Abstract: Modelling Fracture Orientations in Reservoirs with Photo-Elastic Techniques
Keith Rawnsley, Vincent Auzias, Jean Pierre Petit
Fractures within a reservoir can be divided into two types: faults, which result from shear displacement, and tension fractures which result from opening mode displacement. Tension fractures develop perpendicular to the minimal principal stress and so represent an image of the stress field orientation prevailing during their development.
In reservoirs tension fractures are not observed by standard seismic techniques and can only be observed very locally in core or suitable well bore images. Frequently tension fracture orientations vary within and between wells. For this reason the extrapolation of tension fracture orientations between wells can be complicated.
We present a method based on photo-elastic techniques that can model complex tension fracture patterns. Photo-elastic techniques permit the modelling of a stress field in a faulted strata and have shown that variations in tension fracture orientation may often be linked to stress field perturbations due to faults. The photo-elastic model consists of a rectangular plate of transparent polymer into which narrow slots with various frictional properties can be engineered to represent faults. The plate is loaded in a compression cell and light is transmitted through the plate via two polarizers and the directions of the stress field trajectories can be determined.
Applications of photo-elastic modelling of tension fracture orientations to both outcrop and reservoir data are presented. The reservoir example consists of a horizontal well with continuous fracture data obtained from core and Formation Micro-Scanner logs. It is shown that the stress field at the seismic fault scale can be related to the large scale variations in the fracture pattern in the well. Smaller scale variations in the well fracture orientations are simulated by down scaling the photo-elastic model from seismic scale faults to include mostly sub-seismic faults near the well.
AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France