Peter Elkington

Phase-based dispersion analysis for acoustic array borehole logging data

A phase-based dispersion analysis method for velocity (slowness) extraction from guided waves recorded by an acoustic borehole logging tool in a geological formation is presented. The technique consists of acquiring waveforms from an array of receivers distributed along the tool and constructing the dispersion characteristic by processing in the frequency domain and exploiting phase information to measure the travel time for each frequency component. The approach is nonparametric and completely data-driven and provides high resolution estimates that do not rely on velocity guesses or assumptions regarding the type of modes. Results are free of the aliases and spurious modes which are characteristic of some prior approaches. Examples of dispersion estimation curves are presented using synthesized flexural waves and field data from wireline dipole sonic tools; results are compared with those from the weighted spectral semblance (WSS) and amplitude and phase slowness estimation (APES) methods to demonstrate the effectiveness and utility of the proposed method.

Magnetic Flux Leakage Method: Large-Scale Approximation

We consider the application of the magnetic flux leakage (MFL) method to the detection of defects in ferromagnetic (steel) tubulars. The problem setup corresponds to the cases where the distance from the casing and the point where the magnetic field is measured is small compared to the curvature radius of the undamaged casing and the scale of inhomogeneity of the magnetic field in the defect-free case. Mathematically this corresponds to the planar ferromagnetic layer in a uniform magnetic field oriented along this layer. Defects in the layer surface result in a strong deformation of the magnetic field, which provides opportunities for the reconstruction of the surface profile from measurements of the magnetic field. We deal with large-scale defects whose depth is small compared to their longitudinal sizes---these being typical of corrosive damage. Within the framework of large-scale approximation, analytical relations between the casing thickness profile and the measured magnetic field can be derived.

Modified S transform for time-frequency analysis of borehole flexural waves dispersion

Guided wave propagation usually exhibits dispersive behaviour. The time-varying spectral components of dispersive waves have been characterized effectively using the time-frequency approach; the short-time Fourier transform (STFT) and the continuous wavelet transform (CWT) are commonly used for this purpose. However, the resulting energy distributions suffer from poor resolution related to the uncertainty principle, and this complicates the allocation of energy to individual propagation modes especially when the dispersion curves of these modes are close to each other in the time-frequency domain (in which case the separation becomes a challenge). Therefore there is a need for high resolution time-frequency techniques. To meet this challenge we introduce a modified version of the S transform, a method which combines the advantage of the STFT and CWT but with greater resolution. An adaptive algorithm is presented which identifies frequency regions of interest related to different energy modes, and employs ...

Acquiring Microresistivity Borehole Images in Deviated and Horizontal Wells Using Shuttle-Deployed Memory Tools

Borehole images have broad application in geological, petrophysical and geomechanical studies. The advent of the small-diameter memory micro-resistivity imaging tool improves operational efficiency in a broad range of well types. It extends the availability of high-quality images to slim and / or high dogleg wells, and reduces acquisition risk in high angle wells and wells drilled through formations prone to borehole bridging. In spite of the tool’s small size and weight, its novel design provides coverage and image quality that matches or exceeds that of previous generation imaging tools. It is deployed with or without a wireline, and is not constrained by wireline data transmission rates because data are recorded to internal memory. Deploying the tool inside drill pipe on the well shuttle facilitates access into highly deviated wells and past bad hole conditions without compromising borehole coverage.

Noise-Produced Patterns in Images Constructed from Magnetic Flux Leakage Data

Weatherford, East Leake, Loughborough LE12 6JX, UKAbstract. Magnetic flux leakage measurements help identify the positi on, size and shape ofcorrosion-related defects in steel casings used to protectboreholes drilled into oil and gas reser-voirs. Images constructed from magnetic flux leakage data co ntain patterns related to noise inher-ent in the method. We investigate the patterns and their scaling properties for the case of delta-correlated input noise, and consider the implications for the method’s ability to resolve defects.The analytical evaluation of the noise-produced patterns is made possible by model reduction facil-itated by large-scale approximation. With appropriate modification, the approach can be employedto analyze noise-produced patterns in other situations where the data of interest are not measureddirectly, but are related to the measured data by a complex linear transform involving integrationswith respect to spatial coordinates.Key words: magnetic flux leakage, noise-produced patterns, corrosive defectsAMS subject classification: 78A30, 78M34, 60G60

Microresistivity Image Driven Inversion of Nuclear Logs for the Evaluation of Unconventional Reservoirs

Density, photoelectric, neutron, and gamma ray logs are key inputs to the evaluation of unconventional resources. Each has a finite spatial resolution, and the properties of beds thinner than this cannot be measured directly but are (in principle) available from inversion if the logs’ response functions are known, and the thickness of each bed is known from an independent highresolution source. An inversion method is described for beds with apparent dip angles less than about 60 degrees which uses pre-computed response functions from measurement-specific nuclear particle transport code models, combined with bed boundaries picked automatically from high-resolution microresistivity images rendered using a new technique developed for high dynamic range data. The method has application in thinly-bedded formations in general and has particular relevance in coal bed methane reservoirs where multiple thin high-contrast beds are common. Inversion results are known to be sensitive to uncertainty in bed thickness values, and constraining the inversion with information from high-resolution images is found to be highly advantageous in producing reliable results in beds as thin as 0.075m; this compares to the resolution of about 0.6m for standard porosity logs, and 0.2m for the high-resolution field logs. Inverted density logs show good agreement with core density values, and the integrity of inverted photoelectric, neutron porosity and gamma ray logs is judged to be good based on the similarity between field logs and logs reconstructed from inverted results, plus the high degree of agreement with properties of known marker beds. Inverted logs are also shown to provide improved differentiation between litho-types defined from combinations of logs using principal components analysis. The paper addresses an important source of uncertainty in the estimation of reservoir properties - namely finite spatial resolution - and in so doing reduces bias in volumetric calculations and associated net pay calculations.

Dispersion correction for acoustic borehole logging data

Compressional and shear formation velocities are key to the prediction of petrophysical properties from seismic attributes. In fast formations shear velocity may be obtained from monopole source, but in slow formations, it is commonly determined from the flexural mode associated with dipole excitation, which is a dispersive borehole-guided mode who’s low frequency and high frequency asymptote to the formation S-velocity, and to the Scholte-velocity, respectively. The shear slowness is commonly computed from well log dipole flexural mode data using Semblance Time Coherence (STC) processing. Dispersion is handled by restricting the waveforms spectral content to the low frequencies that travel close to the formation’s shear velocity. This restricting may not eliminate the need for a residual dispersion correction. Inversion addresses this difficulty by computing shear slowness directly from observed dispersion characteristics. In order to make the inversion efficient the iterative steps which compare observe...

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.

Phase moveout method for extracting flexural mode dispersion and borehole properties

Among the dispersive modes encountered in acoustic well logging applications is the flexural mode associated with dipole source excitations whose low frequency asymptote provides the only reliable means of determining shear velocity in slow rock formations. We have developed a phase moveout method for extracting flexural mode dispersion curves from with excellent velocity resolution; the method is entirely data-driven, but in combination with a forward model able to generate theoretical dispersion curves, we are able to address the inverse problem and extract formation and borehole properties in addition to the rock shear velocity. The concept is demonstrated using data from isotropic and anisotropic formations.

Phase-based dispersion analysis of borehole acoustic logs

The dispersive behaviour of acoustic waves in boreholes is of interest in the evaluation of reservoir rocks, particularly from the point of view of near wellbore stress distribution. It is also used as a quality control on dipole sonic calculations that estimate formation shear slowness from the low frequency asymptote of the flexural wave slowness. Multiple methods are available for dispersion analysis; the paper reviews the most commonly used, including the Prony and the spectral semblance methods, and proposes a new phase-based analysis technique that has the benefit of improved slowness resolution. The methods are applied to synthetic and real data sets, and results compared. The new method is show to have lower slowness uncertainty for any given frequency, and the upper and lower frequency limits for which dispersion can be calculated is also extended.