Resonance perturbation measurement method and system for core complex dielectric constant

Abstract

The present invention relates to the field of dielectric testing technology for geological samples, and discloses a resonant perturbation measurement method and system for complex dielectric constant of a core. The measurement method comprises the following steps: (1) performing response tests on standard samples made of standard materials with known dielectric constants in four resonant cavities respectively, and obtaining the frequency offset Δ f , establish △ f The quadratic curve relationship between the dielectric constant of the standard material; (2) The relative change of the inverse of the quality factor of the acrylic plunger sample injected with salt water △1 / Q , establish △1 / Q The linear relationship between the dielectric loss and the test core sample frequency deviation △ f And the relative change of the reciprocal of the quality factor △1 / Q , bring in the established fitting curve relationship, and invert the initial complex dielectric constant; (4) perform volume compensation on core samples at different heights to obtain the final accurate complex dielectric constant. The present invention has high accuracy and sensitivity, and can distinguish changes in the water content of 0.1% inside the core.

Description

Technical Field

[0001] The invention relates to a method and a system for measuring the complex dielectric constant of a standard core, belonging to the technical field of dielectric testing of geological samples. Background Art

[0002] Complex water-bearing conditions pose a significant challenge to the interpretation of traditional resistivity logging technology. Dielectric dispersion logging technology relies on the differences in complex dielectric properties between different components such as formation water, oil and gas, and rock skeletons. Through the precise measurement of complex dielectric constants at multiple frequency points, it achieves quantitative evaluation of key parameters such as reservoir porosity, fluid saturation, and mineralization. In a laboratory environment, the precise determination of the complex dielectric constant of core samples provides solid data support and theoretical basis for the in-situ interpretation of dielectric dispersion logging technology.

[0003] Currently, the evaluation of deep and tight reservoirs has become the focus of logging technology research. These reservoirs generally have low porosity, low water content and unknown mineralization, which undoubtedly increases the difficulty of analyzing the dielectric properties of core samples in the laboratory. Therefore, it is particularly important to develop a fast, sensitive and non-intrusive dielectric detection method.

[0004] In the research field of dielectric testing methods for geological samples, many achievements have emerged. As a classic testing method, the parallel plate capacitor method uses two parallel plates as fixtures and electrodes, but its frequency coverage is usually low, below 100MHz, and requires the rock sample to be processed into a thin disc unit. The method for measuring the dielectric constant of coal and rock based on an impedance analyzer disclosed in CN105699787A uses an impedance analyzer combined with a parallel plate capacitor to achieve the dielectric constant measurement of coal mixed with rocks. However, this method has certain limitations in sample preparation. In order to measure different types of samples such as drill cuttings, cores and powder samples, researchers have developed coaxial transmission line fixture units of different sizes. This method can provide a relatively wide frequency band range from hundreds of kHz to GHz, but the rock sample must undergo specific pre-treatment processes such as drilling and polishing to ensure precise matching of the coaxial unit and obtain accurate measurement results. A high-frequency rock dielectric constant measurement method and a measuring fixture disclosed in CN109444174A utilize the transmission reflection characteristics to perform drilling processing on the core sample to achieve the measurement of the high-frequency dielectric constant. The open coaxial probe rule has a wider frequency coverage range, can work in a broadband of more than 1MHz to tens of GHz, and can quickly measure the planar dielectric properties of loose samples or rock samples adjacent to the open end. However, this method can only sense the dielectric properties of a certain point of the sample, and cannot complete the entire dielectric detection on the scale of the plunger sample. The dielectric constant spectrum experimental system and method of loose sediments containing natural gas hydrates disclosed in CN117030808A utilizes the open coaxial probe method to achieve the measurement of the dielectric properties of loose hydrate sediments. Although the above-mentioned broadband scanning method can scan over a wide frequency range, its accuracy and sensitivity are often limited. In contrast, the resonant cavity rule has higher sensitivity, thanks to its high Q value. This method is based on the resonant cavity perturbation principle. It does not need to consider the influence of sample surface roughness and cracks. By measuring the resonant frequency and quality factor of the resonant cavity before and after the sample is placed in the resonant cavity, the dielectric constant of the sample can be calculated. However, the current advanced resonant cavity instruments mainly operate at frequencies above 1 GHz to 10 GHz, and cannot directly measure 1-inch standard cores, which limits its application scope to a certain extent. The rock dielectric constant automatic measurement system disclosed in CN1064548A, although using a closed resonant cavity as a sensor to achieve dielectric constant testing of small-sized cores and container-loaded liquids, also faces the limitation of being unable to directly measure standard cores.

[0005] In summary, the existing dielectric testing methods for geological samples have certain limitations in terms of measurement accuracy, scope of application, and sample preparation. Therefore, developing a complex dielectric testing device suitable for standard plunger sample size with high sensitivity and accuracy is of great significance for promoting the evaluation of deep and tight reservoirs. Summary of the invention

[0006] In view of the shortcomings of the existing dielectric constant measurement technology, the present invention proposes a resonant perturbation measurement method of the complex dielectric constant of the core with high sensitivity and accuracy to meet the needs of dielectric dispersion logging interpretation, so as to realize non-destructive testing of 1-inch standard plunger sample cores and provide more reliable technical support for the dielectric property analysis of geological samples. The present invention also provides a measurement system for realizing the method.

[0007] The resonant perturbation measurement method of the complex dielectric constant of the core of the present invention comprises the following steps:

[0008] (1) Perform response tests on standard samples made of standard materials with known dielectric constants in four resonant cavities, obtain the frequency offset △f of each resonant cavity, and establish a quadratic curve relationship between △f and the dielectric constant of the standard material;

[0009] (2) Test the relative change of the inverse of the quality factor △1 / Q of the acrylic plunger sample injected with salt water, and establish a linear relationship between △1 / Q and dielectric loss;

[0010] (3) The frequency shift △f and the relative change of the inverse of the quality factor △1 / Q of the test core sample are substituted into the fitting curve relationship established in steps (1) and (2) to invert the initial complex dielectric constant;

[0011] (4) Perform volume compensation on core samples at different heights to obtain the final accurate complex dielectric constant.

[0012] The present invention adopts compensation calibration because the volume of the tested plunger sample is relatively large relative to the space in the cavity and cannot fully meet the perturbation conditions of the resonant cavity. The method of relying on the traditional perturbation formula to invert the dielectric constant has limitations in the present invention.

[0013] The standard materials in step (1) are four uniform and dense substances with stable dielectric properties, namely PTFE with a dielectric constant of 2, nylon with a dielectric constant of 3.7, epoxy resin with a dielectric constant of 6.2, and alumina with a dielectric constant of 9.2.

[0014] The standard samples made of standard materials in step (1) are all regular cylinders with a diameter of 1 inch and a height of 30 mm.

[0015] The resonant cavity in step (1) comprises an outer conductor and an inner conductor arranged inside the outer conductor, a top cover is arranged on the outer conductor, an air cavity is formed between the inner conductor and the outer conductor, the height of the inner conductor is lower than the outer conductor, two N-type interfaces are arranged at the bottom of the outer conductor, and the N-type interfaces are connected to the inner conductor. The outer conductor is a shell.

[0016] In the step (1), the electromagnetic field mode in the resonant cavity is close to the transverse electromagnetic (TEM) mode.

[0017] In step (1), the four resonant cavities operate at frequencies of 100 MHz, 300 MHz, 500 MHz and 1 GHz respectively. The resonant cavities at frequencies of 100 MHz, 300 MHz and 500 MHz are provided with bosses on the top surfaces of the inner conductors, so that the resonant cavities become step impedance resonant cavities, so as to reduce the overall volume of the resonant cavities.

[0018] In the step (1), the resonant cavity adopts the equivalent circuit method, and the top cover, the outer conductor, the boss on the upper part of the inner conductor and the air cavity together serve as the lumped capacitor C l , when the total admittance on the reference plane is zero, the resonance condition is met, and the resonance frequency f r The implicit expression of is deduced as , where Z0 is the equivalent impedance, c is the speed of light in a vacuum, and l is the length of the conductor outside the resonant cavity. The load capacitance effectively reduces the length of the cavity without changing the resonant frequency.

[0019] The cross-sectional dimension parameters of the resonant cavity in step (1) satisfy (π / 2(b1+a)) <λ r Conditions, b1 is the diameter of the inner conductor, a is the diameter of the outer conductor, λ r is the wavelength value corresponding to the fixed frequency resonant cavity. This suppresses the high-order transverse electric mode or transverse magnetic mode, further improving the sensing accuracy.

[0020] In step (2), an axial through hole is drilled in the acrylic plunger sample to make its porosity 10%. Specifically, the acrylic plunger sample has a diameter of 25 mm and a height of 30 mm, and is evenly drilled with 16 axial through holes, each of which has a diameter of 2 mm (and a height of 30 mm).

[0021] The dielectric constant and loss of the acrylic plug sample in step (2) are calculated using the complex refractive index (CRIM) model when the pore water saturation and salinity are known, that is, Calculate, where ε * matrix , ε * water and ε * mix represent the complex dielectric constants of the acrylic plug-like skeleton, pore water, and water-filled acrylic plug-like, respectively, saturated is the water-containing porosity; the complex dielectric constant of water ε * water It is given by the Cole-Cole or Debye model, that is, or , where ε s , ε∞ , ε0, τ, α, ω and j are the zero-frequency dielectric constant, optical frequency dielectric constant, vacuum dielectric constant, Debye relaxation time, Cole-Cole factor, test angular frequency and imaginary sign, respectively. , f is the test frequency, where the optical frequency dielectric constant is 4.9 and the vacuum dielectric constant is 8.85×10 -12 , the zero-frequency dielectric constant and the Debye relaxation angular frequency are linearly related to the core temperature T, ε s =A1+B1×T, τ=A2+B2×T, where coefficients A1, A2, B1, and B2 are obtained by referring to published experimental materials or laboratory controlled temperature calibration;

[0022] The △1 / Q of the acrylic plug sample was linearly fitted with the dielectric loss result calculated by the complex refractive index model to obtain the relationship curve between △1 / Q and dielectric loss.

[0023] In step (4), the dielectric constant ε of the core sample originally measured is * raw The dielectric constant, denoted as ε, is inversely proportional to the volume difference between the core sample and the standard sample. * inversed , the adjustment formula is: , V sample is the volume of the core sample, V standard is the volume of the standard sample.

[0024] The resonant perturbation measurement system for the complex dielectric constant of the standard core to realize the above method adopts the following technical solutions:

[0025] The measuring system includes a vector network analyzer, a computer and four resonant cavities working at different frequencies. The vector network analyzer is connected to two N-type interfaces of the resonant cavity through a phase-stable cable and is also connected to the computer. The computer is used to realize automatic control of the vector network analyzer and the resonant cavity, and collect monitoring data and analyze and process the data. The computer adopts a programmable controller.

[0026] The present invention measures the dielectric constant of rock based on the resonant cavity method. The dielectric properties of standard core samples can be tested without secondary cutting and grinding. Compared with other testing methods, this method has extremely high accuracy and sensitivity, and can even distinguish changes in the water content of 0.1% inside the core. It can efficiently carry out experiments on the electrical parameters of the core and obtain rich electrical property information, thereby providing data and theoretical support for dielectric logging interpretation. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 It is a schematic diagram of the structural principle of the resonant perturbation measurement system of the complex dielectric constant of the core of the present invention.

[0028] Figure 2 It is a schematic diagram of the structure of the resonant cavity in the present invention.

[0029] Figure 3 It is a schematic diagram of the electric field distribution of the cross section of the capacitor-loaded coaxial resonant cavity in the present invention.

[0030] Figure 4 This is a flow chart of the complex dielectric constant detection method of a 1-inch standard core based on the resonant cavity perturbation method.

[0031] Figure 5 These are the dielectric constant test results of ten sandstone core dry samples.

[0032] Figure 6 This is the relationship between the core dielectric constant and water porosity when the core is fully saturated with clean water and at a frequency of 100MHz.

[0033] Figure 7 This is a graph showing the relationship between the core dielectric constant and water-containing porosity at a concentration of 20Kppm brine and a frequency of 100MHz.

[0034] Figure 8 This is a graph showing the relationship between the core dielectric constant and water-containing porosity at a concentration of 80Kppm brine and a frequency of 100MHz.

[0035] Fig. 9 This is a graph showing the relationship between the core dielectric constant and water-containing porosity at a brine concentration of 160Kppm and a frequency of 100MHz.

[0036] Fig.10 This is the relationship between the core dielectric constant and water porosity when the core is fully saturated with clean water and at a frequency of 300MHz.

[0037] Fig.11 This is a graph showing the relationship between the core dielectric constant and water-containing porosity at a concentration of 20Kppm brine and a frequency of 300MHz.

[0038] Fig.12 This is a graph showing the relationship between the core dielectric constant and water-containing porosity at a concentration of 80Kppm brine and a frequency of 300MHz.

[0039] Fig.13 This is a graph showing the relationship between the core dielectric constant and water-containing porosity at a concentration of 160Kppm brine and a frequency of 300MHz.

[0040] Fig.14 This is the relationship between the core dielectric constant and water porosity when the core is fully saturated with clean water and at a frequency of 500MHz.

[0041] Fig.15This is a graph showing the relationship between the core dielectric constant and water-containing porosity at a concentration of 20Kppm brine and a frequency of 500MHz.

[0042] Fig.16 This is a graph showing the relationship between the core dielectric constant and water-containing porosity at a concentration of 80Kppm brine and a frequency of 500MHz.

[0043] Fig.17 This is a graph showing the relationship between the core dielectric constant and water-containing porosity at a concentration of 160Kppm brine and a frequency of 500MHz.

[0044] Fig.18 This is the relationship between the core dielectric constant and water porosity when the core is fully saturated with clean water and at a frequency of 1 GHz.

[0045] Fig.19 This is the relationship between the core dielectric constant and water porosity at a concentration of 20Kppm brine and a frequency of 1GHz.

[0046] Fig. 20 This is the relationship between the core dielectric constant and water porosity at a concentration of 80Kppm brine and a frequency of 1GHz.

[0047] Fig.21 This is a graph showing the relationship between the core dielectric constant and water porosity at a concentration of 160Kppm brine and a frequency of 1GHz.

[0048] Fig. 22 This is the dielectric loss information diagram of core sample C4-1(4) at different water porosities.

[0049] Fig.23 This is a trend chart of the dielectric loss of core sample C4-1(4) changing with frequency under fully saturated conditions at different mineralization levels.

[0050] In the figure: 1. Computer, 2. Vector network analyzer, 3. Resonant cavity, 4. Rock sample;

[0051] 31. Outer conductor, 32. Inner conductor, 33. Load capacitor boss, 34. First N-type interface, 35. Second N-type interface, 36. N-type interface conductor. DETAILED DESCRIPTION

[0052] like Figure 1 As shown, the resonant perturbation measurement system of the complex dielectric constant of the core of the present invention comprises a computer 1, a vector network analyzer 2 and a resonant cavity 3, wherein the vector network analyzer 2 is respectively connected to the computer 1 and the resonant cavity 3. The resonant cavity 3 is designed to cover 4 frequency points, namely 100 MHz, 300 MHz, 500 MHz and 1 GHz, which can cover the frequency range of dielectric logging.

[0053] The resonant cavity includes an outer conductor and an inner conductor. An air cavity is formed between the inner conductor and the outer conductor. The height of the inner conductor is lower than that of the outer conductor. Two N-type interfaces are set at the bottom of the outer conductor.

[0054] The structure of the resonant cavity 3 is as follows Figure 2 As shown, it is a cylindrical coaxial structure, that is, a coaxial transmission line with two ends short-circuited by conductor plates. An inner conductor 32 is arranged in an outer conductor 31. The outer conductor 31 is a cylindrical shell with a metal top cover arranged on it. The inner conductor 32 is a cylinder coaxial with the outer conductor 31. A cylindrical air cavity is formed between the inner conductor 32 and the outer conductor 31. The height of the inner conductor 32 is lower than that of the outer conductor. The outer conductor 31 is grounded to reduce external electromagnetic interference. By carefully designing the sizes of the inner and outer conductors, it is ensured that the electromagnetic field mode inside the resonant cavity is close to the transverse electromagnetic (TEM) mode.

[0055] In order to optimize the volume of the low-frequency resonant cavity, the resonant cavity of 100MHz, 300MHz and 500MHz frequency points is further adjusted on the basis of the quarter-wavelength resonant cavity, and a load capacitor boss 33 is set on the top of the inner conductor 32 to optimize the size of the resonant cavity, so that the cavity becomes a step impedance resonant cavity, which changes the equivalent capacitance distribution of the resonant cavity and can achieve the overall volume reduction of the cavity. The finite difference time domain method is used to design and optimize the resonant cavity, so that the electric field energy is concentrated at the position of the load capacitor boss 33, and the magnetic energy in this area is at the weakest level, which ensures the sensitivity of dielectric detection and eliminates the influence of magnetic permeability. Figure 3 The electric field distribution of the capacitor-loaded resonant cavity cross section is given.

[0056] Each resonant cavity 3 reserves two N-type interfaces, namely, a symmetrically arranged first N-type interface 34 and a second N-type interface 35, which are fed by electric excitation method, and the conductor 36 of the N-type interface extends inward and is connected to the inner conductor 32 of the resonant cavity. The two N-type interfaces are connected to the vector network analyzer 2 through a phase-stable cable. For the three resonant cavities with frequencies of 100MHz, 300MHz and 500MHz, the plunger-shaped sample is placed on the load capacitor boss 33 when testing the sample. For the 1GHz resonant cavity, the sample is placed directly on the top surface of its inner conductor 32. The vector network analyzer 2 is connected to the computer 1, and the computer 1 controls the vector network analyzer 2 through the test program, and realizes the reading of data and the inversion of the complex dielectric constant on the computer, so as to realize the automatic control of the entire test system.

[0057] Using the equivalent circuit method, the metal cap, the outer conductor, the boss 33 above the inner conductor and the air cavity are used together as the lumped capacitor C l The resonance condition is satisfied when the total admittance on the reference plane is zero, and the resonance frequency f r The implicit expression of can be derived as: , where Z0 is the equivalent impedance in the circuit, c is the speed of light in a vacuum, and l is the length of the conductor outside the resonant cavity. The load capacitor can effectively reduce the length of the cavity without changing the resonant frequency.

[0058] The cross-sectional dimension parameters of the resonant cavity 3 satisfy (π / 2(b1+a)) <λ r Conditions, b1 is the diameter of the inner conductor, a is the diameter of the outer conductor, λ r The wavelength value corresponding to the fixed frequency resonant cavity is set, thereby suppressing high-order transverse electric mode or transverse magnetic mode, and further improving the sensing accuracy.

[0059] The flow chart of the dielectric constant measurement of the above system is as follows: Figure 4 As shown, the specific steps include the following steps.

[0060] 1. System setup and initial calibration.

[0061] 1. Set the system test frequency, IF bandwidth and test points.

[0062] 2. Select the two-port calibration type, set the reference plane for measurement (this reference plane refers to the measurement reference position determined when the calibration piece is connected to the test port of the vector network analyzer) at the first N-type port 34 and the second N-type port 35 of the resonant cavity, connect the two ports of the vector network analyzer 2 with the open-circuit calibration piece, the short-circuit calibration piece and the load matching piece respectively for calibration, and then connect the two ports with the through calibration piece for through calibration, thus completing the initialization calibration of the entire vector network analyzer 2.

[0063] 2. Use standard materials for compensation calibration and dielectric inversion.

[0064] In order to test near the standard resonant frequency of a single resonant cavity, the method of scanning S parameters twice can be used to improve the test accuracy while ensuring the test speed. First, set a test frequency range that should cover the area near the standard resonant frequency, set a suitable intermediate frequency bandwidth and a small number of test points, and perform the first scan. Through the first scan, a resonant frequency point f0 can be roughly determined, and then the second scan is performed. Add a very small frequency offset on the left and right sides of f0 to form a very narrow frequency range, and increase the number of test points within this range for a more precise scan. In this way, S parameters with higher accuracy can be obtained. It should be noted that the very small frequency offset added during the second scan should be determined according to the specific conditions of each cavity, and the selection principle is that the bandwidth range of the second scan should be able to cover the frequency points where the S21 parameter drops 3dB on both sides of f0. In this way, S parameters (scattering parameters) with higher accuracy can be quickly obtained while ensuring the test speed.

[0065] 1. Test the S parameters of the standard material and obtain the quadratic curve relationship between the △f and dielectric constant of the standard material.

[0066] The standard materials are four uniform and dense materials with stable dielectric properties, namely PTFE with a dielectric constant of 2, nylon with a dielectric constant of 3.7, epoxy resin with a dielectric constant of 6.2, and alumina with a dielectric constant of 9.2. The standard samples made of standard materials are all regular cylinders with a diameter of 1 inch and a height of 30 mm.

[0067] Standard samples made of standard materials with known dielectric constants are tested for response in four resonant cavities to obtain the frequency shift △f of each resonant cavity, and a quadratic curve relationship between the frequency shift △f and the dielectric constant of the standard material is established.

[0068] 2. Test the S parameters of the water-filled acrylic plunger sample and obtain the linear relationship between △1 / Q and dielectric loss of the acrylic plunger sample.

[0069] The acrylic plug has a diameter of 25 mm and a height of 30 mm, and is evenly drilled with 16 axial through holes throughout its height, each with a diameter of 2 mm. The porosity of the acrylic plug reaches 10%.

[0070] 0ppm of deionized water and 10Kppm of sodium chloride brine were injected into the acrylic plug sample respectively to make the acrylic plug sample reach a saturated state, that is, 10% of the pores of the acrylic plug sample were filled with water.

[0071] Using the CRIM model, that is Approximately calculate the dielectric constant and loss of the sample, where ε * matrix , ε * water and ε * mix represent the complex dielectric constants of the acrylic plug-like skeleton, pore water, and water-filled acrylic plug-like, respectively, saturated is the water-containing porosity. The complex dielectric constant of water, ε * water It is given by the Cole-Cole or Debye model, that is, or , where ε s , ε ∞ , ε0, τ, α, ω and j are the zero-frequency dielectric constant, optical frequency dielectric constant, vacuum dielectric constant, Debye relaxation time, Cole-Cole factor, test angular frequency and imaginary sign, respectively. , f is the test frequency, where the optical frequency dielectric constant is 4.9 and the vacuum dielectric constant is 8.85×10 -12The zero-frequency dielectric constant and Debye relaxation angular frequency are established with the core temperature T as ε s =A1+B1×T, τ=A2+B2×T, where the coefficients A1, A2, B1, and B2 can be obtained by referring to published experimental materials or laboratory controlled temperature calibration.

[0072] Since the fluid distribution is inevitably non-uniform when the through-hole is not fully saturated, in order to avoid the interference of this non-uniform distribution on the dielectric results, the relationship between dielectric loss and △1 / Q is studied under fully saturated conditions with salinity of 0ppm and 10Kppm. Linear regression is used to interpolate between these two saturation points to determine the coefficient relationship between dielectric loss and △1 / Q at four different frequency points.

[0073] The △1 / Q of the acrylic plunger sample was linearly fitted with the dielectric loss result calculated by the CRIM model to obtain the relationship curve between △1 / Q and dielectric loss.

[0074] 3. Test rock samples 4 (see Figure 1 ) to obtain △f and △1 / Q, and then bring them into the inversion curves of steps 1 and 2, that is, the quadratic curve relationship between the frequency offset △f and the dielectric constant of the standard material and the relationship curve between △1 / Q and dielectric loss, to invert the initial dielectric constant.

[0075] 4. Perform volume compensation on core samples at different heights to obtain the final accurate complex dielectric constant.

[0076] In order to avoid the influence of small changes in the volume of core sample 4 on the results during the actual production process, a volume compensation algorithm was developed. The compensation algorithm is based on the original measured dielectric constant ε * raw The dielectric constant, denoted as ε, is inversely proportional to the volume difference between the core sample and the standard sample. * inversed .

[0077] Volume compensation is performed based on the slight difference in the volume of rock sample 4, using the volume compensation formula Volume compensation is performed to obtain the final complex dielectric constant, which ensures the accuracy of the inverted dielectric constant estimation. Similarly, dielectric loss information is also obtained through a similar method.

[0078] A series of dielectric property tests were conducted on ten sandstone plug samples from three well sources using the complex dielectric test system of the present invention. The porosity of these ten cores is between 8% and 20%. Deionized water and sodium chloride (NaCl) powder were used as solvent and solute, respectively, and solutions with mineralizations of 0ppm, 20Kppm, 80Kppm and 160Kppm were prepared at room temperature as pore filling fluids. The experimental design was carried out in ascending order of mineralization to avoid the interference of residual pore fluids with higher mineralization on experiments with lower mineralization. The saturation process of the sandstone plug samples was carried out under a constant pressure of 20MPa, and a single saturation lasted for 48 hours to ensure that the core was completely saturated. For the water-containing porosity gradient experiment, it was achieved by combining centrifugation with natural air drying, thereby improving the control accuracy of the dehydration process. A total of 6 different water saturation gradients were designed for each sample corresponding to a single mineralization, and these gradients were basically evenly distributed in the range of 0%-100%. The water-containing porosity during the dehydration process is calculated by weighing and combining the following formula: , where W saturated is the weight of the sample in the saturated state, W dry is the weight of the sample in a completely dry state, ρ wate is the density of pore water, ϕ saturated is the water-containing porosity.

[0079] Figure 5 The figure shows the dielectric dispersion information of 10 dry rock samples. It can be observed that the dielectric values ​​of the dry rock samples obviously lack discreteness and the amplitudes are tightly converged. Even if the dielectric constant of the sample changes by only 0.1, this tiny change can be distinguished by the system. Therefore, the test system of the present invention can effectively detect subtle differences in dry rock samples.

[0080] Figure 6-Figure 21 The statistical relationship between the dielectric constant and water-bearing porosity of sandstone plug samples at four different frequencies is shown. Figure 6 , Figure 7 , Figure 8 and Fig. 9 It is the relationship between the dielectric constant and the water porosity at a frequency of 100MHz, and represents the cases of fully saturated clean water (i.e. water with a salinity of 0ppm), 20Kppm, 80Kppm and 160Kppm salt water respectively. Fig.10 , Fig.11 , Fig.12 and Fig.13 It is the relationship between the dielectric constant and the water porosity at a frequency of 300MHz, representing the cases of fully saturated clean water, 20Kppm, 80Kppm and 160Kppm salt water respectively. Fig.14 , Fig.15 , Fig.16 and Fig.17It is the relationship between the dielectric constant and the water porosity at a frequency of 500MHz, representing the cases of fully saturated clean water, 20Kppm, 80Kppm and 160Kppm salt water respectively. Fig.18 , Fig.19 , Fig. 20 and Fig.21 It is the relationship between the dielectric constant and the water porosity at a frequency of 1 GHz, representing the cases of fully saturated clean water, 20 Kppm, 80 Kppm and 160 Kppm salt water respectively.

[0081] In general, the dielectric constant increases with the increase of water porosity, and is also affected by salinity and frequency changes. In order to more clearly reveal these subtle differences, the CRIM fitting curve is introduced as a reference baseline. Among them, the dielectric constant of salt water is calculated according to the Debye model, while the dielectric constant value of the rock matrix comes from Figure 5 The corresponding values ​​of the dry samples shown in . The results show that the complex dielectric test system designed by the present invention has extremely high detection accuracy and can accurately detect and distinguish small changes in water-containing porosity as low as 0.1%. In terms of consistency with the fitting curve, due to the limitations of the CRIM model itself, it cannot fully reflect the interfacial polarization phenomenon under low-frequency and high-salinity conditions. However, under high-frequency conditions, the fitting results show a high degree of consistency with the actual test results, which further proves the reliability of the test results of the present invention.

[0082] The present invention also performs well in the measurement of dielectric loss. Taking the C4-1(4) sample as an example, under different water porosity and mineralization conditions, its dielectric loss shows an obvious change pattern. When the sample is in a dry state, the dielectric loss is close to zero. As the water porosity gradually increases, the dielectric loss also gradually increases. It is particularly noteworthy that in Fig. 22 Under the condition of 80Kppm high salinity, the slope of the relationship between dielectric loss and water porosity at different frequencies changes, and Figure 8 , Fig.12 , Fig.16 , Fig. 20 The slope of the relationship between the dielectric constant and the water-containing porosity shown in is highly consistent. This further verifies that the measurement results of the test system of the present invention under different parameter conditions have good correlation and reliability. In addition, Fig.23The trend of the dielectric loss of sample C4-1 (4) changing with frequency under completely saturated conditions of different mineralizations is shown. As can be seen from the figure, the dielectric loss decreases with increasing frequency. For high salinity conditions, the dielectric loss value can reach a considerable value, close to 35, but even so, it is still within the effective recognition range of the detection system of the present invention, which fully demonstrates the powerful detection capability and wide applicability of the test system of the present invention under complex conditions such as high salinity. It can be seen from the above embodiments that the complex dielectric test system developed by the present invention has extremely high sensitivity and has great advantages for dielectric testing of some low-porosity cores.

Claims

1. A resonant perturbation measurement method for the complex dielectric constant of a core, characterized by: The following steps are involved: (1) Perform response tests on standard samples made of standard materials with known dielectric constants in four resonant cavities, obtain the frequency offset △f of each resonant cavity, and establish a quadratic curve relationship between △f and the dielectric constant of the standard material; (2) Test the relative change of the inverse of the quality factor △1 / Q of the acrylic plunger sample injected with salt water, and establish a linear relationship between △1 / Q and dielectric loss; (3) The frequency shift △f and the relative change of the inverse of the quality factor △1 / Q of the test core sample are substituted into the fitting curve relationship established in steps (1) and (2) to invert the initial complex dielectric constant; (4) Perform volume compensation on core samples at different heights to obtain the final accurate complex dielectric constant.

2. The resonant perturbation measurement method of the complex dielectric constant of a core according to claim 1 is characterized by: The standard materials in step (1) are four uniform and dense materials with stable dielectric properties, namely PTFE with a dielectric constant of 2, nylon with a dielectric constant of 3.7, epoxy resin with a dielectric constant of 6.2, and alumina with a dielectric constant of 9.

2. The standard samples made of the standard materials are all regular cylinders with a diameter of 1 inch and a height of 30 mm.

3. The resonant perturbation measurement method of the complex dielectric constant of a core according to claim 1 is characterized by: The resonant cavity in step (1) comprises an outer conductor and an inner conductor arranged inside the outer conductor, a top cover is arranged on the outer conductor, an air cavity is formed between the inner conductor and the outer conductor, the height of the inner conductor is lower than the outer conductor, two N-type interfaces are arranged at the bottom of the outer conductor, and the N-type interfaces are connected to the inner conductor.

4. The resonant perturbation measurement method of the complex dielectric constant of a core according to claim 1 is characterized by: In the step (1), the four resonant cavities operate at frequencies of 100 MHz, 300 MHz, 500 MHz and 1 GHz respectively; the resonant cavities at frequencies of 100 MHz, 300 MHz and 500 MHz are provided with bosses on the top surfaces of the inner conductors.

5. The resonant perturbation measurement method of the complex dielectric constant of a core according to claim 1 is characterized by: In step (1), the resonant cavity adopts the equivalent circuit method, and the top cover, the outer conductor, the upper boss of the inner conductor and the air cavity together serve as the lumped capacitor C l , when the total admittance on the reference plane is zero, the resonance condition is met, and the resonance frequency f r The implicit expression of is deduced as , where Z0 is the equivalent impedance, c is the speed of light in vacuum, and l is the length of the conductor outside the resonant cavity.

6. The resonant perturbation measurement method of the complex dielectric constant of a core according to claim 1 is characterized by: The cross-sectional dimension parameters of the resonant cavity in step (1) satisfy (π / 2(b1+a)) <λ r Conditions, b1 is the diameter of the inner conductor, a is the diameter of the outer conductor, λ r is the wavelength value corresponding to the fixed frequency resonant cavity.

7. The resonant perturbation measurement method of the complex dielectric constant of a core according to claim 1 is characterized by: In the step (2), an axial through hole is drilled in the acrylic plunger sample so that its porosity is 10%.

8. The resonant perturbation measurement method of the complex dielectric constant of a core according to claim 1 is characterized by: The dielectric constant and loss of the acrylic plug sample in step (2) are calculated by using the following formula when the pore water saturation and mineralization are known: Calculate, where ε * matrix , ε * water and ε * mix represent the complex dielectric constants of the acrylic plug-like skeleton, pore water, and water-filled acrylic plug-like, respectively, saturated is the water-containing porosity; the complex dielectric constant of water ε * water is through or Given, where ε s , ε ∞ , ε0, τ, α, ω and j are the zero-frequency dielectric constant, optical frequency dielectric constant, vacuum dielectric constant, Debye relaxation time, Cole-Cole factor, test angular frequency and imaginary sign, respectively. , f is the test frequency, where the optical frequency dielectric constant is 4.9 and the vacuum dielectric constant is 8.85×10 -12 , the zero-frequency dielectric constant and the Debye relaxation angular frequency are linearly related to the core temperature T, ε s =A1+B1×T, τ=A2+B2×T, where coefficients A1, A2, B1, and B2 are obtained by referring to published experimental materials or laboratory controlled temperature calibration; The △1 / Q of the acrylic plug sample was linearly fitted with the dielectric loss result calculated by the complex refractive index model to obtain the relationship curve between △1 / Q and dielectric loss.

9. The resonant perturbation measurement method of the complex dielectric constant of a core according to claim 1 is characterized by: In step (4), the dielectric constant ε of the core sample originally measured is * raw The dielectric constant, denoted as ε, is inversely proportional to the volume difference between the core sample and the standard sample. * inversed , the adjustment formula is: , V sample is the volume of the core sample, V standard is the volume of the standard sample.

10. A resonant perturbation measurement system for complex dielectric constant of a core, used to implement the resonant perturbation measurement method for complex dielectric constant of a core according to any one of claims 1 to 9, characterized in that: It includes a vector network analyzer, a computer and four resonant cavities working at different frequencies. The vector network analyzer is connected to two N-type interfaces of the resonant cavity through a phase-stable cable and is also connected to the computer. The computer is used to realize automatic control of the vector network analyzer and the resonant cavity, and to collect monitoring data and analyze and process the data.

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