Impact of salt deposition induced by water evaporation on petrophysical properties and pore structure in underground gas storage through dynamic and static experiments

During the production process of underground gas storage (UGS) with high-salinity formation water, salt deposition has been observed due to formation water evaporation. This work aims at understanding the effect of salt deposition on petrophysical properties and pore structure. Firstly, dynamic and static experiments are carried out to simulate the multi-cycle gas injection and production of the UGS based on the core plugs, rock debris and water from Wen23 gas field. Secondly, Nuclear magnetic resonance (NMR), scan electron microscope (SEM) and energy dispersive spectrum (EDS) are applied to investigate the change of pore structure and salt-crystal morphology. Results indicate that the core permeability and porosity after water evaporation are averagely increased by 39 % to 68 % and 42 % to 66 % respectively compared with the initial water saturation conditions under different water salinity. Salt crystals in the reservoir are mainly deposited in large pores. The dispersed and aggregated salt crystals are observed and most salt crystals are between 1 mu m and 5 mu m in size. Thus, formation water evaporation is beneficial to the improvement of reservoir petrophysical properties without formation water supplement. This work provides an effective method to understand the impact of salt deposition caused by formation water evaporation on the petrophysical properties, fluid flow ability, and gas storage capacity of the UGS and CO2 geological storage.