Effect of injection time and hydrostatic pressure on chloride migration in a porous geothermal reservoir

Few studies have focused on the effects of injection time and hydrostatic pressure on the solute transport in porous geothermal reservoirs to date. The chloride displacement experiments were individually carried out at 35 °C at the injection time of 1 h, 2 h, 3 h, 4 h and 5 h individually through the simulated columns packed with the thermal reservoir fine sand. Column experiments were performed at 35 °C at hydrostatic pressure of 0, 6 and 9 MPa individually. By using the one-dimensional CDE model in the CXTFIT 2.1 software, the migration law of Cl⁻ and its influencing factors in the studied matrix were examined. The results show that the Cl⁻ breakthrough curves under different injection time and hydrostatic pressure are symmetrically distributed, and they can all be well described by the CDE model. Thus, the solute dispersion can conform to the Fick’s law in the simulated low-temperature pore geothermal water. The breakthrough curve and transport parameters of Cl⁻ are highly correlated with the injection time due to the variations of the total amount of solute mass injected, concentration differences and molecular diffusion ability in the studied geothermal water. In addition, the value of D increases from 25.22 cm²/h at 0 MPa to 36.13 cm²/h at 9 MPa, combining with the increasing molecular diffusion coefficient, dispersion coefficient and dispersivity with hydrostatic pressure. Hence, the solute hydrodynamic dispersion in the simulated sandy column are enhanced with the increasing hydrostatic pressure. The results are of great significance to enrich the theory of solute transport in groundwater.