GEOPHYSICS Statoil's ocean bottom seismography aimed at reducing dry hole cost
Hydrocarbons evaluation made easier by ROV-planted seafloor sensors
Dev George
Managing Editor - International
Another step forward has been achieved in improving the predictability of the composition of offshore prospects. The culmination of a five-year research and development program by Den Norske Stats Oljeslskap (Statoil), the new technology, SUMIC, is a seismic acquisition system that employs seabed sensors. The implanted sensors consist of hydrophone and multi-component geophone rods that are embedded in the seafloor by remotely operated vehicles.
Until recently, offshore prospects were at a geophysical disadvantage when compared to those onshore due to the inability of water to sustain shear waves, and therefore the lack of S-wave data in the seismic acquired via traditional streamer hydrophones. Now, however, by utilizing instrumentation embedded in the seafloor, S-wave data acquisition can be conducted simultaneously with P-wave data acquisition just as it can be onshore.
P-waves are pressure waves produced by rock compression and expansion that occur when a rock is sharply compressed, setting off small particle vibrations in the same direction that the compressional waves are traveling. S-waves are shear waves or waves of shearing action such as occurs when a rock is struck from the side. Small rock particle motion is perpendicular to the direction of wave propagation. The speed at which these waves travel is controlled by the mechanical properties of the rock, its density and elastic dynamic constants. In fluid-saturated rocks, these properties depend on the amount and type of fluid present, the composition of the rock grains, and the degree of inter-grain cementation.
The SUMIC submarine seismic system records P-waves and S-waves using SUMIC sticks, Statoil's newly invented seismic instrumentation rods. These rods, containing multi-component geophones as well as hydrophones, are planted to varying degrees of depth in the sea bottom using remotely operated vehicles. The SUMIC sticks are themselves connected to one another with cable, and the full array of implanted instrumentation rods are then connected to a seismic acquisition vessel on the surface.
The vessel maintains a stationary position near the array of seafloor seismic instruments to record the data as well as handle deployment and retrieval of the system. A second seismic vessel is employed to tow and fire the airguns that produce a split-spread inline common-receiver gather.
The combination of P- and S-wave data rather than P-wave alone can yield previously unavailable information about fluids in the pore spaces and improve the potential for identifying the prospect lithology. In addition, S-wave data is also able, in some cases, to image structures that P-waves can not adequately portray, such as reservoirs with gas clouds in porous rocks above the reservoir.
But will it be used?
Acquisition of seismic data via an ocean-bottom geophone/recording system put in place by ROVs is considerably more expensive than acquisition with hydrophones towed at the sea's surface, and it is doubly expensive when both are done simultaneously. Undoubtedly, the system has significant advantages over conventional marine seismic methods, in that it allows the collection of both P-wave and S-wave data in the marine environment, but will the industry use such a system with some regularity, and why would it be used in that environment, when, at this time, even though shear wave technology exists for onshore application, very little use is made of it?
Perhaps it will find its niche as a special tool for special occasions - hard sea bottoms, deepwater prospects, obstructed prospects, and those that underlie difficult-to-penetrate strata such as salt, shallow gas, and gas chimneys. Statoil's goals for SUMIC are to cut offshore exploration costs by improving detection and mapping of stratigraphic traps and thereby reduce the number of dry wells, and to cut development costs with better reservoir descriptions and fewer appraisal wells. The company is continuing its research to develop further applications.
According to Svein Kjellesvik, Vice President of marketing and sales for Geco-Prakla, "The commercialization of this technique will significantly increase the efficiency of operation, making it more attractive for oil companies to use." Geco-Prakla will offer the service this year.
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