--> Abstract: Characterization of a Fractured Carbonate Reservoir Using Multi-Component Seismic Data, by J. H. H. M. Potters; #90933 (1998).

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Abstract: Characterization of a Fractured Carbonate Reservoir Using Multi-Component Seismic Data

Potters, J. H. H. M. - Petroleum Development Oman

Situated in the southern foothills of the Oman Mountains and with an initial three billion barrels of 32 degree API oil, the Natih field is one of the largest oil fields in Oman. It was discovered in 1963 and was brought on stream in 1967. The main reservoir is the fractured and mostly fine-grained limestone of the Natih formation, which has a moderate to high (12 to 28%) porosity. The Natih is a series of regressive marine cycles deposited during the Middle Cretaceous. Because the matrix permeability is low, of the order of 0.5 to 10 milliDarcy, production is almost entirely fracture-related. That is, oil flow rates are essentially limited by the ability to refill the fractures from the matrix. Since the field has been produced for more than 30 years and counts more than a hundred wells, relatively much information is available. This is why it was chosen as the location of a unique seismic experiment aimed specifically at characterizing the fracture network.

This experiment has become known as the Natih 9C3D experiment, and was carried out jointly by Petroleum Development Oman, the main oil producer in Oman, and Shell Research in the Netherlands. The survey was carried out in 1991/92 and involved the acquisition of 28.4 sq. kms. of 3-D data using not only conventional vertical vibrators but also horizontal vibrators. The latter were used to generate two perpendicularly-polarized shear waves, which are sensitive to fractures. Furthermore, special geophones were used which not only detect vertical motion, but also horizontal motion in two perpendicular directions. With three source directions and three receiver directions, this means that a total of nine wavefield components are recorded (hence the 9C3D nomenclature). The sensitivity of shear waves to fractures manifests itself in birefringence, a form of anisotropy which means that shear waves whose polarization is parallel to the fractures propagate faster than those whose polarization is perpendicular to the fractures. Therefore, when two such shear signals are generated, a time difference between the two builds up on the way down across the fractured medium, to which the same amount is added again on the way up. Birefringence is zero in the absence of fractures and increases with increasing fracture density. This implies that the measurement of shear travel time differences provides a direct measure for fracture density. It is not necessary to know the fracture direction beforehand, as it can be derived mathematically from any orientation of shear waves as long as they are perpendicular at the seismic source location.

We measured the direction of the fast and slow waves as well as their time difference over the entire survey area, and displayed these in maps. The wave polarizations were found to be consistent with FMI/FMS, tectonic stress, and tracer data. A very large anisotropy, in excess of 20 percent, was found over a large part of the survey. An independent fault modelling exercise was also undertaken on the conventional 3-D seismic data. Fault/fracture ?domains? were identified on the basis of fault frequency and orientation. A comparison showed that these domains coincide with areas of coherent shear-wave time difference. A review of the production data for the field furthermore indicated that the domains were consistent with well behaviour reflecting the degree of fracturing. This demonstrates the utility of shear data for targeting wells in fractured reservoirs.

In addition there are two pieces of evidence that shear waves provide a Direct Hydrocarbon Indicator in fractured media. Firstly, the shear-wave anisotropy increases on traversing the fracture gas/oil contact. Secondly, the fractured Fiqa shale overburden exhibits a velocity anomaly in the slow shear wave, as a response to a gas chimney above the reservoir. These two observations are supported by new insights in the physics of wave propagation which, moreover, are consistent with our observations that effects are absent on conventional seismic and on the fast shear component.

We conclude that shear seismic data provides more direct information on fracture direction, fracture density and fracture fluid content than conventional seismic data.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil