The use of coiled tubing (CT) in wells containing hydrogen sulfide gas (H 2 S) has associated problems because of the potential for sulfide stress cracking (SSC) of the CT material. Carbon dioxide (CO 2 ) can also contribute to general corrosion or intensify any H 2 S-related corrosion because of chemical reactions resulting in acid. However, with the right precautions, equipment, and procedures, this type of operation can be carried out safely and successfully.The various problems associated with working with CT in H 2 S and CO 2 wells are discussed and a general best practice taken from locations working regularly with H 2 S and CO 2 is presented. This paper shows the equipment, chemical inhibitor, quantity and method of inhibitor application, and other precautions taken to carry out the work safely and successfully. An alternative approach is also highlighted for some situations whereby any H 2 S is bullheaded into the well before coiled-tubing intervention. This alternative approach avoids any contact of H 2 S with CT to prevent corrosion. A number of case histories are shown for different H 2 S and/or CO 2 locations around the world, which detail the type of operations, quantity of H 2 S and CO 2 , procedures used, frequency of operations, and the overall success of these methods in ongoing operations.This paper presents a review of methods and equipment currently being used around the world to work in potentially corrosive and dangerous H 2 S and/or CO 2 wells. Coiled Tubing Corrosion in Acid and Sour EnvironmentsOilfield production fluids containing the acid gases H 2 S and/or CO 2 can be corrosive to CT because of the resultant lowering of the pH of the aqueous phase. Low-pH aqueous fluids accelerate corrosion by providing a plentiful supply of hydrogen ions. Any brines in the production fluid will also increase the overall corrosivity to the CT, as well as provide an aqueous medium for contamination. Thus, H 2 S and/or CO 2 in brine is more corrosive than the same gasses present in oil. Alternatively, because an aqueous phase is necessary, the risk of corrosion or cracking in dry gas wells containing these gases is low.Basically, H 2 S mixed with water will produce sulphuric acid when oxidized or a weaker acid when no oxidizer is present. CO 2 produces Carbonic acid when mixed with water. H 2 S is noncorrosive in the absence of moisture.Other than wall thinning from general corrosion, there are several failure modes that can occur when CT is exposed to H 2 S-containing fluids. These are detailed in Appendix A and can all be directly related to hydrogen entry into the tubing metal structure. Unless otherwise stated, this paper will concentrate on the issues associated with SSC-type failures. Suitable Coiled Tubing SelectionGrade 80 CT will tend to show less susceptibility to SSC as compared to higher-strength grades but is not immune to problems in H 2 S-containing environments. Halliburton individuals: