Pore-space formation in rock under high-pressure deformation

Abstract

1. The present experiments confirm that, as predicted by the statistical theory of rock strength and deformation, there are two zones in the stress field beyond the elastic limit: in zone A, irreversible deformation continues without pore formation; in zone B, irreversible deformation is accompanied by the formation of a pore space, increase in volume, and change in the filtrational properties. 2. The experiments show that, even with a high omnidirectional compressive stress, pore formation in rock is accompanied by negative pressure, creating a pore pressure difference between the deformed volume and the surrounding region. 3. The loading trajectory traversed by the rock in order to reach the final stress state determines the structure of the material and its filtrational properties. 4. A mechanical state diagram showing the position of zones A and B in the stress field allows the variation in stress state of the rock to be planned so that the trajectory to the final stress state includes pore formation at a specified rate in one case, while, in the other, pore formation is eliminated or reduced to a minimum. The particular loading trajectories chosen will be determined by the requirements of the practical problem. 5. Given information on the current stress state of rock at a particular depth and its previous loading trajectory and given the appropriate mechanical state diagram, it is possible to make evaluations regarding the presence and dimensions of pore space in rock, which might provide storage for minerals. This work was supported financially by the Russian Fund for Fundamental Research (Project 93-05-9850).