High-pressure and high-temperature closed geothermal exchanger for a magmatic or metamorphic formation

Abstract

The invention relates to a geothermal exchanger comprising a casing containing a heat-transfer fluid with which it is in direct contact. The casing is flexible such as to be in direct contact with a wall of the borehole containing the exchanger under the effect of the pressure of the heat-transfer fluid.

Claims

1. A closed geothermal exchanger for a magmatic or metamorphic formation, comprising a shell containing a heat-transfer fluid with which it is directly in contact, the shell having a surface end and a borehole end, and a containment device that, at least in part, caps and extends over an upper part of the shell including the shell surface end, the containment device including a harness comprising aramid straps, wherein the shell is flexible so that it can be in direct contact, under the effect of the pressure of the heat-transfer fluid with a wall of a borehole containing the exchanger.

2. The geothermal exchanger as claimed in claim 1, the flexible shell having a shore A hardness of between about 60 and about 90.

3. The geothermal exchanger as claimed in claim 1, the flexible shell comprising silicone elastomer.

4. The geothermal exchanger as claimed in claim 3, the silicone elastomer having a thermal conductivity higher than 3.5 W/(m.Math.K).

5. The geothermal exchanger as claimed in claim 1, further comprising an inner tube into which the heat-transfer fluid is injected, the inner tube comprising a bottom part with an opening that allows the heat-transfer fluid to pass toward the outside of the inner tube.

6. The geothermal exchanger as claimed in claim 5, the inner tube comprising a silicone elastomer having a thermal conductivity of about 0.2 W/(m.Math.K).

7. The geothermal exchanger as claimed in claim 1, the temperature of the heat-transfer fluid being greater than 100 C.

8. The geothermal exchanger as claimed in claim 1, wherein the aramid straps are comprised of poly-paraphenylene terephthalamide.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be better understood from studying the attached figures, which are provided by way of entirely nonlimiting example, in which:

(2) FIG. 1 is a view in longitudinal section of a closed geothermal exchanger according to one embodiment of the invention.

(3) FIG. 2 is a view in longitudinal section of a closed geothermal exchanger according to a second embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

(4) Reference is now made to FIG. 1 which shows a closed geothermal exchanger 1 according to one embodiment of the invention. The geothermal exchanger comprises a shell 2 containing CO.sub.2 in a supercritical state 3 (supercritical CO.sub.2) used as a heat-transfer fluid. The shell 2 and the heat-transfer fluid 3 are in direct contact.

(5) The shell 2 is made of a silicone elastomer which is a flexible material the shore A hardness of which is comprised between 60 and 90. However, it could be made from any other material that gives it the flexibility it needs to come into direct contact, under the effect of the heat-transfer fluid, with the wall of the borehole.

(6) The silicone elastomer used to make the shell 2 has a thermal conductivity higher than 3.5 W/(m.Math.K). This value is similar to the thermal conductivity of granite.

(7) The heat-transfer fluid may be any heat-transfer fluid chemically compatible with silicone elastomer, which may have a temperature ranging as high as at least 250 C. and a pressure of up to at least 150 bar.

(8) The geothermal exchanger 1 is installed in a vertical borehole 4 made in a granite formation 5. Under the effect of the pressure of the supercritical CO.sub.2 fluid 3, the shell 2 is brought into direct contact with the wall of the borehole 4. The injection 6 and return 7 of the heat-transfer fluid 3 are performed at the surface end 8 of the upper part of the shell 2.

(9) The upper part of the exchanger 1 is contained within a containment harness 9 made of Kevlar straps. It may be made of other aramid straps. The straps used have a width of 1 cm and their cumulative width is 40 cm. The straps could have a different width. The containment harness constitutes a containment device the shape of which could be other than that of a harness.

(10) The containment harness 9 extends over the upper part of the shell, including its surface end 8. It extends over a length L1 of 1 m and is wedged between the bladder 2 and the granite formation 5.

(11) The shell 2 extends over a length L2 of 20 m. This length may be different. It may vary from 10 to 20 m. Its diameter d is 20 cm. It may be different and vary from 20 to 30 cm. The thickness of its wall is 10 mm. It may be different and vary from 10 to 30 mm.

(12) For an internal pressure of 120 bar in the shell 2, the load on the upper part of the exchanger 20 cm in diameter in contact with atmospheric pressure is 38 000 daN. The Kevlar straps used have a tensile strength of 1000 daN per cm of width. A cumulative width of around 40 cm of straps distributed over the 62 cm of circumference of the bladder 2 makes it possible to compensate for the load on the upper part of the exchanger. As it is pressurized, when the bladder 2 is not yet pressed firmly against the wall of the hole, the resistance of the bladder 2 to stretching is enough to keep the bladder 2 and the heat-transfer fluid 3 inside the borehole 4. When the bladder 2 is pressed against the wall of the borehole 4 by 120 bar of internal pressure, the bladder 2 is held in place by the resistance of the walls of the borehole and by the resistance of the containment harness 9. As the length of strap wedged between the bladder 2 and the granite formation 5 is 1 m, the area of straps subjected to the pressure of 120 bar is 100 cm.sup.2, which corresponds to a crushing force on the strap of 12 000 daN, which means that a tensile force of 1000 daN on the strap can be withstood.

(13) FIG. 2 shows a closed geothermal exchanger 1 according to another embodiment of the invention. The exchanger 1 comprises a shell 2 containing supercritical CO.sub.2 fluid 3. The shell 2 and the heat-transfer fluid 3 are in direct contact.

(14) The shell 2 is made of silicone elastomer with a shore A hardness comprised between 60 and 90. It could, however, be made from any other material that gives it the flexibility that allows it to come into direct contact, under the effect of the heat-transfer fluid, with the wall of the borehole.

(15) The silicone elastomer used to make the shell 2 has increased thermal conductivity, higher than 3.5 W/(m.Math.K).

(16) The heat-transfer fluid may be any heat-transfer fluid chemically compatible with silicone elastomer, which may have a temperature ranging as high as at least 250 C. and a pressure of up to at least 150 bar.

(17) The exchanger 1 is installed in a vertical borehole 4 made in a granite formation 5. Under the effect of the pressure of the supercritical CO.sub.2 fluid 3, the shell 2 is brought into direct contact with the wall of the borehole 4. The injection 6 and return 7 of the heat-transfer fluid 3 are performed at the surface end 8 of the upper part of the shell 2.

(18) The upper part of the exchanger 1 is contained in a containment harness 9 made of Kevlar straps. It may be manufactured from other aramid straps. The containment harness constitutes a containment device the shape of which could differ from that of a harness. The straps used have a width of 1 cm and their cumulative width is 40 cm. The straps could have a different width. The containment harness constitutes a containment device the shape of which could differ from that of a harness.

(19) The containment harness 9 extends over the upper part of the shell, including its surface end 8. It extends over a length L1 of 1 m and is wedged between the bladder 2 and the granite formation 5.

(20) The exchanger 1 also comprises an inner tube 10 made of silicone elastomer. This inner tube 10 has an opening 11 allowing fluid to pass between the inside of the tube 10 and the outside of the tube 10.

(21) The circulation of heat-transfer fluid 3 in the shell 2 is thus organized from the inside of the tube 10 toward the outside of the tube 10, as indicated by the three arrows in the shell 2. The direction of circulation in the shell 2 may be organized in the opposite direction.

(22) The silicone elastomer of which the inner tube 10 is made has a low thermal conductivity so as to limit internal thermal heat losses. The value of its thermal conductivity is of the order of 0.2 W/(m.Math.K).

(23) The shell 2 extends over a length L2 of 30 m. This length may be different. It may vary from 12 to 30 m. Its diameter is 20 cm. It may be different and vary from 20 to 30 cm. The inside diameter of the inner tube 10 is 120 mm. The diameter of the inner tube may vary according to the diameter d of the geothermal exchanger 1. It may be 200 mm when the diameter d of the geothermal exchanger is 30 cm. The thickness of its wall is 10 mm. It may differ and vary from 10 to 30 mm.

(24) For an internal pressure of 120 bar in the shell 2, the load on the upper part of the exchanger 20 cm in diameter in contact with atmospheric pressure is 38 000 daN. The Kevlar straps used have a tensile strength of 1000 daN per cm of width. A cumulative width of around 40 cm of straps distributed over the 62 cm of circumference of the bladder 2 makes it possible to compensate for the load on the upper part of the exchanger. As it is being pressurized, when the bladder 2 is not yet pressed firmly against the wall of the hole, the resistance of the bladder 2 to stretching is enough to hold the bladder 2 and the heat-transfer fluid 3 in the borehole 4. When the bladder 2 is pressed against the wall of the borehole 4 by 120 bar of internal pressure, the bladder 2 is held in place by the resistance of the walls of the borehole and by the resistance of the containment harness 9. As the length of strap wedged between the bladder 2 and the granite formation 5 is 1 m, the surface area of strap subjected to the pressure of 120 bar is 100 cm.sup.2, which corresponds to a crushing force on the strap of 12 000 daN, which means that a tension of 1000 daN on the strap can be withstood.

(25) The invention is not restricted to the embodiments set out and other embodiments will be clearly apparent to those skilled in the art.