Research Projects VI. RESEARCH APPROACH

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1 RESEARCH APPROACH VI- VI. RESEARCH APPROACH Research at the Erosion/Corrosion Research Center (E/CRC) at The University of Tulsa is guided by member companies through planning sessions at the Advisory Board Meetings and through responses to the E/CRC Member Interest Questionnaires. Based on this input, present E/CRC research is focusing on developing models and computer programs for predicting sand erosion, and erosion-corrosion involving a CO 2 environment with sand. Emphasis is placed on computing penetration rates and threshold velocities where the threshold velocity is the limiting flow velocity corresponding to some tolerable rate of penetration. Research Projects The overall research approach is organized into the three research areas -- erosion, CO 2 corrosion, and erosion-corrosion in a CO 2 environment with sand, each area having both experimental and computational components. As a result of many years of research in these areas, three computer programs have been developed for use of E/CRC members and to direct future research in these areas. One computer program is for calculating sand erosion called SPPS (Sand Production Pipe Saver). Another computer program is developed for calculating CO 2 corrosion rates. The results of the erosion modeling and CO 2 corrosion modeling has been also combined in a computer program for calculating erosioncorrosion involving sand and CO2. The efforts in these areas are briefly discussed below. Erosion Research The research in this area is focused in upgrading and improving the E/CRC erosion calculation computer program SPPS. Current capability of the SPPS includes erosion prediction in: Elbows including long radius elbow (multiphase flow) Tee (limited experimental data) Straight pipe (limited data) Direct impingement (additional data is being gathered in liquid) Sudden expansion (additional work with small particles) Plug Tee Gradual and sudden contractions SPPS 2-D (Limited geometries Single phase) An overview of the research program for upgrading SPPS is given by the flow chart shown in Figure VI- and VI-2. The center stem in these flow chart is headed by experimental November 20

2 VI-2 EROSION/CORROSION RESEARCH CENTER projects for determining the erosion resistances of oilfield materials. A compressed air/solid particle impingement apparatus is available for use for evaluating erosion resistances of various materials and for testing effects of solid particle size, sand shape, impingement velocity and impingement angle on erosion. The results of these experiments are incorporated into the erosion equations that are developed for sand erosion. Flow Model and Validation (LDV) Particle Tracking and Validation (LDV) Particle Impact Velocity Erosion Ratio Testing and Modeling Erosion Ratio Equations Erosion Prediction for an Oilfield Geometry Simplified Mechanistic Models for Oilfield Geometries Erosion Testing in Oilfield Geometries SPPS Figure VI-. Flow Chart for Erosion Research (for Single-Phase flow). November 20

3 RESEARCH APPROACH VI-3 Mechanistic Models For Gas-Liquid Flows (Predict Flow Regime & Flow Velocities) Estimate Impacting Sand Rate and Velocities Erosion Ratio Testing and Modeling Erosion Ratio Equations Erosion Prediction for an Oilfield Geometry Simplified Mechanistic Models for Oilfield Geometries Erosion Testing in Oilfield Geometries SPPS Figure VI-2. Flow Chart for Erosion Research (for Multiphase flow). The left side branches in Figures VI- and VI-2 represent computational and modeling projects. Flow modeling involves utilizing mechanistic models that have been developed for multiphase flows (Figure VI-2) as well as the development of computational models (Computational Fluid Dynamics (CFD) based) for describing the flow conditions (velocities, turbulent kinetic energies, shear stresses, etc.) in piping systems. For example, in SPPS 2-D development Fluent CFD code has been used to describe the flow fields for a variety of conditions. The particle-tracking model then uses the CFD data on flow conditions to track the paths of solid particles in the flow to determine the locations, speeds, and angles of impact of the solid particles with the pipe or fitting boundaries. Erosion ratio data and the results of the particle tracking models are then combined to predict erosion given information about the piping system geometry, the flow parameters, and the type, size, and concentration of solid particles. A facility for experimental verification of computer models for flow patterns, turbulent kinetic energy, and particle tracking has been used by the E/CRC. This facility uses a laser Doppler Velocimeter (LDV) to characterize flow and particle velocity in oilfield geometries. Currently, this equipment is being used to November 20

4 VI-4 EROSION/CORROSION RESEARCH CENTER examine effects of particle size and liquid viscosity in a submerged direct impact geometry. In general, the results from this facility are being used to validate the computational fluid dynamics (CFD) based flow and particle tracking results. Modifications are being examined to the particle tracking models to account for particle-near wall interaction and to be able to accurately predict erosion for small particles and fines. For multiphase flow, mechanistic models for predicting flow regimes and flow velocities are used to obtain information about gas and liquid velocities and holdup. Based on these models and particle tracking in a representative simplified flow region, representative particle impacting velocities and impacting sand rate are estimated. Then erosion ratio equations are used to predict erosion for an oilfield geometry. Erosion testing of oilfield geometries in E/CRC flow loops provides feedback to the computational models for improving and validating the models. Geometries tested so far include a coupling, an elbow, a tee, a choke, a contraction, and several expansion sections for both compressed air and water. Multiphase flow loops are also available to perform erosion tests in oilfield geometries for liquid, gas, or gas-liquid flows containing sand. E/CRC and Tulsa University Sand Management Projects (TUSMP) JIP have a large scale tower-boom multiphase flow loops with 2-, 3-, and 4-inch test sections that are being used for erosion testing and sand monitoring. Another multiphase flow loop with a -inch and 2-inch test sections is available for erosion testing as well as sand monitoring. The multiphase towerboom flow loop has been constructed and is being used to conduct erosion experiments in multiphase flow including, slug, churn, annular and mist flow in horizontal and vertical test sections. These flow loops are being used to validate and improve our erosion computer program (Sand Production Pipe Saver (SPPS)) for multiphase flow. The results of erosion research have been issued to member companies in the form of a user-friendly, erosion prediction computer program called "Sand Production Pipe Saver" (SPPS). Using the computer program provides information on velocity thresholds and penetration rate for several oilfield geometries and for a range of oil and gas production conditions. An Excel based version of SPPS has been developed that includes calculations of penetration rates and threshold velocities in multiphase flow when sand production is anticipated. A new version of this program that includes several additional geometries and a 2-D version option is currently available to the members of E/CRC. CO 2 Corrosion Research A flow chart for our previous research on CO 2 corrosion is shown in Figure VI-3. Most of the experimental data for the CO 2 program has been provided by the CO 2 loop. The November 20

5 RESEARCH APPROACH VI-5 CO 2 Loop is a two-phase flow loop circulating carbon dioxide as the gas phase and a 2% to 8% or higher NaCl brine as the liquid phase. Superficial liquid velocities up to 22 fps and superficial gas velocities up to 50 fps are possible in the loop. Velocity effects on the corrosion of AISI 08, API X65 and API N 80 steels have been examined under CO 2 pressures up to 50 psig and temperatures from 35 to 200 degrees F. Some data for this program was also provided by the Three-Phase Miniloop in which sand can be circulated along with the test fluids. CO 2 Loop Loop Miniloop Testing CO 2 Corrosion Corrosion Electrochemistry and and CO 2 Corrosion Prediction Surface Concentrations Concentrations Scale Formation Figure VI-3. Flow Chart for CO 2 Corrosion Research. Data from the test loops and from the electrochemistry and corrosion literature have been used to develop a model for CO 2 corrosion. The model is a mechanistic model and comprises all the processes involved in CO 2 corrosion. A CO 2 corrosion prediction computer program called SPPS: CO2, which was developed by the E/CRC, is based on the model. The program can predict effects of changes in flow velocity, temperature, ph, piping size, CO 2 partial pressure, etc. An important capability of the CO 2 program is its ability to compute surface concentrations of all the species involved in the corrosion process. With November 20

6 VI-6 EROSION/CORROSION RESEARCH CENTER this information, the program can predict the formation of iron carbonate scales and the penetration rate under scale-forming conditions using a mechanistic model for the formation of the protective scales developed at E/CRC. This capability is important not only because the corrosion rate is greatly affected by the presence of protective scales, but also because this knowledge about scale formation is needed to predict erosion-corrosion in a CO 2 environment with sand. In previous research, mechanistic models for predicting mass transfer coefficients in multiphase flow have been developed to extend the E/CRC model to multiphase flow. Mechanistically-based mass transfer models for vertical annular flow, intermittent flow, and bubbly flow have been developed from generalization of the well-known Chilton-Colburn analogy to multiphase flow using heat transfer data from the literature data and E/CRC measurements of CO 2 corrosion rates. Results of this work, together with some experiments in the CO 2 loop that have been carried out, were used to generalize the E/CRC CO 2 corrosion prediction program SPPS: CO 2 to predict corrosion rates in two-phase flow. In recent research, the E/CRC corrosion prediction model and computer program, SPPS: CO2, have been expanded to handle concentrations of NaCl up to 20 wt% by incorporating the Barta and Bradley correlation (984) for predicting the solubility of CO 2 in water as a function of CO 2 partial pressure, Na, temperature, and fugacity. A result of increased NaCl concentrations in CO 2 saturated systems is slightly reduced corrosion rates. Measured corrosion rate from flow loop experiments and from predictions using the Berta and Bradley correlation in SPPS: CO2 have shown very good agreement. Also updated in SPPS: CO2 is the calculation of the solubility product, K sp, for iron carbonate (FeCO 3 ). The new calculation is from a correlation offered by Sun, et al. (2009) based on experimental work by Silva, et al. (2002) showing that the solubility limit is a function, not only of temperature, but also of ionic strength. Using Sun s correlation increases K sp as concentration of NaCl is increased, for example. This trend reduces the likelihood of forming FeCO 3 scales in high NaCl concentration solutions. Erosion-Corrosion Research The E/CRC currently has 4 recirculating test loops capable of handling CO 2 corrosion and Erosion-Corrosion conditions for a broad range of environmental and flow conditions. One of the loops handles sand in gas/liquid flows while the other three are small-scale flow loops capable of circulating a water phase, a hydrocarbon phase, and sand. The small scale loops are suitable to study inhibitor performance in a CO 2 environment with sand. In addition, one of the loops has been recently modified to provide high single phase November 20

7 RESEARCH APPROACH VI-7 liquid velocities, thus providing a capability to extend the studies of inhibitor performance to very high erosivity conditions. The flow loops provide the experimental database for this research. The E/CRC erosion prediction computer program, SPPS, and the corrosion prediction program SPPS: CO2 are employed together with the experimental work to provide the basis for analytical work and modeling for erosion-corrosion. The results of this research provide guidelines for materials selection and inhibitor use in erosion-corrosion environments. Previously, a user-friendly computer program, SPPS: E-C, based on this research was developed. This program provides threshold velocities corresponding to boundaries between scale formation behavior and uniform corrosion and pitting. Then, given the intended flow velocity, the program predicts the erosion-corrosion penetration rate. This work was originally for flow conditions involving a single fluid with sand. New versions of SPPS and SPPS: CO 2 have been used to expand this program to predict threshold velocity in multiphase flow. Currently multiphase flow data on erosion-corrosion is being collected to evaluate and validate the program. This program uses MS Excel spreadsheet with VBA. Testing of inhibitors in sand-bearing fluids has been conducted in single-phase liquid flows in the Miniloop. Researchers from Baker-Hughes Petrolite have provided the inhibitor and expertise to assist in the research. The inhibitor was tested under two different conditions, iron carbonate scale forming and non-scale forming conditions, commonly encountered in CO2 corrosion of carbon steel. The common approach for both test conditions involved acquiring baseline data without inhibitor followed by conducting tests with the inhibitor. This approach highlighted the effect of sand erosion on inhibitor performance. Previous research in erosion-corrosion was directed toward examining the erosioncorrosion of CRA s and toward evaluating inhibitor performance in erosion-corrosion environments. The effects of flow conditions, sand rate, ph and temperature on the erosioncorrosion of 3Cr, Super 3Cr and 22Cr alloys have been widely studied at the E/CRC. Tests were conducted using the scratch test procedure, and the single phase liquid and multiphase gas/liquid flow loops. At high erosivity conditions, a synergistic effect between erosion and corrosion was confirmed. Synergism seems to occur for the three alloys; however, the degree of synergism is quite different for the three alloys and may not be significant for 22Cr under field conditions where erosivities are typically much lower that those occurring in the small diameter test loop. A procedure to predict penetration rates for erosion-corrosion conditions was developed based on the 2 nd order model behavior observed by the re-healing process of the passive film of CRA s under scratch test conditions. Predictions of the corrosion component November 20

8 VI-8 EROSION/CORROSION RESEARCH CENTER of erosion-corrosion based on scratch test data compared well to test results from multiphase gas/liquid flow loop for the three CRA s at high erosivity conditions. Second-order behavior appears to be an appropriate and useful model to represent the repassivation process of CRA s. Good agreement between the actual and predicted penetration rates was found. Current work on effects of sand erosion on inhibitor performance, supported by Petrobras, involves characterizing the inhibitor s behavior in terms of inhibitor adsorption isotherms. For the imidazoline-based inhibitor presently under study, inhibitor effectiveness has been described best by the Frumkin isotherm. K a d C inh = θ θ e fθ Using the Frumkin isotherm, inhibitor effectivenesses for all environments including ph from 3.5 to 6.5 and temperature from 35 to 200 o F, not including sand erosion or an oil phase, were described by a single value of the coefficient K a/d and a single value of the coefficient f. For cases involving an oil phase, a modification of the isotherm by reducing slightly the value of the coefficient K a/d was found to fit the data very well. Likewise, the effect of sand erosion on the inhibitor s efficiency was described very well by reducing the value of K a/d by an amount that is a function of erosivity and temperature. Modifying the value of the adsorption/desorption coefficient K a/d in this way, and integrating the Frumkin isotherm into the CO 2 corrosion prediction program, SPPS:CO2, allows one to predict inhibited erosion-corrosion rate with good accuracy. Work is currently underway to develop a more mechanistic approach to modeling the iron carbonate (FeCO 3 ) scale precipitation and growth process and the erosion process involved in erosion-corrosion for carbon steel. Scale precipitation requires that [Fe 2+ ][CO 2- ] exceed K sp. And the rate of precipitation increases with increasing supersaturation and with increasing temperature. Iron carbonate specimens have been tested for their erosion resistance and characterized as to thickness and porosity. The E/CRC CO 2 corrosion prediction program, SPPS: CO2, is being modified to compare FeCO3 erosion rate with FeCO3 precipitation rate and, assuming steady-state conditions, to determine the scale thickness and corrosion rate at which the growth rate of the scale is equal to the erosion rate of the scale. Figure VI-4 provides a flowchart for E/CRC s research in erosion-corrosion. November 20

9 RESEARCH APPROACH VI-9 SPPS Erosion-Corrosion SPPS:CO 2 ECR, CR E-C Prediction Computer Program SPPS Sand Erosion 2 Multiphase Flow Experiments Extend to Multiphase Flow Model FeCO3 Growth and Sand Erosion 2 2 Inhibitors and Sand CRA s and Sand 3 Current Project Supported by Petrobras (includes oil) Predict Threshold Velocity Erosion-Corrosion Rate. Previous work 2. Current work 3. Future work Figure VI-4. Flow Chart for Erosion-Corrosion Research. Experimental and computational findings throughout the research program are used to guide the design of experiments in the flow systems. Since erosion or erosion-corrosion effects may not be detectable or quantifiable except after considerable exposure of the materials to the flow, it is important to design the experiments carefully so as to use the test loops as efficiently as possible. In experimental work, the main objective is to collect data that will help generalize the computational models for erosion, CO 2 corrosion, and erosioncorrosion. Effects predicted by the computer programs are compared to test results in the various testing facilities at selected flow system conditions. Based on these comparisons, adjustments are made to the computational models. Through this strategy, efficient use of the experimental facility is made that will lead to guidelines for erosion and erosioncorrosion in terms of the hydrodynamic, mechanical, and geometric variables of the production flow system. Computer programs are issued to member companies as userfriendly tools for making design and operating decisions and are updated periodically as erosion and corrosion technology develops and as our database of reference data expands. Planning Session The Advisory Board meetings include a planning session. During this period, brief presentations are made on the overall plan for each of the three main areas of research. The November 20

10 VI-0 EROSION/CORROSION RESEARCH CENTER floor is then opened up for comments and suggestions from the Advisory Board members on technical issues and on directions that E/CRC should be taking in its research. The objectives of the planning session are to convey both technical and member interest information to the Research Center staff. It is important that, by the conclusion of the planning session, member interest issues be put in question form suitable for inclusion in a Member Interest Questionnaire. Directions or changes in direction, priorities, and schedules that the E/CRC adopts are based on responses to the questionnaire. November 20