A method for predicting movable oil content of a rock based on a nuclear magnet relaxation ratio

Abstract

The application provides a method for predicting movable oil content in rock based on nuclear magnetic relaxation ratio, which comprises the following steps: 1) determining a nuclear magnetic base T2 relaxation spectrum of a core in a dry state and a T2 bulk relaxation parameter of white oil; 2) carrying out a complete saturation experiment on the core by using the white oil, carrying out T2 nuclear magnetic resonance test, and obtaining a T2 relaxation spectrum of the core in a saturated state; 3) subtracting the base signal of the core from the saturated T2 relaxation spectrum to obtain a T2 relaxation spectrum of the white oil in the core, and then obtaining the relationship between the relaxation time and the white oil bulk relaxation fluid ratio and calculating the total amount of the white oil bulk relaxation fluid; 4) centrifuging the obtained saturated core, determining the mass of the core and calculating the centrifugal movable fluid content, and establishing a movable oil content prediction formula based on the nuclear magnetic relaxation ratio. The application has reasonable design, the movable oil content is predicted by calculating the proportion of the nuclear magnetic resonance bulk relaxation signal of the oil in the rock, and the reliability of the nuclear magnetic technology in identifying the fluid distribution of the oil is improved.

Description

Technical Field

[0001] This invention relates to the field of oil and gas exploration technology, specifically to a method for predicting the movable oil content of rocks based on nuclear magnet relaxation ratio. Background Technology

[0002] With the depletion of global conventional oil resources and the reduction of high-quality reserves, the difficulty and technological cost of oil resource development have increased significantly. Against this backdrop, unconventional oil has become an important strategic alternative resource in the global energy transition. However, compared with conventional oil resources, unconventional oil reservoirs exhibit lower porosity and permeability, and are characterized by complex pore structures, varied lithofacies, strong heterogeneity, and diverse mineral compositions. Accurate assessment of their enrichment characteristics and oil-bearing potential is necessary to overcome the challenges in their development.

[0003] Unconventional oils are primarily found within complex, multi-scale, tight rock formations (such as shale and tight sandstone). Different types of unconventional reservoirs exhibit significant differences in the proportion of organic / inorganic pore types, pore size distribution, and fracture connectivity. This often results in over 60% of the unconventional oil remaining trapped within the matrix micropores during extraction, rendering it unusable. Therefore, research on the mobility of oil in unconventional formations has become a key research direction for overcoming development efficiency bottlenecks. Currently, the field of reservoir fluid mobility analysis has developed a multi-faceted experimental evaluation method system, including the oil saturation index (OSI), multidimensional nuclear magnetic resonance (NMR), multi-temperature-level pyrolysis, swelling method, and solvent stepwise extraction method. Among these, NMR technology, by distinguishing between the signal responses of bound and mobile fluids, has been successfully applied to classify the occurrence types of oil adsorbed in organic pores, free oil in inorganic pores, and seeping oil in microfractures, and has achieved refined characterization of the proportion of shale oil in different states.

[0004] However, existing NMR methods still have significant limitations. Conventional NMR experiments typically use the relaxation spectra of oil before and after centrifugation to invert the distribution of oil within the rock pores and thus analyze the movable oil content, neglecting the impact of the difference in volume relaxation time compared to water on the measured relaxation time. Because the NMR signal of oil within the rock involves both surface and volume relaxation mechanisms, a systematic error (ranging from 15% to 30%) occurs when converting the T2 relaxation spectrum to pore size distribution, reducing the reliability of NMR distribution data in characterizing oil pore size distribution. This factor, to some extent, restricts the accurate evaluation of unconventional oil resources. At the same time, since the volume relaxation signal of fluids in rocks originates from fluid molecules far from the pore walls, there is a close relationship between the proportion of volume-relaxed fluids and the movable oil content within oil-bearing rocks. Therefore, there is an urgent need to develop new NMR interpretation models to identify the volume relaxation signal of oil and thus predict its movable oil content. Summary of the Invention

[0005] To address the limitations of NMR signal interpretation in the assessment of unconventional oil mobility in the aforementioned background technologies, this invention proposes a method for predicting the movable oil content in rocks based on the NMR relaxation ratio. This method predicts the movable oil content by calculating the proportion of NMR relaxation signals in the oil within the rock.

[0006] To address the aforementioned technical problems, this invention provides a method for predicting the movable oil content of rocks based on nuclear magnet relaxation ratios, comprising the following steps:

[0007] 1) Select appropriate rock samples and white oil samples, prepare standard rock cores, clean and dry them, record the quality of the rock cores, and measure the T2 relaxation spectrum of the rock core matrix and the T2 volume relaxation parameters of the white oil under dry conditions.

[0008] 2) Use white oil to perform a full saturation experiment on the dried core. When the core is confirmed to be fully saturated, record the core mass and carry out T2 nuclear magnetic resonance testing to obtain the T2 relaxation spectrum of the core in the saturated state.

[0009] 3) Subtract the basement signal from the T2 relaxation spectrum of the core under saturation to obtain the T2 relaxation spectrum of white oil in the core. Then, substitute the relaxation time and volume relaxation parameters of white oil in the relaxation spectrum into the formula for calculating the volume relaxation fluid ratio to obtain the relationship between relaxation time and the volume relaxation fluid ratio of white oil and calculate the total volume relaxation fluid of white oil in the core.

[0010] 4) After the saturated core obtained from the full saturation experiment is centrifuged, the mass of the core after centrifugation is measured and the centrifugation movable fluid content of the core is calculated. Then, based on the linear correlation between the total amount of white oil relaxant fluid in the core and the centrifugation movable fluid content, a rock movable oil content prediction formula based on the nuclear magnetic resonance relaxation ratio is established.

[0011] The method for predicting the movable oil content of rocks based on nuclear magnetic resonance relaxation ratio, wherein: the specific process of step 1) is as follows: select a suitable rock sample to drill a standard core, clean the core appropriately and dry it to ensure that there is no obvious floating dust on its surface; then perform nuclear magnetic resonance T2 test on the processed dry core and the selected white oil to obtain the T2 relaxation spectrum of the dry core and the relaxation data of the white oil.

[0012] The method for predicting the movable oil content of rocks based on nuclear magnetic resonance relaxation ratio, wherein: the T2 nuclear magnetic resonance test in step (2) uses a CPMG pulse sequence suitable for the transverse relaxation time magnetic field, and the relevant parameters used include the echo interval T E =0.132 milliseconds, number of echoes NECH=3788, number of scans n=32, waiting time T w=750 milliseconds and 30 loop steps.

[0013] The method for predicting the movable oil content of rocks based on the nuclear magnetic resonance relaxation ratio, wherein the specific process of the complete saturation experiment in step 2) is as follows: the core is placed in the core saturation device, after vacuuming, white oil is injected and pressurized to 25 MPa, and saturation is carried out for more than 5 days to ensure that the core is completely saturated. After saturation is completed, the core is taken out, the residual fluid on the core surface is removed, and nuclear magnetic resonance testing is performed to obtain the T2 relaxation spectrum under saturation.

[0014] The method for predicting the movable oil content of rocks based on nuclear magnet relaxation ratio, wherein the specific process of obtaining the relationship between relaxation time and the relaxation fluid ratio of white oil body in step 3) is as follows:

[0015] During low-field NMR testing, the transverse relaxation time T2 is mainly dominated by two mechanisms:

[0016]

[0017] In equation (2) above, T 2B For the relaxation time, T 2S For surface relaxation time;

[0018] According to nuclear magnetic resonance theory, during low-field NMR testing, the decay process of the magnetization signal of the hydrogen-containing fluid in a single pore follows a single exponential decay law, and the total magnetization M(t) is equal to the sum of the magnetizations of the individual pores.

[0019]

[0020] In equation (5) above, M(t) is the magnetization signal of the pore measured at time t, M0 is the initial magnetization signal, and v i T represents the proportion of type i pores in the total porosity. 2i For the longitudinal and transverse relaxation times corresponding to the i-th type of pore;

[0021] If only the same type of pores exist, the above equation (5) can be simplified to:

[0022]

[0023] In equation (6) above, M i (t) and M i (0) represents the magnetization signal measured at time t and time 0 for pore i, respectively;

[0024] Based on formulas (5)-(6), we first analyze the relaxation process by considering the combined effects of the two relaxation mechanisms on the relaxation of oil in the pores; at this point, we can let t=1 and introduce formula (2) into formula (6) to further obtain:

[0025]

[0026] In equation (7) above, T 2is Let T be the transverse surface relaxation time of the fluid within pore i; since the volume relaxation process is not affected by the fluid's environment compared to surface relaxation, T in formula (7) is... 2B It can be obtained directly by performing nuclear magnetic resonance (NMR) tests on free-state fluids;

[0027] In addition to considering the combined effects of the two relaxation mechanisms on pore fluid relaxation, the relaxation process of the fluid within the pores can also be viewed as a weighted average of the relaxation processes of two types of relaxable fluids under the influence of the two relaxation mechanisms, i.e., the measured magnetization signal M of the oil within the pores obtained by testing. i (t) is actually a weighted average of the relaxation processes of the two types of relaxant fluids; if, under the influence of the bulk relaxation mechanism, the proportion of fluids that complete the NMR relaxation process via bulk relaxation is γ (0 < γ < 1), then:

[0028]

[0029] In equation (8) above, γ is the measured relaxation time T. 2i The corresponding proportion of the volumetric relaxor fluid;

[0030] Therefore, for the measured relaxation time T 2i For the fluid within the pores, according to formulas (7) and (8), we can obtain:

[0031]

[0032] After rearranging formula (9), the formula for calculating the volume relaxation ratio γ in the multi-mechanism relaxation signal is obtained as follows:

[0033]

[0034] The method for predicting rock movable oil content based on nuclear magnet relaxation ratio, wherein the analysis method for the relationship between the total amount of white oil relaxant fluid in the core and the centrifugation movable fluid content in step 4) includes the following steps:

[0035] 4.1) Determine the content of white oil mobile fluid in the core by centrifugation.

[0036] During the centrifugation of the core, the temperature needs to be controlled within 0-4℃ to ensure that the fluid loss comes entirely from centrifugation. The mass of the core itself needs to be recorded before and after centrifugation to obtain the content of white oil mobile fluid in the core. In addition, broken centrifuged cores do not meet the experimental requirements, cannot be included in the statistical range, and the experiment needs to be repeated.

[0037] 4.2) Establish a formula for predicting the movable oil content of rocks based on the nuclear magnet relaxation ratio.

[0038] By mapping the total amount of relaxed fluid in the white oil body of the core to the centrifugal movable fluid content of the white oil in the core and plotting them on a scatter plot, and performing linear correlation analysis on the results, a quantitative relationship between the total amount of relaxed fluid in the white oil body of the core and the centrifugal movable fluid content can be established, thereby enabling the prediction of the movable oil content in the rock.

[0039] By adopting the above technical solution, the present invention has the following beneficial effects:

[0040] The present invention proposes a reasonable method for predicting the movable oil content of rocks based on the relaxation ratio of nuclear magnetic resonance (NMR). This method overcomes the limitations of NMR signal interpretation in evaluating the fluid mobility of oil-bearing rocks affected by receptor relaxation. This not only improves the reliability of NMR technology in identifying the fluid distribution of oil, but also provides a new standard for assessing the movable oil content of rocks, thus enhancing the engineering applicability of NMR technology in the accurate evaluation of unconventional oil reservoirs.

[0041] The proposed method for calculating the volumetric relaxation ratio of nuclear magnetic resonance (NMR) fluids combines NMR principles with the testing process, classifying the test signals of fluids like oil that are affected by multiple relaxation mechanisms during NMR testing. This overcomes the deficiency in previous NMR studies where the influence of volumetric relaxant fluids could not be quantitatively characterized. Furthermore, based on this NMR relaxation calculation method, the total volumetric relaxant fluid content within the rock can be calculated and compared with the movable fluid content measured by existing experimental methods such as centrifugation, thereby enabling the prediction of movable oil content in the rock. Moreover, this method can serve as a new testing standard in future applications such as multidimensional NMR and NMR logging, further refining existing methods for analyzing the occurrence characteristics and mobility of complex fluids within unconventional rocks. Attached Figure Description

[0042] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0043] Figure 1 This is a flowchart of the method for predicting the movable oil content of rocks based on the nuclear magnet relaxation ratio according to the present invention;

[0044] Figure 2 The T2 relaxation spectra of three core samples in the embodiments of the present invention, which are in a state of complete saturation with No. 5 white oil, in the method for predicting the movable oil content of rocks based on the nuclear magnet relaxation ratio.

[0045] Figure 3 This is a diagram showing the volume relaxation fluid ratio division results corresponding to three fully saturated T2 relaxation spectra of core samples in the embodiment of the present invention, which is used to predict the movable oil content of rocks based on the nuclear magnet relaxation ratio.

[0046] Figure 4 This is a graph showing the relationship between the total amount of white oil relaxant fluid and the centrifugally mobile fluid content of three core samples in the embodiments of the present invention, which are based on the nuclear magnet relaxation ratio for predicting the rock mobile oil content. Detailed Implementation

[0047] The technical solution of the present invention will now be clearly and completely described with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0048] The present invention will be further explained below with reference to specific embodiments.

[0049] like Figure 1 As shown in the figure, this embodiment provides a method for predicting the movable oil content of rocks based on the relaxation ratio of nuclear magnetic resonance (NMR). This method overcomes the limitations of NMR signal interpretation in evaluating the fluid mobility of oil-bearing rocks affected by receptor relaxation. It not only improves the reliability of NMR technology in identifying the fluid distribution of oil, but also provides a new standard for assessing the movable oil content of rocks, thus enhancing the engineering applicability of NMR technology in the accurate evaluation of unconventional oil reservoirs.

[0050] For porous media such as rocks, hydrogen nuclear magnetic resonance (HNMR) can analyze the environment and state of the fluid containing hydrogen molecules based on their relaxation time. The molecular relaxation process is influenced by three mechanisms: (1) molecular motion in the fluid, (2) surface relaxation at the pore walls, and (3) molecular diffusion in the internal gradient. These three mechanisms correspond to the following relaxation processes: volume relaxation, surface relaxation, and diffusion relaxation, respectively, with corresponding relaxation times T0 and T1. B ,T S With T D The transverse relaxation time T2 observed in NMR spectroscopy is mainly dominated by the above three mechanisms:

[0051]

[0052] The above formula is effective for single saturated pores at the rapid diffusion limit. In commonly used low-field NMR tests, the instrument's magnetic field is often set to be uniform, i.e., there is no magnetic field gradient. Therefore, the diffusion relaxation term in formula (1) can be ignored, i.e.:

[0053]

[0054] Among them, T 2B For the relaxation time, T 2S For surface relaxation time;

[0055] For surface relaxation in saturated pores, the reciprocal of the relaxation time is equal to the product of the pore's inherent relaxation rate and the pore surface area-volume ratio:

[0056]

[0057] Where ρ2 is T 2S Let be the longitudinal / transverse surface relaxation rate of the pore, and be the longitudinal / transverse surface relaxation time of the fluid within the selected pore. Considering the relationship between the pore surface area-volume ratio and the pore radius, this formula can be further transformed into:

[0058]

[0059] Where r is the pore radius, F s F is the pore morphology factor; for cylindrical pores, F s Take 2. As can be seen from formula (4), for the same type of pores in the saturated state, the nuclear magnetic relaxation time is positively correlated with the pore radius.

[0060] According to nuclear magnetic resonance theory, during low-field NMR testing, the decay process of the magnetization signal of the hydrogen-containing fluid in a single pore follows a single exponential decay law, and the total magnetization M(t) is equal to the sum of the magnetizations of the individual pores.

[0061]

[0062] Where M(t) is the magnetization signal of the pore measured at time t, M0 is the initial magnetization signal, and v i T represents the proportion of type i pores in the total porosity. 2i denoted as the longitudinal and transverse relaxation times corresponding to the i-th type of pore.

[0063] If only the same type of pores exist, the above equation can be simplified to:

[0064]

[0065] Among them, M i (t) and M i (0) represents the magnetization signal measured at time t and time 0 for pore i, respectively.

[0066] Based on formulas (5)-(6), this invention first analyzes the relaxation process by considering the combined influence of two relaxation mechanisms on the relaxation of oil within pores. At this point, we can let t = 1 and introduce formula (2) into formula (6) to further obtain:

[0067]

[0068] Among them, T 2is Let T be the transverse surface relaxation time of the fluid within pore i. Since the volume relaxation process is not affected by the fluid's environment compared to surface relaxation, T in formula (7) is... 2B It can be obtained directly by performing nuclear magnetic resonance (NMR) tests on free-state fluids.

[0069] Furthermore, this invention suggests that, in addition to considering the combined effects of the two relaxation mechanisms on pore fluid relaxation, the relaxation process of the fluid within the pores can also be viewed as a weighted average of the relaxation processes of two types of relaxed fluids under the influence of the two relaxation mechanisms, i.e., the measured magnetization signal M of the oil within the pores obtained by testing. i (t) is actually a weighted average of the relaxation processes of the two types of relaxant fluids. If, under the influence of the bulk relaxation mechanism, the proportion of fluids completing the NMR relaxation process via bulk relaxation is γ (0 < γ < 1), then:

[0070]

[0071] Where γ is the measured relaxation time T 2i The corresponding proportion of the volumetric relaxor fluid;

[0072] Therefore, for the measured relaxation time T 2i For the fluid within the pores, according to formulas (7) and (8), we can obtain:

[0073]

[0074] By rearranging formula (9), we can obtain the formula for calculating γ:

[0075]

[0076] Formula (10) is the formula for calculating the volume relaxation ratio in a multi-mechanism relaxation signal.

[0077] From formula (10), it can be seen that given the relaxation times corresponding to various types of porous fluids and the fluid's own volume relaxation time, the proportion γ of surface-relaxed fluids can be solved. Therefore, in low-field NMR testing, we can obtain a series of relationships between relaxation times and signal quantities, i.e., fluid content, based on the obtained NMR signal distribution spectrum. By considering each relaxation time and its corresponding signal quantity as corresponding to a type of porous fluid, the fluid surface relaxation proportion at each relaxation time can be calculated based on this relationship. According to this method, the fluid signal in the T2 relaxation spectrum obtained by NMR testing can be divided into two parts: surface relaxation and volume relaxation.

[0078] like Figure 1 As shown, in one embodiment of the present invention, a method for predicting the movable oil content of rocks based on nuclear magnet relaxation ratio is disclosed, comprising the following steps:

[0079] 1) Select appropriate rock samples and white oil samples, prepare standard core samples (hereinafter referred to as cores) from the rocks, clean and dry them, record the quality of the cores, and measure the T2 relaxation spectrum of the core NMR matrix and the T2 volume relaxation parameters of the white oil under dry conditions.

[0080] The specific process is as follows: Select a suitable rock sample to drill a standard core sample, clean the core appropriately and dry it to ensure that there is no obvious dust on its surface; then perform nuclear magnetic resonance T2 tests on the processed and dried core and the selected white oil to obtain the T2 relaxation spectrum of the dried core and the relaxation data of the white oil.

[0081] In this embodiment, standard rock cores (2.5 cm in diameter and 5 cm in height) were prepared by cutting selected rock according to the national standard GB / T 29172-2012 "Core Analysis Methods". The cores were then moderately cleaned and dried at 105°C for 48 hours until their mass remained unchanged to remove residual water. An appropriate amount of white oil (5-10 cm) was then weighed out. 3 The core and white oil were placed in a non-NMR container; then, NMR T2 tests were performed on the core and white oil to obtain their NMR relaxation spectra, which were used as the basement relaxation spectra of the core and the relaxation data of the white oil body.

[0082] 2) A complete saturation experiment was performed on the dried core using white oil. Once the core was confirmed to be fully saturated, the core mass was recorded, and T2 NMR spectroscopy was conducted to obtain the T2 relaxation spectrum of the core under saturation. In this embodiment, the T2 NMR spectroscopy used a CPMG pulse sequence suitable for transverse relaxation magnetic fields. The relevant parameters used were: echo interval T... E =0.132 milliseconds, number of echoes NECH=3788, number of scans n=32, waiting time T w=750 milliseconds, with a cycle count of 30. These parameters are used for all low-field NMR tests in this invention. The specific process of the saturation experiment is as follows: the core is placed in a core saturation device, vacuumed, injected with white oil, and pressurized to 25 MPa. Saturation is carried out for more than 5 days to ensure that the core is completely saturated. After saturation, the core is removed, residual fluid on the core surface is cleaned, and NMR tests are performed to obtain the T2 relaxation spectrum under saturation.

[0083] 3) Subtract the basement signal from the T2 relaxation spectrum of the core under saturation to obtain the T2 relaxation spectrum of the white oil in the core. Then, substitute the relaxation time and the volume relaxation parameter of the white oil in the relaxation spectrum into the formula for calculating the volume relaxation fluid ratio. Calculate the volume relaxation ratio of the white oil corresponding to each relaxation time in the T2 relaxation spectrum, thus obtaining the relationship between relaxation time and the volume relaxation fluid ratio of the white oil. Accumulate each volume relaxation signal quantity, and finally calculate the total volume relaxation fluid of the white oil in the core. The relationship between relaxation time and the volume relaxation fluid ratio of the white oil is obtained through the calculation process of the above formulas (2) and (5)-(10).

[0084] 4) After the saturated core obtained from the complete saturation experiment, centrifuge it, measure the mass of the core after centrifugation, and calculate the centrifugation mobile fluid content. Then, based on the linear correlation between the total volumetric relaxation fluid of the white oil body in the core and the centrifugation mobile fluid content, establish a prediction formula for the mobile oil content of the rock based on the nuclear magnetic resonance volumetric relaxation ratio. The analysis process of the volumetric relaxation signal and mobile fluid content in step 4) includes the following steps:

[0085] 4.1) Determine the content of white oil mobile fluid in the core by centrifugation.

[0086] During the centrifugation of the core, the temperature needs to be controlled to ensure that the fluid loss comes from centrifugation. The mass of the core itself needs to be recorded in a timely manner before and after centrifugation to obtain the content of white oil mobile fluid in the core. In addition, broken centrifuged cores do not meet the experimental requirements, cannot be included in the statistical range, and the experiment needs to be repeated.

[0087] 4.2) Establish a formula for predicting the movable oil content of rocks based on the nuclear magnet relaxation ratio.

[0088] By mapping the total amount of relaxed fluid in the white oil body of the core to the centrifugal movable fluid content of the white oil in the core and plotting them on a scatter plot, and performing linear correlation analysis on the results, a quantitative relationship between the total amount of relaxed fluid in the white oil body of the core and the centrifugal movable fluid content can be established, thereby enabling the prediction of the movable oil content in the rock.

[0089] The present invention will be further described below with reference to specific embodiments.

[0090] This embodiment uses the experiment of saturating and centrifuging three shale core samples with white oil as an example. The experiment must be conducted under a room temperature pressure environment (20℃, 0.1MPa). Two shale samples were selected and labeled SY-1, SY-2, and SY-3. The three shale samples were processed into cores, cleaned, and dried, and the mass of the dried cores was recorded. The processed cores were subjected to T2 NMR spectroscopy to obtain their matrix T2 relaxation spectra.

[0091] Subsequently, a suitable amount of saturating fluid was selected to conduct a core saturation experiment. No. 5 white oil was used as the test fluid in this experiment. Before saturation, 5 ml of white oil was placed in a non-NMR container, and T2 NMR was performed. The centroid relaxation time of the obtained T2 relaxation spectrum was taken as the relaxation time of the white oil volume. In this experiment, the relaxation time of No. 5 white oil was 200 ms.

[0092] A saturation experiment was conducted using core samples to extract white oil from shale cores. The cores were placed in a core saturation apparatus, which was then sealed and saturated to a vacuum of 25 MPa. After five days of saturation, the cores were considered fully saturated. Subsequently, three core samples were removed and weighed again to obtain the mass of white oil within the shale cores under fully saturated conditions. T2 NMR spectroscopy was performed on the fully saturated shale cores, and the resulting T2 relaxation spectra are shown below. Figure 2 As shown. By Figure 2 It is evident that the NMR signals of the white oil are mainly distributed in the range of 0.05-300 ms. Furthermore, morphologically, all three core samples exhibit a certain bimodal distribution characteristic. This suggests that although the three shale samples differ in mineral composition, organic matter type, and content, they all share similar porosity development.

[0093] Using formula (10), the volumetric relaxant fluid proportion corresponding to each relaxation time in the T2 relaxation spectrum of the core can be calculated. The volumetric relaxant fluid content corresponding to different relaxation times calculated according to formula (10) is as follows: Figure 3 As shown. According to Figure 3 The information shown indicates that in all three core samples, the amount of white oil involved in surface relaxation was higher than that involved in volume relaxation. Specifically, the volume relaxation ratio was approximately 30% in SY-1, approximately 6% in SY-2, and approximately 5% in SY-3. The distribution clearly shows that the influence of white oil volume relaxation on the T2 relaxation spectrum is mainly concentrated in the region with higher relaxation times, confirming the mobility of fluids within the open, medium-to-large pores of the shale.

[0094] Subsequently, the three core samples were centrifuged at 10,000 rpm for 3 hours each to ensure that all mobile fluid within the cores was completely removed. By measuring the mass of the cores after centrifugation, the volume of centrifuged fluid in the cores before and after centrifugation could be calculated, i.e., the content of mobile fluid in the cores containing white oil. The calculated mobile fluid contents in the three core samples were 0.35 cm³.3 0.11cm 3 With 0.08cm 3 To further compare the relationship between the content of movable fluids in white oil after centrifugation and the content of the two types of fluids, it is necessary to further compare the correlation between the total volumetric relaxant fluid of white oil in the core before and after centrifugation and the content of movable fluids after centrifugation. By calculating the total volumetric relaxant fluid of white oil in the core and the content of movable fluids after centrifugation and analyzing their correlation, the role of volumetric relaxant fluids in identifying the content of movable fluids can be demonstrated, such as... Figure 4 As shown in the figure, there is a high correlation (R0) between the total volumetric relaxant fluid and the centrifugal movable fluid content. 2 The linear fitting result of >0.99 can be expressed as: y = 1.4625x - 0.0353. This further proves that the ratio of volumetric relaxed / surface relaxed fluid is closely related to the ratio of movable / bound fluid in the core measured by centrifugation experiments. Based on this, it can also be confirmed that the calculation method of the relative content of the two types of relaxed fluids proposed in this study has good reference value when used to determine the movable proportion of fluids in shale.

[0095] This invention overcomes the limitations of NMR signal interpretation in evaluating the fluid mobility of oil-bearing rocks affected by receptor relaxation. It not only improves the reliability of NMR technology in identifying the fluid distribution of oil, but also provides a new standard for assessing the mobile oil content of rocks, thus enhancing the engineering applicability of NMR technology in the accurate evaluation of unconventional oil reservoirs.

[0096] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for predicting the movable oil content of rocks based on nuclear magnet relaxation ratio, characterized in that... This includes the following steps: 1) Select appropriate rock samples and white oil samples, prepare standard rock cores, clean and dry them, record the quality of the rock cores, and measure the T2 relaxation spectrum of the rock core matrix and the T2 volume relaxation parameters of the white oil under dry conditions. 2) Use white oil to perform a full saturation experiment on the dried core. When the core is confirmed to be fully saturated, record the core mass and carry out T2 nuclear magnetic resonance testing to obtain the T2 relaxation spectrum of the core in the saturated state. 3) Subtract the basement signal from the T2 relaxation spectrum of the core under saturation to obtain the T2 relaxation spectrum of the white oil in the core. Then, substitute the relaxation time and volume relaxation parameters of the white oil in the relaxation spectrum into the formula for calculating the volume relaxation fluid ratio to obtain the relationship between relaxation time and the volume relaxation fluid ratio of the white oil, and calculate the total volume relaxation fluid of the white oil in the core. The specific process of obtaining the relationship between relaxation time and the volume relaxation fluid ratio of the white oil is as follows: During low-field NMR testing, the transverse relaxation time T2 is mainly dominated by two mechanisms: (2)； In equation (2) above, T 2B For the relaxation time, T 2S For surface relaxation time; According to nuclear magnetic resonance theory, during low-field NMR testing, the decay process of the magnetization signal of the hydrogen-containing fluid in a single pore follows a single exponential decay law, and the total magnetization M(t) is equal to the sum of the magnetizations of the individual pores. (5)； In equation (5) above, M(t) is the magnetization signal of the pore measured at time t, M0 is the initial magnetization signal, and v i T represents the proportion of type i pores in the total porosity. 2i For the longitudinal and transverse relaxation times corresponding to the i-th type of pore; If only the same type of pores exist, the above equation (5) can be simplified to: (6)； In equation (6) above, M i (t) and M i (0) represents the magnetization signal measured at time t and time 0 for pore i, respectively; Based on formulas (5)-(6), we first analyze the relaxation process by examining the combined effects of the two relaxation mechanisms on the relaxation of oil in the pores. At this point, we can let t=1 and introduce formula (2) into formula (6) to further obtain: (7)； In equation (7) above, T 2is Let T be the transverse surface relaxation time of the fluid within pore i; since the volume relaxation process is not affected by the fluid's environment compared to surface relaxation, T in formula (7) is... 2B It can be obtained directly by performing nuclear magnetic resonance (NMR) tests on free-state fluids; In addition to considering the combined effects of the two relaxation mechanisms on pore fluid relaxation, the relaxation process of the fluid within the pores can also be viewed as a weighted average of the relaxation processes of two types of relaxable fluids under the influence of the two relaxation mechanisms, i.e., the measured magnetization signal M of the oil within the pores obtained by testing. i (t) is actually a weighted average of the relaxation processes of the two types of relaxant fluids; if, under the influence of the bulk relaxation mechanism, the proportion of fluids that complete the NMR relaxation process via bulk relaxation is γ (0 < γ < 1), then: (8)； In equation (8) above, γ is the measured relaxation time T. 2i The corresponding proportion of the volumetric relaxor fluid; Therefore, for the measured relaxation time T 2i For the fluid within the pores, according to formulas (7) and (8), we can obtain: (9)； By rearranging formula (9), the formula for calculating the volume relaxation ratio γ in the multi-mechanism relaxation signal is obtained as follows: (10)； 4) After the saturated core obtained from the full saturation experiment is centrifuged, the mass of the core after centrifugation is measured and the centrifugation movable fluid content of the core is calculated. Then, based on the linear correlation between the total amount of white oil relaxant fluid in the core and the centrifugation movable fluid content, a rock movable oil content prediction formula based on the nuclear magnetic resonance relaxation ratio is established.

2. The method for predicting the movable oil content of rocks based on nuclear magnet relaxation ratio as described in claim 1, characterized in that: The specific process of step 1) is as follows: Select a suitable rock sample to drill a standard core, clean the core appropriately and dry it to ensure that there is no obvious dust on its surface; then perform nuclear magnetic resonance T2 test on the processed dried core and the selected white oil to obtain the T2 relaxation spectrum of the dried core and the relaxation data of the white oil.

3. The method for predicting the movable oil content of rocks based on nuclear magnet relaxation ratio as described in claim 1, characterized in that: The T2 NMR test in step (2) used a CPMG pulse sequence suitable for transverse relaxation time magnetic fields, and the relevant parameters used included the echo interval T. E =0.132 milliseconds, number of echoes NECH=3788, number of scans n=32, waiting time T w =750 milliseconds and the number of loop steps is 30.

4. The method for predicting the movable oil content of rocks based on nuclear magnet relaxation ratio as described in claim 1, characterized in that, The specific process of the complete saturation experiment in step 2) is as follows: the core is placed in the core saturation device, vacuumed, white oil is injected and pressurized to 25 MPa, and saturated for more than 5 days to ensure that the core is completely saturated. After saturation is completed, the core is taken out, residual fluid on the core surface is removed, and nuclear magnetic resonance testing is performed to obtain the T2 relaxation spectrum under saturation.

5. The method for predicting the movable oil content of rocks based on nuclear magnet relaxation ratio as described in claim 2, characterized in that, The analytical method for the relationship between the total amount of relaxor fluid in the white oil body of the core and the content of centrifugally movable fluid in step 4) includes the following steps: 4.1) Determine the content of white oil mobile fluid in the core by centrifugation. During the centrifugation of the core, the temperature must be controlled within 0-4℃ to ensure that the fluid loss comes entirely from centrifugation. Furthermore, the core mass must be recorded before and after centrifugation to obtain the content of mobile white oil fluid. Additionally, broken centrifuged cores do not meet experimental requirements, cannot be included in the statistical range, and the experiment must be repeated. 4.2) Establish a formula for predicting the movable oil content of rocks based on the nuclear magnet relaxation ratio. By mapping the total amount of relaxed fluid in the white oil body of the core to the centrifugal movable fluid content of the white oil in the core and plotting them on a scatter plot, and performing linear correlation analysis on the results, a quantitative relationship between the total amount of relaxed fluid in the white oil body of the core and the centrifugal movable fluid content can be established, thereby enabling the prediction of the movable oil content in the rock.

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