Abstract - The hydraulic properties of faults are included in reservoir flow models using transmissibility multipliers, and their geometric properties are incorporated explicitly through the definition of the depths of each grid-block. While it is simple to discriminate between hydraulic and geometric fault properties in a geocellular flow model in which faults are represented as planar discontinuities, the reality is that faults are complex heterogeneous and anisotropic volumes of variable composition and thickness, and often such a distinction is a factor of the scale and resolution of the flow model rather than of geological fault characteristics.
Transmissibility multipliers for faulted grid-block to grid-block connections are a function of the permeabilities of the grid-blocks as well as the permeability and thickness of the fault zone. A constant multiplier applied to a fault in a sedimentologically heterogeneous reservoir model therefore represents a heterogeneous fault. This implicit fault heterogeneity is perhaps seldom recognised by the reservoir engineer and is of a geologically meaningless form arising entirely from the dependencies embedded in the operation of the multiplier. A recently developed method (Figure 1) for estimating fault transmissibility multipliers in sandstone / shale sequences nests geological fault characterisation within the framework of commercial flow simulators (Walsh et al. 1998, Manzocchi et al. in press). Fault zone permeability is calculated as an empirical function of the throw of the fault and the Shale Gouge Ratio (SGR) of the fault surface at each connection. Fault zone thickness is calculated as a function of throw. Application of the SGR algorithm (e.g. Gibson 1994) at the resolution of the reservoir flow model is easily implemented and provides a geologically significant fault zone heterogeneity structure. The method does, however, contain stringent assumptions about the level and the length-scales of fault zone permeability and thickness heterogeneity. Fault rock permeability and thickness are known to vary considerably over short distances along fault traces (e.g. Foxford et al. 1998), but information about the correlation lengths of fault zone structure is difficult to gather at outcrop, and geostatistical descriptions of fault zones are extremely unsophisticated.
Two basic processes contribute to the thickness and content of a fault zone, and both also generate multiple slip surfaces (Childs et al. 1996). These bifurcation processes operate either at the propagating tip-line of a fault, tip-line bifurcation, or at points on an existing fault surface, asperity bifurcation. The former process results in fault segmentation and the development of relays which are progressively breached with increased fault displacement. The frequency and integrity of relay ramps are a function of the mechanical anisotropy of the faulted sequence as well as of fault displacement. Unbreached relays, irrespective of whether or not they are seismically visible, are generally irresolvable at the discretisation of a reservoir flow model, yet provide continuous high permeability flow paths through the fault. Breached relays may also strongly modify the geometry across the fault, as two slip surfaces do not provide the same juxtapositions as one fault containing the aggregate throw. Asperity bifurcation occurs when an irregularity, on either the hangingwall or footwall bounding slip surface, is mechanically eroded by creation of a new slip surface which isolates it from the wallrock. While irregularities of the bounding slip surfaces may originate early in the fault development, by refraction of the original fracture across boundaries between mechanically contrasting layers, they are also continuously generated throughout the active life of a fault (Figure 2).
While it is problematic to include the full complexity of fault zone juxtapositions and hydraulic properties within the tight framework of a low resolution full-field simulation model, a stochastic representation of both juxtaposition and fault-rock properties increases the realism of the fault representation. Formal definition of the geostatistical properties governing this stochasticity is discussed but will require further integration of theoretical, numerical and outcrop studies of fault zone characteristics.
References
Childs, C., Walsh, J. J., and Watterson, J. 1996. A model for the structure and development of fault zones. Journal of the Geological Society, London, 153, 337-340.
Foxford, K. A., Walsh, J. J., Watterson, J., Garden I. R., Guscott, S. C., and Burley, S. D., 1998. Structure and content of the Moab Fault zone, Utah, USA. In: Jones, G., Fisher, Q. J. and Knipe, R. J. (eds). Faulting, fault sealing and fluid flow in hydrocarbon reservoirs. Geological Society, London, Special Publications 147, 87-103.
Gibson, R. G. 1994. Fault-zone seals in siliciclastic strata of the Columbus Basin, offshore Trinidad. American Association of Petroleum Geologists Bulletin, 78, 1372-1385.
Manzocchi, T., Walsh, J. J., Nell, P. A. R., and Yielding, G. 1999. Fault transmissibility multipliers for flow simulation models. Petroleum Geoscience, 5, 53-63.
Walsh, J. J., Watterson, J., Heath, A. E., and Childs, C. 1998. Representation and scaling of faults in fluid flow models. Petroleum Geoscience, 4, 241-251.
Abstract of talk given to:
EAGE/SPE Iternational Symposium on Petroleum Geostatistics, Tolouse, 20-23 April 1999