Sonic Borehole Logging

Basic Concept

Sonic borehole logging has been used since the 1950’s to estimate seismic and physical properties of the formations surrounding a borehole. Sonic-logging methods can either measure 1) seismic-wave transmission (i.e., sonic, acoustic, acoustic waveform, and full-waveform sonic), 2) cement bonding, or 3) acoustic reflections via acoustic televiewer logging. Sonic-logging tools mainly consider how seismic waves (see seismic reflection, seismic refraction, or vertical-seismic profiling) propagate through formations to estimate porosity and saturation and/or identify fluid-filled or potentially transmissive fractures.

Theory

Sonic-logging tools exploit seismic (or acoustic) wave energy, which can induce oscillations within, interact with, and are influenced by subsurface earth materials. The earliest and simplest sonic-logging tools measured the compressional seismic velocity of the formation by determining the first arrival of a sound source emitted by a transducer on the tool. These early tools automatically picked the first arrivals and calculated seismic velocities.

More advanced sonic tools make use of two or more receivers and an additional transmitter, which, when located opposite the receivers, can compensate for borehole-diameter variations. These sonic tools measure and record the full-wave train at each receiver, which permits the analysis of compressional-wave, shear-wave, fluid, and Stoneley (or tube-wave) velocities. These parameters can be used to infer lithology and estimate porosity, saturation, and geotechnical properties. (Hearst and Nelson, 1985; Paillet and others, 1992)

Sonic-logging tools use a piezoelectric, electrostrictive, or magnetostrictive transducer to generate a pulsed sound-wave source with a frequency typically between 10 and 20 kiloHertz (kHz). The wave travels through the borehole fluid, and, at the critical angle of incidence, moves as head waves more parallel to the borehole wall through the formation(s). The wave propagates from the formation back into the borehole fluid to one or more receivers on the tool.

Piezoelectric crystals in the receivers sense the impinging pressure wave and convert it to an electromagnetic signal that is recorded as a function of time. Digital data for each waveform and each receiver can be analyzed for wave velocity, transmission, amplitude and/or attenuation. Borehole-centralized tools minimize vibrational noise and, because it is equidistant from the borehole wall, allows for travel time through the fluid to be removed from the data.

The amplitude of each recorded signal helps distinguish each wave arrival, which can be estimated for compressional, shear, and/or the lower velocity (i.e., Stoneley or tube) waves. The velocity of a wave through the formation can be computed as the distance between the receivers divided by the difference in arrival times. Sometimes the value of slowness, which is the inverse of velocity, is used to characterize the transmission properties of the formation.

Sonic logs require a fluid-filled borehole to couple the signal with the casing or borehole wall. Additionally, the transmitter and receivers are acoustically isolated, which inhibits the signal from traveling directly through the tool. The frequencies used for full-waveform tools can be optimized for borehole diameter, rock type, mode of interest, and survey objective. Optimal frequencies typically are high-enough amplitude to excite compressional- and shear waves but are low enough to avoid the noise that interferes with first-arrival selection.

Applications

Sonic, acoustic-waveform, and acoustic logging make use of acoustic propagation and wave attenuation that can be applied to geologic, hydrologic, and/or geomechanical properties. The most common application is the estimation of porosity from values of compressional seismic velocity (Vp). Also called the sonic porosity, travel-time log, and continuous-velocity log, this method uses calculated Vp values to estimate porosity by using the Wylie, Rymer-Hunt, or other equation (Paillet and Cheng, 1991).

Such porosity formulas relate the bulk velocity to the velocity of the matrix, velocity of the fluid in the pores, and the volume of pore space. By using the slowness of the compression wave, these formulas can be simplified to the time-average equation. In such, the porosity is equal to the difference in the measured slowness and the slowness of the matrix divided by the difference in the slowness of the fluid and the slowness of the matrix.

Determination of the shear-wave arrivals and calculations of shear-wave velocity (Vs) can be used for correlation with surface geophysical measurements, calculations of the Vp/Vs ratio, and other geotechnical properties. Advanced processing can be done to obtain estimates of Vp and Vs from the digital full-waveform logs. Velocities can be handpicked, picked using an automation algorithm, or a combination of these.

Vp and Vs estimates, along with density information, permit the computation of geomechanical properties such as Poisson’s ratio; shear-, Young’s-, or bulk modulus; and bulk compressibility (e.g., Vishkai and others, 2017). In some cases, however, Vs is difficult to generate or interpret in the full-waveform log. Thus, Vs is sometimes estimated from Poisson’s ratio for various rock types (Paillet and Cheng, 1991) or Stoneley-wave velocity, which can be identified at all frequencies (White, 1983).

Fluid waves (e.g., Stoneley or tube waves), are surface waves that propagate along the borehole wall nearly at the fluid-velocity. Unlike typical seismic methods, certain properties of fluid waves can allow for the determination of critical subsurface features. Fracture locations can be identified by analyzing the amplitudes and energy signatures of Stoneley waves, and their ability to transmit water can be estimated (Tang and Cheng, 1989; Paillet and Cheng, 1991).

A specialized type of sonic log is the cement-bond log (CBL) or bond-attenuation log (BAL) that uses the full-waveform to assess how well casing is bonded to the formation. Several tool designs are available, but they all generally assess amplitude and attenuation of the waveform to evaluate the strength of the formation bond. Micro-cracks or channeling might indicate potential for fluid migration, no bond with the formation is indicated by high-amplitude ringing (Bigelow, 1990; Pilkington, 1992).

Sonic records can be adversely affected by varying diameter, mud invasion, and borehole wall damage. Depending on the tool design, however, some affects can be mitigated (e.g., the compensated sonic tool has two transmitters to correct for borehole damage/diameter). Vibrational noise can be imposed in the record by rough borehole walls but may be reduced by centralizers and an upward-logging direction. Additionally, there are numerous tools that investigate various parameters. Table 1 distinguishes the main types and applications.

Wave component (velocity and amplitude)	Application
Full waveform	Velocity profiling, lithology, fracture detection
	Cement-bond evaluation
	Calibration of surface seismic
Compressional wave velocity (Vp)	Porosity and lithology
	Hydrocarbon resource evaluation
	Cement-bond evaluation
Shear-wave velocity (Vs)	Mechanical properties
	Fracture detection and evaluation
	Permeability Vs/Vp(ratio) for lithology determination and gas detection
Tube (Stoneley) waves	Fracture and permeability evaluation

Table 1. Multiple sonic measurements of transmitted waves and applications (Prensky, 1994).

Thus, understanding survey objectives and required data is key prior to logging. However, if tool selection is based on investigation objectives and logging parameters are optimized to the borehole environment, sonic-borehole logging can aid the following:

Determination of compressional and shear wave velocities
Determination of porosity
Determination of lithology
Estimation of geomechanical properties
Identification of fracture locations
Estimation of fracture permeability
Assessment of cement bond quality
Estimation of cement and/or formation strength
Correlation with surface surveys

Examples/Case studies

Abbas, A.K., Flori, R.E., and Alsaba, M., 2018, Estimating rock mechanical properties of the Zubair shale formation using a sonic wireline log and core analysis: Journal of Natural Gas Science and Engineering, v. 53, p. 359-369, doi:10.1016/j.jngse.2018.03.018.

Abstract: The Zubair Formation is the most prolific reservoir in Iraq, which is comprised of sandstones interbedded with shale sequences. Drilling boreholes in this formation has always been a challenge due to the weak nature of the shale sequence. Historically, over 90% of wellbore problems in the Zubair Formation are due to shale instability. To solve this problem, it is necessary to understand the rock mechanical properties and the response of shale. The main objective of this study is to develop local empirical correlations of rock mechanical properties that can be used to estimate a continuous profile of these properties throughout the depth of the Zubair shale formation directly from a wireline sonic log.

Well-preserved core samples of Zubair shale were run through extensive testing, including a number of shale characterization and rock mechanical tests. Special characteristics of shale were measured and described, including the porosity, structure, texture, and mineralogy, using the free water content method, a scanning electron microscope image, a thin section photograph, and X-ray diffraction analysis. Consolidated undrained triaxial tests were conducted to determine the static rock mechanical properties. Local empirical correlations were established, with the acoustic compressional velocity as a primary input parameter. Thus, sonic wireline logs can be used directly to obtain a continuous profile of the rock mechanical properties through the entire interval of the Zubair shale formation. The accuracy of the newly developed empirical correlations was examined using statistical analysis. Moreover, these correlations were compared with previous correlations from the literature. The results showed that the suggested empirical correlations are highly accurate and reliable, in contrast to those in the literature, which did not adequately fit the Zubair shale data. This highlights the importance of using local correlations to estimate rock mechanical properties.

The predicted continuous rock mechanical profile gives a good indication of the strength and stability of the shale around the wellbore. Consequently, it can be used to solve shale instability problems, optimize drilling processes (i.e., the selection of bit type and drilling parameters), seal integrity evaluation, and otherwise improve fracturing operations across the Zubair shale formation.

Harrison, A.R., Randall, C.J., Aron, J.B., Morris, C.F., Wignall, A.H., Dworak, R.A., Rutledge, L.L., and Perkins, J.L., 1990, Acquisition and Analysis of Sonic Waveforms From a Borehole Monopole and Dipole Source for the Determination of Compressional and Shear Speeds and Their Relation to Rock Mechanical Properties and Surface Seismic Data, in Proceedings, SPE Annual Technical Conference and Exhibition: New Orleans, LA, Society of Petroleum Engineers, doi:10.2118/20557-MS.

Abstract: Classical sonic logging employs the acquisition and analysis of data with a simple monopole source. For this type of source, physics limits shear speed determination to speeds faster than the acoustic velocity of the borehole fluid. A dipole source excites the borehole flexural mode, providing a means to determine shear speed without this limitation. Propagation models and waveforms from computer Propagation models and waveforms from computer simulations of hard and soft formations with both monopole and dipole sources are presented. These simulations are then compared to laboratory scale model data. Real-world borehole data acquired with both monopole and dipole sources, along with the instrumentation employed to acquire the data, are described. In particular, a combined monopole and dipole logging instrument with a high fidelity monopole and dipole receiver array is discussed. A processing algorithm for extracting compressional, shear and Stoneley speeds from monopole and dipole full waveform data is presented. Log samples are presented for hard, soft and presented. Log samples are presented for hard, soft and extremely soft formations from both monopole and dipole sources. Application of log data to evaluation of rock mechanical properties, such as Poisson's ratio, fracture evaluation, and correlation to compressional and shear seismic data, is also discussed.

Hornby, B.E., 1989, Imaging of near-borehole structure using full-waveform sonic data: Geophysics, v. 54, no. 6, p. 747-757, doi:10.1190/1.1442702.

Abstract: The full waveforms recorded by an array of receivers in a modern borehole sonic tool contain secondary arrivals that are reflected from near-borehole structural features. These arrivals are used to form an image of the near-borehole structural features in a manner similar to seismic migration. Possible uses of this technique include horizontal well logging; structural dip and contour determination; fault, salt dome, pinnacle reef, and fracture zone imaging; and EOR steam-flood monitoring. Since both the source and the receivers pass through structures that cross the borehole, the downdip structure and the updip structure can be imaged separately. The technique involves a backprojection of the recorded data into a matrix of accumulation bins representing distances radially out from the borehole and along the borehole axis. Separate matrices are formed for the updip and for the downdip raypaths. The basic technique is illustrated with synthetic data, generated to approximate the case of a sonic tool logging through a dipping bed boundary. Results are shown for a borehole experiment performed in Alaska. The data were acquired with a research sonic prototype tool and specially recorded with a long acquisition time--20 ms per trace instead of the normal 5 ms. This longer acquisition time enabled the acquisition of scattered P and S arrivals to be recorded after most of the direct signal had died out. Images are shown of near-borehole structural features to a distance of 18 m from the borehole. The images are presented against an independently derived formation lithology analysis and a high-resolution synthetic seismic display computed from the measured density and slowness logs.

Lai, J., Kangjun, C., Xin, Y., Wu, X., Chen, X., Yang, K., Song, Q., Wang, G., and Ding, X., 2021, Fracture characterization and detection in the deep Cambrian dolostones in the Tarim Basin, China: Insights from borehole image and sonic logs: Journal of Petroleum Science and Engineering, v. 196, p. 107659, doi:10.1016/j.petrol.2020.107659.

Abstract: Characterization and detection of natural fractures using well logs in ancient deeply buried dolostones with low matrix porosity are very difficult. Core description, thin section analysis, conventional well logs, borehole image and sonic logs are used to document distribution of fractures in deep Cambrian dolostones in the Tarim Basin, China. The SHmax (maximum horizontal stress) direction is determined from drilling induced fractures, borehole breakouts and shear wave birefringence. Natural fractures with various attributes (open to partly open to closed) and orientation (including dip angles) are recognized. Natural fractures are classified into closed fractures, partly closed fractures, vuggy fractures and open fractures, and they can be further classified into high angle fractures, low-medium dip angle fractures, and low angle or horizontal fractures. Natural fractures are mainly associated with the more brittle rocks (dolograinstone, fine-medium-coarse crystalline dolostones), while fractures in dolomicrites and evaporitic gypsums-bearing dolostones are rare. Open to closed fractures are recognized as dark to bright sine waves on image logs, and the presences of open fractures cause minor deviations on the DEN logs, but significant deviations in resistivity values, while AC will be significantly increased in front of fractures. The energy of sonic full-waveforms will be strongly attenuated in front of open fractures. Fractures with strike parallel to the SHmax tend to have large aperture and good connectivity, and are suggested to enhance hydrocarbon productivity. Dolostones adjacent to the deep faults are heavily fractured, and fractures act as the hydrothermal fluid flow pathways, facilitating dissolution process. The abundance of vuggy fractures, and enlargement of the pre-existing microfractures support that fractures had enhanced dissolution. The dissolution along fracture surfaces helps increase fracture porosity. The results of this study about subsurface fracture patterns and their distributions are important for the hydrocarbon exploration and production in the Cambrian dolostones in the Tarim Basin.

Sun, X., Tang, X., Cheng, C.H., and Frazer, L.N., 2000, P- and S-wave attenuation logs from monopole sonic data: Geophysics, v. 65, no. 3, p. 755-765, doi:10.1190/1.1444774.

Abstract: In this paper, a modification of an existing method for estimating relative P-wave attenuation is proposed. By generating synthetic waveforms without attenuation, the variation of geometrical spreading related to changes in formation properties with depth can be accounted for. With the modified method, reliable P- and S-wave attenuation logs can be extracted from monopole array acoustic waveform log data. Synthetic tests show that the P- and S-wave attenuation values estimated from synthetic waveforms agree well with their respective model values. In‐situ P- and S-wave attenuation profiles provide valuable information about reservoir rock properties. Field data processing results show that this method gives robust estimates of intrinsic attenuation. The attenuation profiles calculated independently from each waveform of an eight‐receiver array are consistent with one another. In fast formations where S-wave velocity exceeds the borehole fluid velocity, both P-wave attenuation and S-wave attenuation profiles can be obtained. P- and S-wave attenuation profiles and their comparisons are presented for three reservoirs. Their correlations with formation lithology, permeability, and fractures are also presented.

Vishkai, M., Wang, J., Wong, R.C.K., Clarkson, C.R., and Gates, I.D., 2017, Modeling geomechanical properties in the montney formation, Alberta, Canada: International Journal of Rock Mechanics and Mining Sciences, v. 96, p. 94-105, doi:10.1016/j.ijrmms.2017.04.001.

Abstract: Recently, unconventional reservoirs have received attention particularly in North America. These reservoirs require hydraulic fracturing to be commercially productive. At this point, it remains unclear as to the influence of geomechanical properties on fracture stimulation and its effective permeability, nature of the fractured zone (whether single extensive fracture or network), and extent of fractured zone. An understanding of the geomechanical properties and their spatial heterogeneity can be used to guide well placement and fracturing job design. In most simulation models, geomechanical properties are assumed to be homogeneous throughout the reservoir. In this study, heterogeneity of geomechanical properties is demonstrated by using geomodeling. A three-dimensional (3D) earth model was built by integrating both petrophysical and geological log data. The model includes dynamic elastic properties and rock strength property distributions in both vertical and horizontal directions within the reservoir and provides an ideal basis to understand hydraulic fracturing and wellbore stability. To determine elastic rock properties, changes in compressional and shear velocity through all the layers of the reservoir rock were taken into consideration. A workflow was developed to constrain well properties to derive realistic rock property values and distributions even in areas where only limited well log information exist. The 3D geomechanical earth model demonstrates that (1) the distribution of rock properties depends on formation lithology and (2) high lateral and vertical resolution can be achieved even in the areas with sparse wellbore information.

Wang, H., Tao, G., and Shang, X., 2016, Understanding acoustic methods for cement bond logging: The Journal of the Acoustical Society of America, v. 139, no. 5, doi:10.1121/1.4947511.

Abstract: Well cementation is important for oil/gas production, underground gas storage, and CO2 storage, since it isolates the reservoir layers from aquifers to increase well integrity and reduce environmental footprint. This paper analyzes wave modes of different sonic/ultrasonic methods for cement bonding evaluation. A Two dimensional finite difference method is then used to simulate the wavefield for the ultrasonic methods in the cased-hole models. Waveforms of pulse–echo method from different interfaces in a good bonded well are analyzed. Wavefield of the pitch–catch method for free casing, partial or full bonded models with ultra-low density cement are studied. Based on the studies, the modes in different methods are considered as follows: the zero-order symmetric Leaky–Lamb mode (S0) for sonic method, the first-order symmetric Leaky–Lamb mode (S1) for the pulse–echo method, and the zero-order anti-symmetric Leaky–Lamb mode (A0) for the pitch–catch method. For the sonic method, a directional transmitter in both the azimuth and axial directions can generate energy with a large incidence angle and azimuth resolution, which can effectively generate S0 and break out the azimuth limitation of the conventional sonic method. Although combination of pulse–echo and pitch–catch methods can determine the bonding condition of the third interface for the ultra-low density cement case, the pitch–catch cannot tell the fluid annulus thickness behind casing for the partial bonded cased-hole.

References

Bigelow, E.L., 1990, Cement evaluation: Houston, TX, Atlas Wireline Services, 142 p.

Hearst, J.R., and Nelson, P.H., 1985, Well Logging for Physical Properties: New York, NY, McGraw-Hill, 571 p.

Hornby, B.E., 1989, Imaging of near-borehole structure using full-waveform sonic data: Geophysics, v. 54, no. 6, p. 747-757, doi:10.1190/1.1442702.

Paillet, F.L., Cheng, C.H., and Pennington, W.D., 1992, Acoustic waveform logging- Advances in theory and application: Log Analyst, v. 33, p. 239-258.

Paillet, F.L. and Cheng, C.H., 1991, Acoustic Waves in Boreholes: Boca Raton, FL, CRC Press Inc., 284 p.

Pilkington, P.E., 1992, Cement evaluation - Past, present, and future: Journal of Petroleum Technology, v. 44, no. 2, p. 132-140, doi:10.2118/20314-PA.

Prensky, S.E., 1999, Advances in borehole imaging technology and applications, in Lovell, M.A., Williamson, G., and Harvey, P.K., eds., Borehole Imaging: applications and case histories: London, England, Geological Society of London Special Publications, v. 159, p. 1-43, doi:10.1144/GSL.SP.1999.159.01.01.