Salt Precipitation Law of Formation Water During CO2 Injection into Depleted Gas Reservoirs

After CO2 is injected into the formation, some CO2 will dissolve in the formation water and react with rocks, changing the ion type and content of the formation water, accompanied by water evaporation and salting out. Taking a depleted gas reservoir in the west Sichuan Basin as an example, CMG simulation software was applied to simulate the gas, water and solid phase equilibrium in the process of CO2 injection, based on the PR Henry equation, water-rock reaction kinetics theory, and salting out model. Research showed that: ① During the injection process of CO2, water evaporation caused a gradual decrease in the formation water saturation, leading to a gradual increase in the salinity of the formation water. However, because the small change in water saturation, the increase in salinity of formation water wasn’t significant; ② Water evaporation reduces the water saturation of the formation, confirming an increase in the mole number of H2O component in the gas phase. However, because the mole number of CO2 component in the gas phase increases faster, the mole fraction of H2O component in the gas phase still continues to decline; ③ With the gradual increase of CO2 solubility, calcite which belongs to the carbonate mineral and anorthite, k-feldspar belong to the feldspar mineral were dissolved, kaolinite and illite which belong to the clay mineral were precipitated; ④ Due to the combined action of water-rock reaction and salt-out, the content of OH- ions in water gradually decreases and the content of H+, Ca2+, HCO3−, Al3+, SiO2(aq) ions continuously increases; ⑤ During the CO2 injection process, the precipitation of CaSO4, CaSO4 · 2H2O were inhibited, and CaCO3 was the main precipitation product. The research results have certain reference significance for the study of the salt-out law of formation water during the geological storage process of CO2 in depleted gas reservoirs.