Method for evaluating permeation enhancing effect of ultrasonic wave acting on porous medium

Abstract

The invention provides a method for evaluating the permeation enhancing effect of ultrasonic wave acting on a porous medium, which comprises the following steps: carrying out first NMR test on a dried, vacuumized and saturated sample to obtain a first saturated water T2 spectrum of the sample, and analyzing related parameters of the sample through the first saturated water T2 spectrum; performing a first ultrasonic permeation enhancing effect on the sample subjected to the first NMR test through ultrasonic waves, and performing a second NMR test on the sample every set time in the process to obtain a second saturated water T2 spectrum; after each second NMR test is finished, the sample is vacuumized again for water saturation, and the sample which is vacuumized again for water saturation is subjected to a third NMR test to obtain a third water saturation T2 spectrum; related parameters of the medium after the ultrasonic action at different time points are calculated through the third water saturation T2 spectrum, and the intensity of the ultrasonic cavitation effect is calculated according to the second water saturation T2 spectrum and the third water saturation T2 spectrum; and evaluating the permeation enhancing effect of the ultrasonic waves through the previous and later related parameters. According to the method, the ultrasonic cavitation intensity and the permeation increasing effect can be evaluated at the same time.

Description

Technical Field

[0001] The invention belongs to the technical field of medium permeability after engineering application of ultrasonic wave acting on porous medium, and specifically relates to a method for evaluating the permeability enhancement effect of ultrasonic wave acting on porous medium. Background Art

[0002] Ultrasonic waves refer to sound waves with a frequency higher than 20kHz. Usually, ultrasonic waves propagate in the form of longitudinal waves, because longitudinal waves can propagate in elastic media with stretching and compression capabilities. Common solids, liquids, and gases all have stretching and compression elasticity. When ultrasonic waves propagate in two-phase or multi-phase media, they interact with the media and produce a series of ultrasonic-specific effects. When ultrasonic waves act on the rock skeleton of porous media, they can change its pore size distribution characteristics, increase pore connectivity, and significantly improve the permeability of the medium. When acting on saturated fluids (oil, water, and natural gas) in the pores, they can reduce fluid viscosity and improve the fluidity of saturated fluids. The ultrasonic cavitation effect is the main reason for improving the permeability of porous media. Ultrasonic cavitation refers to the phenomenon that when ultrasonic waves propagate in the fluid, the fluid molecules are alternately compressed, and the tiny bubbles (cavitation nuclei) in the fluid oscillate, expand, compress, and collapse under the action of ultrasonic waves, thereby generating instantaneous high pressure and instantaneous high temperature in the microbubbles and the tiny spaces around them. The instantaneous pressure generated by the rupture of ultrasonic cavitation bubbles can be close to 200MPa. Therefore, under the continuous action of ultrasonic cavitation effect, the high-pressure impact causes the throat of the porous medium to expand, increasing the pore connectivity and thus improving the permeability of the medium. Therefore, this technology is widely used to improve the permeability of low-permeability porous media, thereby achieving the purpose of improving the efficiency of groundwater pollutant remediation and increasing the recovery rate of oil, coalbed methane and natural gas.

[0003] Since the ultrasonic cavitation effect is affected by many factors such as the initial intensity, frequency, duration of action, medium properties and liquid viscosity of the ultrasonic wave, the ultrasonic permeability enhancement effect is also closely related to these factors. When applying ultrasonic permeability enhancement technology in actual engineering, it is often necessary to adjust the above parameters in time according to the changes in the intensity of the cavitation effect and the permeability of the medium in order to achieve the best or target permeability enhancement effect, while also reducing the ultrasonic energy consumption and not causing unnecessary damage to the formation due to excessive permeability enhancement. Therefore, the evaluation of the intensity of the ultrasonic cavitation effect and the permeability enhancement effect is very important.

[0004] At present, the intensity of ultrasonic cavitation effect and permeability enhancement effect are usually evaluated indirectly. For example, cavitation intensity is mainly evaluated by measuring acoustic parameters, observing visual evidence of cavitation phenomena, or detecting chemical or biological effects caused by cavitation, while ultrasonic permeability enhancement is mainly evaluated by indirect parameters such as changes in the pore structure of the medium after ultrasonic action, fluid penetration velocity or flow rate. It can be seen that the evaluation of the two often needs to be carried out step by step, resulting in low consistency and accuracy of the evaluation. Summary of the invention

[0005] In order to solve the problem that the existing evaluation method conducts cavitation intensity and ultrasonic permeability enhancement evaluation in steps, resulting in low consistency and accuracy of the evaluation, the present invention provides a method for evaluating the permeability enhancement effect of ultrasonic waves on porous media.

[0006] In order to achieve the above object, the present invention provides the following technical solutions:

[0007] A method for evaluating the permeability enhancement effect of ultrasonic waves on porous media, comprising:

[0008] The dried sample of the porous medium to be tested is subjected to a vacuum saturation treatment, and the vacuum saturated sample is subjected to a first NMR test to obtain a first saturated T2 spectrum of the sample, and the initial pore size distribution, porosity and permeability coefficient of the sample are obtained by analyzing the first saturated T2 spectrum;

[0009] The sample after the first NMR test is subjected to a first ultrasonic permeation effect by ultrasound, and a second NMR test is performed on the sample at set intervals during the first ultrasonic permeation effect to obtain a second water-saturated T2 spectrum of the sample; after each second NMR test, the sample is re-vacuumed and saturated with water, and a third NMR test is performed on the re-vacuumed and saturated sample to obtain a third water-saturated T2 spectrum of the sample;

[0010] The pore size distribution, porosity and permeability coefficient of the sample after ultrasonic action at different time points were obtained by analyzing the third saturated water T2 spectrum, and the intensity of ultrasonic cavitation effect of the sample after ultrasonic action at different time points was obtained by calculating the second saturated water T2 spectrum and the third saturated water T2 spectrum;

[0011] The initial pore size distribution, porosity and permeability of the sample were compared with the pore size distribution, porosity and permeability of the sample after ultrasonic treatment at different time points. The permeability enhancement effect of ultrasound was evaluated based on the comparison results and the intensity of ultrasonic cavitation effect.

[0012] Preferably, the permeability coefficient, porosity and pore size distribution of the medium are calculated by T2 spectrum, specifically:

[0013] The calculation formula of permeability coefficient k is:

[0014]

[0015] Where: n represents the porosity of the medium, ρ w represents the density of water, g represents the acceleration of gravity, m represents the aperture classification level, N p It is expressed as the number of cylindrical capillaries that make up the water flow path, μ represents the viscosity coefficient, ρ2 represents the transverse surface relaxation strength, T 2i is the T2 value corresponding to position i;

[0016] The calculation formula of porosity n is:

[0017]

[0018] V w is the volume of the aqueous phase, cm 3 ; V c is the volume of the sample, cm 3 ; I is the NMR signal value measured from the saturated sample, dimensionless; f(I) is the product of I and V w The linear relationship between

[0019] The pore size distribution of the medium can be analyzed through the peak position and area of ​​the T2 spectrum.

[0020] Preferably, the intensity of the ultrasonic cavitation effect is specifically:

[0021] The intensity C of the ultrasonic cavitation effect is obtained based on the total signal intensity S2 of the second saturated water T2 spectrum and the total signal intensity S3 of the third saturated water T2 spectrum:

[0022]

[0023] The total signal intensity of the T2 spectrum refers to the sum of all signal intensities in the entire T2 spectrum distribution.

[0024] Preferably, the ultrasonic permeation of the sample is stopped when the T2 signal intensity variation in the second NMR test is less than 1%.

[0025] Preferably, the method further includes changing the ultrasonic power and frequency, repeating the second NMR test and the third NMR test, and evaluating the permeability enhancement effect of different ultrasonic actions on the medium by comparing and analyzing the permeability coefficients of the medium measured under different ultrasonic action parameters and comparing them with the data obtained by the first NMR test;

[0026]

[0027] Where A is the increase rate of the permeability coefficient of the porous medium after a certain ultrasonic action, k i is the permeability coefficient of the porous medium calculated according to the third NMR test after a certain ultrasonic action, and k0 is the permeability coefficient of the porous medium obtained in the first NMR test.

[0028] The method for evaluating the permeability enhancement effect of ultrasonic waves on porous media provided by the present invention has the following beneficial effects:

[0029] The present invention conducts continuous nuclear magnetic resonance testing on the porous medium during the action of ultrasonic waves, and after the continuous nuclear magnetic resonance testing, fills the cavitation bubbles in the porous medium with saturated water, and then conducts nuclear magnetic resonance testing again, and combines the T2 spectrum obtained by the continuous two-aspect nuclear magnetic resonance and the pore characteristic analysis of the porous medium itself to obtain the cavitation effect intensity of the ultrasonic wave in the porous medium and the change of the porosity and permeability characteristics of the medium, and evaluates the cavitation effect intensity and the permeability enhancement effect on the porous medium according to the change of the porosity and permeability characteristics, which solves the problem that the existing evaluation method conducts the cavitation intensity and ultrasonic permeability enhancement evaluation in steps, resulting in low consistency and accuracy of the evaluation, and can provide a better cavitation effect intensity and medium permeability change evaluation method for the application of ultrasonic permeability enhancement technology in actual engineering. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] In order to more clearly illustrate the embodiment of the present invention and its design scheme, the following briefly introduces the drawings required for this embodiment. The drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0031] Figure 1 A schematic flow chart of a method for evaluating the permeability enhancement effect of ultrasonic waves on porous media provided by an embodiment of the present invention;

[0032] Figure 2 Schematic diagram of initial T2 distribution of the medium;

[0033] Figure 3 Schematic diagram of T2 distribution under different conditions; Figure 3 (a) is a schematic diagram of T2 distribution under different ultrasonic power treatments (100W, 200W, 300W, 400W and 500W); Figure 3 (b) Schematic diagram of T2 distribution for re-saturation at each time interval;

[0034] Figure 4 Schematic diagram of pore radius distribution after treatment with different ultrasonic powers (100W, 200W, 300W, 400W and 500W);

[0035] Figure 5 Schematic diagram of the effect of ultrasonic power on medium pore size distribution;

[0036] Figure 6 It is a schematic diagram of the variation trend of the permeability coefficient of the medium with the ultrasonic power;

[0037] Figure 7 It is a schematic diagram of T2 distribution under different time states; Figure 7a is a schematic diagram of T2 distribution at different ultrasonic treatment times (0min, 30min, 60min, 90min and 150min); Figure 7 b is a schematic diagram of T2 distribution for re-saturation at each time interval;

[0038] Figure 8 Schematic diagram of pore radius distribution after different ultrasonic treatment times (0min, 30min, 60min, 90min and 150min);

[0039] Fig. 9 Schematic diagram of the effect of ultrasonic duration on medium pore size distribution;

[0040] Fig.10 It is a schematic diagram of the relationship between the medium permeability coefficient and the duration of ultrasonic action;

[0041] Fig.11 is a schematic diagram of T2 distribution; Fig.11 (a) is a schematic diagram of T2 distribution after ultrasound treatment at different frequencies (10, 40 KHz and 120 KHz); Fig.11 (b) Schematic diagram of T2 distribution for re-saturation at each time interval;

[0042] Fig.12 The pore radius distribution diagram after ultrasonic treatment at different frequencies (0 Hz, 40 KHz and 120 KHz);

[0043] Fig.13 Schematic diagram of the effect of ultrasonic frequency on the pore size distribution of the medium;

[0044] Fig.14 Schematic diagram of the relationship between medium permeability and ultrasonic frequency. DETAILED DESCRIPTION

[0045] In order to enable those skilled in the art to better understand the technical solution of the present invention and implement it, the present invention is described in detail below in conjunction with the accompanying drawings and specific embodiments. The following embodiments are only used to more clearly illustrate the technical solution of the present invention, and cannot be used to limit the scope of protection of the present invention.

[0046] At present, the intensity of ultrasonic cavitation effect and permeability enhancement effect are usually evaluated indirectly. For example, cavitation intensity is mainly evaluated by measuring acoustic parameters, observing visual evidence of cavitation phenomena, or detecting chemical or biological effects caused by cavitation, while ultrasonic permeability enhancement is mainly evaluated by indirect parameters such as changes in the pore structure of the medium after ultrasonic action, fluid penetration velocity or flow rate. It can be seen that the evaluation of the two often needs to be carried out step by step, resulting in low consistency and accuracy of the evaluation.

[0047] Ultrasonic waves can change the pore structure of porous media, increase pore connectivity, and significantly improve the permeability of the medium through mechanical vibration and cavitation. It has significant effects on improving the efficiency of groundwater pollution remediation in low permeability formations, CO2 storage efficiency, and coal, oil and natural gas extraction efficiency. The present invention conducts a series of nuclear magnetic resonance tests (NMR) on porous media during the action of ultrasonic waves, and intends to evaluate the change law of medium permeability based on the measured T2 spectrum. The present invention describes the evaluation method and steps of the evaluation system through theoretical analysis combined with experimental research methods, which can realize the evaluation of the permeability enhancement effect of different porous water-containing media under the action of ultrasonic waves. It is suitable for a variety of porous media such as sandstone, shale, coal and sand, and can provide theoretical guidance for practical engineering applications such as groundwater pollution remediation in low permeability aquifers, CO2 geological storage, and oil, coalbed methane and unconventional natural gas extraction.

[0048] The names of the present invention are explained as follows:

[0049] T2 is the time constant that describes the recovery process of the transverse component of nuclear magnetization intensity, so it is called transverse relaxation time. The transverse relaxation process is caused by the internal exchange of energy in the nuclear spin system, so it is also called spin-spin relaxation time. T2 (T2 spectrum) refers to the distribution of different T2 values ​​in a sample.

[0050] Specifically, the present invention provides a method for evaluating the permeability enhancement effect of ultrasonic waves on porous media. Figure 1 As shown, the following steps are included:

[0051] Step 1: Dry the sample of the porous medium to be tested at 105°C for 12 hours, then evacuate and saturate with water for 24 hours. After saturation, perform the first NMR test on the sample. The test sequence is CPMG, and the test parameters are: repeated sampling interval (T W )=3500ms, echo time (T E )=0.2ms, number of echoes (N E ) = 18,000, cumulative sampling times (Ns) = 8. The first saturated water T2 spectrum of the sample is obtained by testing, and then the initial pore size distribution, porosity and permeability coefficient of the sample are obtained by analyzing the first saturated water T2 spectrum.

[0052] Step 2: After selecting the ultrasonic power and frequency, the sample after the first NMR test is subjected to the first ultrasonic permeation enhancement by ultrasound. During this process, the sample is subjected to a second NMR test every 30 minutes to obtain the second saturated T2 spectrum of the sample.

[0053] Step 3: After each second NMR test, the sample is re-vacuumed and saturated with water for 12 hours, and the third NMR test after saturation is performed to obtain the third saturated T2 spectrum of the sample. The action is stopped until the T2 signal intensity change amplitude in the second NMR test is less than 1%. In the second NMR test, a series of second saturated T2 spectra during the ultrasonic action process are obtained, which can quantitatively describe the intensity of the ultrasonic cavitation effect.

[0054] Step 5: Analyze the third water-saturated T2 spectrum to obtain the permeability, porosity and pore size distribution of the sample after ultrasonic action at different time points, and calculate through the second water-saturated T2 spectrum and the third water-saturated T2 spectrum to obtain the intensity of the ultrasonic cavitation effect of the sample after ultrasonic action at different time points.

[0055] The permeability coefficient, porosity and pore size distribution of the medium after ultrasonic action at different time points are calculated by the third saturated water T2 spectrum, specifically:

[0056] The calculation formula of permeability coefficient k is:

[0057]

[0058] Where: n represents the porosity of the medium, ρ w represents the density of water, g represents the acceleration of gravity, m represents the aperture classification level, N p It is expressed as the number of cylindrical capillaries that make up the water flow path, μ represents the viscosity coefficient, ρ2 represents the transverse surface relaxation strength, T 2i is the T2 value corresponding to position i.

[0059] The calculation formula of porosity n is:

[0060]

[0061] V w is the volume of the aqueous phase, cm 3 ; V c is the volume of the sample, cm 3 ; I is the NMR signal value measured from the saturated sample, dimensionless; f(I) is the product of I and V w The linear relationship between them can be measured using standard samples with known water content.

[0062] The peak position and area of ​​the third saturated water T2 spectrum are used to analyze the pore size distribution of the sample after ultrasonic treatment at different time points. T2 spectrum data can be obtained by nuclear magnetic resonance (NMR) technology. The T2 spectrum is usually displayed as a distribution in which different peaks correspond to pores of different pore sizes. The pore size distribution can be inferred by analyzing the peak position and area of ​​the T2 spectrum. Generally, the smaller the pore size, the faster the relaxation process and the smaller the T2.

[0063] The intensity of ultrasonic cavitation effect is specifically:

[0064] The intensity C of the ultrasonic cavitation effect is obtained based on the total signal intensity S2 of the second saturated water T2 spectrum and the total signal intensity S3 of the third saturated water T2 spectrum:

[0065]

[0066] In nuclear magnetic resonance (NMR) measurements, the total signal intensity of the T2 spectrum refers to the sum of all signal intensities over the entire T2 spectrum distribution.

[0067] Step 6: Compare and analyze the initial pore size distribution, porosity and permeability coefficient of the sample obtained by analyzing the first saturated T2 spectrum with the permeability coefficient, porosity and pore size distribution of the sample after ultrasonic action at different time points, and evaluate the permeability enhancement effect of ultrasound based on the comparison results and the intensity of ultrasonic cavitation effect.

[0068] Change the ultrasonic power and frequency, repeat the above steps 1-6, and evaluate the intensity of the cavitation effect and the final permeability enhancement effect in the medium by comparing and analyzing the T2 spectra measured under different ultrasonic action parameters, and then evaluate the permeability of the medium after ultrasonic action on the porous medium.

[0069]

[0070] Where A is the increase rate of the permeability coefficient of the porous medium after a certain ultrasonic action, k i is the permeability coefficient of the porous medium calculated according to the third NMR test after a certain ultrasonic action, and k0 is the permeability coefficient of the porous medium obtained in the first NMR test.

[0071] Therefore, the present invention provides a method for simultaneously obtaining the ultrasonic cavitation intensity and the permeability enhancement effect, and can directly and quantitatively characterize the cavitation intensity and the permeability enhancement effect, and then evaluate the permeability of the medium after the ultrasonic wave acts on the porous medium through the intensity of the ultrasonic cavitation effect and the permeability enhancement effect.

[0072] The present invention is applicable to various porous media, and directly evaluates the permeability enhancement effect by quantitatively calculating the permeability coefficient rather than evaluating it by other indirect means. In addition, the present invention can also quantitatively evaluate the intensity of the cavitation effect during the permeability enhancement process.

[0073] Example 1

[0074] The following is a low permeability porous medium as an example to verify the evaluation method proposed in the present invention. Specifically, the main component of the low permeability porous medium in this embodiment is quartz sand. The specific operation is as follows:

[0075] The sample of the porous medium to be tested was dried at 105°C for 12 hours, and then vacuumed and saturated with water for 24 hours. After saturation, the sample was tested by NMR to obtain the saturated T2 spectrum of the sample, such as Figure 2 This is a schematic diagram of the initial T2 spectrum distribution of the medium. According to sample analysis, the original porosity of this medium is 0.434 and the permeability coefficient is 1.16×10 -5 m / s, and the median pore radius is 1.43 μm.

[0076] In the NMR-based ultrasonic permeability evaluation, ultrasonic power and ultrasonic time are two important factors that affect the evaluation results. The following is a specific analysis of the effects of ultrasonic power and ultrasonic time on the permeability of the medium through experiments. By analyzing the influence of ultrasonic power, frequency and action time on the permeability of different media, a theoretical basis is provided for the selection of ultrasonic parameters in practical engineering applications.

[0077] 1. Analysis of the influence of ultrasonic power on the permeability enhancement effect of medium based on NMR research

[0078] 1) T2 spectrum analysis

[0079] First, ultrasonic waves of the same frequency and different powers were applied to the saturated medium to evaluate the effect of ultrasonic power on the intensity of cavitation effect and the permeability enhancement effect of the medium. The ultrasonic frequency was selected as 40KhZ, and the powers were 100W, 200W, 300W, 400W, and 500W, respectively. The saturated medium was subjected to ultrasonic waves of the above five different powers for 150 minutes, and the second NMR test was performed on the sample every 30 minutes during this process; after each second NMR test, the sample was re-vacuumed and saturated with water for 12 hours, and the third NMR test after saturation was performed; the action was stopped until the change in the T2 signal intensity in the second NMR test was less than 1%.

[0080] Since the pore size determines the relaxation rate of the NMR signal, specifically, a larger pore size corresponds to a longer T2 time, while a smaller pore size corresponds to a shorter T2 time, the T2 spectrum distribution reflects the pore size distribution of the medium. Figure 3 The second NMR test shows the T2 spectra obtained during the NMR test at different ultrasound powers. Figure 3 The second NMR test shows that with the increase of ultrasonic power, the T2 peak shifts to the left and the peak area decreases, from 77.53ms to 60.80ms. This is because the ultrasonic action causes a large number of cavitation bubbles to form, but NMR cannot detect the signal before the cavitation bubble collapses, so Figure 3The T2 distribution in the second NMR test cannot accurately reflect the actual pore structure of the medium after ultrasonic treatment. Therefore, it is necessary to vacuum saturate the medium after ultrasonic treatment again to discharge the gas in the uncollapsed cavitation bubbles, and perform NMR test after saturation to obtain the T2 distribution curve of the medium re-saturated with water after ultrasonic treatment. Figure 3 The third NMR test shows the T2 distribution curve obtained after re-saturation after each ultrasonic stage. As the ultrasonic time increases, the T2 peak shifts to the right and the peak area gradually increases, from the initial 77.53ms to 107.19ms. This shows that the internal pore size of the medium increases after ultrasonic action, and the pore size increases more significantly with the increase of power, and the pore connectivity is better.

[0081] 2) Pore size distribution characteristics

[0082] According to the NMR relaxation mechanism, the transverse relaxation time T2 can be expressed as:

[0083]

[0084] Where: ρ2 is the surface relaxation intensity of the transverse relaxation time T2 of the medium particle surface; S / V is the pore specific surface area; T 2B is the transverse relaxation time of the bulk fluid; D is the diffusion coefficient of the fluid; γ is the gyromagnetic ratio; G is the magnetic field gradient; T E is the echo time.

[0085] When the rock is saturated with a single fluid, the volume relaxation of the fluid is much slower than the surface relaxation, so the contribution of volume relaxation to T2 can be ignored. E When both are small, the contribution of diffusion relaxation of pore fluid to T2 can also be ignored. At this time, T2 is mainly contributed by the surface relaxation between solid and fluid, and equation (1) can be simplified to:

[0086]

[0087] In the formula, Fs is related to the shape of the pore. If the pore shape is flat, cylindrical or spherical, the corresponding value is 1, 2, 3. Since the cylindrical pore shape is between the flat and spherical shapes, F s Take 2 more.

[0088] Formula (2) can be further simplified as:

[0089]

[0090] ρ2 is the surface relaxation intensity of the transverse relaxation time T2 of the medium particle surface, and γ is the gyromagnetic ratio. From formula (3), it can be seen that the transverse relaxation time T2 is proportional to the pore radius r, that is, the larger the pore, the larger T2; conversely, the smaller the pore, the smaller T2. Therefore, according to the T2 time distribution after the ultrasonic action, the distribution ratio of each pore size interval of the medium can be made, such as Figure 4 As shown in the figure. With the increase of ultrasonic power, the number of pores with a pore size less than 1.6μm gradually decreases; the number of pores with a pore size between 1.6 and 2.5μm remains almost unchanged; the number of pores with a pore size greater than 2.5μm gradually increases. This is because as the ultrasonic power increases, the cavitation effect also increases. Since the instantaneous pressure generated by the rupture of ultrasonic cavitation bubbles can be close to 200MPa, under the continuous action of ultrasonic cavitation effect, the high-pressure impact expands the pore size of the medium, increases the pore connectivity and improves the permeability of the medium.

[0091] In order to facilitate the comparison of the pore size changes at each ultrasonic time, the present invention divides the pore size of the medium into the following categories: r < 1 μm is micropore; 1 μm ≤ r ≤ 10 μm is mesopore; 10 μm ≤ r is macropore. Figure 5 It can be seen that with the increase of ultrasonic power, the proportion of each pore size range gradually changes. The proportion of small pores decreases from 24.17% to 9.23%; the proportion of medium pores increases from 71.8% to 86.28%; the proportion of large pores increases from 4.03% to 4.49%. This shows that the effect of ultrasonic wave on the expansion of small pores inside the medium is the most obvious, and the large pore structure is stable and the change range is not large.

[0092] 3) Permeability coefficient analysis

[0093] When water flows slowly in interconnected pore channels, it satisfies Darcy's law:

[0094]

[0095] Where: v is the average flow velocity; Q is the volume flow rate; r is the channel radius.

[0096]

[0097] Where: h w is the head loss. According to fluid mechanics, h w Equal to the loss along the way h f With local loss h j The sum of. Since h f The value is much larger than h j The value of h w ≈h f Combining equation (4) and equation (5), we can know that:

[0098]

[0099] According to the Poiseuille equation:

[0100]

[0101] Where: Δp is the pressure difference at both ends of the pore; μ is the viscosity coefficient of water. f for:

[0102]

[0103] Where: g is the acceleration due to gravity. From equations (6) to (8), the pore permeability coefficient of the cylindrical capillary fluid can be obtained as:

[0104]

[0105] Where: d is the channel diameter. From formula (9), we can know that the pore permeability coefficient k is calculated as p The key is to determine the representative channel diameter d. The classic capillary model and the Marshall model calculate the pore permeability coefficient of the medium from the distribution probability of the water flow inlet diameter:

[0106]

[0107] Where: m is the aperture classification level; d i is the diameter of the i-th channel; f(d i ) is the probability of the i-th level channel appearing. According to the basic idea of ​​the Marshall model, the smallest capillary channel controls the flow rate of water in the series capillary channels. Therefore, considering that the water flow path is composed of multiple capillaries with different pore sizes, Pick The smaller value. The medium pore permeability coefficient can be obtained as:

[0108]

[0109] Where: N P is the number of cylindrical capillaries that make up the water flow path; Calculate the permeability coefficient of the variable head test medium:

[0110]

[0111] Where: k is the permeability coefficient; a is the cross-sectional area of ​​the variable head tube; L is the permeability length of the sample; A is the cross-sectional area of ​​the sample; t is the time; h1 is the head at the beginning; h2 is the head at the end. A in formula (12) includes the cross-sectional area of ​​the pore channel and the cross-sectional area of ​​the sample skeleton. The permeability coefficient of the medium depends on the cross-sectional area of ​​the pore channel. The permeability coefficient k of the medium is p for:

[0112]

[0113] Where: q is the unit water seepage; Ap is the cross-sectional area of ​​the pore channel; j is the hydraulic gradient; and n is the porosity. Substituting equations (3) and (13) into equation (11), the permeability coefficient of the medium can be calculated using the pore size and distribution measured by nuclear magnetic resonance:

[0114]

[0115] Where: Pick The minimum value in ; The channel diameter The probability of occurrence. The fine particle content of the medium increases with the increase of the plasticity index. The finer the particles, the more complex the pore structure. P Based on the previous experience of studying the influence of plasticity index on permeability coefficient in capillary model, this experiment selected N P is 4.

[0116] It can be seen from formula (14) that the permeability coefficient of the medium can be calculated according to the T2 time distribution. Based on the NMR test data, the permeability coefficient of the medium after being treated with different ultrasonic powers was calculated, and the variation trend of the permeability coefficient of the medium with power was plotted, as shown in Figure 6 As the ultrasonic power increases, the permeability coefficient of the medium gradually increases, and there is a linear relationship between the two.

[0117] 4) Evaluation of cavitation effect intensity and permeability enhancement effect

[0118] The intensity of the ultrasonic cavitation effect can be quantitatively calculated based on the series of T2 spectra obtained in the second NMR test and the third NMR test; based on the series of T2 spectra obtained in the third NMR test and formula (14), the permeability enhancement effect of ultrasonic waves of different powers on the porous medium can be analyzed.

[0119] Due to the ultrasonic effect, a large number of cavitation bubbles are formed. Figure 3 The NMR in () cannot detect the signal before the collapse of the cavitation bubble. The T2 signal decreases with the increase of ultrasonic power. The decrease in the signal corresponds to the intensity of the ultrasonic cavitation effect. As shown in Table 1, the intensity of the cavitation effect shows an obvious upward trend with the increase of power.

[0120] Table 1 Cavitation effect intensity under different ultrasonic powers

[0121]

[0122] In addition, according to the NMR test data and formula (14), the porosity and permeability of the medium under the action of ultrasound at different powers can be calculated. The specific data are listed in Table 2. It can be seen that after 15 minutes of 40KHz, 500W ultrasound, the porosity of the medium increased from 43.43% to 48.10%. The permeability increased from 1.16×10 -5 m / s increased to 3.35×10 -5 m / s, an increase of 188.79%. This experimental result shows that ultrasonic treatment has a significant effect on improving the permeability of the medium, and this improvement becomes more obvious with the increase of ultrasonic power.

[0123] Table 2 Porosity and permeability of media treated with different ultrasonic powers

[0124]

[0125] 2. Analysis of the effect of ultrasonic time on permeability enhancement based on NMR

[0126] 1) T2 spectrum analysis

[0127] Figure 7 The T2 distribution shown in the second NMR test was obtained after the medium was sonicated for different times. Figure 7 From the second NMR test, it can be seen that with the increase of ultrasonic action time, the T2 peak shifts to the left and the peak area decreases, indicating that the intensity of the cavitation effect increases with the increase of ultrasonic action time. Figure 7 The third NMR test shows the T2 distribution curve obtained after re-saturation after each ultrasonic stage. As the ultrasonic time increases, the T2 peak shifts to the right and the peak area gradually increases, indicating that as the ultrasonic action time increases, the pore size of the medium becomes larger and the pore connectivity becomes better.

[0128] 2) Pore size distribution characteristics

[0129] like Figure 8 ,With the increase of ultrasonic time, the proportion of pores with radius between 0 and 1.6 μm gradually decreases, and the proportion of pores with radius larger than 1.6 μm gradually increases. Fig. 9 As shown in the figure, with the increase of ultrasonic action time, the proportion of small pores decreased from 23.08% to 9.23%, the proportion of medium pores increased from 73.15% to 86.28%, and the proportion of large pores increased from 3.77% to 4.79%. It can be seen that consistent with the law of ultrasonic power, the effect of ultrasonic action time on the expansion of small and medium pores is the most obvious.

[0130] 3) Permeability coefficient analysis

[0131] The variation trend of the permeability coefficient of the medium with the duration of ultrasound action is as follows: Fig.10As shown in the figure, with the increase of ultrasound duration, the permeability coefficient of the medium gradually increases, and there is an obvious linear relationship between the two.

[0132] 4) Evaluation of cavitation effect intensity and permeability enhancement effect

[0133] The same principle as above, Table 3 and Table 4 respectively list the cavitation effect intensity and permeability enhancement effect inside the medium under different ultrasonic action time. The results show that increasing the ultrasonic treatment time has a significant promoting effect on increasing the cavitation effect and improving the permeability of the medium.

[0134] Table 3 Cavitation effect intensity under different ultrasonic action time

[0135]

[0136] Table 4 Porosity and permeability of the media at different ultrasonic treatment times

[0137]

[0138] 3. Analysis of the influence of ultrasonic frequency on permeability enhancement based on NMR research

[0139] 1) T2 spectrum analysis

[0140] Fig.11 The T2 distribution shown in Figure 1 was obtained when the medium was treated with two different ultrasonic frequencies at 300 W. Fig.11 It can be seen that compared with the original peak position, the peaks after medium ultrasound are shifted to the left. Fig.11 The third NMR test shows the T2 distribution curve obtained after re-saturation after each ultrasonic stage. As the ultrasonic frequency increases, the T2 peak shifts to the right from 71.49ms to 84.67ms. This shows that as the ultrasonic frequency increases, the pore size inside the medium becomes larger and the pore connectivity becomes better.

[0141] 2) Pore size distribution characteristics

[0142] like Fig.12 and Fig.13 As shown in the figure, with the increase of ultrasonic frequency, the pore size distribution also changed, the proportion of small pores decreased from 24.17% to 19.35%, the proportion of medium pores increased from 71.80% to 76.32%, and the proportion of large pores increased from 4.03% to 4.34%. It can be seen that the expansion effect of the same ultrasonic frequency on small and medium pores is much smaller than the ultrasonic power and the ultrasonic action time.

[0143] 3) Permeability coefficient analysis

[0144] The variation trend of the permeability coefficient of the medium with the ultrasonic frequency is as follows: Fig.14As shown in Figure 2, with the increase of ultrasonic frequency, the permeability coefficient of the medium gradually increases, and there is an obvious linear relationship between the two.

[0145] 4) Evaluation of cavitation effect intensity and permeability enhancement effect

[0146] The same calculation principle as above, Table 5 and Table 6 respectively list the cavitation effect intensity and permeability enhancement effect inside the medium under different ultrasonic frequencies. The results show that increasing the ultrasonic frequency can increase the cavitation effect and improve the permeability of the medium, but the enhancement effect is smaller than the action time of the ultrasonic power meter.

[0147] Table 3 Cavitation effect intensity under different ultrasonic frequencies

[0148]

[0149] Table 4 Porosity and permeability of media under different frequencies of ultrasound

[0150]

[0151] The present invention conducts continuous nuclear magnetic resonance testing on porous media during the action of ultrasonic waves, and after the continuous nuclear magnetic resonance testing, the cavitation bubbles in the porous media are filled with saturated water, and then the nuclear magnetic resonance testing is conducted again, and the T2 spectrum obtained by the continuous two-aspect nuclear magnetic resonance and the pore characteristics of the porous media are combined to obtain the cavitation effect intensity of the ultrasonic wave in the porous media and the change of the pore permeability characteristics of the medium. The overall theoretical analysis and specific experimental research are used to systematically describe the ultrasonic cavitation effect intensity and the permeability enhancement effect on the porous media, solving the problem that the existing evaluation method conducts the cavitation intensity and ultrasonic permeability enhancement evaluation step by step, resulting in low consistency and accuracy of the evaluation. It can provide a better cavitation effect intensity and medium permeability change evaluation method for the application of ultrasonic permeability enhancement technology in actual engineering, so as to achieve the best target permeability enhancement effect, while reducing the ultrasonic energy consumption, and will not cause unnecessary damage to the formation due to excessive permeability enhancement. The evaluation of the permeability enhancement effect of different porous water-containing media under the action of ultrasound can provide a theoretical basis for engineering applications such as groundwater pollution remediation of low permeability aquifers, geological storage, oil extraction, and coalbed methane extraction.

[0152] It should be pointed out that the specific implementation methods described above can enable those skilled in the art to understand the invention more comprehensively, but do not limit the invention in any way. Therefore, although the invention has been described in detail in this specification and embodiments, those skilled in the art should understand that the invention can still be modified or replaced by equivalents; and all technical solutions and improvements that do not deviate from the spirit and scope of the invention are included in the protection scope of the patent for the invention. Any figure mark in the claims should not be regarded as limiting the claims involved. Any simple change or equivalent replacement of the technical solution that can be obviously obtained by any technician familiar with the field within the technical scope disclosed in the present invention belongs to the protection scope of the present invention.

Claims

1. A method for evaluating the permeability enhancement effect of ultrasonic waves on porous media, characterized in that: include: The dried sample of the porous medium to be tested is subjected to a vacuum saturation treatment, and the vacuum saturated sample is subjected to a first NMR test to obtain a first saturated T2 spectrum of the sample, and the initial pore size distribution, porosity and permeability coefficient of the sample are obtained by analyzing the first saturated T2 spectrum; The sample after the first NMR test is subjected to a first ultrasonic permeation effect by ultrasound, and a second NMR test is performed on the sample at set intervals during the first ultrasonic permeation effect to obtain a second water-saturated T2 spectrum of the sample; after each second NMR test, the sample is re-vacuumed and saturated with water, and a third NMR test is performed on the re-vacuumed and saturated sample to obtain a third water-saturated T2 spectrum of the sample; The pore size distribution, porosity and permeability coefficient of the sample after ultrasonic action at different time points were obtained by analyzing the third saturated water T2 spectrum, and the intensity of ultrasonic cavitation effect of the sample after ultrasonic action at different time points was obtained by calculating the second saturated water T2 spectrum and the third saturated water T2 spectrum; The initial pore size distribution, porosity and permeability of the sample were compared with the pore size distribution, porosity and permeability of the sample after ultrasonic treatment at different time points. The permeability enhancement effect of ultrasound was evaluated based on the comparison results and the intensity of ultrasonic cavitation effect.

2. The method for evaluating the permeability enhancement effect of ultrasonic waves on porous media according to claim 1, characterized in that: The permeability coefficient, porosity and pore size distribution of the medium are calculated by T2 spectrum, specifically: The calculation formula of permeability coefficient k is: Where: n represents the porosity of the medium, ρ w represents the density of water, g represents the acceleration of gravity, m represents the aperture classification level, N p It is expressed as the number of cylindrical capillaries that make up the water flow path, μ represents the viscosity coefficient, ρ2 represents the transverse surface relaxation strength, T 2i is the T2 value corresponding to position i; The calculation formula of porosity n is: V w is the volume of the aqueous phase, cm 3 ; V c is the volume of the sample, cm 3 ; I is the NMR signal value measured from the saturated sample, dimensionless; f(I) is the product of I and V w The linear relationship between The pore size distribution of the medium can be analyzed through the peak position and area of ​​the T2 spectrum.

3. The method for evaluating the permeability enhancement effect of ultrasonic waves on porous media according to claim 1, characterized in that: The intensity of the ultrasonic cavitation effect is specifically: The intensity C of the ultrasonic cavitation effect is obtained based on the total signal intensity S2 of the second saturated water T2 spectrum and the total signal intensity S3 of the third saturated water T2 spectrum: The total signal intensity of the T2 spectrum refers to the sum of all signal intensities in the entire T2 spectrum distribution.

4. The method for evaluating the permeability enhancement effect of ultrasonic waves on porous media according to claim 1, characterized in that: When the T2 signal intensity changes by less than 1% in the second NMR test, the ultrasonic permeation of the sample is stopped.

5. The method for evaluating the permeability enhancement effect of ultrasonic wave acting on porous media according to claim 2, characterized in that: The method also includes changing the ultrasonic power and frequency, repeating the second NMR test and the third NMR test, and evaluating the permeability enhancement effect of different ultrasonic actions on the medium by comparing and analyzing the permeability coefficients of the medium measured under different ultrasonic action parameters and comparing them with the data obtained from the first NMR test; Where A is the increase rate of the permeability coefficient of the porous medium after a certain ultrasonic action, k i is the permeability coefficient of the porous medium calculated according to the third NMR test after a certain ultrasonic action, and k0 is the permeability coefficient of the porous medium obtained in the first NMR test.

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