Patent EA-002169-B1 (EA)
Title: A plant for exploiting geothermal energy
IPC: E21B43/30
Filed: 1997-11-24
Abstract: 1. A plant for exploiting geothermal energy by circulating water through a geological formation (1) at least 1000 m below the earth surface (2), comprising at least one supply hole (3) leading from the surface (2) down to said formation (1), at least one return hole (4) for the transport of heated water from said formation to the surface, and a heat absorbing arrangement connecting the supply and return holes (3,4), said arrangement comprising a heat transfer surface across which heat is transferred from said formation to said water, said heat transfer surface comprising at least one drilled heat absorbing hole (5), characterized in that ratio of length of said heat absorbing hole or all holes to a given nominal power in MW after one year of operation is at least 1300 m/MW, preferably equal or more than 2600 m/MW 2. A plant according to claim 1 characterized in that said heat absorbing arrangement comprises a plurality of drilled heat absorbing holes (5) connecting the supply and return holes (3,4) in a parallel flow relationship, a substantial part of each heat absorbing hole (5) lying at least 50 m, preferably at least 100 m from the nearest heat absorbing hole (5), the total length of the heat absorbing holes (5) exceeding 5000 m. 3. A plant according to claim 2 characterized in that said heat absorbing holes (5) extend at an angle (.alpha.) downwards from the supply hole (3) to the return hole (4). 4. A plant according to claim 3, characterized in that said angle (.alpha.) is between about 20 degree C and 50 degree C, preferably 40 degree C. 5. A plant according to any of claims 2 to 4, characterized in that at least a substantial part of the heat absorbing holes (5) extend parallel to each other. 6. A plant according to any of claims 2 to 5, characterized in that the heat absorbing holes (5) are arranged in one layer (14) or in several vertically spaced layers. 7. A plant according to claim 6, characterized in that the heat absorbing holes (5) are arranged in several vertically spaced layers, the distance between adjacent holes (5) in each layer (14) is about the same as the distance between adjacent vertical holes (5). 8. A plant according to claim 6 or 7, characterized in that the said distance between said layers (14) is less than about 150 m,.preferably 150 m. 9. A plant according to any of claims 2 to 8, characterized in that a plurality of supply holes (3) are arranged around a common return hole (4). 10. A plant according to any of claims 2 to 8, characterized in that the upper ends of said supply hole (3) and said return hole (4) are arranged closely to one another, one of said holes extending generally vertically and the other deviating from the vertical, so as to introduce a substantial spacing between the ends of the heat absorbing holes. 11. A plant according to claim 10 wherein the said spacing is at least 500 m, preferably exceeds 1 km. 12. A plant according to any of claims 2 to 11, characterized in that the heat absorbing holes (5) have a diameter of less than 18 cm. 13. A plant according to claim 12, characterized in that the diameter of said heat absorbing holes is less than 14 cm, preferably less than 12. 14. A plant according to claim 12 or 13, wherein the holes (5) are of a nominal 10 cm diameter. 15. A method for use in constructing a plant for exploiting geothermal energy by circulating water through a geological formation (1) at least 1000 m below the earth surface (2), said plant comprising at least one supply hole (3) leading from the surface (2) down to said formation, at least one return hole (4) for the transport of heated water from said formation to the surface, (2), characterized in that the heat absorbing arrangement is formed as a plurality of the heat absorbing holes (5) in a parallel flow, wherein dimensions of said heat absorbing holes (5) using the formula Q  [is approximately equal to]  Cx(TG - TW)xkxlx in accordance with the differential equation for heat transfer from a cylinder of homogenous rock material into water flowing through a central hole in said cylinder, wherein  C is a constant between 0.6 and 2.4, TG is an initial average temperature of a geological formation along the heat absorbing hole, TW is an average temperature of water flowing in a heat absorbing hole, k is thermal conductivity of rock in W/m degree C. l is length of a hole in meters. 16. The method according to claim 15, wherein the dimensions of said heat absorbing holes (5) are defined using the formula Q  [is approximately equal to]  Kxk<0,93>xD<0,2> (0,1 + (t + 1)<-0,1>)(T<G> - TW) xl, wherein  C is a constant between 1.7 and 2.0, D is the diameter of hole in meters, t is duration of the plant operation in years. 17. The method according to claim 15 or 16, characterized in that at least the major parts of the heat absorbing holes are drilled by using a percussion hammer and a coiled tubing drill string.