Erosion Research
At the Erosion/Corrosion Research Center (E/CRC) erosion is being studied by performing both experiments and modeling. The experiments are a combination of direct impingement tests and tests performed in flow loops. The direct impingement tests are used to determine the erosivity of a variety of materials. These tests are performed by shooting the erodent (usually sand) at coupons. The coupon angle can be varied to determine the effect of impact angle on the erosion. Flow loops are used to examine the effect of geometry, fluid properties, and particle properties and size on erosion. Several single-phase and multiphase flow loops are available to cover a broad range of testing conditions. The multiphase flow loops are used to study the effect of flow regime on erosion. The single-phase gas testing apparatus and the multiphase flow loops intended for high gas rates use a once-through approach for the addition of sand. However, the single-phase liquid and multiphase loops intended for relatively high liquid rates recirculate the sand in the test section. The loops that reuse gas and/or liquid use a cyclone separator to separate the sand to protect the equipment downstream. For the loops that recirculate the sand, the sand is reintroduced to the test section through a sand injection nozzle at the base of the separator.
Modeling is used to complement the experimental work. Modeling can be used to interpret experimental data, extend experimental data to conditions than can not be tested with current facilities, and help design future tests. Two types of models are currently being used. The first modeling technique is a computational fluid dynamics (CFD) based erosion model. A flow model is used to determine the velocities in a given geometry as well as other flow information. A particle tracking program then is used to determine the particle trajectories. The particle impingement information along with the pipe material properties is used to determine the erosion of the geometry being modeled. This technique is complicated and time consuming; therefore, simpler mechanistic models are developed that capture the erosion mechanism. The simple models can account for fluid and particle properties and flow rates, particle sizes and shapes, material properties, geometry size and shape. The simple models are converted into user-friendly computer programs for use by the E/CRC member companies. The program that predicts erosion is called SPPS (Sand Production Pipe Saver).
CO2 Corrosion Research
A mathematical model and a computer program for predicting CO2 corrosion rates for scale-forming conditions has been developed. The prediction procedure is based on mechanistic models for the CO2 corrosion process and the scale formation process. In this model, scale is assumed to form whenever Fsat > 1.0 at the pipe wall, where: and where [Fe2+] and [CO32-] are concentrations of the corrosion products at the surface of the metal, and ksp is the solubility product of iron carbonate. The thickness and % coverage of the scale are assumed to grow with each iteration of the computer program until the value of Fsat = 1.0 at the surface of the metal under the scale. This condition defines the equilibrium corrosion rate. Corrosion rates computed using this model have been compared with data from E/CRC test loops, with some field data provided by E/CRC Advisory Board members, and with published test data. In most cases, agreement between the model predictions and the corrosion measurements is very good. For some cases where iron carbonate scales are very protective, some refinements to the model are being made to improve agreement.
The graph at the left shows model predictions of how the corrosion rate changes when temperature is increased for a CO2 partial pressure of 100 kPa, a solution pH of 5.2, a pipe diameter of 89 mm, and an Fe2+ bulk concentration of 12 ppm. Flow velocity is held constant at 2.0 m/s for this example, and temperature is varied from 30 to 120oC. At lower temperatures in this example, corrosion rate increases with increasing temperature because the rates of electrochemical reactions and mass transfer increase with temperature. However, the increases in temperature and corrosion rate also increase Fsat gradually (due to a decrease in Ksp for FeCO3 with increasing temperature, and due to increases in surface concentrations of Fe2+ and ), and at temperatures exceeding about 60oC the model predicts formation of iron carbonate scale and much diminished corrosion rates.
Erosion-Corrosion Research
When sand is produced along with production fluids, erosion-corrosion can occur. In many cases, erosion-corrosion is much more damaging than either erosion or corrosion acting alone. Three erosion-corrosion behaviors have been observed in a CO2 environment when sand is present – scale formation, pitting, and uniform corrosion. For erosion conditions of lowest severity (low erosivity) scale formation is observed with low corrosion rates. For high erosivities, uniform corrosion takes place with high corrosion rates. For intermediate erosivities, pitting is most often the observed behavior. In these cases, penetration rates can be extremely high. The figure at the left shows photographs of several test specimens that developed pitting while being tested in an E/CRC test loop. The test specimens flush mount into an elbow test cell. The direction of flow is shown by the arrow. Notice that the most severe pitting is on the down-stream end of the elbow at sand impingement points. Protective iron carbonate scale is removed or prevented from forming at impingement points, and at these points corrosion can take place at high rates. The “threshold velocity” defines the boundary between the scale formation and pitting regions. In many cases desired production flow velocities are higher than the threshold velocity. This means that pitting is a risk. A method has been developed for computing threshold velocities. The method involves testing in the E/CRC’s Three-Phase Miniloop over ranges of pH, temperature, CO2 partial pressure, and flow velocity to find Erosion-Corrosion Resistances (ECR) for these conditions. This data is used along with our erosion prediction program (SPPS) and with our corrosion prediction program (SPPS: CO2) to compute threshold velocities for the onset of pitting. With this procedure it is possible to compute threshold velocity curves for any temperature, pH, CO2 pressure, pipe size, sand size, sand production rate, and fluid properties. Once the threshold velocity is known, erosion penetration rates and corrosion penetration rates can be computed based on whether the intended flow velocity is above or below the threshold velocity.