Formation-water evaporation and salt precipitation mechanism in porous media under movable water conditions in underground gas storage

Abstract

Underground gas storage (UGS) plays an important role in large-scale energy (natural gas) storage, while salt deposition may affect the storage and production capacity of UGS. However, salt precipitation mechanism of UGS under movable water conditions is not clear. This work aims at understanding salt precipitation on rock properties of an UGS in porous media under movable water conditions. Firstly, core flooding experiments are performed to establish the relationship between dynamic petrophysical properties and the amount of salt crystals. The dynamic process of salt precipitation at the pore-scale is further captured by microscopic visualization experiments, and the location and morphology of salt crystals at micro-scale are also observed using scan electron microscope (SEM). Results indicate that formation-water evaporation and salt precipitation improve the formation properties when the amount of deposited salt is less than 39 mg/g. After the amount of deposited salt is more than 48 mg/g, formation porosity and permeability can be reduced. The size of salt crystals under movable water (>40 μm) is much larger than that under irreducible water conditions (1 μm–5 μm). This work offers a methodology for analyzing the effect of salt precipitation on the UGS in porous media under movable water conditions.