Experimental study about the gas slip flow in Longmaxi shales from the southern Sichuan Basin

Abstract

In order to clarify the gas slip flow effect and its influencing mechanism of shale gas reservoirs, low-temperature nitrogen adsorption measurements were performed on the Silurian Longmaxi Formation shales from the Sichuan Basin. Pore structure characteristics of shales were described based on nitrogen adsorption-desorption isotherms. Non-steady state gas flow experiments on gas shales were carried out to obtain the apparent permeability coefficients of helium and nitrogen. The effects of pore pressure, gas types, confining pressure on gas slippage were discussed. Results show that gas slippage cannot be neglected when the pore pressure is less than 2.5 MPa. The Klinkenberg corrected permeability coefficients of shales are gas-dependent due to the "molecular sieving effect". The Klinkenberg corrected permeability of helium on shales is larger than that of using nitrogen. Gas slippage factors are also related to gas types. Helium slippage factor of shales is about 1.7 times of nitrogen slippage factor. The effective transport pore diameter of helium on shales with the confining pressure being 10-40 MPa ranges from 113 to 166 nm, while that of nitrogen is between 66 and 99 nm. These values are significantly larger than that derived from low-temperature nitrogen adsorption. A power function is utilized to fit the gas slippage factor and permeability of shales and can be used to predict gas flow in shales.

Keywords

• shale
• slippage effect
• permeability
• transport pore diameter
• sichuan basin