GB2589393A

GB2589393A - Downhole energy harvesting

Info

Publication number: GB2589393A
Application number: GB2005905.1A
Authority: GB
Inventors: Sarfraz Sohab; Martin Hudson Steven; david jarvis Leslie
Original assignee: Metrol Technology Ltd
Current assignee: Metrol Technology Ltd
Priority date: 2020-04-23
Filing date: 2020-04-23
Publication date: 2021-06-02
Also published as: GB202005905D0

Abstract

A downhole energy harvesting system 3 comprising a downhole metallic pipe 2 and at least one thermally driven electrical generator module 4 disposed downhole when a first side of the generator module is exposed to a different temperature than a second side of the generator such that a temperature gradient exists. A convection inducing apparatus 6 is disposed in the annulus 23 for inducing convection in fluid in the apparatus. The first side of the generator module is in thermal contact with the pipe. The second side is in thermal contact with the fluid. The convection inducing apparatus comprises a thermally insulated pipe section having first and second openings for allowing the ingress and egress of fluid from the interior of the pipe section. The pipe section is disposed in the annulus.

Description

DOWNHOLE ENERGY HARVESTING

This invention relates to downhole energy harvesting.

There is a desire to have power available downhole in oil and/or gas well installations for a number of reasons. For example, it can be desirable to be able to take readings of parameters downhole and transmit these readings back towards the surface and/or it can be desirable to power and control downhole pieces of equipment. At least in some instances it is desirable to include the facility to provide such control from the surface.

In some limited circumstances, it may be possible to power downhole devices from the surface via, for example, direct cable connections. In other circumstances the use of local battery power is typically the most realistic option. However, this comes with issues since the life of batteries will be limited and the relatively high temperatures typically experienced downhole can severely limit the battery life which is available. Therefore, it is desirable to look to provide different sources of power.

Previous attempts have been made to generate electrical power downhole using thermally driven electrical generators. However, in using such devices problems can arise as the efficiency and the effectiveness of such devices tends to be low with the relatively small temperature differentials which are typically available downhole.

The ideas in the present application are aimed at addressing such issues.

According to one aspect of the present invention there is provided a downhole energy harvesting system for use in a well installation having downhole metallic pipe running in a borehole of the well with an annulus around the downhole metallic pipe, the system comprising: at least one thermally driven electrical generator module disposed downhole for generating electrical power when a first side of the generator module is exposed to a different temperature than a second side of the generator module such that a temperature gradient exists between the first and second sides of the generator module; and convection inducing apparatus disposed in the annulus for inducing convection in fluid which may flow in the convection inducing apparatus, wherein the first side of the generator module is disposed in thermal contact with the downhole metallic pipe and the second side of the generator module is disposed in thermal contact with said fluid which may flow in the convection inducing apparatus such that in use the generator module generates electricity when a temperature gradient exists between the first and second sides of the generator module and heat is transported away from the second side of the generator module by action of the convection.

This can help improve the effectiveness of electricity generation by transporting heat away from the "cold" side of the generator module. The convection cycle can also draw cooler fluid towards the site of the generator module.

The thermally driven electrical generator module may comprise at least one thermoelectric generator. Typically the at least one thermoelectric generator will be arranged to make use of the Seebeck effect.

The thermally driven electrical generator module may comprise a heat transfer portion for providing thermal contact between the downhole metallic pipe and the remainder of the thermally driven electrical generator module. The heat transfer portion may have a contact surface which is profiled so as to correspond to a shape of the wall of the downhole metallic pipe with which the module is to contact. The contact surface may be curved typically so as to be a part cylindrical surface -this may be concave or convex as appropriate. The heat transfer portion may be disposed, or arranged to be disposed, between the downhole metallic pipe and the thermoelectric generator, where present. The heat transfer portion may be of metallic material. The heat transfer portion may comprise a metal block.

The convection inducing apparatus may comprise a thermally insulated pipe section having an interior and first and second openings each for allowing the ingress fluid into the interior of the pipe section and/or the egress of fluid out of the interior of the pipe section, the pipe section being disposed in the annulus so as to extend along the annulus with the second opening at position located further into the well than the first opening.

This can provide a simple and convenient way to induce a convection cycle.

The insulated pipe section may comprise a hollow walled pipe portion. The insulated pipe section may comprise insulating material applied to an inner and/or and outer wall of a pipe portion. The pipe portion in that case may be hollow walled or solid walled. The pipe portion may be of metallic material, say of steel.

The pipe section may be of or comprise thermally insulating material. The pipe section may be of thermally insulating material coated with an insulating material and inserted into a metallic pipe. This may provide structural robustness. The pipe section may comprise a thermally insulating pipe used in standalone form or to coated with another insulating material but not placed in a metallic pipe.

The generator module may be disposed in the region of the second opening of the insulated pipe section.

More generally the insulated pipe section may extend generally upwards along the annulus away from the location of the generator module.

The convection inducing apparatus may comprise at least one valve for controlling the flow of fluid in the insulated pipe section. The valve may be a one way valve, ie a non-return valve, to promote fluid flow in a preferred direction. The at least one valve may be provided to control the ingress fluid into the interior of the pipe section and/or the egress of fluid out of the interior of the pipe section via one of the first and second openings. Two valves may be provided with a first of the two valves arranged to control the ingress fluid into the interior of the pipe section and/or the egress of fluid out of the interior of the pipe section via the first opening and a second of the two valves arranged to control the ingress fluid into the interior of the pipe section and/or the egress of fluid out of the interior of the pipe section via the second opening.

The interior of the insulated pipe section may be in fluid communication with the annulus so that fluid from the annulus may flow in through one of the first and second openings and out of the other opening in the convection cycle.

This allows fluid from the annulus to be used in convection. This may be the simplest system to put in place and can yield benefits especially if cool fluid from higher in the well can be drawn to the site of the generator module.

The convection inducing apparatus may comprise at least one flow guiding portion for guiding flow into and/or out of the insulated pipe section. The at least one flow guiding portion may comprise a cowl portion. The cowl portion may be provided in the region of the generator module. The cowl portion may define two openings, a first opening to which the insulated pipe section is connected and a second opening providing fluid communication with the region local to the generator module. In one embodiment the second opening may be provided so as to be above the first opening in normal use such that heated fluid will tend to leave the cowl via the second opening and cooler fluid from higher in the annulus will tend to be drawn down the insulated pipe section into the cowl. In such a case the second opening may be provided with a non-return valve arranged to resist inward flow via the second opening.

The radial position of the downhole energy harvesting system within the annulus may be chosen to enhance desired results.

In particular the radial position of the generator module and/or the radial position of the convection inducing apparatus, say the insulated pipe section, may be chosen.

At least one of the downhole energy harvesting system, the generator module and/or the convection inducing apparatus may be disposed, or arranged to be disposed, at an offset location within the annulus. That is to say, either towards the inside of the annulus or towards the outside of the annulus.

At least one of the downhole energy harvesting system, the generator module and/or the convection inducing apparatus may be disposed, or arranged to be disposed, against an inner wall of the annulus or against an outer wall of the annulus.

At least one of the downhole energy harvesting system, the generator module and/or the convection inducing apparatus may be disposed, or arranged to be disposed, in the annulus so that there is a fluid flow path along the annulus on one side of the of the downhole energy harvesting system, the generator module and/or the convection inducing apparatus, but no such fluid flow path on the respective other side.

The first side of the generator module may be disposed, or arranged to be disposed, in mechanical contact with the metallic pipe at the inside of the annulus. This may be direct mechanical contact or via mechanical contact spacer.

In such a case the apparatus may be arranged so that there will be no fluid flow path for fluid in the annulus between the generator module and the metallic pipe in the region of the mechanical contact.

Further in such a case the apparatus may be arranged so that there will be a spacing between the second side of the generator module and the outside of the annulus so as to provide a fluid flow path for fluid in the annulus which runs between the generator module and outside of the annulus.

Where the generator module is disposed towards the outside wall of the annulus is typically the greatest temperature difference across the two sides of the generator module will be seen. Further the flow in the annulus tends to be more laminar towards the outside wall of the annulus and towards the inside wall of the annulus. Thus heat transfer into the annulus fluid and/or the setting up of convection currents may be more effective when the generator module is disposed towards zo the inside wall of the annulus or towards the outside wall of the annulus. Thus these offset locations are generally preferred to more central locations within the annulus.

The convection inducing apparatus may comprise a closed loop arrangement which may comprise a pair of heat exchange chambers, a first of which chambers is located in the annulus in the region of the electrical generator and a second of which chambers is located in the annulus at a location nearer to the surface of the well than is the first chamber and at least one thermally insulated pipe section connected to the two heat exchange chambers for providing fluid communication therebetween.

The convection inducing apparatus may comprise a closed loop arrangement comprising a pair of thermally insulated pipe sections comprising said insulated pipe section and a second such pipe section and a pair of heat exchange chambers, a first of which chambers is located in the annulus in the region of the electrical generator and connects first ends of the insulated pipe sections to one another and a second of which chambers is located in the annulus at a location nearer to the surface of the well and connects second ends of the insulated pipe sections to one another so that there is fluid communication between the interiors of the pipe sections and the chambers, and heat exchange fluid provided in the closed loop arrangement for flowing around the loop in a convection cycle.

Often the well installation will be a producing well installation with oil and/or gas product flowing up the borehole within the downhole metallic pipe. Thus heat from the product exiting the well acts as the heat source for electricity generation.

In such cases this flow will be within the metallic pipe -it may be directly within the metallic pipe or within a second run of metallic pipe that itself runs within the metallic pipe of the above aspects. The downhole metallic pipe, might for example be production tubing, or casing.

In some cases the annulus may be defined between the downhole metallic pipe and the formation. In other cases the annulus may be defined between the downhole metallic pipe and another run of metallic pipe. The two runs of metallic pipe might for example be production tubing and casing, or two runs of casing.

In other cases the system may be used in an abandoned well. In such a case there will not be a flow of product towards the surface. However the fluid in the reservoir will still act as a source of heat to drive the system as this heat tends to flow into the fluid held in the capped metallic pipe in the abandoned well installation.

According to another aspect of the present invention there is provided a downhole communication system for use in a well installation having downhole metallic pipe running in a borehole of the well with an annulus around the downhole metallic pipe, the system comprising downhole apparatus comprising first communications apparatus for communicating with a second communications apparatus and a downhole energy harvesting system as defined above for providing electrical energy to the downhole apparatus.

The downhole apparatus may comprise at least one sensor.

The downhole apparatus may comprise at least one actuator and a control unit for controlling the actuator.

The downhole energy harvesting system may be arranged to provide electrical energy to power one or more of: the first communications apparatus, the at least one sensor, the at least one actuator, the control unit.

In a specific example, the downhole apparatus may be arranged to be disposed, or be disposed, in an abandoned well below a sealing plug and may be arranged for monitoring a parameter in the fluid below the plug and transmitting data representative of this parameter back towards the surface. This may be useful, say, to take and communicate temperature and/or pressure readings, which may be at relatively infrequent intervals.

In another example the downhole apparatus, may be arranged to be disposed, or be disposed, in a B annulus and may be arranged for monitoring a parameter in the fluid in the annulus and transmitting data representative of this parameter back towards the surface. This may be useful, say, to take and communicate temperature and/or pressure readings, which may be at relatively infrequent intervals.

The first communication apparatus may comprise an acoustic communication unit disposed in the B annulus for acoustic communication with a communication device including an acoustic communication unit disposed in the A annulus. The communication device in the A annulus may be arranged for communication with an out of hole communication apparatus. Such communication may be via any convenient communication channel -eg via cable, via EM communications, via acoustic communications or any combination of these.

According to another aspect of the present invention there is provided an oil and/or gas well installation having downhole metallic pipe running in a borehole of the well with an annulus around the downhole metallic pipe and comprising a downhole energy harvesting system as defined above or a downhole communication system as defined above.

According to another aspect of the present invention there is provided downhole energy harvesting apparatus for providing an energy harvesting system in a well installation having downhole metallic pipe running in a borehole of the well with an annulus around the downhole metallic pipe, the apparatus comprising: at least one thermally driven electrical generator module locatable downhole for generating electrical power when a first side of the generator module is exposed to a different temperature than a second side of the generator module such that a temperature gradient exists between the first and second sides of the generator module; and convection inducing apparatus locatable in the annulus for inducing convection in fluid which may flow in the convection inducing apparatus, wherein the first side of the generator module is arranged to be disposed in thermal contact with the downhole metallic pipe and the second side of the generator module is arranged to be disposed in thermal contact with said fluid which may flow in the convection inducing apparatus such that in use the generator module generates electricity when a temperature gradient exists between the first and second sides of the generator module and heat is transported away from the second side of the generator module by action of the convection.

According to a further aspect of the present invention there is provided a downhole energy harvesting method for use in a well installation having downhole metallic pipe running in a borehole of the well with an annulus around the downhole metallic pipe, the method comprising: disposing at a downhole location in the well installation at least one thermally driven electrical generator module for generating electrical power when a first side of the generator module is exposed to a different temperature than a second side of the generator module such that a temperature gradient exists between the first and second sides of the generator module; inducing convection in fluid which may flow in convection inducing apparatus provided in the annulus, ensuring that the first side of the generator module is in thermal contact with the downhole metallic pipe and the second side of the generator module is in thermal contact with said fluid which may flow in the convection inducing apparatus so that heat is transported away from the second side of the generator module by action of the convection; and generating electrical energy in the thermally driven electrical generator module due to a temperature gradient existing between the first and second sides of the generator module.

Note that in general each of the optional features following each of the aspects of the invention above is equally applicable as an optional feature in respect of each of the other aspects of the invention and could be re-written after each aspect with any necessary changes in wording. Not all such optional features are re-written after each aspect merely in the interests of brevity.

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 schematically shows a well installation including a downhole energy harvesting system; Figure 2 shows another example of an energy harvesting system which might be to used in a well installation of the type shown in Figure 1; Figure 3 shows another example of an energy harvesting system which might be used in a well installation of the type shown in Figure 1; Figure 4 shows an abandoned well installation including an energy harvesting system of the type shown in Figure 1; and Figure 5 shows a well installation including an energy harvesting system of the type shown in Figure 1 installed in a B annulus.

Figure 1 shows a well installation comprising a well head 1 provided at the surface S and downhole metallic structure 2 comprising downhole metallic pipe running down into a borehole formed in the formation F. In this example there are two runs of downhole metallic pipe 2, namely production tubing 21 and surrounding this casing 22. An annulus 23 is defined between the production tubing 21 and the casing 22. In more general terms there may be multiple runs of casing 22 provided in a well installation with respective annuli defined between each adjacent pair of casing runs as well as the annulus 23 defined between the production tubing 21 and the casing 22. Conventionally the innermost annulus 23 is termed the "A annulus", the next annulus out is termed the "B annulus", and the next one the "C annulus", and so on.

In the present well installation a downhole energy harvesting system 3 is provided in the A annulus 23. This energy harvesting system 3 makes use of convection to help increase an available temperature differential for the thermally driven generation of electricity.

In the present embodiment the energy harvesting system 3 comprises a thermally driven electrical generator module 4 which is disposed so as to be in thermal contact with the outer surface of the production tubing 21. The energy harvesting system 3 further comprises insulation 5 provided on the outer wall of the production tubing 21 in the region of the thermally driven electrical generator module 4. In some cases this insulation 5 may be provided at locations which are circumferentially aligned with or at least close to a location at which the thermally driven electrical generator module 4 is disposed as illustrated in Figure 1. However, in alternatives the insulation 5 might be provided around the whole of the surface of the production tubing 21 in the region of the thermally driven electrical generator module 4. That is to say the insulation might surround the production tubing 21 as a sleeve.

In one particular alternative there may be provided insulation running along the length of the production tubing 21 all the way to the surface from the top end of the thermally driven module 4. This can provide a better thermal shielding higher up in the well from where the colder fluid is drawn down to cool the thermally driven module 4. Similarly, insulation below the thermally driven module 4 position is most advantageous if it runs as close as possible down to the reservoir level in order to shield the thermal loss from occurring below the thermally driven module 4. A combination of both below and above insulation can provide a much larger temperature differential across the thermally driven module 4 in combination with the convection apparatus.

In some circumstances a plurality of thermally driven electrical generator modules 4 might be provided around the circumference of the production tubing 21 either so that these are substantially adjacent with one another or with angular spacing therebetween.

The downhole energy harvesting system 3 further comprises convection inducing apparatus 6. In this embodiment the convection inducing apparatus is in the form of a thermally insulated pipe 61 which is disposed in the A annulus 23 and has open ends 61a, 61b which are in fluid communication with the annulus 23. As will be appreciated in a typical well installation the annulus 23 will be full of an annulus fluid which might typically be brine. Thus this annulus fluid, say brine, may flow into, through and out of the insulated pipe 61.

The thermally driven electrical generator module 4 comprises at least one, and in this embodiment, a plurality of thermoelectric generators 41 and a heat transfer metallic block 42 for transferring heat from the production tubing 21 to a first side of the thermoelectric generators 41. A second side of the thermoelectric generators 41 face towards the annulus 23 and in particular the fluid present in the annulus 23. This second side of the thermoelectric generators may be exposed to the annulus fluid or a suitable housing portion or heat sink portion (not shown) may be provided to effect thermal contact between the second side of the thermoelectric generators 41 and fluid in the annulus 23.

The heat transfer block 42 has a profiled -cylindrical -surface facing the production tubing 21 so as to fit closely against the tubing 21 to give good heat transfer.

In the present embodiment the thermoelectric generators 41 make use of the Seebeck effect for generating electricity when a temperature differential exists across the thermoelectric generators 41. Such thermoelectric generator devices are well known and commercially available. Thus detailed description of the thermoelectric generators is unnecessary in this application and is therefore omitted. As is well understood the amount of electricity which such thermoelectric generator devices can produce is related to the temperature differential which exists across the thermoelectric generator.

In alternatives different types of thermally driven electrical generators might be used -in one example, a device arranged as a heat engine might be provided together with an electrical generator arranged to be driven by the output of the heat engine. Thus say, a Sterling engine might be provided, the mechanical output of which drives an electrical generator such as an alternator. Improvement in the output of such arrangements may also be realised with the present ideas.

In a typical well installation there will be a thermal gradient from the production tubing On particular the product flowing out of the reservoir and up through the production tubing 21) on the one hand and the formation F of the other hand.

Correspondently there will be a temperature difference across the annulus 23 and across the thermoelectric generators 41 when situated as shown in Figure 1.

However, this temperature differential in typical circumstances will be small compared with that which produces effective operation of a thermoelectric generator. Without any other effects in play then a temperature differential across a thermoelectric generator 41 situated as shown in Figure 1 would likely be say 0.1 degree centigrade. This is too small a temperature differential to give a large electrical output from existing thermoelectric generators. However, at these types of temperature differences a small increase in temperature difference can result in a considerable increase in electrical output.

In the present arrangement the provision of the convection inducing apparatus 6 can help give rise to such an increase in temperature differential across the thermoelectric generators 41.

As will be appreciated, in a typical well installation the ambient temperature within the annulus will decrease as one progresses from a downhole location towards the surface. In a typical well installation this change in temperature might be in the order of 3 degrees centigrade per 100 meters of depth within the well.

The provision of the insulated pipe 61 in the annulus 23 serves to induce a convection cell in the annulus 23. Further with the inlet end 61b of the insulated pipe 61 in the region of the second side of the thermoelectric generators 41, heated fluid will tend to flow up the insulated pipe 61 which in turn will tend to drive cooler fluid from the region of the upper end 61a of the insulated pipe 61 down towards the region of the thermally driven electrical generator module 4. This is turn can reduce the temperature in the region of the second side of the thermoelectric generators 41. This is turn can give rise to an increase in temperature differential across the thermoelectric generators 41 and correspondingly give an increase in the electrical power that may be generated by the thermoelectric generators 41.

Note that were the convection cell to set up running in the opposite direction, with colder fluid flowing down the pipe 61, the same benefit would be seen. At least one one way valve might be provided in the pipe 61 if it is desired to control the flow direction of the convection cell.

To give good operation the insulated pipe 61 may have a length, of say, between 100 and 500 meters. The insulated pipe 61 may be formed in a number of different ways. In some circumstances a hollow walled metallic pipe might be used. The void in such a pipe 61 might be evacuated to give 'vacuum' insulation. In addition or alternative to this, insulating material may be applied to either an inside or outside external surface of the pipe or in a cavity in a hollow walled pipe.

As electricity is generated by the thermoelectric generators 41 this may be stored in rechargeable batteries and/or other suitable charge storage devices or used as it is generated by devices provided downhole. The energy harvesting system may comprise one or more charge storage device, such as a battery.

Furthermore it will be appreciated that the electrical energy generated may be used by any one of a number of different downhole devices such as sensors, communication units, control units and actuators for devices such as valves and so on.

It should be noted that whilst the energy harvesting system in the embodiment of Figure 1 is provided in the A annulus 23 there is no reason why such systems might not be provided in other annuli when present. All that is required is the presence of fluid in which convection can be induced -as will be clear from examples further below this need not be annulus fluid itself. Note that the systems may also be used in at least some situations in uncased annuli -ie an annulus formed by downhole pipe at the inside boundary and the wall of the formation at the other boundary.

The arrangement shown in Figure 1 shows at a schematic level what is probably a simplest implementation of the system.

Figure 2 shows an alternative arrangement for the energy harvesting system as may be provided in the annulus 23. Again here is provided a plurality of thermoelectric generators 41 put in thermal contact with the external wall of the production tubing 21 via a thermal contact metallic block 42 and again insulation 5 is provided. Here the insulation 5 is provided on the edges of the thermally driven electrical generator module 4. Furthermore a cowl portion 62 is provided in the region of the thermally driven electrical generator module 4 to help guide the convection flow. The cowl portion 62 has a first opening 62a which is connected to the opening 61b at the lower end of the thermally insulated pipe 61 and a second opening 62b which is provided towards an upper end of the cowl portion 62. A volume or chamber 62c is defined between the cowl portion 62 and the second side of the thermoelectric generators 41. As fluid in this chamber 62c is heated by heat that transfers across the generator module 4 from the fluid in the production tubing 21, this heated fluid can escape from the chamber 62c via the second opening 62b. In turn this will tend to draw cooler fluid down the insulated pipe 61 and into the chamber 62c via the first opening 62a. Again in this way a convection cycle is set up where cooler fluid tends to be driven to the region of the second side of the thermoelectric generators 41 to help maintain a temperature differential thereacross.

Figure 3 shows another downhole energy harvesting system which is similar to that shown in Figure 2 but which is arranged as a closed system. Thus here rather than setting up convection in the annulus fluid itself, convection is set up within fluid provided in the closed system. The system of Figure 3 has the same arrangement of electrical generator module 4 as in the arrangement shown in Figure 2. Further the thermally insulated pipe 61 is still provided. However, in this case rather than the cowl portion 62 a different arrangement is provided.

In the system of Figure 3 a first fluid tank 63 is provided adjacent the generator module 4 and a second fluid tank 64 is provided in the annulus 23 at a location above (that is nearer the surface of the well) the first tank 63. The insulated pipe 61 joins the tanks to one another and allows flow of fluid from the upper tank 64 towards the lower tank and a second thermally insulated pipe 61' is provided to give a return path back from the lower tank 63 towards the upper tank 64.

In operation fluid in the region of the second side of the thermoelectric generators 41 will tend to be heated and rise up the second insulated pipe 61' to the upper tank 63 and fluid from the upper tank 64 will be drawn down the insulated pipe 61 into the lower tank 63. Again as the upper tank 64 is at a higher position in the well the ambient temperature at that point will tend to be lower such that the fluid in the upper tank 64 will cool providing a source of cool fluid to the lower tank 63. In turn this means again that a favourable increase in temperature differential across the thermoelectric generators 41 can be realised.

In a development the fluid provided in the closed system may be selected so as to tend to undergo a phase change at the type of temperatures which will be seen in the lower tank 63. Thus a liquid with a suitable (relatively low) boiling point may be chosen for the fluid in the closed system such that boiling of the fluid may occur in the region of the thermoelectric generators 41 and condensation may tend to occur in the upper tank 64. If this can be achieved it can increase the amount of heat which may be extracted from the hot side of the thermoelectric generators 41 in the lower tank 63. Such a phase change based closed system may, in some cases, be implemented using a single thermally insulated pipe arranged so that the pipe takes the phase changed vapour from a lower tank adjacent to the second side of the thermoelectric generators 41 up to an upper tank acting as a condenser and the condensed liquid runs down the wall of the pipe to the lower tank.

Figure 4 shows another well installation including a downhole energy harvesting system of the type shown in Figure 1. The well installation in Figure 4 again comprises a wellhead 1 with metallic structure 2 running down into the formation F. In the present case the well installation includes a downhole communication system 7 which in turn comprises the energy harvesting system 3. The energy harvesting system 3 is as described in relation to Figure1 and therefore detailed description of it is omitted at this point. Of course other forms of energy harvesting systems such as shown in Figures 2 or 3 could equally be used in the installation shown in Figure 4.

The communication system 7 comprises a downhole tool 71 which is arranged to be powered using energy harvested by the energy harvesting system 3 and an out zo of hole communication unit 72.

In this case the well installation is an abandoned well installation and includes a plug P provided in the downhole metallic structure 2 which seals the well installation against the passage of product or other fluid from the reservoir past the location of the plug P. In such circumstances it can be desirable to monitor parameters below the plug P. In particular it may be desirable to monitor temperature and/or pressure in this region and periodically send details of those readings back towards the surface.

In the present embodiment the downhole tool 71 is arranged for taking pressure and/or temperature measurements and transmitting these back to the out of hole communication unit 72 making use of power from the downhole energy harvesting system 3.

The downhole tool 71 may be arranged for communicating this data back towards the surface using any available and convenient transmission means. For example, wireless EM (electromagnetic) communications may be used where signals are transmitted along the metallic structure itself, or acoustic communications may be used, or communications via the formation or via cables may be used, or any combination of all of these techniques.

This particular implementation is of interest because in an abandoned well situation there can be a desire to take readings over a relatively long period of time from a region which is fundamentally inaccessible from the surface due to the presence of the plug P. Therefore even in a circumstance where the rate at which power may be generated by the energy harvesting system 3 is low, a useful system can result. That is to say energy may be harvested over a prolonged period of time and stored in appropriate charge storage means, such as one or more battery, for use in transmitting a relatively small amount of data say containing temperature and/or pressure readings. Further such transmissions may be necessary only relatively infrequently, say once a month. With the arrangement shown in Figure 4 this type of monitoring and communication may be continued for many years after which any "one-shot" battery driven system would have ceased to operate due to dissipation of battery power.

Figure 5 shows yet another well installation including an energy harvesting system 3. Again the well installation includes a wellhead 1 and metallic structure 2 running down into the formation F. Again the energy harvesting system 3 has the same structure and functioning of that shown in Figure 1 and therefore detailed description of this is omitted. Of course other forms of energy harvesting systems such as shown in Figures 2 or 3 could equally be used in the installation shown in Figure 5.

Again the well installation shown in Figure 5 includes a communication system 7 which includes a first downhole tool 71 which is arranged to draw power from the energy harvesting system 3. In this embodiment the energy harvesting system 3 and first downhole tool 71 are provided in a B annulus 23'.

The communication system 7 again comprises an out of hole communication unit 72 which is arranged for communication with the downhole tool 71. However, in this communication system a second downhole tool 73 is provided, in this case, in the A annulus 23. The second downhole tool 73 comprises an acoustic communication unit 73a for communicating via acoustic communications with the first downhole tool 71 provided in the B annulus 23' (with the first downhole tool 71 including an appropriate acoustic communication unit). The second downhole tool 73 comprises a second communication unit 73b arranged for communicating with the out of hole communication unit 72. In the present embodiment this communication is via a cable 74 although any suitable form of communication may be used for this leg of the communication channel -say as mentioned above in relation to Figure 4. Note what is of interest here is that power is provided in the relatively inaccessible B annulus by the downhole energy harvesting system 3 which may then be used by the first downhole tool 71 for whichever purpose is desired. Thus for example as well as communication with the second downhole tool 73 this energy might be used for taking readings, controlling the operation of devices and/or powering the operation of devices and so on.

In a further alternative which may be used alone or in combination with any of the embodiments described above, a plurality of convection inducing apparatus may be provided in a borehole. These may be provided to operate in series. As an example, in place of a longer single thermally insulated pipe, a plurality of shorter pipes may be provided in an axially spaced series. This can set up multiple convection cells, in effect stacked one on another to drive heat up the well to give say a desired heating or cooling effect at a chosen location. Using a series of pipes may ease manufacture and/or installation compared to using one longer pipe.

In general terms any energy harvesting system as defined and described above may comprise a plurality of convection inducing apparatus which are axially spaced from one another along the borehole. These may be arranged so that an upper end of a convection cycle set up by a first of the convection inducing apparatus is adjacent to a lower end of a convection cycle set up by a second of the convection inducing apparatus. At least one convection inducing apparatus may be circumferentially spaced from a respective axially adjacent convection inducing apparatus. The plurality of convection inducing apparatus may be arranged in a staggered formation in the borehole -each may extend along the direction of the borehole and be axially and circumferentially spaced from one or more respective adjacent convection inducing apparatus. For example, a staggered series of thermally insulated pipes may be provided, for example in an annulus -that is arranged along and around the annulus in a staggered configuration.

Claims

CLAIMS1. A downhole energy harvesting system for use in a well installation having downhole metallic pipe running in a borehole of the well with an annulus around 5 the downhole metallic pipe, the system comprising: at least one thermally driven electrical generator module disposed downhole for generating electrical power when a first side of the generator module is exposed to a different temperature than a second side of the generator module such that a temperature gradient exists between the first and second sides of the generator module; and convection inducing apparatus disposed in the annulus for inducing convection in fluid which may flow in the convection inducing apparatus, wherein the first side of the generator module is disposed in thermal contact with the downhole metallic pipe and the second side of the generator module is disposed in thermal contact with said fluid which may flow in the convection inducing apparatus such that in use the generator module generates electricity when a temperature gradient exists between the first and second sides of the generator module and heat is transported away from the second side of the generator module by action of the convection, and wherein the convection inducing apparatus comprises a thermally insulated pipe section having an interior and first and second openings each for allowing the ingress fluid into the interior of the pipe section and/or the egress of fluid out of the interior of the pipe section, the pipe section being disposed in the annulus so as to extend along the annulus with the second opening at position located further into the well than the first opening.
2. A downhole energy harvesting system according to claim 1 in which the thermally driven electrical generator module comprises at least one thermoelectric 5 generator arranged to make use of the Seebeck effect.
3. A downhole energy harvesting system according to claim 1 or claim 2 in which the insulated pipe section comprises a hollow walled pipe portion.
4. A downhole energy harvesting system according to any preceding claim in which the generator module is disposed in the region of the second opening of the insulated pipe section.
5. A downhole energy harvesting system according to any preceding claim in 15 which the insulated pipe section extends generally upwards along the annulus away from the location of the generator module.
6. A downhole energy harvesting system according to any preceding claim in which the convection inducing apparatus comprises at least one valve for 20 controlling the flow of fluid in the insulated pipe section.
7. A downhole energy harvesting system according to any preceding claim in which the interior of the insulated pipe section is in fluid communication with the annulus so that fluid from the annulus may flow in through one of the first and 25 second openings and out of the other opening in the convection cycle.
8. A downhole energy harvesting system according to any preceding claim in which the convention inducing apparatus comprises at least one flow guiding portion for guiding flow into and/or out of the insulated pipe section.
9. A downhole energy harvesting system according to claim 8 in which the flow guiding portion comprises a cowl portion provided in the region of the generator module, the cowl portion having two openings, a first cowl opening to which the insulated pipe section is connected and a second cowl opening providing fluid communication with the region local to the generator module.
10. A downhole energy harvesting system according to claim 9 in which the second cowl opening is provided so as to be above the first cowl opening in normal use such that heated fluid will tend to leave the cowl via the second cowl opening and cooler fluid from higher in the annulus drawn down the insulated pipe section into the cowl.
11. A downhole energy harvesting system according to claim 10 in which the second opening is provided with a non-return valve arranged to resist inward flow 20 via the second opening.
12. A downhole energy harvesting system according to any preceding claim in which at least one of the downhole energy harvesting system, the generator module and the convection inducing apparatus is arranged to be disposed, at an 25 offset location within the annulus.
13. A downhole energy harvesting system according to any preceding claim in which at least one of the downhole energy harvesting system, the generator module and the convection inducing apparatus is arranged to be disposed, 5 against an inner wall of the annulus.
14. A downhole energy harvesting system according to any preceding claim in which the convection inducing apparatus comprises a closed loop arrangement comprising a pair of thermally insulated pipe sections comprising said insulated io pipe section and a second such pipe section and a pair of heat exchange chambers, a first of which chambers is located in the annulus in the region of the electrical generator and connects first ends of the insulated pipe sections to one another and a second of which chambers is located in the annulus at a location nearer to the surface of the well and connects second ends of the insulated pipe is sections to one another so that there is fluid communication between the interiors of the pipe sections and the chambers, and heat exchange fluid provided in the closed loop arrangement for flowing around the loop in a convection cycle.
15. A downhole communication system for use in a well installation having downhole metallic pipe running in a borehole of the well with an annulus around the downhole metallic pipe, the system comprising downhole apparatus comprising first communications apparatus for communicating with a second communications apparatus and a downhole energy harvesting system according to any preceding claim for providing electrical energy to the downhole apparatus.
16. A downhole energy harvesting system according to claim 15 in which the downhole apparatus is arranged to be disposed, in an abandoned well below a sealing plug and arranged for monitoring a parameter in the fluid below the plug and transmitting data representative of this parameter back towards the surface.
17. A downhole energy harvesting system according to claim 15 in which the downhole apparatus is arranged to be disposed in a B annulus and arranged for monitoring a parameter in the fluid in the annulus and transmitting data representative of this parameter back towards the surface.
18. An oil and/or gas well installation having downhole metallic pipe running in a borehole of the well with an annulus around the downhole metallic pipe and comprising a downhole energy harvesting system as claimed in any one of claims 1 to 14 or a downhole communication system as claimed in any one of claims 15 10 17.
19. Downhole energy harvesting apparatus for providing an energy harvesting system in a well installation having downhole metallic pipe running in a borehole of the well with an annulus around the downhole metallic pipe, the apparatus comprising: at least one thermally driven electrical generator module locatable downhole for generating electrical power when a first side of the generator module is exposed to a different temperature than a second side of the generator module such that a temperature gradient exists between the first and second sides of the generator module; and convection inducing apparatus locatable in the annulus for inducing convection in fluid which may flow in the convection inducing apparatus, wherein the first side of the generator module is arranged to be disposed in thermal contact with the downhole metallic pipe and the second side of the generator module is arranged to be disposed in thermal contact with said fluid which may flow in the convection inducing apparatus such that in use the generator module generates electricity when a temperature gradient exists between the first and second sides of the generator module and heat is transported away from the second side of the generator module by action of the convection, and wherein the convection inducing apparatus comprises a thermally insulated pipe section having an interior and first and second openings each for allowing the ingress fluid into the interior of the pipe section and/or the egress of fluid out of the interior of the pipe section, the pipe section being arranged to be disposed in the annulus so as to extend along the annulus with the second opening at position located further into the well than the first opening.
20. A downhole energy harvesting method for use in a well installation having downhole metallic pipe running in a borehole of the well with an annulus around 20 the downhole metallic pipe, the method comprising: disposing at a downhole location in the well installation at least one thermally driven electrical generator module for generating electrical power when a first side of the generator module is exposed to a different temperature than a second side of the generator module such that a temperature gradient exists between the first and second sides of the generator module; inducing convection in fluid which may flow in convection inducing apparatus provided in the annulus, ensuring that the first side of the generator module is in thermal contact with the downhole metallic pipe and the second side of the generator module is in thermal contact with said fluid which may flow in the convection inducing apparatus so that heat is transported away from the second side of the generator module by action of the convection; and generating electrical energy in the thermally driven electrical generator module due to a temperature gradient existing between the first and second sides of the 10 generator module.