A reservoir radio frequency heating method based on temperature monitoring

Abstract

The present invention discloses a reservoir radio frequency heating method based on temperature monitoring, which relates to the field of reservoir exploitation. The method comprises the following steps: (1) establishing a temperature and characteristic frequency intersection diagram according to the construction area and the target layer; (2) using a well temperature monitoring instrument to measure the reservoir temperature before radio frequency heating of the reservoir; (3) querying the temperature and characteristic frequency intersection diagram 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 period of time, using the well temperature monitoring instrument again to read the downhole reservoir temperature value, and judging whether the current temperature has reached the preset heating temperature; if not, repeating step (3); until the current temperature reaches the preset heating temperature. Compared with the existing fixed-frequency downhole radio frequency heating method, the present invention can effectively improve the heating energy efficiency under the same heating conditions, 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.

Description

Technical Field

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

[0002] The development and utilization of oil shale is gaining increasing attention worldwide. With increasing environmental pressure, in-situ oil shale mining has become an inevitable trend for future large-scale commercial oil shale production. In-situ mining involves heating the oil shale reservoir at high temperatures to convert the solid kerogen in the shale into liquid hydrocarbons. These liquid hydrocarbons are then extracted from the ground using traditional oil and gas drilling and production processes. Currently, oil shale heating methods can be categorized as 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 application with application publication number CN115354990A discloses a device and method for intelligent production of heavy oil reservoirs using directional radio frequency heating. This device effectively reduces the high energy consumption of traditional blanket heating methods by using directional radio frequency heating to heat the areas rich in residual oil. The Chinese invention patent application with application publication number CN108318520A discloses a downhole radio frequency heating simulation test device, which can be used to study the factors affecting the effect of radio frequency heating on oil and gas reservoirs and evaluate the feasibility of radio frequency heating methods in thermal oil recovery. However, traditional fixed-frequency downhole radio frequency heating technology cannot ensure that radio frequency heating always operates at maximum efficiency. 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;

[0009] (3) querying the temperature and characteristic frequency cross-plot 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 period of time, the downhole reservoir temperature value 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 imaginary part of the complex dielectric constant spectrum 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 downhole radio frequency heater to stop heating;

[0017] (6) If not, it is necessary to maintain the set temperature and continue to heat the reservoir at a constant temperature until the heating needs to be stopped according to the construction requirements.

[0018] Preferably, 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 (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 implementation of the technology 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 by adjusting the heating frequency, the reservoir temperature is maintained at the preset temperature for a long time. 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 temperature can be quickly heated to the preset temperature and the set temperature can be 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 part spectrum of the complex dielectric constant of shale after being saturated with NaCl solution of different concentrations;

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

[0024] In-situ electric 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. Traditional radio frequency in-situ electric heating methods for reservoirs use a fixed single-frequency voltage for heating, ignoring the dispersion law of the reservoir dielectric constant and the impact of temperature changes on the reservoir dielectric spectrum during the heating process. The present invention innovatively considers the impact of temperature changes on the reservoir dielectric spectrum during the electric heating process for the first time. 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 electric 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 with reference to the accompanying drawings and research background.

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

[0027] Radio frequency (RF) reservoir heating technology is an in-situ reservoir heating process. RF heaters are placed downhole to generate electromagnetic waves, which increase the reservoir temperature through dielectric heating. This in-situ reservoir heating technology is a technique for improving unconventional reservoirs, effectively increasing the recovery rate of unconventional reservoirs such as shale oil. Under the influence of electromagnetic waves, the heating characteristics of the dielectric medium are primarily determined by the imaginary part of the complex dielectric constant, or the dielectric loss coefficient.

[0028] Radio frequency heating technology typically operates in the frequency range of 300kHz to 300MHz. Existing radio frequency reservoir heating technologies primarily utilize single-frequency excitation, and there are no reports on multi-frequency or variable-frequency radio frequency reservoir heating technologies. The complex dielectric constant of porous media such as rock exhibits significant frequency dispersion, varying with frequency. Rock physics experimental studies have shown that the frequency dispersion of the complex dielectric constant of rock generally conforms to the Cole-Cole model:

[0029]

[0030] Where, 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 permittivity change with frequency. It can be seen that with the increase of frequency, the imaginary part of the relative permittivity first increases and then decreases. At the characteristic frequency, the imaginary part of the relative permittivity reaches its maximum value.

[0032] The principles of radio frequency heating are similar to those of microwave heating. In microwave heating, polar water molecules continuously rotate under the influence of an alternating electromagnetic field. The frequency of this rotation is essentially fixed and independent of their distribution. Therefore, microwave heating typically uses a fixed heating frequency of 2.45 GHz, which is close to the natural frequency of water molecules. However, in the radio frequency range, the dispersion of the complex dielectric constant is dominated by the interfacial polarization effect. Matthew Josh, a researcher at 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 range (between 10^5 and 10^9 Hz), interfacial polarization (the Maxwell-Wagner effect) dominates the dielectric dispersion of rocks.

[0033] Interfacial polarization refers to the phenomenon in which cations and anions in the pore water solution in a porous medium migrate in opposite directions under the influence of an alternating electric field, resulting in accumulation at the solid-liquid interface. When the electric field direction is reversed, the cations and anions reverse their migration directions and accumulate at opposing solid-liquid interfaces. The collisions and friction that occur during ion migration cause the medium temperature to rise. When the reservoir contains metallic minerals, the migration of free electrons within the mineral particles under the influence of an alternating electric field also follows this pattern. When the electric field frequency matches the interfacial polarization cation and anion migration period—that is, when the cations and anions have completed their accumulation at the interface—the electric field direction reverses. At this point, the imaginary part of the relative dielectric constant due to interfacial polarization is maximized, resulting in optimal heating efficiency. Clearly, the characteristic frequency of this matching is dependent on multiple factors, including the pore structure of the porous medium, the concentrations of cations and anions, and the migration rates. Unlike the fixed characteristic frequency of the rotational polarization of water molecules, this matching characteristic frequency is not the same as the rotational 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 of the complex dielectric constant of rock. On the one hand, as temperature increases, the thermal motion of water molecules intensifies, increasing their polarization and rotational freedom, leading to a decrease in the dielectric constant of water. On the other hand, rising temperature accelerates the migration of ions in the rock's pore water solution, enhancing the rock's conductivity. Under an RF electric field, this accelerated ion migration caused by rising temperature decreases the rock's equivalent dielectric constant and affects the interface polarization response characteristics, which in turn affects the magnitude of the interface polarization characteristic frequency. Theoretically, higher reservoir temperature increases the migration rate of anions and cations. Given an unchanged pore structure, anions and cations can accumulate more quickly at the interface, resulting in a higher characteristic frequency. Therefore, even if RF excitation corresponding to the characteristic frequency is initially used for heating, as the reservoir temperature increases, the characteristic frequency will shift toward higher frequencies. Without increasing the heating frequency, the imaginary part of the complex dielectric constant at that frequency will decrease, reducing heating efficiency. In other words, as construction progresses, temperature changes cause the characteristic frequency corresponding to the maximum imaginary part of the complex dielectric constant to change. Traditional fixed-frequency RF heating processes cannot guarantee heating efficiency at the optimal heating frequency operating 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 at the maximum efficiency state, 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. Before construction, the present invention conducts indoor experiments on the core of the construction target layer to measure the complex dielectric constant spectrum of the rock samples at different temperatures. A relationship chart between the temperature of the target layer in the construction area and the optimal construction frequency is established. On this basis, by monitoring the changes in the well temperature during the construction process, the construction frequency of the RF heater is adjusted in real time, thereby keeping the reservoir heating efficiency at the maximum efficiency state, improving the reservoir heating efficiency, and saving 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 imaginary part of the complex dielectric constant spectrum 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.

[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 (such as Figure 1 fb in).

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

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

[0046] (8) If it is necessary to maintain the set temperature and 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 (taking into account the skin effect, low frequency is more likely to obtain a higher penetration depth) (e.g. Figure 1 The heating frequency (fa) in the figure is used to reduce heating efficiency. This prevents further temperature rise while also allowing the temperature to be transferred further, thus increasing the heating range. In other words, once the desired temperature is reached, it is necessary to maintain the temperature and continue heating. To prevent the reservoir temperature from rising further and maintain constant 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] By implementing the technology of this invention, while monitoring reservoir temperature, and adjusting the RF heating frequency in real time according to construction requirements, the downhole RF heater can quickly heat the reservoir to a preset temperature with maximum heating efficiency. By adjusting the heating frequency, the reservoir temperature can be maintained at the preset temperature for a long time. Compared to existing fixed-frequency downhole RF heating methods, this method can effectively improve heating energy efficiency under the same heating conditions, allowing the reservoir to quickly heat to the preset temperature and maintain the set temperature through flexible frequency modulation 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; (3) querying the temperature and characteristic frequency cross-plot 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 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; 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 imaginary part of the complex dielectric constant spectrum at different temperatures are extracted, and a cross-plot of temperature and characteristic frequency is established.

2. 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 downhole radio frequency heater to stop heating; (6) If not, it is necessary to maintain the set temperature and continue to heat the reservoir at a constant temperature until the heating needs to be stopped according to the construction requirements.

3. The reservoir radio frequency heating method based on temperature monitoring according to claim 2, 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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