Predicting 4D permeability evolution in fault zones by numerical-analytical modelling (FRAPtre).

The spatial distribution of subsidiary fault networks and related permeability along fault zones is the result of the complex history of stress and deformation that develops during faulting. The numerous parameters that control the deformation development processes include : 1) the fault geometry and kinematics; 2) the fault slip mechanism; 3) rock type and rheology; 4) boundary stress conditions; 5) pore fluid pressure; 6) structural inheritance. The pathway of fault rocks during displacement along the fault plays a major role in the spatial distribution of the deformation intensity. The behaviour of fault zones has been simulated by a numerical-analytical tool (FRAPtre) that links in both ways the boundary stress conditions and the deformation fabric for a given fault geometry and kinematics. Results include also the static stress conditions evaluation as well as the permeability prediction. The FRAPtre approach couples the usual features used in predicting fault permeability, namely clay content and amount of displacement, with other key parameters as the stress/strength condition along the fault surfaces and the path of the displacement on it. 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 (at each cell) by projecting the resulting stress tensor on the fault cell surfaces, as well as the normal pressure and the shear. Then it is possible to compute the deformation function DF, that is simply the difference between shear and strength on the fault cell surface. The DF parameter measure the disposition of rock to break into a cataclastic rock (or to grind existing grains in clastic rocks) and then start the fault gouge formation process. A more realistic statistical approach is computed, by taking into account the standard variation of rock rheology around the given mean values. The Fault Gouge Index FGI is computed by cumulating the deformation function of all cells along the displacement path, conveniently weighted by the corresponding portion of the displacement, and multiplied by the clay content fraction at that cell. The thickness of the fault gouge is computed taking into account the amount of displacement and the deformation function DF. This will provide thicker fault gouges where the conditions to develop it are more favourable. The fault permeability is computed by a generalised equation that matches the found permeability values available in literature on fault core permeability. The resulting spatial distribution of predicted permeability is then output to interface with commercial software for fluid flow modelling in reservoirs.