A New Method to Accurately Ascertain Hydrogen Fluoride Strength of Acidizing Stimulation Fluids Using an Ion Selective Electrode Meter
Authors: Akaighe Nelson , Mukhopadhyay Sumitra , Lopez Erik 
Published: January 2016

Abstract
Organic acid-HF systems have been widely reported to effectively dissolve chemically reactive formation fines and clays that respond poorly to traditional inorganic mud acids. Some of these organicHF acid systems often minimize the creation of formation damaging precipitates like silica gel, and also prevent dissolution products from reprecipitation.
Accurate determination of HF strength of acidizing fluids via free fluoride measurements especially when prepared with weak organic acids is necessary. Ion selective electrodes (ISE) can accurately measure the strength of hydrogen fluoride (HF) acids used in sandstone acidizing jobs. These measurements are important to ensure that acidizing fluids with the desired acid strength is delivered downhole during well stimulation operations. Unlike inorganic acids such as hydrochloric acid that ionize completely in aqueous solutions, organic acids are weak acids and only partially ionize in aqueous solutions. As a result, organic acids do not completely hydrolyze the fluoride ion source such as ammonium fluoride to generate the desired amount of HF. Furthermore, the presence of multiple dissociable protons with different acid strengths on organic acids such as citric acid and etidronic acid or 1-hydroxyethane 1,1-diphosphonic acid (HEDP) further exacerbates the problem.
In this study, several organic-HF acids were prepared with ammonium fluoride salt solutions and their HF strength determined using the ISE meter equipped with a fluoride ion selective electrode. Several coreflow tests were conducted to investigate the effectiveness of the prepared organic-HF acids to remove aluminosilicates and quartz from clean Buff Berea sandstone cores at 150°F. The coreflow test data with the organic-HF acids were compared with coreflow data of traditional mud acids and fluoroboric acid systems. A combination of X-ray Fluorescence (XRF) and X-ray Powder Diffraction (XRD) techniques were used to identify the mineralogy of Berea core plugs used in the coreflow tests. Coupled Plasma (ICP) technique was utilized to determine the concentrations of Si, Al, Fe, Ca and Mg ions in effluent samples of the coreflow tests.
The results clearly indicated that stronger organic acids with lower pKa values require lesser amount of ammonium fluoride to generate the same amount of free fluoride ions compared to weaker organic acids with higher pka values. The HF strength of sandstone acidizing fluids generated with tri-protic and tetraprotic organic acids, such as citric acid and HEDP respectively, were accurately determined with the aid of the ISE meter. Furthermore, the coreflow test results indicate that contrary to the common belief, very mild organic-HF acid systems with pH range between 3.6 and 5.5 can effectively stimulate sandstone formations similar to traditional mud acid and fluoroboric acid systems with much lower pH values. Finally, accurate measurement of the strength of HF acids generated with organic acids enables the formulator to deliver the desired strength of organic-HF acid downhole that dissolves and/or removes maximum amount of siliceous minerals from sandstone formations.