Slowness Time Coherence Research Articles

Numerical simulation of borehole 3D scanning acoustic imaging using scattered waves

Oil and gas exploration increasingly requires high-resolution imaging of small, irregularly shaped, and highly heterogeneous well-side complex and abnormal geo-bodies. Conventional borehole acoustic imaging is often unable to accurately obtain the position and azimuth of small-scale abnormal geo-bodies. This study presents an inversion method that uses scattered waves for borehole 3D acoustic imaging and an implementation scheme that combines plane and spherical scanning imaging. The finite-difference time-domain method was used to simulate the acoustic fields for borehole azimuthal acoustic imaging of one and two caves next to a well. The proposed inversion method of 3D spatial scanning based on multi-mode wave information was validated through numerical simulations investigating the effect of different parameters on the imaging results. The simulation results show that the cave-scattered waves include the PP-, PS-, SP-, and SS-waves. When plane scanning imaging is performed based on a single wave mode, the other wave modes become interference factors. After the weighted processing of the PP-, PS-, SP-, and SS-waves, plane scanning imaging based on multi-mode scattered acoustic waves is shown to weaken pseudo-solutions, enhance the signal-to-noise ratio, and improve the radial and axial positioning accuracy of scatterers. When the scatterer is close to the borehole axis, the echo received by the receiver is not a real plane wave. In contrast with the 3D slowness time coherence (STC) and beamforming methods, spherical scanning imaging based on single-mode scattered acoustic waves completely considers this fact, which improves its azimuth positioning accuracy. Furthermore, spherical scanning imaging based on multi-mode scattered acoustic waves accurately estimates the azimuth of caves beside a well with a high imaging resolution. Finally, numerical simulation results were validated using the field measurement data of a well, and the actual imaging effect of the new method was tested. Therefore, rather than using single-mode reflected waves with limited information, the proposed method of scanning imaging using scattered acoustic waves can substantially improve the imaging resolution and positioning accuracy of small-scale abnormal geo-bodies beside a well and enhance the detection range.

An analysis for unipole acoustic logging while drilling wavefields with varying eccentered collar positions

Abstract The unipole logging while drilling (LWD) tool is a recently developed acoustic logging tool that uses an azimuthal emitting source to generate acoustic waves in the borehole to measure the longitudinal and transverse wave speeds of the surrounding stratum. In this study, the unipole transmitter transducer is modeled as a surface force source, and the analytical solution for unipole LWD wavefield is derived. The received waveforms are composed of infinite-order contributions owing to the directivity of the excitation of the unipole source. Using up to eight lowest-order modes, the acoustic waveforms excited by the unipole are calculated in both soft and hard formations, and then slowness-time coherence and velocity dispersion analysis are performed. The pure mode waves are obtained using the mode wave decomposition method. The influence of tool eccentricity (off-center positioning) on the wavefields excited by the unipole transmitter is also discussed. Based on the addition theorem of the Bessel function, the analytical solution of the received waveforms when the tool is eccentered can be obtained. In soft and hard formations, the received waveforms with tool eccentricities of 0 to 90% are compared. Tool eccentricity has a smaller effect for the soft formation than the hard formation. The received waveforms on the side closest to the borehole wall are also affected most when the tool is off center, with a sharp increase in amplitude in the fast formation and a late arrival wave group in the soft formation. These conclusions are important for the design and data interpretation of a unipole LWD tool.

The Response Mechanism of Borehole Shear-Horizontal Transverse-Electric Seismoelectric Waves to Fluid Salinity

The converted electric field in the seismoelectric effect can be used to monitor the salinity of the reservoir. Compared to some conventional excitation methods (e.g., Monopole source), the response law of borehole shear-horizontal transverse-electric (SH-TE) seismoelectric waves to fluid salinity is unique. In order to explore its physical mechanism, we study the influence of fluid salinity on borehole SH-TE wave fields in this paper. First, to analyze the effect of salinity on the electric field, we simulate the response for different salinity levels inside and outside the borehole. Then we study the wave fields in case of a radial salinity discontinuity outside the borehole, and simulate the interface response by the secant integral method. Finally, we show the feasibility of using the borehole SH-TE wavefields to estimate the salinity interface position combining the slowness-time coherence (STC) method. The results show that the electric field amplitude changes monotonously with the pore fluid salinity level. However, the borehole fluid salinity has almost no effect on the electric field. This is caused by the excitation method and the low frequency of the sound source. An interface converted electromagnetic wave response is generated when an SH wave passes through the salinity discontinuity interface. The interface position estimation examples show that the borehole SH-TE wave field is a potential method to evaluate the behavior and the location of the interface next to the borehole.

Acoustic evidence for a broad, hydraulically active damage zone surrounding the Alpine Fault, New Zealand

We present an analysis of the wireline sonic logs acquired through Alpine Fault hanging wall rocks during the second phase of the Deep Fault Drilling Project (DFDP-2). Due to inaccuracy in the initial tool-processed slowness logs, a more robust reprocessing of the data using the slowness–time coherence method was required. The reprocessing of the data yielded slowness logs that provide a significantly better estimate of compressional and shear slowness. The average slowness values of compressional and shear waves are 68 μs/ft and 110 μs/ft, respectively, corresponding to velocities of 4.5 km/s and 2.8 km/s, with some excursions related to fractures, borehole conditions, and tool settings, but no systematic downhole trends. The log values are compared to laboratory measurements of P- and S-wave velocity for samples of Alpine schist and mylonitic rocks to examine the scale dependence of the measurement. The average log velocity is 2 to 18% slower than the laboratory measurements due to the presence of mesoscale fractures that are present in situ but are not sampled at the laboratory scale. Using effective medium theory, we estimate the crack density in the hanging wall host rock to be ≤0.4. Comparison to field-scale seismic studies indicates that there is a broad (500–600 m wide) outer damage zone surrounding the intensely deformed inner damage zone of the Alpine Fault. This fractured outer zone facilitates fluid flow through the hanging wall and around the Alpine Fault.

3D slowness time coherence for sonic imaging

Although sonic imaging can provide higher resolution images of the near-wellbore region than lower frequency seismic and borehole seismic measurements, many challenges confront its more widespread use. The traditional sonic imaging workflow of first filtering the borehole modes and then migrating the underlying reflected arrival events ignores a critical interpretation step, namely, characterizing these reflected arrivals in terms of their azimuths, raypath types, and other attributes. Furthermore, using sonic imaging results in subsequent modeling and simulation workflows requires determining the 3D coordinates or at least the true dip and azimuth of these near-wellbore reflectors, and feature extraction from noisy 2D sonic imaging migration images cannot provide either of these. To address these interpretation challenges and develop a means of mapping these reflectors without requiring a migration, a central issue that arises is whether we can determine the slowness and propagation direction of a reflected wavefield using a standard array of receiver sensors mounted around the circumference of a tool sonde. To accomplish this task, we have developed 3D slowness time coherence (STC). We combine an automated time pick with a ray-tracing procedure and our 3D STC processing to evaluate the many candidate arrival events that may be present in the filtered waveform measurements, which leads to a 3D map of the reflectors that can be readily integrated into digital models of the surrounding subsurface as well as logs of reflector true dip and azimuth that can be compared with similar logs produced from borehole images.

Leaky modes and the first arrivals in cased boreholes with poorly bonded conditions

The generation mechanism of the first arrivals in the cased boreholes for the poorly bonded conditions is investigated. Based on the analyses of the Riemann surface structure of the characteristic function, the dispersion features, excitation spectra and contributions of modes excited in the cased boreholes with different cementing types are studied. The phase velocity dispersion studies of leaky modes show that high-order modes form “plateau” regions with one approximate velocity denoted by v separated by their cutoff frequencies, in which the phase velocity changes little with a considerable frequency range, while the group velocity keeps a relatively constant high value. Usually, the operation frequency range of a specific cementing evaluation acoustic logging tool is covered by such a “plateau” region. Mode excitation and contribution analyses show that the first arrivals in the cased boreholes for the poorly bonded conditions are the contributions from leaky modes, where the traveling velocity of the first arrivals processed by slowness time coherence (STC) method is equal to the approximated velocity v. Analyses on generation of leaky modes in the cased boreholes supplement the understanding of the generation mechanism of the first arrivals.

Instantaneous frequency-slowness analysis applied to borehole acoustic data

The methods most frequently used to process borehole acoustic data are based on semblance analysis. Two most commonly utilized semblance implementations are: slowness-time coherence and slowness-frequency coherence. Both of them are relatively robust under noisy well conditions. They deliver slowness value across the receiver array, and, as the quality control measures, coherence peak value and frequency dispersion curve.Semblance processing might be substituted by instantaneous frequency-slowness method based on complex wave form analysis. Instantaneous frequency -slowness delivers rich set of quality control measures. Among them are the velocities, the goodness and standard deviation across the receiver array, and instantaneous frequency and slowness wave forms computed between adjacent receiver pairs. Furthermore, since computations are performed across adjacent receivers, the vertical resolution is limited to the offset between receivers. Thus the effect of multiple semblance peaks observed while the receiver array is passing through the high acoustic impedance contrast is eliminated. Also, the method is capable to detect underperforming receivers. Finally it can help to control mixed acoustic mode conditions.Instantaneous frequency-slowness method delivers robust results under good to moderately noisy well data. The set of quality measures it delivers is much broader than the one generated by the semblance method.

Robust Detection and Estimation for Logging While Drilling Monopole Acoustic Data

In this paper, we present methods and strategies for robust detection and slowness estimation of weak compressional (P) and shear (S) waves in borehole sonic logging in noisy environments such as while logging while drilling, by proposing methods for enhancing the semblance used in the slowness time coherence method used in the logging. In this direction our contributions are two fold. First, we propose a novel class of shrinkage estimators in the discrete Radon transform domain derived from data semblance, for waveform de-noising to combat random additive noise and reflections. In this context we also identify necessary and sufficient conditions for the optimality of the proposed estimator. Our second contribution lies in developing a novel method to cancel energy propagating at the slowness of borehole modes, chiefly Stoneley, and to a lesser extent, shear, that significantly lowers the semblance of the weaker head wave arrivals. This approach is based on representing the data using time-frequency compact space-time propagators. Finally, we present an algorithm that combines the space time shrinkage in the discrete Radon transform domain and the proposed interference cancelation strategies for enhanced compressional and shear detection via semblance processing. The algorithm is validated on synthetic data and demonstrated to enhance performance on real logging while drilling field data sets.

Borehole acoustic array processing methods: A review

In this talk, we review the different borehole acoustic array methods and compare their effectiveness with simulated and real waveform examples: Starting from the slowness time coherence (STC) method, weighted semblance method (WSS), and many other common dispersive processing approaches including: Prony’s method, maximum entropy (ARMA) methods, and predictive array processing and Matrix pencil technique. We also discuss the Methods include phase minimization or coherency maximization and phase-based approaches.

Space-time methods for robust slowness estimation for monopole logging while drilling

In this paper, we present methods for interference cancelation for robust slowness estimation from noisy monopole logging while drilling (LWD) data. The main contributions are two fold. First, we show via tests on real data sets presence of systematic propagative interferences in monopole LWD data, which is the primary reason for loss of compression and shear semblance in the slowness time coherence (STC) processing of the LWD data. This interference in turn is mostly dominated by Stoneley type propagative component, which, unlike the main Stoneley mode, is time persistent over the entire acquisition interval. In addition, we also show that in fast formations the shear wave can significantly interfere with the compressional wave making the compressional slowness estimates quite bad. Second, based on these observations we propose a Successive Interference Cancellation (SIC) algorithm to estimate and cancel these interferences leading to STC enhancement and improved slowness estimation of the head waves. The algorithm exploits a novel representation of borehole acoustic signals using a dictionary of space-time propagators and is more robust compared to traditional slowness filtering methods, especially for the low aperture borehole acoustic array. We show the superior performance of the proposed algorithm on synthetic and real data sets.

Studies on phase and group velocities from acoustic logging

It is still argued whether we measure phase or group velocities using acoustic logging tools. In this paper, three kinds of models are used to investigate this problem by theoretical analyses and numerical simulations. First, we use the plane-wave superposition model containing two plane waves with different velocities and able to change the values of phase velocity and group velocity. The numerical results show that whether phase velocity is higher or lower than group velocity, using the slowness-time coherence (STC) method we can only get phase velocities. Second, according to the results of the dispersion analysis and branch-cut integration, in a rigid boundary borehole model the results of dispersion curves and the waveforms of the first-order mode show that the velocities obtained by the STC method are phase velocities while group velocities obtained by arrival time picking. Finally, dipole logging in a slow formation model is investigated using dispersion analysis and real-axis integration. The results of dispersion curves and full wave trains show similar conclusions as the borehole model with rigid boundary conditions.

Shear slowness measurement by dispersive processing of the borehole flexural mode

In sonic well logging, the flexural mode generated by a dipole transducer allows measurement of shear slowness in slow formations, that is, formations in which the shear slowness is slower than the borehole fluid speed. The flexural mode is a dispersive borehole mode, but many current processing methods do not account for this dispersion. The approximate semblance equation is derived to explain what happens when a dispersive wave is processed nondispersively. The processing described here is a dispersive analog of the slowness‐time coherence (STC) processing used commonly for array sonic waveforms. It back‐propagates waveforms according to model dispersion curves and calculates semblance. It is a specialized version of the maximum likelihood (ML) and least‐mean‐squared‐error (LMSE) estimator for formation shear slowness. The dispersive analog of STC is too slow for depth‐by‐depth logging but may be a helpful analytic tool. A high‐speed variant, dispersive STC (DSTC), eliminates this difficulty by a new window‐positioning technique and can be faster than STC. DSTC results based on data from a real‐axis integration model show that the technique measures the formation shear slowness with small bias because of windowing and other arrivals. Results are presented from two wells logged with a dipole sonic tool. A log from a fast formation shows that shear slownesses measured from the flexural mode by DSTC agree with monopole results. A log from a slow formation is also presented.

Error bars for sonic slowness measurements

Semblance provides a quality indicator for beam‐former array processing methods such as slowness/time coherence (STC) and dispersive STC (DSTC). Semblance does not indicate the slowness variance because the array design, the processing time‐bandwidth parameters, and the signal spectral content are not taken into account. For the classical case of a single propagating wave with added Gaussian noise, semblance is distributed according to the noncentral beta distribution. This distribution is parameterized by the number of receivers, M, the processing time‐bandwidth product, BT, and signal‐to‐noise ratio. Given M and BT, the measured semblance can be inverted to give a high‐quality estimate of the signal‐to‐noise ratio. The Cramer‐Rao lower bounds on slowness variance for the classical case of signals in added Gaussian noise depend on the array dimensions, the signal power spectrum, and the signal‐to‐noise ratio. Estimates of the Cramer‐Rao bounds can be calculated from the estimated signal‐to‐noise ratio and the measured signal power spectrum. Monte‐Carlo simulations show that these bounds approximate the actual slowness variance for high, but practical, signal‐to‐noise ratios. Preliminary results on field data show that error bars based on Cramer‐Rao bounds are credible, and that error bars yield slowness standard deviations comparable to those from repeat runs.