Scale Inhibition Properties And Mechanism Of Carbon Quantum Dots On Barium Sulfate And Strontium Sulfate

Scaling is a common problem in oilfield production process,which can affect the regular production of oilfields.Scale can deposit on the surface of any equipment.Once scale is formed,it will grow and become thicker day after day.As a result,it will increase the resistance of fluid flow,reduce the transport capacity of pipeline,and reduce the efficiency of heat exchanger.What more serious is,it may cause corrosion to decrease the service lifetime of the equipment,and even lead to explosions and stop production.To avoid formation of scales,the general and effective way is to add small amount of scale inhibitors.At present,phosphorous compounds have been effectively used to prevent the generation of scales.However,the use of these phosphorus-containing compounds is limited because they have the potential to deteriorate the environment.Therefore,it is an urgent thing to develop green scale inhibitors containing no phosphorus.On the other hand,since carbon quantum dots(CQDs)were firstly discovered by Xu et al.in 2004,they have attracted extensive research for their good water solubility,photochemical stability,catalytic activity,and good biocompatibility.Moreover,the carbon source is rich,the preparation method is simple and the cost is low.Therefore,CQDs have attracted extensive research in the fields of optics,electricity,catalysis,medicine,bioimaging and so on.CQDs are composed of carbon core and surface groups,and there are a large number of oxygen-containing functional groups on their surface.Compared with the precursors of CQDs,CQDs contain a large number of carboxyl,hydroxyl,carbonyl and epoxy groups.Carboxylic acid group is the main functional group to inhibit scaling.Polycarboxylate is also a commonly used scale inhibitor at present,but its low dispersion hinders its scale inhibition efficiency.Therefore,the CQDs with rich carboxyl groups can not only make full use of the advantages of zero-dimensional carbon nanomaterials,but also have the scale inhibition function of carboxylic acid groups.So that they can have good water solubility and scale inhibition performance,which can solve the problem that carboxylic acid polymer scale inhibitors are difficult to disperse and improving the scale inhibition efficiency.However,as far as we know,no one has applied this method in the field of scale inhibition.In this paper,a new type of scale inhibitor,carboxyl carbon quantum dots(CCQDs),was synthesized by pyrolysis of citric acid without any auxiliary reagents.And the as-prepared CCQDs were characterized by transmission electron microscopy(TEM),Fourier transform infrared spectrometer(FT-IR),X-ray diffraction(XRD),ultraviolet-visible spectrophotometer(UV-Vis)and fluorescence spectrophotometer.The scale inhibition performance of CCQDs,which were acquired at the same temperature but at different pyrolysis time,to BaSO4 and SrSO4 was investigated according to the widely used "Evaluation Method of Scale Inhibitor Performance for Oilfield"(SY/T5673-93).And the effects of the system's pH,temperature,scale ion concentration and salinity on the scale inhibition efficiency of CCQDs scale inhibitor were also be investigated.The results showed that all the CCQDs scale inhibitors which obtained at different carbonization time can make the scale inhibition efficiency of BaSO4 and SrSO4 reach 100%,while the scale inhibition efficiency of critic acid at the same dosages were all zero.In addition,the CCQDs scale inhibitor also has good temperature resistance,alkali resistance and salt resistance.However,the scale inhibition efficiency of CCQDs is greatly influenced by pH and scale ion concentration.The changes of surface composition,stacking morphology and crystal form of BaSO4 and SrSO4 scales before and after adding CCQDs were investigated by means of FT-IR,scanning electron microscopy(SEM)and XRD.And combined with the properties of CCQDs and their scale inhibition performance under different conditions,the scale inhibition mechanism of CCQDs was explored.It can be inferred from the experimental results that CCQDs can bind with Ba2+and Sr2+ and adsorb on the active sites on the surface of BaSO4 and SrSO4 crystals,thus inhibiting the formation and growth of scales and achieving scale inhibition effect.