Communications – electrical: acoustic wave systems and devices – Seismic prospecting – Well logging
Reexamination Certificate
1997-10-09
2001-01-30
Oda, Christine K. (Department: 2862)
Communications, electrical: acoustic wave systems and devices
Seismic prospecting
Well logging
C367S035000, C181S105000
Reexamination Certificate
active
06181642
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an acquisition method and apparatus for seismic exploration. More specifically, the invention relates to a data acquisition method and apparatus for downhole seismics investigating the strata surrounding a borehole.
2. Description of the Related Art
Seismic exploration methods are well established in the art. They are employed to obtain a broad general information about subsurface strata. The known techniques are generally implemented by utilizing an explosive or vibratory source and seismic sensors which are disposed at multiple locations on the surface of the earth or which are towed by a exploration vessel through the water.
More recently, it has be recognized that a combination of seismic exploration methods with selected borehole, measurements may be utilized to obtain more detailed information about subsurface formations and hydrocarbon reservoirs. These methods are often referred to as borehole or downhole seismics, particularly single well seismics and cross-well seismics. They involve the placement of seismic sensors in the borehole, which record the seismic wave field due to a seismic source at another location in the same borehole or in another borehole.
In most applications of downhole seismics, the seismic wave field of greatest interest is the wave field propagating in the rock, i.e., the wave field which would be present in the rock at the sensor location if the borehole itself were absent. Thus, parts of the seismic wave field which are only due to the presence of the borehole itself are usually regarded as noise. In particular, it is desirable to design downhole receiver tools so as to minimize their sensitivity to energy propagating primarily within the fluid-filled borehole. Examples of such waves are known tube waves or low frequency Stoneley waves.
It had been found that the use of hydrophones mounted in elongated cables, so-called “streamers”, placed downhole in a fluid-filled well, are particularly responsive to tube waves. In those cases the detected seismic signal is dominated by the tube wave energy, obscuring arrivals representing seismic waves that have undergone reflection from a reflector in the subterranean formation.
A method for attenuating the tube wave signal in a streamer configuration is known from U.S. Pat. No. 4,789,968. The streamer houses pairs of orthogonally mounted hydrophones, thereby permitting the selective detection of waves incident from any direction of interest.
In other methods the hydrophones have been replaced by downhole wall-locking geophones. These geophones are positioned in direct contact to the borehole wall, typically by elements that may be extended outwardly to the borehole wall from a sonde suspended in the borehole fluid at the end of a wireline. A typical example of this technique is described in the United States patent U.S. Pat. No. 5,212,354.
It has been further proposed in U.S. Pat. No. 4,078,223 to mount geophones in a seismic detector cable and oriented in different directions perpendicular to the cable's longitudinal axis. In operation, the geophones that happen to be vertically oriented at any given moment is sensitive to vertical waves. Those geophones which happen to be horizontally oriented at a given moment are disabled by gravity operated switches or the like.
In view of the known arrangements and methods for detecting seismic signals with a downhole receiver, it is the object of the invention to provide a receiver arrangement that is sensitive to the wave field of interest but reduces the noise caused by symmetric waves guided by the borehole.
SUMMARY OF THE INVENTION
The invention utilizes the different characteristics of motion induced by the tube wave or any other symmetrically guided wave and by waves incident from outside the borehole. The motion induced by the tube wave at points on opposite sides of the borehole is of equal magnitude but opposite direction. However, the motion induced by a wave incident from outside the borehole at opposite sides of the borehole is of essentially equal magnitude and equal direction.
The guided wave can be attenuated by arranging at least two geophones at separate positions along the perimeter of a cross-section essentially perpendicular to the axis of the wellbore and combining their respective output signals.
By introducing cylindrical coordinates (r, &phgr;, z) with z denoting the position (“depth”) along the longitudinal axis of the borehole, r the distance from this axis, i.e., the radius of the borehole, and &phgr; the polar angle in a plane perpendicular to the axis of the borehole, an alternative description of the present invention can be given. The invention can then be characterized as an arrangement of at least two geophones positioned at approximately the same position z at different polar angles &phgr;. The radial positions of the detectors are defined by the diameter of the borehole at z.
It is preferred to restrict the difference in the axial position (&Dgr;z) to values smaller than the wavelength of the tube wave. If the difference is of the order of the wavelength or larger, it is necessary to introduce additional corrections to the signals as measured by the geophones in order to suppress the noise due to the tube wave. In terms of absolute values, the difference preferably should not exceed 1 m or even more preferably 0.5 m. However larger differences may be corrected using programmed microprocessor devices or computers which are capable of reconstructing the diametrically opposed wave field from individually positioned geophones.
The term “geophone” as used herein includes directionally selective or anisotropic transducers which convert the seismic wave into a signal suitable for further processing, e.g., transducers based on the principle of a moving coil or solid state or other accelerometer. Also included are transducers which are sensitive to two or three independent directions in space, such as two-component (2C-) or three-component (3C-) geophones.
In a preferred embodiment of the invention, the geophone arrangement is designed with a radial symmetry, thus facilitating noise cancellation procedure. In a variant of this embodiment, noise cancellation can be achieved by a suitably connecting the output lines of the respective geophones so as to produce a difference signal in which the tube wave contribution is reduced.
In an even more preferred embodiment of the invention the geophones are arranged as two pairs of geophones with the geophones of each pair separated by approximately 180 degrees and one pair positioned perpendicularly to the other. Each geophone of this arrangement is radially oriented in the same direction. Employing this arrangement, seismic signals can be detected irrespective of their angle of incidence. It is noteworthy that such an arrangement is simultaneously applicable for vertically oriented boreholes and for horizontal boreholes without further modifications.
The outputs of the two paired geophones positioned on opposite sides of the borehole are combined so as to minimize the sensitivity with respect to the tube waves while enhancing the sensitivity with regard to the seismic signals from the formation. In principle it suffice to subtract/add both outputs to achieve this result.
It will be appreciated that the above described arrangement consisting of two pairs of one-component geophones each oriented in radial direction covers all possible angles of incidence. Under specific circumstances, however, it may be desirable to give the arrangement a preferential direction, in which case a single pair of one-component geophones could be deployed provided the means to control the angular position of the pair inside the wellbore.
The number of geophone in the arrangement can be reduced by making use of two- or three component geophones oriented not only in radial direction but also in one or two perpendicular directions. A single pair of 2-C geophones, accordingly oriented, is in principle capable of detecting the complete s
Christie Philip Andrew Felton
Coates Richard Timothy
Jeffery Brigitte L.
Jolly Anthony
Oda Christine K.
Schlumberger Technology Corporation
Wang William L.
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