Method for determining nuclear magnetic resonance bi-T2 cutoff values of low-permeability compact sandstone

Abstract

The invention discloses a hypotonic tight sandstone nuclear magnetic resonance bi-T2 cutoff value determination method, which comprises the following steps: carrying out a multistage centrifugal force nuclear magnetic resonance experiment on a target rock core to obtain T2 spectrum data under different centrifugal forces; determining a free fluid displacement centrifugal force limit P1 according to a mutation point where the T2 spectrum form is attenuated from multiple peaks to a single peak, and extracting a corresponding T2cuff1; on the basis of the threshold condition that the absolute value of the accumulated porosity difference value under adjacent centrifugal force is smaller than 0.2%-0.4%, the strong bound water displacement centrifugal force boundary P2 is determined, and corresponding T2cutoff 2 is extracted; and calculating the free water saturation, the weak irreducible water saturation and the strong irreducible water saturation in combination with the T2cuff1 and the T2cuff2. The determination method of the double T2 cutoff values is provided through a multistage centrifugal force nuclear magnetic resonance experiment, triple division of the free water, the weak bound water and the strong bound water is achieved, accurate calculation of the free water saturation, the weak bound water saturation and the strong bound water saturation of the low-permeability sandstone is achieved, and an accurate fluid evaluation basis is provided for low-permeability-compact oil and gas reservoir development.

Description

Technical Field

[0001] The invention belongs to the technical field of petroleum exploration and development, and in particular relates to a method for determining a double T2 cutoff value of nuclear magnetic resonance of low-permeability dense sandstone. Background Art

[0002] In conventional sandstone reservoirs, the pore space types of reservoir rocks are classified into three types according to the connectivity of the stored fluids, namely: free fluid pores with good connectivity, bound fluid pores with poor connectivity, and clay bound water pores with no connectivity. The spatial sizes of the above three types of pores are often directly related to the reservoir quality. Generally, the higher the reservoir quality, the larger the free fluid porosity and the lower the bound fluid porosity and clay bound water pores.

[0003] In conventional oil and gas layers, controlled by the matching of reservoir dynamics and the capillary pressure of reservoir rocks, conventional oil and gas layers of different reservoir qualities under different reservoir dynamics backgrounds have different oil and gas properties. For example, high-quality reservoirs under high reservoir dynamics backgrounds correspond to better oil and gas saturation. Most of the oil and gas in conventional oil and gas layers are stored in free fluid pores with good connectivity. When evaluating the fluid properties of reservoir rocks using nuclear magnetic resonance experiments, the traditional T2 cutoff method can be used to well evaluate the bound water saturation. However, when the exploration and development object changes from conventional oil and gas layers to unconventional low permeability-tight sandstone oil and gas layers, the traditional nuclear magnetic T2 cutoff method no longer meets the evaluation of bound water in low permeability-tight sandstone.

[0004] Conventional reservoirs have a single pore structure and good homogeneity. Therefore, the pore types are mainly free fluid pores with good connectivity, while bound fluid pores with poor connectivity and clay bound water pores without connectivity account for a small proportion of the total pores. Compared with conventional sandstone, unconventional low-permeability-dense sandstone often has the characteristics of complex lithology, poor physical properties, and complex pore structure. Its pore types are mainly secondary solution pores formed by dissolution in the late stage of diagenesis, and a small amount of primary intergranular pores are retained. The above pore combination determines that low-permeability-dense sandstone has poor permeability compared with conventional sandstone and has a complex fluid occurrence state. Therefore, low-permeability-tight sandstone reservoirs usually have a higher irreducible water saturation. Since the basis for defining irreducible water is not clear, a large number of water production cases have been found in the early stages of development of low-permeability-tight oil and gas layers in exploration and development practice. This is because part of the irreducible water in traditional perceptions has changed from a restricted state to a movable state under the action of a production pressure difference higher than the reservoir-forming power. Therefore, the irreducible water boundary of low-permeability-tight sandstone reservoirs cannot be generally defined according to past experience, and a single nuclear magnetic resonance T2 cutoff value cannot be used to distinguish fluid properties into movable fluid and irreducible fluid. Summary of the invention

[0005] The problem to be solved by the present invention is to provide a method for determining the double T2 cutoff value of nuclear magnetic resonance of low-permeability and dense sandstone. The method conducts a gas-water displacement experiment on a water-saturated core by gradually increasing the centrifugal force, and studies the pore structure of the rock and the change law of the fluid occurrence state by analyzing the nuclear magnetic resonance signal of the water-saturated rock under the action of equal-increment centrifugal force displacement, so as to realize the accurate calculation of the double irreducible water saturation of low-permeability and dense sandstone.

[0006] In order to solve the above technical problems, the technical solution adopted by the present invention is: a method for determining the double T2 cutoff value of nuclear magnetic resonance of low-permeability dense sandstone, comprising the following steps:

[0007] S1: Perform multi-stage centrifugal force nuclear magnetic resonance experiments on the target core to obtain T2 spectrum data under different centrifugal forces;

[0008] S2: Determine the free fluid displacement centrifugal force limit P1 according to the mutation point where the T2 spectrum decays from multiple peaks to a single peak, and extract the corresponding T2_cutoff1;

[0009] S3: Based on the threshold condition that the absolute value of the cumulative porosity difference under adjacent centrifugal forces is less than 0.2% to 0.4%, the strong bound water displacement centrifugal force limit P2 is determined, and the corresponding T2_cutoff2 is extracted;

[0010] S4: Combine T2_cutoff1 and T2_cutoff2 to calculate free water saturation, weakly bound water saturation and strongly bound water saturation.

[0011] Further, the S1 comprises the following steps:

[0012] S11: After washing the target core with oil and salt and drying it, a vacuum pressurized water saturation operation is performed;

[0013] S12: Use standard samples to calibrate the NMR experimental instrument, set the equipment detection parameters to match the core physical properties and fluid properties, and unify the measurement parameters;

[0014] S13: using the unified measurement parameters, performing nuclear magnetic resonance measurement on the core saturated with water to obtain a nuclear magnetic T2 spectrum corresponding to the core saturated with water;

[0015] S14: centrifuge the water-saturated core using a centrifuge, increase the centrifugal force by increasing the speed of the centrifuge, and measure the nuclear magnetic T2 spectrum of the water-saturated core after the centrifugal force of 50 to 500 psi is applied.

[0016] Furthermore, in S12, the measurement parameters include but are not limited to the waiting time T W , echo interval T E , the number of echoes is NECH.

[0017] Further, in S14, the centrifugal force of 50-500 psi is set to eight groups including 50 psi, 100 psi, 150 psi, 200 psi, 250 psi, 300 psi, 400 psi and 500 psi.

[0018] Further, the S4 comprises the following steps:

[0019] S41: Divide the cumulative porosity components corresponding to the two T2 cutoff values ​​by the cumulative porosity of the water-saturated rock to obtain the free water limit S w1 and the strongly bound water limit S w2 ;

[0020] S42: Calculate the free water saturation S corresponding to the core when it is saturated with water wfm , Strongly bound water saturation S wsb and weakly bound water saturation S wwb .

[0021] Furthermore, in S41, the free water limit S w1 The calculation formula is as follows,

[0022]

[0023] Furthermore, in S41, the strongly bound water boundary S w2 The calculation formula is as follows,

[0024]

[0025] Furthermore, in S42, the free water saturation S wfm The calculation formula is as follows,

[0026] S wfm =100-S w1 .

[0027] Furthermore, in S42, the strongly bound water saturation S wsb The calculation formula is as follows,

[0028] S wsb =S w2 .

[0029] Furthermore, in S42, the weakly bound water saturation S wwb The calculation formula is as follows,

[0030] S wwb =S w1 -S w2 .

[0031] The advantages and positive effects of the present invention are:

[0032] Aiming at the problem of inaccurate fluid saturation evaluation caused by complex pore structure in low-permeability-tight sandstone reservoirs, the present invention proposes a method for determining double T2 cutoff values ​​(T2_cutoff1, T2_cutoff2) for the first time through innovative design of multi-stage centrifugal force nuclear magnetic resonance experimental process, realizing the triple division of free water, weakly bound water and strongly bound water. This method can realize the accurate calculation of free water saturation, weakly bound water saturation and strongly bound water saturation of low-permeability sandstone, and provide accurate fluid evaluation basis for the development of low-permeability-tight oil and gas reservoirs. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 It is a schematic diagram of the overall process of an embodiment of the present invention.

[0034] Figure 2 This is a graph showing the results of a multi-stage centrifugal force nuclear magnetic resonance experiment of sample 1 of a specific embodiment of the present invention.

[0035] Figure 3 This is a graph showing the results of a multi-stage centrifugal force nuclear magnetic resonance experiment of sample 2 of a specific embodiment of the present invention.

[0036] Figure 4 This is a diagram for determining the double T2 cutoff value of multi-stage centrifugal force nuclear magnetic resonance of sample 1 in a specific embodiment of the present invention.

[0037] Figure 5 This is a diagram for determining the double T2 cutoff value of multi-stage centrifugal force nuclear magnetic resonance of sample 2 in a specific embodiment of the present invention. DETAILED DESCRIPTION

[0038] The technical solution of the present invention will be described clearly and completely below in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0039] The embodiments of the present invention are further described below in conjunction with the accompanying drawings:

[0040] like Figure 1 As shown, a method for determining a double T2 cutoff value of low-permeability dense sandstone nuclear magnetic resonance comprises the following steps:

[0041] S1: Perform multi-stage centrifugal force nuclear magnetic resonance experiments on the target core to obtain T2 spectrum data under different centrifugal forces.

[0042] Specifically, due to different regional geological backgrounds in different regions, low-permeability-tight sandstone reservoirs in different regions have differences in lithology, physical properties, and pore structure characteristics. The double T2 cutoff value of low-permeability-tight sandstone in each region is also different, but low-permeability-tight sandstone in various regions has commonalities. Therefore, when conducting multi-stage centrifugal force nuclear magnetic resonance experiments on low-permeability-tight sandstone in a specific research block, it is necessary to formulate appropriate experimental conditions according to regional characteristics. The specific experimental steps are as follows.

[0043] S11: Perform core preparation work. Specifically, after the target core is washed with oil, salt and dried, a vacuum pressurized saturated water operation is performed.

[0044] S12: Adjust the experimental parameters. Specifically, the standard sample is used to calibrate the nuclear magnetic experiment instrument, the setting of the equipment detection parameters is matched with the core physical properties and fluid properties, and finally the measurement parameters are unified. Preferably, the measurement parameters include but are not limited to the waiting time T W , echo interval T E , echo number NECH, etc.

[0045] S13: Perform nuclear magnetic resonance measurement of the water-saturated core. Specifically, use unified measurement parameters to perform nuclear magnetic resonance measurement on the water-saturated core to obtain the nuclear magnetic T2 spectrum corresponding to the water-saturated core.

[0046] S14: Perform nuclear magnetic resonance measurement of the core after centrifugation. Specifically, the water-saturated core is centrifuged by a centrifuge, and the centrifugal force is increased by increasing the speed of the centrifuge to measure the nuclear magnetic T2 spectrum of the water-saturated core after the centrifugal force of 50psi, 100psi, 150psi, 200psi, 250psi, 300psi, 400psi and 500psi. After each displacement, the core needs to be re-saturated with water, and then the centrifugal force is increased to displace and perform nuclear magnetic measurement. Finally, the nuclear magnetic T2 spectrum of a single low-permeability-tight sandstone core is obtained when it is saturated with water and after 8 increasing displacement centrifugal forces.

[0047] It should be noted that when using a specific centrifugal force to displace water-saturated rocks, the centrifugal time must be guaranteed to ensure that the core reaches the bound water limit state corresponding to the centrifugal force, that is, all movable water has been completely displaced. The upper limit of centrifugal force for low-permeability and tight sandstone in each region is different. It is necessary to formulate a suitable upper limit of centrifugal force according to the actual situation of the block to ensure the timeliness of the experiment while achieving the purpose of the experimental research.

[0048] S2: Determine the free fluid displacement centrifugal force limit P1 according to the mutation point where the T2 spectrum decays from multiple peaks to a single peak, and extract the corresponding T2_cutoff1.

[0049] Specifically, the nuclear magnetic resonance T2 spectra of the low permeability-tight sandstone core under 9 saturation states when saturated with water and after 8 incremental displacement centrifugal forces are plotted on the same map.

[0050] As the centrifugal force gradually increases, the area of ​​the nuclear magnetic T2 spectrum gradually decreases, indicating that part of the movable fluid is displaced under the action of centrifugal force. When the nuclear magnetic T2 spectrum morphology decays from multiple peaks to a single peak, it means that the fluid in the movable pores corresponding to the larger T2 relaxation time has been completely displaced. The minimum displacement centrifugal force corresponding to the decay of the T2 spectrum morphology from multiple peaks to a single peak is defined as the free fluid displacement centrifugal force limit P1. At this time, the cumulative porosity value of the nuclear magnetic T2 spectrum under the displacement centrifugal force P1 is The horizontal coordinate T2 value corresponding to the intersection of the cumulative porosity curve of saturated water rock is the free fluid nuclear magnetic T2 cutoff value T2_cutoff1.

[0051] S3: Based on the threshold condition that the absolute value of the cumulative porosity difference under adjacent centrifugal forces is less than 0.2% to 0.4%, the strong bound water displacement centrifugal force limit P2 is determined, and the corresponding T2_cutoff2 is extracted.

[0052] Specifically, after the displacement centrifugal force reaches P1, as the displacement centrifugal force further increases, it can be found that the nuclear magnetic single-peak signal of the low-permeability-tight sandstone core still weakens to varying degrees. The weakening amplitude gradually decreases with the increase of centrifugal force, and when the displacement centrifugal force increases to a certain extent, the nuclear magnetic T2 spectrum signal basically no longer changes.

[0053] Based on the measurement accuracy of the nuclear magnetic resonance instrument (±0.2%), the threshold is defined as 1 to 2 times the measurement error (0.2% to 0.4%) to ensure that the threshold is significantly higher than the noise level. The displacement centrifugal force corresponding to the single peak signal of the nuclear magnetic T2 spectrum is basically no longer reduced, and the absolute value of the cumulative porosity difference under the action of adjacent centrifugal forces is less than the threshold of 0.2% to 0.4% is defined as the strong bound water displacement centrifugal force limit P2. At this time, the cumulative porosity value of the nuclear magnetic T2 spectrum under the action of the displacement centrifugal force P2 is The horizontal coordinate T2 value corresponding to the intersection of the cumulative porosity curve of saturated water rock is the strongly bound water nuclear magnetic T2 cutoff value T2_cutoff2.

[0054] S4: Combine T2_cutoff1 and T2_cutoff2 to calculate free water saturation, weakly bound water saturation and strongly bound water saturation. Specifically, S4 includes the following steps:

[0055] S41: After obtaining the double T2 cutoff value of low permeability-tight sandstone through the above steps, the cumulative porosity components corresponding to the two T2 cutoff values ​​are divided by the cumulative porosity of saturated water rock to obtain the free water limit S w1 and the strongly bound water limit S w2, the specific formulas are shown in the following formulas (1) and (2).

[0056]

[0057] S42: After obtaining the free water limit and strong bound water limit of a single low-permeability-tight sandstone through nuclear magnetic resonance experiments, the free water saturation S corresponding to the core being saturated with water can be calculated. wfm , Strongly bound water saturation S wsb and weakly bound water saturation S wwb , as shown in the following equations (3), (4) and (5).

[0058] S wfm =100-S w1 Formula (3)

[0059] S wsb =S w2 Formula (4)

[0060] S wwb =S w1 -S w2 Formula (5)

[0061] Free water is water with good mobility that exists in medium-large pore throat structures with good connectivity; strongly bound water is defined as immobile water that exists in micro-small pore throat structures with no connectivity; weakly bound water is fluid that can flow under critical production pressure difference conditions and exists in semi-connected pore throat structures with poor connectivity.

[0062] Compared with the traditional understanding that bound water is generally defined as the sum of capillary bound water and clay bound water, through exploration and development practice, it is realized that part of the water stored in the capillary pore throat structure in low permeability-tight sandstone reservoirs changes from an immobile state to a movable state after reaching the critical production pressure difference, and is produced together with oil and gas. Therefore, the new definition divides the traditional capillary bound water into weakly bound water and strongly bound water, and the clay bound water and the capillary bound water that is always immobile are combined to define strongly bound water.

[0063] Finally, based on a proposed multi-stage centrifuge NMR experimental process and a new method for calculating dual T2 NMR cutoff values, the accurate calculation of strong and weak bound water saturations of low permeability and tight sandstone was achieved, which further clarified the fluid storage state of low permeability and tight sandstone reservoirs, and is of great significance for guiding the exploration and development of low permeability and tight oil and gas reservoirs.

[0064] The present invention is specifically described below by taking the nuclear magnetic double T2 cutoff value confirmation of a low permeability-tight sandstone core in the western South China Sea as an example:

[0065] Conventional physical property tests have been conducted on both Sample 1 and Sample 2. The helium porosity of Sample 1 was measured to be 8.45%, and the absolute gas permeability was 0.29 mD. The helium porosity of Sample 2 was 7.87%, and the absolute gas permeability was 0.491 mD. According to industry standards, both samples are typical low-porosity and low-permeability sandstones.

[0066] S1: Calculate the double T2 cutoff value of NMR, that is, conduct multi-stage centrifugal force NMR experiments on two cores. The specific experimental steps are the same as above. Finally, the T2 spectra of the NMR experiments under 9 water saturation states are plotted on the same map, such as Figure 2 , Figure 3 shown.

[0067] S2: Determine the free fluid NMR T2 cutoff value T2_cutoff1. Figure 2 , Figure 3 As shown in the figure, it is observed that when the NMR T2 spectra of sample 1 and sample 2 change from double peaks to single peaks, the corresponding centrifugal forces are 200psi and 100psi respectively, and the high relaxation time T2 spectrum peak area no longer decreases. At this time, it is believed that the free water in the well-connected pores has been completely displaced. Therefore, the free fluid displacement centrifugal force limit P1 of sample 1 and sample 2 is determined to be 200psi and 100psi respectively.

[0068] Cumulative porosity value of NMR T2 spectrum under displacement centrifugal force P1 The horizontal coordinate T2 value corresponding to the intersection of the cumulative porosity curve of saturated water rock is the free fluid nuclear magnetic T2 cutoff value T2_cutoff1, such as Figure 4 , Figure 5 As shown in the figure, the free fluid NMR T2 cutoff values ​​T2_cutoff1 of sample 1 and sample 2 are 4ms and 0.43ms respectively.

[0069] S3: Determine the strong bound water NMR T2 cutoff value T2_cutoff2. When the centrifugal force exceeds the free fluid displacement centrifugal force limit P1, as the centrifugal force increases, the single peak area of ​​the NMR T2 spectrum of the two samples further decreases, and the attenuation amplitude gradually decreases with the increase of centrifugal force. When the displacement centrifugal force increases to a certain extent, the NMR T2 spectrum signal basically no longer changes, such as Figure 2 , Figure 3 shown.

[0070] The displacement centrifugal force corresponding to the single peak signal of the nuclear magnetic T2 spectrum basically no longer decreases, and the absolute value of the cumulative porosity difference under the action of adjacent centrifugal forces is less than 0.2% to 0.4% is defined as the strong bound water displacement centrifugal force limit P2. At this time, it is believed that the free water and weak bound water in the core have been completely displaced, and only immobile strong bound water exists. The strong bound water displacement centrifugal force limits P2 of the two cores obtained by the above method are 400psi and 300psi respectively.

[0071] Cumulative porosity value of NMR T2 spectrum under the action of displacement centrifugal force P2 The horizontal coordinate T2 value corresponding to the intersection of the cumulative porosity curve of saturated water rock is the strongly bound water nuclear magnetic T2 cutoff value T2_cutoff1, such as Figure 4 , Figure 5 As shown in Figure 2, the strong bound water NMR T2 cutoff values ​​T2_cutoff1 of sample 1 and sample 2 are 0.68ms and 0.27ms respectively.

[0072] S4: Detailed characterization of bound water in two low-permeability-tight sandstones.

[0073] After obtaining the double T2 cutoff values ​​of the two low-permeability-tight sandstones through the above steps, the cumulative porosity components corresponding to the two T2 cutoff values ​​are divided by the cumulative porosity of the water-saturated rock, respectively, as shown in equations (1) and (2), and the free water limit S corresponding to the two cores can be obtained. w1 and the strongly bound water limit S w2 .

[0074] The free water limit S of sample 1 was obtained by calculation w1 The strong bound water limit S is 55.56%. w2 The free water limit of sample 2 is 30.86%. w1 The strong bound water limit S is 34.25%. w2 It is 24.66%.

[0075] After obtaining the free water limit and strong bound water limit of a single low permeability-tight sandstone, the free water saturation S corresponding to the saturated water content of a single core can be calculated. wfm , Strongly bound water saturation S wsb and weakly bound water saturation S wwb , see formula (3), formula (4), and formula (5).

[0076] The free water saturation S of sample 1 was obtained by calculation wfm The strongly bound water saturation S is 44.44%. wsb The weakly bound water saturation S is 30.86%. wwb The free water saturation S of sample 2 is 24.7%. wfmThe strongly bound water saturation S is 65.75%. wsb The weakly bound water saturation S is 24.66%. wwb It is 9.59%.

[0077] The advantages and positive effects of the present invention are:

[0078] Aiming at the problem of inaccurate fluid saturation evaluation caused by complex pore structure in low-permeability-tight sandstone reservoirs, the present invention proposes a method for determining double T2 cutoff values ​​(T2_cutoff1, T2_cutoff2) for the first time through innovative design of multi-stage centrifugal force nuclear magnetic resonance experimental process, realizing the triple division of free water, weakly bound water and strongly bound water. This method can realize the accurate calculation of free water saturation, weakly bound water saturation and strongly bound water saturation of low-permeability sandstone, and provide accurate fluid evaluation basis for the development of low-permeability-tight oil and gas reservoirs.

[0079] The above is a detailed description of an embodiment of the present invention, but the content is only a preferred embodiment of the present invention and cannot be considered to limit the scope of implementation of the present invention. All equivalent changes and improvements made within the scope of the present invention should still fall within the scope of the patent coverage of the present invention.

Claims

1. A method for determining the double T2 cutoff value of nuclear magnetic resonance of low-permeability dense sandstone, characterized by: The following steps are included: S1: Perform multi-stage centrifugal force nuclear magnetic resonance experiments on the target core to obtain T2 spectrum data under different centrifugal forces; S2: Determine the free fluid displacement centrifugal force limit P1 according to the mutation point where the T2 spectrum decays from multiple peaks to a single peak, and extract the corresponding T2_cutoff1; S3: Based on the threshold condition that the absolute value of the cumulative porosity difference under adjacent centrifugal forces is less than 0.2% to 0.4%, the strong bound water displacement centrifugal force limit P2 is determined, and the corresponding T2_cutoff2 is extracted; S4: Combine T2_cutoff1 and T2_cutoff2 to calculate free water saturation, weakly bound water saturation and strongly bound water saturation.

2. The method for determining the double T2 cutoff value of low-permeability dense sandstone nuclear magnetic resonance according to claim 1, characterized in that: The S1 comprises the following steps, S11: After washing the target core with oil, salt and drying, a vacuum pressurized saturated water operation is performed; S12: Use standard samples to calibrate the NMR experimental instrument, set the equipment detection parameters to match the core physical properties and fluid properties, and unify the measurement parameters; S13: using the unified measurement parameters, performing nuclear magnetic resonance measurement on the core saturated with water to obtain a nuclear magnetic T2 spectrum corresponding to the core saturated with water; S14: centrifuge the water-saturated core using a centrifuge, increase the centrifugal force by increasing the speed of the centrifuge, and measure the nuclear magnetic T2 spectrum of the water-saturated core after the centrifugal force of 50 to 500 psi is applied.

3. The method for determining the double T2 cutoff value of low-permeability dense sandstone nuclear magnetic resonance according to claim 2, characterized in that: In S12, the measurement parameters include but are not limited to the waiting time T W , echo interval T E , the number of echoes is NECH.

4. A method for determining double T2 cutoff value of low permeability dense sandstone nuclear magnetic resonance according to claim 2 or 3, characterized in that: In S14, the centrifugal force of 50-500 psi is set to eight groups including 50 psi, 100 psi, 150 psi, 200 psi, 250 psi, 300 psi, 400 psi and 500 psi.

5. A method for determining a double T2 cutoff value of nuclear magnetic resonance of low-permeability dense sandstone according to any one of claims 1 to 3, characterized in that: The S4 comprises the following steps, S41: Divide the cumulative porosity components corresponding to the two T2 cutoff values ​​by the cumulative porosity of the water-saturated rock to obtain the free water limit S w1 and the strongly bound water limit S w2 ; S42: Calculate the free water saturation S corresponding to the core when it is saturated with water wfm , Strongly bound water saturation S wsb and weakly bound water saturation S wwb .

6. The method for determining the double T2 cutoff value of low-permeability dense sandstone nuclear magnetic resonance according to claim 5, characterized in that: In S41, the free water limit S w1 The calculation formula is as follows, 7. The method for determining the double T2 cutoff value of low-permeability dense sandstone nuclear magnetic resonance according to claim 5, characterized in that: In the S41, the strongly bound water boundary S w2 The calculation formula is as follows, 8. The method for determining the double T2 cutoff value of low-permeability dense sandstone nuclear magnetic resonance according to claim 5, characterized in that: In S42, the free water saturation S wfm The calculation formula is as follows, S wfm =100-S w1 。 9. The method for determining the double T2 cutoff value of low-permeability dense sandstone nuclear magnetic resonance according to claim 5, characterized in that: In the step S42, the strongly bound water saturation S wsb The calculation formula is as follows, S wsb =S w2 。 10. The method for determining the double T2 cutoff value of low-permeability dense sandstone nuclear magnetic resonance according to claim 5, characterized in that: In the step S42, the weakly bound water saturation S wwb The calculation formula is as follows, S wwb =S w1 -S w2 。

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