6th International Conference on Gas Hydrates


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Proceedings of the 6th International Conference on Gas Hydrates (ICGH 2008), Vancouver, British Columbia, CANADA, July 6-10, 2008 PETROLEUM HYDRATE DEPOSITION MECHANISMS: THE INFLUENCE OF PIPELINE WETTABILITY Guro Aspenes ∗,1,2, Sylvi Høiland 2, Tanja Barth 1, Kjell Magne Askvik 3, Ramesh A. Kini 4 and Roar Larsen 2 1 University of Bergen, Department of Chemistry, Bergen, Norway 2 SINTEF Petroleum Research, Norway 3 StatoilHydro R&D, Bergen, Norway 4 Chevron Energy Technology Company, Houston, TX, USA ABSTRACT The mechanisms by which hydrates deposit in a petroleum production-line are likely to be related to pipeline surface properties, e.g. pipeline material, surface energy and roughness. In this work, the wettability alteration of pipeline surfaces from contact with oil, as well as the adhesion energy between water and solid in the presence of oil is investigated. Contact angles are determined as a function of solid material and oil composition, for both model oils and crude oils. Although contact angles in oil/brine/solid systems have been extensively reported in the literature, the variety of solids that may mimic a pipeline is limited. In this study, we include various metal surfaces in addition to glass and a coating. Initial results from using near infrared imaging for collecting contact angle data in non-translucent systems are also presented. Keywords: deposition, adhesion, pipeline, metal surface, wettability, surface energy, hydrate NOMENCLATURE Wabc Adhesion energy in a three phase system θ Contact angle γab Interfacial tension between phase a and b INTRODUCTION Pipelines used for petroleum transportation are af- fected over time by the fluids and solids which are contacting them, giving rise to e.g. corrosion, coat- ing by an oil or wax/asphaltene layer or/and general wear of the pipeline. The deposition of hydrates in a production line is likely to be affected by the state of the pipe wall. The molecular forces governing the attraction between two solids, e.g. two hydrate parti- cles or hydrate particle and pipe wall, in the presence of oil and/or water, may be quantified through the adhesion energy. The adhesion energy depends on ∗Corresponding author: Phone:+47 55 54 39 08 Fax: +47 55 54 39 05 Email: guro.aspenes@iku.sintef.no the interfacial tensions between the involved phases. For solid surfaces in contact with crude oil and wa- ter, the interfacial tensions, and hence the wettability of the system, are influenced by adsorption of crude oil components [1], giving wettability states rang- ing from water- to oil-wet. The formation of oil-wet hydrates correlates with non-agglomerating behav- ior and low hydrate plugging tendency [2]. For hydrates that form and grow from a water droplet stuck on the pipe wall, as well as for flow- ing hydrate particles adhering to the wall, the ad- hesion energy depends on pipeline wettability. For instance, a water drop is more likely to adhere to a water-wet than an oil-wet pipe wall. Particle-particle adhesion has been studied to some extent [3], while the effect of wettability of the pipeline wall is, to the best of our knowledge, not studied in the same degree. In the present work, the effect of crude oil on pipeline wettability has been studied. As long as the adhesion between hydrate particles and between hydrate particles and the wall are low, hydrate de- position and plugging are presumably minor prob- lems, unless particle loadings are high. A goal is to make the hydrates flow with the stream as a dis- persion. It has been shown that acid fractions from a non-plugging oil can be added to a plugging oil to change its hydrate plugging tendency into non- plugging [2, 4]. Some biosurfactants have shown to have the same effect [4, 5]. The wettability of a solid can be quantified by the angle θ in the three-phase contact point of a liquid drop in thermal equilibrium on a horizontal surface. The contact angle θ is defined here as the angle mea- sured through the aqueous phase, see Figure 1. Figure 1: Sketch of a sessile drop, oil/water/solid system. The relationship between interfacial tension and contact angle was established by Young [6] and is generally known as Young’s equation cosθ = γso− γsw γwo (1) where γ is the interfacial tensions between the three different interfaces solid/oil, solid/water and wa- ter/oil. Surfaces with contact angles lower than 90 degrees are considered as water-wet, whereas angles larger than 120 degrees correspond to oil-wet sur- faces. For the intermediate angles, the surfaces have no preference for one or the other liquid phases. MATERIALS AND METHODS Materials The solid surfaces that have been investigated are stainless steel (AISI 316 L), aluminum (EN AW 5052), brass (63% Cu, 37% Zn), glass, quartz and two epoxy surfaces coated in two different ways (Epoxy-A and Epoxy B). The surfaces have been washed thoroughly with a detergent (sodosil (RM 01) from Riedel-de Haën) and soaked in distilled water. The surfaces were then flushed with ethanol (p.a. quality) before drying the surface with nitrogen gas directly before use. The model oil consists of petroleum ether (J. T. Baker and Riedel-de Haën, boiling range 60-80 °C) with commercial naphthenic acids (Aldrich) in con- centrations ranging from 0 to 5000 ppm. The aqueous phase is a buffer solution (titrisol from Merck) of pH 6 (± 0.05), consisting of 0.16 mol/l NaOH (aq) and 0.06 mol/l citric acid (aq) (C6H8O7). The crude oil is supplied by StatoilHydro ASA and given the name B4c. Metal surfaces The composition of the different metal alloys was given by the supplier, and has been verified through x-ray element analysis. All the metal surfaces are covered with an oxide layer [7]. The stainless steel alloy consists of approximately 18 % chromium, 12 % nickel, 2 % molybdenium, small traces of carbon and silicon and approximately 68 % iron. The surface composition of the stainless steel surface is dependent on chemical composition, temperature and pH of the aqueous solution with which the solid metal is in contact. At low pH (<6) and in air at room temperature a spontaneous for- mation of a protective surface layer makes the stain- less steel corrosion resistant. The main components of the passive layer are insoluble chromium (Cr2O3 and/or Cr(OH)3) and traces of iron oxide (mostly Fe2O3) [8–10]. The aluminum alloy has approximately 2 % magne- sium and traces of silicon, iron, copper, manganese and zinc with amounts varying from 0.1 to 0.5 %, giving an amount of aluminum of approximately 96 %. Due to the high reactivity of aluminum, with reduction potential of -1.66 volts, the surface con- sists of a thin layer of Al2O3 which protects the sur- face from further reaction and is rapidly self-healing when scratched. Both alloying elements of the brass alloy react with oxygen in air giving copper oxide (CuO) and zinc oxide (ZnO). Glass surfaces The quartz cells mainly consist of silicon oxide (SiO2), while optical glass is silicon oxide contain- ing some impurities such as borate. For the model oil systems glass cells were used, while both quartz and glass were used in the crude oil experiments. Epoxy coated surfaces The epoxy coating delivered from Jotun AS consists of two parts that is mixed shortly before use. Part A consists of bis(oxyranylmethyl)ether Bisphenol F (50-100 %), alcyl (c10-c16) glycidyl ether (2,5-10 %) and benzyl alcohol (2,5-10 %). Part B consists of 3,6-diazaoctanethylenediamin (10-25 %). The final composition consists of 6.5 parts of A and 1 part of B by volume. Jotun AS state that the coating has excellent durability against solvents and chemicals and very good durability against water. The surfaces were rinsed before coating. The coat- ing was applied to all the different metals (Alu- minum, Brass and Steel). However, the type of metal used showed to have no effect on the measured con- tact angles on the surfaces, since the layer of epoxy was fairly thick. When the two parts A and B were mixed the blend behaved like glue, making it diffi- cult to obtain an even and smooth layer. Two meth- ods were used to coat the metal surfaces, named epoxy-A and epoxy-B. Epoxy-A was obtained by simply applying the coating to the surfaces and pol- ish it with sand paper (P80) after hardening, giving a rather smooth surface. Epoxy-B was obtained by diluting the mixed coating in acetone (proportion ∼ 1 g epoxy : 1 ml acetone) and pouring the solution over the surfaces. Epoxy-B was a bit rougher than Epoxy-A from a visual point of view. Methods Contact angle measurements All experiments were performed at room temper- ature, approximately 21 °C. The metal surfaces were aged in the oil solution for approximately 24 hours. For the model oil systems the surfaces were submerged in a glass cell which was filled with petroleum ether containing naphthenic acid with concentrations ranging from 0 to 5000 ppm. A water drop (buffer) was deposited on the surface. For the experiments with crude oils the cuvettes (quartz and glass, 1x1 cm) were aged in the oil for approximately 24 hours at approximately 50 °C to avoid wax precipitation. The same brine used for the model oil systems (buffer solution, pH 6) was employed. The contact angles were determined from image analysis. The camera used is a Retiga Exi fast 1394 from QIMAGING with a spectral response between 400 and 1000 nm. This camera is connected to a mi- croscope (SMZ800) from Nikon. The software used is ImagePro Plus. Near Infrared imaging has been used for collecting contact angle data in non-transparent systems. An optical filter delivered from Edmund Optics with re- flection below 800 nm (RM-90) was used to take advantage of the most transparent region of the oil. The light source used is a Quartz Tungsten Halo- gen lamp delivered from Newport, with a maximum spectral efficiency between 500 and 1500 nm. All angles have been evaluated using an axisymmet- ric drop shape analysis - profile (ADSA-P) method that measures the angle from the complete drop shape profile. The contact angle can also be de- termined by manually setting the tangent, but this method is associated with some degree of subjectiv- ity [11]. The ADSA-P determines the contact angle from combining interfacial tension and gravity prop- erties. Between 8 and 12 parallel measurements were per- formed for each system, and the standard deviation calculated. Interfacial tension The interfacial tensions, γ , were measured by the drop weight method [12], using Harkins-Brown equation [13] γ = (V∆ρg) (2pirF) (2) where V is the drop volume, ∆ρ , is the difference in density of the two phases, g is the acceleration due to gravity (g = 9,81 m/s2), r is the radius of the nee- dle, and F is a correction factor which is based on the radius of the needle and the volume of the droplet. The densities were determined using an Anton Paar DMS60 densitometer connected to an Anton Paar DMA602HT measuring cell. Surface energy The state of a solid surface can be quantified through its surface energy. By approximation, this can be de- termined by using the ”equation of state for interfa- cial tension” (EOS) [14]: γsl = γlv + γsv−2√γlvγsve−β (γlv−γsv)2 (3) β is a constant that has been determined empirically and has an average value of 0.0001247 (mJ/m2)−2 [15]. This equation is used for measurements in solid, liquid, vapour (slv) systems, hence the deno- tions. If the equation above is combined with the Young equation (1), the following relation is ob- tained: cosθ =−1+2 √ γsv γlv e−β (γlv−γsv) 2 (4) The solid surface energy, γsv, can be determined from this equation using various probe fluids with different surface tensions resulting in different con- tact angles, θ . This determination comprises some basic assumptions such as no interaction between the air and solid surface and that the air can be equal- ized with vacuum. The determination also assumes that there is no chemical reaction between solid and probe fluid. One challenge related to this procedure is that the fluid surface tension, γlv, must be larger than the sur- face energy, γsv [15]. This can be explained by the spreading coefficient Sls, given by the equation [16]: Sls = γsv− γlv− γsl (5) If Sls > 0 the liquid will spread on the solid sur- face. Most of the fluids that were tested in this work spread on the surfaces. The choice of fluids not spreading was limited to DMSO, formamide, glyc- erol and water. This indicates that the surfaces in this work are highly energetic, as should be expected. In literature, almost no measurements of surface en- ergies of metals are reported from using this method. Most of the available experimental surface energy data of metals are obtained from surface tension measurements in the liquid state and extrapolated to zero temperature [17]. These values are in the order of 1000 to 2000 mJ/m2 at approximately 300 Kelvin. This gives quite different values than from determin- ing surface energy from solid state with contact an- gle measurements. Adhesion energy The contact angle can be used to determine the ad- hesion energy, which is calculated from rewriting Young’s equation into the Young-Dupré equation [18]: Wswo = γwo(1+ cosθ) (6) The adhesion energy, Wswo, gives the adhesion en- ergy per unit area of a solid surface (s) and water (w) adhering in oil (o), and thus comprises both interfa- cial tension, γwo, between the brine and oil phase, and contact angle, θ , into one parameter. RESULTS AND DISCUSSION Surface energy As mentioned, surface energy showed to be diffi- cult to measure for such highly energetic surfaces as metal surfaces and glass, due to spreading con- ditions for most of the probe fluids. Fluids such as hexane, heptane, decane and benzene with surface tensions between 18 and 28 mJ/m2 spread on all the surfaces while the fluids with higher surface tension than 45 mJ/m2 could be used on some of the sur- faces. An example of surface energy determination (brass) is shown in Table 1. Table 1: Surface energy determined for brass. Probe Surface tension Angle Surface energy fluids γ(mJ/m2) θ (degrees) γ(mJ/m2) Hexane 18.5* Spreading - Heptane 20.3* Spreading - Decane 23.9* Spreading - Benzene 28.9* Spreading - DMSO 45.1 20.6 ± 2 42.3 Formamide 59.2 45.8 ± 2 42.9 Glycerol 65.7 47.5 ± 2 46.4 Water(dist) 73.0 53.2 ± 3 46.9 Water(buffer) 73.6 55.4 ± 3 45.6 Average 45 ± 2 *Values from literature [15]. The ”equation of state for interfacial tension” (EOS) method, see Equation 4, was used for determining the surface energy for all the surfaces , i.e. stain- less steel, glass, aluminum and epoxy coated sur- faces. The average values and standard deviations from these experiments are given in Table 2 and Fig- ure 2. The variation between some of the values obtained was rather large and obvious outliers were eliminated from the data set. Table 2: Surface energies for all the solids. Solid Surface energy Probe fluids surface γ(mJ/m2) used Epoxy-A 24 ± 2 3 Epoxy-B 29 ± 2 4 Brass 45 ± 2 5 Aluminum 59 ± 1 2 Stainless steel 64 ± 5 2 Glass 65 1 2 Pipeline model surfaces Epoxy-A Epoxy-B Brass Aluminum Steel Glass Su rfa ce  e n e rg y (m J/ m 2 ) 0 20 40 60 80 Figure 2: The different surface energies studied in this work. Podgornik et al. [19] recently reported surface en- ergy for steel determined with contact angle mea- surements and probe fluids, giving a value of ap- proximately 30 mJ/m2. This is somewhat lower than the value determined in this work, but it should be noted that our material is stainless steel. The surface energies for the solids range from 24 to 65 mJ/m2. For glass, only one probe fluid could be used. Therefore, the value given for glass has no standard deviation. Model oils As mentioned above, it has been shown that acid fractions can change the hydrate plugging tendency by making the hydrate surface less water-wet [4]. A wettability alteration of the pipeline towards less water-wet behavior from adsorption of e.g. petroleum acids, is likely to reduce the possibility of hydrate deposition, and thus the plugging tendency. The results from the contact angle measurements are presented in Table 3 and Figures 3 and 4. The fig- ures show the contact angles as a function of acid concentration. The results are presented in two sep- arate graphs, Figure 3 and 4, because the acids seem to have different adsorption behavior on the differ- ent surfaces. Figure 3 illustrates the metal surfaces, while Figure 4 illustrates the glass and epoxy coated surfaces. Concentration acids (ppm) 0 1000 2000 3000 4000 5000 6000 An gl e  (de gr e e s) 40 60 80 100 120 140 160 180 Brass Stainless steel Aluminum Figure 3: Contact angles of water drops on metal surfaces in petroleum ether with different concen- tration of naphthenic acids. Concentration acids (ppm) 0 1000 2000 3000 4000 5000 6000 An gl e  (de gr e e s) 0 20 40 120 140 160 180 Epoxy-A Epoxy-B Glass Figure 4: Contact angles of water drops on glass and coated surfaces in petroleum ether with differ- ent concentration of naphthenic acids. From the graphs we can see that the angles increase with increasing concentration of acids, i.e. towards more oil-wet conditions. Thus, the experiments in this work show that petroleum acids are able to change the wettability of the pipe surface, in a sim- ilar manner as it can change the wettability of the Table 3: Measured angles of buffer drops on different surfaces with different concentrations of naphthenic acids in petroleum ether. Concentration Epoxy-A Epoxy-B Brass Aluminum Steel Glass Naphthenic acids Angle Angle Angle Angle Angle Angle ppm θ (degrees) θ (degrees) θ (degrees) θ (degrees) θ (degrees) θ (degrees) 0 145± 2 133± 4 119± 8 71± 3 73± 9 29± 3 500 - 132± 4 - 128± 11 - - 700 157± 4 - 119± 5 - 77± 9 30± 3 1000 158± 3 141± 2 135± 4 150± 4 76± 10 20± 3 1500 158± 4 140± 2 133± 3 - 113± 9 36± 4 2000 163± 4 148± 3 131± 4 152± 2 111± 4 36± 4 5000 174± 2 145± 5 129± 3 142± 8 111± 6 40± 3 hydrate surface. The change in wettability is likely to be caused by adsorption of acids onto the surfaces. The surfaces seem to get saturated with acid molecules at dif- ferent concentrations. The metal surfaces have sat- uration limits at approximately 1000-1500 ppm of acids, while the other surfaces have a rather smooth increase from 0 to 5000 ppm of acids. The alu- minum surface is most influenced by the acids, while brass is the least affected surface as far as the metal surfaces are concerned. The standard deviations of the measurements are rather large, which is true for most contact angle measurements, in general [15]. The two different epoxy coated surfaces give differ- ent contact angles even though they are made of the same material. The two curves are virtually the same but only with an upward shift. The difference can be due to different physical structure of the coating, such as surface roughness. The interfacial tension between petroleum ether, with different concentrations of naphthenic acids, and buffer solution is shown in Figure 5. This can be combined with the contact angles to give an adhe- sion energy, as defined in Equation 6. The value ob- tained indicates the adhesion energy between brine (buffer) and a solid surface in the presence of oil. The adhesion energies are given in Table 4. Adhesion energy as a function of acid concentration is shown in Figures 6 and 7. Figure 6 illustrates the metal surfaces, while Fig- ure 7 illustrates the glass and epoxy coated surfaces. The curves are similar to the results from the con- tact angle measurements, only inverted. The results indicate that the adhesion of water to a solid surface decrease with increasing concentrations of acids in Concentration acids, ppm 0 1000 2000 3000 4000 5000 6000 In te rfa ci a l t e n si o n  (m N /m ) 18 20 22 24 26 28 Figure 5: Interfacial tension between buffer solution and petroleum ether with different concentrations of naphthenic acids. an oil/brine/solid system. In order to fully understand what actually happens for each of the surfaces the surface-acid interactions are important. This is beyond the scope of this present paper. However, some brief considerations are given below. The interaction is believed to be driven by a Lewis Brønsted type acid-base chemisorption between the reactive carboxylic acid head and specific surface re- active sites on the metal surfaces [20–23]. Different chemical reactions results in different ori- entation of acids on the surfaces [23]. The different effect of acids on the contact angles on metal sur- faces has been suggested to occur due to difference in reactivity [24], where aluminum is a more reac- tive metal than the other metal samples tested here. There seems to be a correlation between adhesion Table 4: Adhesion energy between buffer solution and different surfaces, in different concentrations of naph- thenic acids in petroleum ether. Concentration Epoxy-A Epoxy-B Brass Aluminum Steel Glass Naphthenic acids Adhesion Adhesion Adhesion Adhesion Adhesion Adhesion energy energy energy energy energy energy ppm mJ/m2 mJ/m2 mJ/m2 mJ/m2 mJ/m2 mJ/m2 0 5 ± 1 9 ± 1 14 ± 3 36 ± 1 36 ± 4 52 ± 1 500 - 8 ± 1 - 9 ± 4 - - 700 2 ± 1 - 12 ± 2 - 29 ± 4 44 ± 1 1000 2 ± 0 5 ± 1 7 ± 1 3 ± 1 28 ± 4 44 ± 0 1500 2 ± 1 5 ± 1 7 ± 1 - 14 ± 3 40 ± 1 2000 1 ± 0 3 ± 1 7 ± 1 3 ± 0 14 ± 1 39 ± 1 5000 0 ± 0 4 ± 1 8 ± 1 4 ± 2 13 ± 2 36 ± 1 Concentration acids (ppm) 0 1000 2000 3000 4000 5000 6000 Ad he si o n  e n e rg y (m J/ m 2 ) 0 10 20 30 40 Brass Stainless steel Aluminum Figure 6: Adhesion energy between buffer solution and metal surfaces in petroleum ether with different concentrations of naphthenic acids. Concentration acids (ppm) 0 1000 2000 3000 4000 5000 6000 Ad he si o n   e n e rg y (m J/ m 2 ) 0 10 40 50 60 Epoxy-A Epoxy-B Glass Figure 7: Adhesion energy between buffer solution and glass and coated surfaces in petroleum ether with different concentrations of naphthenic acids. energy and solid surface energy. This is shown in Figure 8. The adhesion of water to a solid surface in oil is lower on surfaces with low surface energies, meaning that the aqueous phase has less tendency to stick to these surfaces. The graph also shows the decrease in adhesion energy as the concentration of acids is increased. The surfaces have become more oil-wet. Aluminum deviates somewhat from the other sur- faces, most likely because of surface reaction with the acids. Surface energy (mJ/m2) 20 30 40 50 60 70 A d h e s i o n e n e r g y  ( m J / m 2 ) 0 10 20 30 40 50 60 0 ppm 5000 ppm E p o x y - A E p o x y - B S t e e l A l u m i n u mB r a s s G l a s s Figure 8: Correlation between solid surface energy and adhesion energy of brine in petroleum ether with different concentrations of acids, 0 and 5000 ppm. The relevance of this fundamental work to real- istic pipeline surfaces, being rough and corroded, might be discussed, but the surfaces employed in this work are nevertheless representative for materi- als frequently used in laboratory equipment. Hence, the results are of direct value for understanding the influence of pipeline wettability on hydrate deposi- tion in e.g. wheel tests and bench scale flow loops. From experimental observations during wheel tests, a water drop stuck on the pipe wall has shown to convert to hydrate when entering the relevant P, T domain. Furthermore, the tests will be able to reveal fundamental knowledge of a generic kind. In general, the results from this work suggest that hydrates will stick less to surfaces with low surface energy and when the surrounding oil has a high con- centration of acids. Experiments with model systems can also be useful for explaining results obtained in experiments with crude oils, as shown in the next section. Real oil systems Experiments from a crude oil system are presented in Table 5. The oil tested here gives a contact an- gle larger than any of the model oil experiments performed on glass (see Table 3). The angle and the resulting adhesion energy are compared with the model oil system containing 5000 ppm of acids, which was presented in Tables 3 and 4. The contact angle is larger for the crude oil system compared to the model oil system. However, due to different in- terfacial tensions of the systems, the adhesion ener- gies for the systems are quite similar. Hence, the sur- face is rendered more oil-wet from contact with the crude oil, but the brine does not adhere any stronger to it. Table 5: Contact angles and adhesion energies of brine on glass in crude oil, and in a model oil with 5000 ppm acids. Oil phase Contact Interfacial Adhesion angle tension energy θ (degrees) mJ/m2 mJ/m2 Crude oil 86 ± 6 33 35 ± 3 Model oil 40 ± 3 20 36 ± 1 The amount of extracted acids of the B4c oil is de- termined to be 11.8 mg/g oil [25]. This is a con- centration of 11 800 ppm of acids. At this point no experiments has been performed on higher concen- trations than 5000 ppm. However, the contact an- gle will probably remain unchanged after a certain concentration from the surface sites being filled, as shown for the metal surfaces. CONCLUSIONS The wettabilty of pipeline surface material has been measured after exposure to oil. Petroleum acids are found to render all the surfaces more oil wet. The effect of acids on the adhesion in a brine/oil/solid system has also been tested. The re- sults show that: • The adhesion energy between brine and solid surface in oil depends on the solid surface ma- terial. The adhesion energies can be correlated to the initial solid surface energies of the sur- faces. • Acids affect the adhesion in the sense that an increase in acid concentration leads to a reduc- tion in adhesion and more oil-wet conditions. • Preliminary experiments with epoxy coated surfaces indicate that surface roughness can have an effect on the adhesion in brine/solid/oil systems. • It is indicated that hydrates will stick less to the pipeline surfaces when acids are present in the oil. ACKNOWLEDGMENTS StatoilHydro and Chevron ETC are acknowledged for funding and permission to publish data. The Norwegian Research Council, the Petromaks pro- gram, is thanked for funding. The HYADES project group consists of experienced research personnel from both university, research in- stitution and industry, that actively participates in planning of activities and discussion of results. The in-kind contributions from both university and in- dustry partners are of essential value for the quality of the project, and are thus highly appreciated. The project group consists of the following persons: • SINTEF Petroleum Research: Roar Larsen, David Arla, Sylvi Høiland, and Jon Harald Kaspersen. • University of Bergen: Tanja Barth, Alex Hoff- mann, Pawel Kosinski, Anna E. Borgund, Guro Aspenes (PhD student), Boris Balakin (PhD student), Ziya Kilinc (MSc student), and Håkon Pedersen (MSc student). • Chevron ETC (Houston): Ramesh Kini, Lee Rhyne, and Hari Subramani. • StatoilHydro: Per Fotland and Kjell M. Askvik. Arta AS is acknowledged for providing the metal samples. REFERENCES [1] Høiland Standal S. 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