Research On Chemistry Of Scaling And Descaling Process In Concentrated Alkali ASP Flooding

After years of laboratory research and pilot test in DaQing Oil-field, ASP floodingtechnology was demonstrated to dramatically improve the performance of increasing oil anddecreasing water for Class Ⅰo r Ⅱreservoir in Daqing which could enhance oil recoverymore than20%compared with water flooding. However, scaling problems seriously exposedin the system of lifting process as it would cause pumps stucked. As pumps detection periodshortened it would seriously restrict the development of ASP flooding technology. Therefore,research on the scaling theory and chemical control technology of ASP flooding hadimportant significance.Dissolution-sedimentation mechanism of reservoir minerals, scaling laws, characteristicsof oil-wells scaling and chemical control technology of ASP flooding were researchedsystematically in this paper.1. Experimental research on simulating process of dissolution-sedimentation wasdeveloped. Kind and degree of reservoir minerals dissolution were confirmed. Formationmechanism of ASP flooding mixing scale was improved.Dissolution objects were mainly cements or clay minerals in base-rock dissolutionreaction. The main products were silicate and the rate, extent, chemical composition ofdissolution was related to crystalline types and spatial structure. ASP flooding scale wasformed by the interaction actions of physical and chemical reactions at the same timeincluding carbonates, amorphous silica, silicates and co-deposition of different particles.2. The main factors affecting the stability of silicon ions, control scaling wasexperimental researched in ASP flooding system. Variational characteristics of pH value, TDS,competing reaction of scaling ions in the process of silicates scaling were researched for thefirst time.(1) Temperature, pH value, calcium, magnesium, aluminum, and barium had promotionreaction to the deposition of silicon ions. The maximum influencing factor was pH value.Polyacrylamide and surfactant had a certain equilibrating action on the stability of silicon ions,could reduce the risk of silicon scaling.(2) With the increase of pH value, the scaling speed of produced liquid with high calcium,magnesium, barium accelerated and the scale content also increased. Magnesium ions did notdeposit until the calcium ions reached sedimentation equilibrium in the system.(3) For the process of silicate scaling, the decrease of pH value was the key factor forscaling. It was difficult to form silicate scale if pH value increased. In the system with high content of carbonate and silicate, the main factor caused calcium, magnesium, barium ions toscale was carbonate.3. Chemical composition, existence form, sedimentary characteristics of ASP floodingscale were characterized by infrared spectroscopy, electron spectroscopy, XRD, scanningelectron microscopy and ICP-AES. Scaling characteristics of oilwells in three blocks wereanalyzed.(1) PH value, ions composition, polymer, surfactant, viscosity of produced liquid inmechanical recovery well had a certain change characteristics with the increasing ofinjection-production period. The main reason that causes changing was the physical-chemicalreaction between injection liquid and reservoir rocks.(2) According to the infrared spectrum of oilfield scale, we could determine whetherthere were calcium carbonate, barium carbonate, silica, calcium silicate, magnesium silicate,barium sulfate, calcium sulfate, ferrous, ferric hydroxide, iron oxide in scale sample andcompare the relative content of scale.(3) In the process of sedimentation of the same scale, carbonates deposited first. Thedeposition of carbonates provided attachment point and crystal nucleus for silicate. The roughsurface of carbonates increased the friction between the fluid and pipe wall which caused thedeposition of silicate. The deposition of carbonates belonged to crystal growth pattern and thescale was densified. The deposition of silicon oxide belonged to colloid dehydration scalingand the pore of particles was visible, the scale was loose.4. PESA and MA-AA-AMPS composite scale inhibitor was synthesized to be suitablefor ASP flooding oil wells. The anti-scaling rate was more than85%.(1) The optimum synthesis conditions were as follows: reaction temperature70℃,reaction time1.5h, catalyst dosage3.5%, the molar ratio of sodium hydroxide and maleicanhydride2:1, the mass ratio of hydrogen peroxide and maleic anhydride1.4:1. The yield ofepoxysuccinic acid was88%.(2) The optimum polymerization condition of PESA were as follows: polymerizationtemperature90℃, polymerization time3h, sodium hydroxide dosage12.24%, initiator dosage2.86%.(3) The optimum synthesis conditions of MA-AA-SA copolymer were as follows: maleicanhydride25g, sodium allylsulfonate15g, sodium hypophosphite10g. Quantitative distilledwater was added, heating until the solids dissolved then adding0.2g catalyst.50mL30%hydrogen peroxide and40mL crylic acid were added dropwise afterwards. The dropwiseadding time was1.5h. Reacted2h at95～100℃.(4) The dosage of composite agent was100mg/L, temperature was below55℃, calciumcontent was less than80mg/L, silicon content was less than900mg/L, the anti-scaling rate was more than85%. Increasing the dosage of agent could improve the anti-scale effect.The research results of this paper is useful for understanding the scaling problem ofconcentrated alkali ASP Flooding, provide the theoretical basis for the prediction of scaling,provide a technical basis to field scale prevention and removal, provide the theory basis forthe leadership decision-making.