In-situ quantification and multi-scale positioning characterization method for oil displacement recombination analysis by co2 injection

Abstract

The application discloses an in-situ quantification and multi-scale positioning characterization method for oil displacement by CO2 injection, relates to the technical field of oil and gas exploration and development, and comprises the following steps: initial state calibration and reference collection; gas injection displacement and dynamic damage introduction; in-situ thermal remelting treatment; and multi-dimensional quantitative evaluation.The application has the beneficial effect that the "in-situ thermal remelting" strategy is introduced, the absolute mass of the precipitates can be directly calculated by establishing the relationship between the total signal S hot of the remelting state and the total signal S cold of the precipitating state, a multi-scale characterization system of "microscopic pore + macroscopic core" is established, the specific pore diameter interval where the plugging occurs is accurately identified by using the pore diameter resolution capability of the T2 spectrum, and the non-destructive and full-view monitoring of the distribution of the precipitates is realized by using the "in-situ MRI imaging + quantitative matrix correction" technology, so that the spatial distribution law of the precipitates in the core is directly revealed.

Description

Technical Field

[0001] This invention relates to the field of oil and gas exploration and development technology, and in particular to an in-situ quantitative and multi-scale localization characterization method for CO2 flooding reorganization analysis. Background Technology

[0002] In the oil and gas field development sector, CO2 injection is not only an important method for enhancing oil recovery but also enables geological CO2 sequestration, effectively mitigating the greenhouse effect and possessing significant economic and environmental value. However, during CO2 flooding, when supercritical CO2 comes into contact with formation crude oil, CO2 exhibits a strong extraction ability for light components. As light components are preferentially extracted and carried away, the relative concentration of heavy components (such as waxes, gums, and asphaltenes) in the remaining oil increases. Furthermore, changes in the solubility parameters of the fluid system easily disrupt phase equilibrium, leading to recombination and precipitation in the rock pores. This recombination not only reduces oil recovery but also severely clogs reservoir pore throats, significantly reducing permeability and directly hindering the effectiveness of CO2 injection.

[0003] Therefore, establishing a precise evaluation method is of significant engineering importance. On the one hand, it is necessary to quantitatively obtain the amount of recombinant components precipitated and construct a quantitative relationship between precipitate content and permeability loss to accurately assess the degree of reservoir damage. On the other hand, it is necessary to accurately locate the specific spatial position where the precipitate occurs in order to identify the blockage mechanism and provide a basis for decision-making in formulating targeted unblocking solutions. Currently, the technical means used in the industry to evaluate such recombinant analysis mainly have the following limitations: Conventional core displacement pressure difference method: It can only macroscopically and qualitatively determine whether blockage has occurred by monitoring changes in displacement pressure difference and permeability, treating the core as a "black box system," which cannot quantitatively calculate the specific mass of the precipitate, nor can it determine the microscopic pore location where the blockage occurs.

[0004] Offline analysis methods: Conventional methods such as cross-section observation, solvent extraction, or scanning electron microscopy (SEM) are all destructive and localized detection techniques. Cross-sectioning or microscopic scanning can only observe the precipitation characteristics of a few specific sections or a very small field of view in the core, while the pore structure and degree of damage along the core are often heterogeneously distributed. This method cannot fully reflect the macroscopic spatial distribution of the entire core from the injection end to the exit end, making it difficult to make a holistic assessment of reservoir damage. In addition, this process often requires cutting, crushing, or chemically treating the core, resulting in permanent damage to the core and making it unusable, thus wasting experimental samples. At the same time, its pretreatment process is cumbersome, the analysis cycle is long, and the experimental cost is high, making it difficult to meet the needs of engineering for rapid evaluation of injection and production effects.

[0005] Online NMR: While T1-T2 two-dimensional NMR technology can assist in fluid identification, the solid phase recombinant component signal is in the instrument's detection "blind zone," making direct quantification impossible. This can easily lead to misinterpretation of lost precipitation signals as oil displacement effects, resulting in a significantly inflated recovery rate assessment. Furthermore, two-dimensional spectrum sampling is time-consuming, failing to meet the real-time monitoring requirements of CO2 injection dynamics, and the ambiguity of the inversion algorithm results in significant errors in component quantification calculations. Therefore, existing technologies cannot simultaneously meet the requirements of "in-situ non-destructive," "real-time dynamic," and "precise quantification." The industry urgently needs an analytical method that can overcome the NMR solid phase signal blind zone, accurately distinguish the essential difference between "fluid displacement" and "signal disappearance due to recombinant analysis," and provide in-situ, online, and quantitative characterization of the timing, location, and amount of recombinant analysis during gas injection at the whole core scale, in order to reveal the spatiotemporal evolution of reservoir damage.

[0006] To address the technical challenge of existing nuclear magnetic resonance (NMR) technology in evaluating gas injection (especially CO2 injection) development, which is hampered by the extremely short relaxation time of heavy components (such as wax and asphaltenes) after precipitation and solidification, placing them in the instrument's detection blind zone, and thus making it impossible to distinguish between "fluid displacement" and "retention of heavy components during phase transformation," thereby hindering accurate assessment of reservoir damage, an in-situ, online, and non-destructive method for in-situ quantification and multi-scale localization characterization of reorganization analysis in CO2-assisted oil recovery is urgently needed. Summary of the Invention

[0007] To address the aforementioned technical problems, this invention discloses an in-situ quantitative and multi-scale localization characterization method for recombinant components analyzed during CO2 flooding. This invention utilizes online low-field nuclear magnetic resonance (NMR) technology to perform in-situ quantitative calculations of the precipitation of recombinant components (such as waxes, asphaltenes, and resins) caused by fluid extraction or component differentiation during gas injection (such as CO2, hydrocarbons, etc.) displacement, and to locate their multi-scale distribution in pores and core samples.

[0008] To achieve the above objectives, the present invention adopts the following technical solution:

[0009] The in-situ quantitative and multi-scale localization characterization method derived from CO2 flooded oil recombination analysis includes the following steps:

[0010] Step S1: Initial State Calibration and Reference Acquisition

[0011] The core samples were cleaned, dried, and vacuum saturated with formation crude oil. The first nuclear magnetic resonance T2 spectrum scan was performed under simulated formation temperature and pressure conditions to obtain the fluid distribution characteristics of the core under the initial saturated oil state as a comparison benchmark.

[0012] Step S2, Gas Displacement and Dynamic Damage Introduction

[0013] Gas injection displacement experiments were conducted to induce recombination analysis through the contact extraction of gas and crude oil, which was then retained in the core pores. After displacement equilibrium was reached, scanning was performed to characterize the organic matter precipitation damage caused by gas injection.

[0014] Step S3: In-situ heat remelting treatment

[0015] Keeping the core position unchanged, a set thermal field is applied to it to raise the temperature. The thermal effect is used to induce the precipitated heavy components to undergo phase change remelting or redissolution. The T2 spectrum and nuclear magnetic resonance imaging after heat treatment are collected to record the recovery of fluid signals.

[0016] Step S4: Multidimensional Quantitative Evaluation

[0017] A quantitative-localization evaluation system was constructed based on nuclear magnetic resonance data. The total amount of heavy components precipitated was quantitatively calculated using T2 difference spectroscopy, and multi-level localization identification was implemented: at the microscale, the precipitation characteristics of heavy components in pores of different sizes were clarified based on T2 spectral characteristics; at the macroscale, nuclear magnetic resonance imaging technology was introduced to intuitively locate the enrichment area of ​​precipitates in the core space, revealing the precipitation distribution law of heavy components in CO2 displacement.

[0018] Furthermore, the specific process of step S1 is as follows:

[0019] S11. Load the core to be tested into a non-magnetic variable-temperature core holder, apply confining pressure and back pressure, and establish a simulated formation pressure environment.

[0020] S12. Stabilize the core temperature at the simulated target temperature T using a constant temperature circulation system. res ;

[0021] S13. Inject saturated simulated formation oil containing light and heavy components into the core. After the saturation is uniform, use the initial state nuclear magnetic T2 spectrum and initial nuclear magnetic imaging map.

[0022] Furthermore, in step S12, the temperature is set below the freezing point of the heavy components in the target fluid.

[0023] Furthermore, the specific process of step S2 is as follows:

[0024] S21. Supercritical CO2 gas is injected into the core at a constant speed or constant pressure using an injection pump.

[0025] S22. During the displacement process, the injected gas comes into contact with the formation oil, extracting light hydrocarbon components and supersaturated heavy components, which precipitate in situ as solid phases within the core pores.

[0026] S23. Keeping the pressure constant, collect the T2 NMR spectrum of the precipitated state, and record the total T2 signal quantity at this time as S. cold precipitated state NMR image Im coldAt this point, the intermolecular forces of the precipitated organic solid phase are strong, and their signal contribution to the T2 spectrum is negligible.

[0027] Furthermore, the specific process of step S3 is as follows:

[0028] S31. Maintain the core's position, confining pressure, and pore fluid pressure within the holder;

[0029] S32. Adjust the temperature control system to raise the core temperature to the remelting temperature T. hot_K Once the core temperature is balanced and the NMR signal is stable, the solid phase heavy components undergo a phase transition, and the signal is restored.

[0030] S33. Acquire the remelted NMR T2 spectrum, and denote the total signal quantity as S. hot And the remelted nuclear magnetic resonance imaging, denoted as Im hot .

[0031] Furthermore, in S32, the temperature setting must be higher than the melting point or total melting temperature of the organic solid phase.

[0032] Furthermore, in step S4, using the collected cold and hot state data, the following evaluation and reorganization analysis are conducted to determine the specifics:

[0033] S41, based on experimental verification results, using Curie's law to correct for temperature-induced magnetization vector changes, the amount of recombinant analysis was quantitatively calculated. The calculation formula is as follows:

[0034] (1);

[0035] In the formula, K1 is the signal correction coefficient at different temperatures; K2 is the system calibration coefficient, mg / au, and K2 is 4.24 mg / au at 45℃; S hot S cold αu represents the total signal amplitude in the remelted state and the precipitated state, respectively.

[0036] S42, constructing a complex melting difference spectrum to characterize the distribution of organic solid phases in pores of different sizes;

[0037] First, based on the following equation (2) characterizing the occurrence characteristics of organic solid phases in pores of different size ranges, and according to the principle of nuclear magnetic resonance, the T2 relaxation time is positively correlated with the pore radius, and equation (2) is:

[0038] (2);

[0039] If the peak value of the differential spectrum is concentrated in the long T2 range, it indicates that the precipitate mainly blocks the throat of large pores; if the peak value is concentrated in the short T2 range, it indicates that the precipitation mainly occurs in micropores or exists in the form of surface adsorption.

[0040] Furthermore, based on the qualitative analysis, the pore size distribution is quantitatively characterized using formula (3). By calculating the difference between the amplitude of the remelted state signal and the amplitude of the precipitated state signal at each relaxation time point, the amount of precipitation at the corresponding relaxation time can be obtained. Formula (3) is:

[0041] (3);

[0042] In the formula, This represents the amount of precipitation at relaxation time T2; , α and β are the signal amplitudes at relaxation time T2, representing the remelted state and the precipitated state, respectively;

[0043] Finally, based on the calculation results, a distribution curve of the amount of precipitate as a function of T2 (i.e., pore size) was plotted to achieve accurate quantitative calculation of the amount of recombinant precipitate in different pore sizes.

[0044] S43, the signal intensity of the core NMR image is calculated based on grayscale values, visualized using reverse Jet chromatography, and after temperature correction, the spatial distribution of the precipitated heavy components can be obtained. deposit (x,y), the calculation formula is:

[0045] (4);

[0046] In the formula, I deposit (x,y) represents the normalized recombinant analysis yield; Im hot (x,y) represents the signal intensity of the core NMR image after remelting; Im cold (x,y) represents the signal intensity of the core NMR image before heating; K1 is the signal correction coefficient at different temperatures; Im hot,max This represents the maximum signal intensity value in the image after remelting at higher temperatures.

[0047] The beneficial effects of this invention are that it has the following advantages compared with the prior art:

[0048] 1. An innovative "in-situ thermal remelting" strategy is introduced, utilizing the reversible phase transition characteristics of heavy components to restore the "hidden" solid-phase signal to a "visible" liquid-phase signal. This is achieved by establishing a total remelted state signal (S...). hot ) and total signal of precipitated state (S) cold The relationship between the precipitate and the absolute mass of the precipitate can be directly calculated.

[0049] 2. A multi-scale characterization system of "microscopic pores + macroscopic core" was established, and the specific pore size range where blockage occurred was accurately identified by utilizing the pore size resolution capability of T2 spectrum.

[0050] 3. The innovative use of "in-situ MRI imaging + quantitative matrix correction" technology enables non-destructive, full-view monitoring of precipitate distribution, intuitively revealing the spatial distribution pattern of precipitates within the core. This method not only avoids human error caused by physical cutting, but also intuitively presents the heterogeneous distribution pattern of precipitates at the core scale, providing a basis for optimizing the gas injection development scheme. Attached Figure Description

[0051] Figure 1 These are the signal quantities of a unit mass of sample oil at different temperatures in the embodiments of the present invention;

[0052] Figure 2 The T2 spectra of the core before and after displacement in this embodiment of the invention (without remelting).

[0053] Figure 3 These are the T2 spectra before and after heating following core displacement in this embodiment of the invention;

[0054] Figure 4 The location of the reorganization analysis within the pores of different sizes in the embodiments of the present invention is determined;

[0055] Figure 5 This is a distribution curve of the amount of recombinant analysis as a function of T2 (pore size) in an embodiment of the present invention;

[0056] Figure 6 This is a comparison of nuclear magnetic resonance imaging of core samples before and after heating in an embodiment of the present invention;

[0057] Figure 7 This is a grayscale distribution diagram of the core nuclear magnetic resonance signal before and after heating in an embodiment of the present invention;

[0058] Figure 8 This is a distribution diagram of the amount of core recombinant analysis before and after heating in an embodiment of the present invention;

[0059] Figure 9 This is a physical simulation device used in this embodiment of the invention to characterize the precipitation of organic solid phases in gas injection development reservoirs;

[0060] Among them, 1-first plunger pump; 2-three-way valve; 3-CO2 intermediate container; 4-sample oil intermediate container; 5-first pressure sensor; 6-second pressure sensor; 7-first check valve; 8-second check valve; 9-six-way valve; 10-third check valve; 11-third pressure sensor; 12-low-field nuclear magnetic resonance spectrometer; 13-nuclear magnetic resonance scanning area; 14-non-magnetic variable-temperature core holder; 15-core; 16-ceramic plug; 17-valve; 18-fourth pressure sensor; 19-backpressure valve; 20-metering pump; 21-second plunger pump; 22-third plunger pump; 23-first circulating temperature control system; 24-fifth pressure sensor; 25-second circulating temperature control system. Detailed Implementation

[0061] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. 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.

[0062] In traditional NMR displacement experiments, after heavy components (such as waxes and asphaltenes) precipitate into the solid phase, their T2 relaxation time is extremely short, placing them in the "signal blind zone" of the instrument. This leads to the precipitate signal being mistakenly identified as "displaced oil," directly causing a severely inflated recovery rate calculation and making it impossible to obtain the specific mass of the precipitate. This invention innovatively introduces an "in-situ thermal remelting" strategy (e.g., raising the temperature from the displacement temperature of 28°C to the remelting temperature of 45°C). Utilizing the reversible phase transition characteristics of heavy components, the "hidden" solid phase signal is restored to a "visible" liquid phase signal. By establishing the total remelted state signal (S... hot ) and total signal of precipitated state (S) cold The relationship between the precipitate and the absolute mass of the precipitate can be directly calculated.

[0063] This invention discloses an in-situ quantitative and multi-scale localization characterization method for CO2-assisted oil recovery analysis, comprising the following steps:

[0064] Step 1: Initial state calibration and benchmark acquisition.

[0065] The specific process is as follows:

[0066] S11, the core to be tested 15 is loaded into the non-magnetic variable temperature core holder 14, and confining pressure and back pressure are applied to establish a simulated formation pressure environment.

[0067] S12, the core temperature is stabilized at the simulated reservoir temperature T through a constant temperature circulation system. res The temperature is set below the melting point of the heavy components (such as high-carbon alkanes) in the target fluid.

[0068] S13, saturated simulated formation oil containing light and heavy components in core 15, after saturation and homogenization, initial state NMR T2 spectrum and initial NMR imaging map were collected.

[0069] Step 2: Gas displacement and introduction of dynamic damage.

[0070] The specific process is as follows:

[0071] S21, supercritical CO2 gas is injected into core 15 at a constant speed or constant pressure using an injection pump;

[0072] S22, during the displacement process, the injected gas comes into contact with the formation oil, extracting light hydrocarbon components, leading to supersaturation of heavy components, which precipitate in situ as solid phase in the pores of core 15;

[0073] S23, keeping the pressure constant, collect the T2 NMR spectrum of the precipitated state (cold state), and record its total signal quantity as S. cold The precipitated state NMR image was obtained, and its two-dimensional signal distribution Im was calculated. cold At this point, the intermolecular forces of the precipitated organic solid phase are strong, and their signal contribution to the T2 spectrum is negligible.

[0074] Step 3: In-situ heat remelting treatment.

[0075] The specific process is as follows:

[0076] S31, keep the position, confining pressure and pore fluid pressure of core 15 in the holder unchanged, and prevent the fluid from flowing mechanically;

[0077] S32, adjust the temperature control system to raise the core temperature to the remelting temperature T. hot_K The temperature setting must be higher than the melting point or total melting temperature of the organic solid phase (e.g., for C22 alkanes, T...). hot_K (Typically set to 45℃ or higher); after the core temperature reaches equilibrium and the NMR signal stabilizes, the organic solid phase undergoes a phase transition, and the signal recovers.

[0078] S33, acquire the remelted NMR T2 spectrum, and record its total signal quantity S. hot And the remelted nuclear magnetic resonance imaging, denoted as Im hot .

[0079] Step 4: Multidimensional quantitative evaluation.

[0080] Using the collected cold and hot state data, the following evaluation and reorganization analysis were conducted:

[0081] S41, quantitative calculation of precipitate amount.

[0082] Based on the experimental verification conclusion (i.e., the existence form of the organic solid phase after remelting—pure liquid phase or molten state—has a negligible effect on the hydrogen signal per unit mass), Curie's Law is used to correct the temperature-induced magnetization vector change.

[0083] The quantitative calculation of recombinant analysis yield is performed using the following formula:

[0084] (1);

[0085] In the formula, K1 is the signal correction coefficient at different temperatures, obtained through standard sample calibration; K2 is the system calibration coefficient, mg / au, obtained through standard sample calibration, and K2 is 4.24 mg / au at 45℃; S hot S cold : αu represents the total signal amplitude of the T2 spectrum in the remelted state and the precipitated state, respectively.

[0086] S42, Evaluation of micropore distribution.

[0087] A complex melting difference spectrum was constructed to characterize the distribution of organic solid phases in pores of different sizes.

[0088] First, the occurrence characteristics of organic solid phases in pores of different size ranges are characterized based on the following equation (2). According to the principle of nuclear magnetic resonance, the T2 relaxation time is positively correlated with the pore radius. Based on this, the location of blockage can be qualitatively identified. If the peak value of the differential spectrum is concentrated in the long T2 interval, it indicates that the precipitate mainly blocks the throat of large pores; if the peak value is concentrated in the short T2 interval, it indicates that the precipitation mainly occurs in micropores or exists in the form of surface adsorption. Equation (2) is:

[0089] (2).

[0090] Based on the qualitative analysis, the pore size distribution is quantitatively characterized using formula (3). By calculating the difference between the amplitude of the remelted state signal and the amplitude of the precipitated state signal at each relaxation time point, the amount of precipitation at the corresponding relaxation time can be obtained. Formula (3) is:

[0091] (3);

[0092] In the formula, This represents the amount of precipitation at relaxation time T2; , αu represents the signal amplitude at relaxation time T2 for the remelted state and the precipitated state, respectively.

[0093] Finally, based on the calculation results, a distribution curve of the amount of precipitate changing with T2, i.e., the pore size, was plotted to achieve accurate quantitative calculation of the amount of recombinant precipitate in different pore sizes.

[0094] S43, Evaluation of macroscopic spatial distribution.

[0095] Constructing a spatial distribution map of the precipitated recombinant components I deposit (x, y) is used to determine the specific location of precipitation damage, such as whether it occurs at the injection end, the spurious front, or is uniformly distributed throughout the core. The signal intensity of the core NMR image is calculated based on grayscale values, and visualization is performed using reverse Jet chromatography. After temperature correction, a spatial distribution map of the heavy components is obtained. deposit (x,y), the calculation formula is:

[0096] (4);

[0097] In the formula, I deposit (x,y) represents the normalized recombinant analysis yield; Im hot (x,y) represents the signal intensity of the core NMR image after remelting; Im cold (x,y) represents the signal intensity of the core NMR image before heating; K1 is the signal correction coefficient at different temperatures; Im hot,max This is the maximum signal intensity value in the image after remelting (used for normalization).

[0098] Application examples

[0099] This method was used to apply the displaced core samples (displacement temperature 28℃, remelting temperature 45℃).

[0100] (1) Quantitative calculation of precipitation and correction of recovery rate

[0101] The specific process is as follows:

[0102] First, based on the amount of nuclear magnetic resonance signal in the experimental fluid at different temperatures ( Figure 1 Perform temperature correction and calculate the temperature correction coefficient K1.

[0103] (5);

[0104] (6);

[0105] In the formula, T is the temperature (°C); K3 is... Figure 1 The slope of the fitted line is set to -2.15; T hot_K T cold_K These are the thermodynamic temperatures (°C) for the remelting temperature and the precipitation temperature, respectively.

[0106] Subsequently, CO2 displacement experiments and remelting operations were performed, while nuclear magnetic resonance data were acquired. Figure 3 The total signal quantity S of the T2 remelting state is obtained by integrating the spectral lines. hot The total semaphore S of the extracted state is 2088.8. cold The value is 1725.1. Substituting the above value into formula (1), the amount of recombinant analysis M is obtained. deposit It is 1.7g.

[0107] Subsequently, based on the above precipitation data, the core recovery rate was corrected and evaluated. According to formula (5) mentioned in this invention specification, a linear correspondence between the total nuclear magnetic resonance signal amplitude and the fluid mass M at the experimental temperature was established, denoted as formula (7):

[0108] (7);

[0109] Where S is the total signal amplitude in the core at the experimental temperature, derived from... Figure 2 get.

[0110] Based on this, the uncorrected recovery rate is first calculated using existing conventional methods, as shown in formula (8):

[0111] (8);

[0112] Where R* represents the uncorrected recovery rate, %; M s The mass of the sample oil prepared from the saturated core before displacement, in g; M cold The mass of the remaining sample oil in the uncorrected precursor displacement core is given in g.

[0113] The core recovery rate R* before correction is 31.2%, which can be calculated using formulas (7) and (8).

[0114] Since the above recovery rate R* did not consider the impact of solid phase signal loss caused by recombination analysis, the evaluation result was inflated. Therefore, the calculated recombination analysis amount Mdeposit was used for correction. The correction model is formula (9):

[0115] (9);

[0116] Where R is the corrected recovery rate, %.

[0117] M deposit Substituting (1.25g) into formula (9) yields a corrected true recovery rate R of 15.2%.

[0118] Experimental results show that the corrected recovery rate (15.2%) is significantly lower than the uncorrected value (31.2%). This indicates that conventional methods, by ignoring recombination analysis, result in a larger evaluation error. This invention, however, introduces M... deposit The item was revised to effectively eliminate "pseudo-recovery rates" and restore the true oil displacement efficiency of gas injection development.

[0119] (2) Location of precipitation at the pore size

[0120] Based on the core T2 spectrum ( Figure 3 Based on the characteristics, the pore space is divided into two intervals: micropores (0.01-35.00 ms) and macropores (35.00-1000.00 ms), with 35.00 ms as the boundary. For example... Figure 4As shown, the signal amplitudes of both types of pores showed an upward trend after remelting. Characteristic analysis indicated that precipitation in the small pore region was mainly concentrated on the right side with a longer relaxation time, while precipitation in the large pore region was significantly enriched on the left side with a shorter relaxation time. Further calculation based on formula (2) showed that the difference spectrum integral of the small pore region was 156.0, while that of the large pore region was as high as 207.8. This result confirms that the heavy component mainly precipitated in the large pore region with a long relaxation time.

[0121] Further combining formula (3) with Figure 3 Obtain the curve of the precipitation amount as a function of T2 ( Figure 5 Analysis showed that in the small pore range (0.01-35.00 ms), the amount of recombinant precipitate increased monotonically with the increase of T2 value (i.e. pore size); while in the large pore range (35.00-1000.00 ms), the amount of precipitate showed a "first increase and then decrease" change pattern with the increase of T2 value.

[0122] Compared to conventional core displacement differential pressure methods, which can only vaguely indicate "core blockage" through a decrease in overall permeability or an increase in pressure, this invention achieves precise characterization of the degree of blockage in different pores. Conventional differential pressure methods obtain macroscopic average values ​​and cannot distinguish whether blockage occurs in the large pores that contribute most to conductivity, or in the small pore throats controlled by capillary forces. This invention, however, utilizes the pore size resolution of the T2 spectrum to clearly quantify that precipitates are mainly concentrated in large pores (35.00-1000.00), thus revealing the patterns identified in the reconstructed analysis at the microscopic scale.

[0123] (3) Location of precipitation at the core scale

[0124] To clarify the specific distribution characteristics of heavy components at the macroscopic scale of the core, nuclear magnetic resonance imaging (MRI) visualization analysis was used:

[0125] First, nuclear magnetic resonance imaging data of the core were collected before heating (precipitated state) and after heating (remelted state) (e.g. Figure 6 As shown), and convert the original imaging data into a grayscale distribution map that can quantify the signal intensity (as shown). Figure 7 (As shown).

[0126] Subsequently, using formula (4), matrix operations, temperature correction, and normalization were performed on the gray values ​​of corresponding pixels before and after heating to generate a core reconstruction analysis yield distribution map (e.g., Figure 8 (As shown).

[0127] Quantitative analysis results show that the bright areas representing high precipitation are highly concentrated at the core outlet, confirming that the heavy components mainly precipitate and remain at the core outlet.

[0128] Unlike conventional methods that typically require destructive core cutting for segmented extraction analysis, this invention utilizes "in-situ imaging + quantitative matrix correction" technology to achieve non-destructive, full-view monitoring of precipitate distribution. This method not only avoids human error caused by physical cutting but also visually presents the heterogeneous distribution patterns of precipitates at the core scale, providing a basis for optimizing gas injection development schemes.

[0129] This invention also discloses the in-situ quantitative and multi-scale localization characterization method for the above-mentioned CO2 injection oil displacement and recombination analysis, and the high-temperature and high-pressure online variable-temperature nuclear magnetic resonance displacement system used, such as... Figure 9 As shown, it includes a nuclear magnetic resonance detection unit, an in-situ displacement and temperature control unit, and a fluid injection and control unit. Specifically,

[0130] ① Nuclear magnetic resonance detection unit: It adopts a low-field nuclear magnetic resonance analyzer, equipped with radio frequency coil and gradient coil, for acquiring T2 spectrum and nuclear magnetic imaging (MRI) data.

[0131] ② In-situ displacement and temperature control unit: The core component is a non-magnetic variable-temperature core holder 14 placed inside the nuclear magnetic resonance magnet. The holder is made of special ceramic material to avoid interference with the magnetic field; it is equipped with a circulating temperature control jacket, connected to an external high-precision constant-temperature circulating pump, which supports rapid adjustment of the core temperature within the range of 20℃ to 120℃, with a temperature control accuracy of ±0.1℃.

[0132] ③ Fluid Injection and Control Unit: Used to simulate formation pressure conditions and fluid displacement process, including first plunger pump 1 (high pressure constant speed / constant pressure), intermediate containers (containing formation oil, CO2 or other injection gas respectively), back pressure valve 19, second plunger pump 21 and third plunger pump 22.

[0133] The top of the first plunger pump 1 is connected via pipelines to the bottom of the CO2 intermediate container 3 and the sample oil intermediate container 4. The tops of the CO2 intermediate container 3 and the sample oil intermediate container 4 are connected to a six-way valve 9, which is also connected to one end of a third one-way valve 10. The other end of the third one-way valve 10 is connected to the left end of a non-magnetic variable-temperature core holder 14. The right end of the non-magnetic variable-temperature core holder 14 is connected to a back pressure valve 19, which is also connected to a metering pump 20 and a second plunger pump 21. The top of the non-magnetic variable-temperature core holder 14 is connected to the core 15 inside. The two ends of the core 15 are fixed and sealed by ceramic plugs 16. The non-magnetic variable-temperature core holder 14 is connected to the low-field nuclear magnetic resonance spectrometer 12. The non-magnetic variable-temperature core holder 14 is connected to the first circulation temperature control system 23. The CO2 intermediate container 3 and the sample preparation oil intermediate container 4 are connected to the second circulation temperature control system 25. The low-field nuclear magnetic resonance spectrometer 12 and the core 15 form a nuclear magnetic resonance scanning area 13.

[0134] A three-way valve 2 is installed on the pipeline connecting the first plunger pump 1 to the CO2 intermediate container 3 and the sample oil intermediate container 4. A first check valve 7 is installed on the pipeline between the CO2 intermediate container 3 and the six-way valve 9. A second check valve 8 is installed on the pipeline between the sample oil intermediate container 4 and the six-way valve 9. A valve 17 is installed on the pipeline between the non-magnetic variable temperature core holder 14 and the back pressure valve 19.

[0135] A first pressure sensor 5 is installed on the pipeline between the CO2 intermediate container 3 and the first one-way valve 7; a second pressure sensor 6 is installed on the pipeline between the sample oil intermediate container 4 and the second one-way valve 8; a third pressure sensor 11 is installed on the pipeline between the third one-way valve 10 and the non-magnetic variable-temperature core holder 14; a fourth pressure sensor 18 is installed on the pipeline between the valve 17 and the back pressure valve 19; and a fifth pressure sensor 24 is installed on the pipeline between the non-magnetic variable-temperature core holder 14 and the first circulating temperature control system 23.

[0136] The process of using this device:

[0137] In the initial state of the experiment, all valves 17 were closed.

[0138] First, establish the system temperature and pressure: open valve 2 and use plunger pump 1 to pressurize intermediate containers 3 and 4 to the target pressure; at the same time, start the temperature control system, use temperature control device 25 to maintain the intermediate containers and pipelines at the target temperature, and use temperature control device 23 to set the core holder 14 and coil 15 to the target temperature; then, use plunger pump 22 to apply confining pressure to core holder 14 (set value is 3MPa higher than the target pressure), and use plunger pump 21 to set back pressure valve 19 to the target pressure.

[0139] Next, the core 15 is saturated: connecting valve 2, valve 17, and the passage of the six-way valve 9 to the intermediate container 4, and setting a plunger pump 1 to inject the fluid in the intermediate container 4 into the core holder 14 at a constant flow rate until the core 15 is fully saturated.

[0140] Next, a displacement experiment was conducted: after saturation, the flow path was switched, keeping only valves 2, 17, and the passage between the six-way valve 9 and the intermediate container 3 open, and carbon dioxide was injected at a constant flow rate for displacement, and the produced fluid was collected through container 20.

[0141] Finally, a hot remelting test is performed: after the displacement is completed, all valves are closed, and the system is heated to the set remelting temperature using the temperature control device 23, followed by nuclear magnetic resonance scanning.

[0142] Compared with existing technologies, this invention has significant substantive features and progress, mainly reflected in two core advantages: "in-situ precise quantification" and "multi-scale spatial positioning".

[0143] 1. Overcoming signal blind zones, achieving in-situ precise quantification of recombinant analysis results.

[0144] Existing technical problem: In traditional nuclear magnetic resonance displacement experiments, after heavy components (such as wax and bitumen) precipitate as a solid phase, their T2 relaxation time is extremely short, falling within the instrument's "signal blind zone." This causes the precipitate signal to be mistakenly identified as "displaced oil," directly resulting in a severely inflated recovery rate calculation and making it impossible to obtain the specific quality of the precipitate.

[0145] This invention innovatively introduces an "in-situ thermal remelting" strategy (e.g., raising the temperature from the displacement temperature of 28°C to the remelting temperature of 45°C), utilizing the reversible phase transition characteristics of heavy components to restore the "invisible" solid-phase signal to a "visible" liquid-phase signal, by establishing a total remelted state signal (S). hot ) and total signal of precipitated state (S) cold The relationship between the precipitate and the absolute mass of the precipitate can be directly calculated.

[0146] 2. Breaking through the "black box" blind spot, achieving multi-scale precise positioning of the location obtained from reconstruction analysis.

[0147] To address the challenges of conventional differential pressure methods failing to distinguish microscopic blockage levels and traditional slicing analysis damaging core samples, this invention establishes a multi-scale characterization system combining "microscopic pores + macroscopic core." At the pore scale, the T2 spectrum's pore size resolution accurately identifies the specific pore size range where blockage occurs. Furthermore, at the core scale, an innovative "in-situ MRI imaging + quantitative matrix correction" technique replaces traditional destructive cutting, enabling non-destructive monitoring and directly revealing the spatial distribution patterns of precipitates within the core.

[0148] This invention utilizes the principle of "in-situ thermal remelting" to restore the previously invisible recombinant analysis signals to visible liquid phase signals through temperature control strategies, while maintaining the core displacement state and pressure environment. This enables precise evaluation in the following three dimensions:

[0149] ① Quantitative dimension: Accurately calculate the specific quality of the recombinant analysis product and effectively eliminate the recovery rate calculation error caused by signal loss (solve the problem of quantitative calculation of precipitation).

[0150] ② Microscopic dimension: Clarify the occurrence characteristics of recombinant components in pore throats of different sizes, and reveal their differentiated clogging laws on pores of different sizes (solve the problem of characterizing micropore size distribution).

[0151] ③ Macroscopic dimension: Combining nuclear magnetic resonance imaging technology, it can intuitively present the distribution of heavy components along the core axis and accurately locate the specific zone where damage occurs (solving the problem of macroscopic spatial positioning).

[0152] Of course, the above description is not intended to limit the present invention, and the present invention is not limited to the examples given above. Any changes, modifications, additions or substitutions made by those skilled in the art within the scope of the present invention should also fall within the protection scope of the present invention.

Claims

1. An in-situ quantitative and multi-scale localization characterization method for CO2-assisted oil recovery analysis, characterized in that, Includes the following steps: Step S1: Initial State Calibration and Reference Acquisition The core samples were cleaned, dried, and vacuum saturated with formation crude oil. The first nuclear magnetic resonance T2 spectrum scan was performed under simulated formation temperature and pressure conditions to obtain the fluid distribution characteristics of the core under the initial saturated oil state as a comparison benchmark. Step S2, Gas Displacement and Dynamic Damage Introduction Gas injection displacement experiments were conducted to induce recombination analysis through the contact extraction of gas and crude oil, which was then retained in the core pores. After displacement equilibrium was reached, scanning was performed to characterize the organic matter precipitation damage caused by gas injection. Step S3: In-situ heat remelting treatment Keeping the core position unchanged, a set thermal field is applied to it to raise the temperature. The thermal effect is used to induce the precipitated heavy components to undergo phase change remelting or redissolution. The T2 spectrum and nuclear magnetic resonance imaging after heat treatment are collected to record the recovery of fluid signals. Step S4: Multidimensional Quantitative Evaluation A quantitative-localization evaluation system was constructed based on nuclear magnetic resonance data. The total amount of heavy components precipitated was quantitatively calculated using T2 difference spectroscopy, and multi-level localization identification was implemented: at the microscale, the precipitation characteristics of heavy components in pores of different sizes were clarified based on T2 spectral characteristics; at the macroscale, nuclear magnetic resonance imaging technology was introduced to visually locate the enrichment area of ​​precipitates in the core space, revealing the precipitation distribution law of heavy components in CO2 displacement. In step S4, using the collected cold and hot state data, the following evaluation and reorganization analysis are performed to determine the specifics: S41, based on experimental verification results, using Curie's law to correct for temperature-induced magnetization vector changes, the amount of recombinant analysis was quantitatively calculated. The calculation formula is as follows: （1）； In the formula, K1 is the signal correction coefficient at different temperatures; K2 is the system calibration coefficient, mg / au, and K2 is 4.24 mg / au at 45℃; S hot S cold : represents the total signal amplitude in the remelted state and the precipitated state, respectively, au; S42, constructing a complex melting difference spectrum to characterize the distribution of organic solid phases in pores of different sizes; First, based on the following equation (2) characterizing the occurrence characteristics of organic solid phases in pores of different size ranges, and according to the principle of nuclear magnetic resonance, the T2 relaxation time is positively correlated with the pore radius, and equation (2) is: （2）； If the peak value of the differential spectrum is concentrated in the long T2 range, it indicates that the precipitate mainly blocks the throat of large pores; if the peak value is concentrated in the short T2 range, it indicates that the precipitation mainly occurs in micropores or exists in the form of surface adsorption. Based on the qualitative analysis, the pore size distribution is quantitatively characterized using formula (3). By calculating the difference between the amplitude of the remelted state signal and the amplitude of the precipitated state signal at each relaxation time point, the amount of precipitation at the corresponding relaxation time can be obtained. Formula (3) is: （3）； In the formula, This represents the amount of precipitation at relaxation time T2; , α and β are the signal amplitudes at relaxation time T2, representing the remelted state and the precipitated state, respectively; Finally, based on the calculation results, a distribution curve of the amount of precipitate as a function of T2 (i.e., pore size) was plotted to achieve accurate quantitative calculation of the amount of recombinant precipitate in different pore sizes. S43, the signal intensity of the core NMR image is calculated based on grayscale values, visualized using reverse Jet chromatography, and after temperature correction, the spatial distribution of the precipitated heavy components can be obtained. deposit (x,y), the calculation formula is: （4）； In the formula, I deposit (x,y) represents the normalized recombinant analysis yield; Im hot (x,y) represents the signal intensity of the core NMR image after remelting; Im cold (x,y) represents the signal intensity of the core NMR image before heating; K1 is the signal correction coefficient at different temperatures; Im hot,max This represents the maximum signal intensity value in the image after remelting at higher temperatures.

2. The in-situ quantitative and multi-scale localization characterization method for CO2-assisted oil recovery analysis as described in claim 1, characterized in that, The specific process of step S1: S11. Load the core to be tested into a non-magnetic variable-temperature core holder, apply confining pressure and back pressure, and establish a simulated formation pressure environment. S12. Stabilize the core temperature at the simulated target temperature T using a constant temperature circulation system. res ; S13. Inject saturated simulated formation oil containing light and heavy components into the core. After the saturation is uniform, use the initial state nuclear magnetic T2 spectrum and initial nuclear magnetic imaging map.

3. The in-situ quantitative and multi-scale localization characterization method for CO2-assisted oil recovery analysis as described in claim 2, characterized in that, In step S12, the temperature is set below the freezing point of the heavy components in the target fluid.

4. The in-situ quantitative and multi-scale localization characterization method for CO2-assisted oil recovery analysis as described in claim 1, characterized in that, The specific process of step S2: S21. Supercritical CO2 gas is injected into the core at a constant speed or constant pressure using an injection pump. S22. During the displacement process, the injected gas comes into contact with the formation oil, extracting light hydrocarbon components and supersaturated heavy components, which precipitate in situ as solid phases within the core pores. S23. Keeping the pressure constant, collect the T2 NMR spectrum of the precipitated state, and record the total T2 signal quantity at this time as S. cold precipitated state NMR image Im cold At this point, the intermolecular forces of the precipitated organic solid phase are strong, and their signal contribution to the T2 spectrum is negligible.

5. The in-situ quantitative and multi-scale localization characterization method for CO2-assisted oil recovery analysis as described in claim 1, characterized in that, The specific process of step S3: S31. Maintain the core's position, confining pressure, and pore fluid pressure within the holder; S32. Adjust the temperature control system to raise the core temperature to the remelting temperature T. hot_K Once the core temperature is balanced and the NMR signal is stable, the solid phase heavy components undergo a phase transition, and the signal is restored. S33. Acquire the remelted NMR T2 spectrum, and denote the total signal quantity as S. hot And the remelted nuclear magnetic resonance imaging, denoted as Im hot .

6. The in-situ quantitative and multi-scale localization characterization method for CO2-assisted oil recovery analysis as described in claim 5, characterized in that, In step S32, the temperature setting must be higher than the melting point or total melting temperature of the heavy component solid phase.

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