Building on the tools of 3-D seismic imaging, Shell geoscientists are working on a fourth dimension, production time, to study how reservoirs change as they produce oil and gas. With new techniques that are increasingly more sensitive and accurate, it's possible to view subtle changes due to the migration of fluids, then make adjustments that improve recovery and extend the life of a field. Instead of predicting what a reservoir will do, now we can watch it perform and take action before problems occur.
Glowing images seem to float from the screen. The underground valleys and hills, fissures and ancient river beds look solid enough, until you try to touch them with your hands. With a few keystrokes, the system operator rotates the whole basin, then zooms in to expose thin layers of rock as easily as thumbing through a deck of cards. Next, he or she triggers a sequence of images that shows fluids moving through the reservoir, and the small team of geoscientists studying the field leans forward in their seats. The animation they are watching is 4-D seismic. It is now becoming possible to view time-lapse 3-D movies of reservoir model production simulations and compare them with real measurements made by recording and comparing multiple 3-D seismic surveys of the same area over a period of months or years.
The technology is still new, but already we're seeing an exciting new generation of 4-D seismic monitoring capability emerging. One driver, of course, is to increase production, but there are environmental and safety aspects as well. While many recent technologies are available that should improve recovery and protect the resource, to take full advantage of them we need to see the true distribution of fluids in the reservoir and
the effects of production, almost as they occur.
Traditional 4-D monitoring involves repeating high-definition 3-D surveys and evaluating the differences. The problem is that full 3-D seismic surveys covering large areas of land or sea are expensive and difficult to repeat exactly.
Offshore, for example, changing tides and currents can affect the repeatability of seismic surveys. The relatively variable and soft upper layers of the earth add to the problem by muffling and scattering the sound passing through them. That's even more true on land, where ordinary noise at the surface, from wind, equipment operating or vehicles driving by, may add false signals.
Uncontrolled variability from one survey to the next leaves geoscientists wondering if what they see on the screen is the result of fluids moving through the reservoir or something else. The goal in this case is not to make good 3-D images; it is to be able to measure meaningful and often small changes in the reservoir over time. The sensitivity of seismic monitoring depends upon making extremely repeatable measurements so that slight differences are significant.
Two new approaches have recently been demonstrated to gathering seismic data that are more repeatable and less expensive than traditional 4-D seismic repeat surveys.

Sparse OBC
Ocean bottom cable (OBC) systems record seismic data from receivers (geophones and hydrophones) fixed on the seabed instead of being towed behind seismic vessels during conventional 3-D streamer surveys at sea. The difference is that once the OBCs are in place, additional surveys can be repeated easily and more often. The receivers are also in the same position from one survey to the next, which offers more consistent results than sets of long streamers being towed just below the waves behind a boat and subject to wind and currents. Permanent seismic reservoir monitoring systems using OBCs offer many advantages, but they are expensive to install. A sparse reservoir monitoring approach is offering up some successes. With it, we can minimize the number of cables we deploy, focus on the areas of highest interest and still achieve the accuracy we want.
The trade-off with sparse OBC is the loss of some noise suppression capability compared to a full 3-D survey, but geophysicists have found that they can cancel out repeated noise and still measure reservoir changes through improved repeatability. This is the most essential element of any sparse 4-D system. With permanent OBCs, the biggest problems of recording 4-D surveys from streamers being towed behind seismic vessels are overcome.

Virtual sources
Sparse OBC provides reliable results under the right conditions, but an even better way to acquire 4-D seismic data on land or offshore has been demonstrated that further improves the accuracy and repeatability of sparse OBC. It involves placing geophones inside a slanted or horizontal well bore and recording all the sound wave energy going through them. Using a computer to filter these signals to a pulse for each geophone simulates the effect of a precise and known seismic signal coming from the receiver itself.
The process may be repeated for each geophone to generate a new "Virtual Source" seismic survey, with all of it shot and received deep underground instead of from the surface. The advantage is that these virtual source signals do not have to first travel through hundreds of meters of overburden before they reach the target zone.
This is a near-perfect approach for sensitive monitoring.
The virtual sources may be installed as permanent geophones, and they can be arranged to emit exactly repeatable source pulses. The virtual source approach may also be used to improve OBC surveys.

The benefits
Sparse OBC and the use of virtual source receivers inside a well bore are more accurate than conventional surveys because the signals for every new survey are recorded by geophones that are always in the same position. After the initial installation expenditure these surveys are also less expensive to repeat than full-field surveys. Compared to the effort and volume of seismic data needed for a conventional 3-D survey, sparse OBC and virtual source repeat surveys are faster, cheaper and more repeatable. They can be recorded and processed in days instead of weeks or months.
Seeing what's happening in a field, knowing where and when, helps the operator improve field management and create more accurate models, which both lead to increased recovery. We estimate that the cost of installing permanent 4-D monitoring systems can be recovered quickly, even with modest increases in production.
While low cost and greater accuracy are important selling points, perhaps the largest benefit is that geoscientists are beginning to view seismic data in a different way. With 3-D seismic and well data only, we tend to favor the models and explanations that best support our intuition. We tend to believe and are reluctant to change these models until we are confronted by new measurements that unambiguously contradict our assumptions. This evolution of 4-D monitoring is helping us reevaluate these assumptions and what we think we know.

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