Non-ideal gas effect of gas storage inside confined pores: A case study of Yongye shale

Unlike conventional reservoirs, the phase behavior of gas in shale formations rich in nanopores is no longer the same as that in bulk regions because of the non-ideal gas effect. To investigate the role of the non-ideal gas effect introduced by nanopores, gas content distribution along the burial depth (3772 m-3872 m) of one of the Yongye shale formations was analyzed according to its pore properties of fresh shale core samples. It is found that gas content is not uniformly distributed along the burial depth and it has a similar distribution trend with the specific pore volume and specific surface area, especially with those of mesopores and micropores. In addition, the specific volumes of micropores and mesopores are more significant in organic-rich pores obtained from the burial depth of 3805 m-3870 m, where a large amount of gas was found. Since the non-ideal gas effect (i.e., critical temperature and pressure shift) becomes obvious in pores with diameters of less than 10 nm, more gas is stored in the shale matrix with more micropores and mesopores. Compared with the case without considering the nonideal gas effect, the gas content of the shale formation (with burial depth from 3805 m to 3870 m) under geological conditions increases significantly. A better understanding of the non-ideal gas effect in nanopores may enable a more accurate assessment of the gas-in-place content of shale reservoirs.