Study On Residual Polymer Flocculating Reutilization Technology In Polymer Flooding Of Offshore Oilfield

Aim at the problems of the increasingly rising water cut,high polymer concentration in produced fluid and the heavy burden on sewage treatment in offshore oil wells,as well as the characteristics of lacking freshwater resources,small platform area and demanding environmental requirements,this study carrys out the research of flocculating reutilization technology in the formation of polymer flooding oil fields,develops a type of seawater-based instant salt resistive flocculant,namely salt-tolerant cationic modified starch?CMS?,and the concentration range of which has been optimized.Meanwhile,this study discusses the mechanisms of the interaction between the CMS and residual polymer,establishes the technology boundaries of the application of flocculating reutilization technologies,which provides theoretical basis for the implementation of the technology in the oilfield application.Firstly,through the two-step method of corn starch cationization and sulfopropylation,salt-tolerant cationic modified starch is synthesized,which is characterized by a high degree of substitution of surface hydroxyl and good salt tolerance.As for the starch cationization,semidry method is utilized to study the influence of temperature,reaction time,catalyst dosage and reactant molar ratio on the degree of substitution of surface hydroxyl and reaction efficiency.Eventually,it is demonstrated that 60°C of reaction temperature,3 hours of reaction time,1:1 of reactant molar ratio,and 10:1 of starch and catalyst mass ratio are the optimal conditions.Meanwhile,by measuring the impacts of salt concentration on the apparent viscosity in the starch sulfopropylation,the optimum synthesis conditions are obtained,namely 80°C of reaction temperature,20 hours of reaction time,6%of propane sultone dosage,0.25%of alkali dosage.In addition,the infrared characterization is applied to determine the structure and fundamental properties of the product.Secondly,based on the conditions of Reservoir 36-1 in Suizhong area,this study systematically analyzes the change law of phase behavior of CMS and residual polymer mixtures,and the optimum use of concentration range is optimized to 1.5².0%.The result of laboratory experiment indicates that,there is a good compatibility between CMS and sea water,and a short dissolution time,wonderful injection performance and long-term stability of CMS.The mixtures are equipped with good sealing performance,thermal stability,flushing resistance and the ability to enhance oil recovery and reduce the concentration of polymer in produced fluid.Through the physical simulation experiments on massive plate heterogeneous reservoirs,SEM and Zeta potential and other advanced means,the interaction mechanism of CMS and residual polymer from both macroscopic and microscopic perspectives,and the mechanism of enhanced oil recovery by flocculation profile control are analyzed.The results of Zeta potential and particle size distribution show that through the effect of electrical neutralization and adsorption bridging,CMS and residual polymer can form the flocculation.The result of SEM suggests that when the residual polymer concentration is too high,the mixed system will form the"gel like"with higher viscosity,however,with the decreasing concentration of residual polymer,"gel like"will transform into the flocculation with larger particle size,then it will be adsorpted or retented in the pore throat.In the physical simulation experiments on massive plate heterogeneous reservoirs,the change regulation of oil saturation and pressure has confirmed that the flocculation has good capacity of fluid diversion,which forces the subsequent water flooding into the layers with middle or low permeability,by which we achieve the purpose of enhancing oil recovery.At last,the technology boundaries of application of flocculating reutilization technologies in offshore oil fields are systematically studied with indoor physical simulation and numerical simulation methods.The indoor physical simulation is to study the influence of reservoir factors and the effects of residual polymer properties on flocculating reutilization technologies by orthogonal experiment and the single factor experiment.In the orthogonal experiment,the polymer concentration is the primary control factor.Single factor experiment indicates several suitable parameters,including 45 to 90°C of reaction temperature,below 20000 mg/L of formation water salinity,0.50?m²-8.5?m² of formation permeability,below 200mPa·s of crude oil viscosity,200¹750 mg/L of residual polymer mass concentration,over 300 cm³·g^-1 of intrinsic viscosity,and 22%to 60%of polymer hydrolysis degree.Meanwhile,results of numerical simulation also confirm formation heterogeneity as the principal control factors of flocculation application technology.Meanwhile,results of numerical simulation also confirm several optimal ranges,including 8 to18 of reservoir permeability ratio,60 to 170 mPa·s of crude oil viscosity,5 to 30?m² of formation permeability,0.01 to 0.02 PV of slug volume of water.Predictably,there is an extensive application prospect of the flocculating reutilization technologies applied in offshore oil fields.