A Novel Single-Stage Sandstone Acidizing Fluid

Abstract

Sandstone acidizing operations usually include solvent and acid preflush, main acid treatment, and post-flush stages. However, the acid preflush stage needs good design and execution to prevent formation damage. Moreover, multiple-stage operations require large-volume fluids and pumping time. Therefore, it is challenging to stimulate sandstone formations, especially those with high clay and carbonate content. A novel single-stage acid has been developed to overcome these challenges and improve the stimulation success rate in a cost-effective manner. The application of the new acid system has been studied in laboratory testing. Core flow tests were performed to evaluate the stimulation performance with Berea Gray and Bandera Gray from 160° to 300°F. An inductively coupled plasma (ICP) instrument was used to analyze the ions in the spent acid effluent. The performance was compared with mud acid and organic mud acid. The acid-rock reactions were studied by batch reactor tests. Compatibility with crude oil and mutual solvent was also tested. The results of core flow tests have shown that the new acid was used to treat the sandstone cores effectively at temperatures from 160° to 300°F. The regained permeability range varies from 115% to 400% under different conditions. The new acid provided similar or better performance compared with the combination of acid preflush and mud acid or organic mud acid. High concentrations of Al and Si were observed in the spent acid effluents by ICP analysis, indicating the high dissolution capacity of clays by the new acid. The new acid is highly compatible with carbonate, which was supported by the high concentrations of Ca and Mg in the spent acid. Both core flow tests and batch reactor tests have shown that the new acid stabilizes the problematic ions (Al, Ca, Mg and Fe) in the spent acid. The new acid is compatible with mutual solvent from the core flow tests; therefore, the mutual solvent preflush can be eliminated. The new acid also has good corrosion control due to the relatively high pH compared with mud acid. Overall, the new single stage acid has been used to stimulate the sandstone cores successfully without acid preflush and solvent preflush. A differentiating characteristic of the fluid is that it greatly reduces the risk of treatment failure by reducing primary, secondary, and tertiary precipitation, while maintaining high dissolving power for clays. It uses a different, more cost-effective chemical pathway to stabilize problematic ions compared to traditional single-step sandstone acidizing systems. The new fluid simplifies operation by reducing the total treatment fluid volume, the total number of fluid stages, and the number of fluid types needed at the wellsite.