Sealing assessment of normal faults in clastic reservoirs: Modeling the petrophysical and stress attributes of faults.

Abstract

We describe procedures for estimation of the petrophysical properties of fault rocks and the stress regime in normal faults as incorporated in the PC software (FAULTAP) developed at the Technology Research Center of Japan National Oil Corporation (JNOC/TRC). A quantitative appraisal of the petrophysical properties of fault rocks provides key information for fault sealing assessment and reservoir management. Empirical evidence shows that the permeability of fault rocks appears to be approximately two orders of magnitude lower than that of the host reservoir rocks. Among the deformation processes in clastic rocks, shale smear by faulting seems to be a most efficient mechanism of permeability reduction in fault rocks. The permeability of fault rocks has an inverse relationship with the clay content incorporated from the host rock as well as with the depth of fault rock and fault displacement. The relationship between clay content and pore throat radius in fault rock forms a basis for calculation of the capillary pressure of fault rock and the hydrocarbon column height that a fault can laterally support. The integrity or the failure of fault seals is crucially dependent upon how various portions of the fault surface are subjected to stress accumulation in the recent geological time. Simple methods for calculation of in-situ stresses on normal faults are presented for evaluation of fault failure by slip tendency or dilation tendency. An integration of fault failure and shale smear parameters provides a better assessment of fault sealing or potential leakage in response to the recent stress regime or changes in the pressure regime during field production.