Effect of compaction and dilatancy on the evolution of permeability in porous sandstones
Abstract
This paper reviews recent laboratory work on the evolution of permeability during the deformation of porous sedimentary rocks. In a typical experiment large cylindrical samples 10 cm in diameter by 25 cm in length are deformed under controlled boundary conditions of constant axial strain rate, constant confining pressure, and constant upstream fluid flow rate. The axial load, sample volume, and upstream and downstream fluid pressure, are measured as dependent variables. At the intermediate confining pressures examined so far (tens of MPa) the rocks deform by producing a zone of discrete deformation bands where the number of individual bands and the zone width both increase linearly with inelastic strain. This pattern mimics field observation in porous, quartz-rich sandstones. After the test micro-structural analysis of the deformed samples reveal that the overall volume change depends on the competition between compaction by grain crushing in the poorly-sorted fault gouge and dilatancy caused by secondary porosity in the form of axial microcracks. The evolution of the bulk permeability in the quasi-static post-failure stage can be predicted very well by the microstructural evolution as a function of inelastic strain, assuming the up-scaled or bulk permeability can be determined by a geometric mean of competing local processes of dilation and compaction. The strong pressure seal formed by the gouge results in a bulk permeability-porosity slope that can be positive or negative, even though the local constitutive law has a single positive slope. During dynamic failure the rapid dilatant pore volume change near the fault results in a suction pump effect, where the rapid decrease in pressure near the fault is balanced by a decrease in upstream fluid pressure to maintain constant input flow rate. A coupled model for dilatant slip shows that the drop in upstream pressure does not require a significant change in cross-fault permeability in our tests.
- Publication:
-
EGS - AGU - EUG Joint Assembly
- Pub Date:
- April 2003
- Bibcode:
- 2003EAEJA.....1162M