Oil and gas well drilling using coiled tubing (CT), in general, is continually increasing. Because a significant portion of this drilling activity could involve critical sour environments, the industry and regulatory bodies have recognized a lack of adequate guidelines for under-balanced drilling of wells containing hydrogen sulfide. Drilling using coiled tubing (DUCT) in critical sour wells is currently not permitted pending the development of such guidelines. A key element of safe and reliable drilling operations in sour wells, is the materials integrity of coiled tubing when subjected to a wet hydrogen sulfide environment.
Considerable laboratory testing and investigation of small-scale, standard NACE test specimens, has already been performed, primarily through a prior joint industry project6 (JIP). These tests did not show unequivocally that coiled tubing steels could be used in sour environments without protection by corrosion inhibitors. Considerably more laboratory testing is indicated. A combined industry effort is required that will compile all laboratory test results from which must evolve a meaningful and practical set of operational guidelines for use with actual full sized drill strings and associated equipment. This paper presents the background to these developments, a review of the materials problem, discusses results from new a new testing method and additional laboratory specimen testing that is required to address any deficiencies or lack in essential data.
Analogous to the development of CT fatigue life predictions, full-scale "corrosion fatigue" testing under representative loading and sour environments, is proposed. A methodology is required to integrate the materials response from these test results into the actual systems response of a CT drilling string operating in sour wells.
For readers unfamiliar with the technical issues of corrosion fatigue we included extensive appendices giving information related to CT chemistry, metallury and service conditions in sour environments.