Abstract:

During the process of drilling, well completion and production, the effective stress changes, the pore structure of tight sandstone is complex, and the micro-fracture is developed. When the effective stress increases, the micro-fracture will close, and the pores volume will decrease, which make a difference in the propagation characteristics of sound waves in tight sandstone. The researches on the characteristics of acoustic wave propagation under variable effective stress are significant for the prediction of formation pressure of tight sandstone and the cognition of response characteristics of dynamic mechanical parameters of rock. Taking the typical tight sandstone of upper Paleozoic Permian in Ordos basin as the research object, velocity of P-wave and S-wave and permeability of dry rock samples with the increasingly effective stress were measured. It was proved that when the effective stress（between 0 ~ 15 MPa） was lower than the critical value, the tight sandstone was mainly characterized by the closure of micro-fractures, and there was an exponential relation between effective stress and velocity of P-wave and S-wave respectively; when the effective stress（between 15 ~ 50 MPa） was higher than the critical value, the deformation characteristics of rock gradually transited from linear elastic stage to plastic deformation stage, the interlayer dislocation occured in tight sandstone, the pores volume decreased, and the velocity of P-wave and S-wave changed with the effective stress in a liner relation. Meanwhile, the response of permeability and wave velocity on effective stress matched with each other. When it was below the critical value, it went down exponentially with the effective stress.

Key words: tight sandstone, velocity of P-wave and S-wave, pore structure, stress sensitivity, permeability