Evaluation of a fault zone in a carbonate hydrocarbon reservoir

Deformed subsurface hydrocarbon reservoir rocks normally have porosity and permeability distributions that vary, often significantly, from that of the undeformed rock mass. Hydrocarbon production data from the carbonate reservoir studied here showed unexpected increased permeability near a fault zone.
This study uses a geomechanical simulator SAVFEMTM to (1) develop evolved stress-strain states around this fault zone, (2) to use simulation results to generate porosity and permeability distributions and (3) to assess how these evolved distributions could affect hydrocarbon production. Loads to represent the geologically observed salt doming and lateral extension seen in the reservoir’s geological history are imposed on a geomodel that represents the reservoir and its overburden in the unfaulted state.
Three series of models have been built to look at the effect of the lithostratigraphy and the boundary conditions on the localization of the deformation. Models are initially 100m long and 50m high with a 75MPa confining pressure applied, and are stretched at a constant horizontal velocity. They are divided in three layers for which mechanical properties are derived from data collected on the studied reservoir (Lecomte et al., 2011). A preexisting discontinuity (zero displacement) is introduced in the center of the model cutting through all layers and dipping 60 degree, mimicking the position and orientation of a normal fault seen in the real case. Vertical displacement representing the low amplitude salt doming movement is locally applied at the base of the model and the magnitude is constrained as a function of the horizontal displacement.
The velocity of doming, due to diapirism, and the position of axis of the fold in comparison of that of the fault are also key parameters on the localization of the deformation around the pre-existing discontinuity and on the formation of fault related fractures, as observed in reservoir. Geomechanical simulation results are then used to estimate the impact of deformation on the petrophysical properties of rocks, such as porosity and permeability, and their distributions through the reservoir, assuming isotropic pores and porosity distribution.
Finally, results obtained from geomechanical simulations were analyzed with the damage mechanics approach, allowing the description of the degradation of the elastic properties of the rock mass due to the evolution of fractures. This damage parameterization describes the changes in seismic velocities through the fractured rock mass and then allows to downscale geomechanical simulations’ results, characterizing the fractures array (preferential orientation, spacing or density of fractures) which might develop around a fault zone.