Hydrochloric acid has been extensively used for well stimulation in matrix acidizing and acid fracturing. However, high reaction rate and severity of corrosion problems, especially for wells completed with Cr-based tubulars, limit the usage of the straight acid, especially for deep gas reservoirs. In-situ gelled acids are used to reduce leak-off rate and enhance acid diversion, while mixing organic acids with regular hydrochloric acid reduces the strength of HCl and increases the acid penetration. Literature review shows that very few numbers of studies have addressed the systems that utilize both advantages.

The objective of this work is to investigate the effect of formic acid on the performance of the live and the spent in-situ gelled formic-HCl through viscosity measurements and core flood studies and to measure the rate of reaction of different formic-HCl in-situ gelled acid mixtures with calcite. Pink Desert limestone was utilized for the core flood and the reaction rate experiments. The core flood study was conducted at 250°F using two different rates of injection (2 and 10 cm3/min) and the rotating disk reactor was used to perform the reaction rate study. The effect of formic acid concentration, disk rotational speed, and zirconium cross-linking additive on the reaction rate was examined at 250°F. The formic acid concentration ranged from 0 to 6.31 wt% and the HCl concentration ranged from 0 to 5 wt.%.

The core flood results showed that in this acid concentration range with the selected polymer and zirconium cross linking agent, the in-situ formic-HCl acid is behaving more like gelled acid. Increasing the formic acid concentration decreases the reaction rate with calcite at both low and high disk rotational speed. This was confirmed by the viscosity measurements, which showed that increasing the formic acid concentration increased the live acid viscosity and decreased the viscosity of the spent acid.

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