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Seamlessly integrating technologies creates a new dimension in geophysical capability and clarity.
|The integration of seismic data, gravity data fit, density model, and MMT model, which defines and resolves zones M, R, Q, L, O, and MM and provides a more accurate image of interpreted salt interfaces. Gravity interpretation allows removal of a considerable volume of salt (NN) from the sedimentary column.|
The evolution of geophysical innovation can be tracked by following a series of well-documented scientific discoveries and technological breakthroughs. In almost every case our reach has exceeded our grasp. Geoscientists have suspected the presence of reservoir quality rocks long before they had the ability to image them, and certainly long before they had the ability to evaluate them. The deepwater offshore, in particular, has tenaciously held onto its prizes for many years before the technology to drill and produce wells in that environment emerged. We can appreciate how the industry has evolved by considering three major interdependent barriers it has struggled to overcome: communication, computation, and comprehension.
Taking them in reverse order, the tendency to specialize has clouded our comprehension, yet specialization is the commonly accepted route to excellence. The minute we put the prefix ”geo” in front of the word “scientist,” we create the environment for the development of tunnel vision. In the early days of geoscientific pursuit, distinct career paths emerged. Seismologists were different from physicists; geologists were different from sedimentologists and paleontologists. All were different from engineers.
Specialization grew to the point that separate companies were formed that specialized in one or another scientific endeavor, and collaboration was rare. A more comprehensive view was essential.
The evolution of computation is well known. Perhaps the best way to appreciate it is to quote the famous Moore’s Law: “The number of micro-components that can be placed on an integrated circuit, or microchip, doubles every 18 months.” This axiom, which has held true for more than four decades, has been both a blessing and a curse.
Our ability to grow computational capability almost exponentially is the technological engine that has enabled most of the recent improvements in geophysical imaging. Yet it has stretched our ability to develop applications and workflows to take advantage of this power. The problem is exacerbated when the participants are thinking in a “box.”
The most insidious barrier has been communication. Geoscientists, engineers and financial managers spoke different languages, used different protocols, and answered to different drivers. Some were comfortable dealing with trends and estimates. Others demanded precise answers. Neither group was happy with the specter of risk. The development of 3-D and 4-D visualization has become the universal language that has struck down this formidable barrier.
Together, the three barriers have become the strategic objectives of a geophysical revolution that is taking place worldwide today.
Putting it all together
|A subsalt reservoir illuminated by a four-boat wide-azimuth acquisition (left) is compared with the same volume illuminated by a single boat employing coil shooting (right).|
The geophysical revolution can be embraced by bringing clarity and comprehension to exploration and production science by expanding technological capabilities. By collaborating with sister divisions within Schlumberger, WesternGeco has developed workflows that allow seamless integration of science and engineering to achieve a common goal: enhancement of recovery and reduction of risk.
An enhanced view of the complex sedimentary terrain that exists beneath the floor of the Gulf of Mexico has been enabled by the integration of seismic technology, full-tensor gravimetry (FTG), and marine magnetotellurics (MMT). Called Multimeasurement-Constrained Imaging (MMCI), the technique improves salt body mapping and reduces overall risk in subsalt exploration. Each technology looks at the same volume of rock using a different physical principle, and when the results are combined, synergy is created.
At the same time, acquisition techniques have been made more efficient and cost-effective. The different methods sample either different physical properties or the same property by a different means. The goal is increased resolution. However, each method has relevant strengths and weaknesses. Using a combined interpretation that honors all constraints allows us to focus on each method’s strengths to create a more comprehensive image of the subsurface (Figure 1) When the measurements are combined, uncertainties are drastically reduced.
Building an accurate velocity model from all available models (including seismic) is fundamental to prestack depth migration interpretation. For example, in the Middle East, joint inversion of seismic and electromagnetic surveys help geoscientists map the contours of the base rock buried under hundreds of feet of constantly shifting sand dunes.
This allows velocities to be corrected to yield more accurate formation images. Elsewhere, in Saudi Arabia’s Rub al Khali, seismic and gravity surveys are jointly inverted to resolve the near-surface velocity model. The high-resolution gravity measurements are an excellent tool for mapping the shallow density variations characteristic of the sand dunes. The same joint inversion technique has been used to resolve an extremely complex thrust-belt area along the Oman-UAE border with significant velocity variations.
The superposition of a complex shale body over a deep carbonate platform had masked the imaging of deep carbonate reservoirs of Mesozoic and Paleozoic ages along a plate subduction line.
More definitive data
An example of technology catching up with vision is the recently introduced coil shooting technique. This involves a single acquisition vessel sailing in overlapping circles, shooting continuously to acquire twice as much high-resolution data in the same time as a multi-boat spread. This makes high-resolution geophysical measurements more efficient and affordable. The concept was introduced by William S. French in 1980 but languished on the shelf for a quarter-century while acquisition technology caught up with it. Now, enabled by broadband digital group forming and noise attenuation, steerable streamers, enhanced source/receiver positioning accuracy, and a calibrated marine source, coil shooting produces a rich-azimuth survey in a fraction of time required for a wide- or multi-azimuth survey (Figure 2).
A true image of the real earth
While seismic acquisition has made progress towards illuminating the subsurface and recording fundamentally better measurements, seismic processing is now bringing us one step closer to the real image of the earth. This is almost like changing from a black and white television to a color version. The explosion in computer power has allowed the implementation of very sophisticated geophysical algorithms that rely less on simplifications and assumptions and more on real earth physics and geology.
Being able to remove simplistic assumptions of the earth’s geology and physics required better communication with other geosciences and other measurements to complement seismic measurements and the seismic “vision,” e.g., using borehole information to calibrate anisotropic parameters extracted from seismic or using electromagnetic methods to image the near surface. This again is not new; however, today it happens at a completely different level of precision and accuracy. We have not only been able to introduce and apply extremely advanced algorithms in recent years, but we have gained a significant amount of comprehension with the early attempts of those advanced technologies.
Today computation, communication, and comprehension have not only brought us one step closer to the real image of the earth, but also one step quicker to an accurate image of the earth. A holistic approach is essential so that comprehension is not limited to acquisition and processing: thereby bridging the gap between geology and geophysics, processing, and depth imaging.
A look into the future
Innovation never takes a day off. The current version of Q-Land, WesternGeco’s single-sensor seismic acquisition technology, has 20,000 channels. A new product, UniQ, has up to150,000 live channels, an extreme capacity with highest flexibility enabling wide-azimuth, broad bandwidth reservoir surveys as well as fast-moving fit-for-purpose exploration surveys. In the marine environment, the use of simultaneous sources will provide unprecedented benefits in noise cancellation, more flexible survey geometries, improved sampling efficiency, and the enabling of azimuthal diversity. Some challenges remain, but there is no doubt -- the capability to employ simultaneous sources is a technology whose time has come.
By integrating geophysical services, enhanced reservoir description, characterization, and monitoring can be achieved throughout the life of a reservoir. Geophysics has now been transformed from elemental products and techniques into a more comprehensive approach of solving exploration and production issues, from complex illumination to 4-D time-lapse.