Hydrocarbon heating system

Abstract

A hydrocarbon heating system for a hydrocarbon production and/or transportation system comprising at least one electrical conductor and an alternating current (AC) power source connected to the at least one electrical conductor. The alternating current power source generates heat in the at least one electrical conductor by providing alternating current power to the at least one electrical conductor.

Claims

1. A hydrocarbon heating system for a hydrocarbon production and/or transportation system comprising: a cable comprising at least two electrical conductors separated by an insulator; and an alternating current (AC) power source connected to the at least two electrical conductors, wherein, the alternating current power source generates heat in the cable by providing alternating current power to the cable; the at least two electrical conductors are terminated by an open circuit; the at least two electrical conductors are arranged as a transmission line; and the hydrocarbon heating system is arranged to use multiple AC frequency excitations, having different combinations of AC voltage and frequency, thereby to generate a specific heating profile of the cable.

2. A hydrocarbon heating system as claimed in claim 1, wherein the at least two electrical conductors are terminated together either by an inductive connection or a capacitive connection.

3. A hydrocarbon heating system as claimed in claim 1, wherein the at least two electrical conductors are arranged as a capacitor.

4. A hydrocarbon heating system as claimed in claim 1, wherein the heating along the length of the at least two conductors is controlled by variation of: magnitude of AC voltage of the AC power source; frequency of the AC voltage of the AC power source; or a combination of AC voltage and frequency.

5. A hydrocarbon heating system as claimed in claim 1, wherein the at least two electrical conductors are attached to a tube for carrying the hydrocarbons, such that heat emanating from the at least two electrical conductors is distributed to the hydrocarbons.

6. A hydrocarbon heating system as claimed in claim 1, wherein the at least two electrical conductors are packaged in the form of a heat trace cable.

7. A hydrocarbon heating system as claimed in claim 6, wherein the heat trace cable comprises an existing electrical cable, such as a pump power cable.

8. A well comprising a hydrocarbon heating system as claimed in claim 1.

9. A marine riser comprising a hydrocarbon heating system as claimed in claim 1.

10. The hydrocarbon heating system as claimed in claim 1, wherein the hydrocarbon heating system is arranged to generate a tapered heating profile.

11. A hydrocarbon heating system as claimed in claim 1, wherein parameters of the hydrocarbon production/transportation system are collected and the AC voltage and frequency selected based on those parameters according to a pre-defined algorithm.

12. A hydrocarbon heating system as claimed in claim 11, wherein the parameters include: thermal parameters of the solids and fluids in the hydrocarbon production/transportation system; the waxing temperature(s) of the relevant hydrocarbons; the length of the hydrocarbon production/transportation system; the temperature profile of the hydrocarbon production/transportation system; and the temperature of the relevant hydrocarbons.

13. A method of heating a hydrocarbon production and/or transportation system including: providing a cable comprising at least two electrical conductors separated by an insulator; and providing an alternating current (AC) power source connected to the at least two electrical conductors, wherein, the alternating current power source generates heat in cable by providing alternating current power to the cable; the at least two electrical conductors are terminated by an open circuit; the at least two electrical conductors are arranged as a transmission line; and multiple AC frequency excitations are used, having different combinations of AC voltage and frequency, thereby to generate a specific heating profile of the cable.

14. A method as claimed in claim 13, wherein the at least two electrical conductors are terminated at the end of the conductors either by an inductive connection or a capacitive connection.

15. A method as claimed in claim 13, further including controlling the heating along the length of the at least two conductors by varying: magnitude of AC voltage of the AC power source; frequency of the AC voltage of the AC power source; or a combination of AC voltage and frequency.

16. A method as claimed in claim 13, wherein the at least two electrical conductors are provided adjacent to a tube carrying the hydrocarbons, to encourage more even heat distribution.

17. The method as claimed in claim 13, wherein a tapered heating profile is generated along the length of the at least two electrical conductors.

18. A method as claimed in claim 13 further including collecting parameters of the hydrocarbon production system and selecting the AC voltage and frequency based on those parameters according to a pre-defined algorithm.

19. A method as claimed in claim 18, wherein the parameters include: thermal parameters of the solids and fluids in the hydrocarbon production/transportation system; the waxing temperature(s) of the relevant hydrocarbons; the length of the hydrocarbon production/transportation system; the temperature profile of the hydrocarbon production/transportation system; and the temperature of the relevant hydrocarbons.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) An embodiment of the present invention will now be described, by way of example only, and with reference to the accompanying figures in which:

(2) FIG. 1 is a schematic diagram of current flow in a hydrocarbon production heating system according to one embodiment of the present invention;

(3) FIG. 2 is a sectional diagram of a well borehole according to one embodiment of the present invention;

(4) FIGS. 3(a), 3(b) and 3(c) are graphs relating depth of a well to power and temperature according to an example of one embodiment of the present invention;

(5) FIG. 4 is a schematic flow diagram of a method of determining the AC voltage and frequency to apply to a hydrocarbon production heating system according to one embodiment of the present invention;

(6) FIG. 5 is a graph of well depth to power required to heat according to one example of the present invention;

(7) FIG. 6 is a graph representing changes in frequencies against changes in well depth for the example of FIG. 5;

(8) FIG. 7 is a graph of well depth to power required to heat according to one example of the present invention;

(9) FIG. 8 is a graph representing changes in frequencies against changes in well depth for the example of FIG. 7;

(10) FIG. 9 is a graph of well depth to power required to heat according to one example of the present invention;

(11) FIG. 10 is a graph representing changes in frequencies against changes in well depth for the example of FIG. 9;

(12) FIG. 11 is a graph of well depth to power required to heat (showing the applied voltage and frequency) according to one example of the present invention;

(13) FIG. 12 is a graph of well depth to power required to heat using multiple frequencies, to compare to that of FIG. 11, according to one example of the present invention;

(14) FIGS. 13(a) and (b) are cross-sectional diagrams of a well borehole according to one embodiment of the present invention;

(15) FIGS. 14(a) and (b) are electric circuit models of an insulated well borehole according to one embodiment of the present invention;

(16) FIG. 15 is a graph representing power dissipation against length of electrical conductors with no insulation in a well borehole; and

(17) FIG. 16 is a graph representing power dissipation against length of electrical conductors with insulation in a well borehole.

DETAILED DESCRIPTION

(18) Resistive heating of well boreholes, as is known in the prior art, usually consists of providing a resistive heating element in contact with the production tubing containing the fluids extracted from the reservoir. Disadvantages of this method include: No ability to target different areas which require heating (once a resistive element is installed); Uniform heating of the production tubing in contact with a resistive element; and A specialist resistive heating element requires to be installed in the well borehole in contact with the production tubing.

(19) The present invention does not use resistive heating but, instead reactance heating, also referred to as transmission line heating, through the use of at least one insulated conductor and the application of an alternating current power source. In this manner, the present invention is able to provide specific heating profiles along hydrocarbon tubing to minimise the power required to ensure the temperature of the fluids being extracted remain above the cloud point, waxing temperature or hydrate formation.

(20) As is known, reactance can be positive imaginary (i.e. inductive) or negative imaginary (i.e. capacitive) and under most circumstances these reactances are frequency dependent. A capacitor exhibits a reactance the magnitude of which decreases with frequency and an inductance has a reactance magnitude which increases with frequency. The sign and magnitude of a reactance can thus be modified by manipulation of the AC frequency and at a particular frequency the current induced can be manipulated by modifying the amplitude of the AC voltage.

(21) Furthermore, whilst the remaining description refers to the application of a hydrocarbon production heating system to wells and well boreholes, it should be understood that the invention may be applied to any hydrocarbon transportation conduit such as a well, riser, flowline or pipeline.

(22) Referring to FIG. 1, the present invention, according to one embodiment, uses the fact that a cable 10 (shown schematically), or other conductors separated by an insulator, having more than one electrical conductor 12, 14 contain an intrinsic capacitance 16 between the conductors 12, 14 which must be repeatedly charged and discharged when the cable 10 is excited with AC (Alternating Current) voltages 18. This (dis)charging current I.sub.1, I.sub.2, I.sub.3 heats the cable. If the cable is put in thermal contact with suitable hydrocarbon tubing, the hydrocarbons are, consequently, heated. The charging current decreases as a function of distance on the cable 10 at a specific frequency, meaning that the heating effect on the sections of the cable 10 closest to the power supply 18 is greater than the heating effect at the far end of the cable 10. This is shown in FIG. 1: the current I.sub.1 at the end of the cable 10 nearest to the AC source 18 is larger than the current I.sub.3 at the far end of the cable 10 because more capacitance charging current flows in that end. As a result, the end of cable 10 nearest to the AC source 18 becomes hotter than the end of the cable 10 furthest from the AC source 18.

(23) The temperature and/or heat profile of the cable 10 is typically complex. The temperature and/or heat profile of the cable 10 is normally one or more of user selectable, tuneable and non-linear. More than one frequency of AC power can be supplied to the cable 10, typically the more than one frequency or frequencies are provided successively, that is one after the other. The AC power on the cable is then an average of the more than one frequency or frequencies.

(24) Whilst two conductors are described in the example above, it is also possible to create an AC system using a single wire conductor. In this case, the return path can be provided by earth, or a body of water for that matter. The same effect as described above can be achieved using a single conductor.

(25) In addition, heating provided by the or each conductor can be more evenly distributed to the hydrocarbon tubing through selection of the arrangement of the or each conductor and the hydrocarbon tubing. For example, the or each conductor can be helically wrapped around the hydrocarbon tubing. Helically wrapped hydrocarbon tubing is preferably used in marine riser systems (from the seabed to a surface facility), although other applications may also be applicable.

(26) The amount of heating provided by a cable and the heating profile shape, that is how much of the cable is heated and the relative difference between the near end and the far end (with respect to the AC source) can be controlled by modifying at least three parameters: voltage magnitude of AC source; frequency of AC source (which alters the skin effect and cable reactance) termination at end of cablethis could be open circuit, short circuit, inductive, capacitive or resistive.

(27) In this manner, two conductors, separated by an insulator, which may be in the form of cable, can be deployed in a well borehole to provide heating by connecting the conductors to an AC power source. The depth of the heating can be controlled using the parameters mentioned above such that the amount of power used to heat the well to the required temperature is minimised. Minimising the amount of power increases profitability of the well by reducing operating expenditure (OPEX).

(28) Optimal (i.e. minimum energy) well heating can be determined using the following steps: 1. Identify the temperature requirement of a particular section of a well heating system to achieve the desired heating of the crude oil inside the tubing (initially assuming no oil flow). This determines a target temperature. 2. Determine the relation between power dissipated and temperature in the section of the well heating system to give the target power dissipation in that section. 3. Compute frequency and voltage of the AC excitation to induce the required dissipated power profile along the conductors of the well heating system.

(29) For example, FIG. 2 shows a section of a well borehole 20. The borehole 20 includes a steel/chrome production tube 22 containing crude oil 24, surrounded by brine 26 and encased in a concrete production casing 28. Heating is achieved by running a single heat-trace cable 30 (the cable having two conductors separated by an insulator) down the side of the production tube 22 which makes good thermal contact with the brine 26 and hence with the production tubing 22 through the brine 26.

(30) The thermal parameters (including conductivity and specific heat capacity) of each material (steel/chrome tube, concrete casing, brine and crude oil) are known. Therefore, calculations of the thermal system of the borehole 20 can be calculated.

(31) Well Specific Power (WSP)

(32) The thermal system described above is linear and hence it is useful to introduce the notion of Well Specific Power, or WSP. The WSP is the power required to heat a 1 m (meter) section of well by 1 K in steady state (i.e. no oil flowing and with the boundary condition set such that the earth surrounding the production casing is at constant temperature). Due to the linearity of the thermal model, it is possible to write the power requirement, P.sub.R, of a 1 m section of well in terms of the WSP as:
P.sub.R=P.sub.s.Math.T
where T is the difference between the steady state temperature without heating applied and the minimum required temperature to heat the system to, for example, the waxing temperature. Simulations and experiments show that the WSP was around 6.25 W for a well with a 3 production tubing and a 9 production casing.
Ideal Power Profile (IPP)

(33) The ideal power profile is the minimum power dissipation required per meter of well to prevent waxing plotted as a function of position down the well. That is, it is the required power, P.sub.R, plotted as a function of distance down the well. FIG. 3 shows an example of a typical Kasamene well with a depth of 700 m and a thermal gradient (FIG. 3(a)) of around 5.5 C./100 m, with a waxing temperature of 55 C. FIG. 3(b) shows the required temperature increase in the well borehole to prevent waxing, based on the thermal gradient of FIG. 3(a). As can be seen, no temperature increase is needed below 500 m.

(34) The ideal power profile is then given by multiplying the graph of FIG. 3(b) by the WSP value, to give the plot of FIG. 3(c).

(35) Ideal Required Power (IRP)

(36) Once the analysis above has been performed to evaluate the WSP and plot the IPP, the ideal required power, P.sub.ID, can be found. This is the minimum power required to heat the well to maintain the crude temperature above the waxing temperature and is therefore found by integrating the IPP along the length of the well, giving:

(37) $P_{\mathrm{ID}}=\frac{P_s}{2}\left[\frac{T_{\mathrm{grad}}}{100}{L}^2+\frac{100}{T_{\mathrm{grad}}}{\left(T_{\mathrm{wax}}-T_{\mathrm{res}}\right)}^2+2L\left(T_{\mathrm{wax}}-T_{\mathrm{res}}\right)\right]$
where T.sub.grad is the wells thermal gradient in K/100 m, L is the well length to the reservoir at a temperature T.sub.res and T.sub.wax is the waxing temperature. This ideal required power is a useful quantity, as this power level is the minimum required to heat the well in steady state and is independent of the method used to heat the cable.

(38) The total heating power required for a well will then be close to the IRP, but may be more due to losses. Any heating solution which uses uniform heating methods, such as using resistive heating, will require significantly higher power than a method, such as disclosed herein, which uses a profiled approach.

(39) Having found the ideal power profile (IPP), the next step is to determine the best excitation method for the well heating system which achieves, as closely as possible, the IPP. To generate the required tapered heating profiles along the length of the electrical conductors, an open circuit down-well termination is preferred. This has the advantage of automatically causing a zero current at the deepest part of the electrical conductors of the well heating system, meaning that power is not wasted heating oil that is already well above waxing temperature and means that the down-hole part of the system is as simple and robust as possible.

(40) Now that the above system is understood, simulations are performed which determine the achievable heating profiles in varying lengths of electrical conductors of a well heating system using different excitation frequencies for a standard 100V excitation. For each frequency, the power dissipated is recorded at a sampling distance, such as every 100 m along the cable and a look-up table constructed. As the power is proportional to the square of the excitation voltage, simulations do not need to be run or a look up table constructed for varying input voltages, as the effect of voltage change can be directly calculated.

(41) Calculation of Optimal Frequency and Voltage

(42) Overall, the method for picking the optimum frequency and voltage is performed in two steps. This is illustrated graphically in FIG. 4. S1, or stage 1, of the method is the system described above. The WSP is calculated and, knowing the temperature profile of the well (T.sub.res, T.sub.grad) and the waxing temperature (T.sub.wax), the length of the well L and the required power, P.sub.R, at several sampling points, S, is calculated. These points are then fed into S2, or stage 2, where a least-square algorithm is used to choose the profile in the lookup table that most closely matches the calculated points.

(43) A number of examples of the above method will now be described.

Example 1

(44) T reservoir: 66 C. T gradient: 5.5 C./100 m T waxing: 55 C. Length: 700 m Sample Step: 100 m

(45) The optimal voltage is found to be just over 800V and the optimum frequency of operation to be around 63 kHz. A comparison between the ideal power profile and the realisable power profile is shown in FIG. 5. As can be seen, the actual realised heating power, P.sub.ACT, is greater than the ideal required power, P.sub.IR, by around 20%. As the heating profile achieved depends on knowing the length of the electrical conductors of the well heating system, a robustness analysis has been performed to verify the heating profile against changes in length, as shown in FIG. 6. As can be seen, the profile varies by around 10% for a little over a 1% change in length, meaning that it is advisable for length to be known to within 1% using this method.

Example 2

(46) T reservoir: 62 C. T gradient: 5.5 C./100 m T waxing: 60 C. Length: 700 m Step: 100 m

(47) In this example, shown in FIG. 7 and FIG. 8, the achieved heating power is only around 2% higher than the ideal required power.

Example 3

(48) T reservoir: 73 C. T gradient: 5.5 C./100 m T waxing: 47 C. Length: 700 m Step: 100 m

(49) In this example, shown in FIGS. 9 and 10, the well is overheated by over 100% (required power level of 8.9 kW and heated by 23 kW). However, the well heating system only needs to reach 300 m deep, rather than the complete 700 m depth of the well. If this is known a priori, it is possible to insert a cable just to the required depth, rather than the full depth of the well, as was done in the example.

(50) Static Oil Versus Flowing Oil Analysis

(51) The examples above are based on simulations of steady state thermal properties of a well assuming there is no oil flow. This is equivalent to the start-up condition of a well which has been idle for a significant time. When a producing well is in operation, hot oil flowing up the production tubing tends to heat the system and consequently the static analysis gives a worst-case heating condition (less power will actually be required).

(52) There is also an intrinsic feedback in operation in a heated well because of the change in viscosity of the oil with temperature: if the oil is sufficiently hot, it flows faster but it is already at the desired temperature; if the oil is cooling, it slows down and thus spends more time being heated.

(53) Therefore, it is possible to reduce the power applied to the well heating system to achieve desirable steady state conditions with flowing hydrocarbons.

(54) Multi Frequency Excitation

(55) A further embodiment of the invention that improves the control of the heating profile of the system described above is to use multi-frequency excitation of the electrical conductors.

(56) Calculating the power dissipated in the cable in this case is non-trivial. Normally, to calculate power loss, the current due to each individual excitation would be calculated (with all other frequency sources set to zero) and the resulting currents added. The resulting current could normally be used to calculate the resulting power loss. However, in this case, the resistance value seen by the different currents is different due to the resistance being dominated by the skin effect. The skin effect is the variation seen in resistance of a conductor at higher frequency. That is, at higher frequencies, current tends to travel closer to the outside of a circular cross-section conductor and, therefore, is known as the skin effect, because the current is travelling in the skin of the conductor. This means that (for dual frequency excitation) the power can be written as:

(57) $P_{\mathrm{TOT}}=P_1+P_2+\sqrt{P_1{P}_2}\left(\sqrt{\frac{R_1}{R_2}}+\sqrt{\frac{R_2}{R_1}}\right)$

(58) Where P.sub.1 and P.sub.2 are the individual powers under independent excitation by frequencies 1 and 2, and R.sub.1 and R.sub.2 are the cable resistances at frequencies 1 and 2.

(59) If we write R.sub.1=R.sub.2 then we can write the above as:

(60) $P_{\mathrm{TOT}}=P_1+P_2+\sqrt{P_1{P}_2}\left(\sqrt{}+\frac{1}{\sqrt{}}\right)$
where the term involving a is always greater than 2, but in reality, as the frequencies required to create a good heating profile are close together, so the term will be close to 2.

(61) FIG. 11 and FIG. 12 shows the effect of multi-frequency excitation obtained by varying the weight associated to the overall employed voltage in the minimization problem: if the frequency is increased (FIG. 12) in a multi-frequency excitation, with very similar frequencies, the sum of the two voltages is less than that needed by a single frequency excitation and the overall power is reduced.

(62) Heating Using Existing Well Infrastructure

(63) Rather than adding dedicated electrical conductors, such as a heat trace cable, it is possible to use existing well infrastructure for heating purposes. For example, a pump power cable is typically present in a well borehole in contact with the production tubing. It is, therefore, possible to use the pump power cable as long as AC blocking filters can be installed at either end of the cable to isolate the heating excitation from the pump and 3-phase source (i.e. isolate the original use of the cable from the additional heating use). Simulations have been performed using two of the three cores of a pump cable (as the heating effect in this example is described using single phase, making one of the pump cable cores redundant). The equivalent transmission line modelled as a parallel line conductor is characterized by the following values: capacitance C=2.821010 [F/m], inductance L=8.22108 [H/m], conductance G=1.521023 [S/m]. Comparing the above values with those obtained with a coaxial cable used as the two electrical conductors of a well heating system, namely C=1.561010 [F/m], L=2.01107 [H/m], G=6.231024 [S/m], it can be seen that the pump cable and coaxial cable have very similar electrical quantities. Simulations have shown that the electrical behaviour of a pump cable and coaxial cable are very similar. From the thermal point of view, the only factor preventing the use of a pump power cable may be the maximum achievable temperature, given the pump power cable is not designed for this purpose. A dedicated heat trace coaxial cable would be able to operate at higher temperatures.

(64) Cableless Heating

(65) A further embodiment of the present invention is to heat well boreholes without the need for any form of specifically deployed electrical conductors, such as a heat trace cable or pump cable. In this embodiment, as shown in FIG. 13(a), a production tubing 40 and production case 42 can be used to form a large coaxial cable, with the production tube 40 acting as an inner electrical conductor and the production casing 42 acting as an outer electrical conductor.

(66) In a normal well, brine 44 is used as a filling material between the case 42 and tube 40. The brine 44 acts as a conductor between the inner and outer electrical conductors (40, 42), whereas the desired material between the inner and outer conductors is a dielectric (an insulator). The lack of good insulation between the electrical conductors 40, 42 causes a problem in that the brine acts to short circuit, conducting all the current between tube 40 and case 42 at the top of the well borehole and preventing propagation of the current deep into the well. One solution to this problem, as shown in FIG. 13(b), is to insulate the inner 40 and/or outer 42 electrical conductor with an insulator 46, such as Teflon. This allows the production tube 40 and case 42 to act in the intended manner.

(67) With respect to FIG. 13(b) and FIG. 14(a), the presence of brine 44 in the insulated pipe can be modelled 50 as a capacitor C.sub.i (the dielectric being the insulator 46) in series with a resistance R.sub.b, the resistance set as the conductance of the brine 44. However, to model a transmission, an alternative model 52 is required in which the shunt branch has the resistance R.sub.b and the capacitance C.sub.i are in parallel. Thus, an equivalent parallel circuit can be defined as detailed in FIG. 14(b).

(68) The power dissipation as a function of cable length in an example scenario with input voltage set to 1 kV at 70 kHz, is shown in FIG. 15 in un-insulated form and FIG. 16 in insulated form. As can be seen, in the un-insulated case, the heating effect is constrained to the first few meters of the well, as the electrical conductors are shorted out by the brine. However, when the tubing is insulated, the current can flow in the entire cable length giving the required heating profile.

(69) In the insulated case, power is dissipated mainly in the production tubing, rather than the casing, due to the tubing having a lower surface area (this corresponds to the usual case in a coaxial cable where the bulk of the thermal dissipation occurs in the centre conductor). This is advantageous because the heating occurs closest to the crude, or other fluid flowing in the inner tubing. An additional advantage of using the production case and tube as the coaxial element is that the heating of the crude is more even than in the case in which a cable runs down the side of the production tubing. This even heating reduces the power required to heat 1 m of the well by 1K (Well Specific Power) from 6.25 W to 4.5 W from simulations: 30% less power is required.

(70) Replacement of Brine with Nitrogen

(71) In existing wells, brine is used as a filler material between the production case and production tube. As discussed above, this causes a problem if the production tube and case are used as the heat trace element if the tube and case are not coated with an insulator. However, even if the case and tube are insulated, or if a dedicated heat trace cable is used, the presence of the brine causes increased thermal mass that must be heated to raise the oil above waxing temperature and, in addition, causes increased thermal conductivity between the production tube and case, causing undesirable cooling of the production tubing from the case and surrounding earth. A potential improvement, in terms of power consumption, is to replace the brine with an alternative insulating material or fluid. One preferable example is an inert gas, such as nitrogen, which is a significantly better thermal insulator than brine.

(72) By replacing brine, or other material between the production tubing and casing, with an improved insulating material or fluid, the power required to maintain the well borehole at a temperature above the waxing temperature is decreased even further.

(73) As mentioned above, although the embodiments above are described in relation to a well heating system, the invention can be applied to any hydrocarbon transportation and/or production system via a conductive material. For example, hydrocarbons may also require heating in a pipeline, well production tubing, well casing, flowlines, jumpers, or marine risers (from the seabed to the surface) and the invention is equally applicable to other hydrocarbon production and/or transportation systems, such as these.

(74) Improvements and modifications may be made without departing from the scope of the invention.