| In the process of acid fracturing for carbonate rock formation,cross-linked acid is widely used due to its advantages such as high viscosity,low fluid loss,low friction and easily pumping.The performance of main acid will affect the acid fracturing effect.At present,the main acid commonly used in oil&gas field are hydrochloric acid and organic acid such as formic acid and acetic acid.However,hydrochloric acid exist the problems of fast reaction speed and strong corrosiveness,and some organic acid also exist the problems of poor dissolution ability and secondary precipitate.Aiming at the shortcomings of the existing acid fracturing systems,an environmental-friendly cross-linked acid was developed,which can improve the dissolution ability,reduce secondary damage and corrosion,and achieve deep penetration of the acid system.Degradable organic acid A and organic acid B were compounded due to their synergistic effect on dissolving carbonate rock.Effects of mixing molar ratio,acid concentration and temperature on dissolution ratio and dissolution rate were investigated by single factor method,and the formulation of main acid was preferred:the molar ratio of organic acid A to organic acid B was 1:1,and the acid concentration was 17%,and it was named EF acid.Further,the thickening agent,cross-linking agent and additives which were compatible with main acid were selected,and the dosage of them was determined according to the standard.The formulation of environmental-friendly cross-linked acid was:17%EF acid+1.0%TP0509+0.12%PEI-A+1.5%HLS-1+1.0%CA-1+2.0%COP-1,and it was named EFJL acid.The effects of different cross-linking ratios on temperature resistance,rheological parameters and viscoelasticity of EFJL acid were investigated by rheological measurement.The reaction kinetic equations of hydrochloric acid cross-linked acid,EF acid and EFJL acid were established by rotating rock disk experiments,and the activation energies of three acids were determined at 50,70,90 and 110°C.The experimental results showed that the addition of cross-linking agent can significantly improve the temperature resistance of the system,and the temperature resistance of EFJL acid can reach 110°C.With the increase of cross-linking ratio,the consistency coefficient K of the system increased and the rheological parameter n decreased.The shear resistance was slightly improved;at 60°C,as the cross-linking ratio increased,the elastic modulus gradually became larger than the viscous modulus and dominated.The reaction rate constants K of the three kinds of acid solutions at 90°C were 1.80×10-6,3.37×10-7,1.87×10-7(mol/L)-m/(cm2·s),respectively,and the reaction order m were 0.76,0.70,and 0.34,respectively.It was indicated that the retardation of EFJL acid is excellent;the activation energies of the three acids were 1.97×104,1.44×104 and 1.25×104 J/mol,respectively,indicating that EFJL acid was more prone to acid rock reaction.The comprehensive performance evaluation of EFJL acid showed that the corrosion rate of EFJL acid system was only 0.9455 g/(m2·h),and the corrosion inhibition rate was as high as 99.15%.Compared with hydrochloric acid cross-linked acid,the acid-rock reaction speed of EFJL acid was slow,and the final dissolution ratio was 73.62%;at the same time,it had good fluid loss and sand carrying capacity.The filtration loss coefficient is 1.64×10-4 m/(min)0.5,and the initial filtration loss is3.79×10-2 m3/m2,the proppant settling velocity is 0.75 cm/min;the crosslinked acid system can automatically break the gel after reacting with carbonate rock at 90°C.The viscosity of the breaker was 4.2 m Pa·s,which was easy to flowback and contributed to reduce pollution and damage to oil&gas field;the degradation time was 49h at 90°C,and the evaluation results met the industry requirements. |
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