Formation water evaporation induced salt precipitation and its effect on gas production in high temperature natural gas reservoirs

摘要

To study the pattern of formation water evaporation and salt precipitation, based on the oil-gas-water three phase thermodynamic equilibrium, the principle of salt dissolution/precipitation, and results of formation water evaporation and salt precipitation experiment, a comprehensive salt precipitation model considering formation water evaporation, precipitation of NaCl in water, and reservoir porosity and permeability variations was established to analyze the salt precipitation and its influence factors during the development of high temperature gas reservoir, and some methods preventing and removing salt precipitation were proposed. The study results show that salt precipitation usually occurs in production well area during development of gas reservoir. When the initial formation water saturation is less than the irreducible water saturation, the concentration of precipitated salt from production well area will be lower, and the influence of salt precipitation on reservoir can be ignored. When initial formation water saturation is higher than irreducible water saturation, the flowing formation water constantly carries NaCl to the well bore, leading to massive precipitation of NaCl in the production well area, damaging or even plugging the reservoir completely, and shortening the development life cycle of gas reservoir at last. The increase of reservoir temperature, formation water salinity and reservoir porosity will intensify reservoir damage caused by salt precipitation. But the increase of production rate and reservoir permeability will reduce reservoir damage caused by salt precipitation. The results of this study can guide the prevention and removal of salt precipitation, enhancement of gas reservoir productive capacity and secondary development of high temperature gas reservoir.