51316-7613-Effect of Stress Level on the SCC Behavior of 13Cr Steel during HT Exposure to Formate Brines

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Product Number: 51316-7613-SG

ISBN: 7613 2016 CP

Author: Siv Howard

Publication Date: 2016

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Formate brines have been used successfully for drilling and completion since the early 90’s. The heaviest of the oilfield formate brines cesium formate with a density of up to 2.3 g/cm3 has been used extensively in many HPHT field developments. The applications range from standard drilling and completion to long-term suspension. The lighter formate brines sodium and potassium formate have also been used as packer fluids in many high temperature wells. To date no Stress Corrosion Cracking (SCC) failures have been reported in any wells that have been exposed to formate brines.Unfortunately the good field experience has not always been replicated in laboratory testing. For example the API SC-13 Task Group 6 CRA testing program has resulted in extensive failures of typical tubing material when tested in formate brines for 90 days at high temperatures. The extreme discrepancy between laboratory test results and field experience triggered a review of the test conditions used by the API CRA test group. It was noticeable that the API CRA test group used a stress level of 100% of yield measured at ambient temperature. This corresponds to a stress level well above 100% of yield at actual test temperature. Under high temperature test conditions which are typically applied when testing formate brines this means a significant over-stressing of the metals. Such over-stressing means that the material is tested in the plastic region which is not the condition in which it is designed to be used in the field. For tubing connections which are the most vulnerable to SCC API RP 5C5 recommends the use of stress levels of maximum 95% for testing measured at actual test temperature. Realistic design stresses in both pipe body and couplings are significantly lower.A testing program was designed to investigate the effect of stress level on the SCC behaviour of martensitic stainless steel. Modified 13Cr and super 13Cr were tested at stress levels ranging from 80 to 100% of yield measured at test temperature. Tests were also included where the metals were stressed to 100% of yield measured at ambient temperature. All tests were performed for 90 days at a temperature of 177°C which is right at the upper recommended limit for use of martensitic stainless steel. The test results showed clear evidence that stress levels are very important when testing SCC behaviour in formate brines and it is highly recommended not to overstress the metals.