腐蚀学-二氧化碳 抗化剂

Copyright ©2008 by NACE International. Requests for permission to publish this manuscript in any form, in part or in whole must be in writing to NACE International, Copyright Division, 1440 South creek Drive, Houston, Texas 777084. The material presented and the views expressed in this paper are solely those of the author(s) and are not necessarily endorsed by the Association. Printed in the U.S.A. CO2 CORROSION INHIBITOR PERFORMANCE IN THE PRESENCE OF SOLIDS: TEST METHOD DEVELOPMENT Anette Pedersen,1 Katerina Bilkova,2 Egil Gulbrandsen3, Jon Kvarekvål Institute for Energy Technology (IFE) P.O. Box 40 NO-2027, Kjeller, Norway ABSTRACT The background and development of three test methods for CO2 corrosion inhibitors in presence of solids are described: 1) Inhibitor performance testing in the presence of suspended solids; 2) Inhibitor performance testing on steel covered with sand deposits; and 3) Assessment of sand aggregate formation with oil in the presence of corrosion inhibitors. Key words: Corrosion, Inhibition, Carbon dioxide, Carbon steel, solid particles INTRODUCTION Carbon steel used in combination with corrosion inhibitors is an economically favorable alternative for multiphase pipelines compared to the use of corrosion resistant materials. CO2 corrosion inhibitor formulations contain surface-active compounds that form a protective layer at the pipe wall. The surfactants also adsorb to other surfaces and interfaces in the produced fluids, like solid particles and emulsion droplets, etc.1-9 The solid particles may comprise sand and clay from the reservoir, and corrosion products and mineral scale. Fine solid particles, such as kaolinite clay, can consume a significant amount of corrosion inhibitor by adsorption.2 This consumption may cause the inhibitor concentration to drop below the minimum effective concentration, and may lead to corrosion failure if the depletion of the inhibitor is not properly accounted for.3-4 Solids can affect detrimentally not only inhibitor performance, but many other aspects of the petroleum production as well. Sand particles are known to cause erosion corrosion. They can also deposit in some part of the pipelines or in separators, and cause severe attacks under the deposit.3 Corrosion inhibitor performance testing in presence of solids has received little attention so far. 1 Present address: Det Norske Veritas, NO-1322, Høvik, Norway. 2 Present address: Intecha, 170 00 Praha, Czech Republic. 3 Presenting author, e-mail: egil.gulbrandsen@ife.no 1 Paper No. 08632 The present paper describes parts of the results obtained in a recent joint industry project (see Acknowledgement section), where the objective was to develop test methods and test protocols for laboratory testing of CO2 corrosion inhibitor performance. The paper focuses on the background and development of three methods for corrosion inhibitor testing in the presence of solids. The first method is used to assess inhibitor performance in presence of suspended solids. The second method is used to test inhibitor performance under sand deposits. The third method is used to assess aggregation of sand and oil caused by presence of surface-active corrosion inhibitors. The intention of the work was not to compare inhibitors. The inhibitors were used as example products in the development of the test method and identification of critical parameters for the testing. The presented test methods may also contribute to the development of improved inhibitor products. TESTING OF INHIBITOR PERFORMANCE WITH SUSPENDED SOLIDS This method was developed for screening tests of inhibitors and determination of the effective inhibitor concentration in the presence of suspended solids. The testing method is intended for field cases when solids are continuously produced in amounts that may reduce inhibitor performance. Kaolinite clay was used as a model solid with well-defined surface area. Clay is added prior to inhibitor addition in the test method. This resembles the case where the solids are already present at the point of inhibitor injection. The effect of oil was also studied. Experimental Procedure All the tests were carried out in glass cells with a volume of 3 liters. Electrodes for electrochemical corrosion monitoring and pH measurements, temperature probes and gas inlets/outlets were inserted through stainless steel lids. The glass cell with accessories is schematically represented in Figure 1. A heating plate was used to control the temperature of the test solution, and the solution was gently stirred with a magnetic stirrer. The test specimens were machined from X65 carbon steel. The element analysis and microstructure of the steel are given in Table 1. Cylinder specimens with surface area of 3.14 cm2 were ground to 1000 mesh with wetted SiC paper prior to use. The specimens were wetted with isopropanol, cleaned with technical acetone in ultrasonic bath, and rinsed with ethanol. Then the specimens were blow dried prior to mounting on the specimen holders. The test solutions were prepared from technical grade NaCl and distilled water. The pH was adjusted by addition of AR grade NaHCO3. The test solutions were continuously purged with CO2 grade 4.0. This gas contains less than 10 ppm of O2, corresponding to less than 0.5 ppb dissolved in the solution in equilibrium with the gas at 1 bar and 25 ºC. 2 The kaolinite clay was a reference clay, denoted KGa-2, from The Clay Mineral Society, University of Missouri, USA.9 The clay properties are given in Table 2. The surface area constituted by the clay is expressed in terms of surface area per unit volume of test brine (m2/L). The standard kaolinite clay (KGa-2) has a specific surface area of 23.5 m2/g. A clay concentration of 43 ppm therefore corresponds to an area of 1 m2/L. A scanning electron image of the clay platelets, which were 0.1 µm–0.5 µm thick and 0.5-2 µm long, is shown in Figure 2. The inhibitors used for the testing are listed in Table 3. The inhibitor concentrations are reported as ppm (i.e. mg/L) of blended product based on total liquid volume (water + oil). The properties of the oils used in the tests are listed in Table 4; Oil 1 is a black oil, Oil 2 is a condensate, while Oil 3 is a refined, low-aromatic oil product. The oils were tested with Inhibitor M. Table 5 gives the partitioning of the inhibitor between the phases. A potentiostat with a multiplexer was used for electrochemical measurements. The corrosion rate was monitored by the polarization resistance method (LPR) throughout the test. LPR measurements were performed in three-electrode configuration, i.e. working electrode - carbon steel specimen, reference electrode - Ag/AgCl, auxiliary electrode - Ti ring. The potential ramp for the LPR measurement was –5 mV to +5 mV vs. Ecor, with scan rate 0.1 mV/s. The corrosion currents was calculated as B/Rp . A constant value of B = 20 mV was used, based on polarization curves and mass loss determinations, as described in Refs. 13-14, and Appendix A. The corrosion rates are reported as average penetration rates (1 A/m2 corresponds to 1.16 mm/y). The polarization resistance was compensated for IR drop, determined by means of impedance spectroscopy (EIS). Potentiodynamic polarization curves were measured at the end of the test. The cathodic polarization curve was run from 0 to –250 mV vs. Ecor, and the anodic one from 0 to +150 mV vs. Ecor, both at scan rate 0.1 mV/s. All the specimens were inspected for localized attack by an optical microscope after the test. The tests were carried out at 60 °C, 0.8 bar CO2 and pH 4.5. Most of the tests were performed in 10 %(w/w) NaCl brine. The solution in the glass cell was deoxygenated for 2-3 hours before the immersion of test specimens. Three specimens were introduced to each cell; two specimen for electrochemical measurements and one specimen for weigh loss measurements. The steel specimens were precorroded for 24 hours prior to addition of clay and inhibitor. Kaolinite clay was added to the cell 10 min before the inhibitor addition. In the tests performed with 10 % oil, the oil was de-aerated with 1 bar CO2 for minimum 4 hours and heated to test temperature before it was transferred to the test cell. 300 ml oil was transferred to the 3-litre test cell after start of precorrosion, and minimum 2 hours before the clay and inhibitor was introduced. A peristaltic pump was used to transfer the oil. The inhibitor was added to the cell from a 10 % stock solution, diluted with either oil or water. 3 Results and discussion Inhibitor testing without oil. Examples of the experimental data from one test without kaolinite clay and one test with 2.5m2/L surface area of clay (108 ppm) are given in Figure 3. The baseline corrosion rate was approximately 3 mm/y. Kaolinite clay and 30 ppm of Inhibitor A was added after 24 hours of precorrosion. The inhibitor concentration was increased to 50 ppm towards the end of the test. The addition of the inhibitor resulted in a rapid decrease of the corrosion rate. The corrosion rate in the test with no clay continued to decrease until it stabilized at about 0.06 mm/y. The corrosion rate with clay stabilized at a higher corrosion rate (0.15 mm/y). This shows that the clay reduced the inhibitor performance. Figure 4 summarizes the steady corrosion rates at the end of the tests, and residual concentrations of the active inhibitor compounds vs. the surface area of clay. The concentrations of the active compound given as the corresponding concentration of the formulated product. The residual analysis was carried out by the inhibitor suppliers. The corrosion rates are the steady corrosion rates for 30 ppm dose rate of the respective inhibitor. The data for Inhibitor A illustrates a case when the inhibition effect was remarkably reduced in the presence of clay (Fig. 4a). The inhibited corrosion rate in absence of clay was 0.1 mm/y. The residual inhibitor concentration dropped to 11 ppm with 2.5 m2/L clay surface area. Nevertheless the inhibited corrosion rate increased only slightly to 0.2 mm/y. With 5 m2/L in the solution, the residual inhibitor concentration decreased to 5 ppm. This resulted in an increase of the corrosion rate to 0.4 mm/y. Only 2 ppm of Inhibitor A was left in the solution in the test with 7.5 m2/L clay surface area (323 ppm clay). This depletion was associated with a dramatic rise of the corrosion rate to 2 mm/y, which was nearly the same as the uninhibited corrosion baseline. Similar results were obtained with Inhibitor B (not shown). Figure 4b shows the results obtained with Inhibitor C. Even if the corrosion rate increased slightly with increasing clay concentration, the corrosion rate remained below 0.1 mm/y in presence of even in presence of 7.5 m2/L clay surface area. The residual inhibitor concentration for this inhibitor was determined both for filtered and unfiltered solution sample. Despite some discrepancy between the results obtained with two analytical protocols, both of the data sets indicate that the inhibitor concentration was reduced to less that 20 ppm for 5 m2/L of clay surface area. This shows that inhibitor performance in presence of suspended solids depends strongly on formulation. The results of the tests with increasing dose of Inhibitor B are given in Figure 5. Two tests, one without clay and one with 5 m2/L clay surface area, are shown in the figure. The corrosion rate decreased when 10 ppm of Inhibitor B was added for the test without clay. The corrosion rate continued to decrease with increasing amount of inhibitor until it stabilized when 80 ppm inhibitor was added. The inhibitor performance was negligible for 10 ppm of the inhibitor with 5 m2/L of clay particle surface. With higher inhibitor concentrations the corrosion rate gradually decreased. Eventually at 150 ppm, the corrosion rate dropped to nearly the same value as for the test without clay. This indicates that it is possible to saturate the clay surface with inhibitor by adding sufficient amount of it, and in that way obtain adequate inhibitor performance. 4 Adsorption studies of surfactants on clay5 also showed that there was a limit concentration above which the adsorbed amount of the inhibitor did not increase, thus all the clay surface was saturated with the inhibitor. It was reported that zeta potential of kaolinite clay increased strongly with addition of CO2 corrosion inhibitor, which contained cationic surfactant. Such behavior is consistent with adsorption of cations.2,4 Presence of other cations like Ca2+ can further reduce performance of cationic inhibitors.2 On the other hand anionic inhibitors showed low affinity to clay.2 The problem of parasitic consumption of inhibitor by clay and other fine particles in the produced fluids is a problem of determining and achieving the optimal dose rate. However, the chemistry of the inhibitor plays a key role in the problem. It appears possible to select an inhibitor product with acceptable performance in the presence of solids. Inhibitor testing with oil present. A summary of the inhibitor test results with oil is given in Figure 6. All the experiments were carried out with 30 ppm of Inhibitor M. The corrosion rate generally increased with increasing clay concentration. In presence of clay, the corrosion rate for the tests with Oil 2 was lower than in the tests with the other oils, or those without oil. On the other hand, the corrosion rate with the other oils was higher than the corrosion rate without oil, for most the tests with clay present. The corrosion rate for Oil 1, and Oil 3 with 5 m2/L clay and more was nearly the same as the uninhibited baseline. These results indicate that oil can include components that may enhance or reduce the inhibitor performance, both in absence and presence of clay. The observed effects may rather be related to residual inhibitor concentration, than a direct interaction of oil and clay on the steel surface. The specimen for one test with 10 % of Oil 3 got oil wetted, and the corrosion rate dropped to extremely low values. The oil-wetting phenomenon is not well understood yet; i.e. it is not possible to predict when oil wetting occurs. Conclusions • The inhibitor performance of some inhibitors was reduced in presence of clay due to adsorption. The test protocol allows distinguishing among different inhibitor product in terms of their sensitivity to clay. • The sensitivity of the inhibitor performance to clay depends on the inhibitor chemistry. It appears feasible to formulate inhibitors so that the performance is little affected the present amounts of clay. • For the inhibitors that were affected by the presence of clay, increased inhibitor dosage would efficiently saturate the clay surface and adequate inhibitor performance could be reached. • Laboratory studies to select inhibitors should include experiments in the presence of representative solids at the expected concentration. TESTS OF INHIBITOR PERFORMANCE UNDER SAND DEPOSITS This test method was developed to evaluate CO2 corrosion performance under sand deposits. The objective was to assess the risk of galvanic corrosion, formed due to sand 5 deposition on part of the steel surface, and to evaluate the ability of inhibitors to mitigate this type of corrosion. The test method is intended for application with continuous sand production or with high probability of sand production. Sand deposition prior to inhibitor addition resembles the case of sand deposition under conditions of insufficient inhibition. Inhibitor addition prior to sand deposition represents the case of sand deposition under conditions of adequate inhibition (at least at bare surfaces). The effect of oil has not been included in the test method yet. Experimental Procedure The chemicals and specimen material were the same as described in the previous part of the paper, see Tables 1, 3 and 4. The tests were performed in 10 %(w/w) NaCl aqueous solution. Some tests were carried out at 60 ºC, which correspond to CO2 partial pressure of 0.8 bar. Other tests were performed at 90 ºC with CO2 partial pressure 0.5 bar. The sand used in these tests was analytical grade, acid-washed and calcinated silica sand. The grain size was specified as 200-400 mesh (30-80 micron). However, in-house SEM examination showed particle diameters of 200–600 µm (Figure 7). Estimating by use of a sphere model, the geometric area was less than 0.01 m2/g. Surface area measurements by the BET technique (nitrogen adsorption) indicated ca. 0.1 m2/g, which indicated substantial porosity and roughness of the sand grains. This is to some extent supported by the SEM image. The sand layer thickness was about 5 mm in all tests. The total amount of sand in the cell was approximately 3 g. The sand thus had a total surface area of about 0.3 m2. The test set-up is shown in Figure 8. A specimen assembly consisting of three specimens used in most of the tests; one specimen not covered by sand (abbreviated NS - no sand), and two specimens fully covered by sand (abbreviated as FS1 and FS2 - fully sand covered). Specimens FS1 and NS were galvanically coupled, while the FS2 specimen was not galvanically coupled to other specimens. The area of the NS specimen was ca. 4 cm2, while the areas of each of the FS specimens were 1 cm2. The specimens were molded into epoxy, and pretreated as described in the previous part of this paper. The sand was deposited with negligible air ingress by means of a specially designed glass tube device. The sand was contained between two moveable pistons. In the tests with inhibitor added before the sand, the sand was saturated with inhibitor by flushing with portions of the inhibitor containing test solution. The FS1 and NS specimens were galvanically coupled by means of the ZRA (Zero Resistance Ammeter) facility of the potentiostat. The galvanic current was logged at regular intervals. The corrosion current was measured with the polarization resistance technique in the galvanically coupled mode, with individual ZRA current measurement on each specimen of the couple. The potential was scanned from -5 mV to +5 mV vs. Eoc of the couple, at a scan rate of 0.1 mV/s. The anodic dissolution current calculation is described in Appendix A. The tests were performed in two different ways. The sand was deposited 1-3 days prior to inhibitor addition in the test resembling the case of sand deposition under conditions of insufficient inhibition. In the test resembling the case of sand deposition under conditions 6 of adequate inhibition, the inhibitor was introduced 1-2 days before sand was deposited on the steel surface. The sand surface was then saturated with inhibitor by exposure to portions of the inhibited brine before deposition on the specimen surface. Results Sand deposited before inhibitor addition. The results for a test where sand was added before Inhibitor C are shown in Figure 9. Only NS and FS1 were exposed in this initial test. The baseline corrosion rate without sand was about 2 mm/y. Sand was added 6 hours after specimen immersion. After one day exposure 30 ppm inhibitor C was added. The corrosion rate of the NS specimen (not covered by sand) decreased rapidly due to the inhibition effect of Inhibitor C. The steady corrosion rate for the NS specimen was 0.03 mm/y. The corrosion rate of the FS specimen decreased gradually to about 0.5 mm/y. Increasing the inhibitor concentration to 50 ppm did not have significant effect on the corrosion rates. Sand deposition c