Data-reconciliation of ionic analysis data applied on the Vega field
Co-supervisors: Geir Arne Evjen, Glenn-Ole Kaasa (Statoil)
Background/Motivation: : The Vega field is a subsea gas and condensate field located in the northern part of the Northsea, approximately 80 km outside Florø. The field consist of the reservoirs, Vega Nord, Vega Sentral and Vega Sør, and started its production October 2010. The well stream is transported 29 km from the reservoir to the Gjøa (operated by Gas de France) platform for further processing . The well stream contains some water in the gas phase and due to cooling of the fluid along the production pipeline, water condenses out from the gaseous phase and forms a water phase. MEG – Mono-Ethylene-Glycol is therefore continuously injected into the multiphase fluid to prevent gas-hydrates from forming in the production pipeline. At the Gjøa platform, MEG/water phase (rich MEG) is separated from the multiphase stream and regenerated to 90 weight% MEG (lean MEG) before it is again, re-injected into the multiphase fluid.
The gas from the reservoirs contains CO2 which in contact with water, lowers the pH value of the water/MEG phase and increases the corrosion potential of the production pipeline. To reduce the corrosion potential, chemicals (pH-stabiliser and film forming corrosion inhibitor) are mixed into the MEG before it is injected in the production pipeline.
It is expected that the reservoirs will at some time in the production period, start to produce formation water. The formation water contains several ionic species which in combination with the injected pH-stabiliser, form solid salts (scale precipitation). On the Vega field, formation of scale is prevented by injecting scale inhibitor into the lean MEG, forcing the ionic species from the formation water and from the pH-stabiliser, to remain in the aqueous solution as super saturated ions. However, the desalination unit on Gjøa, does not handle formation water rate above 10 m3/d, and can cause production stop if exceed. It is therefore essential that the formation water rate is monitored properly to prevent production stop.
The water-MEG phase that is separated from the gas/condensate at Gjøa platform, is analysed on a regular basis and the measurements (all ions, density and MEG weight %) are stored into a database together will relevant online measurements from the Vega subsea field. MEGSim is an in-house application that utilises the measurements into a simple stationary process model, calculating among others, the total formation water rate from the Vega subsea field, such that the handling capacity of the desalination unit is not exceeded. The main principle behind the application is to focus on the mass balance on the Vega field, using known rates and concentrations to estimate e.g. the unknown total formation water in bilinear data reconciliation. The utilised method requires that all concentrations of the source streams are known in advance (together with their measurement uncertainties).
The process model of MEGSim and the data reconciliation method therein have difficulties to handle large variations in the measured rich-MEG analysis data. In addition, the process model is purely stationary, and hold-ups both in the production pipeline and lean-MEG injection pipeline, are not accounted for; consequently restricting the model to be used in production periods when the injection rate of lean-MEG and gas production rate is stable. In addition, there have been some experience with carry-over, i.e. the lean-MEG contains some ionic species, typically originating from pH-stabiliser, completion fluids, etc, that cause the process model of MEGSim to fail.
Outline of work: Based on the above, the following tasks will be suitable for a student, where the main goal is to improve the robustness of the process model in MEGSim:
Understand the actual subsea process at Vega
Familiarise with the process model implemented in MEGSim for Vega and its tuning tool
Understand the bilinear data-reconciliation method used
Identify when and where the process model fails
Study the measured analysis data from the Vega field, with particular focus on the following production periods
Start-up of wells
Carry over periods
Transient periods
Identify how uncertainties in specifications influences the reconciled rates of the process model
Retune process model based on experience from measurements and effect of specifications.
Identify possible model adjustments. Implement and retune model.
Model reduction.