Reservoir radio frequency heating method based on temperature monitoring

Abstract

The invention discloses a reservoir radio frequency heating method based on temperature monitoring, and relates to the field of reservoir mining. The method comprises the following steps: (1) establishing a temperature and characteristic frequency intersection chart according to a construction area and a target layer; (2) measuring the reservoir temperature before the radio frequency heating of the reservoir by using a well temperature monitoring instrument; (3) inquiring the temperature and characteristic frequency intersection chart to obtain the characteristic frequency corresponding to the current reservoir temperature, and setting the characteristic frequency as the heating frequency of an underground radio frequency heater; (4) after a period of time, the well temperature monitoring instrument is utilized again to read the underground reservoir temperature value, and whether the current temperature reaches the preset heating temperature or not is judged; if not, repeating the step (3); and the current temperature reaches the preset heating temperature. Compared with an existing fixed-frequency underground radio frequency heating method, the method has the advantages that the heating energy efficiency can be effectively improved under the same heating condition, the temperature of the reservoir is rapidly increased to the preset temperature, and the set temperature is maintained in a flexible frequency modulation mode for continuous heating.

Description

Technical Field

[0001] The invention relates to the field of reservoir exploitation, and in particular to a reservoir radio frequency heating method based on temperature monitoring. Background Art

[0002] The development and utilization of oil shale has attracted more and more attention from countries around the world. With the increasing pressure of environmental protection, in-situ mining of oil shale has become an inevitable development trend for the future commercial large-scale mining of oil shale. In-situ mining refers to the method of converting the solid kerogen in the oil shale into liquid hydrocarbons by heating the oil shale reservoir at high temperature, and then extracting the liquid hydrocarbons from underground through traditional oil and gas drilling and production processes. At present, the heating methods of oil shale layers can be divided into electric heating, fluid heating and radio frequency heating.

[0003] Traditional radio frequency heating basically uses a fixed single-frequency voltage for heating. For example, the Chinese invention patent with application publication number CN115354990A discloses a device and method for intelligent production of heavy oil reservoirs with directional radio frequency heating, which effectively reduces the high energy consumption problem of traditional covering heating methods by heating the area with residual oil enrichment through directional radio frequency heating. The Chinese invention patent with application publication number CN108318520A discloses a downhole radio frequency heating simulation test device, through which the influencing factors of radio frequency heating of oil and gas reservoirs can be studied and the feasibility of radio frequency heating method in thermal oil recovery can be evaluated. However, the traditional fixed-frequency downhole radio frequency heating technology cannot ensure that the radio frequency heating work always maintains the maximum efficiency state. Summary of the invention

[0004] Based on the above technical problems, the present invention proposes a reservoir radio frequency heating method based on temperature monitoring.

[0005] The technical solution adopted by the present invention is:

[0006] A reservoir radio frequency heating method based on temperature monitoring comprises the following steps:

[0007] (1) Establish temperature and characteristic frequency cross-plots based on the construction area and target layer;

[0008] (2) using a well temperature monitoring instrument to measure the reservoir temperature before RF heating of the reservoir;

[0009] (3) querying the temperature and characteristic frequency intersection chart to obtain the characteristic frequency corresponding to the current reservoir temperature, and setting the characteristic frequency as the heating frequency of the downhole radio frequency heater;

[0010] (4) After a certain period of time, the downhole reservoir temperature is read again using the well temperature monitoring instrument to determine whether the current temperature has reached the preset heating temperature; if not, repeat step (3) until the current temperature reaches the preset heating temperature.

[0011] Preferably, step (1) includes the following steps:

[0012] (101) Collect several experimental rock samples according to the construction area and target layer;

[0013] (102) Measure the complex dielectric constant spectrum data of experimental rock samples under different temperature conditions in the laboratory;

[0014] (103) The characteristic frequencies corresponding to the maximum values ​​of the imaginary parts in the complex dielectric constant spectra at different temperatures are extracted, and a cross plot of temperature and characteristic frequency is established.

[0015] Preferably, the method further comprises the following steps:

[0016] (5) If the current temperature has reached or exceeded the preset heating temperature, further determine whether the heating operation needs to be terminated; if so, terminate the power supply to the underground radio frequency heater to stop heating;

[0017] (6) If not, the set temperature needs to be maintained to continuously heat the reservoir at a constant temperature until the heating needs to be stopped according to the construction requirements.

[0018] Preferably, in step (6): according to the relative dielectric constant spectrum obtained at different temperatures, a low frequency far from the optimal heating frequency is selected as the heating frequency (i.e., the heating frequency is reduced); after adjusting the heating frequency for a period of time, step (4) is repeated.

[0019] The beneficial technical effects of the present invention are as follows:

[0020] Through the technical implementation of the present invention, on the basis of reservoir temperature monitoring, the RF heating construction frequency is adjusted in real time according to construction requirements, so that the downhole RF heater can quickly heat the reservoir temperature to the preset temperature with maximum heating efficiency, and the reservoir temperature is maintained at the preset temperature for a long time by adjusting the heating frequency. Compared with the existing fixed-frequency downhole RF heating method, under the same heating conditions, the heating energy efficiency can be effectively improved, so that the reservoir is quickly heated to the preset temperature and the set temperature is maintained in a flexible frequency modulation manner for continuous heating. The present invention can effectively improve the efficiency of in-situ electric heating of the reservoir and reduce engineering energy consumption. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 is the relative dielectric constant dispersion law corresponding to the Cole-Cole model;

[0022] Figure 2 is the imaginary spectrum of the complex dielectric constant of shale after being saturated with NaCl solutions of different concentrations;

[0023] Figure 3It is a schematic diagram of the flow of the reservoir radio frequency heating method based on temperature monitoring of the present invention. DETAILED DESCRIPTION

[0024] In-situ electrical heating of reservoirs is a technology for improving energy recovery. It has obvious advantages in the development of unconventional resources such as heavy oil, shale oil and natural gas hydrates, and has broad application prospects. The traditional radio frequency reservoir in-situ electrical heating method uses a fixed single-frequency voltage for heating, ignoring the dispersion law of the reservoir dielectric constant and the influence of temperature changes on the reservoir dielectric spectrum during the heating process. The present invention innovatively considers for the first time the influence of temperature changes on the reservoir dielectric spectrum during the electrical heating process. By monitoring the reservoir temperature and adjusting the frequency of the radio frequency heating device in real time according to the relationship between the characteristic frequency corresponding to the maximum value of the imaginary part of the dielectric constant and the temperature, the efficiency of in-situ electrical heating of the reservoir can be optimized and frequency modulation and temperature control can be achieved.

[0025] The present invention is described in detail below in conjunction with the accompanying drawings and research background.

[0026] (1) Principle of radio frequency reservoir electric heating and dielectric dispersion of rocks;

[0027] Radio frequency reservoir electric heating technology is an in-situ reservoir heating process. By placing a radio frequency heater underground to generate electromagnetic waves, the reservoir temperature is increased by dielectric heating. In-situ reservoir electric heating is an in-situ improvement technology for unconventional reservoirs, which can effectively improve the recovery rate of unconventional reservoirs such as shale oil. Under the action of electromagnetic waves, the temperature rise characteristics of the dielectric mainly depend on the imaginary part of the complex dielectric constant, that is, the dielectric loss coefficient.

[0028] The frequency range of radio frequency electric heating technology is usually 300kHz ~ 300MHz. The existing radio frequency reservoir electric heating technology is mainly single-frequency excitation, and there are no reports on multi-frequency or variable frequency radio frequency reservoir heating technology. The complex dielectric constant of porous media such as rocks has obvious dispersion characteristics, that is, it changes with the change of frequency. Rock physics experimental research shows that the dispersion law of the complex dielectric constant of rocks basically conforms to the Cole-Cole model:

[0029]

[0030] In the formula, ε * is the complex dielectric constant, ε0 and ε ∞ are the static dielectric constant (when the frequency is close to 0) and the optical frequency dielectric constant (when the frequency tends to infinity), ω is the angular frequency, j is the imaginary unit, τ is the relaxation time constant, and α is the characteristic parameter, with a value between 0 and 1.

[0031] Figure 1 The figure shows the Cole-Cole model when the model parameters are at certain values ​​(ε0=20; ε ∞=0.5; τ=0.000001s; α=0.35) The real and imaginary parts of the relative dielectric constant change with frequency. It can be seen that with the increase of frequency, the imaginary part of the relative dielectric constant increases first and then decreases. At the characteristic frequency, the imaginary part of the relative dielectric constant reaches its maximum value.

[0032] The principle of radio frequency heating is similar to that of microwave heating. In microwave heating, water molecules, as polar molecules, constantly turn under the action of an alternating electromagnetic field. The turning frequency of water molecules is basically fixed and has nothing to do with the distribution of water molecules. Therefore, microwave heating usually uses a fixed 2.45 GHz as the heating frequency, which is close to the natural frequency of water molecules. However, in the radio frequency band, the interface polarization effect dominates the dispersion of the complex dielectric constant. Matthew Josh, a scholar from the Commonwealth Scientific and Industrial Research Organization (CSIRO) of Australia, summarized the various dominant polarization mechanisms that cause rock dielectric dispersion in different frequency bands. It can be seen that in the radio frequency band (between 10^5 and 10^9 Hz), interface polarization (Maxwell-Wagner effect) dominates the rock dielectric dispersion.

[0033] The so-called interfacial polarization refers to the migration of anions and cations in the pore water solution in the opposite directions under the action of the alternating electric field in the porous medium, and the accumulation occurs at the solid-liquid interface. After the direction of the electric field changes, the anions and cations change their migration direction and accumulate at the relative solid-liquid interface. The collision and friction of ions during migration will cause the temperature of the medium to rise. When the reservoir contains metal minerals, the migration of free electrons inside the mineral particles under the action of the alternating electric field also follows this law. When the electric field frequency matches the interfacial polarization anion and cation migration period, that is, when the anions and cations just complete the accumulation at the interface, the direction of the electric field is reversed. At this time, the imaginary part of the relative dielectric constant caused by the interfacial polarization is the largest, and the heating efficiency is optimal. Obviously, this matching characteristic frequency is related to many factors such as the pore structure of the porous medium, the concentration of anions and cations, and the migration rate. It does not have a fixed characteristic frequency like the steering polarization of water molecules. Figure 2 The figure shows the dispersion law of the imaginary part of the complex dielectric constant of shale measured by Roman et al. after being saturated with NaCl solutions of different concentrations. It can be seen that in the radio frequency range, the ion concentration of the pore water solution will affect the characteristic frequency corresponding to the maximum value of the imaginary part of the dielectric constant.

[0034] (3) The effect of temperature on the complex dielectric constant of rocks;

[0035] Experimental studies have shown that temperature affects the dispersion law of the complex dielectric constant of rocks. On the one hand, when the temperature rises, the thermal motion of water molecules intensifies, and their polarization degree and rotational freedom increase, resulting in a decrease in the dielectric constant of water. On the other hand, the increase in temperature will accelerate the migration speed of ions in the pore water solution of rocks and enhance the conductivity of rocks. Under the radio frequency electric field, the accelerated ion migration caused by the increase in temperature will reduce the equivalent dielectric constant of rocks, and affect the interface polarization response characteristics, and then affect the size of the interface polarization characteristic frequency. In theory, the higher the reservoir temperature, the faster the migration rate of anions and cations. Under the condition that the pore structure remains unchanged, anions and cations can complete the accumulation at the interface more quickly, thus having a higher characteristic frequency. Therefore, even if the radio frequency excitation corresponding to the characteristic frequency is initially used for heating, as the reservoir temperature increases, the characteristic frequency will move toward the high frequency direction, and the imaginary part of the complex dielectric constant at the corresponding frequency will become smaller without increasing the heating frequency, resulting in a decrease in heating efficiency. In other words, as the construction progresses, the change in temperature causes the characteristic frequency corresponding to the maximum value of the imaginary part of the complex dielectric constant to change. The use of traditional fixed-frequency radio frequency heating technology cannot ensure that the heating efficiency remains at the optimal heating frequency working point.

[0036] It can be seen that due to the influence of temperature on the dispersion law of the relative dielectric constant of the reservoir, the traditional fixed-frequency downhole RF heating technology cannot ensure that the RF heating work always remains in the state of maximum efficiency, and the energy efficiency is low.

[0037] Considering that temperature will affect the frequency law of the relative dielectric constant of rock, and then affect the optimal RF heating frequency. The present invention conducts indoor experiments on coring the target layer before construction, and measures the complex dielectric constant spectrum of rock samples at different temperatures. Establish a relationship chart between the temperature and the optimal construction frequency applicable to the target layer in the construction area. On this basis, by monitoring the changes in the temperature in the well during the construction process, the construction frequency of the RF heater is adjusted in real time, so as to keep the reservoir heating efficiency always in the maximum efficiency state, improve the reservoir heating efficiency, and save the power output of the downhole heater.

[0038] like Figure 3 As shown, a reservoir radio frequency heating method based on temperature monitoring, the specific steps are as follows:

[0039] (1) Collect several experimental rock samples according to the construction area and target layer.

[0040] (2) Measure the complex dielectric constant spectrum data of rocks under different temperature conditions in the laboratory.

[0041] (3) Based on the experimental data, the characteristic frequencies corresponding to the maximum values ​​of the imaginary parts in the complex dielectric constant spectra at different temperatures are extracted, and a cross-plot of temperature and characteristic frequency is established.

[0042] (4) Use well temperature monitoring instruments to measure the reservoir temperature before RF heating of the reservoir.

[0043] (5) Query the temperature and characteristic frequency cross plot to obtain the characteristic frequency corresponding to the current temperature, and set this frequency as the heating frequency of the downhole RF heater (e.g. Figure 1 in fb).

[0044] (6) After a certain period of time, read the downhole reservoir temperature value again and determine whether the current temperature has exceeded the heating temperature that needs to be maintained. If not, repeat step (5).

[0045] (7) If the current temperature exceeds the preset heating temperature, it is further determined whether the heating operation needs to be terminated. If so, the power supply to the downhole RF heater is terminated to stop heating.

[0046] (8) If the set temperature needs to be maintained to continuously heat the reservoir at a constant temperature, a relatively low frequency far from the optimal heating frequency is selected based on the relative dielectric constant spectrum at different temperatures obtained in the laboratory (considering the skin effect, low frequency is more likely to obtain a higher penetration depth) (e.g. Figure 1 fa) in the figure is used as the heating frequency to reduce the heating efficiency. While preventing the temperature from continuing to rise, it also allows the temperature to be transmitted farther and the heating range to be wider. In other words, after the temperature is reached, it is necessary to maintain the temperature and continue heating. In order to prevent the reservoir temperature from continuing to rise and maintain constant temperature heating, the frequency needs to be reduced.

[0047] (9) After adjusting the heating frequency for a period of time, repeat step (6) until the heating needs to be stopped according to the construction requirements.

[0048] Through the technical implementation of the present invention, on the basis of reservoir temperature monitoring, the RF heating construction frequency is adjusted in real time according to construction requirements, so that the downhole RF heater can quickly heat the reservoir temperature to the preset temperature with the maximum heating efficiency, and the reservoir temperature is maintained at the preset temperature for a long time by adjusting the heating frequency. Compared with the existing fixed-frequency downhole RF heating method, under the same heating conditions, the heating energy efficiency can be effectively improved, so that the reservoir is quickly heated to the preset temperature and the set temperature is maintained in a flexible frequency modulation manner for continuous heating.

Claims

1. A reservoir radio frequency heating method based on temperature monitoring, characterized in that The following steps are involved: (1) Establish temperature and characteristic frequency cross-plots based on the construction area and target layer; (2) using a well temperature monitoring instrument to measure the reservoir temperature before RF heating of the reservoir; (3) querying the temperature and characteristic frequency intersection chart to obtain the characteristic frequency corresponding to the current reservoir temperature, and setting the characteristic frequency as the heating frequency of the downhole radio frequency heater; (4) After a certain period of time, the downhole reservoir temperature is read again using the well temperature monitoring instrument to determine whether the current temperature has reached the preset heating temperature; if not, repeat step (3) until the current temperature reaches the preset heating temperature.

2. A reservoir radio frequency heating method based on temperature monitoring according to claim 1, characterized in that: Step (1) includes the following steps: (101) Collect several experimental rock samples according to the construction area and target layer; (102) Measure the complex dielectric constant spectrum data of experimental rock samples under different temperature conditions in the laboratory; (103) The characteristic frequencies corresponding to the maximum values ​​of the imaginary parts in the complex dielectric constant spectra at different temperatures are extracted, and a cross plot of temperature and characteristic frequency is established.

3. The reservoir radio frequency heating method based on temperature monitoring according to claim 1, characterized in that: The following steps are also included: (5) If the current temperature has reached or exceeded the preset heating temperature, further determine whether the heating operation needs to be terminated; if so, terminate the power supply to the underground radio frequency heater to stop heating; (6) If not, the set temperature needs to be maintained to continuously heat the reservoir at a constant temperature until the heating needs to be stopped according to the construction requirements.

4. The reservoir radio frequency heating method based on temperature monitoring according to claim 3, characterized in that: In step (6): a low frequency far from the optimal heating frequency is selected as the heating frequency according to the relative dielectric constant spectrum obtained at different temperatures; after adjusting the heating frequency for a period of time, step (4) is repeated.

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