CO2-EGR visual monitoring method based on nuclear magnetic resonance

Abstract

The invention discloses a CO2-EGR visual monitoring method based on nuclear magnetic resonance. The CO2-EGR visual monitoring method aims at solving the problem that dynamic observation of the pore scale gas drive process is insufficient in the traditional technology. Multi-sequence nuclear magnetic resonance monitoring is synchronously executed, a T2 spectrum is obtained through a CPMG sequence to analyze CH4 content changes, a layered T2 spectrum is generated through an SE-SPI sequence to represent axial fluid distribution, a one-dimensional frequency signal is collected based on a GR-HSE sequence to track a displacement leading edge in real time, and fluid migration visualization is achieved in combination with an HSE overall imaging sequence. The system integrates a gas injection unit, a high-pressure-resistant nuclear magnetic holder, a back pressure controller and a data acquisition module, and is hermetically connected through a high-pressure-resistant pipeline to form a closed-loop experiment system. According to the method, displacement front recognition, displacement process in-situ analysis and quantitative characterization of the displacement efficiency in the non-intrusive CO2-EGR process are achieved for the first time, the CO2-EGR visualization bottleneck is broken through, and a scientific basis is provided for optimizing gas injection parameters and improving the natural gas recovery rate and the carbon sequestration efficiency.

Description

Technical Field

[0001] This invention relates to the field of oil and gas field development technology, and in particular to a CO2-EGR visualization monitoring method based on nuclear magnetic resonance. Background Technology

[0002] Natural gas, as an important clean fossil energy source, occupies a key position in the global energy structure transformation. Compared with coal and oil, its higher combustion efficiency and lower carbon emission characteristics make it an effective bridge energy source for achieving a low-carbon transition. However, constrained by factors such as reservoir energy depletion, heterogeneity, and bottom water intrusion, the recovery rate of conventional natural gas reservoirs is generally low, severely restricting the efficient development and utilization of resources. Therefore, enhanced gas recovery (EGR) technology is of strategic significance for ensuring energy supply and economic sustainability.

[0003] Among various EGR technologies, carbon dioxide (CO2) injection is widely recognized as the most promising approach due to its dual potential to enhance oil recovery and achieve carbon sequestration. This method restores formation pressure and displaces residual natural gas by injecting CO2 into depleted or near-depleted gas reservoirs, while simultaneously utilizing the reservoir's sealing properties to achieve geological CO2 sequestration. This synergistic strategy is defined as "Carbon Sequestration and Enhanced Gas Recovery (CSEGR)," and has received widespread attention in the international energy sector in recent years. However, practical application remains limited by issues such as differences in reservoir conditions and insufficient verification of long-term sequestration safety.

[0004] Current CO2-EGR research primarily relies on numerical simulations and macroscopic experiments. While numerical models can simulate displacement behavior at the reservoir scale, their accuracy is limited by reservoir simplification assumptions and parameter uncertainties. Experimental studies mostly focus on long core displacement tests, assessing displacement efficiency through macroscopic parameters such as pressure changes and produced gas composition. Although these methods provide engineering references, they struggle to reveal the microscopic mechanisms of gas-gas displacement within porous media. In particular, because CO2 and CH4 are both colorless gases and highly miscible, traditional visualization techniques such as X-ray computed tomography (CT) face significant limitations: their sensitivity to low-density gases is insufficient, their spatial resolution is inadequate to capture pore-scale dynamics, and they cannot distinguish changes in gas composition.

[0005] Low-field nuclear magnetic resonance (NMR) technology exhibits unique advantages in the quantitative characterization of fluid saturation due to its high sensitivity to hydrogen nuclei (¹H). This technique, by detecting the relaxation signal of ¹H in fluids, can non-invasively analyze the distribution and transport behavior of hydrogen-containing fluids in porous media, and has been successfully applied in enhanced oil recovery (EOR) studies in the oil extraction field. However, its potential in the CO2-EGR field has not been fully explored, especially regarding the lack of systematic research on the dynamic mechanism of CO2 displacing CH4 at the pore scale. The limitations of traditional observation techniques lead to insufficient understanding of key processes such as gas competitive displacement and microscopic sweep efficiency, severely restricting the optimization and promotion of CO2-EGR technology. Therefore, there is an urgent need to develop an innovative monitoring method that can achieve in-situ visualization and quantitative analysis of gas-driven processes. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention provides a visual monitoring method for CO2-EGR based on nuclear magnetic resonance.

[0007] To achieve the above-mentioned objectives, the technical solution adopted by the present invention is as follows:

[0008] A method for visual monitoring of CO2-EGR based on nuclear magnetic resonance includes the following steps:

[0009] S1: Place the dry core sample in the nuclear magnetic resonance core holder, evacuate the vacuum, and pressurize it with CH4 in stages. After each stage of pressure equilibration, measure the baseline T2 spectrum.

[0010] S2: Under constant confining pressure and back pressure conditions, CO2 is injected into the core at a preset gas injection rate for displacement.

[0011] S3: Multimodal NMR synchronous monitoring, simultaneously executing the following sequences:

[0012] a) CPMG sequence: T2 spectrum was obtained, and micropore, mesopore and macropore regions were divided according to relaxation time; the CH4 content in the core and its distribution characteristics at different pore size scales were quantitatively assessed.

[0013] b) SE-SPI sequence: The core is divided into multiple layers along the axis to obtain the layered T2 spectrum to locate the displacement front;

[0014] c) GR-HSE sequence: Real-time acquisition of one-dimensional frequency signals to track the displacement front movement;

[0015] d) Overall HSE imaging sequence: Obtain fluid distribution map and distinguish CH4-rich areas from CO2-dominant areas by signal intensity threshold;

[0016] S4: CH4 recovery rate is calculated based on the peak area of ​​the T2 spectrum (the cumulative sum of amplitude values ​​at the discrete relaxation time points of the corresponding peaks), and displacement efficiency is analyzed by combining layered T2 spectra, one-dimensional frequency and imaging results.

[0017] Furthermore, the GR-HSE sequence acquires a one-dimensional frequency signal, and by adjusting the sequence parameters, the acquisition time is made ≤30 seconds / time.

[0018] The spatial resolution of the GR-HSE sequence is ≥2mm.

[0019] Furthermore, the T2 spectrum is used to classify pore types according to relaxation time: T2 < 10 ms is micropore, 10 ms ≤ T2 ≤ 100 ms is mesopore, and T2 > 100 ms is macropore;

[0020] The formula for calculating CH4 recovery rate is: η = (A o -A t ) / A o ×100%, where A o For the initial peak area, A t Let be the peak area at time t.

[0021] Furthermore, the layered T2 spectrum divides the core into 5-10 layers;

[0022] The lower level corresponds to the injection end, and the higher level corresponds to the output end.

[0023] Furthermore, regions with signal intensity >70% in the imaging results represent CH4-rich regions;

[0024] Areas with signal strength <30% represent CO2-dominant regions.

[0025] The present invention also discloses a nuclear magnetic resonance experimental system for implementing the method described in claims 1-5, comprising:

[0026] Gas injection system, including CH4 gas cylinders and CO2 gas cylinders and flow controller;

[0027] Nuclear magnetic resonance core holder, magnetic field strength 0.3±0.03T;

[0028] Back pressure regulator, PID closed-loop control;

[0029] The data acquisition unit integrates CPMG, SE-SPI, GR-HSE and HSE overall imaging sequence control modules;

[0030] Each component is sealed and connected via high-pressure resistant pipelines.

[0031] Furthermore, the nuclear magnetic resonance clamp includes:

[0032] Radio frequency coil, resonant frequency 11.4MHz;

[0033] Gradient coil, maximum gradient intensity 120mT / m (to achieve SE-SPI spatial encoding).

[0034] Compared with the prior art, the advantages of the present invention are as follows:

[0035] 1. Achieve dynamic visualization of pore-scale gas drive processes

[0036] For the first time, the microscopic transport behavior of CO2 displacing CH4 was captured under non-invasive conditions through multi-sequence coordinated monitoring of nuclear magnetic resonance. This breakthrough overcomes the technical bottleneck of traditional techniques for in-situ observation of gas-gas displacement processes and provides direct evidence for revealing the gas competitive displacement mechanism in porous media.

[0037] 2. Quantitative characterization of multi-scale dynamic parameters

[0038] By integrating T2 spectrum, layered T2 spectrum, one-dimensional frequency and imaging analysis, and simultaneously acquiring multi-dimensional parameters such as displacement front, pore fluid saturation, fluid distribution, and breakthrough time, a full-chain displacement efficiency evaluation system from pore to core scale is established.

[0039] 3. Breakthrough in real-time non-destructive monitoring technology

[0040] The optimized GR-HSE sequence enables second-level tracking of the displacement front under continuous gas injection conditions, avoiding fluid redistribution interference caused by pump shutdown, significantly improving the reliability of dynamic data, and providing real-time basis for gas injection strategy adjustment.

[0041] 4. Precise identification of key factors for displacement efficiency

[0042] By analyzing the coupling effect of permeability and gas injection rate, the control law of reservoir physical parameters on gas migration is clarified, the cause of CO2 fingering phenomenon in low-permeability reservoirs is revealed, and a theoretical basis is laid for targeted optimization of injection schemes.

[0043] 5. Analysis of the Synergistic Effect Mechanism between Carbon Sequestration and Harvesting

[0044] The visualization results verify the dynamic correlation between the CO2 sequestration pathway and CH4 utilization during the displacement process, providing experimental support for the geological application of carbon capture, utilization and storage (CCUS) technology and promoting the synergistic achievement of energy development and emission reduction goals.

[0045] 6. Wide technical adaptability

[0046] The method is applicable to core samples from various reservoirs such as sandstone and shale, and has universal monitoring capabilities for geological conditions with large permeability differences (low to medium-high permeability) and strong heterogeneity, significantly expanding the applicable scope of CO2-EGR technology. Attached Figure Description

[0047] Figure 1 This is a core sample image from an embodiment of the present invention.

[0048] Figure 2 This is a schematic diagram of the NMR experimental equipment according to an embodiment of the present invention.

[0049] Figure 3 These are the T2 spectra of Core#1 in this embodiment of the invention: (a) T2 spectrum of CH4 saturation process, and (b) T2 spectrum of CO2 displacement process.

[0050] Figure 4 These are the T2 spectra of Core#2 in this embodiment of the invention: (a) T2 spectrum of CH4 saturation process, and (b) T2 spectrum of CO2 displacement process.

[0051] Figure 5 These are the T2 spectra of Core#3 in this embodiment of the invention: (a) T2 spectrum of CH4 saturation process, (b) T2 spectrum of CO2 displacement process.

[0052] Figure 6 These are the T2 spectra of Core#4 in this embodiment of the invention: (a) T2 spectrum of CH4 saturation process, and (b) T2 spectrum of CO2 displacement process.

[0053] Figure 7 This is a CH4 recovery rate curve diagram of an embodiment of the present invention.

[0054] Figure 8 These are the layered T2 spectra of Core#1 in this embodiment of the invention: (a) 0 min, (b) 1 min, (c) 2 min, (d) 4 min, (e) 6 min, (f) 8 min, (g) 10 min, (h) 14 min.

[0055] Figure 9 This is a one-dimensional frequency distribution diagram of Core#1 in this embodiment of the invention, (a) CH4 saturation state, (b) time distribution of CO2 displacement process, and (c) fluid distribution in the later stage of CO2 displacement.

[0056] Figure 10 This is a one-dimensional frequency distribution diagram of Core#2 in this embodiment of the invention, (a) CH4 saturation state, (b) time sequence distribution of CO2 displacement process, and (c) fluid distribution in the later stage of CO2 displacement.

[0057] Figure 11 This is a one-dimensional frequency distribution diagram of Core#3 in this embodiment of the invention, (a) CH4 saturation state, (b) time distribution of CO2 displacement process, and (c) fluid distribution in the later stage of CO2 displacement.

[0058] Figure 12This is a one-dimensional frequency distribution diagram of Core#4 in this embodiment of the invention, (a) CH4 saturation state, (b) time sequence distribution of CO2 displacement process, and (c) fluid distribution in the later stage of CO2 displacement.

[0059] Figure 13 This is a recovery rate curve based on CH4 saturation in an embodiment of the present invention.

[0060] Figure 14 This is an NMR imaging image of Core#1 in this embodiment of the invention.

[0061] Figure 15 This is an NMR imaging image of Core#2 in this embodiment of the invention.

[0062] Figure 16 This is an NMR imaging image of Core#3 in this embodiment of the invention.

[0063] Figure 17 This is an NMR imaging image of Core#4 in embodiment of the present invention.

[0064] Figure 18 This is a one-dimensional NMR frequency diagram of core #1 under continuous injection conditions according to an embodiment of the present invention. (a) Signal intensity during CO2 displacement, (b) Methane saturation during CO2 displacement.

[0065] Figure 19 This is a one-dimensional NMR frequency diagram of core #2 under continuous injection conditions according to an embodiment of the present invention. (a) Signal intensity during CO2 displacement, (b) Methane saturation during CO2 displacement.

[0066] Figure 20 This is a one-dimensional NMR frequency diagram of core #3 under continuous injection conditions according to an embodiment of the present invention. (a) Signal intensity during CO2 displacement, (b) Methane saturation during CO2 displacement. Detailed Implementation

[0067] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings and examples.

[0068] This invention provides a CO2-EGR visualization monitoring method based on nuclear magnetic resonance, comprising the following steps:

[0069] S1: Place the dry core sample in the nuclear magnetic resonance core holder, evacuate the vacuum, and pressurize it with CH4 in stages. After each stage of pressure equilibration, measure the baseline T2 spectrum.

[0070] S2: Under constant confining pressure and back pressure conditions, CO2 is injected into the core at a preset gas injection rate for displacement.

[0071] S3: Multimodal NMR synchronous monitoring, simultaneously executing the following sequences:

[0072] a) CPMG sequence: T2 spectrum was obtained, and micropore, mesopore and macropore regions were divided according to relaxation time; the CH4 content in the core and its distribution characteristics at different pore size scales were quantitatively assessed.

[0073] b) SE-SPI sequence: The core is divided into multiple layers along the axis to obtain the layered T2 spectrum to locate the displacement front;

[0074] c) GR-HSE sequence: Real-time acquisition of one-dimensional frequency signals to track the displacement front movement;

[0075] d) Overall HSE imaging sequence: Obtain two-dimensional fluid distribution map, and distinguish CH4-rich areas from CO2-dominant areas by signal intensity threshold;

[0076] S4: CH4 recovery rate is calculated based on the peak area of ​​the T2 spectrum (the cumulative sum of amplitude values ​​at the discrete relaxation time points corresponding to the peak value), and displacement efficiency is analyzed by combining layered T2 spectra, one-dimensional frequency, and imaging results. I. Experiment

[0077] 1.1 Materials

[0078] like Figure 1 As shown, four sandstone core samples were used in this experiment. The cores were first dried in an oven at 100℃ for 12 hours to ensure no residual moisture remained inside. Subsequently, the mass, permeability, and geometric dimensions of the samples were measured. The rock physical properties of the cores are detailed in Table 1. Methane (CH4) was used as the representative gas for natural gas in the experiment, and both CO2 and CH4 gases were provided from standard gas cylinders with a purity of 99.99%.

[0079] Table 1. Rock physical properties of core samples

[0080]

[0081] 1.2 Experimental Setup

[0082] This invention employs a low-field nuclear magnetic resonance (NMR) core analysis system (MacroMR12-110H-GS-HTHP, Suzhou Newmai Analytical Instruments Co., Ltd., China) to measure NMR T2 spectra, layered T2 spectra, one-dimensional frequencies, and NMR imaging. The permanent magnet magnetic field strength of this NMR spectrometer is 0.3 ± 0.03 T, with a magnetic field homogeneity ≤ 50 ppm; the resonance frequency is 11.4 MHz, and the inner diameter of the high-temperature, high-pressure core holder is 25 mm.

[0083] Figure 2A simplified schematic diagram of the NMR experimental setup in a CO2-EGR experiment is shown. The setup mainly consists of a gas injection system, a high-temperature and high-pressure nuclear magnetic core holder, a back pressure regulator (BPR), and a data acquisition unit.

[0084] The experimental procedure is briefly described as follows: First, the dried core sample was placed in a core holder, and the pores were evacuated using a vacuum pump. The initial T2 spectrum was then measured. Next, the core was saturated with CH4 at pressures of 4 MPa, 7 MPa, and 10 MPa. After each pressure point was reached, the core was equilibrated at that pressure, and the T2 spectrum, layered T2 spectrum, one-dimensional frequency, and imaging data were repeatedly measured until the results stabilized. After reaching stable saturation at 10 MPa, the back pressure was set to 10 MPa. Subsequently, the injected gas was switched to CO2 and pressurized to 10 MPa, injected at a constant injection rate. After a period of displacement, the displacement pump was stopped, and the following data were recorded using the measurement system: T2 spectrum, layered T2 spectrum, one-dimensional frequency, and imaging results. The experiment ended when the data from multiple consecutive tests became consistent. It is worth noting that the one-dimensional frequency can be obtained in real time during continuous gas injection by adjusting the test parameter without stopping the injection. Therefore, three additional one-dimensional frequency experiments were conducted under continuous gas injection conditions. Detailed experimental design information is shown in Table 2.

[0085] Table 2 Experimental Setup

[0086]

[0087] The NMR transverse relaxation time (T2) can be determined by the following formula:

[0088]

[0089] Where T2,bulk is the bulk relaxation time, which mainly depends on the physical properties of the fluid itself. T2,surface is the surface relaxation time, which is related to the interaction between the fluid and the pore walls. T2,diffusion is the diffusion relaxation time, which is related to the diffusion of the fluid in the magnetic field gradient. For fluids in porous media, surface relaxation is usually the dominant factor, so the formula can be simplified to:

[0090]

[0091] Where ρ is the surface relaxation strength parameter, reflecting the intensity of the interaction between the fluid and the pore wall. S is the surface area of ​​the pore. V is the volume of the pore. For spherical pores, the ratio of surface area to volume, S / V, is inversely proportional to the pore radius r:

[0092]

[0093] Therefore, the relationship between the relaxation time T2 and the pore radius r can be expressed as:

[0094]

[0095] This indicates that when the surface relaxation strength parameter ρ is known, the relaxation time is positively correlated with the radius. This relationship enables the NMR T2 spectrum to effectively reflect information on the pore structure.

[0096] II. Results and Discussion

[0097] 2.1 T2 Spectrum Analysis

[0098] Based on the CPMG sequence, low-field NMR was used to study the CO2 displacement after the CH4 saturation process. NMR characterizes the fluid content by measuring the relaxation process of 1H-containing fluids (such as CH4 and water) under an external magnetic field, during which the nuclei transition from a high-energy non-equilibrium state to a low-energy equilibrium state. Since the core contains no water after drying, the signal monitored in the core indicates the content of CH4. Therefore, an NMR analyzer was used to measure the transverse relaxation time (T2 spectrum) of the core samples to determine the distribution characteristics of the CH4 saturation process at different pressures and throughout the CO2 displacement process.

[0099] The NMR T2 spectra of cores #1, #2, #3, and #4 after CH4 saturation at different pressures are respectively shown in Figure 3 (a), Figure 4 (a), Figure 5 (a), and Figure 6 (a). The T2 distribution provides insights into the pore size distribution and the CH4 distribution in different pore types. According to the relationship between the T2 relaxation time and the pore size (as shown in Equation 4), the pores in the core are divided into three categories: micropores (T2 < 10 ms), mesopores (10 ms < T2 < 100 ms), and macropores (T2 > 100 ms), as indicated by the shaded regions (yellow, green, and blue, representing micropores, mesopores, and macropores, respectively).

[0100] For all four core samples, the T2 spectrum exhibits a dominant peak in the mesopore range, indicating that mesopores are the main storage space for CH4, while the amplitude in the macropore region is relatively weak. The T2 amplitude continuously increases with the increase in pressure, confirming the positive correlation between the CH4 saturation and the signal intensity. In addition, as the pressure changes, the peak position remains consistent, indicating that the pore size distribution remains unchanged during the saturation stage, and the pressure mainly affects the content of CH4 rather than its spatial distribution within the pore system.

[0101] Figure 3 (b), Figure 4 (b), Figure 5 (b), and Figure 6(b) NMRT2 spectra of four cores after initial CH4 saturation at 10 MPa and dynamic displacement with CO2 at a constant injection rate are presented. In all cores, the T2 signal amplitude gradually decreases during displacement, especially in the mesoporous region where CH4 is initially most concentrated. The decrease in amplitude reflects the gradual displacement of CH4 by CO2. The degree and rate of signal attenuation vary with permeability and injection rate. Core #2, due to its low permeability, shows a slower and less significant decrease in T2 amplitude, indicating greater resistance to CO2 and CH4 transport. In high-permeability cores (#1, #3, and #4), the decrease in T2 amplitude is more rapid, indicating more effective CH4 displacement. However, in core #3, despite its high permeability, the lower injection rate results in a slower decrease in CH4 signal intensity compared to core #1, highlighting the influence of injection rate on displacement efficiency. Core #4, with the highest permeability and injection rate, shows the most significant and rapid decrease in T2 amplitude during displacement. This significant attenuation indicates that its displacement efficiency is at its highest, further demonstrating that favorable permeability-injection rate coupling significantly improves CH4 recovery.

[0102] The peak area in the T2 spectrum serves as an indicator of CH4 content in the core. Unlike conventional spectra, T2 spectra are plotted on a logarithmic time scale, and the "peak area" refers to the cumulative sum of amplitude values ​​at discrete relaxation time points corresponding to the peak value. The CH4 recovery curve in CO2 displacement experiments is estimated based on the peak area at various time points, such as... Figure 7 As shown in the figure. The results indicate that CO2 effectively displaced CH4 in the core, and the CH4 recovery rate gradually increased with the extension of CO2 injection time. Core #1, with its high permeability and relatively high CO2 injection rate, achieved the highest CH4 recovery rate of 87.41% at 14 minutes. In contrast, Core #2, with its lower permeability and the same injection rate, achieved a recovery rate of only 44.31% in the same time period. Core #3 had a permeability similar to Core #1, but a lower injection rate of 0.1 mL / min, achieving a recovery rate of 70.55% in the same time period. Core #4, combining high permeability and the highest injection rate, exhibited the most effective performance, achieving the highest CH4 recovery rate of 90.17% in just 12 minutes.

[0103] These experimental results confirm the effectiveness of CO2 injection in displacing CH4 from porous media, supporting the feasibility of CO2-EGR technology. NMRT2 spectroscopy provides a valuable tool for understanding the dynamic process of CO2 displacing CH4 in porous structures. Furthermore, these findings lay the foundation for subsequent spatial analysis using layered T2 spectroscopy, which will be discussed in the next section.

[0104] 2.2 Layered T2 Spectrum Analysis

[0105] Based on the SE-SPI sequence, layered T2 spectrum distributions of cores at different spatial locations were obtained using NMR scanning. In the layered T2 spectrum, the lower layers correspond to the entrance side of the core, while the upper layers represent the exit side. The overall layered T2 spectrum of the four cores was generated by superimposing the T2 spectra of each individual layer. The amplitude of the T2 signal reflects the relative CH4 content in each layer.

[0106] The core was initially saturated with CH4 at 10 MPa, followed by CO2 displacement at a constant injection rate. Stratification allowed for the temporal and spatial monitoring of CH4 distribution across different core strata. Each subplot illustrates the stratified T2 response at specific time intervals (i.e., 0, 1, 2, 4, 6, 8, 10, and 14 minutes), capturing the dynamic evolution of CH4 saturation under CO2 displacement.

[0107] Layered T2 spectra of core #1 as follows Figure 8 As shown in the figures, the dashed lines in each figure represent the CH4 distribution before displacement, while the solid lines represent the distribution during different stages of CO2 injection. At the initial time (0 minutes), the T2 spectrum exhibits a dominant single peak at all levels, indicating that CH4 is mainly stored in pores of similar size. As CO2 injection progresses, a clear spatial variation trend gradually emerges: the lower layers (inlet side) show a rapid decrease in peak amplitude, while the upper layers (outlet side) initially show a temporary increase. This indicates that CH4 at the core inlet is being displaced and pushed towards the outlet. Between 1 and 2 minutes, CH4 displacement mainly occurs at the core inlet, with a significant decrease in signal amplitude at the lower layers, indicating this phenomenon. Between 4 and 6 minutes, this downward trend gradually extends to the upper layers and throughout the entire core. By 8 minutes, even the highest layers begin to show a significant decrease in CH4 signal, indicating that CO2 has reached the outlet and a breakthrough has occurred.

[0108] 2.3 One-dimensional (1D) NMR profile analysis

[0109] Based on the GR-HSE sequence, one-dimensional (1D) signal intensity along the core axis was obtained by NMR scanning, which can effectively capture the fluid transport front within porous media. The one-dimensional NMR profiles of cores #1, #2, #3, and #4 are shown in [the table / images]. Figures 9 to 12 This study reveals the spatial distribution of CH4 saturation along the entire length of the core. The red arrows in the figure indicate the flow direction of CH4 during saturation and subsequent CO2 displacement. Notably, the high amplitude regions at the inlet or outlet are partly attributed to CH4 migration into the gaps between the core and the end plugs. Furthermore, to quantitatively characterize CH4 saturation, the maximum signal amplitude at each location was considered fully saturated, thus constructing CH4 saturation curves for each core.

[0110] Figure 9One-dimensional NMR profile results of core #1 are presented. The core has a permeability of 10.3 mD and a CO2 injection rate of 0.3 mL / min. After saturation, CH4 is relatively uniformly distributed within the core, except for a slight increase at both ends due to minute gaps between the core and end plugs. Figure 9 (a)]. During CO2 injection, the CH4 signal at the inlet segment decreases significantly within the first 1 to 4 minutes, indicating the beginning of the displacement front, while the signal attenuation at the outlet segment begins around 4 to 6 minutes, marking the occurrence of CO2 breakthrough. Figure 9 (b)~(c)]. By the 14th minute, the overall CH4 content in the core decreased significantly, with only a certain degree of residual saturation remaining near the outlet. The average residual saturation was 30.83%, indicating that under these permeability and injection conditions, the displacement process exhibited piston-like characteristics and had high displacement efficiency.

[0111] The one-dimensional NMR profile results of core #2 are as follows Figure 10 As shown. The core has a low permeability of 1.0 mD, and the CO2 injection rate is 0.3 mL / min. Figure 10 As shown in (a), the CH4 distribution is relatively uniform after saturation, with a slight gap between the inlet and the end plug. During CO2 injection, the signal at the inlet section begins to decay after about 1 minute, but the propagation is slow and mainly limited to a local area. Figure 10 [(b)~(c)] By the 4th minute, the displacement front had only slightly exceeded the core point. CO2 breakthrough occurred between the 8th and 10th minutes, at which point CH4 in the middle and outlet sections remained largely unchanged. After 18 minutes, the average residual saturation of CH4 was 54.63%, indicating a slow and inefficient displacement process. These results suggest that limited pore connectivity and high flow resistance severely inhibited CO2 propagation and CH4 migration.

[0112] Figure 11 One-dimensional NMR profile results of core #3 are presented. This core has a permeability of 10.8 mD and a low CO2 injection rate of 0.1 mL / min. Figure 11 As shown in (a), the initial CH4 saturation distribution is relatively uniform, with a slightly higher signal amplitude in the central region. During CO2 displacement, the CH4 signal at the inlet begins to decrease after approximately one minute, marking the start of displacement. Figure 11 [(b)~(c)] The displacement front advanced slowly along the core, with the breakthrough occurring approximately between 4 and 6 minutes, similar in timing to core #1. However, the overall signal attenuation was relatively slow, reflecting a sluggish fluid transport process.

[0113] By minute 14, the average residual saturation of CH4 was 41.53%. These results indicate that, despite sufficient permeability, the lower injection rate slowed the advance of the displacement front, limiting recovery. Although the axial advance rate was comparable to that of core #1, the lower injection rate resulted in lower displacement efficiency.

[0114] Figure 12 One-dimensional NMR profile results of core #4 are presented. This core has a permeability of 10.0 mD and a relatively high CO2 injection rate of 0.5 mL / min. Figure 12 As shown in (a), the CH4 after saturation is relatively uniformly distributed along the axial direction, indicating that the saturation state has been well established. During CO2 injection, the CH4 signal amplitude at the inlet section drops sharply in the first 1 to 3 minutes, reflecting the start of efficient displacement. Figure 12 (b)~(c)]. A distinct migration front was formed during this process, steadily advancing towards the outlet, exhibiting typical piston-like displacement characteristics. Around the 3rd minute, signal attenuation extended to the outlet segment, indicating that CO2 breakthrough occurred earlier than in other cores due to the higher injection rate. By the 12th minute, the CH4 signal had significantly decreased throughout the core, leaving only a small amount near the outlet. The average residual CH4 saturation was 14.04%, the lowest among all tested cores. These results indicate that, under certain permeability conditions, a higher injection rate can promote the rapid advancement of the displacement front and improve displacement efficiency.

[0115] One-dimensional nuclear magnetic resonance (NMR) profiles further confirmed the above findings, clearly demonstrating the evolution of the CO2 displacement front and the spatial distribution of CH4 at different time points. The CH4 recovery rate at each time point was estimated based on the average residual saturation, and the results were plotted as follows: Figure 13 and show with Figure 7 A consistent trend of change.

[0116] A comparison of the four core samples clearly shows that permeability and injection rate have a crucial impact on the displacement effect. Core #1 exhibits typical piston-like displacement characteristics with a moderate recovery rate; Core #2, due to its lower permeability, has a slow displacement front advance and poor displacement efficiency; Core #3, due to its lower injection rate and delayed breakthrough time, has a lower recovery rate than Core #1; while Core #4, under the synergistic effect of high permeability and high injection rate, achieved the most efficient displacement process and the highest CH4 recovery rate among the four core samples.

[0117] These results highlight the strong coupling between permeability and injection rate. High permeability facilitates CO2 transport, while a higher injection rate helps maintain a stable front and improve displacement efficiency. Optimal recovery is achieved when both are present simultaneously, while limitations in either factor can lead to incomplete displacement and increased CH4 retention. Overall, one-dimensional frequency techniques are a powerful tool for assessing axial fluid distribution, enabling quantitative analysis of saturation dynamics and precise identification of the displacement front and breakthrough time, providing strong support for subsequent displacement behavior in heterogeneous porous media. To further validate these displacement modes and provide intuitive evidence, the following sections will introduce relevant analyses using NMR imaging.

[0118] 2.4 Nuclear Magnetic Resonance Imaging

[0119] Figure 14 , 15 Images 16 and 17 show the NMR imaging results of cores #1, #2, #3, and #4 during the CO2 displacement of CH4. These images capture the evolution of the 1H signal amplitude during CO2 injection, providing visualization information on CH4 distribution and displacement dynamics. The signal amplitude is positively correlated with CH4 content, with red indicating high intensity and blue indicating low intensity.

[0120] Core #1 exhibits moderate permeability and a moderate injection rate, demonstrating a relatively efficient displacement process. Its NMR imaging results are as follows: Figure 14 As shown, after CH4 saturation, the core generally exhibited a strong signal amplitude, with a slight increase near the outlet end. This is mainly attributed to the slight CH4 accumulation in the gap between the core and the plug. According to existing research, the CO2 lateral displacement of natural gas can be divided into three characteristic regions: the CO2 storage zone, the CO2-natural gas transition zone, and the natural gas zone. With CO2 injection, the red color near the inlet gradually changes to yellow and green, indicating that CH4 is being gradually displaced. This color gradient reflects the piston-like displacement front and the development of the CO2-CH4 transition zone; while the high-amplitude areas in the middle and tail of the core correspond to the natural gas zone. Around 4–6 minutes, the signal at the tail of the core begins to weaken, marking the transition from the natural gas zone to the CO2-CH4 transition zone. By the 8th minute, a CO2 storage zone briefly appears at the inlet end. Between 10 and 14 minutes, most of the CH4 is displaced from the core, and the remaining CH4 is redistributed in the pore structure, eventually forming a relatively uniform spatial distribution inside the core.

[0121] Core #2 has the lowest permeability of all samples, indicating a slow and incomplete CH4 displacement process. Its imaging results are as follows: Figure 15As shown, after CH4 saturation, the core exhibited a strong and uniformly distributed signal amplitude along the axial direction. After CO2 injection, the gradual fading of the red area at the intake end indicated the formation of a CO2-CH4 transition zone near the intake end, but this change was initially limited to a localized area. Between 6 and 10 minutes, the signal amplitude in the middle and tail of the core gradually weakened, and the CO2-CH4 transition zone gradually expanded, eventually covering the entire core. By the 14th minute, some areas turned green or blue, indicating a decrease in CH4 content. The residual CH4 redistributed within the pore structure, forming a relatively uniform spatial distribution within the core. These phenomena suggest that poor pore connectivity and high flow resistance led to poor displacement continuity.

[0122] Core #3 has similar permeability to core #1, but a lower CO2 injection rate. Its imaging results are as follows: Figure 16 As shown, after CH4 saturation, the central region of the core exhibits a strong signal amplitude, with slightly higher signals near the inlet and outlet ends, originating from a small amount of CH4 accumulation in the gap between the core and the end plug. After CO2 injection, the red area near the inlet gradually decreases, indicating gradual CH4 displacement, forming a CO2-CH4 transition zone near the inlet. This transition zone gradually expands between 6 and 8 minutes. Between 10 and 14 minutes, some CH4 is displaced, and the remaining portion is redistributed within the core, eventually forming a relatively uniform spatial distribution inside the core. In contrast, the lower injection rate of core #3 resulted in a less pronounced displacement front, lower displacement efficiency, and higher residual CH4 saturation.

[0123] Core #4 exhibits high permeability and the highest CO2 injection rate, demonstrating a rapid CH4 displacement process. Its imaging results are as follows: Figure 17 As shown, after CH4 saturation, the core exhibited a uniformly distributed strong signal. During CO2 injection, the signal at the intake end rapidly weakened, indicating the formation of a CO2-CH4 transition zone in that region. By the 3rd minute, most of the CH4 had been displaced to the tail end of the core; by the 5th minute, three distinct regions had formed within the core: a CO2 reservoir at the intake end, a CO2-CH4 transition zone in the middle, and a CH4 zone at the outlet end. By the 8th minute, as the CH4 zone was completely displaced, the CO2-CH4 transition zone further advanced towards the middle and rear. At the 12th minute, most of the CH4 in the core had been displaced, and the transition zone exhibited a narrow and stable characteristic, with a clear displacement front, indicating high CH4 migration efficiency. In the later stages, the overall signal distribution tended to be uniform, indicating less residual CH4 accumulation. These results confirm that when the injection rate is high and permeability is good, the displacement rate and completeness of CH4 can be significantly improved.

[0124] NMR imaging results from four cores provide intuitive visual evidence of CH4 displacement behavior under CO2 injection, revealing the spatiotemporal evolution of signal amplitude and highlighting key differences in displacement dynamics. In all cores, CH4-rich regions gradually transition from red to green and blue, forming CO2-CH4 transition zones of varying width and clarity. These transition zones reflect the continuity and effectiveness of the displacement process. However, due to the small scale of individual cores, it is difficult to observe all three regions simultaneously within the same core.

[0125] In summary, the imaging results indicate that permeability and injection rate play crucial roles in controlling the width of the transition zone, the spatial distribution of CH4 residues, and the overall displacement efficiency. Higher permeability facilitates CO2 penetration into porous media, while higher injection rates enhance displacement dynamics and inhibit the expansion of the transition zone. Combining NMR imaging with T2 spectroscopy, layered T2 spectroscopy, and one-dimensional frequency response enables comprehensive visualization of fluid redistribution and transition characteristics, providing valuable insights into the pore-scale mechanism of CO2-EGR in heterogeneous porous media.

[0126] 2.5 Real-time one-dimensional frequency under continuous CO2 injection

[0127] To further capture the real-time gas displacement dynamics during the CO2-EGR process, a fast one-dimensional frequency technique was employed by adjusting the parameters of the GR-HSE pulse sequence. While this adjustment sacrifices a small amount of spatial resolution and measurement accuracy, it significantly reduces acquisition time, enabling continuous monitoring without stopping the displacement pump during gas injection. This method allows for dynamic observation of the CH4 displacement process while minimizing gas diffusion caused by pump shutdown.

[0128] Although the amplitude curves of the adjusted GR-HSE parameters fluctuated significantly due to the reduced signal-to-noise ratio, the overall trends in CH4 migration and displacement behavior remained clearly visible. The temporal evolution of CH4 distribution within the core provided valuable insights into the advancement of the CO2 displacement front and residual gas saturation.

[0129] like Figure 18 As shown, in core #1, at the start of CO2 injection, the CH4 signal amplitude at the inlet gradually decreased, indicating that CH4 at the core tip was mobilized and gradually displaced towards the outlet. A significant CO2 breakthrough was observed at approximately 800 seconds, followed by continued CH4 displacement until a steady state was reached at approximately 2,240 seconds. Ultimately, the average residual CH4 saturation was 25.48%, and the estimated CH4 recovery was 74.52%. This indicates an effective displacement process, exhibiting piston-like flow characteristics with minimal finger-like flow or channel effects.

[0130] like Figure 19As shown, the displacement behavior of core #2 initially exhibited a similar trend to that of core #1. Despite having the same injection rate as core #1, the lower permeability led to greater flow resistance and more pronounced CH4 retention. CO2 breakthrough occurred at approximately 480 seconds, but the subsequent displacement process became slower and less efficient, eventually reaching a steady state at approximately 1,360 seconds. The average residual CH4 saturation was 40.24%, and the estimated CH4 recovery was 59.76%. This indicates that the reservoir's physical properties hindered the uniform advancement of the gas front.

[0131] like Figure 20 As shown, core #3 exhibited a slower displacement process due to its lower injection rate. The CH4 signal amplitude steadily decreased from the inlet, and the CO2 breakthrough occurred at approximately 1,600 seconds, significantly later than other cores. The displacement process reached equilibrium at approximately 2,560 seconds, with a final average residual CH4 saturation of 38.47%, corresponding to a CH4 recovery rate of 61.53%. Although the permeability of core #3 was comparable to that of core #1, the low injection rate limited the displacement efficiency, highlighting the crucial balance between permeability and injection rate in optimizing CO2-EGR performance.

[0132] Overall, these results validate the effectiveness of continuous NMR one-dimensional frequency technology in capturing real-time gas migration. This technique can clearly identify the displacement front, track the spatial distribution of CH4 saturation, and quantitatively assess displacement performance. Core differences at different permeabilities and injection rates further highlight the interaction between reservoir characteristics and injection design, providing important insights for optimizing in-situ CO2-EGR strategies.

[0133] Those skilled in the art will recognize that the embodiments described herein are intended to help the reader understand the implementation methods of the present invention, and should be understood that the scope of protection of the present invention is not limited to such specific statements and embodiments. Those skilled in the art can make various other specific modifications and combinations based on the technical teachings disclosed in this invention without departing from the spirit of the invention, and these modifications and combinations are still within the scope of protection of the present invention.

Claims

1. A visual monitoring method for CO2-EGR based on nuclear magnetic resonance, characterized in that, Includes the following steps: S1: Place the dry core sample in the nuclear magnetic resonance core holder, evacuate the vacuum, and pressurize it with CH4 in stages. After each stage of pressure equilibration, measure the baseline T2 spectrum. S2: Under constant confining pressure and back pressure conditions, CO2 is injected into the core at a preset gas injection rate for displacement. S3: Multimodal NMR synchronous monitoring, simultaneously executing the following sequences: a) CPMG sequence: T2 spectrum was obtained, and micropore, mesopore and macropore regions were divided according to relaxation time; the CH4 content in the core and its distribution characteristics at different pore size scales were quantitatively assessed. b) SE-SPI sequence: The core is divided into multiple layers along the axis to obtain the layered T2 spectrum to locate the displacement front; c) GR-HSE sequence: Real-time acquisition of one-dimensional frequency signals to track the displacement front movement; d) Overall HSE imaging sequence: Obtain fluid distribution map and distinguish CH4-rich areas from CO2-dominant areas by signal intensity threshold; S4: CH4 recovery rate is calculated based on the peak area of ​​the T2 spectrum, and displacement efficiency is analyzed by combining layered T2 spectrum, one-dimensional frequency and imaging results.

2. The CO2-EGR visualization monitoring method according to claim 1, characterized in that: The GR-HSE sequence acquires a one-dimensional frequency signal, and by adjusting the sequence parameters, the acquisition time is made ≤30 seconds / time. The spatial resolution of the GR-HSE sequence is ≥2mm.

3. The CO2-EGR visualization monitoring method according to claim 1, characterized in that: The T2 spectrum is used to classify pore types according to relaxation time: T2 < 10 ms is micropore, 10 ms ≤ T2 ≤ 100 ms is mesopore, and T2 > 100 ms is macropore; The formula for calculating CH4 recovery rate is: η = (A o -A t ) / A o ×100%, where A o For the initial peak area, A t Let be the peak area at time t.

4. The CO2-EGR visualization monitoring method according to claim 1, characterized in that: The layered T2 spectrum divides the core into 5-10 layers; The lower level corresponds to the injection end, and the higher level corresponds to the output end.

5. The method according to claim 1, characterized in that: In the imaging results, regions with signal intensity >70% represent CH4-rich areas; Areas with signal strength <30% represent CO2-dominant regions.

6. A nuclear magnetic resonance experimental system for implementing the CO2-EGR visualization monitoring method according to claims 1-5, characterized in that, include: Gas injection system, including CH4 gas cylinders and CO2 gas cylinders and flow controller; Nuclear magnetic resonance core holder, magnetic field strength 0.3±0.03T; Back pressure regulator, PID closed-loop control; The data acquisition unit integrates CPMG, SE-SPI, GR-HSE and HSE overall imaging sequence control modules; Each component is sealed and connected via high-pressure resistant pipelines.

7. The nuclear magnetic resonance experimental system according to claim 6, characterized in that: The nuclear magnetic resonance gripper includes: Radio frequency coil, resonant frequency 11.4MHz; Gradient coil, maximum gradient intensity 120mT / m.

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