Effect of Hydrodynamics and Fault Zone Heterogeneity from Membrane Seal Capacity

Julian Strand¹, Jim Underschultz¹, Karsten Michael¹, Brett Freeman², and Graham Yielding^2
1Petroleum Resources, CSIRO, Bentley, WA, Australia
²Badleys Geosciences Ltd, Spilsby, United Kingdom

The use of Shale-Gouge-Ratio (SGR) methods to predict across-fault seal capacity relies on a calibration of the methodology against field examples. Existing calibrations have plotted across-fault pressure difference or buoyancy pressure against in situ SGR to define a fault-seal failure envelope. Recent work on hydrodynamics and seal capacity has provided insight on fine-tuning the calibration methodologies that should in turn lead to improved fault seal capacity predictions. A situation not fully addressed, however, is the impact of fault zone heterogeneity on the hydrodynamic characteristics of a fault and thus the membrane seal capacity.

For a fault that defines a hydraulic head discontinuity at the reservoir scale, there exists a hydraulic head gradient or distribution within the fault-zone that is determined by the detailed permeability distribution within the fault zone. As a result, the capillary threshold pressure varies across the fault. When compared with the hydraulic head, the fault zone seal capacity can be estimated at various locations within the fault zone. Theoretical examination of membrane seal capacity for various permeability distributions can be used to understand parameters that control the location of the critical leak point for a membrane fault seal. This can also be extended to examine possible up-fault leakage.

A range of permeability distributions are examined for a theoretical fault zone. Assuming a given across fault pressure difference in the aquifer, the internal fault zone seal capacity is determined to demonstrate the various controls on a faults critical leak point.

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