Jahir Pabon

A New Modular Sonic Tool Provides Complete Acoustic Formation Characterization

An improved estimation of sonic slownesses and a comprehensive mechanical characterization of the wellbore rock rely on a complete characterization of the compressional and shear slowness in terms of their radial, azimuthal, and axial variations. The new modular sonic tool accomplishes this by incorporating improved monopole and cross-dipole transmitter technology while featuring an extensive receiver array incorporating 13 axial levels of 8 azimuthal sensors each. Each receiver is individually digitized resulting in 104 waveforms per transmitter firing leading to an extremely reliable and accurate slowness estimation. This comes about through improved borehole mode extraction/rejection and enhanced wavenumber resolution at all frequencies. Formations exhibit wide, and sometimes complex, acoustical behaviors ranging from isotropic, anisotropic with its various mechanisms and significant radial slowness gradients. Radial rock property variations arise because of non-uniform stress distributions and mechanical or chemical near-wellbore alteration due to the drilling process. Anisotropy can be caused by intrinsic shale properties or external differential stresses. The critical data required to invert for these rock parameters underlying these acoustic behaviors are derived from the new tool through the use of broadband dispersion curves associated with propagating borehole acoustic modes. In this paper, we highlight tool features that have an important impact on seismic, borehole seismic, and sonic applications. The acquired high quality waveforms and advanced processing techniques lead to improved compressional and shear slowness estimates, radial profiling of shear and compressional slowness, enhanced anisotropy detection and mechanism identification, and reliable through casing slowness measurements. Examples are shown from several wells in Norway and Mexico.

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

Inversion of guided-wave dispersion data with application to borehole acoustics.

The problem of inferring unknown geometry and material parameters of a waveguide model from noisy samples of the associated modal dispersion curves is considered. In a significant reduction of the complexity of a common inversion methodology, the inner of two nested iterations is eliminated: The approach described does not employ explicit fitting of the data to computed dispersion curves. Instead, the unknown parameters are adjusted to minimize a cost function derived directly from the determinant of the boundary condition system matrix. This results in an efficient inversion scheme that, in the case of noise-free data, yields exact results. Multimode data can be simultaneously processed without extra complications. Furthermore, the inversion scheme can accommodate an arbitrary number of unknown parameters, provided that the data have sufficient sensitivity to these parameters. As an important application, we consider the sonic guidance condition for a fluid-filled borehole in an elastic, homogeneous, and isotropic rock formation for numerical forward and inverse dispersion analysis. We investigate numerically the parametric inversion with errors in the model parameters and the influence of bandwidth and noise, and examine the cases of multifrequency and multimode data, using simulated flexural and Stoneley dispersion data.

Inversion of borehole dispersions for formation elastic moduli

An inversion technique for the estimation of formation elastic parameters from measured sonic borehole Stoneley and flexural dispersions is described. The inversion technique consists of minimizing a cost function derived directly from the determinant of the boundary conditions at the fluid-solid interface. This inversion model allows the possibility of simultaneously using multiple dispersive arrivals for the estimation of some of the elements of a parameter vector containing the geometrical and material constants of the formation model. In the case of noise-free measured dispersions, convergence to the exact values of inverted elastic constants is rapid and independent of the bandwidth. Results from the parametric inversion with uncertain formation elastic parameters show the influence of bandwidth and noise in the measured dispersions on the accuracy of the inverted elastic moduli. In the presence of noise, the inversion accuracy is largely dependent on the intrinsic sensitivity of the measured dispersion and far less dependent on the dispersion bandwidth.

Borehole dipole and quadrupole modes in anisotropic formations

Sonic measurements while drilling are made in the presence of a drill collar (in the form of a thick steel pipe) that provides an additional path for the acoustic energy propagating from the transmitter to an array of receivers. Low-frequency asymptotes of both dipole and quadrupole modes yield the formation far-field shear slowness. Intrinsic or stress-induced anisotropy of surrounding formation in a vertical well is generally investigated by a wireline sonic tool using a borehole dipole mode that exhibits cos/spl theta/ azimuthal dependence in their associated acoustic field propagating along the borehole axis. The fast and slow dipole shear polarizations are orthogonal to one another in the presence of any azimuthal anisotropy. The two flexural modes are excited by rotating the dipole transmitter by 90/spl deg/ from one of the primary shear polarization directions. However, borehole quadrupole modes appear to be more promising for estimating the far-field formation shear slowness in the presence of a drill collar. Formation anisotropy can also be investigated using such quadrupole modes that exhibit cos2/spl theta/ azimuthal dependence in their acoustic field propagating along the borehole axis. The two canonical quadrupole modes are excited by rotating the quadrupole transmitter by 45/spl deg/ from one of the primary shear polarization directions. Borehole flexural dispersions exhibit a crossover in the presence of a uniaxial stress in a plane perpendicular to the borehole axis. In contrast, a perturbation analysis of quadrupole modes in the presence of a uniaxial stress does not show any such crossovers. Both the orthogonal quadrupole modes corresponding to the transmitter oriented parallel and 45/spl deg/ to the uniaxial stress direction asymptote to the same shear slowness at low frequencies. This shear slowness is essentially the same as the fast shear slowness measured by a dipole transmitter parallel to the uniaxial stress direction. However, there are increasing differences between the two orthogonal quadrupole mode slownesses at higher frequencies. Quadrupole waves with positive polarization parallel to the uniaxial stress direction always exhibit larger slownesses than those polarized 45/spl deg/ to the uniaxial stress direction. Even though stress-induced anisotropy in shear slownesses at very low frequencies is not predicted the orthogonal quadrupole modes, significant differences at higher frequencies can be used as an indicator of stress-induced anisotropy magnified by the near-wellbore stress concentrations.

Borehole flexural waves in formations with radially varying properties

Elastic wave propagation in a fluid-filled borehole is affected by near-wellbore alteration of formation properties. Near-wellbore alteration can be caused by several sources, such as overbalance drilling, borehole stress concentrations, shale swelling, near- wellbore mechanical damage and supercharging of permeable formations. Optimal completions of a well for production require both identification and estimation of the radial extent of alteration in reservoir intervals. Measured borehole flexural dispersions in the presence of radial gradients in formation properties can be inverted to estimate the radial extent of mechanical alteration. However, the presence of a tool structure that carries the acoustic transmitters and hydrophone receivers also introduces certain amount of bias on the measured borehole flexural dispersions. This paper describes the Backus-Gilbert inversion of synthetic borehole flexural data for radial variation in formation shear slowness (slowness is inverse of velocity). The inversion algorithm accounts for the tool bias on the measured data by introducing an equivalent structure of a heavy-fluid column placed concentrically with the borehole axis. This simple structure enables computation of the eigensolution for a reference homogeneous and isotropic formation that are used for calculating the data kernel in the perturbation integral equation. The solution of this integral equation yields the radial variation in the formation shear modulus in terms of fractional differences in the measured and reference dispersion at various wavenumbers. Results are presented for both radially increasing and decreasing shear slownesses away from the borehole.

Simulating Sonic Scanner responses in an interactive Web‐based High Performance Computing environment

Sonic Scanner logging tool collects a wealth of data about the geological formation. The drawback of this is that we are now discovering new features on the acoustic logs that have never been observed before; only rigorous modeling can help properly interpret the data. Invariably, it is difficult to learn quickly how to run a modeling code, set the parameters properly, and be able to detect possible errors in the input. In addition, complex modeling requires high power computing resources, which are not always easily accessible the user. To address these problems we developed a multi‐tier Web‐based log modeling environment where the Sonic Scanner simulator is easily accessible from the common Web browser. The user builds the model in an intuitive AJAX‐like interface and submits the simulation to a remote High Performance Cluster. The computed waveforms are played back in the browser using Scalable Vector Graphics in a variety of customizable displays. The Web application is easily available to any user with an Internet access. In addition, a programmatically accessible Web service is available to application developers who desire to build their own interpretation applications using the Sonic Scanner simulator engine.