STRUCTURAL INTERPRETATION, GEOMECHANICAL MODELING, AND FRACTURE PREDICTION IN THE CLAIR FIELD, WEST OF SHETLANDS BASIN, UK

The Clair Field lies 75 km west of the Shetland Islands on the North Atlantic rifted margin. The main structure includes a large, rotated rift block and second-order fault terraces with complex, internal faulting. Oil production comes from Devonian sandstones with locally good matrix properties. However, well performance and both horizontal and vertical connectivity suggest that fractures contribute substantially to subsurface flow.

Predicting fracture distribution and effectiveness requires an integrated approach. Early in field development we tested a structural-geomechanical method in a pilot area around one of the first production wells. Our method begins with a high fidelity structural framework from the initial seismic interpretation. This framework contains faults that have complete splay and intersection geometries. The fault network also provides detailed offsets of mapped horizons with hanging wall and footwall cutoffs and throw distributions.

Elastic dislocation models based on the structural framework, combined with rock properties and boundary conditions, yield distributions of stress and strain parameters and predicted fractures. These agree very well with fractures interpreted from image logs in the production well. Applying the present day stress field discriminates fractures that more likely contribute to flow. The predicted stress-enhanced fractures cluster near the heel of the horizontal well, where production data suggests most flow enters the well bore.

Following the initial pilot success, we then extended the integrated fracture prediction across all of the Phase 1 production area. Coupled with additional geologic and geophysical parameters, the predicted fracture permeability has proven an effective guide for reservoir development and management.