TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractLaboratory testing of the effects of acidic fluids on metallic samples representative of those used under downhole conditions is the normal procedure to establish additive requirements. Under sour conditions (Hydrogen Sulfide gas contaminated production), test methods utilizing differing amounts of H 2 S in the gas phase with subsequent contamination of the test solution under bottomhole temperatures is required.Underbalanced acid washing combined with follow-up acid squeezes is becoming more and more prevalent in carbonate wells with sour production. Typically, these treatments are conveyed using Coiled Tubing strings. The thin walled nature of these tubulars makes corrosion control of the utmost importance.Corrosion testing using modified NACE cells under pressures varying from 500 to 6,000 psi and magnetic stirring systems can test various acid systems at temperatures varying from 27° to 315°C (80° to 600°F) over any exposure time. Safety factors, weight loss and pitting guidelines are employed to attempt to ensure the continued integrity of the Coiled Tubing through many treatments. In addition, evaluations of sections of Coiled Tubing after various treatments using the pearlite layer at the concentric center of the Coiled Tubing determine material losses both on the outside and the inside. Surface inspections are also performed to search for pitting or other acid corrosion associated defects.This paper presents the comparison of laboratory acid corrosion testing and the actual effects on Coiled Tubing strings employed. Data from wells treated that produce from 23 to 60% H 2 S and have bottomhole temperatures ranging from 86° to 110°C (187° to 230°F) are presented. Treatments consisted of acid washes followed by acid squeezes. The varying material deterioration of actual coils, specifically, a loss in wall thickness and surface corrosion features and how this compare to laboratory test data is discussed.