Abstract

Within any faulted reservoir, there are large numbers of faults that are below the resolution of seismic surveys. Some of these faults are encountered in wells, but most of them remain undetected. Such subseismic faults can significantly influence the flow of hydrocarbons during production. The size distribution of subseismic faults can be predicted by extrapolating the size distribution measured at the seismic scale down to the subseismic scale. However, the positions and orientations of the subseismic faults are more difficult to determine. A method based on mechanical modeling is described here to constrain the positions and orientations of subseismic faults. The large, seismically resolvable faults are brought into a three-dimensional (3-D) numerical mechanical model to determine the stress conditions near these faults at the time of faulting. The stress field is then combined with a Coulomb failure criterion to predict the orientations and densities of the smaller faults. This information is represented on a pair of grids (i.e., a density and strike grid). The grids are then used to condition two-dimensional or 3-D stochastic models of faulting, which use a power-law distribution and/or stochastic growth processes to simulate subseismic faults. Two contrasting stochastic methods are used: (1) a method in which the subseismic faults are placed in the volume as fully grown structures and (2) a method in which the faults are allowed to grow and interact. The Oseberg Sør reservoir, northern North Sea, is used as an example of the application of these methods. Methods for incorporating modeled subseismic faults into the reservoir-flow simulation are also discussed.

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