Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Earth science
Reexamination Certificate
2003-08-29
2004-09-07
Shah, Kamini (Department: 2863)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Earth science
C703S010000
Reexamination Certificate
active
06789018
ABSTRACT:
BACKGROUND OF INVENTION
1. Field of Invention
The present invention relates to mapping the presence and the location of the subsurface hydrocarbon reservoirs using the changes in the spectral and amplitude characteristics of the seismic pulse, due to elastic nonlinearity as the seismic pulse propagates through the porous and permeable reservoir rocks. The seismic wave or seismic pulse goes through a nonlinear transformation, and its spectral and amplitude characteristics change. Spectral broadening of the seismic pulse takes place as it is reflected, refracted, and transmitted through the porous reservoir rocks. Due to the elastic nonlinearity effects of the reservoir rocks new frequencies are generated. The generation of the new frequencies and their presence in the reflected signals is used to map the porous and permeable reservoir rocks. Reservoir rocks, which are highly permeable, also generate a Slow-Wave. The Slow-Wave travels at a much lower velocity than the Compressional Wave. The presence of the Slow-Wave also affects the amplitude and the spectrum of the reflected signals. The changes in the spectrum of the seismic reflected signals are used as an indicator to detect the presence of reservoir rocks and measure the reservoir rock properties. This invention would be extremely useful in detecting stratigrapic hydrocarbon traps that are difficult to map using structural information.
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 significant portion of the hydrocarbon reserves.
New technologies and more sensitive methods of measuring the reservoir characteristics are to be developed and introduced to identify and map the higher porosity and fractured reservoir rocks, which may contain a large portion of the 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 reservoir rocks 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 of velocity, attenuation etc. [Reference: Donskoy, McKee, 1977; Paul Johnson, 1997].
The current seismic reflection methods use the response of the earth's subsurface formations to the seismic waves for mapping the structural geology of the hydrocarbon reservoirs. For seismic reflection recording, the seismic source generates a seismic impulse, which propagates through the earth and the reflection response of the subsurface is recorded. The reflected signal characteristics depend on the acoustic and elastic properties of the rock formations. When the seismic wave encounters abrupt changes in the acoustic properties of the subsurface formations, it is reflected and refracted as it travels through the earth. The seismic measurements of the travel times and the amplitudes of the reflected signals define the subsurface geometry and provide an estimate of the acoustic impedances related to the subsurface rock velocities and densities. The seismic reflection record is basically the result of the convolution of the source-generated seismic pulse with the reflection coefficient series of the subsurface rock formations. The amplitude and the phase of the reflected and refracted signals are related to the elastic properties of different elastic mediums. During the downward propagation, there is a loss of higher frequencies, and the amplitude frequency response shows narrowing of the spectrum with depth, where the high frequency limit is imposed by the attenuation of the earth. Earth filtering effects are a major weakness of the current seismic methods.
The current seismic practice make two incorrect assumption when dealing with seismic wave propagation. First, they generally ignore the effects of elastic nonlinearity and treat sedimentary rocks as elastically linear. This is quite often to avoid extremely complex and cumbersome mathematics necessary when dealing with nonlinear behavior. Implicit in the assumption of linearity is the fact that the seismic wave or pulse recorded after being reflected and refracted can contain only those frequencies present in the input signal—the original seismic pulse that was initially transmitted. In the assumption of an elastically linear system no new frequencies can be generated.
The second incorrect assumption made is that the contribution of the Slow-Wave in the reflected and refracted signals from a porous and permeable rock formation is negligible and can be ignored. In reality, the reflected and refracted signals from a porous and permeable rock formation have two components. Part of the propagating energy is reflected and refracted from the rock matrix and part of the energy is reflected and refracted from the pore fluids that are contained in the rock formation. The compressional energy in the permeable rocks, which travels through the pore fluids, is known as a Slow-Wave. Due to the presence of this Slow-Wave in the permeable rock formation, the character and the amplitude of the reflected and refracted signals is affected and changed. The measurement of these changes provides us with a new seismic attribute that can be used to map the reservoir rock properties.
The current seismic methods assume that the earth is elastically linear, and the seismic wave or seismic pulse, as it travels through the earth subsurface formations, experiences no interaction between the frequencies generated by the seismic source. It is assumed that all the changes in the frequency spectra are caused due to attenuation, dispersion, and the reflection tuning effects. With the current assumptions of the earth being an elastically linear system, the recording equipment is designed and configured to handle the band limited seismic signals. Most seismic recordings for hydrocarbon exploration are made with a bandwidth of 6 to 8 Hz on the lower end of the seismic frequency spectrum and 70 to 80 Hz on the upper end of the frequency spectrum. The current recording practices are designed for a linear earth model. This limited frequency bandwidth is not adequate for recording the elastically nonlinear effects of the reservoir rocks. Elastic nonlinearity effects in a porous and permeable reservoir rock generate harmonics and sum-and-difference frequencies. These newly generated frequencies have to be preserved and recorded so that their presence can be detected and measured for mapping the porosity and permeability of the reservoir. To achieve that, seismic data has to be recorded with a bandwidth that has lower frequencies all the way down 0 Hz and higher frequencies to at least twice the frequency that is being currently used.
The current seismic bandwidth limitations are imposed by the recording characteristics of the receivers, amplifiers, and digitizers. These recording frequency bandwidth limitations are accepted because of the current assumption that the earth behaves linearly to the seismic signal. Higher frequencies are limited by the source generated signal and the earth's attenuation. The limit on the lower frequency is quite often determined by the seismic source and the response of the receivers. However, if the earth model is modified to an elastically nonlinear system, then one will expect lower frequencies down to Zero Frequency signals being generated at the lower frequency end of the spectrum and the harmonics of the highest usable frequency being generated in the elastically nonlinear earth. At present, the recording equipment and the knowledge of using that equipment exists in the industry to modify the current systems, for accomm
Khan Tawassul A.
McGuire Sofia Khan
Nonlinear Seismic Imaging Inc.
Nonlinear Seismic Imaging, Inc.
Shah Kamini
LandOfFree
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