With North Sea oil output declining 8-10% per year it will soon not be economical to continue to use these platforms and alternate uses needs to be found.

Date:   2/10/2006 11:04:51 AM ( 18 y ago)

Recycling spent oil and gas wells - Having seen the problems disposing of the Brent Spar oil platform, it is time to look for alternate uses for offshore platforms when it becomes uneconomical for them to continue production.

One alternate use could be the extraction of geothermal energy. The continental shelf in the UK, where these platforms are situated, has a relatively thin earth's crust, giving the wells high bottom hole temperatures. Heat from these wells can be utilised to generate electricity and, by the use of submersible cables, help power the national grid.

The extraction system utilises geothermal heat transferred from a hydrothermal reservoir into a combined Ultra-Large-Scale Heat Pipe/High Speed Organic Rankine Cycle Turbine. The system, which is called GEESOR (Geothermal Energy Extraction System Organic Rankine), overcomes the problems of reinjection, two-phase fluid movement and environmental pollution by using a clean working fluid in a closed cycle. The creation of a reservoir, fracturing and primary fluid movement will also be covered. The system describes an economical alternative to fossil fuels and nuclear power as the main source of electrical power generation for the next millennium.

Governments around the world are constantly looking for new innovative forms of energy. Geothermal energy has by far the greatest potential when it's put in context with the reserves of all forms of energy.

When one looks at the planet on which people live, One see that the entire fossil fuels, i.e. coal, oil and gas, come from the earth's crust. The crust makes up only 0.4% of the total mass of the planet, the remaining 99.6% being hotter than 500°C within the crust, increasing to 5000°C at the core.

The pressures are constantly generating the heat naturally. This means that geothermal energy is infinite in its nature, as it is naturally renewable.

Geothermal energy is natural nuclear energy - The way the planet is constructed, in the form of a number of layers or shells, means that the heat can reach the surface without any trace of nuclear contamination. The radius of the Earth is 6000 kilometres. This distance from the centre to the surface forms a natural shield and is infinitely safer than any man made nuclear reactor.

The principles of the GEESOR - The system has been designed to extract geothermal energy out of spent oil and gas wells by the transfer of heat from the hydrothermal field using Heat Pipe technology. The heat pipe transfers heat to the surface. The working fluid from the heat pipe is utilised directly in a high-speed organic rankine turbine system.

The main innovation is the combination of these two separate cycles, the heat pipe connecting directly to the organic rankine cycle without the need for a heat exchanger. Work undertaken by Dr. G. Rice of Reading University in the 1980’s has theoretically proved the compatibility of the two cycles and has shown very large increases in efficiency and power output, combined with reduced construction costs and maintenance.

The theory of operation - Special fluids operating in a hermetically sealed pipe system transfer the heat from the bottom of the well to the surface, by vaporising and moving at near sonic velocity. This vapour pressure is used directly to drive the turbo generator and is condensed, in an offshore situation by cold seawater, before returning via the feed pump to the base of the well. The minimum depth of the well needed to drive turbo generators depends on the geothermal gradient. In some cases this could be as small as 900 meters. It may be better to look at the minimum bottom hole temperature: this would be in the region of 150°C, although efficiency would be greatly improved with a bottom hole temperature of 400°C.

The bottom 1000m of the well is normally fractured to allow the heat from the reservoir to transfer to the heat pipe. In a hydrothermal field, heat is transferred by convection circulation over the 1,000 metres of exposed formation. Additional angled drilling and hydraulic fracturing to aid this process could be utilised.

The bottom 1,000 meters of the heat pipe is either rifled or has a capillary liner to increase the surface area and allow rapid expansion of the working fluid. The working fluid is sprayed from a return tube where condensed fluid is reintroduced to the evaporator surface. This allows the conversion of liquid to vapour to take place over a very large surface area. The vaporised fluid flows up the heat pipe to the turbine. Cooling water in the condenser provides a strong pressure differential across the turbine thus generating maximum power. A small feed pump on the turbine shaft reinjects the working fluid to complete the cycle.

20 years ago George Lockett known now as HealerGeorge took out a patent on this system and planned to develop the idea into a working demonstration plant but the fall in crude oil prices prevented this. Now crude oil prices have almost doubled and the oil industry is in World decline; it is time to start to look at this system again.

 

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