| Popis: |
Carbonate reservoirs are prone to natural fracturing. Fractures can act as enhanced permeability pathways, which may increase, decrease, or complicate reservoir production and development; healed fractures contribute to reservoir compartmentalization. A primary focus is placed upon the predictability of fracture set patterns and orientations, which vary according to carbonate lithofacies and the stress field(s) under which different types of fractures form. Extension fractures can form at the surface or at reservoir depths. Certain types of extension fracture sets (e.g., syndepositional, regional, and—to a lesser extent—karst-related fracture sets) exhibit predictable patterns and orientations with respect to the stress field under which they originated. Surface outcrops commonly exhibit multiple fracture sets; these are most frequently related to relaxation of compaction and/or thermal cooling. Such fracture sets are considered unlikely to resemble fracture sets in nearby reservoirs at depth; therefore, the use of surface fracture patterns as analogs for same-formation reservoirs, without comparative analysis of burial stress histories, is risky. Fault-related fractures have very high permeability potentials when newly formed, but their resulting role as fluid conduits typically leads to rapid healing, and therefore a higher likelihood of causing reservoir compartmentalization. These fractures typically cut across multiple beds. Fold-related fracture patterns are complex, typically consisting of both extension and conjugate shear-pair fractures, and show variable orientations in space and/or over time. However, they tend to follow the geometries of individual beds and are often confined to single beds, rather than aligning according to overall structural axes. Ekofisk Field, a naturally fractured North Sea chalk reservoir, is presented as an illustrative case of fold-related fracture abundance and effectiveness in enhancing fieldwide permeability parameters, without the drawback of creating major production problems during waterflooding. |
