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Offshore wind could help revive old oil wells
Offshore wind power is a proven concept, and turbines at sea are already feeding power into electricity grids in several European countries. Now classification society DNV GL is proposing a new concept: using turbines to breathe life into oil and gas fields
by Malcolm Latarche
DNV GL’s WINd powered Water Injection (WIN WIN) project combines mature water injection technology (in which water is injected into a reservoir to increase pressure and stimulate production) with the latest developments in offshore wind power to achieve more cost-effective enhanced oil recovery (EOR). DNV GL has invited other industries to join in developing the concept further in a joint industry project. Initial studies show it will reduce the cost of water injection, avoid costly topside modifications and reduce greenhouse gas emissions. DNV GL studies suggest there are opportunities for a new generation of autonomous injection systems used to increase the reservoir pressure. A turbine could be installed where it is required without costly retrofitting on the platform, providing access to systems normally located subsea, increase the flexibility of injection location and reduce the installation time. The system would also be easy to move and use at new locations after the closure of a well or field. Nearly all offshore installations generate their own electrical power by running gas turbines to drive compressors and pumps at the installations. Crude oil separation, gas compression and purification, wastewater treatment, seawater injection and oil and gas export systems are all examples of activities that require energy and also result in considerable emissions. Approximately 80 per cent of nitrogen oxide (NOx)
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emissions from offshore oil and gas installations originate from these turbines, so for Norway at least, the concept could help offset NOx fund levies. DNV GL’s concept is to integrate
the compressor and water treatment equipment into the substructure of a floating wind turbine. The power produced by the turbine can be used with a variety of water injection technologies, ranging from raw seawater injection as the simplest to ‘LowSal’ water injection (low salinity water from a reverse osmosis process) as the most complex. The distance between the satellite injection well (subsea) and the host platform is to a large extent decisive for the costs of the operations, and at certain locations, the distances can be long – in the order of several kilometres. Long power cables and water injection flowlines are major cost drivers in the traditional solutions using fully processed injection water and can be associated with operational risks. Not only is procurement and installation expensive, the systems are also prone to damage that can result in loss of production and need for expensive repair. Furthermore, with longer distances to transport the processed injection water, the power requirements increases. Commonly, water injection systems typically have pump power requirements in the order of 5 megawatts (MW). The wind turbine rating should be in the order of 5–6MW, and such turbines can be sourced from several major turbine
Artist’s impression showing a floating wind turbine providing power for an offshore installation
manufacturers. The subsea injection pumps inject water into the reservoir, and the configuration can, for example, be a single 5MW induction motor driving a single pump or two 2.5MW motors driving two smaller pumps. The benefit of the latter arrangement is better availability, as one of the pumps can continue to inject water in case of failure of the other. However, this redundancy is only at the load level.
The water injection pumps should ideally inject a specific total volume of water over the lifetime of the project to maximise oil recovery. From the perspective of reservoir recovery, this water injection need not be delivered at a constant rate and can therefore be powered by a variable source of energy like a wind turbine. There are pump systems available today that would allow for the variable power provided by a wind turbine generator. Injection of raw seawater (if
reservoir characteristics allow for it) offers a solution for water flooding that minimises the water injection flowline. The solution has been successfully implemented on the Norwegian Continental Shelf and internationally. The Tyrihans field is one example where the injection water is not pumped from the platform but withdrawn directly from the sea by a first-of-its-kind seawater injection subsea unit, powered and remotely operated from the Kristin platform 31km away. OWJ
Offshore Wind Journal I 3rd Quarter 2014 I 39
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