Up-scaling Permeability of Fault Damage Zones In Siliclastic Rocks
Abstract
Major faults are surrounded by damage zones composed of clusters of smaller faults. In silicastic rocks such damage zones represent significant barriers to fluid flow. Here we use a statistical model of fault damage zone geometry based on observations from nature to provide input to a discrete fracture flow model which is used to investigate bulk rock permeability properties of fault damage zones. The statistical model incor- porates many features of natural fault damage zones including fault shape, power law fault length distribution, orientation distribution and random to hierarchically clus- tered spatial distributions. Three dimensional fault damage zone models containing 1.5 million faults are generated from which 2D sections are sampled ranging in size from 5 to 50 m. These 2D sections are used as input to the flow model with a per- meability contrast between rock matrix and fault rock of four orders of magnitude, typical of deformation bands in sandstones. The 'efficiency' of the fault damage zone as a barrier to flow is defined by comparing it with the bulk permeability of a region of similar size with a single spanning fault, placed perpendicular to the flow direction, which contains the same proportion of fault rock. In aquifers or hydrocarbon reser- voirs, the proportion of fault rock present in a fault damage zone can be estimated from core or bore hole logs. The model results show that 50 m regions of the fault damage zone are around 50% efficient perpendicular to, and 1 to 10% efficient per- pendicular to, the fault and that the results are relatively insensitive to the exponent of the power law length distribution. In addition, bulk rock permeability shows a power law relationship with fault rock proportion with an exponent of 1.2, indicating that the fault damage zone become less efficient as the fault density increases. This relation- ship provides a method of predicting fault damage zone bulk permeability from core or bore hole log information.
- Publication:
-
EGS General Assembly Conference Abstracts
- Pub Date:
- 2002
- Bibcode:
- 2002EGSGA..27..749O