GB2478920A

GB2478920A - Power supply for downhole gas compression

Info

Publication number: GB2478920A
Application number: GB1004770A
Authority: GB
Inventors: Julian Reed
Original assignee: Corac Group PLC
Current assignee: Corac Group PLC
Priority date: 2010-03-22
Filing date: 2010-03-22
Publication date: 2011-09-28
Anticipated expiration: 2030-03-22
Also published as: GB2478920B; GB201004770D0

Abstract

An apparatus for gas lift includes an electronics unit 101 for installation, together with a plurality of gas compressors 301 and a plurality of motors for driving the respective gas compressors 301, in a well of a gas reservoir. The electronics unit 101 comprises a plurality of modules 113 and a channel 109 for transmitting production gas from the gas-producing formation to the plurality of gas compressors 301. Each module 113 comprises an electrical power system for powering a respective one of the motors. The electrical power system in each module 113 is operative to receive a DC power input and comprises a power electronics inverter to provide a controlled AC power output. The channel 109 and the plurality of modules 113 are adjacent, such that production gas flowing through the channel 109 serves to cool the power electronics inverter in each module 113. The electronics unit 101 is located in the well upstream of the compressors 301 and motors.

Description

POWER SUPPLY FOR DOWNHOLE GAS COMPRESSION

Background of the Invention

The present invention relates to a unit for installation in a well of a natural gas reservoir or depleted oil reservoir with significant gas retained in it, for the purpose of enhancing the recovery of gas from the well. In particular, the present invention relates to an electronics unit for installation, together with a plurality of gas compressors and a plurality of motors for driving the respective gas compressors, in a well of a natural gas reservoir or depleted oil reservoir.

Field of the Invention

The present invention relates to downhole gas compressors. Installing a series of compressors inside the casing of a gas well offers significant advantages over surface compressor installations, particularly at low flow rates and well pressures. Delivery of gas to the surface may be severely reduced once the friction losses inside the gas well production tubing reduce the capability of the reservoir pressure to force it to the surface.

Surface compression, in this instance, is not effective, as conventional gas compressors cannot have a zero or below zero suction pressure. In the case of downhole gas compression, the compressors are inserted in the casing of the well, ideally near the base of the well, where they can benefit from drawing on the gas at near the reservoir pressure.

It is also known in the art that the gas flowing from a well drilled into a gas reservoir carries with it a burden of vapour and liquid droplets. The pressure of gas at the base of a well falls as gas is extracted. Consequently the flow velocity of the gas in the production tubing also falls. Eventually, the flow velocity becomes too low to carry its load of condensed liquids. This problem is commonly known in the field as liquid loading". As a result, liquid accumulates at the base of the well, the gas flow decreases and eventually ceases. Gas production ceases to be economically effective before the gas flow ceases and operators will normally abandon a well long before the gas supply is exhausted.

It has previously been proposed to install into the well an electrically or hydraulically powered gas compressor. For example, in W097/33070, a downhole multistage rotary compressor is provided, driven by a brushless permanent magnet motor. The effect of the compressor is to accelerate production and increase the ultimate recovery from the reservoir. In this instance, the compressor acts to reduce the static pressure at its inlet which increases the pressure difference between the reservoir and the well, so as to stimulate greater flow. In addition, by increasing the gas pressure, the compressor increases the average density, which leads to a reduction in flow velocity and hence in a * 2 reduction in the pressure losses along the length of the well. A further effect of the compression is to raise the temperature of the gas and thereby delay condensation of vapour. This reduces the liquid loading problem.

Original prior art surface installed compressors typically operated at speeds of up to 5000 rpm. Therefore, conventional motors used in compressor or pump applications also typically rotate at 5000 rpm, and use a conventional 50 to 100 Hz AC (Alternating Current) supply. More recently developed compressors, however, such as those designed by the applicant of the present application, typically operate at speeds of between 45,000 and 60,000 rpm. Such speeds in excess of 45,000 rpm require frequencies an order of magnitude greater, typically of the order of 0.5 to 5 kHz.

Summary of the Invention

The inventors of the present invention have appreciated that, for operation at rotation speeds in excess of 45,000 rpm requiring frequencies of 0.5 to 5 kHz, there are problems associated with supplying power to the downhole gas compressors. Typically, the well may be 2 to 5 km deep. Transmitting this frequency along such long length cables would result in an unacceptable power loss. An additional significant problem is the high temperatures and pressures reached downhole; the inlet gas temperature may be greater than 100 °C and the resultant compression of the gas will increase that temperature further.

The inlet gas pressure may be as high as 60 to 80 atmospheres. These factors require a bespoke electrical cable design to resist the temperatures and also avoid the potential for explosive decompression of the insulation. Another significant problem is the small amount of space available in the well; production tubing for lowering into a well has a number of standard diameters which are designed to fit inside the well casing diameter. The casings are typically 4 to 13 inches in diameter and the production tubing is correspondingly at a smaller diameter to fit inside, with clearance to enable its removal. Therefore space is at a premium.

According to a first aspect of the invention, there is provided an electronics unit for installation, together with a plurality of gas compressors and a plurality of motors for driving the respective gas compressors, in a well of a gas reservoir, the electronics unit comprising: a plurality of modules, each module comprising an electrical power system for powering a respective one of the motors, the electrical power system in each module operative to receive a DC power input and comprising a power electronics inverter to provide a controlled AC power output; and a channel for transmitting production gas from the gas-producing formation to the plurality of gas compressors, wherein the channel and the plurality of modules are adjacent such that production gas flowing through the channel serves to cool the power electronics inverter in each module, and wherein the electronics unit is located in the well upstream of the compressors and motors.

Therefore the principle of the invention is to transmit high voltage DC along the main sub surface cables, minimising the power losses, and to generate the high frequency AC adjacent the motors themselves. Each module in the electronics unit has at least one DC input and comprises an inverter to provide the required AC input to the respective motor. This may be at any desired voltage and frequency, typically 0.5 to 10 kV and 0.5 to kHz. This means that DC power can be transmitted down the well, reducing the large losses typically encountered when transmitting AC power over such long distances. It can also assist with motor control.

In order to avoid destruction of the electronics modules at the high temperatures encountered in the well as the gas is compressed, the electronics unit is installed upstream of the compressors and motors. The incoming gas, which may be at a temperature in excess of 100 °C (as opposed to the much higher temperatures after compression), flows through the channel which is adjacent the electronics and serves to cool the electronics, in particular the inverters. Embodiments of the invention not only require the power electronics to operate at elevated temperatures, but also at elevated pressures, typically up to 100 bar. This may be tolerated by pressure balancing the enclosure of the power electronics to the bore of the assembly, external hydrostatic pressures being withstood by the casing of the well.

The DC power input is preferably supplied via a cable which passes between the casing and production tubing annulus. The DC power input may, although not necessarily, be at a variable voltage. The DC supply is preferably provided using a converter system at the surface, which rectifies the available surface power supply voltage and frequency to the voltage and current required by the DC link.

Preferably, each inverter comprises at least one power semiconductor device arranged to operate in switching mode. In that case, the AC output provided for the respective motor is controlled by switching signals from the power semiconductor device or devices. The switching mode may produce high switching losses which generates heat.

Additionally, heat is developed through other effects in the power electronics and the flow of production gas through the channel serves to dissipate that generated heat.

Each inverter may comprise a single power semiconductor device. More usually, however, each inverter comprises a plurality of power semiconductor devices. In one embodiment, each power semiconductor device is an insulated-gate bipolar transistor (!GBT).

Preferably, the unit further comprises a plurality of electrical storage devices. The electrical storage devices smooth the DC and assist with the switch commutation. The electrical storage devices may comprise capacitors. The particular type, number and values of the electrical storage devices will be determined by the characteristics of the particular installation, power demand or motor behaviour.

Having a plurality of electronics modules, each for powering a respective motor increases the flexibility of the unit. In particular, each motor can be controlled independently of the others. This means that the control power and frequency can be adapted as desired in accordance with the properties of the natural gas reservoir and the well.

In a preferred embodiment, the plurality of modules is arranged towards the outside of the unit and the channel is formed towards the centre of the unit.

Even more preferably, the electronics unit is shaped approximately as an elongate prism, with each electrical power system of a module being mounted on a face of the prism and the channel for the production gas formed substantially through the centre of the prism.

The channel for the production gas extends in the axial direction. In that case, the number of faces of the prism will be determined by the number of motors and compressors downstream of the electronics unit. For example, if there are six motors and six compressors, the electronics unit may be shaped as an elongate hexagonal prism. On each of the six faces of the electronics unit, one electrical power system may be mounted.

Other numbers of faces may be provided, for example, three (a triangular prism) if there are three motors and compressors, four (a square or rectangular or diamond prism) if there are four motors and compressors, five (a pentagonal prism) if there are five motors and compressors, seven (a heptagonal prism) if there are seven motors and compressors, eight (an octagonal prism) if there are eight motors and compressors, or any other suitable number. One or more faces of the prism may be unused. For example, if there are five motors and compressors, the electronics unit may be formed as an elongate hexagonal prism, with one face of the prism being unused.

The advantage of that arrangement is that each face of the elongate prism can be designed to be substantially identical. This maximises manufacturing efficiency.

In this embodiment, the channel is formed in the axial direction through the centre of the electronics unit. In that case, the electrical power system, in particular the inverters, is preferably in thermal contact with the channel, so that the flow of production gas through the channel dissipates heat generated by the module. In one embodiment in which the electronics unit is formed as an elongate prism, at least some of the inverters and other heat generating components and assemblies are mounted on a face of the prism, extending towards the centre of the electronics unit into the channel. * 5

If the electronics unit is formed as an elongate prism, preferably, the inverters on each face of the prism, or heat sinks on which the inverters are mounted, extend towards the centre, into the channel, at axially spaced locations. That is to say, the inverters on a first face may extend into the channel downstream or upstream of the inverters on a second face, the inverters on the second face may extend into the channel downstream or upstream of the inverters on a third face and so on. This means that space is used most efficiently, which is a great advantage for downhole applications, where space is at a premium. The inverters may be mounted on heat sinks which can extend almost fully into the channel thereby maximising heat dissipation. This may mean that the channel is not formed as a bore with a regular cross section, but this will not detrimentally affect gas flow.

Each face of the prism can still be manufactured as substantially identical, or at least similar, but the faces may be shifted with respect to one another in the axial direction.

Alternatively, the inverters or heat sinks may not extend into the channel, and the main chassis of the unit itself may be used as the heat sink. Other variants which maximise the thermal contact between the gas and the main body of the unit may be utilised, using finned or unfinned heatsinks, central or external flow of gas through the unit.

If the electronics unit comprises a plurality of electronic storage devices, the electronic storage devices may be arranged on the faces of the elongate prism, downstream of the inverters.

In an alternative embodiment, the plurality of modules is arranged towards the centre of the unit and the channel is formed towards the outside of the unit. The electronics unit may be shaped approximately as an elongate prism, with each electrical power system of a module being located towards the longitudinal axis of the prism, and the channel for the production gas being formed as a substantially annular channel around the electrical power systems. As described above, the rotational symmetry of the prism will preferably be determined by the number of motors and compressors downstream of the electronics unit.

The advantage of this arrangement is again that each sector of the elongate prism can be designed to be substantially identical.

In this alternative embodiment, the channel is formed as a substantially annular channel around the electronics unit. Again, the electrical power system, in particular the inverters, is preferably in thermal contact with the channel, so that the flow of production gas through the annular channel dissipates heat generated by the module. In one arrangement, the inverters, or heat sinks on which the inverters are mounted, extend from the centre towards the outer channel. They may extend outwards at axially spaced locations. This may mean that the channel is not formed as an annulus with a regular cross * 6 section, but this will not detrimentally affect gas flow. Alternatively, the main chassis of the unit itself is used as the heat sink.

Preferably, the components of the electronics unit are axially aligned in an elongate housing, having dimensions suitable for installation in the casing of a well of the natural gas reservoir. The unit will then find application as a compressor system to be deployed into a gas well enclosed inside the well casing and deployed using the production tubing to convey the system into the well. The invention may also be used with systems deployed using coil tubing, wire line or any other technique that can insert equipment into a well casing.

Preferably, the electronics unit is arranged such that the motors and compressors are arranged in series downstream of the electronics unit. This is discussed further below.

According to a second aspect of the invention, there is provided a unit for installation in a well of a gas reservoir, the unit comprising: a plurality of gas compressors; a plurality of motors, each motor for driving a respective one or more of the gas compressors; a plurality of modules located in the well upstream of the compressors and motors, each module comprising an electrical power system for powering a respective one of the motors, the electrical power system in each module comprising a power electronics inverter to provide the controlled power; and a channel adjacent the plurality of modules, for transmitting production gas from the gas-producing formation to the plurality of gas compressors, wherein production gas flowing through the channel serves to cool the power electronics inverter in each module.

In both aspects of the invention, the compressors and motors may be of the form described in the applicant's UK patent application GB 2384274A. That is to say, each motor may comprise an electric motor having a stator with stationary windings and a rotor supported on gas bearings for rotation relative to the stator, the gas bearings being arranged at the upstream and downstream opposite ends of the motors. Each compressor may be an axial compressor or another type of rotary compressor that produces axial and/or radial flow, in particular axial and centrifugal compressors. Each compressor may have a wheel mounted on an overhanging end of the motor rotor that projects beyond the gas bearing at the upstream end of the motor, whereby all the gas bearings of the compressor and the electric motor are arranged on the downstream side of the dynamic compressor.

With this arrangement, because gas always enters and leaves each compressor axially, it is possible to use a modular approach in which a number of compressor modules are close coupled (aerodynamically in series). That is to say, preferably the motors and compressors are arranged in series downstream of the plurality of modules. Further * 7 modules and/or a set of modules in series, may be disposed at various depths in the casing of a well in order to optimise the upward movement of droplets and inhibit condensation.

Features described in relation to one aspect of the invention may also be applicable to the other aspect of the invention.

Brief Description of the Drawings

The invention will now be described further, by way of example only, with reference to the accompanying drawings, of which: Figure 1 is a schematic axial section of an electronics unit according to one embodiment of the present invention; Figure 2 is a cross section through line Il-Il of Figure 1; Figure 3 is a schematic axial section of a downhole gas compressor, which may be used in conjunction with the electronics unit of Figure 1; and Figure 4 is a schematic view of the electronics unit of Figure 1 and a plurality of gas compressors according to Figure 3 arranged and located in a well.

Figure 1 is a schematic axial section of an electronics unit according to one embodiment of the present invention and Figure 2 is a cross section through line Il-Il of Figure 1. Referring to Figures 1 and 2, the electronics unit 101 is contained in an outer sleeve 103 for installation into the casing of a well of a natural gas reservoir. The electronics unit comprises an input portion 105, an upstream portion lOla, a downstream portion lOib and an output portion 107. A central bore 109 is provided extending through the entire electronics unit, for channelling the production gas. The flow of production gas is shown by the arrows in Figure 1. The central bore 109 is shown by dotted lines in Figures 1 and 2. However, as will become apparent from the discussion below in relation to Figure 2, the central bore may not have a regular cross section.

A plurality of capacitors 111 are mounted on the upstream portion lOla.

In this embodiment, the downstream portion lOlb is formed as an elongate hexagonal prism, as can be seen most clearly in Figure 2. Each face of the prism, labelled A to F in Figure 2, is substantially identical. (In Figure 1, faces B and C can be fully seen and faces A and D can be partially seen.) Each face has mounted thereon an electrical power system including a number of components, indicated schematically by 113. The components 113 include IGBTs (or other switching devices) 115 operating in switching mode. Each electrical power system powers an individual motor and compressor. That is to say, in this example, six motors and compressors are provided upstream of the unit. Any suitable number of electrical power system, and hence faces of the electronics unit, may be provided, however. * 8

As shown clearly in Figure 2, each IGBT (or other switching device) 115 on a face of the elongate hexagonal prism is mounted on an extended heat sink which extends into the central bore 109, shown schematically by dotted lines in Figure 2. The IGBTs on each face are axially spaced. In Figure 2, the IGBT 115 on face A extends into the central bore 109 at the line Il-Il of cross section and is therefore shown in solid lines. The IGBTs on the other faces A to F also extend into the channel, but at different locations, downstream of the IGBT on face A. These IGBTs are therefore shown in dotted lines. Spacing the IGBTs out in the axial direction maximises efficient use of space and heat dissipation. The IGBTs generate the most heat by their switching mode operation, so are the components for which heat dissipation is most important. However, other electrical components may also be mounted on heat sinks which extend into the central bore 109 in order to be cooled by the flow of production gas.

Figures 1 and 2 show one embodiment of the invention but variations are, of course, possible. For example, the IGBTs and other components may be mounted centrally, extending towards the outside of the unit. In that case, no central bore is provided and the production gas flows around the components, in an annular channel formed between the components and the outer sleeve. In addition, the module may be designed to not require specific heat sinks protruding into the gas stream and in this instance use the main chassis of the module as the heat sink itself.

Figure 3 is a schematic axial section of a downhole gas compressor, which may be used in conjunction with the electronics unit of the invention, such as the electronics unit shown in Figures 1 and 2. The downhole gas compressor 301 in Figure 3 is like the downhole gas compressor described in the present applicant's earlier patent application GB 2384274A. In Figure 3, reference numeral 302 designates the outer casing of a well, numeral 303 designates the outer sleeve that contains the assembly, and numeral 305 designates the casing of an electric motor. In this embodiment, the motor is a high frequency permanent magnet motor and is supplied with high frequency current via an umbilical (not shown). Typically the speed of the motor is 45,000 rpm. The preferred electric motor has a stator 311 and a permanent magnet rotor 313. However, it would be possible to use an alternative form of electric motor, such as a squirrel cage or switched reluctance motor. The rotor runs in journal bearings 317, 319, and thrust is taken by a thrust bearing having a collar 321. The motor drives the wheel 323 of the dynamic compressor which has runner blades 325. The direction of the flow of gas, and the direction, in which the compressor augments the pressure of the gas, is shown by the arrows. * 9

With this arrangement, because gas always enters and leaves each compressor axially, it is possible to use a modular approach in which a number of compressor modules are close coupled (aerodynamically in series). Further modules and/or a set of modules in tandem, may be disposed at various depths in the production tube of a well in order to optimise the upward movement of droplets and inhibit condensation. In Figure 3, a complete module is spanned by "W", a next module downstream of W is indicated at "X".

"Y" is an inlet nose fairing to be fitted to a single module or to the first of a number of coupled modules. The cone "Z" is a diffusing cone to be fitted at the exhaust of a module or at the exhaust of the last of a number of modules connected in series.

Figure 4 is a schematic axial section of the electronics unit of Figure 1 coupled to a plurality of gas compressors according to Figure 3. Figure 4 is not to scale. Figure 4 shows downhole electronics unit 101 upstream of four downhole gas compressors 301. The downhole electronics unit 101 and downhole gas compressors 301 are contained in a sleeve 401 and are shown in Figure 4 located in a well 403 of a natural gas reservoir. As shown, the downhole gas compressors 301 are arranged in series downstream of the downhole electronics unit 101. Four downhole gas compressors are shown in Figure 4 but any number of downhole gas compressors could be provided, with the electronics unit having the corresponding number of electrical power systems.

The arrangement described in relation to Figures 1 to 4 has a number of advantages. It avoids the losses resulting from transmitting AC power down a long well bore. It allows the AC power to be supplied downhole, despite the high temperatures, by providing an electronics unit which is cooled by flow of the production gas itself. This approach also assists in providing power and control to machines in remote or aggressive environments. It also reduces the number of power cables or conductors that need to be run to a multiple machine system. The electronics module is arranged so as to maximise efficient use of the space and to maximise efficient cooling. In particular, the embodiment in which the electronics unit is provided as an elongate prism having a number of faces equal to the number of compressors, provides an elegant solution to the problems of the prior art. * 10

Claims

CLAIMS1. An electronics unit for installation, together with a plurality of gas compressors and a plurality of motors for driving the respective gas compressors, in a well of a gas reservoir, the electronics unit comprising: a plurality of modules, each module comprising an electrical power system for powering a respective one of the motors, the electrical power system in each module operative to receive a DC power input and comprising a power electronics inverter to provide a controlled AC power output; and a channel for transmitting production gas from the gas-producing formation to the plurality of gas compressors, wherein the channel and the plurality of modules are adjacent such that production gas flowing through the channel serves to cool the power electronics inverter in each module, and wherein the electronics unit is located in the well upstream of the compressors and motors.
2. An electronics unit according to claim 1, wherein each inverter comprises at least one power semiconductor device arranged to operate in switching mode.
3. An electronics unit according to any preceding claim, wherein the plurality of modules is arranged towards the outside of the unit and the channel is formed towards the centre of the unit.
4. An electronics unit according to claim 3, shaped approximately as an elongate prism, with each electrical power system of a module being mounted on a face of the prism and the channel for the production gas formed substantially through the centre of the prism.
5. An electronics unit according to claim 1 or claim 2, wherein the plurality of modules is arranged towards the centre of the unit and the channel is formed towards the outside of the unit.
6. An electronics unit according to any preceding claim, wherein the components of the electronics unit are axially aligned in an elongate housing, having dimensions suitable for installation in the casing of a well of the natural gas reservoir. * 11
7. An electronics unit according to any preceding claim, arranged such that the motors and compressors are arranged in series downstream of the electronics unit.
8. A unit for installation in a well of a gas reservoir, the unit comprising: a plurality of gas compressors; a plurality of motors, each motor for driving a respective one or more of the gas compressors; a plurality of modules located in the well upstream of the compressors and motors, each module comprising an electrical power system for powering a respective one of the motors, the electrical power system in each module comprising a power electronics inverter to provide the controlled power; and a channel adjacent the plurality of modules, for transmitting production gas from the gas-producing formation to the plurality of gas compressors, wherein production gas flowing through the channel serves to cool the power electronics inverter in each module.
9. A unit according to claim 8, wherein the motors and compressors are arranged in series downstream of the plurality of modules.