Geological faults are planar structures oriented in three-dimensional space on which shear displacement has occurred. Faults can be barriers to flow, conduits, or combinations of the two and their hydraulic properties vary considerably over both space and time. Current field and borehole data collection techniques are unable to predict their hydraulic behaviour and a high level of risk is attached to such assessments in the petroleum and waste disposal industries. This paper presents the first published research investigating the Spatial and temporal evolution of fault permeability in the damage zone surrounding a fault using numerical modelling. The model (MOPEDZ) uses the finite element method to simulate the coupled physical and chemical processes inherent in permeability evolution, and generates a two dimensional description of the damage zone both in terms of its structural architecture and its constituent permeabilities. We present the theoretical and computational model so far alongside two preliminary applications.
Faults are very complex hydrogeologically. Faults can be barriers to flow, conduits, or combinations of the two and their hydraulic properties vary considerably over both space and time (Hooper 1991; Caine et al. 1996). It is, therefore, critical for prediction of both future and historical fluid migration through fault zones, to be able to assess their spatial and temporal hydraulic evolution. Due to the large timescales Involved, this is particularly relevant when modelling migration history in oil fields, or when simulating the transportation of radio nuclides following the burial of radioactive waste.
This paper presents the first published research Investigating the spatial and temporal evolution of fault permeability in the damage zone surrounding a fault using numerical modelling. Fault permeability is governed by two main components: the permeability of the fault core and the permeability of the surrounding damage zone. In crystalline rock, the damage zone is generally heavily fractured and dominates the fault's overall hydraulic behaviour. In these environments, the permeability of the fault is thought to be governed by fracture frequency and heterogeneity within the damage Zone, and by the subsequent rate of mineralization of fracture surfaces.
In the case of sedimentary environments, a number of juxtaposition models exist in the literature for predicting fault core permeability (Jones et al. 1998). These models predict transmissibility of the core based on lithological juxtaposition across the fault or the ratio of shale smear. However these models do not take into consideration the damage zone. The technique works reasonably well for sandstone-shale juxtapositions, but in faults where displacement is small and the same lithology is present on either side of the fault, e.g. sandstone is juxtaposed to sandstone, the oil industry has no realistic means of predicting fault permeability (Jan Konstanty, Senior New Opportunities Geoscientist, NAM/Shell International EP, pers. comm.). For these latter small-displacement faults, permeability is thought to be heavily influenced by the structure of the damage zone surrounding the fault. In high poroity sandstone this damage zone may comprise a complex network of low porosity deformation bands and slip surfaces