Assessing Fault Hydraulic Behavior Through Analytical/Numerical Modeling of the Interaction Between Fault History and Present-Day Stresses (Trafur Project, Univ. Roma Tre & Petrobras)

Lima, Claudio C.¹
 Salvini, Francesco²
 Moriss, Mathieu³
 Cabral, Leonardo⁴

Faults induce dramatic changes in fluid pathways and can control reservoir compartmentalization. The TRAFUR project was designed to provide a tool to preliminary predict fault permeability on the basis of outcrop analysis and numerical modeling. The inner fault core zone is characterized by fault breccias and gauge in chaotic assemblages resulting from clast grinding induced by the fault movement. The outer fault zone is the damage zone, where brittle fracture cleavage and limited clast rotation prevail, and where rocks still preserve the original geometry. In the model, the capability of a fault to develop these two zones depends on the value of the Deformation Function (DF), the difference between the predicted acting stress and the corresponding rock strength. Positive values of DF represent areas where brittle deformation and clast grinding will develop. Negative values of DF indicate areas where the fault activity was insufficient to induce the development of associated brittle deformation, i.e. no fault core/damage zone will develop. A software (FRAP) was prepared to analytically compute scenarios of fault evolution. For a given scenario, the acting total stress includes the regional stress, the fluid pressure, and the stress induced by the friction during fault movement. The total deformation associated to each fault cell will be the integration of all the positive values of DF along the path of the cell on the fault surface, along its pathway induced by the fault slip. Results were tuned to the real world by the comparison between field evidence of faults (from

) and their modeling with the FRAP. In this way, empirical relationship was established between the deformation history and fault permeability. Modeling results suggested that, as function of fault development, after an initial increase of permeability (up to 4 times the original one), the fault core behaves as hydraulic barriers, permeability going rapidly to extremely low values. The damage zone showed a similar behavior with the persistence of a higher permeability band along their outer zones. In this way a well developed fault will be responsible of a loss in fluid circulation across it, and a relatively higher permeability along. The actual fault hydraulic behavior was eventually computed by calculating its reactivation under the present-day stress conditions. The modeling helped to explain key features of the production history of a