Transporting oil and gas mixture in gathering system at White Tiger oil field

Abstract

Abstract Transport of oil and gas from producing wells to separation units in two-phase flow by one pipeline could significantly reduce the budget for building the product gathering system. However, practice and studies showed that it would be effective only if the gas flow rate ratio ß 0.7. JV Vietsovpetro crude oil, mainly being produced from the basement of southern part of the White Tiger field, is transported in form of two-phase flow from Satellite Platforms (SP) to Central Processing Platform (CPP) for gas separation through submarine pipeline (without thermal insulation). The crude has high wax content (up to 27% by weight), high pour point temperature 33–34°C, and average GOR of 166 m3/m3 @ 20°C & 100 kPa(a). The transport and gas separation processes encountered many difficulties due to pressure and flow rate fluctuations. Increasing pressure in the pipelines to 2200–2500 kPa(g) improved the situation, but not enough. A consequence was also that the well flow rates were decreased. This paper describes the results from a full-scale field test on transportation of 5848 m3/day (@ 20°C & 100 kPa(a)) well product from SP to CPP as an oil and gas mixture. Studies were carried out on subsea un-insulated pipeline with OD 0.324m and length around 2000m, with gas flow rate ratios, varying from 0.623 to 0.868. Developed on the above basis the technology of preliminary gas separation on the SP and then transport of the gas and oil by two separate pipelines has assisted JV Vietsovpetro in elimination of a/m difficulties. Application of this technology would gain significant benefits when most of the wells on the SP moved to gaslift production. Introduction Currently, JV Vietsovpetro operates three oil fields offshore Vietnam: White Tiger, Dragon and Big Bear. Oils being produced have high wax content and high pour point temperature (see tables #1, 2&3). Most of the pipelines in the in-field and inter-field transportation system do not have thermal insulation. Average temperature of seawater surrounding the pipelines is 26°C, and minimum can reach 22°C. Moreover, there is strong current in the region, up to 1.27m/s, as maximum. In such conditions, transportation of well product of high waxy crude at JV Vietsovpetro White Tiger and Dragon fields encounters not only flow rate and pressure oscillations, but also serious wax deposition problems. This paper presents the results obtained during testing conducted on two subsea pipelines connecting two offshore installations. The lines serve as transportation lines from one installation to other, where separation and following pumping out to a FSO for storage and offloading to the buyers (see fig. # 1) take place. The tests reported are from wells that naturally flowing with reservoir fluid from the granite basement, playing role as the main P-zone at the White Tiger oil field. Each of the tested wells from SP produces more than 1200 m3/day of stock tank oil @ 20°C&100 kPa(a). Wellhead temperature of product is about 100–110°C. To transport product from SP to CPP, initially, JV Vietsovpetro started with one pipeline used. The pressure of the first stage separation at CPP is at 1100–1200 kPa(g), and is dictated by requirement to transport the gas to the Central Compressor Platform (CCP) where the inlet pressure should not be lower than 1000 kPa(g). To provide this pressure an inlet pressure to the header at 200–1300 kPa(g) should be sufficient. However, because of unstable operation of the separators at CPP JV Vietsovpetro has had to increase this pressure by a factor of two, leading to higher wellhead pressures, and finally to reduction in well flow rates. In addition, before being directed to CCP the separated gas would need to be cooled as much as possible at CPP for removal of liquid particles. If so, it would require setting up a gas-cooling unit on the CPP. Finding an appropriate solution for above problems was the main purpose of running the tests.