Smaine Zeroug

Ultrasonic leaky-lamb wave imaging through a highly contrasting layer

This article presents an innovative and effective ultrasonic technique to image through a steel layer with immersion transducers. The purpose is to detect scatterers and external interfaces and evaluate the acoustic properties of materials bonded to the steel layer. The technique is based on the lowest-order antisymmetric leaky lamb wave propagating in the steel layer and is implemented through a pitch-catch transmitter-receiver arrangement. The main application presented herein is for a measurement that takes place in an oilfield well to evaluate the quality of the cement sheath that fills the annular region between steel casing and a rock formation. Current ultrasonic imaging techniques for such an application are based on the pulse-echo mode with a single trans-receiver. Because of the large acoustic impedance contrast between the steel and its surrounding media: fluid on the side of the transducers and cement, or fluid/mud on the other, pulse-echo techniques have proven to be limited to probing only the region adjacent to the casing-cement interface. Using field data acquired with an experimental device, we show that the proposed leaky-lamb wave-based imaging technique provides reflection echoes that allow for probing of the entire cement sheath and in particular of the imaging of the external cement-rock interface geometry.

Identifying formation response using sonic dispersion curves

Summary The combination of monopole and cross-dipole sonic logging with advanced frequency domain processing provides the unique capability to characterize the acoustic state of the formation around the borehole: isotropy versus anisotropy (azimuthal dependence) and homogeneous versus inhomogeneous (radial dependence). Frequency domain processing (i.e., dispersion analysis) augments traditional time-based semblance processing to yield a more complete description of the formation. Field examples, in both shales and reservoir rocks, are shown which highlight the four canonical formation behaviors identifiable from dispersion analysis. Comparisons of measured data to modeled calculations of dipole flexural and Stoneley augment the classification method. Frequency domain processing complements traditional sonic log processing results, leading to improved quality control (QC) procedures. The key to this new evaluation is wideband acquisition of both monopole and cross-dipole sonic data coupled with advanced frequency domain processing

Monopole radial profiling of compressional slowness

Estimation of near-borehole formation compressional slowness is of significant value for petrophysical and geomechanical applications. Probing the near-borehole (shallow) formation and measuring the radial variation of slowness near-borehole can help identify damaged or altered zones, which is valuable information for wellbore stability and optimal well completion. To obtain the properties of the nonaltered zone, it is necessary to probe the formation as deeply as possible, which requires using an acoustic tool with a sufficiently long source-to-receiver (TR) spacing and a large array aperture. A recently developed sonic tool implements these characteristics and hence meets these objectives (i.e., probing shallow and deep). In addition to these hardware enhancements, a robust and automatic inversion scheme that provides a twodimensional (2D) image of the formation compressional slowness near-borehole has been developed. This technique is based on the inversion of transit times estimated from the waveform recorded by the tool. This inversion scheme is based on an analytical approach, making the implementation of the algorithm fast, robust, and suitable for the wellsite environment. In this paper, we demonstrate how the theoretical improvements, summarized here, enable presentation of a robust and reliable 2D image of the formation compressional slowness variation in the nearborehole in real time and with minimal user interaction. A real field data example will be presented and discussed to illustrate this new profiling technique.

Increasing Spatial and Temporal Bandwidth with Multi-component Streamer Data

Increasing bandwidth is not only about temporal frequencies but also about spatial wavenumbers, in particular those which are poorly sampled in the cross-line direction with streamer separations of 16 to 24 times the inline sampling interval. In this talk, we present results from a test with a mini-3D array of prototype 4C marine streamers in which we use, in addition to the pressure, the vertical and crossline gradients of the pressure wavefield in order to reconstruct and 3D deghost the wavefield at arbitrary points within the aperture. From the experimental 3D survey, we show examples of spatial and temporal enhancement of wavefields reconstructed using a generalised matching pursuit algorithm, comparing pressure-only and multi-component reconstructions. We find that multicomponent reconstruction is able to de-alias high wavenumber diffractions, that are completely missed by a pressure-only matching pursuit algorithm with priors, and generate broad-band unmigrated timeslices with excellent resolution.

Impact of Streamer Spacing on the Reconstruction Using Multimeasurement Seismic Data - Study Before and After Imaging

Reconstruction of the seismic 3D wavefield onto a fine receiver grid within coarse spreads of marine streamers is enabled by multimeasurement streamers that measure crossline and vertical particle accelerations in addition to the pressure. In this work, we evaluate the survey design of such multimeasurement marine seismic acquisitions, addressing the important practical aspect of the streamer separation. We use matching-pursuit-based techniques and employ various combinations of multimeasurement data to reconstruct a 3D full-bandwidth seismic wavefield onto a fine receiver grid for a range of cable spacings. Our results confirm the critical role of the Y acceleration in the crossline reconstruction with antialiasing power and suggest what practicality the value of the Y acceleration could translate into, enabling either better seismic acquisition quality for a given streamer spacing, or equivalent quality from larger streamer separations when compared to reconstructions that do not use the Y acceleration.

Influence of Breakouts On Borehole Sonic Dispersions

Borehole breakouts are commonly encountered during underbalance drilling in the presence of large tectonic stresses. The characteristics of borehole sonic data may be affected by strong departures from cylindrical geometry. A finite-difference time domain (FDTD) formulation with a perfectly-matched layer (PML) enables a study of the influence of breakouts on the borehole Stoneley, flexural and quadrupole dispersions. Breakout azimuths are oriented perpendicular to the maximum horizontal stress direction. The FDTD formulation yields synthetic waveforms at an array of receivers produced by a monopole, dipole or quadrupole source placed on the borehole axis. The borehole cross-section can be modified to simulate the breakout geometry. Synthetic waveforms are then processed by a modified matrix pencil algorithm to isolate both non-dispersive and dispersive arrivals in the wavetrain. While the axi-symmetric Stoneley dispersion is marginally affected by the presence of a breakout, there are discernible changes in both the flexural and quadrupole dispersions that can help in the analysis of such borehole failures. Computational results indicate that the presence of a symmetric breakout causes both the flexural and quadrupole wave splitting in the intermediate frequency band similar to the case of an elliptic hole. The two canonical dispersions approximately correspond to the largest and smallest diameters of the distorted borehole cross-section. These characteristic changes in the Stoneley, flexural and quadrupole dispersions can be used as indicators of the presence of breakouts and need to be accounted for in the sonic data inversion and interpretation.

Theoretical and experimental investigations of acoustic waves in embedded fluid-solid multi-string structures

Current acoustic measurements provide viable inspection for single cased wells, yet their interpretation for complicated multi-string wellbores where, for instance, two or more nested steel strings are deployed, is largely hampered by a lack of knowledge of the measured acoustic wave fields. This letter reports on theoretical and experimental investigations of the acoustic wave propagation in fluid-filled double string systems. Experimental measurements show excellent agreement with the theoretical predictions by a Sweeping Frequency Finite Element Method. The results lead to the identification of acoustic signatures that are crucial for an effective diagnosis of cement conditions in double-string cased wellbores.