Understanding the Impact of Corrosion on Gas Wells Past its Design Life

Abstract

Field X produces dry gas from a carbonate reservoir with moderate aquifer support consisting of 16 wells most of which were drilled between 1986 to 1987. Up until 2021, these wells would have been in service for approximately 35 years, past its theoretical design life. Based on production forecast, the field is expected to continue producing for another 10 to 18 years. In view of the prolonged life extension, there is a need to review the integrity status of the well to ensure safe production until the end of field life. Wellhead preventive maintenance are conducted on a six-monthly basis to assure safety critical equipment (SCE) functionality and performance. Maintenance data collected since 2008 provides a good view on surface and subsurface valve integrity. In recent years, observations on external corrosion have received some attention. At the same time, there were efforts in determining and verifying the corrosion rates for production tubing, wellhead and x-mas tree. For well tubing and production casing, load based assumptions were used to estimate the minimum allowable tubular thickness to establish a basis for remaining life estimates. To verify the remaining life estimates, in 2019, a multi-finger caliper log was ran across the tubing of Well B to measure actual metal loss around the pipe. A magnetic log was also run in Well B in the same year to obtain quantitative measurement of remaining metal thickness in 13-3/8", 9-5/8" and 7" tubing. The observations from these exercises indicate that calculated estimates are more conservative as they do not account for the impact of the highly dynamic conditions downhole. While logging provided an independent view of the condition of the tubing and casing, no inspection of the x-mas tree cavity was carried out. An opportunity to close this gap and obtain information on the internal condition of the x-mas tree body is presented to Field X with the abandonment campaign on two idle wells in quarter three of 2021. The x-mas tree was retrieved, and internal inspection was conducted. The same is done for retrieved tubing from the abandoned wells. This study and its findings will enhance understanding on well design life, especially for vintage wells of over 30 years and provide assurance that wells are safe to produce.