The Evolution of Fracturing and Related Permeability Patterns in Numerical-Analytical Models (FRAPtre) of Fault Zone Growth
Salvini, Francesco, Fabrizio Balsamo, Fabrizio Storti, Università degli Studi Roma Tre, Rome, Italy
The interaction of many parameters controls the evolution of deformational architectures during faulting, including the fault geometry and kinematics, the fault slip mechanism, the amount of fault displacement, the rock type and rheology, the boundary stress conditions, the pore fluid pressure, and the structural inheritance. Substantial improvements of our predictive capability of fracture distributions in hydrocarbon exploration and development require to implement these parameters in modelling tools. We developed a numerical-ana-lytical tool (FRAPtre) that links these parameters to the deformation pattern for a given fault geometry and kinematics. Results include also the static stress conditions evaluation as well as the permeability prediction. The fault surfaces are gridded and stress/strength conditions at each cell are analytically computed by a series of stress tensor additions. The strength values are then computed by projecting the resulting stress tensor on the fault cell surfaces, as well as the normal pressure and the shear. These parameters allow to compute the deformation function DF i.e. the difference between shear and strength on the fault cell surface. DF measures the disposition of rock to break into a cataclastic fabric and then to form gouge layers. The Fault Gouge Index FGI is computed by cumulating the deformation function of all cells along the displacement path and multiplied by the clay content fraction at that cell. Finally, fault permeability is computed by a generalised equation that matches fault core permeability values available in the literature. Application to natural examples validates the proposed numerical-analytical approach.