Acoustics – Geophysical or subsurface exploration – Well logging
Reexamination Certificate
2001-05-18
2003-12-16
Dang, Khanh (Department: 2181)
Acoustics
Geophysical or subsurface exploration
Well logging
C181S112000, C367S025000, C166S250010
Reexamination Certificate
active
06662899
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to the production of hydrocarbons from subsurface formations and to in-hole seismic data acquisition to map advancing fluid fronts and the depletion of hydrocarbons within a hydrocarbon producing formation around a single well bore or outside a well bore on the surface of the earth or on the ocean bottom. The invention relates specifically to an autonomous carrier providing a movable obstruction, the carrier providing a seismic source or a discontinuity or obstruction to convert tube waves to body waves, the carrier being movable inside and outside the well bore and on the earth's surface or on the ocean bottom.
2. Description of the Related Art
In order to relatively precisely map advancing fluid fronts within a field or around a single well bore requires the use of deep reading measurements at spatial resolutions of less than five (5) meters but with the spatial extents of several hundred meters, depending upon the reservoir location, size and the number of wells in the field. Conventional three dimensional (“3D”) seismic acquisition and repeated 3D seismic acquisitions (also referred to as the 4D seismic acquisition) and seismic data acquisition techniques known as vertical seismic profiling (“VSP”), 3D VSP and Reverse VSP or Reverse 3D VSP are often utilized to model the reservoirs and/or to determine the advancing fluid fronts in the producing formations. The conventional 3D and 4D surface seismic acquisitions are performed by deploying detectors at or near the earth's surface and the survey area is usually substantially large. The conventional 3D and 4D surveys provide data with limited spatial resolution and no near real-time ability to utilize results because of the lengthy time span required to acquire and process the data, which can take several months. The subsurface VSP and 3D VSP also suffer from long data processing cycles and have limited spatial extent. Water breakthrough can occur rapidly, especially after a new horizontal well is drilled. Reservoir engineers can take timely action if timely fluid front information is available.
Another related problem is the expense of acquiring repeat 3D seismic data over a relatively small geographical area, such as between 10-20 Km
2
. The current seismic surveying vessels using surface towed cables are designed to acquire vast volumes of data over a large region. Ocean bottom cable surveys, wherein seismic sensor or detector cables are deployed on the sea bottom, provide an alternative surveying method but are more expensive than the towed streamer cable acquisition methods.
The term “signature” as used herein, means the variations in amplitude, frequency and phase of an acoustic waveform (for example, a Ricker wavelet) expressed in the time domain as displayed on a time scale recording. As used herein the term “coda” means the acoustic body wave seismic energy imparted to the adjacent earth formation at a particular location. The coda associated with a particular seismic energy source point or minor well bore obstruction in this invention will be the seismic signature for that seismic energy source point. The term “minor borehole obstruction”, “obstruction”, “borehole discontinuity” or “discontinuity” means an irregularity of any shape or character in the borehole such that tube wave energy transiting the wellbore will impart some energy to the irregularity in the borehole and thus radiate body wave energy into the surrounding earth formation while also transmitting and reflecting some the tube wave energy as well.
The term “impulse response” means the response of the instrumentation (seismic sensors and signal processing equipment) to a spike-like Dirac function or impulse. The signal energy of an acoustic wave field received by seismic sensors depends upon the texture of the rock layers through which the wave field propagated, from which it was reflected or with which it is otherwise associated, whether along vertical or along lateral trajectories. The term “texture” includes petrophysical parameters such as rock type, composition, porosity, permeability, density, fluid content, fluid type and inter-granular cementation by way of example but not by way of limitation.
From the above considerations, it is reasonable to expect that time-lapse seismic monitoring, that is, the act of monitoring the time-varying signature of seismic data associated with a mineral deposit such as an oil field 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 as 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 weather and culture may affect the outcome. If time-lapse tomography or seismic monitoring is to be useful for quantitative oil-field reservoir monitoring, instrumentation and environmental influences that are not due to changes in reservoir characteristics must be transparent to the before and after seismic data sets. Successful time-lapse tomography requires careful preliminary planning.
One way to avoid many time-dependent environmental changes and updated state-of-the-art instrumental changes is to permanently install seismic sources and seismic detectors in one or more boreholes in and around the area of economic interest. Identical processing methods are applied to the data throughout the monitoring period using cross-well (cross-borehole) tomography rather than conventional surface type operations. One such method is disclosed in patent application Ser. No. 08/949,748, filed Oct. 14, 1997 and assigned to the assignee of this invention and which is incorporated herein by reference as a teaching of cross-well tomography.
U.S. Pat. No. 5,406,530, issued Apr. 11, 1995 to Tokuo Yamamoto, teaches a nondestructive method of measuring physical characteristics of sediments to obtain a cross sectional distribution of porosity and permeability values and variations and of shear modulus and shear strength. A pair of bore holes has bore hole entries spaced apart from each other at a predetermined distance and a plurality of hydrophones are spaced at predetermined known locations. A pseudo random binary sequence code generator as a source of seismic energy is placed in another bore hole and activated to transmit pseudo random wave energy from the source to the hydrophones. Seismic wave characteristics are measured in a multiplicity of paths emanating from the source to the hydrophones using cross-well tomography.
The Yamamoto teaching is primarily directed to use in shallow bore holes for engineering studies. Such holes are less than 100 meters deep, as opposed to oil-field bore holes, which may be two to five kilometers deep. The requirement for an active source to be placed at various levels in the bore hole is problematic because the source can damage the hole and interfere with production. Since the seismic equipment must be moved up and down the bore hole, it is difficult, using conventional methods, to precisely locate and/or position seismic equipment to maintain identical recording conditions over an extended time period.
G. W. Winbow in U.S. Pat. No. 4,993,001 issued Feb. 12, 1991, describes a method and apparatus for converting tube waves into down hole body waves for seismic exploration. The equipment comprises a rotary-valve tube wave source for producing swept-frequency tube waves that are injected into tubing or well bore fluid.
Aronstam Peter
Norris Michael
Baker Hughes Incorporated
Dang Khanh
Madan Mossman & Sriram P.C.
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