude of each mode. The dashed curve indicates the theoretical dispersion curve without the tool. Agreement between the modeling and the experiment is good, and the two dispersion curves are almost on top of each other. Because the LWD tool occupies a significant area of the borehole cross section, we can see noticeable bias from the dispersion curve without tool. The effect of the tool presence must be accurately predicted and included in the processing to extract formation properties from the dispersion curve. SOPHISTICATED ACOUSTICS

As mentioned above, the geometry of the new tool was carefully designed to ensure that the borehole modes are well established before reaching the bottom receivers and the aperture of the array is long enough to accurately extract the required information for the long wavelengths featured by these modes. The tool features 12 axial receiver stations separated by 4-inch inter receiver spacing for a total aperture of 3.67 ft for the receiver array. Four azimuthal receivers are located every 90 degrees around the tool and the 48 total sensors enable us to accurately decompose the quadrupole mode. Figure 3 shows waveforms of P&S mode acquired in the Gulf of Mexico with the new LWD tool and their semblance (Kimball, 1984). We can see very coherent compressional and shear arrival. The 4-inch inter receiver spacing of this tool is the finest granularity in the industry for any sonic logging tool. The field tests to date have demonstrated the advantage of fine acoustic sampling featured by the new LWD sonic tool.

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Figure 3: P&S waveforms at 12 receiver stations acquired in the Gulf of Mexico.

The receiver array is the key to the next generation of

multipole LWD sonic tools. Large efforts have been spent in the feasibility study, the design and qualification of the digital receiver array. Borrowing from our success with our latest generation wireline sonic, we decided to digitize the analog signals close to the sensors and digitally multiplex the signals to decrease the interconnection complexity between the inside and outside of the collar and to maximize reliability.

From the acoustic viewpoint, we want to make the structure of the receiver module soft enough so that it does not become a waveguide by itself. However, the receiver module must have durability for harsh downhole environments, and various restrictions are imposed in the design and choice of the materials. We have refined the design with much iteration, and the resulting high-fidelity receivers have been verified to provide a stable response under the difficult operating environments common in LWD.

The transmitter for the new tool is a single unit multipole and broadband type transmitter. It excites the borehole monopole, dipole, and quadrupole modes over frequency bands from 1 to 30 kHz. The multipole transmitter design is based on the current generation LWD monopole transmitter technology, as the present transmitter is a proven technology that works in LWD conditions. Each of the four transmitter quadrants has its own dedicated driver circuit and they can be excited for all the modes in phase to excite monopole and with sectors in phase opposition to excite dipoles and quadrupole.

HIGH QUALITY MEASUREMENT

Monopole P&S - The new LWD sonic tool has been run in the Schlumberger testing and drilling facility in Cameron, TX. The log shown in Figure 4 (a) is derived from monopole high frequency waveforms acquired while drilling. The compressional slowness ranges from 60 µs/ft to 110 µs/ft in the zone. About a half of the interval in the log is a fast formation or an intermediate formation and they are suitable for evaluating monopole response. A wireline sonic logging tool (Sonic Scanner from Schlumberger) (Pistre, 2005) was run after the drilling job to provide a reference of the sonic log (Figure 4 (b)). If we overlay the two logs of Figure 4, their agreement is considered very good.

Figure 4: Monopole compressional and shear logs acquired with the new LWD sonic tool and a wireline sonic tool (Sonic Scanner) at the Schlumberger test facility.

The wireline logging was conducted one week later than the drilling job, because another LWD tool was tested in the same well after the test of the new LWD sonic tool. This makes the comparison of the LWD log and the wireline log more interesting. The LWD log looks more coherent and continuous from 2520 ft to

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2550 ft, for example. This means that the borehole was less damaged at the time of LWD logging compared to the wireline logging, and it plainly demonstrates some of the effects of time on sonic measurements.

Figure 5 shows an example of the monopole high frequency waveforms and the semblance derived from them. They are acquired while drilling from a horizontal well in sand and shale located in US Land. The compressional arrival cannot be seen in the wave trains at this scale; however it can be clearly seen in the Slowness-Time-Coherence (STC) plane. Drilling noise is largely reduced by 16 times stacking. The arrival with the largest amplitude is Stoneley wave and their waveform shape varies along the array due to its slight dispersions.

Figure 5: Waveforms of P&S mode acquired in US Land with the new LWD tool and their semblance. They were acquired while drilling a horizontal well in sand and shale. The left track displays the STP projection over an 800 foot interval.

Figure 6: Comparison of the real-time log and the recorded mode log in the same interval of the horizontal well in US Land. Real time log - Comparison of the real-time log and the recorded mode log in the same well as above is shown in Figure 6. The slowness and the coherence of three peaks (compressional, shear and Stoneley) are computed in the downhole processing and then they are sent to the surface with mud pulse telemetry in real time. These data can be reconstructed into a pseudo projection log on the surface as shown in Figure 6 (a). The comparison demonstrates how the downhole processing is robust and the important features of the formation can be sent to the surface in real-time. Quadrupole - When the formation S-wave is slower than the acoustic speed of borehole fluid (slow formation), no shear head wave is developed in the fluid. Therefore, with a monopole source, physics does not allow us to determine shear slowness in a slow formation. On the other hand, a dipole source excites the borehole flexural mode, and it provides a way to determine shear slowness without the above limitation (Sinha, 2004). A dipole source is widely used to determine slow-shear for a wireline tool. A sonic logging tool has its own flexural mode and the tool for dipole measurement is designed so that the tool flexural mode does not interfere with the borehole flexural mode. A wireline dipole logging tool is not as rigid as an LWD sonic logging tool, and the tool flexural mode can be slow enough for the measurement.

The major structural part of the LWD tool is a rigid drill collar and the LWD tool flexural mode is much faster than that for a wireline tool. The LWD tool flexural mode and the borehole flexural mode are strongly coupled and make it difficult to determine slow-shear with a dipole source.

It is well known that a quadrupole source can be used to extract shear in a slow formation (Kurkjian, 1986). Chen (1989) verified that a quadrupole source is capable of direct S-wave logging in laboratory scale-model experiments. They have demonstrated that the low

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frequency portions of the borehole quadrupole mode (a.k.a. screw mode) propagate at the formation shear speed.

In terms of determining slow shear with an LWD sonic tool, a quadrupole source has clear advantage over a dipole source. The LWD tool quadrupole mode is generally faster than a borehole quadrupole mode and their frequency bands are rather shifted. In other words, the two modes are well separated in a frequency-slowness plane and the coupling is considered to be fairly weak.

Figure 7 shows an example of the quadrupole waveforms and the semblance derived from them. They are acquired while drilling in the Gulf of Mexico. The bottom panel shows the slowness dispersions extracted with a modified matrix pencil algorithm (Ekstrom, 1995). We can see strong dispersions from 3 kHz to 6 kHz and it explains the change in waveform shape across the array in the top panel. The left track displays the STP projection over a 1600 foot section.

Figure 7: Waveforms of quadrupole mode acquired in the Gulf of Mexico with the new LWD tool, their semblance and the dispersions. They are acquired while drilling in sand. The left track displays the STP projection over a 1600 foot section.

The LWD tool has significant effects on the dispersions as mentioned above and the dispersions in Figure 7 are already biased due to the tool presence. As we have studied the tool response in various conditions using finite-difference acoustic modeling, the tool effects can be predicted with a tool model. Therefore, the tool effects can be included in the processing to extract formation properties. Stoneley - The estimation of the formation fluid mobility from the analysis of the Stoneley wave slowness and attenuation has been carried out with wireline tools for more than a decade (Brie, 2000). Thanks to broadband type transmitter and driver electronics, large size memory for the waveform storage, and a large number of receivers, the new LWD tool is capable of recording Stoneley waves over a wide frequency band with higher signal to noise ratio and finer acoustic sampling. Moreover, Stoneley waves measured with an LWD tool can have higher sensitivities to the fluid mobility compared to a wireline tool.

Figure 8: Waveforms from the monopole low frequency firing acquired in the North Sea with the new LWD tool, their dispersions, and the spectrum. The left track displays a VDL (variable density log) image of the waveform at receiver 7 over a 500 foot interval.

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Although the presence of the LWD tool has a significant effect on the propagation of Stoneley waves, just like the quadrupole mode, the tool bias can be predicted with a tool model again. Accurate and reliable computation of fluid mobility and formation permeability will be enabled with the new LWD sonic tool right after drilling.

The analysis of the transmitted and reflected energy of the Stoneley wave crossing a fracture plane provides information to detect open fractures crossing the borehole. High quality Stoneley waveforms from the new LWD sonic tool could be utilized for fracture evaluation.

Figure 8 shows an example from the low frequency firing acquired in the North Sea with the new LWD tool. The middle and bottom panel show their dispersions and the spectrum. The dispersion curve is continuous from 1.5 kHz to 9 kHz in this case and this quality of the dispersions is considered excellent. The high quality dispersions and the tool modeling will potentially enable the Stoneley answer products mentioned above.

The new LWD sonic tool also has potential for

cement bond analysis such as top-of-cement detection. Furthermore, the new tool can record signals at all four azimuths around the tool to enable accurate imaging of reflectors and fractures. BARS (Borehole Acoustic Reflection Survey) acquisition during stand additions could be a tremendous imaging opportunity in the future.

SUMMARY

A new LWD sonic tool, which can acquire monopole P&S, Stoneley, and quadrupole modes, has been presented. The tool was carefully designed so that it enables high quality measurement for all the propagation modes as well as robust compressional and shear logs. The new tool is equipped with high-fidelity receivers, a powerful broadband type transmitter and driver electronics, and highly optimized attenuator. The new LWD tool breaks the barrier of its predecessors and it can robustly determine slow shear with a quadrupole source.

The acoustic response of the new LWD tool has been accurately characterized in several environments and the tool effect has become predictable. We believe the combination of high quality data acquired with the new tool and the predictable tool effect will expand the applications of LWD sonic measurements. REFERENCES

Brie, A., Endo, T., Johnson, D.L., and Pampuri, F., 2000, Quantitative formation permeability evaluation from Stoneley waves: SPE Reservoir Evaluation and Engineering, vol. 3, 109-117.

Chen, S.T., 1989, Shear-wave logging with quadrupole sources: Geophysics, vol. 54, 590-597. Ekstrom, M.P., 1995, Dispersion estimation from borehole acoustic arrays using a modified matrix pencil algorithm: 29th Asilomar Conference on Signals, Systems, and Computers, Pacific Grove, Calif., October 31. Kimball, C.V., and Marzetta, T.L., 1984, Semblance processing of borehole acoustic array data: Geophysics, vol. 49, 274-281. Kurkjian, A.L., and Chang, S-K., 1986, Acoustic multipole sources in fluid-filled boreholes: Geophysics, vol. 51, 148-163. 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 (Azimuth, Radial, and Axial) formation acoustic properties: SPWLA 46th Annual Logging Symposium, paper P. Sinha, B., and Asvadurov, S., 2004, Dispersion and radial depth of investigation of borehole modes: Geophysical Prospecting, vol. 52, 271-286. ABOUT THE AUTHORS Toshihiro Kinoshita is a Senior Engineer at Schlumberger K.K., Japan. Since 2001, he has involved in sonic logging tool development. He conducts acoustic modeling and studies of the tool physics. Before Schlumberger, he was working on computational fluid dynamics in Cray Research Japan. He has a BE degree in aeronautics from the University of Tokyo and PhD degree in aerospace engineering from Tohoku University, Japan. He is a member of SEG. Takeshi Endo is a Project Manager for Advanced Sonic Answer Products at Schlumberger K.K. in Japan. He joined Schlumberger in 1985 as an engineer in the seismic department of the reservoir modeling group in Japan. Since 1991, he has been leading sonic application developments in the Sonic Engineering Group. He earned BS, MS and Dr. Sc. degrees in geophysics from the University of Tokyo, Japan. He is a member of SPWLA, SEG, and EAGE. Hiroshi Nakajima is a Project Manger for a new LWD Sonic project at Schlumberger K.K. in Japan. He joined Schlumberger in 1989 as a mechanical engineer. Since then he has been working on various engineering projects for downhole system such as seismic and sonic. He earned BE degree in Mechanical Engineering from the Nihon University, Japan. He is a member of SPE and SEG.


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Hiroaki Yamamoto is Measurement Evaluation group leader in the Sonic Product Line at Schlumberger K.K., Japan. He graduated from Kyoto University in 1985 with a M.Eng. in Eng. Geophysics. He is a member of SEG, SEGJ and SPWLA. Alain Dumont is an Engineering Advisor at Schlumberger K. K. Since 1979, he has pioneered for Schlumberger the development of enabling technologies for a variety of new logging tools, in France, Texas and Japan. He holds engineering degrees from École Centrale de Lyon and École Supérieure d'Électricité in France. Andy Hawthorn, Drilling and Measurements HQ Domain Head for Acoustics and Geophysics at Schlumberger based in Sugar Land Texas. Andy is responsible for the LWD sonic and seismic services both existing commercial and upcoming engineering and research projects. Andy joined Schlumberger in 1990 as a field engineer in Norway. Since then he has held numerous positions around the world. He has a BS degree in Geology and an MSc in Geological Engineering from the University of Durham in England.

next generation lwd sonic tool - home, schlumberger

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