Properties of fault damage zones in siliclastic rocks: a modelling approach
Published:January 01, 2005
N. E. Odling, S. D. Harris, A. Z. Vaszi, R. J. Knipe, 2005. "Properties of fault damage zones in siliclastic rocks: a modelling approach", Understanding the Micro to Macro Behaviour of Rock–Fluid Systems, R. P. Shaw
Download citation file:
Major faults are surrounded by damage zones of minor faults that, in siliclastic rocks, can form barriers to flow in their own right. Reservoir flow simulation — now a routine part of reservoir management — requires equivalent hydraulic parameters on the scale of the whole fault, while reservoir geological models, from which flow simulator grids are generated, require information on the 3D characteristics of fault populations. Here, a stochastic model of fault damage zone architecture is generated and used to explore the impact of damage zone architecture on extrapolation from 1D (fault throw) and 2D (fault length) to 3D fault population characteristics. Sampling of the simulated damage zone models shows that clustering of faults causes deviations from simple laws relating particularly 1D samples to 3D population power-law exponents, with differences between expected and observed values of up to 0.25. The stochastic model is used to generate input for a 2D discrete fracture flow model for the case where minor (isotropic) fault permeability is four orders of magnitude lower than that of the host rock and, thus, forms partial barriers to flow. The flow model is used to explore the impact of fault damage zones on bulk fault permeability. The damage zone is shown to be around 50% efficient, i.e. a simple estimate of bulk permeability can be made using the harmonic average of fault rock and host-rock permeability weighted by thickness in 1D traverses (e.g. core, well logs), where only half the observed thickness of fault rock in the fault damage zone is assumed. Considering the contributions of the damage zone and the major slip zone, the fault damage zone is likely to make a significant contribution to the bulk permeability of the fault as a whole when the permeability of minor faults in the damage zone is similar to, or at most, one order of magnitude greater than that of the slip zone fault rocks.
Figures & Tables
Understanding the Micro to Macro Behaviour of Rock–Fluid Systems
Understanding how fluids flow through though rocks is very important in a number of fields. Almost all of the world's oil and gas are produced from underground reservoirs. Knowledge of how they got where they are, what keeps them there and how they migrate through the rock is very important in the search for new resources, as well as for maximising the extraction of as much of the contained oil/gas as possible. Similar understanding is important for managing groundwater resources and for predicting how hazardous or radioactive waste or carbon dioxide will behave if stored or disposed of underground. Unravelling the complex behaviour of fluids as they flow through rock is difficult, but important. We cannot see through rock, so we need to predict how and where fluids flow. Understanding the type of rock, its porosity, the character and pattern of fractures within it and how fluids flows through it are important. Some contributors to this volume have been trying to understand real rocks in real situations and others have been working on computer models and laboratory simulations. Put together, these approaches have yielded very useful results, many of which are discussed in this volume.