Use of Exclusion Zones in Mapping and Modeling Fracture Corridors

Abstract

SummaryFracture corridors are fault-related, subvertical, tabular fracture clusters that traverse the entire reservoir vertically and extend for several tens or hundreds of meters horizontally. Conductive fracture corridors may have significant permeability and may profoundly affect reservoir-flow dynamics. Therefore, it is important to map conductive fracture corridors deterministically for reservoir evaluation and well planning. Deterministic mapping of fracture corridors requires locating fracture corridors and assigning to them length, orientation, fluid conductivity, and connectivity. Estimation of orientation, length, and—especially—connectivity is a major challenge in fracture-corridor mapping.An exclusion zone is a region that cannot have a conductive fault or fracture corridor passing through. Borehole images, open-hole logs, flow profiles, and lost-circulation data can be used to identify horizontal wells with no fracture-corridor intersection. Well tests, production/injection history, Kh ratio (permeability times thickness) well-test/core ratio, first water arrival, and oil-column-thickness maps can be used to identify vertical “matrix” wells that do not intersect fracture corridors. Adjacent matrix wells may be surrounded by inferred exclusion zones. The confidence level of inferred exclusion zones depends on factors such as interwell distance, matrix permeability, width, orientation, and spacing of fracture corridors. Overlapping of exclusion zones from independent data sources such as well testing and oil-column thickness have higher confidence than non-overlapping zones.Only borehole images provide orientation and only well tests provide length of fracture corridors. In the absence of well testing and borehole imaging, exclusion zones provide constraints and aid both in locating fracture corridors and assigning them orientation and length. Perhaps the most significant contribution of exclusion zones to fracture-corridor mapping is in identifying interconnected and isolated fracture corridors. An interconnected network of fracture corridors may extend laterally for several kilometers as major fracture permeability pathways, which not only improve pressure support, bottom upsweep of oil, but also cause rapid water breakthrough.