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Acoustic logging while drilling
Ultrasonic caliper measurements while drilling were introduced principally for improving neutron and density measurements.
Caliper transducers consist of two or more piezoelectric-crystal stacks placed in the wall of the drill collar. These transducers generate a high-frequency acoustic signal, which is reflected by a nearby surface (ideally, the borehole wall). The quality of the reflection is determined by the acoustic-impedance mismatch between the original and reflected signals. Often, there are difficulties in obtaining caliper measurement in wells with high drilling-fluid weights. Compared to the wireline mechanical caliper, the ultrasonic caliper provides readings with much higher resolution.
Acoustic-velocity data are important in many lithologies for correlation with seismic information. These data can be a useful porosity indicator in certain areas. Shear-wave velocity can be measured, and used to calculate rock mechanical properties. Four main challenges in constructing an logging while drilling (LWD) acoustic tool are described as follows:
- Preventing the compressional wave from traveling down the drill collar and obscuring the formation arrival. Unlike wireline tools, the bodies of LWD tools must be rigid structural members that can withstand and transmit drilling forces down the BHA. Therefore, it is impractical to adopt the wireline solution of cutting intricate patterns into the body of the tool to delay the arrival of the compressional wave. Isolator design is crucial and is still implemented to enable successful signal processing in a wide variety of formations, particularly the slower ones [those having a compressional delta time (ΔtC) slower than approximately 100 μsec].
- Mounting transmitters and receivers on the outside diameter (OD) of the drill collar without compromising their reliability.
- Eliminating the effect of drilling noise from the measurement.
- Processing the data so that they can be synthesized into a single ΔtC and that this data point can be transmitted by mud pulse. This is particularly challenging given the large quantity of raw data that must be acquired and processed.
In its most basic form, an acoustic-logging device consists of a transmitter with at least two receivers mounted several feet away. Additional receivers and transmitters enhance the measurement quality and reliability. The transmitters and receivers are piston-type piezoelectric stacks that operate at a higher frequency than typical drilling noise. Drilling noise has been shown to be concentrated in the lower frequencies (Fig. 1). A data-acquisition cycle is performed as the transmitter fires, and the waveforms are measured and stored. Arrival time is measured from the time the transmitter fires until the wave arrives at each receiver. From this acoustic-velocity information, the tool’s downhole data-processing electronics, using digital signal-processing techniques, calculate the formation slowness or ΔtC. This value is the reciprocal of velocity and is expressed in units of μsec/ft. Waveforms are stored in tool memory for later processing at the surface when the memory is dumped. Developments in acoustic LWD have focused on increasing the array of transmitters and receivers, and operating with dual frequencies. These have shown much better ability to provide shear measurements when the shear velocity is greater than the mud velocity. When the converse condition exists, there is no shear-wave arrival, and corrections have to be applied to other modes to derive shear. The processing required both at surface and downhole has become ever more sophisticated.
Acoustic slowness measurements are a relatively recent addition to the suite of LWD measurements. Similar to wireline devices, LWD tools consist of transmitter, isolator, and receiver-array sections contained in either a single or separate drill-collars and use monopole-type (axisymmetric) transmitters. However, unlike wireline devices, which are small relative to the borehole size, the rigid collar, structural design, and large diameter of acoustic LWD drill collars may actually interfere with the physics of acoustic-energy propagation, making it more difficult to decouple the transmitter-to-receiver signal traveling along the tool body and complicating generation of borehole guided modes, such as:
Downhole processing provides acoustic slowness data in real time and also allows for storage of raw waveform data in downhole memory that can be downloaded during bit trips. The receiver array provides data redundancy and is coupled with a much narrower recording bandwidth to enhance the signal-to-noise ratio in the presence of the drilling-related noise. This results in more accurate measurements of acoustic slowness. As with all types of LWD tools, the primary advantage of LWD acoustic measurements is the acquisition of data before significant fluid invasion or alteration of the formation can occur, and providing the data in sufficient time to influence drilling decisions for improved:
Table 1 summarizes the advantages and disadvantages of LWD acoustic measurements. LWD acoustic data recorded in casing during bit trips offers the potential for cased-hole evaluation, e.g. formation velocity and cement-evaluation, during bit trips.
Multipole tools (monopole and pseudo-dipole or quadrupole) have recently been introduced and continue to be developed. These devices use the monopole transmitter for compressional-slowness and shear-slowness in fast formations. Similar to wireline tools, these tools may include an azimuthal receiver array. With these tools, the compressional-slowness measurement is obtained in fast and slow formations by summing the azimuthal measurements at each receiver level. Fig. 2 illustrates the excellent agreement between LWD data and wireline data. Measurement of compressional and shear slowness in slow formations is especially impacted as the diameter of the LWD drill collar approaches the borehole diameter. Because the drilling environment precludes propagation of a true dipole flexural mode, different methodologies are being used to obtain a pseudo-shear measurement using other borehole-guided modes, either through:
- Tool design
- Data processing
- Both of the above
Current solutions include:
- Multiple-frequency and higher-frequency dipole-type transmitters
- Quadrupole transmitters
Acoustic LWD can provide a limited real-time seismic-while-drilling capability in combination with a surface source. There is also great interest among drillers and geophysicists to develop a viable drilling-based system in which the drillbit acts as the acoustic source and the sensors (ruggized geophones) are located in the borehole, as part of an LWD bottomhole assembly. Both types of systems provide checkshot surveys for time-depth correlation with pre-drill surveys and look-ahead seismic capability.
Frequency dispersion issue
The log in Fig. 3 shows an example of a log processed at the surface from waveforms stored downhole. Here, the ΔtC values have been reprocessed from the stored waveforms. When compared with a wireline log, this log is clearly less affected by the washout below the shoe and in the shale at X235 measured depth. LWD acoustic devices, by nature of their size, fill a much larger portion of the borehole than wireline devices, and are less susceptible to the effects of borehole washout. Synthetic seismograms can be produced when acoustic and density data are combined, which yield valuable correlations with seismic information. Nevertheless, synthetic seismograms derived from LWD suffer from the same frequency-dispersion issues as wireline when making comparisons with data acquired from surface seismic.
Industry solution to this issue
To deal with this issue, LWD-tool designers have made progress in developing seismic-while-drilling systems that can be used to provide seismic checkshots. In this system, sensitive instruments are placed in a downhole sub connected to the telemetry system. A surface gun is located on the surface. If the well is vertical or near vertical, this might be on the rig; otherwise, it will be on a boat located above the receivers. When the gun is fired, which is typically at a connection to ensure quiet conditions, the arriving waveform is detected by the instrumentation and stored in memory. Processing is carried out, and information is sent to the surface, from which the one-way seismic travel time can be derived. One of the key challenges in seismic while drilling is overcoming the lack of an electrical link between surface guns and downhole receivers.
Seismic while drilling has the potential to reduce the positional uncertainty in the earth model. The main applications are in exploration wells or where there is limited confidence in the velocity model. Data can be acquired at connections either while drilling or while pulling out the hole. The cost of locating a boat on a station with guns for the duration of drilling may be an impediment to routine operations of this sort in deviated wells. Data quality is not currently thought to be adequate for processing for vertical-seismic-profile purposes.
- Aron, J., Masson, J.P., Plona, T.L. et al. 1994. Sonic Compressional Measurements While Drilling. Presented at the SPWLA 35th Annual Logging Symposium, Tulsa, Oklahoma, USA, 19-22 June.
- Market, J., Althoff, G., Deady, R. et al. 2002. Processing And Quality Control Of LWD Dipole Sonic Measurements. Presented at the SPWLA 43rd Annual Logging Symposium, Oiso, Japan, 2-5 June. SPWLA-2002-PP.
- Aron, J. et al. 1994. Sonic Compressional Measurements While Drilling, paper SS. Trans., 1994 Annual Logging Symposium, SPWLA, 1–17.
- Minear, J. et al. 1995. Compressional Slowness Measurements While Drilling, paper VV. Trans., 1995 Annual Logging Symposium, SPWLA, 1–12.
- Hsu, K. et al. 1997. Interpretation and Analysis of Sonic While Drilling Data in Overpressured Formations, paper FF. Trans., 1997 Annual Logging Symposium, SPWLA, 1–14.
- Hashem, M. et al. 1999. Seismic Tie Using Sonic-While-Drilling Measurements, paper I. Trans., 1999 Annual Logging Symposium, SPWLA, 1–13.
- Market, J., Schmitt, D., and Deady, R. 2004. LWD So
- Varsamis, G. et al. 1999. A New MWD Full Wave Dual Mode Sonic Tool Design and Case Histories, paper F. Trans., 1999 Annual Logging Symposium, SPWLA, 1–12.
- Varsamis, G.L. et al. 2000. LWD Shear Velocity Logging in Slow Formations—Design Decisions and Case Histories, paper O. Trans., 2000 Annual Logging Symposium, SPWLA, 1–13.
- Joyce, B. et al. 2001. Introduction to a New Omni-Directional Acoustic System for Improved Real Time LWD Sonic Logging—Tool Design and Field Test Results, paper SS. Trans., 2001 Annual Logging Symposium, SPWLA, 1–14.
- III, J.V.L., Dubinsky, V., Patterson, D. et al. 2001. Field Test Results Demonstrating Improved Real-Time Data Quality in an Advanced LWD Acoustic System. Presented at the SPE Annual Technical Conference and Exhibition, New Orleans, Louisiana, 30 September-3 October 2001 Cite error: Invalid
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- Tang, X. et al. 2003. Shear-Velocity Measurement in the Logging-While-Drilling Environment—Modeling and Field Evaluations. Petrophysics 44 (2): 79–90.
- Market, J. et al. 2002. Processing and Quality Control of LWD Dipole Sonic Measurements, paper PP. Trans., 2002 Annual Logging Symposium, SPWLA, 1–14.
- Dubinsky, V., Tang, X.M., Bolshakov, A. et al. 2003. Engineering Aspects of LWD Quadrupole Measurements and Field Test Results. Presented at the SPE Annual Technical Conference and Exhibition, Denver, Colorado, 5-8 October 2003. SPE-84248-MS. http://dx.doi.org/10.2118/84248-MS
- Goldberg, D. et al. 2003. Analysis of LWD Sonic Data in Low-Velocity Formations, paper BH 1.5. Expanded Abstracts, 2003 Annual Meeting Technical Program, SEG, 301–304.
- Boonen, P. and Yogeswaren, E. A Dual-Frequency LWD Sonic Tool Expands Existing Unipolar Transmitter Technology to Supply Shear Wave Data in Soft Formations, paper X. Trans., 2004 Annual Logging Symposium, SPWLA, 1–11.
- Endo, T., Yoneshima, S., and Valero, H.-P. 2004. Analysis of LWD Sonic Data in Very Slow Formation—Compressional Processing and Indirect Vp/Vs Estimation, paper G. Proc., 2004 Formation Evaluation Symposium of Japan, SPWLA Japan Chapter, 1–8.
- Tang, X., Zheng, Y., and Dubinsky, V. 2005. Logging While Drilling Acoustic Measurement in Unconsolidated Slow Formations, paper R. Trans., 2005 Annual Logging Symposium, 1–13.
- Zhu, Z. et al. 2005. Experimental Studies of Multipole Acoustic Logging with Scaled Borehole, paper BG 2.6. Expanded Abstracts, 2005 Annual Meeting Technical Program, SEG, 376–379.
- Tang, X. et al. 2003. Logging-While-Drilling Shear and Compressional Measurements in Varying Environments, paper II. Trans., 2003 Annual Logging Symposium, SPWLA, 1–13.
- Wang, T., and Tang, X. 2003. Investigation of LWD Quadrupole Shear Measurement in Real Environments, paper KK. Trans., 2003 Annual Logging Symposium, SPWLA, 1–12.
- Haugland, S.M. 2004. Frequency Dispersion Effects on LWD Shear Sonic Measurements in Acoustically Slow Environments. Presented at the SPE Annual Technical Conference and Exhibition, Houston, Texas, 26-29 September 2004. SPE-90505-MS. http://dx.doi.org/10.2118/90505-MS
- Huang, X., Zheng, Y., and Toksoz, N.M. 2004. Effects of Tool Eccentricity on Acoustic Logging While Drilling (LWD) Measurements, paper BG 1.4. Expanded Abstracts, 2004 Annual Meeting Technical Program, SEG, 290–293.
- Underhill, W., Esmersoy, C., Hawthorn, A. et al. 2001. Demonstrations of Real-Time Borehole Seismic From an LWD Tool. Presented at the SPE Annual Technical Conference and Exhibition, New Orleans, Louisiana, 30 September-3 October. SPE-71365-MS. http://dx.doi.org/10.2118/71365-MS
- Haldorsen, J.B.U., Esmersoy, C., Hawthorn, A. et al. 2003. Optimizing the Well Construction Process: Full-Waveform Data From While-Drilling Seismic Measurements in the South Caspian Sea. Presented at the SPE/IADC Drilling Conference, Amsterdam, Netherlands, 19-21 February 2003. SPE-79844-MS. http://dx.doi.org/10.2118/79844-MS
- Esmersoy, C., Hawthorn, A., Durrand, C., and Armstrong, P. 2005. Seismic MWD: Drilling in time, on time, it's about time. The Leading Edge 24(1): 56–62. http://dx.doi.org/10.1190/1.1859702
- Althoff, G., Cornish, B., Varsamis, G. et al. 2004. New Concepts for Seismic Surveys While Drilling. Presented at the SPE Annual Technical Conference and Exhibition, Houston, Texas, 26-29 September 2004. SPE-90751-MS. http://dx.doi.org/10.2118/90751-MS
Noteworthy papers in OnePetro
Minear, John W. et al. 1996. Initial Results from an Acoustic Logging-While-Drilling Tool, SPE Annual Technical Conference and Exhibition, 6-9 October. 36543-MS. http://dx.doi.org/10.2118/36543-MS
Tepper, P., Boonen, P. et al. 1998. Drilling Applications of a New Logging-While-Drilling Slim Sonic Tool: Two Case Studies, SPE Annual Technical Conference and Exhibition, 27-30 September. 49137-MS. http://dx.doi.org/10.2118/49137-MS