Analysis of acid returns improves efficiency of acid stimulation: a case history

Techniques are demonstrated to evaluate acid stimulation treatments based o n chemical analysis of the return fluids. The techniques can provide inform ation about the source of formation damage, the amount and type of scale in the casing, the efficiency of the acid treatment, and the amount of iron c ontrol chemicals required for similar treatments. As an example, these meth ods were used for the acid stimulation treatment of a typical seawater inje ction well (Well A) in a large carbonate reservoir in Saudi Arabia. Over a 2-day period, more than 50 field samples were collected. More than 700 chem ical analyses were eventually made on these samples. Analysis of the casing pickle fluids showed that 235 kg of iron from corrosion products, as well as a significant amount of acid-insoluble iron carbide, Fe3C, was produced. Measurement of iron (II) and iron (III) concentrations showed that the iro n corrosion products in the casing contained mostly iron oxides such as rus t or mill scale. The pickle treatment prevented the injection of this mater ial into the formation around the wellbore. Otherwise, the main acid treatm ent would have flushed all of this material into the reservoir and reduced injectivity. Analysis of the matrix acid flowback samples showed that iron control chemical concentrations in the acid could be reduced with significa nt cost savings. The extent of acid dilution and the amount of unreacted ac id were also determined. The results showed that 125 kg of calcium sulfate was dissolved from the near-wellbore region during flowback. Acid stimulati on of Well A increased the injectivity index (II) of the well from 0.12 to 0.21 m(3)/day/kPa (5 to 9 bbl/day/psi), an increase of 75%. The acid stimul ation was successful and the evaluation methods provided useful information to improve the success of future treatments. (C) 2000 Elsevier Science B.V . All rights reserved.