Communications – electrical: acoustic wave systems and devices – Wellbore telemetering – Through well fluids
Reexamination Certificate
2003-02-14
2004-09-21
Moskowitz, Nelson (Department: 3663)
Communications, electrical: acoustic wave systems and devices
Wellbore telemetering
Through well fluids
C367S083000, C367S911000, C181S104000, C181S106000, C175S050000
Reexamination Certificate
active
06795373
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to the downhole generation of seismic waves for use in investigation and monitoring of earth formation reservoir characteristics surrounding a well borehole. In particular, the invention relates to a method and system for monitoring seismic energy radiated from well boreholes into surrounding earth formations. The seismic body waves radiated into the surrounding earth formation, which are generated within well bore structures, are used for cross well type projects and reverse vertical seismic profile (RVSP) type projects to investigate and monitor hydrocarbon or other mineral deposits over the productive lifetime of a producing reservoir.
This is a system for monitoring the distribution of the contents of a subsurface mineral deposit over its economic life span for long-term resource management. The system employs pressure waves generated within well bore structures, which are radiated as seismic waves into surrounding earth formation. Time varying changes of selected attributes of those seismic waves that have transmitted the deposit between boreholes or between boreholes and seismic sensors, may be indicative of the temporal changes in the mineral content.
BACKGROUND OF THE INVENTION
In the oil and gas industry, geophysical prospecting techniques are commonly used to aid in the search for and evaluation of subterranean hydrocarbon deposits. Generally, a seismic energy source is used to generate a seismic signal which propagates into the earth and is at least partially reflected by subsurface seismic reflectors (i.e., interfaces between underground formations having different acoustic impedances). The reflections are recorded by seismic detectors located at or near the surface of the earth, in a body of water, or at known depths in boreholes, and the resulting seismic data may be processed to yield information relating to the location of the subsurface reflectors and the physical properties of the subsurface formations.
Geophysical surveys are used to discover earth structure, mineral deposits, and the subsurface extent of mineral deposits such as oil, natural gas, water, sulphur, etc. Geophysical methods may also be used to monitor changes in the deposit, such as depletion resulting from production of the mineral over the economic lifetime of the deposit. The usefulness of a geophysical study depends on the ability to quantitatively measure and evaluate some geophysical analogue of petrophysical parameters related to the presence of the mineral under consideration.
Seismic methods may be applied to production-management monitoring as well as to exploration of hydrocarbon reservoirs. As is well known to geophysicists, an acoustic seismic source at or near the surface of the earth is caused periodically to radiate a seismic wavefield into the earth at each of a plurality of source survey stations. Acoustic seismic sources are usually of the impulsive or swept-frequency type. An impulsive source produces a very sharp minimum-phase wave of very short duration and that somewhat simulates the generation of an impulse. An explosion is an example of such a source.
The swept-frequency or chirp type seismic source may to generate a controlled wavetrain to form a relatively long pilot signal such as 2 to 30 seconds to assure sufficient energy is imparted to the earth. The swept-frequency or chirp type source method relies on signal compression to compress the signal and ensure sufficient vertical resolution to resolve the position of subsurface reflectors. Signal compression generally is called deconvolution, with many techniques well known in the art of seismic data processing. Deconvolution of sweep or chirp signals compresses the source signal into a much shorter signal representative of a subsurface reflective boundary. The accuracy and effectiveness of any deconvolution technique is directly related to how well the source signal is known or understood. Most deconvolution operators are derived from statistical estimates of the actual source waveform.
Swept frequency type sources emit energy in the form of a sweep of regularly increasing (upsweep) or decreasing (downsweep) frequency in the seismic frequency range. In addition to upsweeps and downsweeps, various alternative forms of swept frequency signals are well known in the art, for example, so called random sweeps, pseudo-random sweeps or nonlinear sweeps. In a nonlinear sweep, more time may be spent sweeping high frequencies than low frequencies to compensate for high-frequency attenuation in the signal's travel through the earth, or to shape to a desired wavelet. The vibrations are controlled by a control signal, which can control the frequency and phase of the seismic signals.
The acoustic seismic wavefield radiates in all directions to insonify the subsurface earth formations. The radiated wavefield energy is reflected back to be detected by seismic sensors (receivers) located at designated stations also usually located at or near the surface of the earth, but which may also be in the subsurface, for example, in well boreholes (herein, also called wellbores). The seismic sensors convert the mechanical earth motions, due to the reflected wavefield, to electrical signals. The resulting electrical signals are transmitted over a signal-transmission link of any desired type, to instrumentation, usually digital, where the seismic data signals are archivally stored for later processing.
The travel-time lapse between the emission of a wavefield by a source and the reception of the resulting sequence of reflected wavefields by a receiver is a measure of the depths of the respective earth formations from which the wavefield was reflected. The relative amplitudes of the reflected wavefields may be a function (an analogue) of the density and porosity of the respective earth formations from which the wavefields were reflected as well as the formations through which the wavefields propagated. The phase angle and frequency content of returned signals in the reflected wavefields may be influenced by formation fluids, the sought-for minerals or other formation characteristics.
The processed seismic data associated with a single receiver are customarily presented as a one-dimensional time scale recording displaying rock layer reflection amplitudes as a function of two-way wavefield travel time. A plurality of seismic traces from a plurality of receivers sequentially distributed along a line of survey at intervals, such as 25 meters, may be formatted side by side to form a two dimensional (2-D) analog model of a cross section of the earth. Seismic sections from a plurality of intersecting lines of survey distributed over an area of interest provide three-dimensional (3-D) imaging. A series of 3-D surveys of the same region made at successive time intervals, such as every six months, would constitute a 4-D, time-lapse study of the subsurface that would be useful to monitor, for example, the fluid-depletion rate of hydrocarbon reservoir.
From the above considerations, it is reasonable to expect that time-lapse seismic monitoring, that is, the act of monitoring the time-varying characteristics of seismic data associated with a mineral deposit such as a hydrocarbon reservoir of oil or gas over a long period of time, would allow monitoring the depletion of the fluid or mineral content, or the mapping of time-varying attributes such the advance of a thermal front in a steam-flooding operation.
Successful time-lapse monitoring requires that differences among the processed data sets must be attributable to physical changes in the petrophysical characteristics of the deposit. This criterion is severe because changes in the data-acquisition equipment and changes in the processing algorithms, inevitable over many years may introduce differences among the separate, individual data sets from surveys that are due to instrumentation, not the result of dynamic reservoir changes.
In particular, using conventional surface exploration techniques, long-term environmental changes in field conditions such as weathe
Baker Hughes Incorporated
Madan Mossman & Sriram P.C.
Moskowitz Nelson
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