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The invention relates to a line shaft pump for extracting a fluid from a geothermal production well and a method for installing this pump.
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Geothermal energy is deemed a long-term usable and essentially carbon-neutral energy source on a global scale, wherein its reserves are stored in the upper three kilometers of the crust of the earth. In the recent years two main strategies of geothermal energy use emerged, first, the use of the heat itself, and second, indirect use which involves converting the heat into electricity in a geothermal power plant, while in some cases today, both strategies are integrated in combined heat and power systems to optimize efficiency. While in some places, this energy naturally emerges for example in form of thermal springs, geysers, or volcanoes, most industrially usable sites require some kind of drilling and often extraction and re-injection of water as a medium for heat transport is necessary.
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The first step in geothermal water extraction is identifying suitable geothermal reservoirs, which are underground zones where heat from the earth's interior has naturally heated water or, depending on the pressure conditions, even steam. Typically, the closer these reservoirs are in the vicinity of tectonic plate boundaries, volcanic activity, or hotspots where magma is close to the crust of the earth, the higher the achievable temperature of extracted water or steam. However, most of the geothermal heat extraction today relies on geothermal wells with heated water as a fluid. Once a geothermal reservoir is identified, the next step involves drilling a geothermal well, wherein specialized drilling rigs bore into the crust of the earth to reach depths from a few hundred meters to several kilometers, depending on the location and the type of geothermal resource. Generally, geothermal wells consist of multiple layers of casing, typically made of concrete, steel, or composite structures thereof to prevent collapsing of the well and to protect the groundwater from contamination. Additional cementing is often used to seal the space between the casing and the surrounding rock. Furthermore, to maintain a balance of the reservoirs, for every extraction well, one or more re-injection wells are used to re-inject the extracted steam or water, re-heating the reinjected water or steam. However, a sufficient distance between extraction and re-injection wells needs to be maintained for preventing shortcuts and thus decreasing the efficiency of the geothermal reservoir.
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In in steam-dominated reservoirs, the pressure is often sufficient to propel steam to the surface without the need for pumping, whereas in reservoirs, where liquid components dominate the composition of the fluid in the reservoir, pumps are often used to bring the hot water to the surface. One option is the installation of submersible pumps within the well to lift the water, which suffer from short lifetimes, and high long-term maintenance and replacement cost due to their application in high temperature environments, which poses increased risk to many, in particular electrical, components. Another, initially more costly, yet long term more cost-efficient option is the use of line shaft pumps, wherein the main electrical components such as the motor, are arranged above ground and the suction and impeller parts are arranged well below ground, connected by a long line shaft and tubing to transport the fluid to the surface. Next to their robust design, efficiency, and adaptability to the demanding conditions often encountered in geothermal energy production, line shaft pumps further are able to resist corrosive environments with minimal maintenance, thus enabling reliable long-term operation.
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However, the rotating line shaft in such pumps can generate friction and thus additional heat and vibrations, which, also considering the length of the shaft, could be significantly detrimental to the pump and the well. To mitigate this friction, lubricants are typically applied to the line shaft and other related parts, such as bearings and sleeves. One of the most common methods for applying lubrication to a line shaft involves adding the fluid above ground, so that gravity or a dedicated pumping system distribute the lubrication along the line shaft, resulting in a downwards flow of lubricant. At the end of the lubricated line shaft, the lubricant is today usually discharged into the borehole, thus either contaminating the pumped water, or accumulating in the borehole, which requires regular evacuation of the lubricant during pump standstills. This leads to increased operating costs and negative environmental impacts.
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Various publications have already suggested bringing the oil directly back to the surface via a separate return line. However, no system has yet been brought to market maturity or tested in a real facility, as typically, the space between the inner diameter of the pump geothermal well, i.e., the borehole and its casing, and the outer diameter of the pump riser tube or the coupling of the individual riser tube segments is commonly regarded as too small. In particular, in standard geothermal wells, the well has a diameter of 13 5/8" or 13 3/8" and an inner diameter of 314.33 mm. However, a typical coupling of riser tube segments has an outer diameter of 298.45 mm. This results in a gap of only 7.9 mm between the pump and the wall, making a return line impossible.
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Further, even if installation of very small piping in such narrow gaps would be possible, a non-destructive installation of the pump assembly would also not be possible, as typically, and related to the typical length up to 750 m and the inertia of the pump, parts of the assembly could hit the wall the geothermal production well, in turn damaging an outer return line with the slightest movement during installation.
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On the other hand, installation on the inside of the riser tube is commonly regarded as impossible as flow induced vibrations could lead to significant damage and environmental regulations could prohibit such solutions due to the potential risk upon damage.
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In summary, while line shaft pumps offer significant advantages in geothermal applications, the lack of a functioning oil return system poses a critical challenge. This issue can result in oil leakage into the geothermal fluid, raising serious environmental concerns by contaminating the extracted water and surrounding ecosystems.
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Starting from this state of the art, it is therefore an object of the invention to propose a line shaft pump for extracting a fluid from a geothermal well and a method for its installation, which enable reliable and continuous operation of the line shaft pump while enabling optimized operational costs.
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The subject matter of the invention satisfying this object is characterized by the features of the independent claims.
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Thus, according to the invention a line shaft pump for extracting a fluid from a geothermal production well is proposed, wherein the geothermal production well comprises an inner well surface, wherein the line shaft pump comprises a drive unit and a hydraulic unit with a rotor, wherein the hydraulic unit comprises an inlet section for receiving the fluid at a suction pressure, an outlet section comprising a discharge head for discharging the fluid at a discharge pressure, and a riser tube, wherein the rotor comprises at least one impeller for conveying the fluid from the inlet section to the outlet section, wherein the impeller is arranged within an impeller section, wherein the rotor further comprises a line shaft for rotating about an axial direction, wherein each impeller is fixedly connected to the line shaft, wherein the line shaft comprises at least one line shaft segment and extends from a first end to a second end, wherein the first end is arranged at the drive unit, wherein the line shaft is configured for transmitting rotational energy from the drive unit to the at least one impeller, wherein the riser tube comprises a first outer surface and a first inner surface, wherein the riser tube is arranged vertically between the impeller section and the discharge head and configured to supply the fluid from the impeller section to the discharge head, wherein an essentially impermeable lubricant retention tube for transporting a lubricant is arranged within the riser tube and configured to surround the line shaft at a distance, providing an essentially vertical channel delimited in radial direction by the line shaft and the lubricant retention tube. In this context, the essentially impermeable lubricant retention tube is to be understood as a tube, designed and built for creating an impermeable barrier between the pumped fluid on the outside and the lubricant on the inside of the lubricant retention tube, however, since the tube can be realized by joining a plurality of segments, either by welding or by coupling, i.e. the tube might not be made as a single piece, it can only be described as essentially impermeable, which for the skilled person entails it to be as impermeable as possible. One advantageous option to realize this tubing is for example coil tubing. Further, the hydraulic section comprises at least one centralizer arranged to surround the riser tube forming an axial through hole for receiving the riser tube, wherein each centralizer is in intimate contact to the first outer surface, wherein the radial dimensions of each centralizer are smaller than the radial dimensions of the inner well surface, wherein each centralizer is configured such that the radial distance between the first outer surface and the inner well surface is essentially constant in all radial directions at the axial position of each centralizer, wherein each centralizer comprises at least one additional axial through hole.
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One advantage of using a line shaft pump according to the invention is that the at least one centralizer provides a stable space between the inner well surface and the first outer surface, which enables installation and operation of the pump while allowing for additional piping on the outside of the riser tube. This protection by the at least one centralizer is crucial to the functioning of any additional piping, whether it being a lubricant return line, probing lines of any sort, suction lines for transporting any component above ground by a negative pressure applied from the top, or supply lines for additives, as any of these potential lines could otherwise be easily crushed and damaged during operation and installation of the pump. Therefore, this space provided by the centralizers enables reliable and continuous operation of the line shaft pump without required regular maintenance stops for lubricant recovery and with increased compliance with environmental standards, thereby improving optimizing operational costs.
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In a preferred embodiment, each centralizer comprises at least two segments in radial direction, which facilitate the installation of each centralizer around the riser tube.
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Further, in a preferred embodiment, each centralizer comprises at least one damping element, wherein the material of each damping element is selected such that its hardness is lower than the hardness of the inner well surface. Examples of suitable materials for the damping element comprise PEEK, PPS, PTFE, rubber, or vulcanite.
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An advantageous configuration is that each damping element is configured to cover the radially outermost edges of each centralizer. This way, the possibility of the centralizer transmitting vibrations from the pump to the inner well surface, which could damage the production well over time and lead to the loss of the geothermal resource, is minimized.
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Further, an advantageous configuration is that the radial distance between the first outer surface and the inner well surface is greater than 8 mm, preferably greater than 19 mm, and most preferably greater than 34 mm.
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In another preferred embodiment, a collector section is arranged at the end of the impeller section facing the riser tube, wherein the collector section comprises a lubricant collection chamber for receiving the lubricant from the channel, wherein the lubricant collection chamber is in fluid communication with a lubricant return line, wherein the lubricant return line is routed to the outlet section and arranged on the outside of the riser tube.
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The larger radial distance as described above helps to create the necessary space for the lubricant return line, which can either be realized by a larger production well as the standard diameter, or a smaller diameter of the riser tube or impeller section. While this approach is widely considered inefficient and counterintuitive as it reduces the maximum diameter for the pump, and therefore its capacity, in a given production well, this measure enables long term reliable application of the pump through efficient recovery of the lubricant during operation. The return line can for example be realized as a tube routed with or without direct contact with the riser tube, as a tube intimately attached to the riser tube on essentially the full length, as a U-profile attached to the riser tube such that the riser tube forms a part of the wall of the return line, as a milled section of the riser tube, closed on the top side by sheets, or any other means, which allow for transport of the lubricant from the collector section to the outlet section.
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In a further preferred embodiment, the collector section further comprises a sealing device arranged adjacent and vertically below the collection chamber, wherein the sealing device is configured to prevent intrusion of lubricant from the channel or the collection chamber into the impeller section.
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In another embodiment, the riser tube comprises a plurality of riser tube segments, arranged axially adjacent to each other, wherein each riser tube segment is connected to at least one adjacent riser tube segment through a segment coupling. This configuration particularly advantageous for easy installation of the line-shaft pump, particularly in deep wells.
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Further, an advantageous configuration is that a protector element is arranged adjacent to the impeller section, axially extending towards the collector section, wherein the protector element is configured to extend beyond the lowermost end of the lubricant return line, wherein the distance between the radially outermost edge of the protector element and the inner well surface is larger than the distance between the radially outermost edge of the centralizer and the inner well surface. This protector element is particularly helpful for preventing damage to the lubricant return line during installation of the line-shaft pump, while the smaller diameter, compared to that of each centralizer, ensures that the protector element does not touch the inner well surface during operation of the line-shaft pump, and thus also prevents damage to the inner well surface.
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A further beneficial configuration is that the outlet section comprises a reducer, wherein the reducer comprises a first axial opening, and a second axial opening, wherein the first axial opening is arranged and configured for connecting to the discharge head and the second axial opening is arranged and configured for connecting to the production well, wherein the radial dimension of the first axial opening is smaller than the radial dimension of the second axial opening, wherein the first axial opening is configured for receiving the riser tube, wherein the riser tube extends through the reducer, wherein the reducer comprises at least one axial through hole for receiving tubing outside of the riser tube. This configuration on one hand provides the connection between the production well, the riser tube and the discharge head, and on the other hand enables easy piping of the lubricant return line.
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In another embodiment, a pressure regulation device is configured to control the pressure within the channel or the lubricant return line. This pressure regulation device allows for precise control of the lubricant flow rate and thus enables cost and energy efficient operation while guaranteeing sufficient lubrication of the line-shaft pump in all operating modes.
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In a further embodiment, a separator is arranged and configured for receiving the lubricant from the lubricant return line and removing contaminants from the lubricant.
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Further, according to the invention, a method is proposed for installing a line shaft pump, comprising the steps of
- lifting a component, section, or segment of the line shaft pump from a storage unit, centralizing the component, section, or segment axially above the geothermal production well, lowering the section or segment into the geothermal production well using a centralizing device for preventing damage of the inner well surface and the pump component, section, or segment;
- using a holding device to hold all lowered components, sections, or segments in place for joining the next component, section, or segment, for forming an assembly, after the component, section, or segment has been lifted, centralized, and lowered;
- lifting the assembly for removing the holding device and lowering the assembly for reinstallation of the holding device and repeating of the previous steps until completion of the pump installation.
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Further advantageous measures and embodiments of the invention will become apparent from the dependent claims.
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The invention will be explained in more detail hereinafter with reference to embodiments of the invention and with reference to the drawings.
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In the drawing is shown:
Fig. 1.: A schematic drawing of a first embodiment of a line shaft pump for extracting a fluid from a geothermal well.
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One embodiment of a line shaft pump for extracting a fluid from a geothermal well according to the invention is shown in Fig. 1, wherein a line shaft pump 1 for extracting a fluid from a geothermal production well 2 is shown. The geothermal production well 2 comprises an inner well surface 3. The line shaft pump 1 comprises a drive unit D and a hydraulic unit H with a rotor 4, wherein the hydraulic unit H comprises an inlet section I for receiving the fluid at a suction pressure, an outlet section O comprising a discharge head 5 for discharging the fluid at a discharge pressure, and a riser tube 6, wherein the rotor 4 comprises at least one impeller 7. The impeller 7 is arranged within an impeller section I', wherein the rotor 4 further comprises a line shaft 8 for rotating about an axial direction A.
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Each impeller 7 is fixedly connected to the line shaft 8, wherein the line shaft 8 extends from a first end to a second end, wherein the first end is arranged at the drive unit D, wherein the line shaft is configured for transmitting rotational energy from the drive unit D to the at least one impeller 7, wherein the riser tube 6 comprises a first outer surface 9 and a first inner surface 9'. In this embodiment, the riser tube 6 comprises to segments in axial direction, wherein both segments are joined by a coupling, wherein the outer surface of the coupling forms forms one surface with the first outer surface 9, even though the connection between the coupling and riser tube segments can comprise steps. In other embodiments, this connection might be formed without steps, yet in the scope of this invention, all types of coupling connections with the riser tube segments are suitable. The riser tube 6 is arranged vertically between the impeller section I' and the discharge head 5 and configured to supply the fluid from the impeller section I' to the discharge head 5. A lubricant retention tube 10 for transporting a lubricant is arranged within the riser tube and configured to surround the line shaft at a distance, providing an essentially vertical channel delimited in radial direction by the line shaft and the lubricant retention tube. The hydraulic section H comprises at least one centralizer 11 arranged to surround the riser tube 6 forming an axial through hole for receiving the riser tube, the centralizer 11 being in intimate contact to the first outer surface 9. Notably, the radial dimensions of each centralizer 11 are smaller than the radial dimensions of the inner well surface 3, and each centralizer 11 is configured such that the radial distance between the first outer surface 9 and the inner well surface 3 is essentially constant in all radial directions at the axial position of each centralizer 11. In this embodiment, the centralizer 11 comprises one additional axial through hole 12, which can be used for routing additional piping on the outside of the riser tube. In other embodiments, ach centralizer 11 can comprise a plurality of additional axial through holes.
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In other embodiments, each centralizer 11 comprises at least two segments in radial direction.
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Further, the first embodiment of a line shaft pump according to the invention shows that each centralizer 11 comprises at least one damping element, wherein the material of each damping element is selected such that its hardness is lower than the hardness of the inner well surface 3. In this embodiment, each centralizer 11 is made from one material only, which is the material of the damping element, however, in other embodiments, only parts of each centralizer are made from the damping material, wherein other materials can be selected to support structural integrity.
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Further, in the first embodiment, each damping element is configured to cover the radially outermost edges of each centralizer.
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In this embodiment, the radial distance between the first outer surface 9 and the inner well surface 3 is greater than 34 mm, whereas in other embodiments, the radial distance between the first outer surface 9 and the inner well surface 3 is greater than 8 mm, and preferably greater than 19 mm.
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Further, in the first embodiment, a collector section C is arranged at the end of the impeller section I' facing the riser tube 6, wherein the collector section C comprises a lubricant collection chamber 60 for receiving the lubricant from the channel, wherein the lubricant collection chamber 60 is in fluid communication with a lubricant return line 61, wherein the lubricant return line 61 is routed to the outlet section O and arranged on the outside of the riser tube 6. The collector section C further comprises a sealing device 70 arranged adjacent and vertically below the collection chamber 60, wherein the sealing device 70 is configured to prevent intrusion of lubricant from the channel or the collection chamber 60 into the impeller section I'. A protector element 90 is arranged adjacent to the impeller section I', axially extending towards the collector section C, wherein the protector element 90 is configured to extend beyond the lowermost end of the lubricant return line 61, wherein the distance between the radially outermost edge of the protector element 90 and the inner well surface 3 is larger than the distance between the radially outermost edge of the centralizer 11 and the inner well surface 3. In this embodiment, the proctor element 90 is formed as a collar surrounding the line shaft pump, to provide protection in all radial directions, however in other embodiments, the protector element 90 can comprise single or multiple parts for protecting in specific directions.
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Further, in the first embodiment, the outlet section O comprises a reducer 100, wherein the reducer 100 comprises a first axial opening, and a second axial opening, wherein the first axial opening is arranged and configured for connecting to the discharge head 5 and the second axial opening is arranged and configured for connecting to the production well 2, wherein the radial dimension of the first axial opening is smaller than the radial dimension of the second axial opening, wherein the first axial opening is configured for receiving the riser tube 6, wherein the riser tube 6 extends through the reducer 100, wherein the reducer 100 comprises at least one axial through hole for receiving tubing outside of the riser tube 6. In other embodiments, this reducer can be integrated in the discharge head.