Method and device for the long-term seismic monitoring of an und

Details Method and device for the long-term seismic monitoring of an und Method and device for the long-term seismic monitoring of an und

1996-08-26

1998-03-03

Moskowitz, Nelson

Communications, electrical: acoustic wave systems and devices

Seismic prospecting

Land-reflection type

367 25, 367 77, 181106, 181111, 181401, G01V 140, G01V 128

Patent

active

057243115

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to a method for the long-term seismic monitoring of an underground area containing fluids and more particularly of an underground gas storage reservoir.
2. Description of the Prior Art
It is well-known to test the filling of an underground reservoir intended for the storage of natural gas at successive times of its loading, by means of a seismic system including a vibrator or an impulsive source for applying disturbances to the ground surface and a reception device comprising alignments of sensors arranged at the surface or in a well and coupled with the ground. As the injection of gas significantly changes the speed of sound, the density and therefore the acoustic impedance of the reservoir formation, the variations of the time interval between two reflectors, one being placed above, the other below the reservoir, can be determined on the seismic records. The filling evolution can also be monitored by measuring the amplitude variation of the waves reflected at successive times at the level of the reservoir layers at successive times of the filling.


BACKGROUND OF THE INVENTION

The published French patent applications 2,593,292; 2,642,849; 2,656,034; 2,688,896 and 2,689,647 notably describe systems including one or several servicing arrays installed in a fixed position in wells connected to the surface by one or several transmission channels. These servicing arrays can be seismic receivers and also possibly be one or several seismic wave sources arranged in wells. The arrays of receivers or emitters can be placed outside casing pipes. They are coupled with the surrounding formations by the cement injected into the annular space around these pipes after they have been set.
The arrays of receivers can also be associated with a tubing lowered into a well and coupled with the formations surrounding the well by means of a casing pipe for example.
Subsurface seismic exploration is generally performed by coupling seismic sources and receivers with the ground according to different combinations where the sources and/or the receivers are arranged at the surface or close thereto or in one or several wells through the formation explored.
French patent applications 2,703,457; 2,703,470 and 2,674,029 describe methods of seismic monitoring of underground reservoirs by means of arrays of receivers placed in wells and of seismic sources placed at the ground surface or possibly in these wells. A method of seismic monitoring of a gas storage reservoir is also described for example by E. Blondin et al in Geophysical Prospecting 34, 73-93, 1986.
A series of seismic emission-reception cycles are performed by changing the time of activation of a seismic source in relation to the axis of the well where the arrays of receivers are installed, according to the so-called "walk-away" technique, and by recording the time arrivals at receivers R1 to Rn as a function of the time of transit t of sound thereto. The source can be shifted along a radial axis OX and a two-dimensional seismic display (X,t) or (X,Z) is obtained. It can also be shifted all around the well so as to obtain a 3D seismic display (X,Y,t) or (X,Y,Z), Z standing for the depth, axis OY being perpendicular to OX and OZ. In practice, the source S can be shifted on either side of the well over a distance (or a radius) of the same order of magnitude as the underground area to be monitored.
Deposit monitoring and gas storage reservoir filling are generally long-lasting operations. The volume variations of the gas bubbles stored for example, due to later samplings, are slow. The seismic monitoring operations are often spaced out in time. In practice, the surface seismic equipment has to be reinstalled at each new seismic recording session and the emission conditions of the previous seismic operations preferably have to be reproduced.
Reinstallations are not always possible. For the production of a deposit or of a storage reservoir, various installations and material are set and the site is

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