51314-3870-The Physical Chemistry Nature of Hydrogen Sulfide Gas as it Affects Sulfide Stress Crack Propagation in Steel

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Product Number: 51314-3870-SG

ISBN: 3870 2014 CP

Author: Bruce Miglin

Publication Date: 2014

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Industry standard ISO 15156 / NACE MR0175 provides guidance on proper material selection in sour hydrogen sulfide exposures. The guidance is based upon the partial pressure of the acid gasses which have most often been correlated to equivalent partial pressures in laboratory studies conducted at a low total pressure – much lower than that experienced in wells. However higher pressures i.e. above 200 bar (3000 psi)- and temperature among other factors seen in the wells will impact the behavior of acid gasses in the water phase in some instances reducing the electrochemical (corrosive) impact by a factor of 3-10 depending upon the potential critical parameters of hydrogen sulfide activity or solubility. As conditions in real world wells deviate from idealized lab conditions especially high-pressure high-temperature conditions the appropriateness of design based on partial pressure needs to be re-evaluated.This paper describes a laboratory testing program that studied the physical chemistry of hydrogen sulfide (H2S) and its impact on the sour material properties of low alloy steel (alloy steel). In particular this testing program investigated various parameters of H2S to determine the most influential environmental factor in sulfide stress crack propagation. The program used an alloy steel typically installed in mildly sour wells to see if H2S gas concentration (ppm) gas partial pressure (pH2S psia) gas fugacity (fH2S psia) aqueous solubility (xH2S ppmaq) or aqueous chemical activity (related to gas fugacity) most strongly influenced crack propagation of the steel - while other factors such as temperature pH and chloride concentration were held constant.Meeting the objective of this testing entailed: a) Computer modeling of the physical chemistry of H2S gas as a function of pressure and gas mixture compositions; b) Selection of test parameters based on the computer models to provide sufficient distinction amongst the physical chemistry parameters to be seen in sour material toughness tests; c) Selection of an alloy steel grade with demonstrated modest susceptibility to sulfide stress cracking for testing in quantitative manner; d) Use of a modified standard test method providing quantifiable sour performance results e) Selection and prove-up of a test facility to execute the required testing; and f) Rigorous statistical data analysis of the results to establish 95% confidence limits on the conclusions.