Study On Scaling Model Of Calcium Sulfate Based On Thermodynamic Equilibrium

Water injection is one of the important measures taken to improve the productivity of oil wells in the process of oil production.However,due to the change of reservoir environmental factors and the incompatibility between injected water with formation water,scale may be created in reservoirs and wellbore during the water injection process.Usually,calcium sulfate is very difficult to remove because of its low solubility,which will seriously affect the normal production of oil fields.Therefore,aiming at the characteristics of calcium sulfate scale,we should use and improve corresponding dynamic scale prediction model in laboratory conditions,and study the influence of scaling on reservoir core,which is of positive significance for the implementation of scale prevention and scale removal in oilfield production.Firstly,In order to study the prediction model of calcium sulfate scale in depth,the scale prediction model at home and abroad is investigated,the mechanism and prediction of calcium sulfate sealing are studied,and the characteristics and main influencing factors of calcium sulfate scale are summarized.At the same time,the theoretical support of calcium sulfate scaling mainly affected by temperature has been obtained.The experiment of calcium sulfate scaling was carried out and the activation energy needed in the induction period of calcium sulfate scaling was obtained with the prediction formula of calcium sulfate scaling induction period.Secondly,the application of calcium sulfate scaling experiment to determine calcium sulfate fouling induction time.The experimental study on the solubility of calcium sulfate in salt solution was carried out,and the change of solubility of calcium sulfate in various salt solution experiments with the concentration of the system solution was measured.The nucleation rate of calcium sulfate scale and the number of nucleation are obtained according to the calculation formula of nucleation rate and the number of nucleation.Then,through the investigation of various domestic and foreign documents,the development and application of calcium sulfate scaling model are studied.The NRTL activity coefficient model of the multi component electrolyte system was used to calculate the solubility and activity coefficient of calcium sulfate in various salt solution experiments with the temperature change.The change curves of the above two coefficients are fitted out.Then,the saturation index of calcium sulfate is calculated and the trend of scaling is predicted.Finally,by combining the sulphate scaling prediction formula with the displacement experiment,the scale distribution of calcium sulfate is calculated.And the influence of calcium sulfate scaling on the reservoir core was studied by displacement experiment.Further,the influence of dimensionless diffusivity and dimensionless kinetic constants on the distribution of calcium sulfate scale and calcium sulfate scaling in reservoir core is studied,and the influence of scaling is also analyzed.