Experimental determination of dynamic pore-throat structure characteristics in a tight gas sandstone formation with consideration of effective stress

In this study, a robust and pragmatic technique has been developed to experimentally quantify the dynamic characteristics of microscopic pore-throat structure with consideration of effective stress in a tight gas sandstone formation. More specifically, porosity and permeability of highly heterogeneous core samples collected from a tight gas reservoir were measured, and then thin-section analysis as well as other analyses with the scanning electron microscopy and x-ray diffraction techniques were performed to identify and classify the particle composition, grain size, clay minerals, and pore and throat types. Subsequently, pore-throat structure such as its connectivity, radius and size distributions were determined by conducting pressure-controlled mercury injection and rate-controlled mercury injection tests, respectively. In addition, the nuclear magnetic resonance (NMR) technique is employed to examine effect of stress sensitivity on porosity and permeability as well as pore and throat radius under different pore pressures by decreasing the pore pressure from 30.0 to 1.0 MPa, and the corresponding effective stress was increased from 0 to 29.0 MPa. As such, the pores and throats become smaller and the average proportion of volume controlled by nano-scale throat with the radius less than 100 nm increased from 48.01% to 48.90%, while the average volumetric proportion controlled by micron-scale and submicron-scale throats was decreased from 46.57% to 46.37%. The shrinkage of pores and throats resulted in an average reduction for porosity and absolute permeability from 10.43% to 6.84% and from 0.01237 to 0.00501 mD, respectively. Moreover, samples with a lower permeability showed more serious permeability damage, illustrating that they experience a stronger stress sensitivity than those with a higher permeability.