| In recent years,with the gradual maturity of acid fracturing technology,it has become an indispensable means of exploring heterogeneity and developing oil and gas.Foam acid has become an important part of acid fracturing process because of its high viscosity,high flowback rate and small filtration loss.The conventional foam acid system only uses a single foaming agent.Although the foaming property is good,the foam stability is poor due to the influence of factors such as high temperature and high pressure in the actual formation environment,which leads to the short half-life of the foam.The retarding performance of the foam acid is seriously reduced,which ultimately affects the construction effect.In this paper,through a large number of literature research in the early stage,the foaming agent suitable for the foam retarded acid system was selected.Using acrylamide(AM),methacryloyloxyethyl trimethyl ammonium chloride(DMC)and cetyl dimethyl allyl ammonium chloride(C16DMAAC)as raw materials,a high temperature resistant polymer foam stabilizer SM-1 was obtained by aqueous solution polymerization in free radical polymerization.The conversion rate and intrinsic viscosity were used as evaluation criteria.The synthesis process was optimized by the control variable method to determine the optimum reaction conditions of the system.The product was characterized to determine that the final product was the target product;the performance of the system was evaluated and the following conclusions were drawn:(1)A nonionic surfactant AEO-7 with certain salinity resistance was selected as foaming agent for acid solution.AEO-7 has good foam performance and strong acid resistance in 20%hydrochloric acid solution.Under high salinity,the foam half-life decreases by less than 25%,and the foam height is almost unchanged.The optimum dosage in 20%hydrochloric acid solution is 0.9%.(2)The optimum synthesis process of SM-1 polymer foam stabilizer:the total amount of monomer accounts for 35%of the total mass of the system(solid content 35%),the mass ratio of the three monomers is:m(DMC):m(AM)m(C16DMAAC)=1:1.5:0.02(cationic degree 60%),the initiator system is(NH4)2S2O8-NaHSO3-V50,the initiator mass fraction is 0.6%,the initiation temperature is 6℃,and the system reaction pH=6.The final product was characterized by Fourier transform infrared spectroscopy and fluorescence spectroscopy of pyrene.The results show that the experimental product is consistent with the target product;(3)The compatibility of foam retarded acid base solution with corrosion inhibitor WR-19 and iron ion stabilizer HJZ-801 is good,and there is no stratification,flocculation and precipitation.At the same time,the half-life and height of the prepared foam are basically consistent with the main formula.Finally,the formula of high temperature resistant foam retarded acid system was determined as follows:20%HCl+0.9%AEO-7+0.08%SM-1+6%WR-19+4%HJZ-801.The foam half-life of the system at room temperature was 59 min,and the foam half-life at 140℃ was 29 min.The foam mass at different temperatures was greater than 70%,and the foam state was stable.Compared with the average acid-rock reaction rate of blank hydrochloric acid,the static acid-rock reaction rate of the system at 140℃ decreased by two orders of magnitude,7.029×106mg/(cm2·s),and the slow rate reached 92.3%.(4)The average initial filtration loss of the foam retarded acid system between 80℃ and 110℃ was<4.24(10-2m3/m-2),and the average filtration coefficient was<0.86(10-4m/min1/2).After acid-rock reaction at 140℃ for 2 h,the residual acid viscosity is 1.876 MPa·s,the residual acid surface tension is 14.54 mN/m,and the residual acid interfacial tension is 0.94 mN/m.The average damage rate of the core is 19.01%.The performance test results meet the industry standard.The experimental results show that the high temperature foam retarded acid system has excellent retarding performance,which can effectively reduce the acidrock reaction rate at 140℃ and achieve deep acidification. |
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