Effective Gas Densities of Shale Formations in North America

In the matrix of a shale formation, a significant fraction of the pores has a size that is smaller than 100 nm. The fluid behavior inside a nano-size conduit deviates from the conventional behavior observed at unconfined conditions at an identical pressure and temperature. With this in mind, we determine the effective shale gas density at the core scale in the present study. We determine the effective density using the pore-throat and pore-body size distributions of different shales in North America. The effective density is determined based on a simplified local density function for different pore geometries (slit and cylindrical conduits) as a function of pore pressure. The pore-throat size distribution is derived by analyzing mercury injection capillary pressure measurements, and the pore-body size distribution is derived from nitrogen adsorption–desorption. We use the tree-like pore model to account for the effective connectivity of the pore space at the core scale. Our study shows that, in the matrix, the effective pore-body sizes of the shale formations examined are mostly larger than 20 nm, as opposed to the pore-throat size. The gas density deviates significantly from the nominal value in a conduit smaller than 10 nm. Our study reveals that using the pore-throat size distribution overestimates the effective density at in situ conditions by 8%, and the effective density using the pore-body size distribution remains close to the nominal value. The deviation of the gas storage from the nominal value without confinement is also determined: the deviation can reach 5% only if we use the pore-throat size, whereas the gas storage is similar to the nominal value based on the pore-body size. Our study clarifies the importance of considering a realistic pore size at the core scale. There is no need to account for the effect of the pore confinement when we determine the properties controlled by the pore-body size. This study has major applications in estimating the original gas in place based on petrophysical measurements and in building a realistic reservoir model for unconventional reservoirs.