Method for separating the nmr signal of water in miscible solutions in porous media

Abstract

The application discloses a method for separating the nuclear magnetic signal of water in a miscible solution in a porous medium, and relates to the technical field of fluid flow research, and comprises the following steps: S1, selecting a porous medium core sample to be measured, and obtaining core gas measured porosity and permeability; S2, injecting ethanol into a water-saturated core to simulate a displacement process, and obtaining the relaxation time distribution of the core; S3, injecting ethanol into a dry core, and obtaining the relaxation time distribution of the core; S4, injecting heavy water into the water-saturated core, and obtaining the relaxation time distribution of the core; S5, obtaining the relaxation time distribution of water in a mixed solution of ethanol and water; S6, obtaining the ethanol injection PV critical value of a difference spectrum method; S7, obtaining the ethanol injection PV critical value of a time domain analysis method; and S8, selecting the difference spectrum method or the time domain analysis method to separate the nuclear magnetic signal of water in the mixed solution of ethanol and water in the porous medium, so that the problem that the content and distribution change of water in the porous medium in a miscible displacement process cannot be accurately measured by using a conventional displacement experiment method is solved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of petroleum engineering and fluid flow research, in particular to a method for separating the nuclear magnetic signal of water in miscible solution in porous media. BACKGROUND

[0002] During the development of gas reservoirs, capillary imbibition phenomenon is prone to occur in the reservoirs due to small throat radius, containing original water and micro-cracks, etc. Meanwhile, various water-based working fluids such as drilling fluid, completion fluid, well flushing fluid, workover fluid and fracturing fluid, etc. will contact and invade the formation during the development of gas reservoirs, forming liquid phase retention, resulting in a large number of pore throats being occupied by liquid, reducing the effective diameter of the gas flow channel in the reservoir and sharply increasing the flow resistance, which shows obvious water lock effect and greatly limits the economic and efficient development of gas reservoirs.

[0003] In view of this problem, some experts and scholars have proposed to inject dry agent to rapidly react with the formation water in the reservoir near the wellbore to achieve the purpose of consuming formation water and reducing the resistance of the channel. Ethanol is used as a carrying agent for the dry agent, and ethanol and water are mutually miscible when the dry agent reacts with water. For miscible fluids, it is difficult to accurately measure the content and distribution of water in the porous media by using conventional displacement experiment methods, especially when chemical reactions occur during the miscible process. Nuclear magnetic resonance technology has been widely used in reservoir engineering and reservoir geology due to its simplicity, speed and accuracy. Based on the nuclear magnetic resonance technology and principle, the present application proposes a method for separating the nuclear magnetic signal of water in the mixed solution of ethanol and water in the porous media by using nuclear magnetic resonance technology.

[0004] Since it is difficult to accurately measure the content and distribution of water in the porous media when ethanol displaces the water-containing core by using conventional displacement experiment methods, it is urgent to develop a convenient and accurate displacement experiment method to simulate the content and distribution of water in the porous media when ethanol displaces the water-containing core, thereby providing theoretical support for related research. SUMMARY

[0005] The present application aims to provide a method for separating the nuclear magnetic signal of water in miscible solution in porous media, which can quantitatively analyze the content and distribution of water in the porous media during miscible displacement by using nuclear magnetic resonance technology.

[0006] The present application is realized by the following technical scheme:

[0007] A method for separating the nuclear magnetic signal of water in miscible solution in porous media, comprising the following steps:

[0008] S1, selecting a porous media core sample to be tested, drying the core to measure the dry weight of the core, and measuring the gas porosity and permeability of the dry core;

[0009] S2, vacuumize the dry core, then pressurize, inject saturated pure water, connect the NMR online displacement device, and place the water-saturated core into the NMR coil, wrap the core with a heat shrink tube, add confining pressure, then inject ethanol into the water-saturated core at a constant speed to simulate the displacement process, and obtain the T2 relaxation time distribution of the core at different times, denoted as y1;

[0010] S3, place the core after the experiment in step S2 into an oven to dry, measure the weight of the dried core, wrap the dry core with a heat shrink tube, place it into the NMR coil, wrap it with a heat shrink tube, add confining pressure, then inject ethanol into the dry core at a constant speed to simulate the displacement process, and obtain the T2 relaxation time distribution of the core at different times, denoted as y2;

[0011] S4, place the core after the experiment in step S3 into an oven to dry, measure the weight of the dried core, vacuumize the dry core, then pressurize, inject saturated pure water, wrap the water-saturated core with a heat shrink tube, place it into the NMR coil, add confining pressure, then inject heavy water into the water-saturated core at a constant speed to simulate the displacement process, and obtain the T2 relaxation time distribution of the core, denoted as y3;

[0012] S5, use the difference spectrum method to subtract the T2 relaxation time distribution y2 of ethanol in the core at the same displacement time point from the mixed T2 relaxation time distribution y1 of ethanol and water 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, denoted as y4;

[0013] S6, compare the relaxation time y4 of water in the core at the same time point with the relaxation time y3 of heavy water and water, and it can be known that when the ethanol injection amount is less than a certain critical value, the NMR signal y4 of the separated water in the mixed solution of ethanol and water in the porous medium obtained by the difference spectrum method is most accurate, and this critical value of the ethanol injection PV is the ethanol injection PV critical value for the difference spectrum method;

[0014] S7, the ethanol injection PV critical value when the NMR signal of the separated water in the mixed solution of ethanol and water in the porous medium obtained by the time domain analysis method is accurate;

[0015] S8, according to the range of the ethanol injection PV critical value within which the NMR signal of the separated water in the mixed solution of ethanol and water in the porous medium needs to be as large as possible, it is necessary to select the difference spectrum method or the time domain analysis method to separate the NMR signal of water in the mixed solution of ethanol and water in the porous medium according to the size of the core porosity.

[0016] Further, the NMR test parameters of the NMR detector are as follows: the minimum echo interval TE is 0.1 ms, the waiting time TW is set to 8000 ms, and the number of echoes NECH is the maximum 6000.

[0017] Further, in step S1, the dry core is placed in the vacuumizing and pressurizing saturation device, vacuumized for 6 hours, then saturated with pure water at 20 MPa for 6 hours, and the saturated core is weighed to obtain the saturated porosity of the core.

[0018] Further, in step S2, the method for obtaining the T2 relaxation time distribution of the mixture of ethanol and water in the core is as follows: the online displacement device of nuclear magnetic resonance is connected, the core is placed in the 70 mm nuclear magnetic resonance coil, the core is wrapped with a heat shrink tube, the fluorine oil is pressurized to 4 MPa, the ethanol is injected into the saturated core at a constant displacement rate of 0.02 mL / min, the displacement test is performed for 3 hours, the nuclear magnetic instrument is used for testing, the T2 relaxation time distribution of the lithology is tested at intervals, and the result is recorded as y1.

[0019] Further, in step S3, the method for obtaining the T2 relaxation time distribution of the ethanol in the core at the same displacement time point as y1 is as follows: the core after the completion of step S2 is placed in an oven, dried at 100℃ for 12 hours, and the weight of the dried core is tested, the dried core is wrapped with a heat shrink tube, placed in a 70 mm nuclear magnetic resonance coil, the core is wrapped with a heat shrink tube, the fluorine oil is pressurized to 4 MPa, the ethanol is injected into the dried core at a constant displacement rate of 0.02 mL / min, the displacement time is 3 hours, the nuclear magnetic instrument is used for testing, the T2 relaxation time distribution of the lithology is tested at intervals, and the result is recorded as y2.

[0020] Further, in step S4, the method for obtaining the T2 relaxation time distribution of the heavy water and water in the core at the same displacement time point as y1 is as follows: the core is dried at 100℃ for 12 hours, and the weight of the dried core is tested, the dried core is placed in the vacuumizing and pressurizing saturation device, vacuumized for 6 hours, then saturated with pure water at 20 MPa for 6 hours, the saturated core is wrapped with a heat shrink tube and placed in a 70 mm nuclear magnetic resonance coil, the fluorine oil is pressurized to 4 MPa, the heavy water is injected into the saturated core at a constant rate of 0.02 mL / min, the displacement is performed for 3 hours, the T2 relaxation time distribution of the lithology is tested at intervals, and the result is recorded as y3.

[0021] Further, in step S6, the smaller the mass ratio of ethanol in the mixed solution of ethanol and water in the porous medium, the closer the NMR signal of water separated by the difference spectrum method in the mixed solution of ethanol and water to the NMR 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 water mass to ethanol mass in the mixed solution of ethanol and water in the porous medium, and when the mass ratio of water to ethanol in the porous medium is greater than the value, the T2 relaxation time of water separated by the difference spectrum method in the mixed solution of ethanol and water in the porous medium is the most accurate, and if it is less than the critical value, the T2 relaxation time of water separated by the difference spectrum method in the mixed solution of ethanol and water in the porous medium has a larger error. That is, when ethanol displaces the water-containing core, there is a critical value of ethanol injection PV, and when the amount of ethanol injection is less than the critical value, the T2 relaxation time of water separated by the difference spectrum method in the mixed solution of ethanol and water in the porous medium is the most accurate, and when the amount of ethanol injection is greater than the critical value, the T2 relaxation time of water separated by the difference spectrum method in the mixed solution of ethanol and water in the porous medium has a larger error.

[0022] Further, in step S8, when the NMR signal of water in the mixed solution of ethanol and water in the porous medium is separated by the NMR method, there is a critical mass ratio of water mass to ethanol mass, and when the mass ratio of water to ethanol in the porous medium is greater than the critical mass ratio, the separated NMR signal of water is the most accurate, and when it is less than the critical mass ratio, the separated NMR signal of water has a larger error. That is, when ethanol displaces the water-containing core, there is a critical value of ethanol injection PV, and when the amount of ethanol injection is less than the critical value, the T2 relaxation time of the separated water is the most accurate, and when the amount of ethanol injection is greater than the critical value, the T2 relaxation time of the separated water has a larger error. When ethanol displaces the water-containing core, it is necessary 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 value of ethanol injection PV for time-domain analysis is larger; when the core porosity is less than 9.3%, the critical value of ethanol injection PV for the difference spectrum method is larger, so it is necessary to reasonably select the difference spectrum method or the time-domain analysis method to separate the NMR signal of water in the mixed solution in the porous medium according to the size of the core porosity.

[0023] Further, when the T2 relaxation time distribution of the core is obtained, the relaxation time T2 is tested every 20 min.

[0024] Further, the difference spectrum method is to set different waiting times according to fluid relaxation properties in the process of nuclear magnetic resonance testing, for example, the longitudinal relaxation time of oil and gas as a non-wetting phase in the rock (usually several seconds) is much longer than that of water in the rock (usually several hundred milliseconds), so the T2 relaxation time measured by long waiting time can contain the nuclear magnetic signals of oil, gas and water, and the T2 relaxation time obtained by testing the same sample by short waiting time can only include the nuclear magnetic signal of water, and the nuclear magnetic signal of oil and gas can be reserved by subtracting the T2 relaxation time, that is, the T2 spectrum corresponding to the nuclear magnetic signal decay curve of two different waiting times is obtained, and then the T2 spectrum of long waiting time is subtracted from the T2 spectrum of 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 echo train obtained is inverted into T2 relaxation time distribution. For example, the echo train signal containing the nuclear magnetic signals of oil, gas and water by long waiting time is subtracted from the echo train signal containing only the nuclear magnetic signal of water by short waiting time, and the echo train difference of oil and gas is obtained, and then the T2 signal distribution of oil and gas is obtained by multi-exponential inversion of the echo train difference.

[0025] Compared with the prior art, the present application has the following advantages and beneficial effects:

[0026] Firstly, in the present application, based on the principle of nuclear magnetic resonance, a method for separating the nuclear magnetic signal of water in a porous medium containing ethanol and water is established, and the distribution and content of water in the porous medium during miscible displacement can be quantitatively analyzed by this method. This method has the advantages of convenience and speed, and does not need to change the fluid properties and the pore structure of the porous medium.

[0027] Secondly, in the present application, 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 ethanol injection PV critical value when the model is used to displace the water-containing core according to the method described in the present application. Generally speaking, for a core with a porosity greater than 9.3%, the time domain analysis method is selected to separate the nuclear magnetic signal of water in the core, and when the porosity of the core 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 separated water nuclear magnetic signal in the core is the most accurate, but also ensures that the amount of ethanol injected is larger, which can guarantee the water nuclear magnetic signal separation requirement in a larger range of ethanol displacement experiments. The problem that it is difficult to accurately measure the content and distribution change of water in the porous medium during miscible displacement in the existing conventional displacement experiment method is solved. BRIEF DESCRIPTION OF DRAWINGS

[0028] Figure 1 The flowchart of the present application.

[0029] Figure 2is a T2 spectrum of water in the ethanol and water mixed solution in the porous medium separated by the difference spectrum method at the ethanol displacement time of 0 min in Example 1, compared with the T2 relaxation time of heavy water and water with the same mass ratio (i.e. the same displacement time).

[0030] Figure 3 is a T2 spectrum of water in the ethanol and water mixed solution in the porous medium separated by the difference spectrum method at the ethanol displacement time of 60 min in Example 1, compared with the T2 relaxation time of heavy water and water with the same mass ratio (i.e. the same displacement time).

[0031] Figure 4 is a T2 spectrum of water in the ethanol and water mixed solution in the porous medium separated by the difference spectrum method at the ethanol displacement time of 160 min in Example 1, compared with the T2 relaxation time of heavy water and water with the same mass ratio (i.e. the same displacement time).

[0032] Figure 5 is a relationship curve between the ethanol injection time of the core and the sum of squares of differences in dimensionless NMR signals in Example 1.

[0033] Figure 6 is a T2 spectrum of water in the ethanol and water mixed solution in the porous medium separated by the time domain analysis method at the ethanol displacement time of 0 min in Example 1, compared with the T2 relaxation time of heavy water and water with the same mass ratio (i.e. the same displacement time).

[0034] Figure 7 is a T2 spectrum of water in the ethanol and water mixed solution in the porous medium separated by the time domain analysis method at the ethanol displacement time of 60 min in Example 1, compared with the T2 relaxation time of heavy water and water with the same mass ratio (i.e. the same displacement time).

[0035] Figure 8 is a T2 spectrum of water in the ethanol and water mixed solution in the porous medium separated by the time domain analysis method at the ethanol displacement time of 160 min in Example 1, compared with the T2 relaxation time of heavy water and water with the same mass ratio (i.e. the same displacement time).

[0036] Figure 9 is a relationship curve between the ethanol injection time of the core and the sum of squares of differences in dimensionless NMR signals in Example 1.

[0037] Figure 10 is a relationship diagram between the core porosity and the critical value of ethanol injection PV. DETAILED DESCRIPTION

[0038] The application will be further described in conjunction with the examples below, but the embodiments of the application are not limited thereto.

[0039] In the prior art, it is difficult to accurately measure the content and distribution change of water in a porous medium in a miscible displacement process by using a conventional displacement experiment method, especially for alcohol-water miscible solution, the conventional displacement experiment method is not applicable. The nuclear magnetic resonance technology has the advantages of simplicity, speediness and accuracy, and has been applied in the displacement experiment of oil-water immiscible fluid, and has been widely applied in oil reservoir engineering and oil reservoir geology. Based on the existing mature nuclear magnetic resonance technology, the method for separating the nuclear magnetic signal of water in the miscible solution in the porous medium by using the nuclear magnetic resonance technology is further optimized and improved, so as to facilitate the quantitative analysis of the content and distribution of water in the porous medium in the miscible displacement process. The experimental theoretical basis and method are provided for related research, for example, the water drying effect in the porous medium and the alcohol alcohol damage removal effect can be conveniently quantitatively evaluated.

[0040] Example 1

[0041] In order to facilitate the public to understand the present application, the present embodiment is further illustrated by specific examples.

[0042] In the present scheme, three porous medium core samples are designed, which are numbered as core I-18, C-6 and K-5. The nuclear magnetic signal of water in the mixed solution of ethanol and water needs to be separated and compared and analyzed.

[0043] The nuclear magnetic test parameters are set: the minimum echo interval TE is 0.1 ms, the waiting time TW is set to 8000 ms, and the echo number NECH is 6000. The hydrogen nucleus in the porous medium can be completely relaxed, that is, the fluid is fully relaxed, and as many nuclear magnetic signals as possible can be collected to avoid signal acquisition loss.

[0044] Based on the principle of nuclear magnetic resonance, hydrogen atoms can be regarded as a nucleus with a current ring rotating around the core direction, which can generate a magnetic moment, called nuclear magnetic moment (μ), also known as magnetic dipole. A single hydrogen nucleus has a nuclear magnetic moment, but a group of hydrogen nuclei does not exhibit a magnetic moment. When an external static magnetic field Bo exists, the entire nucleus group system is magnetized and exhibits a macroscopic magnetization vector. 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:

[0045] M=(Nγ 2 hI(I+1)Bo) / 3KT (1)

[0046] In the formula, k is the Boltzmann constant; γ is the magnetic spin ratio, the magnetic spin ratio of hydrogen nucleus is 26.7519×10 7 (rad T -1 S -1T - absolute temperature, K; h - Planck's constant; I - spin quantum number of the nucleus, I = 0, 1 / 2, 1, 3 / 2...

[0047] From the above formula (1), it can be seen that the macroscopic magnetization vector M is the direct object of measurement of nuclear magnetic resonance, and in the case of selected atomic nuclei, its value is determined by the number of spins per unit volume N, the applied magnetic field strength Bo and the temperature T.

[0048] Due to the nature of nuclear magnetic resonance technology, at present, nuclear magnetic resonance fluid identification can be divided into two categories: the first category is a fluid identification method based on pulse scanning sequence and time parameter; the second category is a fluid identification method based on an external agent. Among them, the first kind is mainly to distinguish the nuclear magnetic signals of different fluids by selecting appropriate scanning pulse sequences and test parameters, because the diffusion ability of different fluids under the applied gradient field is different, so that the relaxation time of different fluids is also different, and this difference can be highlighted by changing the echo interval, and then the nuclear magnetic signal is inverted and processed by 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 determination of the waiting time of proton polarization during the nuclear magnetic resonance test process, that is, the time between the end of a CPMG pulse test and the start of a new CPMG pulse sequence. The value of the waiting time needs to be large enough to ensure the complete proton polarization. If TW is too small, some spin nuclei have not completely recovered to the thermal equilibrium state, and are subjected to a 90-degree pulse to start nuclear magnetic resonance, so that the detected nuclear magnetic signal will be weakened. When the TW value is reduced to a certain extent, the nuclear magnetic signal of the spin nuclei that cannot be completely polarized cannot be completely detected. By setting different TW values to measure the T2 relaxation time of mixed oil, gas and water, the nuclear magnetic signal of a certain fluid can be retained by subtracting the relaxation times, that is, the T2 spectrum corresponding to the two nuclear magnetic signal decay curves of two different waiting times is obtained, and then the T2 spectrum of the long waiting time is subtracted from the T2 spectrum of the short waiting time, and the difference spectrum obtained is the T2 spectrum distribution of the remaining fluid. The time domain analysis method refers to the use of the echo train signal of long waiting time TWL to subtract the echo train signal of short waiting time TWS to obtain the echo train difference of another fluid, and then use multi-exponential inversion to invert the echo train difference to obtain the T2 signal distribution of this fluid. The time domain analysis method and the difference spectrum method both 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 operation in the time domain and inverts the echo train difference. The time domain analysis method is more accurate and has less noise than the difference spectrum method.

[0049] Based on the fluid separation method based on the nuclear magnetic resonance technology, the nuclear magnetic signal is only related to the number of hydrogen nuclei, the magnetic spin ratio, the applied magnetic field and the temperature, that is, the nuclear magnetic signal amount of the ethanol and water mixed solution can be regarded as the collection of the nuclear magnetic signal amounts of the same volume of ethanol and the same volume of water. Based on the nuclear magnetic resonance recognition method of oil, gas and water in the prior art and the nuclear magnetic resonance principle, it is known that the nuclear magnetic signal of the ethanol and water mixed solution can be regarded as the sum of the nuclear magnetic signals of the water and the ethanol with the same mass, and therefore the nuclear magnetic signal of the ethanol with the same mass can be deducted from the mixed solution, so as to separate the nuclear magnetic signal of the water in the mixed solution. Figure 1 The scheme of the present application is further described below.

[0050] First step:

[0051] Three low-permeability core samples were selected, dried to measure the dry weight of the core, and the dry core gas porosity and permeability were measured. The dry core was placed in a vacuum pressurized saturation device and vacuumed for 6 hours, then pressurized to 20 MPa and saturated with pure water for 6 hours. The saturated core was weighed to obtain the saturated porosity of the core. The physical properties table of the experimental core is shown in Table 1.

[0052] Table 1

[0053] Core Porosity / % Permeability / mD Critical time of ethanol injection / min Critical value of ethanol injection PV 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

[0054] Second step:

[0055] The dry core was vacuumed again, connected to the nuclear magnetic resonance online displacement device, and the core was placed in a 70mm nuclear magnetic resonance coil. The core was wrapped with a heat shrink tube, and fluorine oil was added to provide a surrounding pressure of 4MPa. Ethanol was injected into the saturated core at a constant displacement rate of 0.02mL / min, and the displacement time was 3h. The T2 relaxation time distribution of the core was tested every 20min using the above nuclear magnetic resonance test parameters, and was recorded as y1.

[0056] Third step:

[0057] The core after completing the experiment in the second step was placed in an oven at a temperature of 100℃ for continuous drying for 12h, and the weight of the dried core was measured. The dry core was wrapped with a heat shrink tube and placed in a 70mm nuclear magnetic resonance coil. The core was wrapped with a heat shrink tube, and fluorine oil was added to provide a surrounding pressure of 4MPa. Ethanol was injected into the dry core at a constant displacement rate of 0.02mL / min, and the displacement time was 3h. The T2 relaxation time distribution of the core was tested every 20min using the above nuclear magnetic resonance test parameters, and was recorded as y2.

[0058] Fourth step:

[0059] The core after the experiment in the third step is put into an oven, dried at 100℃ for 12h, and the weight of the dried core is measured. The dried core is put into a vacuum pressurized saturation device, vacuumized for 6h, then saturated with pure water at 20MPa for 6h. The water-saturated core is wrapped with a heat shrink tube, put into a 70mm NMR coil, and surrounded with fluorine oil at 4MPa. Heavy water is injected into the water-saturated core at a constant speed of 0.02mL / min, and the displacement time is 3h. The T2 relaxation time distribution of the core is measured every 20min, and is recorded as y3.

[0060] Through the second to fourth steps, the T2 relaxation time distribution of the mixed solution of ethanol and water in the core at the same displacement time point y1, the T2 relaxation time distribution of ethanol in the core at the same displacement time point y2, and the NMR signal of the mixed solution of heavy water and water at the same displacement time point y3 are obtained respectively.

[0061] The NMR signal of water in the mixed solution of ethanol and water in the porous medium is obtained by using the time domain analysis method and the difference spectrum method respectively, and is compared with the NMR signal of the mixed solution of heavy water and water at the same displacement time point y3. The comparison results of the T2 spectrum of water in the mixed solution of ethanol and water in the porous medium and the T2 relaxation time of the mixed solution of heavy water and water with the same mass ratio (i.e. the same displacement time) by using the difference spectrum method are shown in Figures 2-4 ; the comparison results of the T2 spectrum of water in the mixed solution of ethanol and water in the porous medium and the T2 relaxation time of the mixed solution of heavy water and water with the same mass ratio (i.e. the same displacement time) by using the time domain analysis method are shown in Figures 6-8 .

[0062] Step 5:

[0063] Then, the T2 relaxation time distribution of heavy water in the mixed solution of ethanol and water in the porous medium is obtained by using the difference spectrum method, i.e. subtracting the T2 relaxation time distribution of ethanol in the core at the same displacement time point y2 from the T2 relaxation time distribution of the mixed solution of ethanol and water in the core at the same displacement time point y1, and is recorded as y4.

[0064] Step 6:

[0065] According to the NMR signal of water separated from the mixed solution of ethanol and water in the porous medium obtained by using the difference spectrum method, the critical value of PV of ethanol injection is accurate.

[0066] According to the foregoing steps, in the porous medium, the T2 relaxation time distribution of the separated water in the mixed fluid of ethanol and water gradually increases with the decrease of the proportion of water in the porous medium, and the difference between the T2 spectrum distribution and signal amount of water in the mixed solution of ethanol and water and the T2 relaxation time distribution of the mixed solution of heavy water and water with the same mass ratio (i.e. the same displacement time) gradually increases, that is, there is a critical mass ratio of water and ethanol in the separation of the nuclear magnetic signal of water in the mixed solution of ethanol and water in the porous medium. When the mass ratio of water and ethanol in the mixed solution of ethanol and water in the porous medium is greater than the critical mass ratio, the T2 relaxation time of the separated water in the mixed solution of ethanol and water in the porous medium is the most accurate, and if it is less than this critical value, the T2 relaxation time error of the separated water in the mixed solution of ethanol and water in the porous medium is larger, as shown in Figures 2-4

[0067] 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 evaluated by the size of the square sum of the dimensionless nuclear magnetic signal difference between them. The turning point of the square sum of the dimensionless nuclear magnetic signal difference between the separated water signal in the mixed solution of ethanol and water in the porous medium and the nuclear magnetic signal of the mixed solution 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 and the sharply changing region of the square sum of the dimensionless nuclear magnetic signal difference, and the intersection point of the two fitting curves is the turning point of the square sum of the nuclear magnetic signal difference, which is also the PV critical value of ethanol injection (i.e. the critical time of ethanol injection) when the nuclear magnetic signal of the separated water in the mixed solution of ethanol and water in the porous medium is accurate, as shown in Figure 5 and Table 2.

[0068] Table 2

[0069] Core Porosity / % Permeability / mD Critical time of ethanol injection / min Critical value of ethanol injection PV 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

[0070] Step 7:

[0071] According to the time domain analysis method, the PV critical value of ethanol injection when the nuclear magnetic signal of the separated water in the mixed solution of ethanol and water in the porous medium is accurate.

[0072] ​Referring to the sixth step, the NMR signal y4 of water in the ethanol and water mixed solution in the porous medium obtained by the time domain analysis method, and the NMR signal y3 of the heavy water and water mixed solution with the same mass ratio (i.e. the same displacement time) can be known. In the porous medium, the T2 relaxation time distribution of the separated water in the ethanol and water mixed fluid decreases with the decrease of the proportion of water in the porous medium. The difference between the T2 spectrum distribution and the 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 a critical mass ratio of water and ethanol for the NMR signal separation of water in the ethanol and water mixed solution in the porous medium. When the mass ratio of water and 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 separated water in the ethanol and water mixed solution in the porous medium is the most accurate. If the mass ratio is less than the critical value, the error of the T2 relaxation time of the separated water in the ethanol and water mixed solution in the porous medium is large, as shown in FIG. 8. Figures 6-8

[0073] The NMR signal y4 of water in the ethanol and water mixed solution in the porous medium obtained by the time domain analysis method, and the NMR signal y3 of the heavy water and water mixed solution with the same mass ratio (i.e. the same displacement time) are evaluated by the size of the square sum of the dimensionless NMR signal difference between the two to evaluate the NMR signal separation accuracy of water in the ethanol and water mixed solution in the porous medium. The turning point of the square sum of the dimensionless NMR signal difference between the separated water signal in the ethanol and water mixed solution in the porous medium and the NMR signal of the heavy water and water with the same mass ratio (i.e. the same displacement time) corresponds to the fitting curves of the stable region and the sharply changing region of the square sum of the dimensionless NMR signal difference. The intersection of the two fitting curves is the turning point of the square sum of the NMR signal difference, which is also the PV critical value of the ethanol injection (i.e. the critical time of the ethanol injection) when the NMR signal of the separated water in the ethanol and water mixed solution in the porous medium is accurate, as shown in FIG. 9 and Table 3. Figure 9

[0074] Table 3

[0075] Core Porosity / % Permeability / mD Critical time of ethanol injection / min Critical value of ethanol injection PV 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

[0076] Step 8:

[0077] For different physical properties of the core, the PV critical value (or the critical time of the ethanol injection) obtained by the difference spectrum method and the time domain analysis method is not equal, as shown in Table 4 and Table 5.

[0078] Table 4: PV critical value of ethanol injection when the NMR signal of water separated from the ethanol and water mixed solution in the porous medium by the difference spectrum method is accurate

[0079] Core Porosity / % Permeability / mD Critical time of ethanol injection / min Critical value of ethanol injection PV 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

[0080] ​​Table 5: Critical PV values ​​of ethanol injection when the NMR signal of water in ethanol / water mixture in porous media is accurate using the time domain analysis method

[0081] Core Porosity / % Permeability / mD Critical time of ethanol injection / min Critical value of ethanol injection PV 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

[0082] Since the accuracy of separating the nuclear magnetic signals of ethanol and water in porous media is positively correlated with the mass of water, the critical value of ethanol injection PV obtained by the difference spectrum method and time domain analysis method is also correlated with the porosity of the core. When the nuclear magnetic resonance method is used to separate the nuclear magnetic signals of water in a mixed solution of ethanol and water in porous media, that is, when ethanol displaces a 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 error of the T2 relaxation time of the separated water is large. When using ethanol to displace a water-bearing core, it is necessary to inject as much ethanol as possible, that is, the critical value of ethanol injection PV is as large as possible. Figure 10 As shown in the figure, when the core porosity is greater than 9.3%, the ethanol injection PV critical value of the time domain analysis is larger; when the core porosity is less than 9.3%, the ethanol injection PV critical value of the differential spectrum method is larger. Therefore, it is necessary to reasonably choose the differential spectrum method or the time domain analysis method to separate the nuclear magnetic resonance signals of water in the porous medium mixed solution according to the core porosity.

[0083] 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 scope of protection of the present invention.

Claims

1. A method for separating the nuclear magnetic signal of water in miscible solutions in a porous medium, characterized in that, The method comprises the following steps: S1, selecting a porous medium core sample to be tested, drying the core sample to measure the dry weight of the core sample, and measuring the dry core gas porosity and permeability; S2, vacuumizing the dry core sample, then pressurizing and injecting saturated pure water, connecting a nuclear magnetic resonance online displacement device, and placing the water-saturated core sample into a nuclear magnetic resonance coil, wrapping the core sample with a heat shrink tube, applying confining pressure, then injecting ethanol into the water-saturated core sample at a constant speed to simulate the displacement process, and obtaining the T2 relaxation time distribution of the core sample at different time points, denoted as y1; S3, placing the core sample after the experiment in step S2 into an oven to dry, measuring the weight of the dried core sample, wrapping the dry core sample with a heat shrink tube, placing the core sample into a nuclear magnetic resonance coil, wrapping the core sample with a heat shrink tube, applying confining pressure, then injecting ethanol into the dry core sample at a constant speed to simulate the displacement process, and obtaining the T2 relaxation time distribution of the core sample at different time points, denoted as y2; S4, placing the core sample after the experiment in step S3 into an oven to dry, measuring the weight of the dried core sample, vacuumizing the dry core sample, then pressurizing and injecting saturated pure water, wrapping the water-saturated core sample with a heat shrink tube, placing the core sample into a nuclear magnetic resonance coil, applying confining pressure, then injecting heavy water into the water-saturated core sample at a constant speed to simulate the displacement process, and obtaining the T2 relaxation time distribution of the core sample, denoted as y3; S5, using the difference spectrum method, subtracting the T2 relaxation time distribution y2 of ethanol in the core sample at the same displacement time point from the mixed T2 relaxation time distribution y1 of ethanol and water in the core sample 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, denoted as y4; S6, comparing the relaxation time y4 of water in the core sample at the same time point with the relaxation time y3 of heavy water and water, when the ethanol injection amount is less than a certain critical value, the nuclear magnetic signal y4 of the separated water in the mixed solution of ethanol and water in the porous medium obtained by the difference spectrum method is the most accurate, and the ethanol injection amount critical value is the ethanol injection PV critical value of the difference spectrum method; S7, according to the time domain analysis method, when the nuclear magnetic signal of the separated water in the mixed solution of ethanol and water in the porous medium is accurate, the corresponding ethanol injection amount critical value is the ethanol injection PV critical value of the time domain analysis method; S8, when ethanol is used to displace the water-containing core sample, as much ethanol as possible needs to be injected, that is, the ethanol injection PV critical value should be as large as possible, therefore, the difference spectrum method or the time domain analysis method needs to be selected to separate the nuclear magnetic signal of water in the mixed solution of ethanol and water in the porous medium according to the size of the core sample porosity.

2. The method for the separation of the NMR signal of water in miscible solutions within porous media according to claim 1, characterized in that: The nuclear magnetic test parameters of the nuclear magnetic detector are as follows: the minimum echo interval TE is 0.1 ms, the waiting time TW is set to 8000 ms, and the echo number NECH is 6000 at the maximum.

3. The method for separating the NMR signal of water in a miscible solution within a porous medium according to claim 1, wherein: In step S1, the dry core sample is placed into a vacuumizing and pressurizing saturation device to be vacuumized for 6 hours, then pressurized to 20 MPa to saturate pure water for 6 hours, and the water-saturated core sample is weighed to obtain the water-saturated porosity of the core sample.

4. The method for separating the nuclear magnetic signal 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 T2 relaxation time distribution of the mixture of ethanol and water in the core is as follows: connecting the online displacement device of nuclear magnetic resonance, placing the core into the 70mm nuclear magnetic resonance coil, wrapping the core with a heat shrink tube, surrounding the core with fluorine oil at a pressure of 4MPa, injecting ethanol into the water-saturated core at a constant displacement rate of 0.02mL / min, performing displacement test for 3h, testing the T2 relaxation time distribution of the core every certain time interval by using the nuclear magnetic instrument, and recording the result as y1.

5. The method for separating the nuclear magnetic signal 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: placing the core after step S2 into an oven, drying the core at a temperature of 100℃ for 12h, testing the weight of the dried core, wrapping the dried core with a heat shrink tube, placing the core into the 70mm nuclear magnetic resonance coil, wrapping the core with a heat shrink tube, surrounding the core with fluorine oil at a pressure of 4MPa, injecting ethanol into the dried core at a constant displacement rate of 0.02mL / min, performing displacement for 3h, testing the T2 relaxation time distribution of the core every certain time interval by using the nuclear magnetic instrument, and recording the result as y2.

6. The method for separating the nuclear magnetic signal of water in a miscible solution in a porous medium according to claim 5, characterized in that, In step S4, the method for obtaining the T2 relaxation time distribution of the mixture of heavy water and water in the core at the same displacement time point as y1 is as follows: drying the core at a temperature of 100℃ for 12h, testing the weight of the dried core, placing the dried core into the vacuum pressurization saturation device, vacuumizing the core for 6h, then pressurizing the core to 20MPa to saturate the core with pure water for 6h, wrapping the water-saturated core with a heat shrink tube, placing the core into the 70mm nuclear magnetic resonance coil, surrounding the core with fluorine oil at a pressure of 4MPa, injecting heavy water into the water-saturated core at a constant rate of 0.02mL / min, performing displacement for 3h, testing the T2 relaxation time distribution of the core every certain time interval, and recording the result as y3.

7. The method for NMR signal separation of water in miscible solutions within porous media of claim 1, wherein: In step S6, the smaller the mass ratio 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 difference spectrum method in the mixed solution of ethanol and water 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 water to ethanol in the porous medium. When the mass ratio of water to ethanol in the porous medium is greater than the value, the T2 relaxation time of water separated by the difference spectrum method in the mixed solution of ethanol and water in the porous medium is the most accurate. If the mass ratio is less than the critical value, the T2 relaxation time of water separated by the difference spectrum method in the mixed solution of ethanol and water in the porous medium has a larger error. The critical value is the critical PV value of ethanol injection for the difference spectrum method.

8. The method for the separation of NMR signals of water in miscible solutions within porous media according to claim 1, wherein: 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 mixed solution of the porous medium; 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 mixed solution of the porous medium.

9. The method for the separation of NMR signals of water in miscible solutions within porous media according to claim 1, wherein: When obtaining the T2 relaxation time distribution of the core, the relaxation time T2 is tested every 20min.

10. The method for the separation of NMR signals of water in miscible solutions within porous media according to claim 1, wherein: The difference spectrum method is that the CPMG pulse sequence needs to determine the waiting time of proton polarization in the process of nuclear magnetic resonance test, that is, the time before a new CPMG pulse sequence starts after a CPMG pulse test ends; the time domain analysis method is that the echo train signal of a long waiting time TWL is subtracted from the echo train signal of a short waiting time TWS to obtain the echo train difference of another fluid, and then the T2 signal distribution of the fluid is obtained by using multi-exponential inversion to invert the echo train difference.

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