Nuclear magnetic signal separation method for water in miscible solution in porous medium

Abstract

The invention discloses a nuclear magnetic signal separation method for water in a miscible solution in a porous medium, and relates to the technical field of fluid flow research, and the method comprises the following steps: S1, selecting a porous medium core sample to be detected, and obtaining the gas logging porosity and permeability of the core; s2, injecting ethanol into the saturated water rock core to simulate a displacement process, and obtaining relaxation time distribution of the rock core; s3, injecting ethanol into the dried rock core, and obtaining relaxation time distribution of the rock core; s4, heavy water is injected into the saturated water rock core, and relaxation time distribution of the rock core is obtained; s5, obtaining relaxation time distribution of water in a mixed solution of ethanol and water; s6, acquiring an ethanol injection PV critical value of a differential spectrum method; s7, obtaining an ethanol injection PV critical value of a time domain analysis method; and S8, selecting a differential spectrum method or a time-domain analysis method to separate a nuclear magnetic signal of water in a mixed solution of ethanol and water in the porous medium. The problem that the content and distribution change of water in the porous medium in the miscible-phase displacement process are difficult to accurately measure by an existing conventional displacement experiment method is solved.

Description

Technical Field

[0001] The invention relates to the technical field of petroleum engineering and fluid flow research, and in particular to a method for separating nuclear magnetic signals of water in a miscible solution in a porous medium. Background Art

[0002] During the development of gas reservoirs, the reservoirs are prone to capillary self-imbibition due to the small throat radius, primary water and micro-cracks in the reservoirs. At the same time, during the development of gas reservoirs, various water-based working fluids such as drilling fluids, completion fluids, well washing fluids, workover fluids and fracturing fluids will contact and invade the formations, forming liquid phase retention, resulting in a large number of pore throats being occupied by liquids, which reduces the effective diameter of the gas flow channel in the reservoir, increases the flow resistance sharply, and shows a significant water lock effect, which greatly limits the economic and efficient development of gas reservoirs.

[0003] In response to this problem, some experts and scholars have proposed that a desiccant can be injected to react quickly with the formation water in the reservoir near the wellbore, so as to consume the formation water and reduce the resistance of the channel. Ethanol is used as a carrier for the desiccant, and when the desiccant reacts chemically with water, ethanol and water are miscible with each other. For miscible fluids, conventional displacement experimental methods are difficult to accurately measure the content and distribution of water in porous media, especially when chemical reactions occur during the miscibility process. Nuclear magnetic resonance technology has been widely cited in reservoir engineering and reservoir geology due to its advantages of simplicity, speed and accuracy. Based on nuclear magnetic resonance technology and principles, the present invention proposes a method for separating the nuclear magnetic signal of water in a mixed solution of ethanol and water in a porous medium using nuclear magnetic resonance technology.

[0004] Since it is difficult to accurately measure the changes in water content and distribution in porous media when ethanol displaces water-containing cores using conventional displacement experimental methods, it is urgent to develop a convenient and accurate displacement experimental method to simulate the changes in water content and distribution in porous media when ethanol displaces water-containing cores, so as to provide theoretical support for related research. Summary of the invention

[0005] The purpose of the present invention is to provide a method for separating nuclear magnetic resonance signals of water in a miscible solution in a porous medium. By using nuclear magnetic resonance technology, the content and distribution of water in a porous medium during a miscible phase displacement process can be quantitatively analyzed.

[0006] The present invention is achieved through the following technical solutions: A method for separating nuclear magnetic signals of water in a miscible solution in a porous medium comprises the following steps: S1. Select a porous medium core sample to be tested, dry it to measure the dry weight of the core, and measure the porosity and permeability of the dry core by gas measurement; S2, evacuate the dry core, then pressurize and inject saturated pure water, connect the nuclear magnetic resonance online displacement device, put the saturated core into the nuclear magnetic resonance coil, wrap the core with a heat shrink tube, add confining pressure, and then inject ethanol into the saturated core at a constant rate to simulate the displacement process, and obtain the T2 relaxation time distribution of the core at different times, recorded as y1; S3, putting the core after the experiment in step S2 into an oven for drying, measuring the weight of the core after drying, wrapping the dry core with a heat shrink tube, putting it into a nuclear magnetic resonance coil, wrapping the core with a heat shrink tube, adding confining pressure, and then injecting ethanol into the dry core at a constant rate to simulate the displacement process, and obtaining the T2 relaxation time distribution of the core at different times, recorded as y2; S4, putting the core after the experiment in step S3 into an oven for drying, measuring the weight of the core after drying, evacuating the dry core, pressurizing it, injecting saturated pure water, wrapping the saturated core with a heat shrink tube, putting it into a nuclear magnetic resonance coil, adding confining pressure, and then injecting heavy water into the saturated core at a constant rate to simulate the displacement process, and obtaining the T2 relaxation time distribution of the core, recorded as y3; S5, using the difference spectrum method, the T2 relaxation time distribution y1 of the mixture of ethanol and water in the core at the same displacement time point is subtracted from the T2 relaxation time distribution y2 of ethanol in the core at the same displacement time point to obtain the relaxation time distribution of water in the mixed solution of ethanol and water in the porous medium, which is recorded as y4; S6. Compare the relaxation time y4 of the water in the core at the same time point with the relaxation time y3 of heavy water and water. It can be seen that when the ethanol injection amount is less than a certain critical value, the differential spectrum method is the most accurate in obtaining the nuclear magnetic signal y4 of the water separated from the mixed solution of ethanol and water in the porous medium. The critical value of the ethanol injection amount is the ethanol injection PV critical value of the differential spectrum method. S7, when the nuclear magnetic signal of water separated from the mixed solution of ethanol and water in the porous medium is obtained accurately according to the time domain analysis method, the critical value of the ethanol injection PV; S8. When separating the NMR signal of water from the mixed solution of ethanol and water in the porous medium, the range of the ethanol injection PV critical value should be as large as possible. Therefore, it is necessary to select the difference spectrum method or time domain analysis method to separate the NMR signal of water from the mixed solution of ethanol and water in the porous medium according to the core porosity.

[0007] Furthermore, the nuclear magnetic resonance test parameters of the nuclear magnetic resonance detector are: the minimum echo interval TE=0.1 ms, the waiting time TW is set to 8000 ms, and the number of echoes NECH takes a maximum of 6000.

[0008] Furthermore, in step S1, the dry core is placed in a vacuum and pressurized saturation device and vacuumed for 6 hours, and then pressurized with 20 MPa saturated pure water for 6 hours. The saturated core is weighed to obtain the saturated porosity of the core.

[0009] Furthermore, in step S2, the method for obtaining the mixed T2 relaxation time distribution of ethanol and water in the core is: connecting the nuclear magnetic resonance online displacement device, and placing the core in a 70mm nuclear magnetic resonance coil, wrapping the core with a heat shrink tube, adding fluorine oil to the confining pressure to 4MPa, and injecting ethanol into the saturated core at a constant rate of 0.02mL / min displacement speed. The displacement test is 3h, and the test is carried out using a nuclear magnetic resonance instrument. The T2 relaxation time distribution of the rock type is tested once at a certain interval, recorded as y1.

[0010] Furthermore, in step S3, the method for obtaining the T2 relaxation time distribution of ethanol in the core at the same displacement time point as y1 is as follows: the core that has completed the step S2 experiment is placed in an oven, and dried at 100°C for 12 hours, and the weight of the core is measured after drying. The dry core is wrapped with a heat shrink tube and placed in a 70mm nuclear magnetic resonance coil. After the core is wrapped with a heat shrink tube, fluorine oil is added with a confining pressure of 4MPa, and ethanol is injected into the dry core at a constant rate of 0.02mL / min for a displacement time of 3h. The core is tested using a nuclear magnetic resonance instrument, and the T2 relaxation time distribution of the lithology is tested once at regular intervals, recorded as y2.

[0011] Further, in step S4, the method for obtaining the relaxation time distribution of heavy water and water in the core at the same displacement time point as y1 is: at a temperature of 100°C, continue drying for 12 hours, and test the weight of the core after drying, place the dry core in a vacuum pressurized saturation device for 6 hours, and then pressurize it with 20MPa saturated pure water for 6 hours, wrap the saturated core with a heat shrink tube and place it in a 70mm nuclear magnetic resonance coil, add fluorine oil to increase the confining pressure to 4MPa, inject heavy water into the saturated core at a constant rate of 0.02mL / min, displace for 3 hours, and test the T2 relaxation time distribution of the lithology once at regular intervals, recorded as y3.

[0012] Furthermore, in step S6, the smaller the mass proportion of ethanol in the mixed solution of ethanol and water in the porous medium, the closer the nuclear magnetic signal of water separated by differential spectroscopy in the mixed solution of ethanol and water is to the nuclear magnetic signal of water in the mixed solution of heavy water and water containing the same mass of water, that is, there is a critical mass ratio of the mass of water to the mass of ethanol in the mixed solution of ethanol and water in the porous medium. When the ratio of the mass of water to the mass of ethanol in the porous medium is greater than this value, the T2 relaxation time of water separated by differential spectroscopy in the mixed solution of ethanol and water in the porous medium is the most accurate. If it is less than this critical value, the T2 relaxation time of water separated by differential spectroscopy in the mixed solution of ethanol and water in the porous medium has a large error. That is to say, when ethanol displaces the water-containing core, there is a critical PV of ethanol injection. When the ethanol injection amount is less than this critical value, the T2 relaxation time of water separated by the differential spectrum method in the mixed solution of ethanol and water in the porous medium is the most accurate. When the ethanol injection amount is greater than this critical value, the T2 relaxation time of water separated by the differential spectrum method in the mixed solution of ethanol and water in the porous medium has a large error.

[0013] Further, in step S8, when the nuclear magnetic resonance method is used to separate the nuclear magnetic signal of water in the mixed solution of ethanol and water in the porous medium, there is a critical mass ratio of the mass of water to the mass of ethanol. When the mass of water to the mass of ethanol in the porous medium is greater than the critical mass ratio, the nuclear magnetic signal of the separated water is the most accurate. When it is less than the critical mass ratio, the nuclear magnetic signal of the separated water has a large error. That is to say, when ethanol displaces the water-bearing core, there is a critical value of ethanol injection PV. When the injection amount of ethanol is less than this critical value, the T2 relaxation time of the separated water is the most accurate. When the injection amount of ethanol is greater than this critical value, the T2 relaxation time of the separated water has a large error. When ethanol displaces the water-bearing core, considering the need to inject as much ethanol as possible, that is, the critical value of ethanol injection PV is as large as possible. When the core porosity is greater than 9.3%, the critical PV value of ethanol injection by time domain analysis is larger; when the core porosity is less than 9.3%, the critical PV value of ethanol injection by differential spectrum method is larger. Therefore, it is necessary to reasonably choose differential spectrum method or time domain analysis method to separate the nuclear magnetic resonance signal of water in porous medium mixed solution according to the core porosity.

[0014] Furthermore, when obtaining the T2 relaxation time distribution of the core, the relaxation time T2 is tested every 20 minutes.

[0015] Furthermore, the difference spectrum method is to set different waiting times according to the relaxation properties of the fluid during the nuclear magnetic resonance test. For example, as a non-wetting phase, the longitudinal relaxation time of oil and gas in the rock (generally a few seconds) is much longer than the longitudinal relaxation time of water in the rock (generally hundreds of milliseconds). Therefore, the T2 relaxation time measured with a long waiting time can include the nuclear magnetic signals of oil, gas and water, and the T2 relaxation time obtained by testing the same sample with a short waiting time may only include the nuclear magnetic signal of water. By subtracting the T2 relaxation times of the two, the nuclear magnetic signals of oil and gas can be retained, that is, the attenuation curves of the two nuclear magnetic signals with different waiting times are inverted to obtain the corresponding T2 spectra, and then the T2 spectrum with a long waiting time is subtracted from the T2 spectrum with a short waiting time, and the difference spectrum obtained is the T2 spectrum distribution of oil and gas; the time domain analysis method refers to the subtraction of echo train signals, and the obtained echo train is then inverted into the T2 relaxation time distribution. For example, the echo train signal with a long waiting time containing nuclear magnetic signals of oil, gas and water can be subtracted from the echo train signal with a short waiting time containing only nuclear magnetic signals of water to obtain the echo train difference of oil and gas, and then the echo train difference can be inverted using multi-exponential inversion to obtain the T2 signal distribution of oil and gas.

[0016] Compared with the prior art, the present invention has the following advantages and beneficial effects: 1. In the present invention, based on the principle of nuclear magnetic resonance, a method for separating the nuclear magnetic signals of water in two mutually miscible fluids, ethanol and water, in a porous medium is established. This method can quantitatively analyze the distribution and content of water in the porous medium during the miscible displacement process. This method has the advantages of being convenient and fast, and does not require changing the fluid properties and the pore structure of the porous medium.

[0017] 2. In the present invention, in actual research, a model with the same (similar) physical properties can be prepared according to the characteristics of the porous medium core in the gas reservoir to be tested, and then the model can be used to obtain the ethanol injection PV critical value when ethanol displaces the water-containing core according to the method recorded in the present invention. Generally speaking, when the core porosity is greater than 9.3%, the time domain analysis method is selected to separate the nuclear magnetic signal of water in the core. When the core porosity is less than 9.3%, the difference spectrum method is selected to separate the nuclear magnetic signal of water in the core. The ethanol injection PV critical value obtained in this way not only ensures that the water nuclear magnetic signal separated from the core is the most accurate, but also ensures that the injection amount of ethanol is larger, which can guarantee the separation requirements of water nuclear magnetic signal in ethanol displacement experiments in a larger range. It solves the problem that it is difficult to accurately measure the content and distribution changes of water in porous media during miscible displacement in existing conventional displacement experimental methods. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 It is a flowchart of the present invention.

[0019] Figure 2It is a diagram comparing the T2 spectrum of water in a mixed solution of ethanol and water in a porous medium separated by time difference spectroscopy when the ethanol displacement time is 0 min in Example 1 and the T2 relaxation time of heavy water and water at the same mass ratio (ie, the same displacement time).

[0020] Figure 3 It is a diagram comparing the T2 spectrum of water in a mixed solution of ethanol and water in a porous medium separated by time difference spectroscopy when the ethanol displacement time is 60 min in Example 1 and the T2 relaxation time of heavy water and water at the same mass ratio (ie, the same displacement time).

[0021] Figure 4 It is a diagram comparing the T2 spectrum of water in a mixed solution of ethanol and water in a porous medium separated by time difference spectroscopy when the ethanol displacement time is 160 min in Example 1 and the T2 relaxation time of heavy water and water at the same mass ratio (ie, the same displacement time).

[0022] Figure 5 It is a curve diagram of the relationship between the core ethanol injection time and the sum of squares of the dimensionless nuclear magnetic signal differences in Example 1.

[0023] Figure 6 It is a diagram comparing the T2 spectrum of water in a mixed solution of ethanol and water in a porous medium separated by a time domain analysis method when the ethanol displacement time is 0 min in Example 1, and the T2 relaxation time of heavy water and water at the same mass ratio (ie, the same displacement time).

[0024] Figure 7 It is a diagram comparing the T2 spectrum of water in a mixed solution of ethanol and water in a porous medium separated by time domain analysis when the ethanol displacement time is 60 min in Example 1, and the T2 relaxation time of heavy water and water at the same mass ratio (ie, the same displacement time).

[0025] Figure 8 It is a diagram comparing the T2 spectrum of water in a mixed solution of ethanol and water in a porous medium separated by time domain analysis when the ethanol displacement time is 160 min in Example 1 and the T2 relaxation time of heavy water and water at the same mass ratio (ie, the same displacement time).

[0026] Fig. 9 It is a curve diagram of the relationship between the core ethanol injection time and the sum of squares of the dimensionless nuclear magnetic signal differences in Example 1.

[0027] Fig.10 It is the relationship between the core porosity and the critical PV value of ethanol injection. DETAILED DESCRIPTION

[0028] The present invention is further described in detail below in conjunction with examples, but the embodiments of the present invention are not limited thereto.

[0029] In the prior art, it is difficult to accurately measure the content and distribution changes of water in porous media during miscible displacement by conventional displacement experimental methods, especially for alcohol-water miscibles, conventional displacement experimental methods are not applicable. Nuclear magnetic resonance technology has the advantages of simplicity, speed and accuracy. It has been applied in displacement experiments of oil-water immiscible fluids and has been widely used in reservoir engineering and reservoir geology. The present invention is based on the existing more mature nuclear magnetic resonance technology, further optimized and improved, and proposes a method for separating the nuclear magnetic signals of water in miscible solutions in porous media using nuclear magnetic resonance technology, which is convenient for quantitative analysis of the content and distribution of water in porous media during miscible displacement. It provides an experimental theoretical basis and methods for related research, such as being able to conveniently and quantitatively evaluate the drying effect of water in porous media, and the effect of alcohols in removing water lock damage.

[0030] Example 1 In order to facilitate the public to understand the present invention, this embodiment is further described with specific examples.

[0031] In this scheme, three core samples of porous media were designed, numbered as cores I-18, C-6, and K-5, respectively. The NMR signals of water in the mixed solution of ethanol and water needed to be separated and compared and analyzed.

[0032] Set the NMR test parameters: minimum echo interval TE = 0.1ms, waiting time TW is set to 8000ms, and the maximum number of echoes NECH is 6000. Ensure that the hydrogen nuclei in the porous medium can relax completely, that is, ensure that the fluid is fully relaxed, and collect as many NMR signals as possible to avoid signal acquisition loss.

[0033] Based on the principle of nuclear magnetic resonance, a hydrogen atom can be regarded as a spin nucleus with a current loop rotating around the core. This nucleus can generate a magnetic moment, called the nuclear magnetic moment (μ), also known as a magnetic dipole. A single hydrogen nucleus has a nuclear magnetic moment, but a group of hydrogen nuclei does not exhibit a magnetic moment. When there is an external static magnetic field Bo, the entire group of atomic nuclei is magnetized and will exhibit a macroscopic magnetization vector as a whole. This macroscopic magnetization vector is parallel to the external magnetic field Bo. According to Curie's law, the macroscopic magnetization vector M of the atomic nucleus in the Bo magnetic field is: M = (Nγ 2 hI(I+1)Bo) / 3KT (1) Where k is the Boltzmann constant; γ is the magnetic gyroscopic ratio. The magnetic gyroscopic ratio of hydrogen nucleus is 26.7519×10 7 (rad T -1 S -1 ) ; T—absolute temperature, K; h—Planck constant; I—nuclear spin quantum number, I=0, 1 / 2, 1, 3 / 2….

[0034] From the above formula (1), it can be seen that the macroscopic magnetization vector M is the direct measurement object of nuclear magnetic resonance. In the case of a selected atomic nucleus, its value is determined by the number of spins per unit volume N, the external magnetic field strength Bo and the temperature T.

[0035] Due to the nature of NMR technology, the identification of different fluids by NMR can be divided into two categories: the first category is the fluid identification method based on pulse scanning sequence and time parameters; the second category is the fluid identification method based on external reagents. Among them, the first type mainly distinguishes the NMR signals of different fluids by selecting appropriate scanning pulse sequences and test parameters. This is because different fluids have different diffusion abilities under an external gradient field, which makes the relaxation times of different fluids different. This difference can be highlighted by changing the echo interval, etc., and then the NMR signals are inverted and processed by the difference spectrum method (DSM) and time domain analysis method (TDA) to obtain the relaxation time distribution of different fluids. The difference spectrum method refers to the waiting time for the CPMG pulse sequence to determine the proton polarization during the NMR test, that is, the time after a CPMG pulse test ends and before a new CPMG pulse sequence starts. The waiting time value needs to be large enough to ensure that the proton polarization is complete. If TW is too small, some spin nuclei have not yet fully recovered to the thermal equilibrium state, and they will be subjected to a 90-degree pulse and begin to undergo nuclear magnetic resonance. In this way, the detected nuclear magnetic signal will be weakened. When the TW value decreases to a certain extent, the nuclear magnetic signal of this part of the spin nuclei that cannot be fully polarized cannot be detected at all. By setting different TW values ​​to measure the T2 relaxation time of the mixed oil, gas and water, the nuclear magnetic signal of a certain fluid can be retained by subtracting the relaxation time. That is, the two nuclear magnetic signal attenuation curves with two different waiting times are inverted to obtain the corresponding T2 spectrum, and then the T2 spectrum with a short waiting time is subtracted from the T2 spectrum with a long waiting time. The difference spectrum obtained is the T2 spectrum distribution of the remaining fluid. The time domain analysis method refers to subtracting the echo train signal of the short waiting time TWS from the echo train signal of the long waiting time TWL to obtain the echo train difference of another fluid, and then using multi-exponential inversion to invert this echo train difference to obtain the T2 signal distribution of this fluid. Both the time domain analysis method and the difference spectrum method belong to the double TW method, and the time domain analysis method is an extension and improvement of the difference spectrum method. It performs subtraction operations in the time domain and inverts the echo string difference. The time domain analysis method is more accurate and has less noise than the difference spectrum method.

[0036] It is known from the fluid separation method based on nuclear magnetic resonance technology that the nuclear magnetic signal is only related to the number of hydrogen nuclei, magnetic gyrometry, external magnetic field and temperature, that is, the nuclear magnetic signal of a mixed solution of ethanol and water can be regarded as the collection of the nuclear magnetic signal of the same volume of ethanol and the same volume of water. Based on the nuclear magnetic resonance identification methods and nuclear magnetic resonance principles of oil, gas and water in the prior art, it can be known that the nuclear magnetic signal of a mixed solution of ethanol and water can be regarded as the sum of the nuclear magnetic signals of separate masses of water and ethanol. Therefore, the nuclear magnetic signal of the same mass of ethanol can be deducted from one of the mixed solutions, thereby achieving the purpose of separating the nuclear magnetic signal of water in the mixed solution. Reference Figure 1 , the scheme of the present invention is further described below.

[0037] first step: The three low-permeability core samples selected above were dried to measure the dry weight of the cores, and the gas-measured porosity and permeability of the dry cores were measured. The dry cores were placed in a vacuum-pressurized saturation device for 6 hours, and then pressurized with 20 MPa saturated pure water for 6 hours. The saturated cores were weighed to obtain the saturated porosity of the cores. The physical properties of the experimental cores refer to Table 1.

[0038] Table 1 Core Porosity / % Permeability / mD Ethanol injection critical time / min Ethanol injection PV critical value I-18 12.42 5.15 89.90 0.26 C-6 13.72 9.83 66.07 0.10 K-5 10.5 2.2 94.76 0.29 Step 2: Then, the dry core was evacuated, connected to the NMR online displacement device, and placed in a 70 mm NMR coil, wrapped with a heat shrink tube, and the fluorine oil was added to the confining pressure of 4 MPa. Ethanol was injected into the saturated core at a constant displacement rate of 0.02 mL / min for 3 h. The T2 relaxation time distribution of the core was tested every 20 min using the above NMR test parameters, recorded as y1.

[0039] Step 3: Then put the core after the experiment in the second step into the oven, dry it at 100℃ for 12h, and measure the weight of the core after drying. Wrap the dry core with heat shrink tubing, put it into a 70mm NMR coil, wrap the core with heat shrink tubing, and add fluorine oil to a confining pressure of 4MPa. Inject ethanol into the dry core at a constant displacement rate of 0.02mL / min for 3h. Use the above NMR test parameters to test the T2 relaxation time distribution of the core every 20min, recorded as y2.

[0040] Step 4: Then put the core after the experiment in the third step into the oven, dry it at 100℃ for 12h, and measure the weight of the core after drying. Put the dry core into the vacuum pressurized saturation device for 6h, then pressurize it with 20MPa saturated pure water for 6h, wrap the saturated core with heat shrink tubing, put it in a 70mm nuclear magnetic resonance coil, and add fluorine oil to the confining pressure of 4MPa. Inject heavy water into the saturated core at a constant rate of 0.02mL / min, the displacement time is 3h, and the T2 relaxation time distribution of the core is tested every 20min, recorded as y3.

[0041] Through the second to fourth steps, the T2 relaxation time distribution y1 of the mixed solution of ethanol and water in different cores at the same displacement time point, the T2 relaxation time distribution y2 of ethanol in the cores at the same displacement time point, and the nuclear magnetic resonance signal y3 of the mixed solution of heavy water and water at the same displacement time point are obtained respectively.

[0042] The NMR signals of water in the mixed solution of ethanol and water in the porous medium were obtained by time domain analysis and differential spectrum method, and compared with the NMR signals y3 of the mixed solution of heavy water and water at the same displacement time point. The T2 spectrum of water in the mixed solution of ethanol and water in the porous medium separated by differential spectrum method was compared with the T2 relaxation time of heavy water and water at the same mass ratio (i.e. the same displacement time), as shown in Fig. Figure 2-4 ; The T2 spectrum of water in the mixed solution of ethanol and water in porous media separated by time domain analysis method is compared with the T2 relaxation time of heavy water and water with the same mass ratio (i.e. the same displacement time), see Figure 6-8 .

[0043] Step 5: Then, using the difference spectrum method, the T2 relaxation time distribution y1 of the mixture of ethanol and water in the core at the same displacement time point is subtracted from the T2 relaxation time distribution y2 of ethanol in the core at the same displacement time point to obtain the relaxation time distribution of heavy water in the mixed solution of ethanol and water in the porous medium, which is recorded as y4.

[0044] Step 6: The critical PV value of ethanol injection is obtained when the nuclear magnetic signal of water separated from the mixed solution of ethanol and water in the porous medium is accurate according to the difference spectrum method.

[0045] According to the above steps, it can be known that: in a porous medium, as the proportion of water in the porous medium decreases, the difference between the T2 spectrum distribution and signal amount of water in the ethanol and water mixed solution and the T2 relaxation time distribution of the heavy water and water mixed solution with the same mass ratio (i.e., the same displacement time) gradually increases, that is, there is also a critical mass ratio of water to ethanol in the nuclear magnetic signal separation of water in the ethanol and water mixed solution in the porous medium. When the mass ratio of water to ethanol in the ethanol and water mixed solution in the porous medium is greater than the critical mass ratio, the T2 relaxation time of the water separated from the ethanol and water mixed solution in the porous medium is the most accurate. If it is less than this critical value, the T2 relaxation time error of the water separated from the ethanol and water mixed solution in the porous medium is relatively large. Figure 2-4 shown.

[0046] The nuclear magnetic signal y4 of water in the mixed solution of ethanol and water in the porous medium obtained by the difference spectrum method and the nuclear magnetic signal y3 of the mixed solution of heavy water and water with the same mass ratio (i.e., the same displacement time) are used to evaluate the accuracy of the separation of the nuclear magnetic signal of water in the mixed solution of ethanol and water in the porous medium by the size of the square sum of the difference between the dimensionless nuclear magnetic signals. The turning point of the drastic change in the square sum of the difference between the water signal separated from the mixed solution of ethanol and water in the porous medium and the nuclear magnetic signal of heavy water and water with the same mass ratio (i.e., the same displacement time) corresponds to the fitting curves of the stable region of the square sum of the difference of the dimensionless nuclear magnetic signal and the drastic change region, respectively. The intersection of these two fitting curves is the turning point of the square sum of the difference of the nuclear magnetic signal, which is also the PV critical value of ethanol injection (i.e., the critical time of ethanol injection) when the nuclear magnetic signal of water separated from the mixed solution of ethanol and water in the porous medium is accurate, as shown in Figure 5 And as shown in Table 2.

[0047] Table 2 Core Porosity / % Permeability / mD Ethanol injection critical time / min Ethanol injection PV critical value I-18 12.42 5.15 89.90 0.26 C-6 13.72 9.83 66.07 0.10 K-5 10.5 2.2 94.76 0.29 Step 7: The critical PV value of ethanol injection is obtained when the nuclear magnetic signal of water separated from the mixed solution of ethanol and water in the porous medium is accurate according to the time domain analysis method.

[0048] Referring to the sixth step, the nuclear magnetic signal y4 of water in the ethanol and water mixed solution in the porous medium obtained by the time domain analysis method, and the nuclear magnetic signal y3 of the heavy water and water mixed solution with the same mass ratio (i.e., the same displacement time), it can be seen that in the porous medium, as the proportion of water in the porous medium decreases, the difference between the T2 spectrum distribution and signal amount of water in the ethanol and water mixed solution and the T2 relaxation time distribution of the heavy water and water mixed solution with the same mass ratio (i.e., the same displacement time) gradually increases, that is, there is also a critical mass ratio of water to ethanol in the nuclear magnetic signal separation of water in the ethanol and water mixed solution in the porous medium. When the mass ratio of water to ethanol in the ethanol and water mixed solution in the porous medium is greater than the critical mass ratio, the T2 relaxation time of the water separated from the ethanol and water mixed solution in the porous medium is the most accurate. If it is less than this critical value, the T2 relaxation time error of the water separated from the ethanol and water mixed solution in the porous medium is large, such as Figure 6-8 shown.

[0049] The nuclear magnetic signal y4 of water in the mixed solution of ethanol and water in the porous medium obtained by the time domain analysis method and the nuclear magnetic signal y3 of the mixed solution of heavy water and water with the same mass ratio (i.e., the same displacement time) are used to evaluate the accuracy of the separation of the nuclear magnetic signal of water in the mixed solution of ethanol and water in the porous medium by the size of the square sum of the difference between the dimensionless nuclear magnetic signals. The turning point of the drastic change in the square sum of the difference between the water signal separated from the mixed solution of ethanol and water in the porous medium and the nuclear magnetic signal of heavy water and water with the same mass ratio (i.e., the same displacement time) corresponds to the fitting curves of the stable region of the square sum of the difference between the dimensionless nuclear magnetic signal and the drastic change region, respectively. The intersection of these two fitting curves is the turning point of the square sum of the difference of the nuclear magnetic signal, which is also the PV critical value of ethanol injection (i.e., the critical time of ethanol injection) when the nuclear magnetic signal of water separated from the mixed solution of ethanol and water in the porous medium is accurate, as shown in Fig. 9 And as shown in Table 3.

[0050] Table 3 Core Porosity / % Permeability / mD Ethanol injection critical time / min Ethanol injection PV critical value I-18 12.42 5.15 107.06 0.37 C-6 13.72 9.83 95.22 0.28 K-5 10.5 2.2 102.86 0.34 Step 8: For cores with different physical properties, the critical PV values ​​(or critical time of ethanol injection) obtained by the difference spectrum method and the time domain analysis method are not equal, as shown in Tables 4 and 5.

[0051] Table 4: Critical value of PV when the NMR signal of water in ethanol / water mixed solution in porous media is accurately separated by differential spectrum method Core Porosity / % Permeability / mD Ethanol injection critical time / min Ethanol injection PV critical value I-18 12.42 5.15 89.90 0.26 C-6 13.72 9.83 66.07 0.10 K-5 10.5 2.2 94.76 0.29 Table 5: Critical PV value of ethanol injection when the NMR signal of water in ethanol / water mixed solution in porous media is accurate by time domain analysis Core Porosity / % Permeability / mD Ethanol injection critical time / min Ethanol injection PV critical value I-18 12.42 5.15 107.06 0.37 C-6 13.72 9.83 95.22 0.28 K-5 10.5 2.2 102.86 0.34 Since the accuracy of NMR signal separation of ethanol and water in porous media is positively correlated with the mass of water, the critical value of ethanol injection PV obtained by differential spectrum method and time domain analysis method is also correlated with the porosity of the core. When the NMR signal of water in a mixed solution of ethanol and water in porous media is separated by the NMR method, that is, when ethanol displaces water-containing cores, there is an ethanol injection PV critical value. When the injection amount of ethanol is less than this critical value, the T2 relaxation time of the separated water is the most accurate. When the injection amount of ethanol is greater than this critical value, the error of the T2 relaxation time of the separated water is large. When ethanol is used to displace water-containing cores, it is necessary to inject as much ethanol as possible, that is, the critical value of ethanol injection PV should be as large as possible. Fig.10 As shown in the figure, when the core porosity is greater than 9.3%, the critical PV value of ethanol injection by time domain analysis is larger; when the core porosity is less than 9.3%, the critical PV value of ethanol injection by differential spectrum method is larger. Therefore, it is necessary to reasonably choose differential spectrum method or time domain analysis method to separate the nuclear magnetic signal of water in porous medium mixed solution according to the core porosity.

[0052] The above description is only a preferred embodiment of the present invention and does not limit the present invention in any form. Any simple modification or equivalent change made to the above embodiment based on the technical essence of the present invention shall fall within the protection scope of the present invention.

Claims

1. A method for separating nuclear magnetic signals of water in a miscible solution in a porous medium, characterized in that: The following steps are involved: S1. Select a porous medium core sample to be tested, dry it to measure the dry weight of the core, and measure the porosity and permeability of the dry core by gas measurement; S2, evacuate the dry core, then pressurize and inject saturated pure water, connect the nuclear magnetic resonance online displacement device, put the saturated core into the nuclear magnetic resonance coil, wrap the core with a heat shrink tube, add confining pressure, and then inject ethanol into the saturated core at a constant rate to simulate the displacement process, and obtain the T2 relaxation time distribution of the core at different times, recorded as y1; S3, putting the core after the experiment in step S2 into an oven for drying, measuring the weight of the core after drying, wrapping the dry core with a heat shrink tube, putting it into a nuclear magnetic resonance coil, wrapping the core with a heat shrink tube, adding confining pressure, and then injecting ethanol into the dry core at a constant rate to simulate the displacement process, and obtaining the T2 relaxation time distribution of the core at different times, recorded as y2; S4, putting the core after the experiment in step S3 into an oven for drying, measuring the weight of the core after drying, evacuating the dry core, pressurizing it, injecting saturated pure water, wrapping the saturated core with a heat shrink tube, putting it into a nuclear magnetic resonance coil, adding confining pressure, and then injecting heavy water into the saturated core at a constant rate to simulate the displacement process, and obtaining the T2 relaxation time distribution of the core, recorded as y3; S5, using the difference spectrum method, the T2 relaxation time distribution y1 of the mixture of ethanol and water in the core at the same displacement time point is subtracted from the T2 relaxation time distribution y2 of ethanol in the core at the same displacement time point to obtain the relaxation time distribution of water in the mixed solution of ethanol and water in the porous medium, which is recorded as y4; S6. Compare the relaxation time y4 of the water in the core at the same time point with the relaxation time y3 of heavy water and water. It can be seen that when the ethanol injection amount is less than a certain critical value, the differential spectrum method is the most accurate in obtaining the nuclear magnetic signal y4 of the water separated from the mixed solution of ethanol and water in the porous medium. The critical value of the ethanol injection amount is the ethanol injection PV critical value of the differential spectrum method. S7, when the nuclear magnetic signal of water separated from the mixed solution of ethanol and water in the porous medium is obtained accurately according to the time domain analysis method, the critical value of the ethanol injection PV; S8. When separating the NMR signal of water from the mixed solution of ethanol and water in the porous medium, the range of the ethanol injection PV critical value should be as large as possible. Therefore, it is necessary to select the difference spectrum method or time domain analysis method to separate the NMR signal of water from the mixed solution of ethanol and water in the porous medium according to the core porosity.

2. The method for separating nuclear magnetic signals of water in a miscible solution in a porous medium according to claim 1, characterized in that: The nuclear magnetic resonance test parameters of the nuclear magnetic resonance detector are: the minimum echo interval TE=0.1ms, the waiting time TW is set to 8000ms, and the number of echoes NECH takes a maximum of 6000.

3. The method for separating nuclear magnetic signals of water in a miscible solution in a porous medium according to claim 1, characterized in that: In step S1, the dry core is placed in a vacuum and pressurized saturation device and vacuumed for 6 hours, and then pressurized with 20 MPa saturated pure water for 6 hours. The saturated core is weighed to obtain the saturated porosity of the core.

4. The method for separating nuclear magnetic signals of water in a miscible solution in a porous medium according to claim 1, characterized in that: In step S2, the method for obtaining the mixed T2 relaxation time distribution of ethanol and water in the core is as follows: connect the nuclear magnetic resonance online displacement device, place the core in a 70mm nuclear magnetic resonance coil, wrap the core with a heat shrink tube, increase the confining pressure of fluorine oil to 4MPa, and inject ethanol into the saturated core at a constant displacement rate of 0.02mL / min. The displacement test is carried out for 3h, and the T2 relaxation time distribution of the rock type is tested once at regular intervals, which is recorded as y1.

5. The method for separating nuclear magnetic signals of water in a miscible solution in a porous medium according to claim 4, characterized in that: In step S3, the method for obtaining the T2 relaxation time distribution of ethanol in the core at the same displacement time point as y1 is as follows: put the core that has completed the step S2 experiment into an oven, dry it at 100°C for 12 hours, and measure the weight of the core after drying. Wrap the dry core with a heat shrink tube and put it into a 70mm nuclear magnetic resonance coil. After wrapping the core with a heat shrink tube, add fluorine oil and a confining pressure of 4MPa, and inject ethanol into the dry core at a constant rate of 0.02mL / min for 3 hours. Use a nuclear magnetic resonance instrument to test, and test the T2 relaxation time distribution of the lithology once at regular intervals, recorded as y2.

6. The method for separating nuclear magnetic signals of water in a miscible solution in a porous medium according to claim 5, characterized in that: In step S4, the relaxation time distribution of heavy water and water in the core at the same displacement time point as y1 is obtained by: drying at 100°C for 12 hours, and testing the weight of the core after drying, placing the dry core in a vacuum pressurized saturation device for 6 hours, and then pressurizing it with 20MPa saturated pure water for 6 hours, wrapping the saturated core with a heat shrink tube and placing it in a 70mm nuclear magnetic resonance coil, adding fluorine oil to the confining pressure to 4MPa, injecting heavy water into the saturated core at a constant rate of 0.02mL / min, displacing for 3 hours, and testing the T2 relaxation time distribution of the lithology once at regular intervals, recorded as y3.

7. The method for separating nuclear magnetic signals of water in a miscible solution in a porous medium according to claim 1, characterized in that: In step S6, the smaller the mass proportion of ethanol in the mixed solution of ethanol and water in the porous medium, the closer the nuclear magnetic signal of water separated by the differential spectroscopy in the mixed solution of ethanol and water is to the nuclear magnetic signal of water in the mixed solution of heavy water and water containing the same mass of water. There is a critical mass ratio of the mass of water to the mass of ethanol in the mixed solution of ethanol and water in the porous medium. When the ratio of the mass of water to the mass of ethanol in the porous medium is greater than this value, the T2 relaxation time of water separated by the differential spectroscopy in the mixed solution of ethanol and water in the porous medium is the most accurate. If it is less than this critical value, the T2 relaxation time error of water separated by the differential spectroscopy in the mixed solution of ethanol and water in the porous medium is large. This critical value is the ethanol injection PV critical value of the differential spectroscopy.

8. The method for separating nuclear magnetic signals of water in a miscible solution in a porous medium according to claim 1, characterized in that: In step S8, when the core porosity is greater than 9.3%, the time domain analysis method is selected to separate the nuclear magnetic signal of water in the porous medium mixed solution; when the core porosity is less than 9.3%, the difference spectrum method is selected to separate the nuclear magnetic signal of water in the porous medium mixed solution.

9. The method for separating nuclear magnetic signals of water in a miscible solution in a porous medium according to claim 1, characterized in that: When obtaining the T2 relaxation time distribution of the core, the relaxation time T2 is tested every 20 minutes.

10. The method for separating nuclear magnetic signals of water in a miscible solution in a porous medium according to claim 1, characterized in that: The difference spectrum method is used in the process of nuclear magnetic resonance testing, when the CPMG pulse sequence needs to determine the waiting time for proton polarization, that is, the time after a CPMG pulse test is completed and before a new CPMG pulse sequence is restarted; the time domain analysis method refers to subtracting the echo train signal with a short waiting time TWS from the echo train signal with a long waiting time TWL to obtain the echo train difference of another fluid, and then inverting the echo train difference using multi-exponential inversion to obtain the T2 signal distribution of this fluid.

Images