Coiled Tubing Material Selection for Velocity Strings in Sour Brine Service

Abstract

Installing coiled tubing velocity strings is a common deliquification strategy used in gas wells in which the flowrate has dropped below the critical rate required to remove produced liquids from the well. Initially in Shell’s Groundbirch unconventional gas development, a standard carbon steel coiled tubing grade was used for these applications. Due to elevated levels of chlorides, carbon dioxide and in some cases H2S, a long-term inhibition schedule was then needed to minimize corrosion. Recently, however, a special CRA material has been employed, that Shell had previously helped develop for subsea applications (Li et al, 2002), which has eliminated the need for costly long term inhibition. This paper outlines the selection process and implementation of this material change. Standard 13Cr alloys are well known in the industry but could not be used in the Groundbirch fields due to the high chlorides levels of the produced water as well as H2S levels sometimes above NACE thresholds. On this basis, higher chromium coiled tubing materials were evaluated based on their ability to resist corrosion, sulphide and chloride stress cracking, as well as their mechanical properties. A fit for purpose material testing program was used, combining standard NACE test protocols with unconventional specimen shapes and crevice corrosion assessments in simulated well environments. Factors such as the direction of primary loading stresses, mechanical deformation during installation, as well as coiled tubing manufacturing characteristics such as the longitudinal seam, bias welds, and orbital (butt) welds were also considered. From this fit-for-purpose testing, not only was the preferred corrosion-resistant material, Nitronic 19D duplex-steel, selected for the coiled tubing strings, but an operating envelope has been developed for it, as well as manufacturing specifications have been defined for the strings for optimum performance. Installation guidelines to reduce the mechanical damage inflicted on the strings have also been created. Although the campaign startup was delayed due to manufacturing issues, once the campaign did start, the installations have been efficient, with future campaigns planned. By selecting a single, robust material grade for CT Velocity String material for a variety of potential conditions (high salinity, CO2, H2S), and also developing some guidelines for their use, installations can be done efficiently in Groundbirch in large campaigns, with long term cost savings from eliminating the need for continuous inhibition. Furthermore, the long term integrity of the strings should benefit, ultimately leading to longer completion life, less production deferment, and less intervention issues due to corroded and failed CT strings.