An Experimental Study on Acid Treatment Using Autogenic Acid Fluid System in Ultrahigh-Temperature Carbonate Reservoirs

In the acid fracturing stimulation of ultra-high temperature carbonate reservoirs, conventional acid has problems such as fast acid-rock reaction rate, and inability to achieve deep penetration. Therefore, this paper uses methyl acetate (agent A) and ammonium chloride (agent B) as the matrix of the autogenic acid. The acid is an in-situ sustained-release acid with a temperature resistance of 180 °C. The volume ratio of autogenic acid system is optimized using standard sodium hydroxide solution titration experiment. The effects of different temperature and reaction time on the ability of autogenic acid to acid production are studied. The acid-rock kinetics of autogenic acid is studied using the cores of the Maokou limestone and Dengying dolomite in the Sichuan Basin. The acid-rock reaction kinetics parameters of autogenic acid are measured using a self-made high temperature and high pressure rotating reactor. The results show that mixing the agent A and the agent B at a 1.5:1 volume ratio provide the optimal results as the amount of the acid production is the highest. The autogenic acid has a low acid production capacity at low temperatures and only exhibits excellent acid production ability at high temperatures of 180 °C.The effective H+ concentration released from the autogenic acid at 180 °C is maintained at 2.24 mol/L after 4 h. The reaction time of autogenic acid is long, meeting the requirements of sustained-release at ultra-high temperatures.The acid-rock reaction rate of autogenic acid with the core of the limestone at 180 °C is 9.40522 × 10−7 mol/(s·cm2) and with the core of the dolomite is 6.85752 × 10−7 mol/(s·cm2).The autogenic acid reacts with both carbonates at similarly low rates, verifying that the acid has some retardation properties. The autogenic acid system has a slow acid rock reaction rate and a long acid production time, while it is not affected by carbonate lithology. It can realize the purpose of undifferentiated deep acidizing of ultra-high temperature carbonate reservoirs.