Abstract

Formate brines first entered into service as high performance High Pressure High Temperature (HPHT) drilling, completion, and workover fluids in the mid 90’s. Over the years the range of applications has extended to include use as long-term HPHT well suspension fluids and HPHT well packer fluids. Throughout this time there have been no reported occurrences of Stress Corrosion Cracking (SCC) failures, despite lengthy exposures to acid gases under extreme hydrothermal conditions. This field experience, however, has unfortunately not always been replicated in the laboratory. For example an API(1) task group has reported instances of SCC failures of samples of martensitic stainless steel (MSS) tubing materials after 90-days exposure to formate brines in the presence of acid gases at high temperatures. This discrepancy between laboratory test results and field experience triggered a review of the test conditions used by the API group. It was noticeable that the API group stressed their MSS samples to 100% of actual yield stress (AYS) at ambient temperature. This represents a stress level well above 100% AYS at actual test temperature. Such over-stressing means that the material is tested in the plastic region, which is not where it is designed to be used. This paper describes a laboratory investigation into the effect of stress level on the 90-days SCC behavior of 13Cr MSS exposed to formate brines and CO2 at high temperature. The SCC tests were conducted at stress levels ranging from 60 to 100% AYS measured at test temperature. Tests were also included where the 13Cr MSS alloys were ‘over-stressed’, i.e. stressed to 100% AYS measured at ambient temperature. All tests were performed for 90 days at a temperature of 177°C (350°F). The test results showed clear evidence that stress levels are very important when testing SCC behavior in formate brines, and it is highly recommended to take stress-levels into consideration when designing the test.

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