American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc.
Discussion of this paper is invited. Three copies of any discussion should be sent to the Society of Petroleum Engineers office. Such discussion may be presented at the above meeting and, with the paper, may be considered for publication in one of the two SPE magazines.
An experimental program has been performed to determine the effect of drill fluid pH on the corrosion-fatigue performance of Grades D and E drillpipe material. Actual drill fluids were used as the corrosive environment through the entire test program. Test data showed that drill fluid pH level (ranging from 8.0 to 10.0 for nondispersed bentonite mud and 9.0 to 11.0 for dispersed lignosulfonate mud) is not a valid measurement for corrosion control.
The performance of metals under the simultaneous occurrence of cyclic stress and corrosion is governed by the phenomenon of corrosion fatigue. Unlike the air fatigue, the corrosion fatigue of metal can no longer be characterized by what is commonly referred to as "endurance limit." Instead, the term "fatigue limit" is used to characterize the performance of the metal. The corrosion-fatigue performance of the metal. The corrosion-fatigue limit is an arbitrary quantity and is defined as the maximum value of stress at which failure no longer occurs after a certain number of cycles.
In corrosion fatigue the corrosive environment plays a major role in governing the corrosion-fatigue behavior of metals. Specifically, in drill fluid environment, the presence of various dissolved gases in combination presence of various dissolved gases in combination with the pH level becomes an extremely significant factor in governing the corrosion fatigue life expectancy of the drillpipe material in an actual drill fluid environment. Most investigators have used brine solution as the corrosive environment. No conclusive data have been found to substantiate the recent modification of RP7G by the API Task Group on Drill Stem Design and Operating Limits, which states in Sec. 6.2, "The fatigue life of drill pipe will be considerably decreased when it is used in water base drilling fluids with low pH. Although it is difficult to determine the exact lower limit of the pH for precluding this possibility, where pH is the primary method of corrosion fatigue control, many users consider that a mud pH of less than 9.5 will shorten the fatigue life of the drill stem." Jackson et al. showed that a 7 % sodium chloride brine adding just enough NAOH to raise the pH to 9 to 10 range decreased the fatigue life of the steel specimen. Uhlig has stated, "Within the range of about pH 4 to 10 the corrosion rate is independent of pH, and depends only on how rapidly oxygen diffuses to the metal surface." Crawford has shown that in some water raising the pH to 9.5 has increased the corrosion rate. He went on stating that continuing to raise the pH reduced the corrosion rate, but still at a pH reduced the corrosion rate, but still at a pH of 10.5 the rate was higher than at a pH of pH of 10.5 the rate was higher than at a pH of 4.