The invention relates to a hole in the ground for transferring geothermal energy from the ground around the hole to an energy carrier fluid which is introduced into the hole.

In US 4 290 266 it is disclosed that a pipe loop can be placed in a blind bore in the earth and that a fluid can be passed down into the earth via a first section of the pipe loop and up from the earth via a second section of the pipe loop. A disadvantage of this prior art technique is that the said pipe loop sections extend very close to each other whereby upwardly flowing warm fluid is cooled by the cold, downwardly flowing fluid, with the result that the total efficiency is low.

Furthermore it is known that there are a number of deep bore holes or wells which have been used or are being used for the recovery of oil or gas together with associated installations above ground. As a consequence of the strain on the environment which these installations often represent, for environmental and other reasons increasing demands have been made for such installations to be removed or dismantled and the wells sealed after the oil or gas extraction has ceased, which is a very expensive process.

The object of the invention is to provide a hole of the above-mentioned type whereby the above-mentioned disadvantages can be avoided.

The characteristics of the hole according to the invention are presented by the characteristic features indicated in the claims.

The invention will now be described in more detail with reference to the drawing whose only figure schematically illustrates a hole according to the invention.

As illustrated in the figure there is located on the seabed 5 a platform 1 whose upper section projects up above the surface of the sea 6. From the seabed 5 near the platform there extend two substantially vertical hole sections, viz. a first hole section 2 and a third hole section 4 to locations 7

and 8 respectively in the ground formation from which oil or gas have been extracted and brought up via the hole sections 2,4.

Instead of removing the platform and sealing the hole sections 2,4 after the oil or gas recovery has ceased, it is proposed according to the invention that an additional or second hole section 3 should be drilled which connects the locations 7 and 8 to each other, thus providing a hole loop comprising the first, second and third hole sections 2,3 and 4 respectively.

By passing a suitable fluid, such as water, down into the first hole section 2, this fluid can flow on through the second hole section 3, where it is heated by the surrounding ground, and subsequently up through the third hole section 4, without the heated fluid in the third hole section 4 being cooled by the down¬ wardly flowing, cold water which is passed down into the first hole section 2.

If the substantially vertical hole sections 2 and 4 extend from the same platform 1 as illustrated in the figure, the additional hole section 3 can extend in a relatively large loop away from one location 7 before it extends towards the second section 8, thereby ensuring that the fluid is heated to close to the temperature of the ground around the second hole section 3 before flowing upwards and out of the third hole section 4, even though the through-flow rate is high.

There can be provided on the platform a known per se installation (not shown) whereby the heat energy may be converted to another suitable energy form, e.g. electrical energy, and supplied to a consumer. This installation may comprise, e.g. heat exchangers, turbines, generators etc. and it is assumed that it will be familiar to a person skilled in the art.

As it will be understood, the invention permits a re-use of installations and wells, the decommissioning of which would have entailed major expenses for the owner. The invention provides energy whose release does not pollute the environment, and which is obtained from an almost inexhaustible source.

In order to illustrate the costs involved for the energy amount which can be provided, the following information is provided.

At a depth of between 3500 m and 6000 m below the seabed the temperature in the earth is approximately 90 - 150 degrees C.

Two bore holes each having a length of approximately 3000 m for extracting oil or gas at present cost approximately NOK 150 million. An additional deviated hole which has a length of approximately 1000 m and which connects these two bore holes costs approximately NOK 20 million.

For example, the drilling field Statfjord B in the North Sea comprises approximately 40 wells which together with the platform will probably be closed down around the year 2003. By applying the invention, however, these wells could form part of 20 hole loops for the production of geothermal energy, and the platform could be used as the mounting location for the above-mentioned installation.

It will be understood that in the hole sections there can be inserted pipes in which the fluid can flow.

For this oilfield the following example can be used:

Internal pipe diameter 0.215 m

Volume of water 700 mVh

Water velocity 5.32 m/s

Inlet temperature of the water upstream of the pump 20°C Outlet temperature of the water downstreamof the pump 40°C

Outlet temperature of the water from the well 90°C

Hole length 7000 m

Pump power 1.72 MW

Thermal power 40 MW

At a rough estimate this thermal power can provide an effective output of approximately 5 MW. The 20 hole loops in this oilfield would thus be capable of providing a total output of 100 MW.

Even though it has been stated in the above description of the invention that two hole sections have been interconnected by means of an additional hole section, it will be understood that more than two hole sections can be interconnected.