Corrosion Evaluation of Temporarily Installed 'Well Zone Isolation Equipment' after Long Term Exposure to a Caesium Formate Mud System in a North Sea HPHT Well

摘要

A Well Zone Isolation Equipment or "Straddle System" was installed during a temporary well Plug and Abandonment (P&A) for a period of 448 days within a North Sea High Pressure High Temperature (HPHT) well. This straddle system comprises of two packers separated in between by a number of tubing joints. The two packers were manufactured from UNS N07718, super martensitic stainless steel UNS S41426 and the tubing joints were manufactured in super martensitic stainless steel UNS S41426. The straddle system was run and its packers installed or set whilst a 1.85 s.g. (specific gravity) oil based mud was located in the well, thus trapping this mud system in the annular space between the well casing and the straddle systems tubing joints. In preparation for this temporary P&A of this well then the oil based drilling mud system was displaced with a 2.0 s.g. caesium formate mud. This would then leave the inner surface of the straddle system exposed to the caesium formate mud for the duration of the temporary P&A of this well. The temperature at the location of the straddle system was approximately 155 °C. During the re-completion of this HPHT well the straddle system was retrieved and, when received onshore, visually inspected for external corrosion damage. Then specific items were selected for further in-depth corrosion analysis. This paper presents the results of the in-depth investigation performed on the materials used in the construction of this straddle system.