Acoustics – Geophysical or subsurface exploration – Seismic source and detector
Reexamination Certificate
2001-03-26
2003-10-14
Nappi, Robert E. (Department: 2837)
Acoustics
Geophysical or subsurface exploration
Seismic source and detector
C181S105000, C181S122000, C367S038000, C367S039000, C367S040000, C367S041000, C367S042000, C367S035000, C367S032000
Reexamination Certificate
active
06631783
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The invention is related to mapping the hydrocarbon reservoir formations that display dynamic elastic nonlinearity to the seismic signals, which propagate through them. The main reason for this nonlinear behavior in the reservoir rocks is their bulk rock property: the porosity, fractures, grain-to-grain contacts and the pore fluids. The measurements of the interaction of the two seismic waves as they propagate through the elastically nonlinear formations of a reservoir are used to measure their bulk rock properties.
2. Description of the Prior Art
The current state-of-the-art seismic technologies that are being used to map the reservoir characteristics include 3-D seismic reflection surveys, seismic attribute analysis, signal amplitude extraction and coherency techniques. In spite of all the recent progress in seismic data acquisition and seismic data processing, results are quite often non-unique and ambiguous and fail to identify the higher porosity and fractured zones that contain a major portion of the hydrocarbon reserves.
New technologies and more sensitive methods of measuring the reservoir characteristics have to be developed and introduced to identify and map the higher porosity and fractured reservoir rocks, which may contain unproduced hydrocarbon reserves. In the past, the seismic industry has ignored the effects of elastic dynamic nonlinearity of the reservoir rocks. The measurement of the dynamic elastic nonlinearity of the rock is a sensitive tool because the porosity induces an orders of magnitude change for the nonlinear coefficients and a few percent change for linear parameters (velocity, attenuation etc.). Ref: Donskoy, McKee (1977); Paul Johnson (1997).
The dependence of the dynamic elastic nonlinear parameters of the rock on its bulk porosity and its fluid content has an important practical application. The correlation between the measurable effective nonlinear parameters and the structural parameters of the porous and fractured media can be used as a diagnostic tool for reservoir characterization. The measurement of elastic nonlinearity of the reservoir formations using seismic waves directly correlates with its bulk porosity, micro-fractures and the fluid content.
Two compressional waves, as they propagate through a porous rock that acts as an elastically nonlinear medium, interact with each other. Due to this interaction, the sum and difference frequencies of the two primary waves are created. These new frequencies constitute an ‘interaction’ wave that travels along with the primary waves. The amplitude of the summed frequencies or the ‘interaction’ wave is a function of the amplitudes of the two primary waves and the propagation distance through the nonlinear rock. The amplitude of the ‘interaction’ wave is proportional to the product of the primary wave amplitudes. Its amplitude grows with propagation distance due to nonlinearity and decays with distance due to attenuation. Reference U.S. Pat. No. 6,175,536 (Khan), where the interaction of the two crosswell seismic signals was successfully recorded as they propagate through the nonlinear reservoir formations.
This invention uses the measurement of the summed and differenced frequencies that are created due to the interaction of the two seismic (waves) or signals as they propagate through the porous and heterogeneous reservoir rocks. One of the signals is a vibratory ‘sweep’ commonly used for seismic recording; the frequency is swept over the seismic band from low to high or high to low over a period of several seconds. The concept is well known in the industry and is the current art.
The second signal is a mono-frequency sinusoidal signal, which has the same time duration as the vibratory sweep. Both the seismic signals or waves are generated, and transmitted using standard vibratory sources from a single source array, that behave as a single surface source location. The combined seismic wave is used for seismic reflection recording. It propagates through the surface formations and is transmitted and reflected at the formation boundaries that provide acoustic impedance contrasts. The reflected seismic signals are recorded using multiple detector arrays, located on the surface or in different well bores or both. The recording procedures are known in the current art.
In this invention, the interaction of the two-seismic waves as they propagate through the reservoir rocks is measured to map their nonlinear characteristics that correspond due to their bulk porosity, heterogeneity, and fluid contents. The data, which are recorded, have two different sets of information. The cross-correlation with the standard ‘sweep’ provides the normal data-set that is used for normal reflection processing similar to current 2-D and 3-D seismic processing that is universally practiced and known in the art. The second set of information is extracted, by generating two new sweep signals. These new signals are generated by adding and differencing the mono-frequency with the ‘sweep’ frequencies, thus providing two ‘modified-sweeps’ and cross-correlating the recorded data with these ‘modified-sweeps’.
This new set of data, which results after cross-correlation with the two ‘modified-sweeps’ and contains newly generated frequencies, represents the result of interaction between the mono-frequency wave and the ‘sweep’ frequency wave, as they propagate through the nonlinear reservoir rocks. The processing parameters for this new data-set are similar to the parameters used for the data generated after cross-correlation with the primary ‘sweep’ signal. Conventional 2-D and 3-D seismic processing sequence can be used for both sets of data to provide the reflection seismic image of the subsurface. The integration and interpretation of the two results, one based on the primary sweep, and the other based on the two modified-sweeps, highlights and identifies the subsurface formations that are nonlinear due to porosity, microfractures and fluid saturation. The results based on the two ‘modified-sweeps’ will display the reflected signals from higher porosity and micro-fractured formations at relatively higher amplitudes compared to the reflections from homogeneous and non-porous formations.
The unique contribution of this invention is that it provides a method of differential illumination of the subsurface formations that are of greater interest to the hydrocarbon producers. Clays and shales are normally less porous, more homogeneous and behave more linearly in comparison with high porosity sandstones and limestones. As a result, shales and clays generate a weaker ‘interaction’ signal and will show less prominent response on the nonlinearity seismic section.
The seismic results based on the second data-set that are produced after correlation with the two ‘modified-sweeps’ identify and high light the zones that have higher nonlinearity due to higher porosity, microfractures or their fluid content, thus identifying the formations that have greater potential for increasing the hydrocarbon reserves.
SUMMARY OF INVENTION
Briefly, the present invention provides a new and improved method of mapping the subsurface formations that are heterogeneous, that have higher porosity or that have fractures. Two surface vibratory sources are used: (a) One surface source that transmits a conventional sweep, where the frequencies in the seismic bandwidth are swept from low to high or high to low; and (b) the other surface source generates a mono-frequency sinusoidal signal that is predetermined. Both sources transmit their signals from the same source location, and their start timings are synchronized.
The simultaneous transmission of the mono-frequency and the swept signal generates a combination of two waves that travel through the earth formations, and are reflected and refracted from the acoustic boundaries as they propagate through the earth. The reflected signals are recorded by the seismic detectors that are located on the surface, ocean bottom, in one or more well bores or any different combinations o
Khan Tawassul A.
Martin Edgardo San
McGuire Sofia Khan
Nappi Robert E.
Nonlinear Seismic Imaging Inc.
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