Materials Performance

MAY 2017

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45 NACE INTERNATIONAL: VOL. 56, NO. 5 MATERIALS PERFORMANCE MAY 2017 While the nonpolar molecules (e.g., hydro- carbons, N 2 , and O 2 ) interact via weak dis- persive forces, stronger interactions occur between dipolar molecules such as H 2 O and H 2 S, and quadrupolar molecules such as CO 2 . All of these interactions contribute to the nonideality of the fluid, which directly affects the thermodynamic properties and corrosive behavior of aggressive species. The nonideality of a fluid is best quantified using the compressibility factor z, which is defined in Equation (1): z Pv RT PV n RT T = = (1) where P is the pressure, v is the molar vol- ume, R is the gas constant, T is the temper- ature, V is the total volume, and n T is the total number of moles. The compressibility factor is equal to 1 for ideal gases (i.e., those in which the molecules do not interact). W hile the ideal gas ser ves as a useful approximation for gases at low pressures, the compressibility factor can be much lower than 1 for real f luids. Typically, the compressibility factor ranges from 0.2 to 0.3 at the critical point, and it can be sub- stantially lower for subcritical liquids at any pressure and supercritical f luids at elevated pressures. The fugacity coefficient is a direct mathematical consequence of the compressibility factor. 1 While the com- pressibility factor has a direct relevance to volumetric properties, it is the fugacity coefficient that determines phase equilib- ria, including the partitioning of gases. Aqueous Solutions The ideal gas state is too remote from the properties of aqueous systems to be of use for practical en gin e erin g mo d el s. Instead, a different reference condition is needed. This reference is defined as a pure liquid solvent (usually water), in which the ions are at infinite dilution. The advantage of this reference is that the ions remain as separate, noninteracting entities and do not form ion pairs or other complex species at infinite dilution. The activity coefficient represents the interactions between ions at finite concen- FIGURE 1 Thermodynamic quantities and measurable H 2 S solubilities in the presence and absence of NaCl at 50 °C. The vertical three-phase line separates the vapor-liquid equilibrium (VLE) and liquid-liquid equilibrium (LLE) regions. The H 2 S fugacity (thick solid line) and activity (thick dotted line) extend over the VLE and LLE regions, whereas the partial pressure is limited to the VLE region. trations. At higher concentrations, the activity coefficient also ref lects specific interactions between ions and short-range interactions between neutral molecules. Reviews of methods for the computation of activity coefficients are available in the literature. 2-4 Laboratory Testing Corrosion testing performed to evalu- ate the corrosivity of oilfield production ideally would simulate both the production conditions and the f luid compositions as closely as possible. Unfortunately, many current and future fields operate at tem- peratures and pressures that exceed the rated capacity of most current laboratory test equipment. Additional complications arise from issues such as the safety aspects of handling high concentrations of H 2 S, the flammability of heavy liquid hydrocarbons that are supercritical at reservoir tempera- tures, and the problems associated with the removal of highly insoluble scales (e.g., CaF 2 and BaSO 4 ) that may precipitate in the equipment during the test. Thus, many laboratories make compro- mises while tr ying to remain within the practical operating envelope of the test e q u i p m e n t . T h e s e c o m p r o m i s e s m ay include: • Reducing f lammability by removing the heavy hydrocarbons from the test environm ent and replacing th em with nitrogen. This step does not usu- ally consider that removal of the hydrocarbons changes the compress- ibility factor for the gas phase. If com- pressibility of both the field and test gases is not considered, changing the gas composition without adjusting the test pressure could result in the fugacity of the acid gases being off by as much as an order of magnitude. • Achieving the desired CO 2 and H 2 S partial pressures by working with a gas phase that is composed of only CO 2 and H 2 S. Often this allows the overall pressure to be much lower and further reduces flammability. To do this correctly, the compressibility of the gas stream in the field and the CO 2 -H 2 S test gas must both be calcu- lated and adjusted so that the fugac- ity of the CO 2 and the H 2 S in the test matches the CO 2 and H 2 S fugacities in the field. Matching thereby ensures the proper concentrations of CO 2 and H 2 S in the aqueous phase. An accurate pH determination for both the production and the test conditions should be performed to confirm that the corrosivity of the test environment approx- imates the pH of the f luids found in the field. This can become extremely compli- cated when considering supercritical con- ditions, complex brine chemistry, nonideal multicomponent brines, the solubility of

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