Fault Seal Prediction in Porous Sandstone: Assessing the Approach of Using Fluid Flow Properties of Deformation Bands to Represent Seismic-Scale Faults in Production Simulation Models

Tueckmantel, Christian ¹; Fisher, Quentin ¹; Knipe, Rob ^2
1 Centre for Integrated Petroleum Engineering and Geoscience, University of Leeds, Leeds, United Kingdom.
² Rock Deformation Research Limited, University of Leeds, Leeds, United Kingdom.

A study of normal faults in the Nubian Sandstone Sequence, from the eastern Gulf of Suez rift, has been conducted to investigate the relationship between the microstructure and petrophysical properties of cataclasites developed along seismic-scale faults (slip surface cataclasites) and smaller offset faults (cataclastic deformation bands) found in their damage zones. This was to quantify the uncertainty associated with predicting the fluid flow behaviour of seismic-scale faults by analysing small faults in core, a common procedure in the petroleum industry. The microstructure of the cataclasites was analysed as well as their single-phase permeability, threshold pressure and grain-size distribution. Faulting occurred at a maximum burial depth of ~1 km. Our results show that the lowest measured deformation band permeabilities provide a good estimate for the arithmetic mean permeability of slip surface cataclasites. This suggests that cataclastic permeability reduction is mostly established early in the deformation history. Stress at the time of faulting rather than final strain seems to be the critical factor.

The experimentally obtained results were used as input into a production simulation model to quantify the error associated with using permeability and threshold pressure values obtained from deformation bands to represent seismic-scale faults. To this end a simplified reservoir model with a single fault separating two compartments was created using the Eclipse^TM 100 black oil simulator. The host rock properties were derived from typical Nubian Sandstone samples and the fault was incorporated as a series of transmissibility multipliers (TMs) based on the measured single-phase permeability of sampled fault rocks. We ran two series of models. For the first case, fault TMs were calculated based on deformation band permeability. For the second case, fault TMs were calculated based on the permeability of slip surface cataclasites. By comparing the production history of the two cases we attempt to quantify some of the errors inherent with incorporating petrophysical data obtained from cored small-scale faults to represent seismic-scale faults. We also investigate how such errors could be amplified once multi-phase flow properties are considered.

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009