A multi-dimensional nuclear magnetic resonance T1-T2* imaging method

Abstract

The application provides a multi-dimensional nuclear magnetic resonance T1-T2* imaging method. The technology can provide a fast imaging method for nuclear magnetic resonance instruments under different field strengths and different configurations, and is used for precisely measuring organic matter content and spatial distribution in a dense shale oil and gas reservoir sample. Through special data processing of the collected data, a high-dimensional three-dimensional T 1、 T 2 * The atlas result can be used for characterizing important information such as internal structure, micropore characteristics and fluid saturation of the sample. Therefore, the technology has important application value in analysis of complex samples of unconventional oil and gas reservoirs.

Description

Technical Field

[0001] This invention relates to a multidimensional nuclear magnetic resonance (NMR) T1-T2 system for rapid, accurate, and non-destructive detection of organic matter content in shale samples. * The principle and implementation process of the imaging method. This method will greatly improve the detection speed and efficiency of nuclear magnetic resonance imaging technology, and has the potential to solve the problems of long testing cycles and low accuracy of conventional organic matter content testing, enabling its field application in unconventional oil and gas reservoirs. Background Technology

[0002] Nuclear magnetic resonance imaging (NMR) is an advanced non-destructive testing technique with wide applications in medicine, biology, energy, materials, agriculture and forestry, food, safety monitoring, and chemical engineering. Taking biomedicine as an example, due to the differences in longitudinal relaxation times of different types of tissue fluids, NMR T1 imaging can provide the most direct and effective evidence for in-situ detection of pathological mechanisms in biological tissues, making it a conventional NMR weighted imaging method. However, conventional methods have a long detection time for longitudinal relaxation time T1, typically requiring the hydrogen-containing proton spin system in the tissue to reach thermal equilibrium before further measurement can be performed. Therefore, the overall measurement time for the sample using T1 imaging is very long. Meanwhile, T2... * Differences can also be used to some extent to distinguish sample components and identify fluid types and phases.

[0003] This invention, from a quantum mechanical perspective, elucidates fundamental theories and rationally arranges and optimizes corresponding pulses within different time periods of a pulse sequence, thereby significantly shortening the operation time of MRI T1 imaging technology; simultaneously, it adds T2 to segments in rapid T1 editing. * Parameter detection, through T1 / T2 in two-dimensional spectra * This method quantitatively identifies the organic matter and mobile fluid content in shale. The data obtained by this method differs from conventional methods and requires special attention during actual data processing. This invention provides a method for rapid nuclear magnetic resonance T1-T2... * Data processing methods and interpretation workflows for projection and stereoscopic imaging technologies. Summary of the Invention

[0004] The purpose of this invention is to elucidate the proposed multidimensional nuclear magnetic resonance T1-T2 * Imaging methods and corresponding data processing procedures.

[0005] To achieve the above objectives, the present invention adopts the following technical solution:

[0006] A multidimensional nuclear magnetic resonance T1-T2* imaging method is provided, which is divided into a projection imaging method and a stereo imaging method. Both the projection imaging method and the stereo imaging method use pulse sequences for data acquisition.

[0007] Preferably, the projection imaging method includes the following steps:

[0008] Step 1: Apply a 90° pulse to the hydrogen-containing proton spin system of the sample under test in the TRS channel to flip the macroscopic magnetization vector M0 to the transverse plane;

[0009] Step 2: After waiting for a very short time τ, apply a 180° pulse to the spin system of the sample under test on the TRS channel to reunite the transverse planar magnetization vector after phase re-aggregation.

[0010] Step 3: After waiting for a very short time τ again, apply a 90° pulse to the spin system of the sample under test on the TRS channel to rotate the refocused transverse planar magnetization vector 90° to the longitudinal axis (in the same direction as the static magnetic field).

[0011] Step 4: Apply a gradient of constant amplitude to the sample under test on the GRD1 channel. This gradient is used to eliminate the magnetization vector that still remains in the transverse plane of the current spin system.

[0012] Step 5: Next, apply a small angle to the TRS channel. An RF pulse is applied, followed by a phase-coded gradient pulse with a width of [missing information]. The height is from -g max to +g max Changes in m steps;

[0013] Step 6: Next, apply a phase-coded gradient pulse in another direction to the GRD2 channel. This pulse also has a width of [missing information]. The height is from -g max to +g max After m steps of change, the gradient directions of GRD1 and GRD2 determine the projection direction of the final image;

[0014] Step 7: After applying the gradients on GRD1 and GRD2 channels, repeatedly apply 180° RF pulses on the TRS channel. After a certain time, acquire a free induction attenuation signal FID on the ACQ channel. In practice, it is necessary to record the entire waveform of FID, with S recording points and a time interval between points of [missing information]. ;

[0015] Step 8: After the signal acquisition on the ACQ channel is completed, apply another 180° RF pulse on the TRS channel to flip the magnetization vector remaining in the longitudinal direction. Then, apply a gradient of constant amplitude on the GRD1 channel to eliminate the magnetization vector that still remains in the transverse plane of the spin system.

[0016] Step 9, from the first small angle on the TRS channel The entire timing duration, from the start of the RF pulse to the last constant-amplitude gradient on the GRD1 channel, is [duration missing]. The acquisition system continuously loops this timing sequence N times, acquiring N FID signals in the ACQ channel at a time. The phase encoding gradient amplitude values ​​in GRD1 and GRD2 are changed m times each, ultimately acquiring m*m*N FID signals in the ACQ channel. Each FID signal contains S data points. NMR data processing is performed on the acquired FID signals to obtain the required Fast NMR T1-T2 signals. * Imaging results

[0017] Preferably, the stereo imaging method includes the following steps:

[0018] Step 1: Apply a 90° pulse to the hydrogen-containing proton spin system of the sample under test in the TRS channel to flip the macroscopic magnetization vector M0 to the transverse plane;

[0019] Step 2: After waiting for a very short time τ, apply a 180° pulse to the spin system of the sample under test on the TRS channel to reunite the transverse planar magnetization vector after phase re-aggregation.

[0020] Step 3: After waiting for a very short time τ again, apply a 90° pulse to the spin system of the sample under test on the TRS channel to rotate the refocused transverse planar magnetization vector 90° to the longitudinal axis (in the same direction as the static magnetic field).

[0021] Step 4: Apply a gradient of constant amplitude to the sample under test on the GRD1 channel. This gradient is used to eliminate the magnetization vector that still remains in the transverse plane of the current spin system.

[0022] Step 5: Next, apply a small angle to the TRS channel. The radio frequency pulse, a soft pulse with layer-selective characteristics, is used in conjunction with the layer-selective gradient applied in the GRD3 channel to achieve layer-selective measurement of the sample under test. Subsequently, a phase-coded gradient pulse with a pulse width of [missing information] is applied to the GRD1 channel. The height is from -g max to +g max Changes in m steps;

[0023] Step 6: Next, apply a phase-coded gradient pulse in another direction to the GRD2 channel. This pulse also has a width of [missing information]. The height is from -g max to +g max After m steps of change, the gradient directions of GRD1 and GRD2 determine the final imaging direction;

[0024] Step 7: After applying the gradients on GRD1 and GRD2 channels, repeatedly apply 180° RF pulses on the TRS channel. After a certain time, acquire a free induction attenuation signal FID on the ACQ channel. In practice, it is necessary to record the entire waveform of FID, with S recording points and a time interval between points of [missing information]. ;

[0025] Step 8: After the signal acquisition on the ACQ channel is completed, apply another 180° RF pulse on the TRS channel to flip the magnetization vector remaining in the longitudinal direction. Then, apply a gradient of constant amplitude on the GRD1 channel to eliminate the magnetization vector that still remains in the transverse plane of the spin system.

[0026] Step 9, from the first small angle on the TRS channel The duration of the entire timing sequence, from the start of the RF soft pulse to the last constant-amplitude gradient on the GRD1 channel, is [duration missing]. The acquisition system continuously loops this timing sequence N times, acquiring N FID signals in the ACQ channel at a time. The phase encoding gradient amplitude values ​​in GRD1 and GRD2 are changed m times each, ultimately acquiring m*m*N FID signals in the ACQ channel. Each FID signal contains S data points. NMR data processing of the acquired FID signals yields the desired slice-selected fast NMR 2D T1-T2 signal. * By adjusting the size of the selected layer gradient pulse on the GRD3 channel, the spatial position of the selected layer can be changed, and the two-dimensional imaging result can be reconstructed into a three-dimensional result.

[0027] Beneficial effects: This invention provides a rapid imaging method for nuclear magnetic resonance instruments with different field strengths and configurations, used to precisely measure the organic matter content and spatial distribution in tight shale oil and gas reservoir samples. Through data processing of the data acquired using this technology with patent-specific descriptions, high-dimensional three-dimensional T1 and T2 values ​​of tight shale samples can be obtained rapidly and non-destructively. * The spectral results can further characterize important information such as the internal structure, micropore characteristics, and saturated fluid occurrence of the sample. Therefore, this technology has important application value in the analysis of complex samples from unconventional oil and gas reservoirs. Attached Figure Description

[0028] Figure 1 The multidimensional nuclear magnetic resonance T1-T2 provided in the embodiments of the present invention * Flowchart of projection imaging technology;

[0029] Figure 2 The multidimensional nuclear magnetic resonance T1-T2 provided in the embodiments of the present invention * Flowchart of stereoscopic imaging technology;

[0030] Figure 3 The multidimensional nuclear magnetic resonance T1-T2 provided in the embodiments of the present invention * Imaging data processing flowchart;

[0031] Figure 4 The multidimensional nuclear magnetic resonance T1-T2 provided in the embodiments of the present invention * Schematic diagram of rock sample detection using imaging technology and the results;

[0032] Figure 5 The multidimensional nuclear magnetic resonance T1-T2 provided in the embodiments of the present invention * A schematic diagram of imaging technology used to identify the organic matter content and mobile fluid content at a specific location in shale formations.

[0033] Figure 6 The multidimensional nuclear magnetic resonance T1-T2 provided in the embodiments of the present invention * A schematic diagram of the organic matter content and mobile fluid content of shale on the projection plane obtained by imaging technology;

[0034] Figure 7 The multidimensional nuclear magnetic resonance T1-T2 provided in the embodiments of the present invention * The final three-dimensional schematic diagram of the organic matter content and movable fluid content of shale obtained by imaging technology. Detailed Implementation

[0035] The specific embodiments of the present invention will be described with reference to the accompanying drawings. First, the basic principles, concepts, and theories of nuclear magnetic resonance will be introduced.

[0036] refer to Figure 1-7 As shown, the nuclear magnetic resonance detection technology involved in this patent includes: nuclear magnetic resonance pulse sequence design, nuclear magnetic resonance data acquisition and data processing, nuclear magnetic resonance data interpretation and organic matter association.

[0037] Among them, the multidimensional nuclear magnetic resonance T1-T2 * Projection imaging technology employs, for example Figure 1 Data acquisition is performed using the pulse sequence shown.

[0038] The multidimensional nuclear magnetic resonance T1-T2 * Stereoscopic imaging technology employs, for example Figure 2Data acquisition is performed using the pulse sequence shown.

[0039] The multidimensional nuclear magnetic resonance T1-T2 * Imaging technology adopts Figure 3 The process shown is for data processing;

[0040] The multidimensional nuclear magnetic resonance T1-T2 * The correlation between the imaging results of imaging technology and the spatial location of rock sample measurements is as follows: Figure 4 As shown;

[0041] The multidimensional nuclear magnetic resonance T1-T2 * T1-T2 of each spatial layer obtained by imaging technology * Diagrams and principles used to explain shale organic matter, such as... Figure 5 As shown;

[0042] The multidimensional nuclear magnetic resonance T1-T2 * The imaging technique ultimately yielded results showing the organic matter content and mobile fluid content of shale on the projection plane, as follows: Figure 6 As shown;

[0043] The multidimensional nuclear magnetic resonance T1-T2 * The final results of the imaging technology for the three-dimensional organic matter content and mobile fluid content of shale are as follows: Figure 7 As shown.

[0044] The static magnetic field B0 is provided by a magnet and determines the signal-to-noise ratio of the NMR signal. When the sample is placed in the static magnetic field, energy level splitting occurs within the spin system, generating a macroscopic magnetization vector M0 along the direction of the static magnetic field. M0 is determined by parameters such as the static magnetic field strength B0 and temperature. The magnet materials are usually either permanent magnets or superconductors. Permanent magnets are mainly used for low-field NMR measurements; superconductors are usually used in medical imaging and chemical spectroscopy analysis of high-field instruments in the laboratory, requiring the use of liquid helium and liquid nitrogen to maintain a constant magnet temperature.

[0045] Radio frequency (RF) magnetic field B1 and the pulse: The RF pulse is an electromagnetic signal, usually generated by a coil. The magnetic field generated by the RF pulse is the RF magnetic field. The direction of the RF magnetic field is perpendicular to the direction of the static magnetic field, realizing the manipulation of the magnetization vector formed in the static magnetic field. The manipulation angle is: θ = γB1t p Where B1 is the radio frequency magnetic field strength, t p The duration of the radio frequency pulse is the key factor. Therefore, the spin angle can be changed by controlling the amplitude or duration of the radio frequency pulse. The nuclear magnetic resonance pulse sequence is composed of radio frequency pulses of different numbers and frequency attributes in a set sequence. By adjusting the time interval between pulses, the pulse angle, and the frequency selectivity of the pulse, the relaxation and diffusion of the spin system can be measured.

[0046] Magnetic Field Gradient and Imaging: The pulsed magnetic field gradient is generated by a gradient coil. During application, the eddy current effect between the pulsed gradient coil and the radio frequency coil is usually considered, and shielding effectiveness is crucial. By understanding the relationship between the spatial magnetic field strength and the gradient value, the sample under test can be encoded with corresponding spatial phase, frequency, and layer selection, enabling spatial imaging in different dimensions. For a specific direction in space, taking z as an example, after applying a gradient pulse of amplitude g in that direction, the proton Larmor frequencies at different spatial locations are:

[0047]

[0048] Therefore, the relationship between the acquired echo signal and the spatially determined imaging proton density is as follows:

[0049]

[0050]

[0051] Where k is a defined wave function, which is related to the parameters of the gradient pulse. When the system uses frequency coding mode for imaging experiments, When phase encoding is used, As can be seen from the above formula, and Since it is a Fourier transform pair, the imaging result can be obtained by performing a Fourier transform on the acquired echo signal. By combining frequency gradient coding and phase gradient coding modes and methods, high-dimensional magnetic resonance imaging results can be obtained.

[0052] Free Induction Decay Signal (FID): FID is one of the most common signals in NMR measurements. First, a 90° pulse is applied to the sample, shifting the magnetization vector M0 to a transverse plane perpendicular to the static magnetic field. Due to molecular diffusion and the spatial inhomogeneity of the static magnetic field, the magnetization vector M0 undergoes dephasing. If the signal acquisition channel is opened during this period, the free decay signal can be obtained. During acquisition, the width of the acquisition window is controlled by adjusting parameters such as the number of acquisition points and the time interval between acquisition points. The decay rate of the FID signal is controlled by parameter T2. * In nuclear magnetic resonance (NMR) applications, the following are important applications: (1) Using the duration of the FID signal to evaluate the uniformity of the magnetic field and the shimming strategy; (2) Using the FID signal to obtain the chemical spectrum, which is used to evaluate the composition and content of each functional group; (3) Using the attenuation of the FID signal to obtain the difference in magnetization coefficient, which characterizes the magnetic characteristics of the sample's solid skeleton; (4) Using the attenuation of the FID signal to obtain the phase difference, which characterizes the mobility and viscosity of the components.

[0053] Relaxation: The process by which a spin system recovers from a resonant state to a thermal equilibrium state. This process is characterized by the longitudinal relaxation time T1 or the transverse relaxation time T2 in different directions. T1 is also called the spin-lattice relaxation time, reflecting the energy exchange between the spin system and the external environment, while T2 is called the spin-spin relaxation time, reflecting the energy loss within the spin system. The relaxation process of the spin system can be described by the Bloch equation. The longitudinal relaxation time T1 can be measured using a saturation recovery pulse sequence, by changing the time interval T between two pulses. W Record the signal amplitude to reflect the evolution of the longitudinal magnetization vector at different editing times:

[0054]

[0055] The above methods require a relatively long time, with each step taking T... W Each of these processes requires the proton spin system to wait for a considerable period of time and reach thermal equilibrium before the next experiment can proceed, making the data collection process extremely slow.

[0056] The imaging technology in this patent is based on a rapid T1 editing method, employing a small angle. The final measurement result is obtained by acquiring a series of radio frequency pulses. By applying this pulse train containing N small-angle radio frequency pulses, the components of the magnetization vector of the spin system of the sample under test can be expressed as:

[0057]

[0058]

[0059] By employing a relevant adaptive RF pulse phase cycle and adjusting the amplitude of the acquired signal, each small angle can be obtained. The magnetization vector under the radio frequency pulse is:

[0060]

[0061] Where N is the number of small-angle radio frequency pulses, This represents the time interval between two adjacent small-angle pulses. Compared to the conventional T1 measurement method, the above method can complete the T1 measurement in a shorter time because it only operates on the longitudinal magnetization vector once.

[0062] For T2 * This parameter can be measured by processing each acquired FID signal. Its mathematical correlation can be expressed as:

[0063]

[0064] Relaxation imaging

[0065] In practical measurement applications, it has been found that simply obtaining the proton density information of the sample, i.e., imaging, is far from sufficient for analyzing the microscopic information of the sample. Therefore, combining the acquisition of relaxation information with imaging technology can perfectly realize cross-scale observation of the sample at both the macroscopic and microscopic levels.

[0066] In actual dense porous materials, such as the dense shale sample referred to in this patent, the spin system environments of the mobile fluid and the hydrogen-containing components in the organic matter differ, resulting in different components exhibiting longitudinal relaxation time T1 and characteristic transverse relaxation time T2. * The differences are relatively obvious, therefore it can be selected in this patent as a combination of weighted information and imaging technology, if conventional T1 and T2 are used. * The combination of measurement methods and imaging results in a relatively long actual measurement time, which is not conducive to rapid dynamic observation of the sample's own information. Therefore, in this invention, the rapid longitudinal relaxation time T1 and the rapid lateral relaxation time T2 are measured separately. * By integrating measurement methods with relevant imaging techniques and optimizing and adjusting relevant parameters, a rapid multidimensional nuclear magnetic resonance (NMR) T1-T2 imaging method is presented. * Feasible solutions for imaging technology.

[0067] Based on the measured longitudinal relaxation time T1 and transverse relaxation time T2 of the sample... * The relationship between spatial gradient coding and design, such as Figure 1 The fast nuclear magnetic resonance T1-T2 diagram shown * For the imaging pulse sequence, assuming the emission gradients of GRD1 and GRD2 are in the x and y directions respectively, the following response formula can be obtained from the acquired signal:

[0068]

[0069] The specific forms of the four kernel functions K1, K2, K3, and K4 are as follows:

[0070]

[0071]

[0072]

[0073]

[0074] for Figure 1 and Figure 2 The phase encoding mode shown, Multidimensional data is obtained by changing the gradient amplitude step number m, the number of small-angle pulses N in T1 editing, and the number of acquisition points S in each FID signal. The obtained data is then processed using subsequent data inversion steps to obtain the T1-T2 of the tested sample. *Imaging results can also be used to adjust the T1-T2 phase of nuclear magnetic resonance imaging by changing the gradient directions on GRD1 and GRD2. * The projection plane of the image; simultaneously, in the stereo imaging results, by changing the magnitude of the GRD3 gradient, and thus adjusting the selected layer gradient to change the selected layer position and selected layer thickness, a series of two-dimensional T1-T2 layers can be obtained. * The imaging results can be used to reconstruct the corresponding three-dimensional T1-T2. * Imaging results.

[0075] Data processing steps

[0076] This method involves relatively many spatial and data dimensions, therefore, targeted processing is required during actual computation. This invention combines... Figure 3 For the proposed fast nuclear magnetic resonance T1-T2 * The data processing steps for imaging are explained in detail:

[0077] Step 1: Determine whether slice-selective soft pulses were used during the experiment. If no soft pulses were used, organize the acquired NMR data. The dimensions are determined to make the final data size m*m*N*S, where m is the number of phase encoding steps, N is the number of FID signals collected in each phase encoding step, and S is the number of collection points in each FID signal;

[0078] Step 2: For The data is first subjected to a two-dimensional Fourier transform, and then decompiled along the imaging dimension to obtain m*m original data. For decaying data, the Fourier transform is a linear transform, which is a non-ill-conditioned problem, so it will not be discussed further here;

[0079] Step 3: Perform data fitting on the decoded two-dimensional data at each location in the space to obtain T1-T2. * The specific data fitting algorithms are divided into several types, including single exponential fitting, multi-exponential fitting, and inverse Laplace inversion, to obtain the final T1-T2 values ​​at different spatial locations. * Since the Inverse Laplace inversion is an ill-conditioned problem, this section briefly describes the Inverse Laplace transformation. A regularization term will be introduced to invert this data matrix. To obtain a stable and accurate solution F, the Tikhonov regularization method is typically used, introducing a smoothing term to solve the problem.

[0080]

[0081] Where s is the regularization factor, which is related to the signal-to-noise ratio of the collected data. The term represents the Frobenius norm of the matrix. The introduced regularization term determines the stability and accuracy of the solution. If the regularization factor is too large, although the distribution obtained is more stable, the accuracy of the solution is worse, i.e., oversmoothing. If the regularization factor is too small, the solution is more accurate, but the stability of the solution decreases and more spurious signals appear, i.e., undersmoothing. Therefore, considering both the authenticity and stability of the solution, using a reasonable regularization factor is the key to this method. Through the non-negative constraint step, the non-negative constraint solution F(T1,T2) for each spatial layer under a specific regularization factor s can be obtained. * );

[0082] Step 4: Repeat the above steps m*m times, performing ILT data processing on the data at each point in space sequentially, ultimately obtaining continuous (T1-T2) data on a certain projection plane. * The distribution is the final two-dimensional T1-T2. * Projection imaging;

[0083] Step 5: By adjusting the gradient directions of GRD1 and GRD2, the direction of the projected image can be changed, thus obtaining two-dimensional T1-T2 on different planes. * Projection imaging results;

[0084] Step 6: If soft pulses were used during the acquisition process, first perform a two-dimensional Fourier transform on the data to decode the data in both imaging dimensions. Then, perform a two-dimensional Inverse Laplace inversion on the decoded data using the Inverse Laplace transform method described in Step 3 to obtain the two-dimensional T1-T2. * The imaging results, by changing the selected layer position and thickness through the selected layer gradient along the GRD3 direction, can yield a series of two-dimensional T1-T2 images. * The imaging results can be used to reconstruct the corresponding three-dimensional T1-T2. * Imaging results.

[0085] Data interpretation steps

[0086] Taking the projection imaging result as an example, the final obtained F(x,y,T1,T2) * The result is as follows Figure 4 As shown, the grid density within the xy, zx, and zy projection planes represents the spatial resolution, determined by the magnitude and number of gradient steps of GRD1 and GRD2. Within each pixel, the (T1-T2) values ​​of the corresponding rock pixel can be obtained. * )distributed.

[0087] Furthermore, within each rock pixel (T1-T2) * Distribution as follows Figure 5As shown, due to the completely different homonuclear and heteronuclear coupling of the organic hydrogen nuclei in the shale sample compared to the mobile fluid components, the final T1 / T2 ratio of these two components... * The ratios are different, so this parameter difference can be used to quickly and non-destructively identify organic components. By accumulating the signals displayed in the spectrum, the relative contents of organic matter and mobile fluid at that layer can be obtained.

[0088] Furthermore, taking the xy-plane projection as an example, by continuously identifying and calculating the organic matter and mobile fluid content in m*m maps within the xy-plane, the organic matter content profile and mobile fluid profile distribution of shale within the xy-projection plane can be obtained, as shown in the following figure. Figure 6 As shown.

[0089] Higher dimensions, when adopting Figure 2 The three-dimensional solid T1-T2 shown * The imaging sequence acquires data, and the data is processed according to step 6 in the data processing section. In each xy-plane, the organic matter and mobile fluid content in m*m maps are continuously identified and calculated. This step is repeated along the z-axis to obtain the overall three-dimensional profile of shale's organic matter and mobile fluid distribution. The results are as follows: Figure 7 As shown.

[0090] Conventional methods for detecting shale organic matter require grinding and crushing the sample, followed by multiple pyrolysis and acid washing processes to determine the organic matter content. Therefore, these methods are inefficient and have poor accuracy. Furthermore, they only provide the volumetric organic matter mass, not the spatial distribution. This invention proposes a multidimensional nuclear magnetic resonance (NMR) technique that correlates two characteristic relaxation times with an imaging editing segment to effectively acquire the organic matter content at spatial imaging layers. Simultaneously, the imaging technology enables spatial detection of organic matter content, making it an effective and rapid detection method for unconventional tight oil and gas reservoir samples.

[0091] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method of multi-dimensional nuclear magnetic resonance T1 - T2* imaging, characterized by, The imaging method is divided into a projection imaging method and a stereoscopic imaging method, both of which adopt a pulse sequence for data acquisition, the projection imaging method comprising the following steps: Step 1, applying a 90° pulse to a hydrogen-containing proton spin system of a measured sample on a TRS channel, the measured sample being placed in a static magnetic field, energy level splitting occurring in the spin system, a macroscopic magnetization vector M0 being generated along the direction of the static magnetic field, and the macroscopic magnetization vector M0 being turned to a transverse plane; Step 2, wait for a very short time Step 4, applying a gradient with a constant amplitude to the measured sample on a GRD1 channel, the gradient being used to eliminate the magnetization vector still remaining in the transverse plane in the current spin system; After, a 180° pulse is applied to the spin system of the sample under test on the TRS channel, rephasing the transverse plane magnetization vector after the dispersion phase; Step 3, again wait for a very short time Step 8, after signal acquisition on the ACQ channel is completed, applying a 180° radio frequency pulse to the measured sample on the TRS channel again, the magnetization vector remaining in the longitudinal direction being flipped, and then applying a gradient with a constant amplitude to the measured sample on the GRD1 channel again, the gradient being used to eliminate the magnetization vector still remaining in the transverse plane in the current spin system; After that, a 90° pulse is applied to the spin system of the measured sample on the TRS channel, and the reassembled transverse plane magnetization vector is turned 90° to the longitudinal axial direction. The stereoscopic imaging method comprises the following steps: Step 5, next a small angle is applied on the TRS channel A radio frequency pulse is applied, followed by a phase encoding gradient pulse on the GRD1 channel, with a pulse width of , the height varying from -g max to +g max in m steps; Step 6, next a phase encoding gradient pulse of another direction is applied on the GRD2 channel, which is also of width , height varying from -g max to +g max m steps, the gradient directions of GRD1 and GRD2 determine the projection directions of the final imaging; Step 7, after the gradient application on the GRD1 and GRD2 channels is finished, a 180° radio frequency pulse is repeatedly applied on the TRS channel, and a free induction decay signal FID is acquired on the ACQ channel after a certain time, in practice, the FID needs to be recorded in full waveform, the number of recording points is S, and the time interval between points is ; Step 4, applying a gradient with a constant amplitude to the measured sample on a GRD1 channel, the gradient being used to eliminate the magnetization vector still remaining in the transverse plane in the current spin system; Step 9, the first small angle from the TRS channel The entire timing duration is from the start of the radio frequency pulse to the last constant amplitude gradient on the GRD1 channel The acquisition system continuously cycles this part of the timing N times, and N FID signals are acquired in the ACQ channel each time. The phase encoding gradient amplitude value in the GRD1 and GRD2 gradients is changed m times, and m*m*N FID signals are finally acquired in the ACQ channel, each FID signal containing S data points. The acquired FID signals are subjected to nuclear magnetic resonance data processing, and the desired fast nuclear magnetic resonance T 1- T 2 * imaging results; Step 8, after signal acquisition on the ACQ channel is completed, applying a 180° radio frequency pulse to the measured sample on the TRS channel again, the magnetization vector remaining in the longitudinal direction being flipped, and then applying a gradient with a constant amplitude to the measured sample on the GRD1 channel again, the gradient being used to eliminate the magnetization vector still remaining in the transverse plane in the current spin system; Step 1, applying a 90° pulse to the hydrogen proton spin system of the sample under test on the TRS channel to tip the macroscopic magnetization vector M 0 tipped to the transverse plane; Step 2, wait for a very short time ​ After, a 180° pulse is applied to the spin system of the sample under test on the TRS channel, rephasing the transverse plane magnetization vector after the dispersion phase; Step 3, again wait for a very short time ​ After that, a 90° pulse is applied to the spin system of the sample under test on the TRS channel, and the reassembled transverse plane magnetization vector is turned 90° to the longitudinal axial direction. ​ Step 5, continue to apply a small angle on the TRS channel RF pulse, which is a soft pulse with a selected layer characteristic, used in cooperation with the selected layer gradient applied in the GRD3 channel to achieve the purpose of selected layer measurement of the sample being measured, and then a phase encoding gradient pulse is applied on the GRD1 channel, the pulse width is , the height changes from -g max to +g max m steps; Step 6, next a phase encoding gradient pulse of another direction is applied on the GRD2 channel, which is also of the same width , the height varies from -g max to +g max m steps, the gradient direction of GRD1 and GRD2 determines the final imaging direction; Step 7, after the gradient application on the GRD1 and GRD2 channels is finished, a 180° radio frequency pulse is repeatedly applied on the TRS channel, and a free induction decay signal FID is acquired on the ACQ channel after a certain time. In practice, the FID needs to be recorded in full waveform, the number of recording points is S, and the time interval between points is ; ​ Step 9, from the first small angle on the TRS channel The entire timing duration is from the start of the radio frequency soft pulse to the last constant amplitude gradient on the GRD1 channel The acquisition system continuously cycles this part of the timing N times, and N FID signals are acquired in the ACQ channel each time. The phase encoding gradient amplitude value in the GRD1 and GRD2 gradients is changed m times, and m*m*N FID signals are finally acquired in the ACQ channel, each FID signal containing S data points. The acquired FID signals are subjected to nuclear magnetic resonance data processing, and the selected layer fast nuclear magnetic resonance two-dimensional T 1- T 2 * imaging results, by adjusting the size of the selected layer gradient pulse on the GRD3 channel, changing the spatial position of the selected layer, and finally reconstructing the two-dimensional imaging results into three-dimensional results.

Images