Study on Salt Crystallization Characteristics in Evaluating Reservoir Damage by Potassium Formate Completion Fluids

Conventional core flow experiment cannot be used to accurately measure the salt crystallization characteristics of high-density solids-free saltwater completion fluids in downhole working conditions. To address this problem, an evaluation method integrating gas flooding of core and dynamic visualization analysis was proposed. Using nitrogen flooding to simulate the actual flowback environment, and microscopic visualization technology to in-situ observe salt-out behavior, the influences of temperature (80 – 120 ℃), sudden pressure change (3.5 MPa – atmospheric pressure) and gas flowback on salt crystallization were investigated, and an optimized experimental method was established to analyze the crystallization characteristics of potassium formate completion fluid in reservoir environment. The experimental results show that temperature and sudden change of pressure are the main causes of salt-out damage from potassium formate completion fluid, among which the flash evaporation effect induced by gas flowback is the dominant trigger; at 120 ℃, the amount of salt precipitated can reach 8 – 9 g/L. When a single factor of temperature and pressure changes, the percent core permeability recovery is about 65%. Under the simultaneous action of temperature drop and pressure reduction, core damage was aggravated, the percent core permeability recovery drops to 62.33%. The main mechanisms of salt-out in porous media are the bridging growth of crystals at the narrow pore throats and their deposition on the walls of pores. After optimizing the experimental method, salt-out can be suppressed by maintaining constant temperature and constant pressure (120 ℃, 1 MPa), and the percent recovery of permeability can be increased to 75.3%, 10 – 21% higher than that under unsteady conditions. The optimized research method provides a theoretical and methodological guidance for accurately evaluating downhole salt-out damage characteristics and optimizing completion fluids.