Abstract
The permeability of tight sandstone is of great significance in oil and gas exploration and development, which is deeply influenced by environmental temperature and in-situ stress. Based on the self-designed triaxial test system, the gas permeability of sandstone after different thermal treatment temperatures under different confining pressures is analyzed. It is found that the mass decreases with temperature increasing, while the wave velocity reaches a peak at 105°C, and then decreases with the increase of temperature. The maximum permeability of the sandstone is less than 1.2×10−17 m2. The gas permeability decreases with the increase of confining pressure, and it increases during the unloading process, but it still stays at a lower level. Irreversible change occurs in the change in permeability with confining pressure. The gas permeability increases with the temperature under the same confining pressure, it is approximately 2.4 times as large as the initial value when the temperature rises from 20 °C to 400°C. SEM images show that as the temperature increases, the sandstone particles expand to cause the primary cracks and pores to be closed. When the temperature reaches 400°C, some large cohesive particles peel off resulting in sharply increase in the number of smaller particles. The formation of new cracks and development of initial cracks can be observed.
With the global oil and gas exploration from conventional to unconventional, and from shallow to deep or even ultra-deep, low permeability rocks such as tight sandstone gas reservoirs become one of the important energy sources. The tight sandstone are usually characterized by a low porosity (less than 10%), a low gas permeability (less than 10−16m2) and a strong sensitivity to in-situ stress (Duan et al., 2014; Yi et al., 2018). Analyzing gas permeability is of great practical important to exploration and exploitation of tight gas.
The evolution of permeability for tight sandstone is strongly related to the stress as the cracks and joints close under loading (Fu et al., 2015). Moreover, it is known that water will escape with high temperature, in addition, mineral composition and the structure of sandstone are changed as water escape, which results in a change in permeability (Sun et al., 2015; Zhang et al., 2016).
