An Excel-based tool was developed that utilizes cubic equation of state (EOS) and thermodynamic electrolyte chemistry modelling. The software enables operators to assess sour production streams from a reservoir, through production tubing, pipelines and facilities to an export pipeline within a range of temperature and pressure conditions. The tool enables quick and accurate sour-production- stream assessment, improving the operator's understanding of the H2S- related integrity risks to ensure safe and stable production. The tool is designed to support materials of construction suitability reviews and provide input to optimize scavenger applications, using the most at-risk components and/ or locations in the system to determine if wells and facilities are safe to operate under current conditions.
This development represents a unique combination of functionality because it combines three elements to derive accurate partition coefficients across a range of operational conditions and gas/liquid compositions: thermodynamic modelling of water chemistry; equation-of-state hydrocarbon modelling; and the basic physical principles at work in production streams.
The approach is employed to assess the integrity risk posed to system components on Chevrons' Alba field in the North Sea. At the time of the modelling, the asset team planned to bullhead three production wells with base oil because of the significant well integrity concerns resulting from high H2S levels. The ability to quickly determine accurate partial pressure and sulphide partitioning profiles for the three production wells ultimately led to the wells remaining open, and saving the operator from deferring in excess of 3,000 bbl/day of oil production from the at-risk wells.
The modelling results were confirmed by a third-party analysis of sulphide concentrations in three phases in the test separator, indicating that the calculated total sulphide per-unit-volume in the system was correct, and the partial-pressure and three-phase concentration profiles for each well were also correct.
The tool was developed so that non chemists, field personnel, engineers and any other relevant stakeholders can make their own integrity assessments.