Waxy crude oil extraction often comes along with wax gelling and deposition issues in the sub sea transportation pipeline. Once the fluid temperature falls below the fluid’s cloud point ( known as Wax Appearance Temperature, WAT), wax crystals begins to form and snowball if temperature persists to fall. This usually happens when waxy oil is subjected to cold ambient sub sea environment and has to be transported over a long distance.
As the wax accumulates in the pipeline, it will eventually decrease the available flow cross-sectional area, restricting flow and limits the maximum possible production received at onshore. Complete blockage of pipeline is rare, but when it does, it could cause millions of dollars to repair. Long downtime would also incur considerable economic loss as the production prompts to come to a halt. The blockage happens when there is a significant amount of wax deposited in the pipeline and pig has a low pressure to push the large wax solid ahead. Therefore, prevention of wax or any solids deposition is to ensure smooth flow of hydrocarbon from reservoir to receiving facility at onshore.
Wax forms when the heavy hydrocarbons (i.e. paraffins with carbon number of 18 and above) in the oil precipitates out at temperature below WAT. Wax deposition at the pipeline wall happens by mass diffusion as the high wax concentrations tends to be in the bulk fluid than the fluid near the wall. When the waxy oil flows through the pipeline, the fluid at the pipe inner wall surface tends to be colder than the bulk fluid, this temperature gradient facilitates the mass diffusion and causing wax deposits on the wall. These factors cause the wax in the crude oil to form a structured network, which we called “gel” and it gets hardens if it is left long enough in the pipeline.
Once wax has deposited along the pipeline, it is usually removed by mechanical means such as pigging. “How frequent should a pipeline to be pigged?” is among a commonly asked, and this could be answered by conducting flow assurance analysis which can help to determine the pigging required. Pipeline pigging can then be optimized based on engineer’s judgement.
By simulating the wax deposition environment in the pipeline, the operator can visualize, predict and plan ahead for its wax control strategy.
In general practice, maintenance pigging is suggested to be done every 90 days (i.e. 3 months) if no wax deposition is observed. Wax deposition rate can be faster than one thought, depending on the wax content in the crude oil. Industry typically limits the wax thickness in the pipeline to not exceed certain threshold – 4 mm (based on engineering practice) or wax volume in front of pig to maintain below 50 barrel to prevent any pipeline blockage issues. Depending on different operator’s requirement, the amount of wax expect to form and its deposition rate can be determined by using wax deposition prediction models available in the market to study the wax behavior in the pipeline.
Besides pipeline pigging, there are other remediation strategies:
- Chemical injection (wax inhibitors)
- Maintain pipeline fluid at temperature above WAT
- Pipeline insulation
- Thermal heating
Production operator will have to decide the wax control method at design stage after considering its feasibility and the necessary cost that may incur (i.e. capital/operational). Hence, flow assurance analysis on wax management is required before making the decision.
Complete prevention is not always impossible. A more accurate method would be a close real-time monitoring on the condition inside the pipeline. However, such wax monitoring or detection technology is still under researched and not available for application in oil industry. This makes the wax modelling study more convincing to allow the production operator to predict the wax deposition in order to do any engineering judgement for its wax control method. This is the reason why wax management study is essential, especially for waxy crude oil.
- Wax Deposition Prediction
- Efficient Wax Control Strategy
- Optimize Pigging Requirement
- Minimize Potential Pipeline Blockage
Flow Assurance, n.d., viewed on 12 December 2020 <http://hmf.enseeiht.fr/travaux/bei/beiep/book/export/html/1770>