An experimental study of the breakthrough pressure of unsaturated low-permeability sandstone: a case study of the Carboniferous sandstone in the eastern Qaidam Basin

Breakthrough pressure is a significant parameter in the processes of gas reservoir exploration and cap rock evaluation. This study investigates the breakthrough pressure of unsaturated low-permeability sandstones through a series of laboratory experiments. Three core samples of low-permeability sandstone were obtained from the Carboniferous sandstone formation in the eastern Qaidam Basin. The breakthrough pressure of each of the samples was tested under six different levels of water saturation. The mineral and clay compositions of the samples were analyzed using the X-Ray Diffraction and X-Ray Fluorescence method. Porosities and pore size distribution of the samples were measured with the He double chamber and mercury intrusion methods. CH4 was used in the seepage simulation experiments to measure the absolute permeability of the samples. The results indicate that porosity of the low permeability sandstones ranges from 9 02% to 10 96%. The average pore radius ranges from 0 1082 μm to 0 3709 μm. The permeability ranges from 0 008 mD to 0 012 mD. The breakthrough pressure for the dry sandstone ranges from 0 05 MPa to 0 19 MPa. The breakthrough pressure for the fully saturated sandstone ranges from 1 51 MPa to 2 73 MPa. The pore structure will not be prominently changed by the swelling of the caly mineral and therefore the swelling has little effect on the breakthrough pressure. The pore structure and water saturation are the main factors controlling the breakthrough pressure. The results suggest that there is a negative correlation between the breakthrough pressure and the pore radius, i.e., the increasing average pore radius or medium pore radius will lead to a decreasing breakthrough pressure. A quantitative relationship between the breakthrough pressure and water saturation is obtained. The breakthrough pressure increases exponentially with the water saturation.