Underground Gas Storage Capacity Estimation Considering Water Evaporation and Salt Precipitation in Depleted Gas Reservoirs: Method and Case Study in W23 UGS
Authors: He Youwei , Qiao Yu , Tang Yong , Wang Ning , Tang Liangrui , Qin Jiazheng 
Published: October 2023

Abstract
Global gas crisis has reinforced the acceleration of underground gas storage (UGS)construction. Salt precipitation due to water evaporation has become a challenge for the UGS especially with salt layers or high salinity formation water. The impact of salt precipitation and water evaporation on formation properties and UGS capacity the cannot be ignored. This work aims to estimate the gas storage capacity considering formation water evaporation and salt deposition in depleted gas reservoirs.
This work focuses on quantitatively evaluating the gas storage capacity considering salt precipitation and water evaporation in depleted gas reservoirs combining experiments, simulation and analytical methods. Firstly, rock permeability and porosity are tested during multi-cycle injection and production to quantitatively analyze the impact of salt precipitation and formation water evaporation on rock properties under variable water salinity. Secondly, a numerical model considering water evaporation is developed to determine the salt precipitation radius around the gas wells. Thirdly, an analytical model is derived to calculate the natural gas storage capacity through experimental and simulation results. Finally, the proposed approach has been applied into the biggest UGS with high-salinity formation water in China.
Both formation porosity and permeability in near-well region may be finally improved by water evaporation and salt precipitation under irreducible water condition. The formation porosity and permeability of the near-well region may be firstly improved and finally reduced due to continuous salt precipitation in porous media caused by strong evaporation of movable formation water. Salt-precipitation radius is defined as the distance between the wellbore and the farthest completed evaporation region of formation water, and salt-precipitation radius achieves 28.5 m after ten cycles. The UGS capacity is 101.11×108m3 without considering salt precipitation after irreducible-water evaporation, 1.11% higher than original geological reserves. The UGS capacity considering both irreducible-water evaporation and salt precipitation is 101.03×108m3. However, the supplement of movable water may cause negative impact on the UGS capacity after the integrated water evaporation and salt precipitation. When the moveable-water evaporation volume increases to 10Virr and 15Virr, the final UGS capacity is further decreased to 99.37 and 98.55×108m3, which are 0.63% and 1.45% lower than original reservoir reserves. The UGS capacity has decreased by 0.82%, 1.64%, and 2.45% compared with irreducible water. Under the strong supplement of movable water with high salinity, the continuous evaporation and salt deposition will reduce the gas storage capacity and cause damage to the UGS.
This paper offers an approach to quantitatively analyze the effect of water evaporation and salt precipitation on rock properties and gas storage capacity. Operators may obtain the optimal gas injection-production schedule and perforation layers to avoid severe damage due to salt precipitation, which is significant to achieve safe and efficient operation of the UGS.