Method for predicting rock movable oil content based on nuclear magnet relaxation proportion

Abstract

The invention provides a method for predicting rock movable oil content based on a nuclear magnet relaxation ratio. The method comprises the following steps: 1) measuring a rock core nuclear magnetic substrate T2 relaxation spectrum in a dry state and a T2 body relaxation parameter of white oil; 2) carrying out a complete saturation experiment on the rock core by using white oil, and carrying out a T2 nuclear magnetic resonance test to obtain a T2 relaxation spectrum of the rock core in a saturated state; 3) deducting a base signal from the saturated T2 relaxation spectrum of the rock core to obtain a T2 relaxation spectrum of white oil in the rock core, solving a relationship between relaxation time and a relaxation fluid proportion of the white oil body, and calculating the total amount of the relaxation fluid of the white oil body; and 4) centrifuging the obtained saturated rock core, measuring the mass of the rock core, calculating the centrifugal movable fluid content, and establishing a rock movable oil content prediction formula based on the nuclear magnet relaxation ratio. The method is reasonable in conception, the movable oil content is predicted by calculating the proportion of the relaxation signal of the nuclear magnetic resonance body 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] The present application relates to the technical field of oil and gas exploration, and particularly relates to a method for predicting movable oil content of rock based on nuclear magnetic body relaxation ratio. BACKGROUND

[0002] With the consumption of global conventional oil resources and the reduction of high-quality reserves, the difficulty and technical cost of oil resource development have increased significantly. Under this background, unconventional oil has become an important strategic replacement resource in the process of global energy transformation. However, compared with conventional oil resources, unconventional oil reservoirs exhibit lower porosity and permeability, with complex pore structure, variable lithofacies, strong heterogeneity, and diverse mineral composition, and thus it is necessary to accurately evaluate its enrichment characteristics and oiliness to overcome the challenges in the development process.

[0003] Unconventional oil mainly exists in complex multi-scale tight rock reservoir spaces (such as shale and tight sandstone). Different types of unconventional reservoirs have significant differences in the proportion of organic / inorganic pore types, pore size distribution, and fracture connectivity, which leads to more than 60% of unconventional oil remaining in the matrix micropores and being unable to be effectively produced during the production process. Therefore, the research on the mobility of oil in unconventional rock has become a key research direction to break through the bottleneck of development efficiency. At present, the analysis field of the mobility of reservoir fluid has formed a multi-experimental evaluation method system including oil saturation index (OSI), multi-dimensional nuclear magnetic method, multi-temperature stage pyrolysis method, swelling method, and solvent step extraction method. Among them, the nuclear magnetic resonance technology has been successfully applied to the classification of the occurrence types of organic pore adsorbed oil, inorganic pore free oil, and microfracture seepage oil, and has realized the fine characterization of the proportion of different state shale oil content.

[0004] However, the existing nuclear magnetic method still has significant limitations. The conventional nuclear magnetic experiment usually directly uses the relaxation spectrum of oil before and after centrifugation to invert the distribution of oil in the rock pore size and analyze the movable oil content, but ignores the influence of the difference between the lower bulk relaxation time of oil and water on the measured relaxation time. Since the nuclear magnetic signal of oil in the rock has the joint action of surface relaxation and bulk relaxation mechanism, the corresponding relationship between relaxation time and pore size appears systematic error (error range can reach 15%-30%) when converting the T2 relaxation spectrum into pore size distribution, thereby reducing the reliability of the nuclear magnetic distribution data in characterizing the pore size distribution of oil. This factor restricts the accurate evaluation of unconventional oil resources to some extent. However, since the bulk relaxation signal of fluid in the rock comes from fluid molecules far away from the pore wall, there is also a close relationship between the bulk relaxation fluid proportion and the movable oil content in the oil-bearing rock. Therefore, it is urgent to develop a new nuclear magnetic interpretation model to identify the bulk relaxation signal of oil and predict the movable oil content. SUMMARY

[0005] In view of the above background, the present application provides a method for predicting the content of movable oil in rocks based on the relaxation ratio of nuclear magnetic bodies, which realizes the prediction of the content of movable oil by calculating the proportion of the nuclear magnetic resonance relaxation signal of oil in rocks.

[0006] To solve the above technical problems, the present application provides a method for predicting the content of movable oil in rocks based on the relaxation ratio of nuclear magnetic bodies, which comprises the following steps:

[0007] 1) Select appropriate rock samples and white oil samples, prepare the rocks into standard cores and clean and dry them, record the mass of the cores, and measure the T2 relaxation spectrum of the cores in a dry state and the T2 bulk relaxation parameters of the white oil;

[0008] 2) Perform a complete saturation experiment on the dried cores using white oil, record the mass of the cores when they reach complete saturation and carry out T2 nuclear magnetic resonance testing to obtain the T2 relaxation spectrum of the cores in a saturated state;

[0009] 3) Subtract the T2 relaxation spectrum of the cores in a saturated state from the base signal to obtain the T2 relaxation spectrum of the white oil in the cores, and then substitute the relaxation time and the bulk relaxation parameters of the white oil into the bulk relaxation fluid proportion calculation formula to obtain the relationship between the relaxation time and the proportion of the white oil bulk relaxation fluid and calculate the total amount of white oil bulk relaxation fluid in the cores;

[0010] 4) Centrifuge the saturated cores obtained after the complete saturation experiment, measure the mass of the cores after centrifugation and calculate the content of centrifugal movable fluid in the cores, and then establish a formula for predicting the content of movable oil in rocks based on the relaxation ratio of nuclear magnetic bodies based on the linear correlation between the total amount of white oil bulk relaxation fluid in the cores and the content of centrifugal movable fluid.

[0011] The method for predicting the content of movable oil in rocks based on the relaxation ratio of nuclear magnetic bodies, wherein the specific process of step 1) is to select appropriate rock samples, drill standard cores, and dry the cores after moderate cleaning to ensure that their surfaces are free of obvious dust; thereafter, perform nuclear magnetic resonance T2 testing on the treated dry cores and the selected white oil to obtain the T2 relaxation spectrum of the dry cores and the bulk relaxation data of the white oil.

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

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 specific process for determining the relationship between relaxation time and the proportion of white oil relaxant fluid in step 3) is as follows: During low-field NMR testing, the transverse relaxation time T2 is mainly dominated by two mechanisms: 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. 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: 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: 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: 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: After rearranging formula (9), the formula for calculating the volume relaxation ratio γ in the multi-mechanism relaxation signal is obtained as follows:

6. 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 relaxant 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 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. 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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