Effects of stress, cementation and hot pressing on permeability: Experimental observations and network modeling.

摘要

High pressure experiments, microstructural observations and network modeling were conducted to investigate effects of stress, cementation and hot pressing on permeability. A suite of sandstones with various nominal porosities were chosen. The experimental work contains two major parts: the overall permeability evolution as a function of porosity change and the development of permeability anisotropy induced by deviatoric stress.;Under conventional triaxial compression, permeability is measured using either steady state flow technique or pulse transient technique, while porosity change is recorded simultaneously by a volumometer. All the experiments were conducted at room temperature, and the range of effective pressure was sufficiently broad so that the failure mode underwent the brittle-ductile transition. Since permeability is sensitive to the stress-induced anisotropy in pore structure, I also studied anisotropic permeability behaviors during cataclastic flow by conducting both conventional triaxial extension and hybrid triaxial compression experiments.;A systematic permeability-porosity relationship has been elucidated. In the brittle faulting regime, shear-induced dilation and shear localization have complex influences on the evolutions of permeability and porosity. In the cataclastic flow regime, the permeability and porosity changes closely track one another. Drastic permeability decrease was triggered by the onset of shear-enhanced compaction caused by grain crushing and pore collapse. As the results of the interplay of pre-existing pores and stress-induced microcracks, permeability anisotropy in porous rocks is significant and transient during the development of cataclastic flow.;The tested samples at different stages of deformation were studied under both optical and scanning electron microscopes to investigate the micromechanical processes. On the basis of quantitative microstructural observations, a network percolation model was developed to simulate the experimental results on permeability and porosity changes which provided valuable insight on the physical mechanisms. Network modeling was also used to study the permeability evolution during cementation and hot isostatic pressing. The simulations successfully predicted permeability change and pore statistics as a function of porosity during hydrothermal and mechanical compaction. This work provides a fundamental understanding of permeability evolution in relation to the brittle-ductile transition in porous rocks. It also sheds some light on the physical basis of fluid transport mechanisms in many seismotectonic settings.