Techno-Economic Study with Integrated CO₂e Emission Model of Novel Pseudo Dry Gas Concept for Deepwater Gas Field with Long Tieback

Deepwater gas field development with long tieback poses flow assurance challenges due to the liquid dropout, causing high backpressure and extensive compression and pumping energy requirements, leading to high OPEX, CAPEX, and CO₂ emission. A novel Pseudo Dry Gas (PDG) concept has been developed as an inline gas-liquid separator (liquid removal system) which separates liquids into a dedicated liquid line from the gas to induce hydraulically ‘dry gas’ behavior within a wet gas pipeline over the operating envelope. The work aims to demonstrate the techno-economic benefits of a field development study undertaken jointly between oil and gas operators and service companies; with particular focus on the integrated life cycle CO₂ assessment showing the ‘energy intensity’ of the technology in comparison with various well-established concepts.

A widely accepted industry design software has been used to generate data for a range of various development concepts based on deepwater (2,000m of water depth) gas field data with 140km tieback. An advanced flow assurance design technique is developed to evaluate the number of PDG units required along the pipeline and to select the most optimum section to locate the PDG unit(s) for effective separation. The performance of the PDG units are predicted by linking the field data with PDG test data. The test has been conducted on a 6″ flow loop with 6″ PDG prototype and pilot scale unit, covering the operational pressure, temperature and anticipated fluid properties. The CO₂ emission assessment is undertaken in accordance with ISO 14044, linking the reservoir production to the power demand of pumps and compressors when they are required to support production. The power demand is then converted into the equivalent CO₂ emission based on the generation type. Finally, the carbon intensity (CO₂ tonnes/ MMScf) for various development concepts is calculated by combining the accumulative production and total CO₂ emissions over the field life.

The results show a significant reduction in carbon intensity for PDG concept, in excess of 75% when compared to other potential concepts. Moreover, this gain in carbon intensity reduction is accompanied by other economic benefits i.e., without loss of recoverable reserves, enhanced ability to mitigate production risks associated with long-distance subsea gas tiebacks leading to lower life cycle cost and higher Net Present Value (NPV) as well as Internal Rate of Return (IRR). These observations are the net result of changing the internal resistance curve of the gas tieback from a quadradic function to a linear function, enabling the more efficient use of downhole pressure to reduce the overall carbon intensity (CO₂ tonnes/ MMScf) of upstream gas production.