In high pressure high temperature (HPHT) gas condensate wells the use of high density brines is required to ensure safe well operations during both well drilling and completion phases. Caesium formate brine is the latest high-density brine to be used for this application, with the claim that it is both environmentally friendly and none corrosive.
This works presents the results of both corrosion and stress corrosion cracking tests performed with caesium formate brine in simulated HPHT conditionsThe materials tested were low alloy steel grades P-110 and C-110, five supermartensitic stainless steels
(S13Cr) grade 110 ksi, galvanized carbon steel wireline, duplex stainless steel wireline and alloy 718. These materials were tested as single corrosion coupons and four point bent beam specimens.
The test conditions were selected to represent future HPHT Field developments in the Norwegian Continental Shelf.
The results of the test program undertaken show that relatively low corrosion rates were observed on low alloy steels and somewhat higher than expected corrosion rates on the S13Cr in the caesium formate brines. Furthermore, the susceptibility to stress corrosion cracking and localized corrosion was negligible..
In HPHT gas condensate wells the use of high density brines are essential to ensure safe well operations during both well drilling and completion phases. But, several well component failures have been reported recently 1,2 due to stress corrosion cracking in high-density brine applications in HPHT wells.
SCC risk increases when there is a leakage into the production annulus allowing gas containing CO2 and H2S to acidify the packer brine3. In the Arun Field4, annulus corrosion was observed in the presence of CaCl2 brine used as packer fluid. Penetration rate calculated in the range 1.3 to 2.5 mm/yr and a maximum pit depth of 7.6x10-3 (60% of wall thickness). The explanation of this phenomenon was the reaction of CaCl2 and CO2 due to the leakage into the annulus. The bottom hole temperature was 178 °C.
Therefore, it is essential as part of the brine selection process to perform corrosion and stress corrosion cracking tests for brines in an attempt to avoid such catastrophic failures.
Caesium formate and mixtures of caesium/potassium formate have recently been introduced as an attractive alternative because of its environmentally friendly characteristics and reported low corrosivity.
The specific gravity for these brines can be up to 2.3, so they are a suitable replacement for the more corrosive ZnBr2, ZnBr2/CaBr2, ZnBr2/CaBr2/CaCl2, high-density brines.
In order to approve the use of this brine type for several HPHT gas condensate Fields in the Norwegian Sector of the North Sea a test program was embarked upon. The following describes the test program and the results obtained.