Erosion

Sand erosion of wellhead piping.

Erosion has been long recognised as a potential source of problems in oil and gas production systems. It refers to the loss of materials on internal surface of various parts of a production system due to production fluid mixture (especially with sand) that impacting the inner surface at an angle and typically at high velocity. Erosion has been long recognised as a potential source of problems in oil and gas production systems with numerous previous dangerous erosion failures that resulted in loss of containment.

Erosion is a complex process that is affected by numerous factors and small or subtle changes in operational conditions can significantly affect the damage it causes. This can lead to the scenario in which high erosion rates occur in one production system, but very little erosion occurs in other seemingly very similar systems. Detection of erosion damage is also difficult because good monitoring of the internal condition of the pipework is challenging, especially for subsea production systems. This makes erosion management difficult, especially for those unfamiliar with the manner in which erosion occurs.

Erosion mechanisms

Hydrocarbon wells produce a complex multiphase mixture of components, typically including:

• Hydrocarbon liquids – oil, condensate, bitumen
• Hydrocarbon solids – waxes, hydrates, asphaltenes
• Hydrocarbon gases (natural gas)
• Other gases – hydrogen sulfide, carbon dioxide, nitrogen
• Produced water with salts
• Sand and proppant particles

Depending on the characteristics of a system, erosion is typically classified to the following mechanisms. It is generally accepted that particulates (sand and proppant) are the most common source of erosion problems in hydrocarbon systems. However, all of the other mechanisms are equally aggressive under the right conditions.

Sand and particulate erosion

Particulate erosion mechanisms have been extensively studied and there has been some success in predicting particulate erosion rates. Important factors determining the rate of particle erosion are:

Sand production rate, flow rate, flow behavior

The nature of the sand and the way in which it is produced and transported also determines the rate of erosion within a production system. The sand production rate of a well is determined by a complex combination of geological factors, and can be estimated by various techniques, for example those described by Marchino. Often, new wells produce a large amount of sand and proppant as they “clean up”. Typically sand production then stabilises at a relatively low level before increasing again as the well ages and the reservoir formation deteriorates. Sand production is typically erratic and sand concentration typically ranges from 1 to 50 parts per million by mass upstream of the first stage separators. If a well produces less than 5 to 10 lb/day it is often regarded as being sand­ free. However, this does not eliminate the possibility that erosion may be taking place.

The sand transport mechanism is an important aspect controlling erosion within productions systems. Gas systems generally run at high velocities (>10 m/s) making them more prone to erosion than liquid systems. However, in wet gas systems sand particles can be trapped and carried in the liquid phase. Slugging in particular can generate periodically high velocities that may significantly enhance the erosion rate. If the flow is unsteady or operational conditions change, sand may accumulate at times of low flow, only to be flushed through the system when high flows occur. This and other flow mechanisms may act to concentrate sand, increasing erosion rates in particular parts of the production system pipework.

Velocity, viscosity and density of the fluid

The particle erosion rate is highly dependent on the particle impact velocity. It is generally accepted that the erosion rate is proportional to the particle impact velocity raised to a power of n (typically n ranges between 2 and 3 for steels). In cases where erosion is an issue the particle impact velocity will be close to the velocity of the fluid carrying the particle. Therefore erosion is likely to be worst where the fluid flow velocity is the highest. Small increases in fluid velocity can cause substantial increases in the erosion rate when these conditions prevail. In dense viscous fluids particles tend to be carried around obstructions by the flow rather than impacting on them. In contrast, in low viscosity, low density fluids particles tend to travel in straight lines, impacting with the walls when the flow direction changes. Particulate erosion is therefore more likely to occur in gas flows, partly because gas has a low viscosity and density and partly because gas systems operate at higher velocities.

Sand shape, size and hardness

Particle size mostly influences erosion by determining how many particles impact on a surface. Very small particles (~10 microns) are carried with the fluid and rarely hit walls. Larger particles tend to travel in straight lines and bounce off surfaces. Very large particles (~1mm+) tend to move slowly or settle out of the carrying fluid and therefore they are unlikely to do much harm.

It is well established that hard particles cause more erosion than soft particles. There is also evidence to show that sharp particles do more damage than rounded particles. However, it is not clear whether the variability of sand hardness and sharpness causes a significant difference between the erosion rate in production systems associated with different wells or fields.

Liquid droplet erosion

The droplet erosion mechanism is less well understood than particulate erosion. Droplet erosion is obviously confined to wet gas and multiphase flows in which droplets can form. The erosion rate is dependent on a number of factors including the droplet size, impact velocity, impact frequency, and liquid and gas density and viscosity. As many of these values are unknown for field situations, it is very difficult to predict the rate of droplet erosion. Salama & Venkatesh state that solids-free erosion only occurs at extremely high velocities that cause unacceptably high pressure losses. Therefore the conditions required for droplet erosion are unlikely to occur in correctly designed production pipework systems.

Erosion-corrosion

Erosion corrosion at a pipe elbow.

Erosion damage and corrosion damage can usually be distinguished by inspection of the damaged pipework and by consideration of the operating conditions. Erosion often causes localised grooves, pits or other distinctive patterns in locations of elevated velocity. Corrosion is usually more dispersed and identifiable by the scale or rust it generates. Erosion-corrosion is the combined effect of particulate erosion and corrosion. The progression of the erosion-corrosion process depends on the balance between the erosion and corrosion processes as demonstrated by Shadley et al amongst others.

In a purely corrosive flow, without particulates in it, new pipework components typically corrode very rapidly until a brittle scale develops on the surfaces exposed to the fluid. After this scale has developed it forms a barrier between the metal and the fluid that substantially reduces the penetration rate. In erosion environments, this protective barrier can be removed by particulates, exposing new metal surface for corrosion.

Erosion-corrosion mechanisms are potentially very complex, combining as they do two mechanisms that can be quite case specific. This makes prediction of erosion-corrosion penetration rates for a particular field situation very difficult. Erosion-corrosion can be avoided by ensuring that operating conditions do not allow either erosion or corrosion.

Cavitation

When liquid passes through a restriction low pressure areas can be generated, for example downstream of a sudden step. This phenomenon is called cavitation. If the pressure is reduced below the vapour pressure of the liquid, bubbles are formed. These bubbles then collapse generating shock waves. These shock waves can be of sufficient amplitude to damage pipework. Cavitation is rare in oil and gas production systems as the operating pressure is generally much higher than liquid vaporisation pressures. Evidence for cavitation is sometimes found in chokes, control valves and pump impellers, but is unlikely to occur in other components.

Components prone to erosion

Regardless of the erosion mechanism, the most vulnerable parts of production systems tend to be components that experience the following:

• the flow direction changes suddenly
• high flow velocities occur caused by high volumetric flowrates
• high flow velocities occur caused by flow restrictions

Components and pipework upstream of the primary separators carry multiphase mixtures of gas, liquid and particulates and are consequently more likely to suffer from particulate erosion, erosion-corrosion and droplet erosion.

The vulnerability of particular components to erosion heavily depends on their design and operational conditions. However, the following list is suggested as a rough guide to identify which components are most vulnerable to erosion (the first on the list being most likely to erode):

1. Chokes
2. Sudden constrictions
3. Partially closed valves, check valves and valves that are not full bore
4. Standard radius elbows
5. Weld intrusions and pipe bore mismatches at flanges
6. Reducers
7. Long radius elbows, mitre elbows
8. Blind tees
9. Straight pipes

Impact of material choice on erosion

Material properties have a significant effect on erosion. In oil and gas production systems nearly all components will be made of ductile metals; predominantly steels. Plastics, rubbers, elastomers, composites and similar materials may also be present. If erosion problems are suspected, special erosion-resistant materials such as tungsten carbide may also be used.

Erosion prone ductile metals and other common materials

Steels, other metals and most plastics generally show ductile erosive properties. Particulate erosion in ductile materials erosion is primarily caused by a process known as micro­ machining. In this process particles impacting at an angle to the surface scoop away material. At high impact angles, particle impacts on ductile surfaces tend to generate craters, but they do not remove as much material. The relationship between material properties and droplet impingement and cavitation erosion mechanisms are less well understood.

The primary factor controlling erosion in ductile materials is the material hardness. Consequently steels are more resistant than softer metals. Different steels have different hardness values. However, there is some debate as to whether this variation is sufficient to cause much variation in erosion resistance. Haugen at al suggest that the difference between different grades of steel is negligible for impact velocities of less than 100 m/s. Plastics and composites are generally less resistant than metals, although rubber and some polymers are quite resistant to particulate erosion because they absorb the energy of impacting particles.

Erosion resistant materials

Special materials such as tungsten carbides, coatings and ceramics are often used in chokes and highly vulnerable components. These materials are generally hard and brittle. Brittle materials erode in a different manner. Impacts on brittle materials fracture the surface and erosion increases linearly with impact angle, being a maximum for perpendicular impacts. This will affect the shape of the erosion scar and the position of maximum wear. Most of these materials have a superior erosion resistance compared to steel (often orders of magnitude better).

Sand erosion prediction

The most sophisticated modern particulate erosion models consider the erosion process in three stages. Initially the flow of the carrier fluid through the elbow or fixture is modelled or in some way approximated. This flow prediction is used to derive the drag forces imparted by the fluid on the particles, hence the trajectories of a large number of particles are predicted. When individual particles impact on a wall the damage done is calculated using a material­ specific empirical, or a theoretically derived impact damage model. The average impact damage of a large number of particles can then be used to predict the distribution and depth of erosion damage on a surface.

Erosion management

A number of measures can be taken to monitor and avoid erosion. These include:

Reduction of production rate

Reducing the production rate reduces both the sand production rate and the flow velocity through the pipework. However this has obvious financial implications.

Pipework design

Pipework should be designed to minimise flow velocities and avoid sudden changes in flow direction (e.g. at elbows, constrictions and valves). The use of full bore valves and blind tees in place of elbows can also reduce erosion problems. Slugging flows can be particularly damaging therefore the inclusion of slug catchers and drains may be appropriate for certain installations.

Thick-walled pipes are often used to increase the wear life of pipework. However, care should be taken, when doing this, as increasing wall thickness reduces the pipe bore, elevating flow velocities and increasing the erosion rate, particularly with small bore pipework.

Sand control

Main article:Sand control. Downhole sand screens and gravel packs are often used to stop sand entering the production system. These tend to be used on new wells in which sand production has been identified as an issue.

Occasionally sand separation devices, such as hydrocyclones and other types of desander, are used to reduce erosion in components downstream of the wellhead. These devices can be very effective at protecting chokes in particular. However, they do not protect downhole equipment. As with sand exclusion measures, the inclusion of a sand separation device in the production stream is likely to have an adverse impact on production economics.

Use of erosion resistant material

Special materials such as tungsten carbides, coatings and ceramics are often used in chokes and highly vulnerable components.