Reservoir in-situ self-adaptive variable-frequency electric heating system and method

Abstract

The invention provides an in-situ self-adaptive variable-frequency electric heating system and method for a reservoir. The system comprises a heating electrode and control unit, a frequency-adjustable alternating-current power supply and a galvanometer, the heating electrode is connected with the frequency-adjustable alternating-current power supply through a cable, the frequency-adjustable alternating-current power supply is connected with the galvanometer, and the control unit is connected with the frequency-adjustable alternating-current power supply and the galvanometer; the method comprises the following steps: placing a heating electrode into a production well, and lowering the heating electrode to a corresponding depth of a target layer to be heated; the frequency-adjustable alternating-current power supply is turned on through the control unit, and the reservoir in-situ self-adaptive variable-frequency electric heating system starts to work; the power supply frequency of the frequency-adjustable alternating-current power supply is adjusted, and the control unit records the current of the galvanometer; analyzing the peak value of the current square according to the recorded current of the galvanometer; and searching the optimal heating frequency according to the peak value of the current square. According to the method, the optimal power supply frequency can be searched in real time under the condition that the power supply voltage is not increased, and the optimization of the in-situ electric heating efficiency of the reservoir is realized.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of reservoir in-situ electric heating, in particular to a reservoir in-situ self-adaptive frequency conversion electric heating system and method. BACKGROUND

[0002] The reservoir in-situ electric heating technology is a new energy recovery technology, and heavy oil and oil sand can be realized in-situ fluidized mining through the effect of temperature increase and viscosity reduction; solid natural gas hydrate can be decomposed into natural gas and liquid water which are easy to mine under the action of electric heating. The in-situ conversion of low-maturity shale oil through electric heating can produce light oil and natural gas, and the waste gas and slag are left underground, achieving the ideal goal of green mining. The resistance type in-situ electric heating technology is the simplest and most stable in-situ electric heating technology.

[0003] The principle of the traditional resistance type in-situ electric heating technology is shown in Figure 1 The heating electrode is connected with an alternating current power supply through a cable. The heating electrode, the cable, the target layer and the alternating current power supply form a closed loop, and the target layer is regarded as a resistor to realize in-situ heating of the target layer by converting electric current into heat energy.

[0004] The above-mentioned traditional resistance type in-situ electric heating technology has obvious deficiencies: the existing technology regards the heated target reservoir as a resistor for heating, ignores the dielectric properties of the reservoir, and ignores the influence of temperature change and reservoir fluid phase change on impedance. Studies have shown that as a porous medium, the impedance of rock will not only show resistance characteristics, but also show capacitance characteristics due to various polarization phenomena, and its impedance needs to be represented by a complex number.

[0005] Z*=Z′+jZ″ (1)

[0006] In the formula, Z* is the impedance of the rock, Z' and Z" are the real part and the imaginary part of the impedance respectively, and j is the imaginary unit.

[0007] The existence of the dielectric properties of the formation leads to the fact that when a fixed voltage alternating current is applied to the two heating electrodes, the formation has reactive power, the active power cannot reach the maximum, and the electric heating efficiency cannot reach the maximum. When considering the dielectric properties of the rock, the rock impedance is usually replaced by an equivalent circuit, and the most classic rock equivalent circuit is the Debye model equivalent circuit, the structure of which is shown in Figure 2 The Debye model equivalent circuit is composed of a resistor R1 and a capacitor C in series, and a resistor R2 in parallel.

[0008] The impedance of the Debye model equivalent circuit is:

[0009] Z*=R2||(R1+1 / (jωC)) (2)

[0010] After arrangement, we have:

[0011]

[0012] where A = ωR1C, B = ωR2C.

[0013] The active power P consumed by the rock when a voltage of fixed frequency and amplitude U is applied to it:

[0014]

[0015] where |Z*| is the modulus of impedance, is the impedance angle, and Re(Z*) is the real part of impedance.

[0016]

[0017]

[0018] It can be seen that the active power consumed by the rock is related to the frequency of the applied signal and the impedance characteristics of the rock. Figure 3 The variation of the real and imaginary parts of the impedance of the Debye model equivalent circuit with frequency is shown (the model parameters corresponding to the curves in the figure are R1 = 10 Ω, R2 = 100 Ω, and C = 1 μF). The real part of the impedance of the Debye model equivalent circuit decreases with increasing frequency, and the imaginary part increases first and then decreases with increasing frequency.

[0019] Figure 4 The active power of the Debye model equivalent circuit with different frequencies of the voltage signal applied to the above parameter model (the model parameters corresponding to the curves in the figure are R1 = 10 Ω, R2 = 100 Ω, and C = 1 μF) is shown. It can be seen that the active power increases with increasing frequency, and this process can be divided into three stages. Stage I: the active power is very low and basically unchanged with increasing frequency; Stage II: the active power increases rapidly with increasing frequency, and the rate of increase changes from small to large and then to small; Stage III: the active power reaches a maximum value and hardly continues to increase with increasing frequency.

[0020] The frequency spectrum characteristics of the active power change with changes in the Debye model parameters. Figure 5 The frequency spectrum of the active power when the voltage signal applied is 500 V, R1 = 10 Ω, R2 = 100 Ω, and C is 1 μF and 0.1 μF, respectively, is shown. It can be seen that as the capacitance C decreases, the overall active power spectrum shifts to the right, and the peak value of the active power remains unchanged. Experimental studies have shown that the impedance spectrum of the formation changes with changes in the saturation of each phase fluid in the pore and the temperature of the reservoir, so during the reservoir electric heating process, the increase in temperature and the phase change of the fluid will cause changes in the impedance spectrum of the reservoir, and under the condition that the frequency of the heating voltage does not change, the active power will also change.

[0021] According to the analysis result, there are two ways to improve the heating active power of the reservoir in the reservoir electric heating process: (1) improving the heating signal frequency; (2) improving the heating signal voltage. Improving the heating voltage will improve the safety risk of construction, and bring pressure to the safety management on site. In theory, the higher the frequency of the electric heating signal is, the better. As long as the frequency of the heating signal is high enough, the active power of the reservoir to be heated can be maximized. However, under the action of high-frequency alternating current, the skin effect (under alternating conditions, due to the consumption of electromagnetic field energy inside the conductor, the current is concentrated on the surface of the conductor) occurs, the current is concentrated on the top and bottom interfaces of the reservoir, and cannot reach the inside of the reservoir, thereby affecting the heating effect of the reservoir. And the higher the current frequency is, the more obvious the skin effect is, so the frequency of the heating voltage is not the higher the better. The best heating frequency should be at the junction frequency of the middle stage II and stage III. The technical solution of the present application will search for the best heating frequency in the heating process and adaptively adjust the frequency of the heating voltage to maintain the best heating effect and heating power. Figure 4 Summary The present application aims to provide a reservoir in-situ adaptive variable frequency electric heating system and method, which searches for the best heating frequency in the heating process and adaptively adjusts the frequency of the heating voltage to maintain the best heating effect and heating power.

[0022] The traditional reservoir in-situ electric heating technology cannot dynamically adjust the heating frequency according to the impedance spectrum change of the reservoir, which may result in that the heating power cannot be maximized (the frequency is too low), or the heating effect of the reservoir is reduced due to the skin effect (the frequency is too high). The present application aims to provide a reservoir in-situ adaptive variable frequency electric heating system and method, which searches for the best heating frequency in the heating process and adaptively adjusts the frequency of the heating voltage to maintain the best heating effect and heating power.

[0023] The present application provides a reservoir in-situ adaptive variable frequency electric heating system, which comprises a heating electrode control unit, an adjustable frequency alternating current power supply and a current meter.

[0024] The heating electrode is connected to the adjustable frequency alternating current power supply through a cable, and the adjustable frequency alternating current power supply and the current meter are connected, and the control unit is connected to the adjustable frequency alternating current power supply and the current meter.

[0025] Further, the heating electrode is placed in the production well and is lowered to the corresponding depth of the target layer to be heated.

[0026] Further, the heating electrode has a plurality of electrodes.

[0027] Further, the above-mentioned reservoir in-situ adaptive variable frequency electric heating system is used to realize a reservoir in-situ adaptive variable frequency electric heating method, which comprises the following steps:

[0028] (1) placing the heating electrode into the production well and lowering it to the corresponding depth of the target layer to be heated;

[0029] (2) opening the adjustable frequency AC power supply by the control unit, and starting the reservoir in-situ adaptive variable frequency electric heating system to work;

[0030] (3) adjusting the power supply frequency of the adjustable frequency AC power supply, and the control unit recording the current size of the ammeter;

[0031] (4) according to the recorded current size of the ammeter, analyzing the peak value of the current square as Imax;

[0032] (5) according to the recorded results, searching for the corresponding frequency when the current square is approximately equal to M*Imax, and recording the frequency as fb; M is a set threshold value;

[0033] (6) adjusting the power supply frequency of the adjustable frequency AC power supply to fb, and the frequency is the optimal heating frequency.

[0034] Further, the frequency setting when opening the adjustable frequency AC power supply in step (2) is the lowest value.

[0035] Further, the power supply frequency of the adjustable frequency AC power supply is adjusted from low to high in step (3).

[0036] Further, the power supply frequency of the adjustable frequency AC power supply is adjusted from low to high at certain logarithmic intervals.

[0037] Further, in step (4), the peak value of the current square is analyzed by drawing a graph of the current square and the frequency.

[0038] Further, in step (5), the value of M is selected so that the value of fb is close to the demarcation value between stage II and stage III in the active power frequency spectrum characteristic map of the Debye model circuit; wherein:

[0039] Stage II: the active power increases with the increase of the frequency, and the rate of increase changes from small to large and then to small;

[0040] Stage III: the active power reaches the maximum value and no longer continues to increase with the increase of the frequency.

[0041] Further, the reservoir in-situ adaptive variable frequency electric heating method further comprises:

[0042] (7) after a certain time interval, as the temperature and the reservoir phase state change, the impedance changes, and steps (2)-(6) are repeated to reset the optimal power supply frequency fb.

[0043] As described above, due to the adoption of the above technical solutions, the beneficial effects of the present application are:

[0044] The present application can search for the optimal power supply frequency in real time without increasing the power supply voltage, and can adaptively adjust the power supply frequency of the reservoir in-situ electric heating to optimize the efficiency of the reservoir in-situ electric heating. Attached Figure Description

[0045] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings in the embodiments will be briefly described below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0046] Figure 1 This is a schematic diagram of the traditional resistance in-situ electric heating technology.

[0047] Figure 2 This is the equivalent circuit diagram of the Debye model.

[0048] Figure 3 This is a characteristic diagram of the impedance spectrum of the equivalent circuit in the Debye model.

[0049] Figure 4 This is a characteristic diagram of the active power spectrum of the equivalent circuit in the Debye model.

[0050] Figure 5 The active power spectrum characteristics of the equivalent circuit of the Debye model for different C values ​​are shown in the figure.

[0051] Figure 6 This is a schematic diagram of the in-situ adaptive frequency conversion electric heating of the reservoir in an embodiment of the present invention.

[0052] Figure 7 This is a schematic diagram showing the changes in active power and current square as a function of frequency for the equivalent circuit of the Debye model under AC voltage excitation with a fixed amplitude.

[0053] Icons: 1-Production Well 1, 2-Cable 1, 3-Cable 2, 4-Production Well 2, 5-Upper Surrounding Rock, 6-Target Layer to be Heated, 7-Lower Surrounding Rock, 8-Heating Electrode 1, 9-Heating Electrode 2, 10-Control Unit, 11-Adjustable Frequency AC Power Supply, 12-Ammeter. Detailed Implementation

[0054] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations.

[0055] The following detailed description of embodiments of the application provided in the accompanying drawings is not intended to limit the scope of the application as claimed, but merely represents selected embodiments of the application. Based on the embodiments in the application, all other embodiments obtained by those of ordinary skill in the art without creative labor are within the scope of protection of the application.

[0056] Embodiments

[0057] As Figure 6 shown, taking two production wells as an example, the embodiment proposes a reservoir in-situ adaptive frequency conversion electric heating method, including the following steps:

[0058] (1) Place heating electrode one 8 and heating electrode two 9 into production well one 1 and production well two 4 respectively, and lower them to the corresponding depth of the target layer 6 to be heated;

[0059] (2) Turn on the adjustable frequency AC power supply 11 through the control unit 10, set the frequency to the lowest value, and start the reservoir in-situ adaptive frequency conversion electric heating system;

[0060] (3) Adjust the power supply frequency of the adjustable frequency AC power supply 11 from low to high at certain logarithmic intervals, and record the current size of the ammeter 12 by the control unit 10;

[0061] Supplementary explanation:

[0062] According to the principle of circuit theory, the current amplitude is equal to the ratio of voltage to impedance amplitude, so:

[0063]

[0064] It can be seen that the ratio of active power and the square of current amplitude is the real part of the formation impedance:

[0065]

[0066] Therefore, during the increase of frequency, the change of the square value of the current size of the ammeter is similar to the change characteristics of the active power. Figure 7 As shown in the figure, when an AC power with an amplitude of 500V is applied to the Debye model equivalent circuit with model parameters R1=10Ω, R2=100Ω, and C=1μF, the active power and the square of the current amplitude change with frequency, and it can be seen that the change patterns are basically the same.

[0067] (4) According to the recorded current size of the ammeter 12, draw a graph of the relationship between the square of the current and the frequency, and analyze the peak value of the square of the current, denoted as Imax;

[0068] (5) According to the recorded results, search for the frequency corresponding to the square of the current being approximately equal to 90%*Imax, denoted as fb, which is approximately equal toFigure 4 The boundary value between middle stage II and stage III;

[0069] (6) The power supply frequency of the adjustable frequency AC power supply 11 is adjusted to be fb, which is the optimal heating frequency;

[0070] (7) After a certain time interval, as the temperature changes the reservoir phase state, the impedance changes accordingly, and steps (2)-(6) are repeated to reset the optimal power supply frequency fb.

[0071] The above only describes the preferred embodiments of the present application and is not intended to limit the present application. Various modifications and changes can be made by those skilled in the art. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A reservoir in-situ self-adapting frequency-variable electric heating system, characterized in that, The heating electrode is connected to the adjustable frequency AC power supply through a cable, the adjustable frequency AC power supply is connected to the current meter, and the control unit is connected to the adjustable frequency AC power supply and the current meter. The heating electrode is placed in a production well and lowered to the depth corresponding to the target layer to be heated.

2. The reservoir in-situ self-adapting frequency-variable electric heating system according to claim 1, characterized in that, The heating electrode has multiple.

3. The reservoir in-situ self-adapting frequency-variable electric heating system according to claim 1, characterized in that, The method comprises the following steps:

4. A reservoir in-situ adaptive frequency electric heating method, implemented by using the reservoir in-situ adaptive frequency electric heating system of any one of claims 1-3, characterized in that, (1) placing the heating electrode into the production well and lowering it to the depth corresponding to the target layer to be heated; (2) starting the in-situ adaptive variable frequency electric heating system of the reservoir by turning on the adjustable frequency AC power supply through the control unit; (3) adjusting the power supply frequency of the adjustable frequency AC power supply, and the control unit records the current of the current meter; (4) analyzing the peak value of the current square recorded by the current meter, and recording it as Imax; (5) searching for the corresponding frequency when the current square is approximately equal to M*Imax, and recording it as fb; M is a set threshold value; (6) adjusting the power supply frequency of the adjustable frequency AC power supply to fb, which is the optimal heating frequency. The frequency setting when the adjustable frequency AC power supply is turned on in step (2) is the lowest value.

5. The reservoir in-situ self-adapting frequency-variable electric heating method according to claim 4, characterized in that, In step (3), the power supply frequency of the adjustable frequency AC power supply is adjusted from low to high.

6. The reservoir in-situ self-adapting frequency-variable electric heating method according to claim 5, characterized in that, The power supply frequency of the adjustable frequency AC power supply is adjusted from low to high at certain logarithmic intervals.

7. The reservoir in-situ self-adapting frequency-converted electric heating method according to claim 6, characterized in that, In step (4), the peak value of the current square is analyzed by drawing a graph of the relationship between the current square and the frequency.

8. The reservoir in-situ self-adapting frequency- variable electric heating method according to claim 4, characterized in that, In step (5), the value of M is selected so that the value of fb is close to the dividing value between stage II and stage III in the Debye model circuit active power frequency spectrum characteristic map; wherein:

9. The reservoir in-situ self-adapting frequency- variable electric heating method according to claim 4, characterized in that, Stage II: the active power increases with the increase of frequency, and the rate of increase changes from small to large and then to small; Stage III: the active power reaches the maximum value and no longer increases with the increase of frequency. The method further comprises:

10. The reservoir in-situ self-adapting frequency- variable electric heating method according to claim 4, characterized in that, (7) After a certain time interval, as the temperature and the phase state of the reservoir change, the impedance changes accordingly, and steps (2) to (6) are repeated to reset the optimal power supply frequency fb. ​

Images