The most exciting technical advance I've been involved with recently is the use of high-quality seismic velocities to predict rock stresses and pore pressure. This information can significantly reduce the cost of drilling. These velocities are obtained using seismic tomography, which is like a medical X-ray that is shone through the body to build up a picture of its internal structure. Similarly, seismic reflection tomography allows us to make maps of the Earth's interior to visualize the velocity field, which in turn reveals variations and distribution of pore pressure. The cube on the left above was produced using conventional analysis. It indicates the presence of overpressure in this area but is not sufficiently accurate for drilling purposes. The cube on the right is a tomographically refined model. It gives us a much better understanding of the magnitude and spatial distribution in pore pressure.

Colin Sayers
Principal Geophysicist
Houston, Texas

Most geophysicists in the oil industry are concerned with determining the structure of subsurface formations as part of the exploration process. By contrast, my work is focused on using seismic data to determine rock and fluid properties during the production process. My clients include drillers and reservoir engineers as well as geophysicists.

Seismic energy tells us about the composition, fluid content, extent and geometry of rocks in the subsurface. Our key sources of information are compressional (P wave) and shear waves (S wave).

Another important way of understanding the reservoir is to use time-lapse seismic surveys, know as 4D seismic, in which the seismic survey is repeated over time. This monitoring allows us to see changes in the movement of reservoir fluids due to production. This is of particular interest to the reservoir engineer who only has production data at the wells. The availability of seismic data between wells promises to dramatically enhance the reservoir engineer’s understanding of the reservoir.

	Above: The Geco Eagle conducting marine seismic surveys. The vessel tows cables, known as streamers, packed with hydrophone receivers. Air guns generate acoustic vibrations that travel into the earth, pass through strata with different seismic responses and filtering effects, and return to the surface to be recorded as seismic data by the receivers.
Above right: Recording shear waves while they are still in the earth at the sea floor. Recording compressional (P) waves and shear (S) waves at the sea floor for oil and gas exploration is now done with cables that are packed with receivers, similar to streamers that are towed in conventional marine surveys. The cable is laid on the sea floor by the recording vessel, and another vessel activates the sources. This method brings with it several significant technical breakthroughs including: 1) the ability to image reservoirs in regions inaccessible to P-waves, such as those below gas clouds; 2) imaging of reservoirs that are poor reflectors of P-waves; and 3) the ability to distinguish between variations in rock type and variations in reservoir fluids.

The most exciting technical advance I’ve been involved with recently is the use of high-quality seismic velocities to predict how rock deforms during the life of a field. This information can significantly reduce the cost of drilling. Using seismic tomography, a system that allows us to make maps of the Earth’s interior, deep into the mantle, we are able to visualize the velocity field, which in turn reveals variations and distribution of pore pressure. This is where I work closely with the geomechanicists to come up with an accurate picture for the driller.

Although I am a geophysicist, I recognize that no single technique offers all the answers, and one of my main interests is integration of data acquired with different methods and on different length scales. Only then will we have true integrated solutions in which the high resolution of data acquired at wells can be combined with seismic data between the wells to deliver the true value of the different data sets.

* Mark of Schlumberger