The objective of the laboratory experiments described in this paper was to examine how exposure to a combination of H2S and CO2 would impact on environmental cracking, hydrogen charging and loss of ductility of CRAs and Alloy 718 immersed in calcium bromide brines and high density buffered potassium/cesium formate brines at 40°C (104°F) and 160°C (320°F).
The test results indicate that modified 13Cr-2Mo is susceptible to environmental cracking at 160°C (320°F) in high-density brines contaminated with a mixture of CO2 and H2S. In calcium bromide brine the cracking occurred both at high (160°C / 320°F) and low (40°C / 104°F) temperatures. In the buffered formate brine, cracks were observed on cross-sections at high temperature only.
In contrast, 22Cr and 25Cr seemed to be resistant to environmental cracking in high-density brines under the specific conditions of the experiments. For the bromide brine this was unexpected as a previous test conducted under exactly the same conditions, but with 1% added chloride and without H2S, caused cracking of 22Cr.
The fracturing of an Alloy 718 specimen in calcium bromide brine at 160°C (320°F) needs further investigation.
Tubular goods fabricated from Corrosion Resistant Alloys (CRAs) are commonly used in high-pressure high-temperature (HPHT) oil and gas wells. These alloys must retain their integrity for many years in the face of corrosive attack from production fluids and gases under HPHT conditions. They must also be resistant to corrosion of their annular surfaces by the brines used as completion, workover and packer fluids. It is particularly important that any contamination of the brines during use does not compromise the compatibility between the brines and the CRA. Typical accidental contaminants might be oxygen picked up during handling at the surface, and acid gases (CO2 and H2S) from sub-surface influxes via connection or packer leaks. The accidental influx of air into a packer fluid following depletion of the oxygen scavenger is a known source of tubing failure. The addition of various types of corrosion inhibitors to halide brines are examples of deliberate contamination events that could (paradoxically) introduce new chemistries capable of causing localized corrosion or even cracking.
Operators involved in HPHT well constructions that need high-density brines for well control purposes during completion or workover currently have a choice between using bromide brines or buffered formate brines. The traditional high-density brine system is usually a blend of calcium chloride, calcium bromide and zinc bromide. These acidic blends are corrosive under HPHT conditions and therefore require formulation with corrosion inhibitors which themselves may thermally degrade over time and be a cause of corrosion problems. The more benign and less corrosive alternative is a blend of potassium formate and cesium formate brine. The high-density buffered formate brines do not require corrosion inhibitors and they have the distinct advantage of being capable of formulation into combined HPHT drill-in and completion fluids 1-5. In the North Sea, where the use of buffered formate brines facilitates the construction of long high-angle HPHT wells fitted with sand screens6, the buffered cesium formate brine systems have completely replaced zinc bromide brines in completion and workover operations.