Geothermal Potential In North Sea
Oil and Gas Industries. Recorded 24th May 2012 AECC, Aberdeen, Scotland at
SINGLE BOREHOLE GEOTHERMAL ENERGY EXTRACTION SYSTEM
FOR ELECTRICAL POWER GENERATION
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 described in this paper is 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 we live, we see that all the 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 hydro-thermal 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 University1, 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
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 sea water, 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
One Moving Part
pipe effectively forms the heat exchanger between the contaminated fluids in the
hydrothermal field, and only the clean working fluid is brought to the surface
and recycles. The system should therefore have a working life of about 25 years,
as there is only one moving part, that is to say, the combined turbine,
alternator and feed pump floating on air bearing.
The advantage of the single borehole system
Most geothermal power stations are based on
two-hole extraction systems. This is because geothermal fluids are not pure and
usually need re-injecting. There are many natural contaminants in the rocks,
which if brought to the surface and released would cause environmental
pollution. This removal of fluids itself causes a whole host of problems:
corrosion, silting of wells, two-phase fluid movement, subsidence and many other
With the single borehole system, pressure in the geothermal field is maintained.
This has many advantages: hydrothermal fluids do not turn corrosive, and solids
stay in suspension, hence no well silting. However, the most important claim is
that there is no environmental pollution, unlike fossil fuels, which cause acid
rain, and nuclear power, which produces radioactive waste.
Creation of a reservoir
There are many areas of the world where
reservoirs exist naturally2. These are usually associated with fault lines
between continents and volcanic areas where hot springs, geysers and fumeroles
are common. Recent research carried out in Russia in the Kola Peninsula has
revealed moving fluids and open fractures at depths in excess of 12 kilometres.
This discovery has led to a review of current deep geological thinking and has
opened up the development of the single borehole geothermal energy extraction
Foster3 has carried out work on the use of single borehole geothermal energy
extraction systems in hot dry rocks.
Economic advantages of the single borehole
One of the major advantages of the single
borehole system is that wells drilled for other purposes can be utilised for
geothermal power generation. Some examples are unsuccessful or dried-up oil and
gas wells and test boreholes. Normally, the high cost of these wells is written
off, where as a simple test could show suitability for use as a geothermal power
station before the well is plugged.
The Single Borehole Geothermal Energy Extraction System makes use of low boiling
point organic fluids. Therefore, wells which are drilled in areas not normally,
known to be geothermally rich, with relatively low bottom hole temperatures, can
In areas like the North Sea, UK oil and gas drilling is carried out from
platforms. Normally up to 20 wells are drilled from a single platform.
Each year the oil output of these well falls by 5% and
at some point in time they will be uneconomic to continue use for oil
platforms have the advantage that they are on the continental shelf, where the
earth's crust is thin. High bottom hole temperatures combined with large
quantities of cold seawater make them ideal locations for geothermal energy
power generation. If it makes economic sense to connect the UK to France by
cables to utilise cheap French nuclear power then it must also make economic
sense to connect these platforms to the mainland to utilise geothermal power.
1. Rice, G. (1985), "Predicted Performance of
a Heat Pipe/Geothermal Energy System for Energy Soft Computer Systems Limited".
2. Freeston, D.H. (1985), "The Application and Design of Downhole Heat
Exchangers": Geothermics, Vol. 14, 2/3, pp. 343-351.
3. Foster, J.W. (1984), "Method for Producing a Geothermal Reservoir in Hot Dry
Rocks Formation for the Recovery of Geothermal Energy": Author, Illinois State