Patent Application
20090261832
The invention relates generally to seismic and electromagnetic (EM) measurements, and in particular, to the use of an EM receiver functioning as a seismic receiver.
Electromagnetic (EM) logging tools are commonly used to measure conductivity of rock formations, providing the means to identify the presence of water or hydrocarbons. Seismic tools on the other hand, measure the propagation velocity of mechanical waves through different rock formations as means to detect geological structures and rock properties such as porosity. Both Electromagnetic logging tools and seismic logging tools are common in the industry and have been patented.
In existing systems, EM surveys are logged without seismic surveys. When seismic information is required, then a fully separate profile and service is required such as cross-well seismic survey taken with a seismic tool such as Schlumberger's Versatile Seismic Imager™ tool. Electromagnetic and seismic measurements are complementary and help in the processing and interpretation of a reservoir.
In the following description, numerous details are set forth to provide an understanding of the present invention. However, it will be understood by those skilled in the art that the present invention may be practiced without these details and that numerous variations or modifications from the described embodiments are possible.
The present disclosure pertains to a system that makes EM and seismic measurements substantially simultaneously using an EM receiver array for both measurements. The substantially simultaneous EM/seismic measurements are accomplished based on the fact that the receivers measure a varying magnetic field. The variation in magnetic field sensed by the receiver has at least three sources: 1) an alternating source (Electromagnetic Transmitter); 2) receiver motion in the presence of an EM field; and 3) motion of the formation relative to the Receiver (when the transmitter string is mechanically insulated from the formation).
The alternating source, i.e., the EM Transmitter is used as part of existing EM logging tools, whereas the variation in field due to motion of the receiver and motion of the formation provides the bases on which the technological advances of the present invention are based. The advantages enabled by the present disclosure include considerable reduction in field equipment, rig time, personnel, accurate co-location of EM and seismic sensor and the possibility to modulate the EM signals with mechanical energy with its associated benefits.
In a standard cross well EM tomography (shown in the prior art
In the illustrative EM-Seismic system shown
The EM-seismic system described above with respect to
Alternative Embodiments may include the following:
Receivers may be mechanically isolated from or tightly coupled to the formation. For example, the EM receiver array may be mechanically isolated from the formation. In such an embodiment, the EM receiver array is suspended by the wireline cable to the surface and kept mechanically isolated from the wellbore walls through soft centralizers. With such an embodiment, the seismic signals measured by the EM receiver array are the product of the motion from the formation relative to the receiver.
In such an embodiment, the rock formation (water saturated for example) produces a secondary EM field, and when the seismic waves produce motion in the formation, the secondary field reveals a variation with the characteristics (i.e., amplitude and phase) of the seismic wave. While the measurement of the secondary field is being made, the EM transmitter is powered ON at a determined power level and frequency (generating a primary field). The EM receiver array measures a varying EM signal which is “modulated” by the mechanical seismic waves produced by the seismic source as well as any seismo-electric conversions.
The amplitude of the EM transmitter can be adjusted in order to control the level of sensitivity of the EM receiver array to the seismic waves. Such an embodiment is simple to deploy since each receiver is coupled to the others mechanically by the wireline cable (i.e., in the standard wireline method) while being mechanically isolated from the formation, allowing each receiver to independently sense the response of the formation surrounding it.
In still another embodiment, the EM receiver array may be mechanically clamped to the formation. In such an embodiment, each receiver may have tight mechanical coupling with the adjacent rock formation. Such coupling is achieved through a mechanical clamping mechanism (such as that used with Schlumberger's Versatile Seismic Imager™ tool) and each receiver de-coupled from the others by connecting each receiver with a soft cable. Such an embodiment seeks to measure the seismic signals by directly coupling the mechanical energy into the receiver. When each receiver in the array is in the presence of a magnetic field (such as the Earth's magnetic field), then the motion response obtained is similar to that of an EM signal varying at the frequency of the mechanical motion, vibration and/or rotation.
In still another embodiment, the type and position of the seismic source(s) respect to the receivers can be varied from that shown in
In still another embodiment, the type and position of the EM transmitter sources and position respect to the receivers can be varied from that shown in
The EM-Seismic system and method presented here can be used but is not limited to the following applications:
While the invention has been disclosed with respect to a limited number of embodiments, those skilled in the art, having the benefit of this disclosure, will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover such modifications and variations as fall within the true spirit and scope of the invention.
| Number | Date | Country | |
|---|---|---|---|
| 61046826 | Apr 2008 | US |
