Improved recovery of oil from fractured reservoirs often requires understanding how the intensity and orientation of natural fractures vary throughout the reservoir or field. Image logs or core provide excellent fracture data on these fracture parameters in the vicinity of a well, but these often still leave a large portion of the reservoir unknown. Modern 3D land-based seismic can be used to help constrain fracture intensity and directionality in the critical regions between wells, but acquisition and processing may be technically difficult or uneconomic, particularly in older onshore fields where optimized tertiary recovery schemes could increase recovery.
Many important fractured oil reservoirs form in a compressional setting in which there have been various episodes of folding and faulting. The faulting can be quite complex, and involve reverse or thrust faulting, as well as extensional (normal) faulting. Because these folding and faulting events probably represent the peak strain events that the reservoir has experienced, it is likely that the reservoir-scale fracturing developed in response to one or more of these events. Thus, if the strain history could be technically and economically mapped and compared to fracture data from image logs or core, it would be possible to constrain the orientations and intensity of fracturing throughout the reservoir.
This process was carried out at the Circle Ridge Field, in which the fracturing in the Tensleep and Phosphoria Formations controls the efficiency of recovery processes. A 3D palinspastic reconstruction of the strain history for the folding and faulting of this Field was carried out. The strain pattern developed after each stage of deformation was compared to image log and surface fracture data. It was found that the strain produced by the initial folding event predicts fracture orientations and intensity variations quite accurately throughout various structural blocks, and that later faulting events do not seem to have influenced the fracture pattern except in the immediate vicinity of the large faults. This geomechanically-based model of fracturing is being used as the basis for designing tertiary recovery processes for the Field.