By giving reservoir engineers the freedom to experiment with new ideas, one oil company has found 4-D projects to be worth the risk.

Eighteen months ago, Shell had few 4-D or time-lapse projects in progress. At a time when the industry recognized the enormous potential of this technology, the company decided it would turn this situation around and strive to become an industry leader. As a result, about 80 4-D projects have been launched in the past 2 years.
The turnaround came in 1999 when Shell recognized its weakness in the application of certain technologies. This included 4-D reservoir imaging - a technique still in its relative infancy. To address this, a 2-year program began in May 2000 to focus heavily on the delivery of results.
Hans de Waal, global implementation manager for 4-D, said, "The first stage of the program was to set about identifying the fields that showed the potential for 4-D to bring value. These fields would form the basis of our 4-D portfolio."
The outcome was three-tiered. The first level was "safe bets" - relatively easy thick sandstone reservoirs and the use of proven technologies. The second tier was "good chance." Included were fields that contained more difficult carbonate structures, gas reservoirs and stacked reservoirs. In this tier, the benefits of knowledge acquired from around the world would be drawn upon to increase the chances of positive outcome. The third and final tier was "the unknown" - applying tomorrow's technologies with almost no preconceived ideas of what they might find or how they might perform.
The need for speed
This focus on results put teams in Shell's operating and associate companies under pressure to name, in advance, the 4-D projects they would embark upon. Fifteen projects were put forward for implementation during the next 2 years; 25 dedicated 4-D surveys were acquired at the end of the first year with a similar number expected during the second year - estimated to be twice as many as any other company. Shell also participates in several nonoperated surveys.
The kick start
In 1998, 4-D applications in Norway's Draugen field and the United Kingdom's Gannet-C developed into success stories and provided the impetus to include 4-D reservoir imaging as one of the new technology focus areas. In both fields, oil was on the move and being replaced by water. In Draugen, a new well was planned based on the anticipated direction of the flow. Before going ahead, however, an additional seismic survey was carried out and the data added to previous seismic interpretations. The results showed that the fluids were not moving where expected. The new models demonstrated that a fault in the field was blocking water flow, which then changed direction. The teams involved had to drastically change the position of the planned infill well. The cost of the additional survey was in the region of US $4 million. The gains, still being realized, were an additional 12 million bbl of oil - a total value of more than $80 million. For Gannet-C, the value is associated with derisking the infill well and avoiding a planned gas injector.
Risky business?
The portfolio of assets being addressed by 4-D is carefully balanced between those that are 90% certain to show a benefit (the safe bets), those that have high potential (good chance) and those that are high-risk (the unknown). But are they really high-risk? With an initial expectation that risky projects are unlikely to show benefits, any value gained is, therefore, a plus. But the true value is found in what is learned from such ventures - knowledge about reservoirs and previously untested technologies that might otherwise never see the light of day.
"You don't know what you don't know," said de Waal. By giving reservoir engineers the freedom to play around with new technologies, high-risk projects can turn up surprising results. "If you don't learn anything new from these projects, it means you probably haven't taken enough risk," he said.
Five of Shell's operating units are involved in 4-D projects in this unknown category. A percentage of the initial startup cost is picked up centrally to reduce the financial exposure of the individual companies around the world.
Technology tools
The technologies used within 4-D are grouped into data acquisition, data processing and the interpretation stage, integrated subsurface modeling. Seismic technology, which has been evolving for 80 years, still forms the basis of most data gathering. But it is the processing and modeling of the data that has spawned the greatest advance in technologies for understanding reservoirs and reducing the uncertainties in understanding.
Virtual reality is playing a big role in viewing the subsurface. With Shell's nine virtual reality centers around the world running the company's own 123DI software, subsurface teams can immerse themselves in the 3-D images of reservoirs - looking around, under and over them before adding the fourth dimension. Shell has developed or adapted a vast range of interpretation, simulation and modeling software technologies to manage and manipulate the massive mathematical computations required for static and dynamic models of the subsurface.
Models of the subsurface can be static (building a detailed geological image of reservoir architecture) or dynamic (allowing engineers to predict the movements reservoir fluids). They are developed using a variety of applications created by Shell or bought off the shelf. Xstream is Shell's latest application that integrates the static and dynamic functionalities of other Shell proprietary applications and off-the-shelf technologies such as OpenSpirit, INT, Java and Corba. The Xstream environment was launched in early 2001 and will be developed with further functionalities in the coming years, eventually replacing several single-function applications.
Case studies
Sarawak. During production of gas from the Sarawak carbonate reservoir, a tilted gas-water contact developed, causing some wells to produce water. A theoretical feasibility study showed it was possible that 4-D would give usable results.
Two-dimensional seismic profiles recorded in 2001 were compared with their 1990 equivalents. The results exceeded expectations, the difference between the two surveys revealing the original and present gas-water contacts.
The test showed that time-lapse techniques also can be used with 2-D data, provided the data are recorded with techniques that match each other closely.
Maui. New Zealand's largest gas field, Maui, started production in 1979. A second part of the field came onstream in 1994. A few production surprises have occurred, with some wells experiencing early water breakthrough.
Reprocessing of 3-D seismic data from 1991 showed that 4-D could be feasible. A 2001 test program, recording wide 2-D seismic swaths, which simulate 3-D, was processed in Shell's Netherlands technology center and compared with the 1991 data. This test provided valuable information about water movements and indicated a full time-lapse survey could reveal much more.
Gulf of Mexico. While 4-D is not yet applied widely in the Gulf of Mexico, as production moves into increasingly deeper waters, it is recognized as potentially valuable.
Managing reservoirs by using time-lapse imaging is far less costly than intervention into wells, especially where subsea production facilities are used. Three Shell 4-D surveys were acquired in 2001, and up to five per year will be recorded in the next 5 years.
Oman. A novel application of 4-D reservoir imaging involves placing sequences of seismic detectors down boreholes. Either 2-D or 3-D vertical seismic profiles can be recorded to monitor changes in the rocks or detect fluid movement during oil production. Alternatively, the detectors can just listen in passive mode to detect very small acoustic events (microseismic), which are induced during hydrocarbon extraction due to the changes in reservoir stress. These surveys are relatively inexpensive and can be repeated frequently.
Five projects are ongoing in Oman to monitor steam flooding, water injection and natural depletion. Oman is an ideal area in which to test these applications with downhole geophone arrays as conventional surface seismic data in this region typically lacks the resolution associated with marine seismic. Placing the geophones subsurface dramatically increases the data quality and hence facilitates reservoir monitoring.
Acknowledgements
Shell International thanks Petroleum Development Oman; the Ministry of Oil and Gas, Sultanate of Oman; Sarawak Shell Berhad; Petronas; Shell Todd Oil Services Ltd.; and Shell UK Expro for permission to reproduce the examples shown in this article.

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