Performance Comparison of Retarded Acid with Emulsified and HCl/Formic Acid Recipes for Carbonate Acidizing

Abstract

Various acids and chemicals have been developed to achieve retardation properties to enhance oil and gas production. Improving acid retardation through emulsification or polymer gelling agents has drawbacks. Retarded acid recipes including HCl, organic acids, and/or chemical retarders showed comparable retardation efficiency. This paper aims to compare the performance of non-viscous retarded acids and benchmark them against emulsified acids (Hall-Thompson et al. 2020). The proposed acid recipes are either a combination of HCl, formic acid with or without inorganic/organic retarders. Emulsified HCl acids at 15 wt% were prepared as a retardation baseline. Carbonate dissolution was assessed using solubility testing with calcite discs. Coreflood testing was conducted using Indiana limestone core plugs to assess the pore volume profile at temperatures up to 300 °F. This study was supported by Computed Tomography, to evaluate the propagation behavior as a result of the fluid/rock reaction. The solubility calcite discs in the retarded acid recipes showed acid retardation comparable to emulsified acid. The surface tension values of retarded acid recipe were between 19.5-38.5 dynes/cm at 70-300°F. The coreflood results showed retardation performance comparable to emulsified acids at 200 and 300°F. The retarded recipes were capable to prevent nearly 1,500 mg Fe/L from precipitation in spent acid. The corrosion loss and pitting indices were within <0.02-0.09 lb/ft2 using retarded acid recipes at 170-300°F (i.e., <0.02-0.09). The optimum injection rates of retarded acid recipes were nearly 1 and 2 cm3/min at 200 and 300°F, respectively. The results obtained with HCl/Formic acid recipes showed generally competitive retardation performance compared to retarded acid recipes with limitations discussed in the paper.