Application of a New Measurement Technique for Organic Acid Additives in Stimulation Fluids

Abstract

An analytical method was implemented to simultaneously measure the concentrations of acetic, citric and formic acids in stimulation fluids. This method was not previously available for acid stimulation treatments. Capillary electrophoresis is used for rapid and accurate measurement of these organic acids in concentrated hydrochloric acid and in highly contaminated spent acidizing fluids. Commonly used titration methods are not effective at low organic acid concentrations or when more than one organic acid is present. Acetic, citric and formic acids have been used extensively in Saudi Aramco fields over the last few years as stimulation fluids and as acid additives. Citric and acetic acids are frequently used in acid treatments as iron control agents. The following conclusions are made:A method for the simultaneous measurement of acetic acid, citric acid and formic acid was developed and evaluated.The method is linear in the range of 2 to 200 mg/l of organic acid in diluted samples. Field samples will normally be diluted by a factor of 200 to provide a range of 0.02 to 2.0 wt% of organic acid.The method can be used at concentrations from 200 to 1667 mg/l, but has a nonlinear response in this region.Sample requirement is less than 5 ml in most cases.Reproducibility of the method was excellent. Relative standard deviation of organic acid standards was less than 1% at 200 mg/l, less than 3% at 20 mg/l and less than 10% at 2 mg/l.The detection limits of the method are 0.25 mg/l for acetate, 0.4 mg/l for formate, and 1 mg/l for citrate, based on a signal to noise ratio of 3 to 1.For acetate and formate measurement, no interference occurred with chloride up to 1000 mg/l, calcium up to 565 mg/l, iron (II) or iron (III) up to 500 mg/l, or sulfate up to 859 mg/l.For citrate measurement, no interference occurred with chloride up to 1000 mg/l, calcium up to 350 mg/l, or sulfate up to 859 mg/l. Iron (II) and iron (III) caused interference at concentrations of 10 mg/l. The method was used to simultaneously measure organic acid concentrations in coreflood samples and in field samples from acid stimulation treatments. Introduction Organic acids have been used extensively in Saudi Aramco fields over the last few years. For example, a mixture of acetic acid (5 wt%) and hydrochloric acid (5 wt%) was successfully used to stimulate water supply and injections wells in a sandstone field in Central Arabia.1,2 In addition, citric and acetic acids are frequently used with hydrochloric acid as iron control agents.3 Citric, acetic and formic acids are used in field treatments at concentrations up to 10 wt%.4 Formic acid is used as a corrosion inhibitor intensifier.5 To effectively evaluate stimulation treatments using organic acids, it is critical to measure their concentrations in the injected fluids and in the well flowback samples. Analysis of these samples can be difficult because they can contain high concentrations of chemical additives, dissolved solids, suspended solids and oil.