Calcite deposition in formation and wellbore may significantly reduce the inflow performance of geothermal wells where CO2 is present in brine. The key operational and reservoir parameters influencing the magnitude of impairment by calcite deposition were identified through dynamic simulation modelling based on the rate of calcite deposition, effective tubing radius reduction and its effect on flow rate.
Chemical reactions were modeled to generate calcite in the liquid phase. Subsequently, the flowing calcite may convert to the solid phase due to temperature and pressure effects. A method called flexible wellbore modelling (FlexWell) was used to implicitly solve wellbore equations on a separate grid, which allows the well to undulate through multiple layers to more accurately follow the real trajectory. The method also allows the chemical reactions and calcite deposition to be modeled both inside the wellbore and in the reservoir. FlexWell performs a dynamic calculation of the wellbore's hydraulic radius as well as permeability reduction in the reservoir. This permits the investigation of calcite deposition processes on well inflow performance for specific reservoir conditions and well operations.
The results of dynamic simulation modelling show that the rate of calcite scaling in the wellbore is directly proportional to the hydraulic diameter of the well. For a given flow rate, the degree of calcite deposition in the wellbore and around the wellbore may be significantly reduced by decreasing the drawdown pressure (or pressure gradient) near the well and/or increasing the hydraulic diameter (effective wellbore radius) through well-stimulation techniques. Generally reducing the flow rate will increase the well production time before flow is significantly impaired by deposited calcite.