Evaluation of underground gas storage capacity in the depleted gas reservoir with water evaporation and salt precipitation

Abstract

Water evaporation and salt precipitation will occur in gas reservoirs and underground gas storage (UGS) with high-salinity formation water during production and operation. This phenomenon seriously affects the efficient and safe operation and gas storage capacity. This work focuses on quantitatively evaluating the UGS capacity based on depleted gas reservoirs considering water evaporation and salt precipitation. The core flooding experiments are conducted to analyze the influence of water evaporation and salt precipitation on rock properties under different water salinity during multi-cycle injection-production. A numerical model considering water evaporation is established to determine the salt precipitation radius around the gas well. According to the experimental and simulation results, a new analytical model is developed to calculate the underground natural gas storage capacity based on material balance principles. The field case study is presented to show the practical potential of the established model. Both formation porosity and permeability in near-well region may be finally improved by water evaporation and salt precipitation under irreducible water condition. In the presence of movable-water, formation porosity and permeability of the near-well region may be firstly improved and finally reduced due to continuous salt precipitation in porous media. Salt-precipitation radius achieves 28.5 m after ten cycles. The UGS capacity is 101.11 × 108 m3 without considering salt precipitation, 1.11% higher than original geological reserves. The UGS capacity considering both irreducible-water evaporation and salt precipitation is 101.03 × 108 m3. However, when the moveable-water evaporation volume increases to 15Virr, the final UGS capacity is further decreased to 98.55 × 108 m3. The UGS capacity has decreased by 0.82%, 1.64%, and 2.45% compared with irreducible water. The continuous evaporation of movable water and salt deposition will reduce the gas storage capacity and cause damage to the UGS. This work provides an effective method to investigate the quantitative impact of water evaporation and salt precipitation on UGS capacity.