• Jones, R. M.

Abstract

Failure envelopes for well-lithified cataclastic fault rocks from the Otway Basin, Australia, where fault reactivation is a significant risk to trap integrity, have been determined through triaxial testing. Geomechanical analyses indicate that cemented cataclasites exhibit significant cohesive strength and that fault reactivation and trap breach is influenced by the development of shear, tensile and mixed-mode fractures. The mechanics of the fracturing process are influenced by grain strength and cataclasite morphology. Cemented cataclasites are more prone to failure than reservoir sandstones under low differential stress conditions as a result of a relatively low cohesive strength and higher friction coefficient. As such, the geomechanical property differential between cataclastic faults and undeformed reservoir strata may impact significantly on seal integrity during reactivation. Intact cataclasite seal capacity exceeds 2400 psi (16.5 MPa). Following reactivation seal capacity is reduced ~95% as a result of the development of a highly connected fracture network. The tensile strength of these cataclastic faults allows failure to occur by shear, tensile and mixed mode fracturing. This suggests that geomechanical tools used to predict trap breaching by reactivation that assume cohesionless frictional failure may significantly underestimate seal risk. Determination of fault seal risk can, therefore, be significantly enhanced by multi-disciplinary research efforts combining field and laboratory scale geomechanical analysis with microstructural and petrophysical property description. (Author supplied)

Topics

• impedance spectroscopy
• morphology
• grain
• strength
• tensile strength