Method for determining radius and volume of pores inaccessible to polymer flooding based on nuclear magnetic resonance

Abstract

The invention relates to a method for determining the radius and volume of polymer flooding inaccessible pores based on nuclear magnetic resonance, and establishes a method for determining the volume of the polymer flooding inaccessible pores of a rock core based on an original high-pressure mercury injection test result in combination with nuclear magnetic resonance test results before and after polymer flooding. The method aims at accurately reflecting the radius and the volume of pores inaccessible to rock core polymer flooding. According to the method, the advantages of nondestructive testing, high speed measurement and the like of nuclear magnetic resonance testing are utilized, the nuclear magnetic resonance testing is used as a basic technical means for testing differences before and after polymer flooding, a high-pressure mercury injection testing result and a nuclear magnetic resonance testing result are combined to obtain a conversion coefficient of relaxation time and pore radius, and the size and the volume of pores inaccessible to polymer flooding are given. The method effectively solves the problem that calculation results of an existing empirical formula method and a numerical simulation analysis method have large errors, and therefore the method has wide development prospects.

Description

Technical Field

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

[0002] Most polymer molecules used in polymer flooding are high molecular weight polymers, which have a certain spatial size after dissolution. During the polymer flooding process, there are smaller porous media pores in the oil layer, which make it impossible for them to pass through, thus forming an inaccessible pore volume. The evaluation basis of polymer flooding is the inaccessible pore volume. During the polymer flooding process, the inaccessible pore volume will not only reduce the recoverable reserves, but also reduce 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, the accuracy of the final calculated value will be affected.

[0003] At present, most of the methods for measuring the inaccessible pores for polymer flooding are to calculate the gyration radius of polymer molecules through empirical formulas, and then determine the rock pore radius through high-pressure mercury injection experiments to obtain the inaccessible pore volume for 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 law of rock pore changes, the anisotropy of the rock itself often leads to large errors between the simulation results 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 core polymer flooding based on nuclear magnetic resonance to solve the problem that the calculation results of the current empirical formula method and numerical simulation analysis method have large errors.

[0005] To achieve the above-mentioned invention object, the inventive concept of the method for determining the inaccessible pore volume of core polymer flooding based on nuclear magnetic resonance is:

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

[0007] Based on the above inventive concept, in a first aspect, the method for determining the inaccessible pore radius of polymer flooding based on nuclear magnetic resonance described in the present disclosure comprises:

[0008] The pore radius distribution curve of the core is obtained by high-pressure mercury injection testing;

[0009] The T2 spectrum of the core before and after polymer flooding is obtained by nuclear magnetic resonance testing;

[0010] By comparing the pore radius distribution curve of the core with the saturated water stage nuclear magnetic resonance curve in the T2 spectrum of the core before and after polymer flooding, a conversion coefficient between the fluid relaxation time T2 in the core pores and the pore radius is obtained;

[0011] The inaccessible pore radius of the core polymer flooding is determined by using the T2 spectrum before and after polymer flooding in combination with the conversion coefficient.

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

[0013] The pore radius distribution curve of the core is drawn by changing the injection pressure.

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

[0015] Injecting mercury into the core pores under different injection pressure conditions, the injection pressure being the capillary force, the capillary force corresponding to a capillary radius, and the capillary radius being equal to the core pore radius;

[0016] The pore radius of the core is calculated by the following formula to obtain the pore radius distribution curve of the core:

[0017]

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

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

[0020] After the core is saturated with water, the rock pore volume is measured and the nuclear magnetic resonance signal is tested to obtain the nuclear magnetic resonance signal amplitude at the water saturation stage, and a nuclear magnetic resonance test curve of the core at the water saturation stage is drawn;

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

[0022] The cumulative nuclear magnetic resonance signal amplitude curve is drawn using the nuclear magnetic resonance test curve of the core saturated water stage.

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

[0024] Make a comparison chart of the pore radius distribution curve of the core and the nuclear magnetic resonance test curve of the saturated water stage, so that 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 of the saturated water stage respectively, and calculate the conversion coefficient through the T2 value and pore radius value of the corresponding point.

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

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

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

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

[0029] The inaccessible pore radius r of polymer flooding is determined by any method described in the first aspect. 不可及 , combined with the conversion coefficient, calculate the inaccessible pore radius r 不可及 The corresponding T2 value;

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

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

[0032]

[0033] Where: b is the conversion coefficient of 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 accumulated 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 the 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 disclosed method for determining the inaccessible pore radius and volume of polymer flooding based on nuclear magnetic resonance establishes a method for determining the inaccessible pore volume of core polymer flooding based on the original high-pressure mercury injection test results combined with the nuclear magnetic resonance test results before and after polymer flooding, aiming to accurately reflect the inaccessible pore radius and volume of core polymer flooding; the method utilizes the advantages of nuclear magnetic resonance testing such as non-destructive testing and fast speed measurement, and uses it as a basic technical means to distinguish before and after testing polymer flooding, combines the high-pressure mercury injection test results with the nuclear magnetic resonance test results to obtain the conversion coefficient between relaxation time and pore radius, thereby accurately giving the inaccessible pore size and volume of polymer flooding, effectively solving the problem of large errors in the calculation results of the current empirical formula method and numerical simulation analysis method, so the method has broad development prospects. BRIEF DESCRIPTION OF THE DRAWINGS

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

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

[0041] Figure 2 The T2 spectrum of the core before and after polymer flooding and its comparison with the pore radius distribution in the embodiment of the present disclosure; DETAILED DESCRIPTION

[0042] Various exemplary embodiments, features and aspects of the present disclosure will be described in detail below with reference to the accompanying drawings. The same reference numerals in the accompanying drawings represent elements with the same or similar functions. Although various aspects of the embodiments are shown in the accompanying drawings, the drawings are not necessarily drawn to scale unless otherwise specified.

[0043] To solve the background technical problem, the present disclosure provides a method for determining the inaccessible pore volume of core polymer flooding based on nuclear magnetic resonance, and the general inventive concept thereof is:

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

[0045] Based on the general inventive concept, the present disclosure provides a specific and implementable technical solution, which is described in detail below through preferred embodiments to reflect 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 core polymer flooding based on nuclear magnetic resonance described in this embodiment has the following steps:

[0047] Step 10: Core preparation:

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

[0049] Step 20: Draw a core pore radius distribution curve through high-pressure mercury injection testing, as follows:

[0050] The core was cut into 2 cm long cores by wire cutting, and the remaining core was kept for later use. The dried rock samples were subjected to high pressure mercury injection testing, and the core pore radius distribution curve was obtained by changing the injection pressure.

[0051] Assume that several capillary bundles of unequal diameters constitute a porous medium in the rock. Since mercury is a non-wetting phase liquid for the surface of the reservoir rock, when mercury is injected 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 when entering 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, the pressure point corresponding to mercury entering the pore has a corresponding capillary pressure equilibrium point. After mercury is injected into the core pores, the capillary pressure curve is drawn according to the advancing and retreating mercury 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 It refers to the capillary pressure, in MPa; r is the pore radius, in μm; σ is the interfacial tension of mercury, in N / m; θ is the wetting angle between mercury and the core surface, in degrees.

[0055] The operating procedure of the high pressure mercury injection test is as follows:

[0056] 1) Determine the core mass, diameter, permeability and other relevant physical parameters in the initial state;

[0057] 2) Place the core in the experimental instrument and inject mercury at the set pressure. After the pressure stabilizes, record the pressure value and the volume of mercury injected. Increase the injection pressure and repeat the above experiment.

[0058] 3) Mercury needs to overcome capillary force to enter the core pores, and the injection pressure at this time is the capillary force. At this time, it will correspond to a capillary radius equal to the core pore radius. The core pore radius distribution curve is drawn by changing the injection pressure.

[0059] Step 30: Draw the T2 spectrum before and after polymer flooding through nuclear magnetic resonance testing, as follows:

[0060] 1) Select the remaining cores for NMR testing, place the remaining cores in an intermediate container, evacuate, pressurize and saturate with water after evacuation, pressurize and saturate with water at 20 MPa, measure the mass of the cores every 24 hours, and when the change in the mass of the cores between two consecutive measurements is less than 1%, the cores are saturated with water. Measure the rock pore volume V and test the NMR signal, and draw the NMR test curve of the core saturated with water stage.

[0061] 2) Use heavy water to prepare a polymer solution of a certain concentration. In order to eliminate the influence of injection pressure on the lower limit of polymer pore radius, a constant pressure injection method is used to carry out displacement experiments. A certain amount of polymer solution is injected at a constant pressure, and then heavy water is used as tail liquid to continue injection, test the nuclear magnetic resonance signal, and draw the nuclear magnetic resonance test curve after polymer flooding;

[0062] 3) The accumulated NMR signal amplitude at the end of the core saturated water stage corresponds to the pore volume V of the core. Then, based on the NMR test curve of the core saturated water stage, a cumulative NMR signal amplitude curve can be drawn. Any point on the cumulative NMR signal amplitude curve represents the sum of the NMR signal amplitudes before that point.

[0063] The core water-saturated stage NMR test curve, the NMR test curve after polymer flooding and the cumulative NMR signal amplitude curve constitute the T2 spectrum before and after core polymer flooding, such as Figure 2 shown.

[0064] Step 40: Determine the radius and volume of inaccessible pores in the polymer flooding core according to the experimental results of steps 20 and 30, as follows:

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

[0066] In the core NMR experiment, 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 in the core pores can be approximately expressed as:

[0069]

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

[0071]

[0072] The ratio S / V in formula (4) is the specific area of ​​pores. It can be seen from formula (4) that the size of T2 of rocks with intergranular pores is mainly determined by the lithology and the specific area of ​​pores 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 throat is composed of ideal cylinders, then S / V = 2 / r. Formula (4) can be rewritten as

[0073]

[0074] Where: C is the lateral conversion coefficient; r is the pore radius; F S is the pore shape factor, for spherical pores F S =3, for cylindrical throat 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 embodiment of the present disclosure, the method for determining the proportional ratio of T2 to r is:

[0076] The pore radius distribution curve obtained in step 20 and the saturated water stage NMR test curve obtained in step 30 are put into one graph, as shown in FIG. Figure 1As shown, the pore radius 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, and a comparison diagram of the pore radius and the nuclear magnetic signal amplitude is drawn. Then, the conversion coefficient can be calculated by the T2 value and the pore radius r value of the corresponding point. Obviously, the conversion coefficient is the proportional value of T2 and r. In the embodiment disclosed in the present disclosure, the preferred corresponding points are the maximum pore radius point, the minimum pore radius point, and the pore radius point with the highest distribution frequency.

[0077] pass Figure 2 The T2 spectra of the core before and after polymer flooding can be used to obtain the mobilization law of the fluid in the core pores. It is found that when the pore radius is less than a certain radius, the nuclear magnetic resonance test curve of the core in the water-saturated stage and the nuclear magnetic resonance test curve after polymer flooding are overlapped, and the T2 spectra before and after polymer flooding do not change significantly. When the pore radius is greater than this radius, the nuclear magnetic resonance test curve of the core in the water-saturated stage and the nuclear magnetic resonance test curve after polymer flooding are separated, and the T2 spectra before and after polymer flooding change significantly. According to the definition of inaccessible pores and accessible pores for polymer flooding, it can be determined that this radius is the inaccessible pore radius of polymer flooding.

[0078] In summary, the T2 value corresponding to the point where the overlap between the nuclear magnetic resonance test curve of the core in the water-saturated stage and the nuclear magnetic resonance test curve after polymer flooding ends and begins to separate, and the pore radius corresponding to the T2 value multiplied by the conversion coefficient, is the inaccessible pore radius of the core polymer flooding.

[0079] Obviously, the T2 spectrum before and after polymer flooding can be divided into the inaccessible pore area and the accessible pore area according to the inaccessible pore radius.

[0080] Because the maximum value of the cumulative NMR signal intensity represents the pore volume V of the core, the inaccessible pore radius r 不可及 The T2 value is calculated by using the conversion coefficient. Then, on the cumulative NMR signal amplitude curve, the inaccessible pore volume conversion coefficient b of the core is calculated using the cumulative NMR signal amplitude at the corresponding T2 value. The calculation formula is shown in formula (6):

[0081]

[0082] Where: b is the conversion coefficient of 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 accumulated nuclear magnetic signal amplitude.

[0083] Finally, the inaccessible pore volume is calculated by the inaccessible pore volume conversion coefficient, see formula (7):

[0084] V 不可及=b×V (7)

[0085] 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.

[0086] Example

[0087] The following takes a specific core as an example to illustrate the detailed process of determining the inaccessible pore radius and volume of the core by polymer flooding using the disclosed method, as follows:

[0088] 1. Rock sample preparation

[0089] Select core samples with a core diameter of 2.5 cm. Dry the cores according to GB / T 29172-2012 Core Analysis Methods.

[0090] 2. High-pressure mercury injection test

[0091] The core was cut into 2 cm long cores by wire cutting, and the remaining core was kept for later use. The dried rock samples were subjected to high pressure mercury injection testing, and the core pore radius distribution curve was obtained 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 It refers to the capillary pressure, unit is MPa; r is the pore radius, unit is μm; σ is the interfacial tension of mercury, unit is N / m; θ is the wetting angle between mercury and the core surface, unit is °.

[0095] The operation process is as follows:

[0096] 1) Determine the core mass, diameter, permeability and other relevant physical parameters in the initial state;

[0097] 2) Place the core in the experimental instrument and inject mercury at the set pressure. After the pressure stabilizes, record the pressure value and the volume of mercury injected. Increase the injection pressure and repeat the above experiment.

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

[0099] 3. Magnetic resonance imaging

[0100] 1) Select the remaining cores for NMR testing, place the remaining cores in an intermediate container, evacuate, pressurize and saturate with water after evacuation, pressurize and saturate with water at 20 MPa, measure the mass of the cores every 24 hours, and when the change in the mass of the cores between two consecutive measurements is less than 1%, the cores are saturated with water. Measure the rock pore volume V and test the NMR signal, and draw the NMR test curve of the core saturated with water stage.

[0101] 2) Use heavy water to prepare a polymer solution of a certain concentration. In order to eliminate the influence of injection pressure on the lower limit of polymer pore radius, a constant pressure injection method is used to carry out displacement experiments. A certain amount of polymer solution is injected at a constant pressure, and then heavy water is used as tail liquid to continue injection, test the nuclear magnetic resonance signal, and draw the nuclear magnetic resonance test curve after polymer flooding.

[0102] 3) Using the core water saturated stage NMR test curve, draw the cumulative NMR signal amplitude curve. Complete the drawing of the T2 spectrum of the core before and after polymer flooding, see the attached Figure 2 .

[0103] 4. Experimental Data Processing and Analysis

[0104] By comparing the core pore radius distribution with the T2 spectra before and after polymer flooding, as shown in the attached Figure 2 As shown, 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, by Figure 2 From the T2 spectra of the core before and after polymer flooding, we can see that when the pore radius is less than 1.12μm, the nuclear magnetic resonance test curve of the core in the water-saturated stage coincides with the nuclear magnetic resonance test curve after polymer flooding, and the T2 spectra before and after polymer flooding do not change significantly. However, when the pore radius is greater than 1.12μm, the nuclear magnetic resonance test curve of the core in the water-saturated stage separates from the nuclear magnetic resonance test curve after polymer flooding, and the T2 spectra before and after polymer flooding change significantly. It can be obtained that the inaccessible pore radius of polymer flooding is 1.12μm.

[0106] The inaccessible pore radius divides the T2 spectrum into two parts, as shown in the attached Figure 2 As shown, it is divided into inaccessible pore area and accessible pore area.

[0107] Then, based on Figure 2 The cumulative NMR curves in the inaccessible pore radius r 不可及 The inaccessible pore volume conversion coefficient b of the core is calculated according to the cumulative nuclear magnetic signal intensity under the corresponding T2 value. The calculation formula is shown in formula (6):

[0108]

[0109] Where: b is the conversion coefficient of 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 accumulated nuclear magnetic signal intensity.

[0110] Finally, the inaccessible pore volume of the core polymer flooding is calculated by the inaccessible pore volume conversion coefficient. The calculation formula is as follows:

[0111] V 不可及 =b×V (7)

[0112] 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. The final calculation results are shown in Table 1:

[0113] Table 1 Calculation results

[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 the above-mentioned various method embodiments mentioned in the present disclosure can be combined with each other to form a combined embodiment without violating the principle logic. Due to space limitations, the present disclosure will not go into details.

[0116] The embodiments of the present disclosure have been described above, and the above description is exemplary, not exhaustive, and is not limited to the disclosed embodiments. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments. The selection of terms used herein is intended to best explain the principles of the embodiments, practical applications, or technical improvements to the technology in the market, or to enable other persons of ordinary skill in the art 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: include: The pore radius distribution curve of the core is obtained by high-pressure mercury injection testing; The T2 spectrum of the core before and after polymer flooding is obtained by nuclear magnetic resonance testing; By comparing the pore radius distribution curve of the core with the saturated water stage nuclear magnetic resonance curve in the T2 spectrum of the core before and after polymer flooding, a conversion coefficient between the fluid relaxation time T2 in the core pores and the pore radius is obtained; The inaccessible pore radius of the core polymer flooding is determined by using the T2 spectrum before and after polymer flooding in combination with the conversion coefficient.

2. The method for determining the inaccessible pore radius of polymer flooding based on nuclear magnetic resonance according to claim 1, characterized in that: The method for obtaining a pore radius distribution curve of a rock core by high-pressure mercury injection testing comprises: The pore radius distribution curve of the core is drawn 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, characterized in that: The method for drawing the pore radius distribution curve of the core by changing the injection pressure comprises: Injecting mercury into the core pores under different injection pressure conditions, the injection pressure being the capillary force, the capillary force corresponding to a capillary radius, and the capillary radius being equal to the core pore radius; The pore radius of the core is calculated by the following formula to obtain the pore radius distribution curve of the core: Where: p C It refers to the capillary force, unit is MPa; r is the pore radius, unit is μm; σ is the interfacial tension of mercury, unit is N / m; θ is the wetting angle between mercury and the core surface, unit is °.

4. The method for determining the inaccessible pore radius of polymer flooding based on nuclear magnetic resonance according to any one of claims 1 to 3, characterized in that: The method for obtaining the T2 spectrum before and after polymer flooding by nuclear magnetic resonance testing comprises: After the core is saturated with water, the core pore volume is measured and the nuclear magnetic resonance signal is tested to obtain the nuclear magnetic resonance signal amplitude at the water saturation stage, and a nuclear magnetic resonance test curve of the core at the water saturation stage is drawn; Use heavy water to prepare a polymer solution of a set concentration, inject a set amount of polymer solution into the water-saturated core at a constant pressure, and then continue to inject heavy water as tail liquid, test the nuclear magnetic resonance signal, and draw the nuclear magnetic resonance test curve after polymer flooding; The cumulative nuclear magnetic resonance signal amplitude curve is drawn using the nuclear magnetic resonance test curve of the core saturated water stage.

5. The method for determining the inaccessible pore radius of polymer flooding based on nuclear magnetic resonance according to claim 4, characterized in that: The method for obtaining the conversion coefficient between relaxation time and pore radius by comparing the pore radius distribution curve of the core with the water saturated stage nuclear magnetic resonance test curve comprises: Make a comparison chart of the pore radius distribution curve of the core and the nuclear magnetic resonance test curve of the saturated water stage, so that 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 of the saturated water stage respectively, and calculate the conversion coefficient through the T2 value and pore radius value of the corresponding point.

6. The method for determining the inaccessible pore radius of polymer flooding based on nuclear magnetic resonance according to claim 5, characterized in that: The corresponding point is, for example, a maximum pore radius point, a minimum pore radius point, or a pore radius point with the highest distribution frequency.

7. The method for determining the inaccessible pore radius of polymer flooding based on nuclear magnetic resonance according to any one of claims 1 to 3, 5 or 6, characterized in that: The method for determining the inaccessible pore radius of the core polymer flooding by using the T2 spectrum before and after the polymer flooding in combination with the conversion coefficient comprises: On the T2 spectrum before and after polymer flooding, find the T2 value corresponding to the separation point between the nuclear magnetic resonance test curve of the core saturated water stage and the nuclear magnetic resonance test curve after polymer flooding, and multiply the T2 value by the conversion coefficient to obtain the polymer flooding inaccessible pore radius.

8. A method for determining the inaccessible pore volume of polymer flooding based on nuclear magnetic resonance, characterized in that: include: Determine the inaccessible pore radius r of polymer flooding by the method described in any one of claims 1 to 7 不可及 , combined with the conversion coefficient, calculate the inaccessible pore radius r 不可及 The corresponding T2 value; According to the cumulative nuclear magnetic signal amplitude curve in the T2 spectrum before and after polymer flooding, the inaccessible pore radius r 不可及 The corresponding T2 value is used to determine the cumulative NMR signal amplitude and calculate the inaccessible pore volume conversion coefficient b of the core; The calculation formula for the inaccessible pore volume conversion coefficient is: Where: b is the conversion coefficient of 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 accumulated nuclear magnetic signal amplitude.

9. The method for determining the volume of inaccessible pores in polymer flooding based on nuclear magnetic resonance according to claim 8, characterized in that: The inaccessible pore volume is calculated by the inaccessible pore volume conversion coefficient, and the calculation formula is: In 不可及 =b×V; 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.

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