Evolution of strength and hydraulic connectivity during dehydration: Results from a microcrack model

A two-dimensional cellular automaton model based on the elastic mechanics o f tensile (mode I) microcracks was developed to investigate the evolution o f rock strength and hydraulic connectivity during progressive dehydration. Fluid produced by dehydration is assumed to be accommodated by microcracks, which propagate through a simulated rock matrix owing to elevated fluid pr essures. Crack propagation affects rheology by stress relaxation and intera ction, and it affects hydrology by permitting fluid flow between neighborin g cracks. Numerical simulations with undrained boundary conditions show tha t reactions releasing small quantities of fluid (< 0.25 wt %) in a rock mat rix with zero initial hydraulic connectivity induce large strength reductio ns (approximately 80-90 %). Strength reduction occurs abruptly at the onset of dehydration and continues until approximately 10 % reaction, when a low -strength plateau is reached. Subsequent reaction causes almost no further effect on rock strength until the percolation threshold is attained, at whi ch point the strength drops to zero. Results with drained boundary conditio ns yield similar strength reductions before hydraulic connectivity of the c rack network is achieved. Thereafter, fluid drainage allows partial strengt h recovery. The results indicate that the dominant rheological response ind uced by dehydration is caused by the generation of fluid overpressures and is unrelated to the establishment of hydraulic connectivity coinciding with the percolation threshold. Although rocks characterized by zero initial hy draulic connectivity retain additional strength relative to rocks with init ial hydraulic connectivity, the magnitude of this additional strength is sm all (< 20 % original strength).