Schlumberger Technology Corporation

Christopher V Kimball

Pawel Lewicki

Chung Lung Chang

Andrew J Hayman

Sevig Ayter

Ralph Michael D'Angelo

Benoit G Froelich

Thomas J Plona

Kenneth William Winkler

Sergio Kostek

Fred E Stanke

Curtis J Randall

David J Scheibner

Thomas L Marzetta

A method of determining acoustic properties of borehole earth formations, comprising: (a) transmitting acoustic energy from a transmitter location and receiving acoustic energy (a portion of which traveled through the formations) at a plurality of receiver locations in the borehole; (b) deriving data signals from the received acoustic energy; (c) selecting model values of the acoustic properties; (d) producing an intermediate matrix; (e) performing a singular value decomposition on the intermediate matrix to obtain a basis matrix of eigenvectors; (f) deriving a propagator matrix as a function of the model values; (g) producing a reduced propagator matrix from the propagator and basis matrices; (h) producing a test statistic using the data signals and the reduced propagator matrix; and (i) repeating steps (c)-(h) for different combinations of model values, and selecting, as output acoustic property values, the combination of model values resulting in a maximized test statistic.

Sonic well logging with multiwave processing utilizing a reduced propagator matrix

A method of analyzing Stoneley waveforms obtained in a well logging operation, includes analyzing the waveforms by means of multiparameter inversion using one or more externally provided parameters to obtain borehole fluid slowness and borehole fluid attenuation. and analyzing the Stoneiey waveforms by multiparameter inversion using the borehole fluid slowness and borehole fluid attenuation and the externally provided parameters so as to determine a parameter, such as mobility, related to permeability of the formation being logged. Complex conjugate back propagation [using a maximum likelihood/least mean squares error estimator or] by applying Prony's method and fitting model-derived dispersion curves can be used to obtain mobility.

Methods of analyzing stoneley waveforms and characterizing underground

A method of analyzing Stoneley waveforms obtained in a well logging operation, includes analyzing the waveforms by means of multiparameter inversion using one or more externally provided parameters to obtain borehole fluid slowness and borehole fluid attenuation, and analyzing the Stoneley waveforms by multiparameter inversion using the borehole fluid slowness and borehole fluid attenuation and the externally provided parameters so as to determine a parameter, such as mobility, related to permeability of the formation being logged. Complex conjugate back propagation using a maximum likelihood/least mean squares error estimator or by applying Prony's method and fitting model-derived dispersion curves can be used to obtain mobility.

Methods for determining an optimal frequency band for processing of flexural waves according to bias-corrected STC or QSTC are disclosed. A preliminary estimate of the formation shear slowness S and a determination of the borehole diameter D are first obtained, and the flexural waves are processed in a frequency band having a center frequency f.sub.c which is a function of preliminary estimate of S and the product of that estimate of S and diameter D. Where the preliminary estimate of S indicates a slow formation, f.sub.c is chosen as 0.6/SD. Where the estimate indicates a fast formation, f.sub.c is chosen as 0.2/SD. In other or all formations, a smooth function such as [0.2+((0.8/.pi.) arctan ((S-100)/100))/SD] can be used to choose the center frequency. Alternatively, if errors in the environmental parameters can be estimated statistically, they can be used to select an optimum center frequency. Once f.sub.c is chosen, the filter parameters which define the bandwidth of the filter used in the Fourier transform of QSTC and in bias-corrected STC are preferably normalized by scaling the bandwidth BW to a specified fraction of f.sub.c ; BW=f.sub.c /Q. Similarly, the window length T.sub.w used in semblance processing is preferably normalized to scale with f.sub.c so that the processing window contains the same number of cycles regardless of the frequency of the center frequency.

Sonic well logging methods and apparatus for processing flexural wave in

Apparatus and methods for determining an indication of a nonlinear property of a formation traversed by a borehole are provided. An acoustic tool is used in a borehole to generate first and second signals of different frequencies. As a result of nonlinearities in the formation, the mixing of the first and second signals results in a third signal having a frequency equal to the difference of the frequencies of the first and second signals. The amplitude of the third signal is measured by a detector, and the indication of nonlinearity of the formation is determined according to a relationship which relates the measured amplitude at the measured frequency to the amplitudes of the first and second signals, the frequency of measurement, the velocity of the measured wave, the distance of the receiver from the source, and a function of the nonlinear and linear parameters of the formation. The indication of nonlinearity may then be utilized to give an indication of the relative consolidation of the formation surrounding the borehole.

Measurement of formation characteristics using acoustic borehole tool

Methods and related apparatus for conducting shear slowness logging are set forth. The methods broadly comprise: detecting dispersive waves such as flexural or Stoneley waves; Fourier transforming the received signals; backpropagating the Fourier transformed signals according to equations using different dispersion curves; stacking the backpropagated signals; and finding semblances in order to choose the dispersion curve of maximum semblance and thereby identifying the shear slowness of the formation. Formation shear slowness may be plotted over borehole depth. Different embodiments are set forth. In one preferred embodiment (QDSTC), prior to Fourier transforming, the signals are stacked according to a previous estimation of slowness (S.sub.0), and are windowed for maximum energy. The reduced set of data in the window are then extracted for Fourier transformation, and prior to backpropagation, multiplied by S.sub.0 to reset them for backpropagation and stacking. In another preferred embodiment (DSTC), after the signals are backpropagated, the backpropagated signals are inverse fourier transformed and windowed. In DSTC, semblance values may be plotted as a function of slowness and time. Regardless of embodiment, the methods of the invention account for non-dispersive waves by using dispersion curves of constant value during backpropagation.

Sonic well logging methods and apparatus utilizing dispersive wave

A geophysical prospecting method comprises injecting signal energy in the form of at least two pseudo-random pulse sequences of the complementary sequence type into an underground formation. Next, the signal energy is detected after it has been attenuated by passing through the underground formations in the form of response signals. The response signals are correlated with reference signals comprising at least two pseudo-random binary sequences to yield correlations. The correlations are summed so that the resultant signal which corresponds to the subsurface response signal does not have secondary lobes. The method is applicable to seismic prospecting and more particularly to cross-well seismic surveying. It is also applicable to acoustic prospecting for logging measurements. In a preferred embodiment, the pseudo-random sequences comprise a pair of Golay sequences.

Geophysical prospecting method using pseudo-random pulse sequences

Methods for characterizing acoustic transducers and using a characterized acoustic transducer for measuring cement impedance of a cased well are disclosed. The method for characterizing the acoustic transducer generally comprises: arranging the acoustic transducer a predetermined distance from a calibration target of known thickness; transmitting a pulse from the acoustic transducer through a known medium and toward the calibration target and measuring a return signal received therefrom; and fitting, via minimization, a return signal calculation generated by a model of the return signal which has a limited number of parameters, to the received return signal to determine a value for at least one of the parameters and thereby characterize the transducer. The preferred parameters for characterization are the transducer radius and stand-off, such that the transducer may be characterized as having an effective stand-off and an effective radius. The method for measuring cement impedance behind the casing in a well utilizes the method for characterizing the transducer, where the calibration target is preferbly of the same radius and thickness as the casing of the well. Then, downhole, a pulse is transmitted toward the casing in the well and the return signal is measured. Using the effective stand-off and effective radius previously determined as fixed values, the model of the return signal, which utilizes reflective coefficients which, inter alia, are a function of the cement impedance, is fit to the actual return signal to determine an indication of the cement impedance.

Method for making cement impedance measurements with characterized

The present invention is directed to a method of calculating the impedance of a material behind the section of a casing. An acoustic excitation pulse is directed towards a section of the casing, resulting in a return waveform having a reverberation segment and an initial reflection segment. The return waveform is analyzed to choose a resonance frequency indicative of the casing's nominal thickness. The return waveform is bandpass filtered about the chosen frequency. A time window of the reverberation segment is selected, and the energy content in the time window of the filtered reverberation segment is calculated. Both the bandpass filter and time windows are selected based on the chosen resonance frequency, thereby removing variations in the thickness of the casing. The energy content calculation produces a cementation signal indicative of the impedance of the cement behind the section of the casing. The impedance value is indicative of the cement quality. In accordance with another aspect, a time portion of the initial reflection segment is chosen, and the energy content in the time window of the filtered initial reflection segment is calculated. This energy is used to normalize the cementation signal calculated from the reverberation segment. In accordance with another aspect of the present invention, the impedance of the material behind the casing is determined.

Determining impedance of material behind a casing in a borehole

Disclosed is sonic well logging which gives good results both in open and in cased boreholes. New use is made of the energy content output from a multireceiver tool to find a coherence measure for the received sonic energy and to find the peaks of that coherence measure at each depth level so as to produce a number of new logs of parameters of those peaks. Examples are a slowness/time coherence log for those peaks, as well as new logs related to the arrival times of the coherent energy associated with the peaks and the energy attenuation for those arrival times.

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West Redding, CT, United States of America

Christopher V Kimball