High Pressure Testing of a Pilot Subsea Compact Flotation Unit

Abstract

Produced water is the largest volume waste stream in oil and gas production and is a particular problem for late-life operation of oil fields. The huge waste volumes result in two obvious consequences and cost drivers; fast declining reservoir pressure and tremendous power used to handle it. Produced water reinjection has been used as a main strategy for both managing produced water volumes and supporting reservoir pressure. Treating produced water subsea as close as possible to the production wells has several advantages such as smaller topside footprint, minimizing the energy demand and associated emissions, reducing the well back pressure and increasing production rates. While some work has been done in the past on subsea use of deoiling hydrocyclones, robust high performance subsea produced water treatment equipment and processes have not yet been made available. The Compact Flotation Unit (CFU) is a well proven technology for topside use and has been considered promising for subsea use, but never before been tested at subsea relevant pressures. Aker Solutions with partners Aker BP, Equinor and Total and support from Research Council of Norway have successfully run a joint industry project to qualify this technology for subsea application. Pilot testing of a subsea CFU was carried out in Equinor's large scale test facilities in Porsgrunn, using crude oil, natural gas and synthetic produced water. The pilot CFU was tested with pressures significantly above that used for conventional topside CFUs, and at various temperatures. Gas bubbles were injected by use of ejectors designed to provide the desired bubble size. Oil droplet size, flow rate and oil in water concentration were the major control variables in addition to pressure and temperature. The effect of injection of flocculant was also investigated. The main topic of interest was the oil removal efficiency of the CFU under high pressure. The analyses of the results showed that the CFU's efficiency improved with increase of the operational pressure. This was likely to be related to the changes of physical properties of gas, oil and water as pressure increases, and the changing balance between physical phenomena in the process. The CFU also demonstrated robust performance with a large turndown ratio. Test of flocculant at high pressure showed instant and substantial improvement in the CFU efficiency, and the effect of flocculant was not compromised by the high pressure. The promising results suggest that CFU performs well at elevated pressure and is a viable solution for subsea produced water treatment.

Gas Flotation of Petroleum Produced Water: A Review on Status, Fundamental Aspects, and Perspectives

“…In fact, it has been demonstrated that separation can be more efficient when the process is conducted at the seabed conditions. 42 Higher collision probability can also be realized with a large sweep factor (SF). 20 The sweep factor describes the number of times per unit time a given volume of water is swept by a gas bubble and is defined as follows (eq 3): 56 A F A SF where A gas is the total cross-sectional area of bubbles per unit volume, F gas is the volumetric flow rate, and A cell is the crosssectional area of the flotation cell.…”

“…Illustration of design and components in the CFU module design. This figure was reproduced with permission from ref . Copyright 2020 Society of Petroleum Engineers.…”

“…Furthermore, water treatment aided by flocculants can be needed if the flotation process is placed a long distance from the well, but flocculants are not necessary required if the process is close to the production well (for example, subsea). In fact, it has been demonstrated that separation can be more efficient when the process is conducted at the seabed conditions …”

“…As mentioned previously, some subsea separators and hydrocyclones are applied technologies, while subsea CFUs are still in the development stage. The available subsea hydrocyclones present turndown issues with low flow rates and are additionally plagued by small reject orifices, susceptible to blockage and requiring frequent backflush . In general, minimal intervention and maintenance are vital for subsea processes, and there is a need for alternative technologies.…”

Combined Influence of High Pressure and High Temperature on the Removal of Crude Oil from Water during Laboratory-Scale Gas Flotation

“…17 Subsea produced water treatment can also avoid CO 2 emissions since less energy is required for pumping of fluids to the topside level. 18 Currently, subsea oil−water separation installations are the Troll-Pilot and Tordis gravity separators at the Norwegian Continental Shelf, and the hydrocyclones, 19 installed at the Brazilian Marlim field. 20 It was reported that the gravity separators give an oil outlet concentration of 500−600 mg/L, which is too high for the PW to be disposed to the sea.…”