The Synthesis And Performance Evaluation Of Surface Cross-linked Acid Viscosifier

For carbonate reservoirs,complicated lithology reservoirs and calcareous sandstone reservoirs with special properties,conventional acid has already can’t meet the demands of long crack and high fracture conductivity in acid fracturing measures.Surface cross-linked acid is a new acid system developed in recent years,which significantly increases acid viscosity,reduces the rate of acid-rock reaction and acid fluid loss by cross-linking effect.It is suitable for deep acid fracturing.By monomer optimization,acrylamide（AM）and 2-acrylamido-2-methyl propanesulfonic acid（AMPS）were selected to have polymerization reaction.The reaction was initiated by redox potassium persulfate/sodium bisulfite,whose concentration was 0.01%of total solution quality.Monomer concentration was 20%of total solution quality,and mole ratio of AM and AMPS was 3:1.The acid viscosifier synthesis operated under the condition that the initiation temperature was 10 centigrade and pH of reaction system was 7.And the basic performance of the viscosifier was tested.Then optimized crosslinker and additives were added to compose the surface cross-linked acid system.According to the standards of oil and gas industry,the properties of surface cross-linked acid were evaluated.The results showed that after two hours’shearing at 60 and 90centigrade,170s^-1,the viscosity of acid was about 100 mPa·s and 80 mPa·s,which proved good temperature and shear resistance.The reaction rate of acid-rock showed that:the initial rate of hydrochloric acid was 2.5 times as fast as surface cross-linked acid and gelled acid was 1.7 times,which improved good retarding property of the surface cross-linked acid.The phenomenon of single prop-carrying property and the sand-carrying ratios were 20%or 30%showed that the cross-linked acid system had good prop-carrying ability.Broken breakers made the cross-linked acid broken within half an hour to an hour,which satisfied the request of fast flowing back after the acid fracturing treatments.In short,the surface cross-linked acid made up of experimental synthetic viscosifier had good integrated performance,which had referential and directive significance to acid research on site.