PERMEABILITY REDUCTION IN A DILATING ROCK - NETWORK MODELING OF DAMAGE AND TORTUOSITY

There are fundamental differences between low- and high-porosity rocks in the evolution of permeability in the brittle faulting regime. In a rock with porosity less than similar to 5%, dilatancy and permeabilit y enhancement were observed prior to the peak stress. In porous rocks, the behavior is more complex. Before the onset of dilatancy C', both permeability and porosity decrease with increasing effective mean stre ss. Beyond C', permeability may decrease while the pore space dilates. After the peak stress has been attained, the development of a relativ ely impermeable shear band caused an accelerated decrease of permeabil ity. A network model (with pore space statistics constrained by micros tructural data) was developed to simulate the interplay of pre-existin g tubular pores and stress-induced cracks in influencing the permeabil ity evolution. The simulations show that while the accumulation of dam age in form of microcracks may dilate the pore space, it causes the fl ow path to be more tortuous and as a result, the permeability may actu ally decrease.