Acid retardation for deeper stimulation—Revisiting the chemistry and evaluation methods

Abstract

AbstractAcid fracturing treatment in upstream oil and gas production is done to extend the connectivity of the wellbore deep into the reservoir. This is achieved through a dissolution process using acid in conjunction with hydraulic fracturing. A key to success is to impart favourable reaction kinetics between the acid and rock matrix. If the reaction proceeds too rapidly, it results in large voids in the near‐wellbore area but insufficient dissolution in the reservoir. Hydrochloric acid (HCl) is ubiquitous in the petroleum industry for acidizing applications due to its high dissolving capacity towards carbonate minerals, soluble reaction products, and low cost. Its corrosive nature coupled with rapid reaction kinetics have forced the industry to search for mechanisms to address these limitations. The concepts for slowing down reaction rate centre around (1) reducing mass transfer of hydrogen ions, H+; (2) limiting H+ ion dissociation; and (3) altering the wetting property of the rock surface. Reaction kinetics data is critical to guiding the selection of the suitable acid formulations for acid fracturing process. They are commonly measured in the lab using instruments such as a rotating disk reactor, flow cells, and diffusion cell. Though routinely applied, these experimental methods, including their setups and procedures, are rarely questioned. The experimental artefacts can lead to wrong conclusions. Deeper understanding of the fundamental assumptions behind the apparatus design and test procedures is critical. This paper discusses the chemistries employed in acid retardation, and more importantly the attention needed when extending the experimental data to field treatment.