Experiments and Theory for Stress Dependent Elastic and Transport Properties of Tight Gas Sandstones

New laboratory measurements of the elastic-wave velocity in dry tight gas sandstone samples indicate a very strong dependence of the elastic-wave velocities and Poisson's ratio on the confining stress with a noticeable reduction of Poisson's ratio as the stress decreases. These data are supported by theoretical modeling conducted under the assumption that thin cracks open as the stress is reduced. The additional porosity introduced by these cracks is very small, yet it strongly contributes to the observed changes. The absolute permeability decreases by almost two orders of magnitude as the stress increases from almost zero to about 50 MPa. This effect is also presumably driven by the closure of thin cracks at high stress, the cracks that serve as flow conduits at low stress. The porosity introduced by these cracks is very small, yet their effect on rock properties is dramatic. Our data open a way of directly linking permeability to the elastic properties during differential (confining minus pore pressure) stress variations.