Method for Determining Inaccessible Pore Radius and Volume of Polymer Flooding Based on Nuclear Magnetic Resonance

Abstract

The present disclosure relates to a method for determining the inaccessible pore radius and volume of polymer flooding based on nuclear magnetic resonance. A method for determining the inaccessible pore volume of polymer flooding in a core is established based on the original high-pressure mercury injection test results and combined with the nuclear magnetic resonance test results before and after polymer flooding, aiming to accurately reflect the inaccessible pore radius and its volume of polymer flooding in the core. This method utilizes the advantages of non-destructive testing and fast velocity measurement of nuclear magnetic resonance testing, and uses it as the basic technical means for distinguishing before and after polymer flooding. By combining the high-pressure mercury injection test results with the nuclear magnetic resonance test results, the conversion coefficient between the relaxation time and the pore radius is obtained, and the inaccessible pore size and volume of polymer flooding are given, effectively solving the problem of large errors in the calculation results of the current empirical formula method and numerical simulation analysis method. Therefore, this method has broad development prospects.

Description

Technical Field

[0001] The present disclosure relates to the field of exploration and development of three types of oil reservoirs, and particularly to a method for characterizing the inaccessible pore volume of polymer flooding in three types of oil reservoirs based on nuclear magnetic resonance. Background Art

[0002] Most of the polymer molecules used in polymer flooding are high molecular polymers, which have a certain spatial size after dissolution. There are small porous medium pores in the oil reservoir during the polymer flooding process, resulting in their inability to pass through, thus forming an inaccessible pore volume. The evaluation basis of polymer flooding is the inaccessible pore volume. During polymer flooding, the inaccessible pore volume will not only reduce the recoverable reserves but also decrease its swept volume. In addition, when studying the seepage law of polymers, it is usually necessary to calculate the thickness of the depleted layer and the effective viscosity. If the inaccessible pore volume is not considered, it will affect the accuracy of the final calculated value.

[0003] Currently, most of the methods for measuring the inaccessible pores of polymer flooding calculate the gyration radius of polymer molecules through empirical formulas, and then determine the pore radius of the rock through high-pressure mercury injection experiments to obtain the inaccessible pore volume of polymer flooding; there is also a method of using numerical simulation to analyze the pore changes inside the rock. Although this method can intuitively reflect the pore change law of the rock, the anisotropy of the rock itself often leads to a large error between the simulation result and the actual process. Summary of the Invention

[0004] In view of this, the present disclosure proposes a method for determining the inaccessible pore volume of polymer flooding in a core based on nuclear magnetic resonance, which solves the problem that there are large errors in the calculation results of the current empirical formula method and numerical simulation analysis method.

[0005] To achieve the above invention purpose, the inventive concept of the method for determining the inaccessible pore volume of polymer flooding in a core based on nuclear magnetic resonance is as follows:

[0006] Based on the original high-pressure mercury injection test results of the core, combined with the nuclear magnetic resonance test results before and after polymer flooding of the core, the conversion coefficient between the relaxation time and the pore radius is obtained, and finally the inaccessible pore radius and volume of polymer flooding are determined.

[0007] Based on the above inventive concept, in the first aspect, the method for determining the inaccessible pore radius of polymer flooding according to the present disclosure includes:

[0008] Obtaining the pore radius distribution curve of the core through high-pressure mercury injection test;

[0009] Obtaining the T2 maps of the core before and after polymer flooding through nuclear magnetic resonance test;

[0010] Compare the pore radius distribution curve of the core with the nuclear magnetic resonance curve in the saturated water stage in the T2 maps of the core before and after polymer flooding to obtain the conversion coefficient between the fluid relaxation time T2 and the pore radius in the core pores;

[0011] Using the T2 maps before and after polymer flooding and combining the conversion coefficient, determine the inaccessible pore radius of the core during polymer flooding.

[0012] In the present disclosure and possible embodiments, the method for obtaining the pore radius distribution curve of the core through high-pressure mercury injection testing includes:

[0013] Draw the pore radius distribution curve of the core by changing the injection pressure.

[0014] In the present disclosure and possible embodiments, the method for drawing the pore radius distribution curve of the core by changing the injection pressure includes:

[0015] Inject mercury into the core pores under different injection pressure conditions. The injection pressure is the capillary force, and the capillary force corresponds to a capillary radius, which is equivalent to the core pore radius;

[0016] Calculate the pore radius of the core through the following formula to obtain the pore radius distribution curve of the core:

[0017]

[0018] where: p C refers to the capillary force, with the unit of MPa; r is the pore radius, with the unit of μm; σ is the interfacial tension of mercury, with the unit of N / m; θ is the wetting angle between mercury and the core surface, with the unit of °.

[0019] In the present disclosure and possible embodiments, the method for obtaining the T2 maps before and after polymer flooding through nuclear magnetic resonance testing includes:

[0020] After saturating the core with water, measure the rock pore volume and test the nuclear magnetic signal to obtain the nuclear magnetic signal amplitude in the saturated water stage, and draw the nuclear magnetic resonance test curve of the core in the saturated water stage;

[0021] Prepare a polymer solution with a set concentration using heavy water, inject a set amount of the polymer solution into the water-saturated core at a constant pressure, and then continue to inject using heavy water as the tail fluid, test the nuclear magnetic signal, and draw the nuclear magnetic resonance test curve after polymer flooding;

[0022] Use the nuclear magnetic resonance test curve of the core in the saturated water stage to draw the cumulative nuclear magnetic signal amplitude curve.

[0023] In the present disclosure and possible embodiments, the method for obtaining the conversion coefficient between the relaxation time and the pore radius by comparing the pore radius distribution curve of the core and the nuclear magnetic resonance test curve in the water saturation stage includes:

[0024] Make a comparison chart of the pore radius distribution curve of the core and the nuclear magnetic resonance test curve in the water saturation stage, so that the bottom value and the peak value of the pore radius distribution curve correspond to the bottom value and the peak value of the nuclear magnetic resonance test curve in the water saturation stage respectively, and calculate the conversion coefficient through the T2 value and the pore radius value of the corresponding points.

[0025] In the present disclosure and possible embodiments, the corresponding points are, for example, the maximum pore radius point, the minimum pore radius point, or the pore radius point with the highest distribution frequency.

[0026] In the present disclosure and possible embodiments, the method for determining the inaccessible pore radius of the core polymer flooding by using the T2 maps before and after the polymer flooding and combining the conversion coefficient includes:

[0027] On the T2 maps before and after the polymer flooding, find the T2 value corresponding to the separation point between the nuclear magnetic resonance test curve in the water saturation stage of the core and the nuclear magnetic resonance test curve after the polymer flooding, and multiply this T2 value by the conversion coefficient to obtain the inaccessible pore radius of the polymer flooding.

[0028] In a second aspect, the method for determining the inaccessible pore volume of polymer flooding based on nuclear magnetic resonance includes:

[0029] Use the method described in any item of the first aspect to determine the inaccessible pore radius r of the polymer flooding 不可及 , and combine with the conversion coefficient to calculate the T2 value corresponding to the inaccessible pore radius r 不可及 ;

[0030] According to the cumulative nuclear magnetic signal amplitude curve in the T2 maps before and after the polymer flooding, use the T2 value corresponding to the inaccessible pore radius r 不可及 to determine the cumulative nuclear magnetic signal amplitude, and calculate the conversion coefficient b of the inaccessible pore volume of the core;

[0031] The calculation formula for the conversion coefficient of the inaccessible pore volume is:

[0032]

[0033] In the formula: b is the conversion coefficient of the inaccessible pore volume, S r is the cumulative nuclear magnetic signal intensity at the T2 value corresponding to the inaccessible pore radius, S max is the maximum value of the cumulative nuclear magnetic signal amplitude.

[0034] In the present disclosure and possible embodiments, the inaccessible pore volume is calculated by the inaccessible pore volume conversion coefficient, and its calculation formula is:

[0035] V 不可及 = b×V;

[0036] Where: V 不可及 is the inaccessible pore volume, b is the inaccessible pore volume conversion coefficient, and V is the rock pore volume.

[0037] The beneficial effects of the present invention are:

[0038] The method for determining the inaccessible pore radius and volume of polymer flooding in a core based on nuclear magnetic resonance in the present disclosure establishes a method for determining the inaccessible pore volume of a core during polymer flooding based on the original high-pressure mercury injection test results and in combination with the nuclear magnetic resonance test results before and after polymer flooding, aiming to accurately reflect the inaccessible pore radius and its volume of the core during polymer flooding; this method utilizes the advantages of non-destructive testing and fast velocity measurement in nuclear magnetic resonance testing and uses it as the basic technical means for distinguishing before and after polymer flooding. By combining the high-pressure mercury injection test results with the nuclear magnetic resonance test results, the conversion coefficient between the relaxation time and the pore radius is obtained, so that the inaccessible pore size and volume during polymer flooding can be accurately given, effectively solving the problem of large errors in the calculation results of the current empirical formula method and numerical simulation analysis method. Therefore, this method has broad development prospects. Description of the Drawings

[0039] The drawings here are incorporated into the specification and form a part of this specification. These drawings show embodiments consistent with the present disclosure and are used together with the specification to illustrate the technical solutions of the present disclosure.

[0040] Figure 1 It is a comparison chart of the core pore radius distribution curve and the nuclear magnetic resonance test curve at the saturated water stage for the embodiments of the present disclosure;

[0041] Figure 2 It is the T2 map of the core before and after polymer flooding and its comparison chart with the pore radius distribution for the embodiments of the present disclosure; Detailed Embodiments

[0042] The following will detail various exemplary embodiments, features, and aspects of the present disclosure with reference to the drawings. The same reference numerals in the drawings denote elements with the same or similar functions. Although various aspects of the embodiments are shown in the drawings, the drawings do not have to be drawn to scale unless otherwise specified.

[0043] To solve the problems in the background art, the present disclosure provides a method for determining the inaccessible pore volume of a core during polymer flooding based on nuclear magnetic resonance. The general inventive concept is:

[0044] Based on the original high-pressure mercury injection test results of the core and combined with the nuclear magnetic resonance test results before and after polymer flooding in the core, the conversion coefficient between the relaxation time and the pore radius is obtained, and finally the inaccessible pore radius and volume of polymer flooding are determined.

[0045] Based on the above general inventive concept, the present disclosure provides specific implementable technical solutions. The following are detailed descriptions of the technical solutions through preferred embodiments to illustrate and prove the beneficial effects of the present invention.

[0046] The following is a preferred embodiment of the present disclosure. The method for determining the inaccessible pore volume of polymer flooding in a core based on nuclear magnetic resonance is as follows:

[0047] Step 10: Core preparation:

[0048] Select several cores with a core diameter of 2.5 cm, and dry the cores according to GB / T 29172-2012 "Core Analysis Method".

[0049] Step 20: Draw the pore radius distribution curve of the core through high-pressure mercury injection test, specifically as follows:

[0050] Use wire cutting to cut the core into cores with a length of 2 cm, and reserve the remaining core part; conduct high-pressure mercury injection test on the dried core sample, and obtain the pore radius distribution curve of the core by changing the injection pressure.

[0051] Assume that a number of capillary bundles with unequal diameters constitute the porous medium in the rock. Since mercury is a non-wetting liquid on the surface of the reservoir rock, when injecting mercury into the reservoir rock, the required injection pressure is higher than the capillary pressure in the rock pores. Since mercury needs to overcome the capillary force to enter the pores, the injection pressure at this time corresponds to the capillary force, the capillary radius corresponds to the pore radius, and the volume of injected mercury reflects the pore volume. Therefore, there is a corresponding capillary pressure balance point for each pressure point when mercury enters the pores. After injecting mercury into the core pores, draw the capillary pressure curve according to the mercury injection and withdrawal saturation and the applied displacement pressure.

[0052] In the high-pressure mercury injection method, mercury is injected into the core pores under different injection pressure conditions. During this process, the Washburn equation is satisfied, as shown in formula (1):

[0053]

[0054] Where: p C refers to the capillary pressure, unit MPa; r is the pore radius, unit μm; σ is the interfacial tension of mercury, unit N / m; θ is the wetting angle between mercury and the core surface, unit °.

[0055] The operation process of the high-pressure mercury injection test is as follows:

[0056] 1) Measure relevant physical parameters such as the mass, diameter, and permeability of the core under the initial state;

[0057] 2) Place the core in the experimental instrument, inject mercury under the set pressure, record the pressure value and the mercury injection volume after the pressure stabilizes, increase the injection pressure, and repeat the above experiment.

[0058] 3) Mercury needs to overcome the capillary force to enter the core pores, and the injection pressure at this time is the capillary force. At this time, there will be a capillary radius corresponding to the core pore radius. By changing the injection pressure, draw the core pore radius distribution curve.

[0059] Step 30: Through nuclear magnetic resonance testing, draw the T2 spectra before and after polymer flooding, specifically as follows:

[0060] 1) Select the remaining part of the core for nuclear magnetic resonance testing. Place the remaining part of the core in the intermediate container, evacuate it, pressurize it to saturate with water after evacuation, saturate with water under a pressure of 20 MPa, measure the mass of the core once every 24 hours. When the change in the core mass between two adjacent measurements is less than 1%, the core is saturated with water at this time. Measure the pore volume V of the rock and test the nuclear magnetic signal, and draw the nuclear magnetic resonance test curve in the core water saturation stage.

[0061] 2) Prepare a polymer solution with a certain concentration using heavy water. In order to eliminate the influence of the injection pressure on the lower limit of the pore radius swept by the polymer, the displacement experiment is carried out using the constant pressure injection method. Inject a certain amount of polymer solution at a constant pressure, and then continue to inject using heavy water as the tail fluid, test the nuclear magnetic signal, and draw the nuclear magnetic resonance test curve after polymer flooding;

[0062] 3) Correlate the cumulative nuclear magnetic signal amplitude at the end of the core water saturation stage with the pore volume V of the core. Then, based on the nuclear magnetic resonance test curve in the core water saturation stage, draw the cumulative nuclear magnetic signal amplitude curve. Any point on this cumulative nuclear magnetic signal amplitude curve represents the sum of the nuclear magnetic signal amplitudes before this point.

[0063] The nuclear magnetic resonance test curve in the core water saturation stage, the nuclear magnetic resonance test curve after polymer flooding, and the cumulative nuclear magnetic signal amplitude curve constitute the T2 spectra of the core before and after polymer flooding, as Figure 2 shown.

[0064] Step 40: Determine the inaccessible pore radius and volume of the polymer-flooded core through the experimental results of Step 20 and Step 30, specifically as follows:

[0065] Since the content of hydrogen nuclei in the closed reservoir core space is fixed, the fluid relaxation time of the core pores is related to the characteristics of the core.

[0066] In the nuclear magnetic resonance experiment of the core, the relaxation time of the fluid in the reservoir can be calculated by the following formula (2):

[0067]

[0068] Under the condition of rapid fluid diffusion, the fluid relaxation time T2 of the core pores can be approximately expressed as:

[0069]

[0070] In the formula: T 2B is the inherent relaxation time of the fluid, in ms; ρ2 is the surface relaxation rate of the pores where the fluid is located, in μm / ms; S / V is the ratio of the surface area to the volume of the pores where the fluid is located, which is inversely proportional to the pore size. T 2B is usually above 3000 ms. When the core pore size is small, T 2B is much larger than T2. At this time, the first term on the right side of formula (3) can be ignored, so there is:

[0071]

[0072] The ratio S / V in formula (4) is the specific surface area of the pores. It can be seen from formula (4) that for rocks with intergranular pores, the size of T2 is mainly determined by the lithology and the pore specific surface area S / V. If it is assumed that the pores are composed of ideal spheres, then S / V = 3 / r; if it is assumed that the throats are composed of ideal cylinders, then S / V = 2 / r, and formula (4) can be rewritten as

[0073]

[0074] In the formula: C is the transverse conversion coefficient; r is the pore radius; F S is the pore shape factor. For spherical pores, F S = 3, and for columnar throats, F S = 2. For a rock sample, the surface relaxation rate ρ2 can be approximately regarded as a constant.

[0075] It can be seen from formula (5) that T2 is proportional to r. In the embodiments of the present disclosure, the method for determining the proportional ratio of T2 to r is:

[0076] Put the pore radius distribution curve obtained in step 20 and the nuclear magnetic resonance test curve in the saturated water stage obtained in step 30 into one graph, as Figure 1As shown, make the bottom value and peak value of the pore radius distribution curve correspond to the bottom value and peak value of the nuclear magnetic resonance test curve in the saturated water stage, draw the mapping graph of pore radius and nuclear magnetic resonance signal amplitude. Then, through the T2 value and pore radius r value of the corresponding points, the conversion coefficient can be calculated. Obviously, this conversion coefficient is the proportional value of T2 and r. In the embodiment of the present disclosure, preferably, the corresponding points are the point of the maximum pore radius, the point of the minimum pore radius, and the point of the pore radius with the highest distribution frequency.

[0077] Through Figure 2 The T2 maps of the core Figure 2 before and after polymer flooding can obtain the law of fluid utilization in the core pores. It is found that when the pore radius is less than a certain radius, the nuclear magnetic resonance test curve in the saturated water stage of the core and the nuclear magnetic resonance test curve after polymer flooding coincide, and there is no obvious change in the T2 maps before and after polymer flooding. However, when the pore radius is greater than this radius, the nuclear magnetic resonance test curve in the saturated water stage of the core and the nuclear magnetic resonance test curve after polymer flooding are separated, and there is an obvious change in the T2 maps before and after polymer flooding. According to the definitions of inaccessible pores and accessible pores in polymer flooding, it can be determined that this radius is the inaccessible pore radius of polymer flooding.

[0078] Generally speaking, the T2 value corresponding to the point when the nuclear magnetic resonance test curve in the saturated water stage of the core coincides and then starts to separate from the nuclear magnetic resonance test curve after polymer flooding, and the pore radius corresponding to the T2 value multiplied by the conversion coefficient is the inaccessible pore radius of the core in polymer flooding.

[0079] Obviously, through the inaccessible pore radius, the T2 maps before and after polymer flooding can be divided into an inaccessible pore area and an accessible pore area.

[0080] Because the maximum value of the cumulative nuclear magnetic resonance signal intensity represents the pore volume V of the core, so the T2 value can be calculated through the inaccessible pore radius r 不可及 and the conversion coefficient. Then, on the cumulative nuclear magnetic resonance signal amplitude curve, using the cumulative nuclear magnetic resonance signal amplitude corresponding to the corresponding T2 value, calculate the conversion coefficient b of the inaccessible pore volume of the core. The calculation formula is shown in formula (6):

[0081]

[0082] In the formula: b is the conversion coefficient of the inaccessible pore volume, S r is the cumulative nuclear magnetic resonance signal intensity corresponding to the T2 value of the inaccessible pore radius, S max is the maximum value of the cumulative nuclear magnetic resonance signal amplitude.

[0083] Finally, calculate the inaccessible pore volume through the conversion coefficient of the inaccessible pore volume, as shown in formula (7):

[0084] V 不可及= b × V (7)

[0085] Where: V 不可及 is the inaccessible pore volume, b is the conversion coefficient of the inaccessible pore volume, and V is the pore volume of the rock.

[0086] Example

[0087] Taking a specific core as an example, the detailed process of using the method of the present disclosure to determine the inaccessible pore radius and volume of the core is described as follows:

[0088] 1. Rock sample preparation

[0089] Select a core sample with a required core diameter of 2.5 cm. Dry the core according to GB / T 29172-2012 "Core Analysis Method".

[0090] 2. High-pressure mercury injection test

[0091] Use wire cutting to cut the core into cores with a length of 2 cm, and reserve the remaining core part; conduct a high-pressure mercury injection test on the dried rock sample to obtain the pore radius distribution curve of the core by changing the injection pressure.

[0092] In the high-pressure mercury injection test, mercury is injected into the core pores under different injection pressure conditions. During this process, the Washburn equation is satisfied, as shown in formula (1):

[0093]

[0094] where p C refers to the capillary pressure, with the unit of MPa; r is the pore radius, with the unit of μm; σ is the interfacial tension of mercury, with the unit of N / m; θ is the wetting angle between mercury and the core surface, with the unit of °.

[0095] The operation process is as follows:

[0096] 1) Measure the relevant physical parameters of the core such as mass, diameter, and permeability in the initial state;

[0097] 2) Place the core in the experimental instrument, inject mercury at the set pressure, record the pressure value and the mercury injection volume after the pressure is stable, increase the injection pressure, and repeat the above experiment.

[0098] 3) Mercury needs to overcome the capillary force to enter the core pores, and the injection pressure at this time is the capillary force. At this time, there will be a capillary radius equivalent to the core pore radius. The pore radius distribution curve of the core obtained by changing the injection pressure is shown in the appendix Figure 1 .

[0099] 3. Nuclear magnetic resonance test

[0100] 1) Select the remaining core samples for nuclear magnetic resonance (NMR) testing. Place the remaining core samples in an intermediate container, evacuate it, and then pressurize to saturate with water at 20 MPa. Measure the mass of the core samples every 24 hours. When the change in the mass of the core samples between two adjacent measurements is less than 1%, the water saturation of the core samples is completed. Measure the pore volume V of the rock and test the NMR signal, and plot the NMR test curve during the water saturation stage of the core samples.

[0101] 2) Prepare a polymer solution with a certain concentration using heavy water. To eliminate the influence of injection pressure on the lower limit of the pore radius swept by the polymer, a constant-pressure injection method is used to conduct the displacement experiment. Inject a certain amount of polymer solution at a constant pressure, and then use heavy water as the tail fluid to continue injection. Test the NMR signal and plot the NMR test curve after polymer flooding.

[0102] 3) Using the NMR test curve during the water saturation stage of the core samples, plot the cumulative NMR signal amplitude curve. Complete the drawing of the T2 spectra of the core samples before and after polymer flooding, as shown in the appendix Figure 2 .

[0103] 4. Experimental data processing and analysis

[0104] By comparing the pore radius distribution of the core samples with the T2 spectra before and after polymer flooding, as shown in the appendix Figure 2 shows that the pore radius corresponding to the relaxation time of 1 ms is 0.062 μm, that is, the conversion coefficient is 0.062.

[0105] In this embodiment, from the T2 spectra of the core samples before and after polymer flooding in Figure 2 it can be seen that when the pore radius is less than 1.12 μm, the NMR test curve during the water saturation stage of the core samples coincides with the NMR test curve after polymer flooding, and there is no obvious change in the T2 spectra before and after polymer flooding. However, when the pore radius is greater than 1.12 μm, the NMR test curve during the water saturation stage of the core samples separates from the NMR test curve after polymer flooding, and there is an obvious change in the T2 spectra before and after polymer flooding. Thus, the inaccessible pore radius of polymer flooding can be obtained as 1.12 μm.

[0106] And this inaccessible pore radius divides the T2 spectra into two parts, as shown in the appendix Figure 2 shows, which are divided into the inaccessible pore region and the accessible pore region.

[0107] Then, based on the cumulative NMR curve in Figure 2 , through the cumulative NMR signal intensity at the T2 value corresponding to the inaccessible pore radius r 不可及 , calculate the conversion coefficient b of the inaccessible pore volume of the core samples. The calculation formula is shown in Equation (6):

[0108]

[0109] In the formula: b is the conversion coefficient of the inaccessible pore volume, S r is the cumulative nuclear magnetic resonance signal intensity corresponding to the T2 value of the inaccessible pore radius, S max is the maximum value of the cumulative nuclear magnetic resonance signal intensity.

[0110] Finally, the inaccessible pore volume of the core polymer flooding is calculated through the conversion coefficient of the inaccessible pore volume, and the calculation formula is as shown in formula (7):

[0111] V 不可及 = b × V (7)

[0112] In the formula: V 不可及 is the inaccessible pore volume, b is the conversion coefficient of the inaccessible pore volume, and V is the rock pore volume. The final calculation results are shown in Table 1:

[0113] Table 1 Calculation Results Table

[0114] <![CDATA[r 不可及 / μm]]> <![CDATA[S r / a.u]]> <![CDATA[S max / a.u]]> b <![CDATA[V / cm 3 > <![CDATA[V 不可及 / cm3]]> 1.12 2748.35 14193.39 0.19 5.91 1.12

[0115] It can be understood that, without violating the principle logic, the above-mentioned method embodiments mentioned in the present disclosure can be combined with each other to form a combined embodiment. Due to space limitations, the present disclosure will not elaborate further.

[0116] The above has described the embodiments of the present disclosure. The above description is exemplary and not exhaustive, and is not limited to the disclosed embodiments. Many modifications and variations are obvious to those of ordinary skill in the art in the technical field without departing from the scope and spirit of the described embodiments. The choice of terms used herein is intended to best explain the principles of the embodiments, practical applications, or technical improvements to the technologies in the market, or to enable other ordinary technical personnel in the technical field to understand the embodiments disclosed herein.

Claims

1. A method for determining the inaccessible pore radius of polymer flooding based on nuclear magnetic resonance, characterized in that, Comprising: Obtaining the pore radius distribution curve of the core through high-pressure mercury injection testing; Obtaining the T2 maps of the core before and after polymer flooding through nuclear magnetic resonance testing; Comparing the pore radius distribution curve of the core with the nuclear magnetic resonance curve in the saturated water stage in the T2 maps of the core before and after polymer flooding to obtain the conversion coefficient between the fluid relaxation time T2 and the pore radius in the core pores; Using the T2 maps before and after polymer flooding and combining with the conversion coefficient to determine the inaccessible pore radius of the core during polymer flooding; The method for obtaining the conversion coefficient between the relaxation time and the pore radius by comparing the pore radius distribution curve of the core and the nuclear magnetic resonance test curve in the saturated water stage includes: Making a comparison chart of the pore radius distribution curve of the core and the nuclear magnetic resonance test curve in the saturated water stage, making the bottom value and peak value of the pore radius distribution curve correspond to the bottom value and peak value of the nuclear magnetic resonance test curve in the saturated water stage respectively, and calculating the conversion coefficient through the T2 value and pore radius value of the corresponding points; The method for determining the inaccessible pore radius of the core during polymer flooding by using the T2 maps before and after polymer flooding and combining with the conversion coefficient includes: On the T2 maps before and after polymer flooding, finding the T2 value corresponding to the separation point between the nuclear magnetic resonance test curve in the saturated water stage of the core and the nuclear magnetic resonance test curve after polymer flooding, and multiplying this T2 value by the conversion coefficient to obtain the inaccessible pore radius during polymer flooding.

2. The method for determining the inaccessible pore radius of polymer flooding based on nuclear magnetic resonance according to claim 1, wherein The method for obtaining the pore radius distribution curve of the core through high-pressure mercury injection testing includes: Drawing the pore radius distribution curve of the core by changing the injection pressure.

3. The method for determining the inaccessible pore radius of polymer flooding based on nuclear magnetic resonance according to claim 2, wherein The method for drawing the pore radius distribution curve of the core by changing the injection pressure includes: Injecting mercury into the core pores under different injection pressure conditions. The injection pressure is the capillary force, and the capillary force corresponds to a capillary radius, which is equivalent to the core pore radius; Calculating the pore radius of the core through the following formula to obtain the pore radius distribution curve of the core: where: p C refers to capillary force, with the unit of MPa; r is the pore radius, with the unit of μm; σ is the interfacial tension of mercury, with the unit of N / m; θ is the wetting angle between mercury and the core surface, with the unit of °.

4. The method for determining the inaccessible pore radius of polymer flooding based on nuclear magnetic resonance according to any one of claims 1-3, characterized in that, The method for obtaining the T2 maps before and after polymer flooding through nuclear magnetic resonance testing includes: After saturating the core with water, measuring the pore volume of the core and testing the nuclear magnetic signal to obtain the nuclear magnetic signal amplitude in the saturated water stage, and drawing the nuclear magnetic resonance test curve in the saturated water stage of the core; Preparing a polymer solution with a set concentration using heavy water, injecting a set amount of the polymer solution into the water-saturated core at a constant pressure, then using heavy water as the tail fluid to continue injecting, testing the nuclear magnetic signal, and drawing the nuclear magnetic resonance test curve after polymer flooding; Using the nuclear magnetic resonance test curve in the saturated water stage of the core to draw the cumulative nuclear magnetic signal amplitude curve.

5. The method for determining the inaccessible pore radius during polymer flooding based on nuclear magnetic resonance according to claim 1, characterized in that: The corresponding points are such as the maximum pore radius point, the minimum pore radius point, or the pore radius point with the highest distribution frequency.

6. A method for determining the inaccessible pore volume in polymer flooding based on nuclear magnetic resonance, characterized in that, Comprising: Determine the inaccessible pore radius r of polymer flooding using the method described in any one of claims 1-5 不可及 , and calculate the inaccessible pore radius r in combination with the conversion coefficient 不可及 corresponding T2 value; According to the cumulative nuclear magnetic signal amplitude curve in the T2 spectrum before and after polymer flooding, using the inaccessible pore radius r 不可及 corresponding T2 value, determine the cumulative nuclear magnetic signal amplitude, and calculate the conversion coefficient b of the inaccessible pore volume of the core; The calculation formula for the inaccessible pore volume conversion coefficient is: Where: b is the conversion coefficient of the inaccessible pore volume, S r is the cumulative nuclear magnetic resonance signal intensity at the T2 value corresponding to the inaccessible pore radius, S max is the maximum value of the cumulative nuclear magnetic resonance signal amplitude.

7. The method for determining the inaccessible pore volume during polymer flooding based on nuclear magnetic resonance according to claim 6, characterized in that: Calculate the inaccessible pore volume through the inaccessible pore volume conversion coefficient, and its calculation formula is: V 不可及 = b × V; Where: V 不可及 is the inaccessible pore volume, b is the conversion coefficient of the inaccessible pore volume, and V is the pore volume of the rock.

Images