CN112129801A - A fast measurement method for layer-selective T2 relaxation spectrum of porous media - Google Patents

Abstract

The invention discloses a fast measurement method for layer-selective T2 relaxation spectrum of porous media. By designing a T2 relaxation spectrum test pulse sequence with layer-selective function, under a constant layer-selective gradient, the CPMG pulse sequence is used to simultaneously measure spatial position points. The layer selection coding is performed to realize the test and analysis of the nuclear magnetic resonance T2 relaxation spectrum of different layer selection positions in the porous medium, and the physical parameter characteristics of different positions of the porous medium can be obtained.

Description

A fast measurement method for layer-selective T2 relaxation spectrum of porous media

technical field

The invention belongs to the technical field of nuclear magnetic resonance, and relates to a method for rapidly testing a layer-selective T2 relaxation spectrum of a porous medium.

Background technique

Nuclear magnetic resonance T2 relaxation spectroscopy has the advantages of fast, non-destructive, pollution-free, and the same number of test parameters. At present, it is widely used as the main test method for porous media in petroleum exploration and development, coalbed methane occurrence and migration, and cement building materials. In the NMR test process, the CPMG NMR pulse sequence is used to test the echo train signal generated by the fluid in the sample, and the T2 relaxation spectrum is obtained by mathematical inversion of the echo train signal. , the sample is excited as a whole during the analysis, and the porosity, permeability, oil saturation, water saturation, gas saturation, movable fluid distribution, and Analysis of dynamic displacement process, etc.

Porous media often have a certain degree of heterogeneity, especially for samples with low porosity and low porosity, which have a chaotic and complex structure and serious bedding stacking. Without spatial information, the inhomogeneity of the entire core is often ignored, which cannot reflect the huge differences inside the core, and it is difficult to accurately describe the core information. Therefore, it is difficult to obtain the variation law of physical parameters at different positions of the core using the existing methods.

SUMMARY OF THE INVENTION

In order to overcome the above-mentioned technical problems that the existing nuclear magnetic resonance T2 relaxation spectrum testing technology has no spatial information and is difficult to obtain the physical property parameter variation characteristics of different positions of the core, the present invention provides a nuclear magnetic resonance method for layer selection of porous media (such as core test samples). The rapid T2 relaxation spectrum measurement method realizes the NMR T2 relaxation spectrum analysis of the selected layers in different spatial positions of the porous medium by designing the frequency encoding gradient along the sample axis, and obtains the variation characteristics of physical parameters in different positions of the porous medium. The technical scheme of the present invention is:

A fast measurement method for layer-selective T2 relaxation spectrum of porous media, which reduces scanning time by simultaneously performing layer-selective coding on spatial position points with CPMG (Carr-Purcell-Meiboom-Gill) pulse sequence under a constant layer-selective gradient.

The method for testing the T2 spectrum of rapid layer selection provided by the present invention is used to test the T2 relaxation spectrum of different layer selection positions of a porous medium test sample, and includes the following steps:

1) Obtain the layer selection T2 pulse sequence of the porous medium test sample;

The layer-selective T2 pulse sequence of the porous medium is shown in FIG. 1 , which includes an NMR waveform diagram of an RF (Radio Frequency, radio frequency) channel, a GREAD (Gradient in Read, gradient in read) gradient channel, and a READ acquisition channel. In specific implementation, the layer selection of the porous medium is sliced at different positions of the core test sample.

2) Execute the layer-selecting T2 pulse sequence, and start a single scan, which includes steps 2)-7). On the GREAD gradient channel, a constant-amplitude frequency-encoding gradient is applied to the porous medium test sample, through which one-dimensional layer-selective encoding is performed along the length of the test sample.

a) Usually, the gradient is a three-dimensional vector, but in the present invention, we only need the one-dimensional layer selection result along the axial (length direction) of the test sample, so the GREAD gradient in the present invention selects the axial direction of the test sample.

b) In order to avoid the eddy current generated by the gradient from interfering with the nuclear magnetic resonance signal generated by the test sample, the present invention adopts a gradient with a ramp-up shape, and the gradient climbs to a preset amplitude G (the value range of G is 10-300mT/m, such as the preset G=60mT /m), the gradient will continue until the end of a single sweep.

3) On the RF radio frequency channel, apply a 90° radio frequency pulse to the test sample.

a) The 90° radio frequency pulse is a shapeless hard pulse, the duration is t ₉₀ , and the value is 11us.

b) The 90° radio frequency pulse rotates the macroscopic magnetization vector M ₀ by 90°, so that the macroscopic magnetization vector M ₀ is in the transverse plane.

4) On the RF radio frequency channel, apply a 180° radio frequency pulse to the test sample to rephase the macroscopic magnetization vector M ₀ in the transverse plane.

a) The 180° radio frequency pulse is a shapeless hard pulse with a duration of t ₁₈₀ and a value of 22us.

b) from the center of the "90° radio frequency pulse applied on the RF channel" in step 3) to the center of the "180° radio frequency pulse applied on the RF channel" in step 4, for a duration of tau, tau can take a value of 500us;

5) In the READ collection channel, a series of discrete spin echo signals are collected.

a) This signal is the NMR spin echo signal of the test sample, which is generated by the action of the 90° radio frequency pulse and the 180° radio frequency pulse on the hydrogen-containing proton spin system of the test sample. The amplitude of the spin echo signal is determined by Weakness becomes stronger and weaker again.

b) The acquisition process of the signal is discrete acquisition, the acquisition interval is DW, the number of acquisition points is SI, and the duration of the entire acquisition process is DW*SI. DW range is 1-40us; SI range is 16-256.

c) Starting from the center of "180° radio frequency pulse applied on the RF radio frequency channel" in step 4, to the center of "signal acquisition on the READ acquisition channel" in step 5, the entire time sequence duration is tau;

6) Repeat steps 4) and 5), repeat the cycle N times, and the time interval of the cycle is 2*tau. The value range of N is 256-25600.

a) In the N repeated cycles of step 6), the cycle time interval of the 180° radio frequency pulses cyclically applied on the RF radio frequency channel is 2*tau.

b) In the N repeated cycles of step 6), the cycle time interval of cyclically collecting a series of discrete spin echo signals on the READ acquisition channel is 2*tau

7) Close the GREAD gradient channel, and the single scan ends.

a) On the READ acquisition channel, sequentially acquire N series of discrete spin echo signals, the time interval of each series of discrete signals is 2*tau, and the number of points of each series of discrete signals is SI;

b) In the READ acquisition channel, SI*N signal amplitude values are finally acquired, and a set of echo train signals S_1(n, tau) is obtained, where 1≤n≤N.

8) Repeat steps 2)-7) M times cyclically, and perform a single scan repeatedly. The value range of M is 1-32.

a) In the READ acquisition channel, M groups of echo train signals S_m(n, tau) are obtained, where 1≤m≤M.

b) Perform signal accumulation on M groups of echo train signals S_m(n, tau) to obtain accumulated echo train signals S(n, tau).

9) Perform nuclear magnetic resonance data processing on the echo train signal S(n, tau) obtained in step 8).

a) First, the echo train signals ρ(z, n*tau) of different selected layers are obtained. Where z is the direction coordinate of the selected layer.

b) Finally, the layer-selected T2 relaxation spectrum P(z, T ₂ ) is obtained.

Through the above steps, the rapid test of the nuclear magnetic resonance T2 relaxation spectrum of the layer selection of the porous medium is realized.

Compared with the prior art, the beneficial effects of the present invention are:

In view of the fact that the prior art can only analyze the statistical T2 relaxation spectrum of porous media, but cannot analyze the physical property parameters of different positions of the porous media sample by anisotropy and non-uniformity, the method of the present invention designs a T2 relaxation spectrum with layer selection function. The test pulse sequence realizes the test and analysis of the nuclear magnetic resonance T2 relaxation spectrum at different positions in the porous medium, and can obtain the physical property parameters of the different positions of the porous medium.

Description of drawings

Fig. 1 is the layer-selective T2 pulse sequence designed by the present invention.

FIG. 2 is a schematic diagram of layer selection of a sample to be tested in an embodiment of the present invention.

FIG. 3 is an echo train signal obtained from a READ acquisition channel in an embodiment of the present invention.

FIG. 4 shows echo train signals of different selected layers obtained after data processing in an embodiment of the present invention.

FIG. 5 is the T2 relaxation spectrum of different selected layers obtained after data processing in the embodiment of the present invention.

Detailed ways

Below in conjunction with the accompanying drawings, the present invention is further described by means of embodiments, but the scope of the present invention is not limited in any way.

The invention realizes the nuclear magnetic resonance T2 relaxation of different positions in the porous medium by designing the T2 relaxation spectrum test pulse sequence with the function of layer selection, and using the CPMG pulse sequence to simultaneously perform layer selection coding on the spatial position points under the constant layer selection gradient. The test and analysis of the relaxation spectrum can obtain the physical parameter characteristics of different positions of the porous medium, which can be used for the T2 relaxation spectrum of the different layer selection positions of the porous medium test sample.

In the embodiment, a sandstone core is selected as the porous medium test sample, the porosity is 15.8%, the permeability is 8 millidarcy, and the core is in the shape of a cylinder with a diameter of 25mm*50mm.

Use distilled water (H ₂ O) to configure simulated formation water, and then saturate the core samples according to the method specified in SY/T5336.

1. Test sample preparation.

a) Use organic solvent to wash the core with oil, so that the fluorescence level of the core is less than level 3.

b) Dry the core at 116°C to constant weight, put it in a desiccator to cool to room temperature, and keep the core sample under vacuum.

c) According to SY/T5336-2006, the helium porosity of the test core is 15.8%, the permeability is 8 millidarcy, the core diameter is 25mm, and the length is 50mm.

d) Use distilled water (H ₂ O) to configure simulated formation water, then vacuum the simulated formation water according to the method specified in SY/T5336, and perform saturation treatment on core samples.

2. Steps 3 to 10 use nuclear magnetic resonance equipment to run the layer-selective T2 relaxation spectrum testing method provided by the present invention. The nuclear magnetic resonance equipment is a nuclear magnetic resonance imaging system (SPEC series) produced by a company, the magnet is a permanent magnet, and the field strength is 1.0 T. The layer-selecting T2 pulse sequence is shown in FIG. 1 , including an RF (Radio Frequency, radio frequency) channel, a GREAD (Gradient in Read, read gradient) gradient channel, and an NMR waveform diagram of a READ acquisition channel. .

3. Execute the layer-selecting T2 pulse sequence, and start a single scan, which includes steps 3)-8). On the GREAD gradient channel, a gradient of constant magnitude is applied to the test sample for 1D layer-selective encoding along the length of the test sample.

a) Usually, the gradient is a three-dimensional vector, but in the present invention, we only need the one-dimensional layer selection result along the axis (length direction) of the test sample, so the GREAD gradient selects the axis of the test sample.

b) The present invention adopts a gradient in the shape of a ramp-up. The gradient climbs to a preset amplitude G (G=60mT/m) and remains constant. The gradient will continue until the end of a single scan.

4. On the RF channel, apply a 90° RF pulse to the test sample.

a) The 90° radio frequency pulse is a shapeless hard pulse, the duration is t ₉₀ , and the value is 11us.

b) The 90° radio frequency pulse rotates the macroscopic magnetization vector M ₀ by 90°, so that the macroscopic magnetization vector M ₀ is in the transverse plane.

5. On the RF radio frequency channel, apply a 180° radio frequency pulse to the test sample to rephase the macroscopic magnetization vector M ₀ in the transverse plane.

a) The 180° radio frequency pulse is a shapeless hard pulse with a duration of t ₁₈₀ and a value of 22us.

b) From the center of the "90° RF pulse applied on the RF channel" in the step 4 to the center of the "180° RF pulse applied on the RF channel" in step 5, the entire timing duration For tau, the value of tau can be 500us;

6. In the READ acquisition channel, a series of discrete spin echo signals are acquired.

a) This signal is the NMR spin echo signal of the test sample, which is generated by the action of the 90° radio frequency pulse and the 180° radio frequency pulse on the hydrogen-containing proton spin system of the test sample. The amplitude of the spin echo signal is determined by Weakness becomes stronger and weaker again.

b) The acquisition process of the signal is discrete acquisition, the acquisition interval is DW, the number of acquisition points is SI, and the duration of the entire acquisition process is DW*SI. The value of DW is 4us; the value of SI is 16.

c) From the center of the "180° RF pulse applied on the RF channel" in the step 5 to the center of the "signal acquisition on the READ acquisition channel" in the step 6, the entire timing duration is tau

7. Repeat steps 5 and 6, repeat the cycle N times, and the time interval of the cycle is 2*tau. The value of N is 12800.

a) In the N repeated cycles of step 7, the cycle time interval of the 180° radio frequency pulses cyclically applied on the RF radio frequency channel is 2*tau.

b) In the N repeated cycles of step 7, the cycle time interval of cyclically collecting a series of discrete spin echo signals on the READ acquisition channel is 2*tau

8. Turn off the GREAD gradient and end a single scan.

a) On the READ acquisition channel, sequentially acquire N series of discrete spin echo signals, the time interval of each series of discrete signals is 2*tau, and the number of points of each series of discrete signals is SI;

b) In the READ acquisition channel, SI*N signal amplitude values are finally acquired, and a set of echo train signals S_1(n, tau) is obtained, where 1≤n≤N.

9. Repeat steps 3-8, repeat the cycle M times for the timing sequence, and perform a single scan repeatedly. M takes the value 16.

a) In the READ acquisition channel, M groups of echo train signals S_m(n, tau) are obtained, where 1≤m≤M.

b) Perform signal accumulation on M groups of echo train signals S_m(n, tau) to obtain the accumulated echo train signals S(n, tau), as shown in FIG. 3 .

10. Perform nuclear magnetic resonance data processing on the echo train signal S(n, tau) obtained in step 9.

a) First to the echo train signals ρ(z, n*tau) of different selected slices, where z is the coordinate of the slice selection direction. Figure 4.

b) Finally, the layer-selected T2 relaxation spectrum P(z, T ₂ ) is obtained, as shown in Figure 5 .

The NMR data processing used in step 10 will be described in detail below;

In the signal acquisition process of the READ acquisition channel, the signal is S(n, tau), which is expressed as formula (1):

S(n,tau)=∫ρ(z,n*tau)exp[i2π(kk ₀ )z]dz (1)

in,

In formula (1), S(n, tau) is the acquired signal, z is the layer selection direction coordinate; i is a complex number; k is the nuclear magnetic resonance K space wave vector; k ₀ is the additional wave vector caused by the gradient; ρ( z, n*tau) is the macroscopic magnetization signal obtained by the excitation of the H nuclear spins of the test sample, as a function of the layer selection direction (z) coordinate and each acquisition time (n*tau) of the READ acquisition channel. In formulas (2) and (3), γ is the gyromagnetic ratio, G is the corresponding value that remains constant after the gradient climbs to the preset amplitude in step 3, t is the acquisition time, and t is the start of each acquisition in the READ acquisition channel. is zero, and then increases to the acquisition process duration DW*SI of step 6(b), where T is half of the maximum value of t.

Between 180° RF pulses, the wave vector (kk ₀ ) is eliminated, and the NMR signal reaches the maximum signal of the spin echo. After this, the magnetization dissipates again and a new process of generating the next spin echo is started by the 180° radio frequency pulse.

Through the inverse Fourier transform of formula (1), namely formula (4), ρ(z, n*tau) can be obtained.

ρ(z, n*tau)=∫S(n,tau)exp[-i2π(kk ₀ )z]dk (4)

Equation (1) and Equation (4) are pairs of Fourier transforms.

In practical applications, as in step 6(b), the acquisition process of the signal S(n, tau) is discrete acquisition, so the inverse Fourier transform in formula (4) is solved by the fast Fourier transform (FFT) algorithm .

Subsequently, ρ(z, n*tau) obtained by FFT is used as a function of the layer selection direction (z) coordinate, and the corresponding multiple layers are shown in FIG. 2 (including layers 1 to 7). We can analyze the decay of ρ(z, n*tau) over time (n*tau) for each selected layer, and the results are shown in Figure 4. Fit the decay at each point on the selected layer with an exponential function, as in Equation (5).

Among them, P(z, T ₂ ) is called the layer-selected T2 relaxation spectrum, and the T2 relaxation spectra of the 1-selected layer to the 7-selected layer are shown in FIG. 5 .

It should be noted that the purpose of publishing the embodiments is to help further understanding of the present invention, but those skilled in the art can understand that various replacements and modifications are possible without departing from the spirit and scope of the present invention and the appended claims. of. Therefore, the present invention should not be limited to the contents disclosed in the embodiments, and the scope of protection of the present invention shall be subject to the scope defined by the claims.

Claims (8)

1. A fast measurement method for layer selection T2 relaxation spectrum of porous media, by using CPMG pulse sequence to simultaneously perform layer selection coding on spatial position points under a constant layer selection gradient, it is used for the detection of different layer selection positions of porous media test samples. T2 relaxation spectrum; including the following steps: 1) Obtain the layer selection T2 pulse sequence of the porous medium test sample; the layer selection T2 pulse sequence of the porous medium includes the radio frequency channel, the reading gradient channel, and the nuclear magnetic resonance waveform of the reading channel; Execute the layer-selected T2 pulse sequence and start a single scan, including steps 2)-7): 2) On the reading gradient channel, a gradient of constant amplitude is applied to the porous medium test sample, and one-dimensional layer selection coding is performed along the length direction of the porous medium test sample through the gradient; Specifically, the gradient of the ramp-up shape is used, and the gradient remains unchanged after the gradient climbs to the preset amplitude G until the end of a single scan; 3) On the radio frequency channel, a 90° radio frequency pulse is applied to the porous medium test sample; the 90° radio frequency pulse is a shapeless hard pulse with a duration of t ₉₀ , and the macroscopic magnetization vector M ₀ is rotated by 90° to make the macroscopic magnetization The intensity vector M ₀ is in the transverse plane; 4) On the radio frequency channel, a 180° radio frequency pulse is applied to the porous medium test sample to rephase the macroscopic magnetization vector M ₀ in the transverse plane; the 180° radio frequency pulse is a shapeless hard pulse with a duration of t ₁₈₀ ; From the center of the 90° radio frequency pulse applied on the RF radio frequency channel in step 3) to the center of the 180° radio frequency pulse applied on the RF radio frequency channel in step 4), the duration is tau; The 90° radio frequency pulse and the 180° radio frequency pulse act on the hydrogen-containing proton spin system of the test sample to generate a series of discrete spin echo signals; this signal is the nuclear magnetic resonance spin echo signal of the porous medium test sample; the spin echo signal The magnitude of the value changes from weak to strong and then weak; 5) The string of spin echo signals is collected in the read channel; the collection interval is DW, the number of collection points is SI, and the duration of the collection process is DW*SI; From the center of the 180° radio frequency pulse applied on the RF radio frequency channel in step 4) to the center of the signal acquisition on the READ acquisition channel in step 5), the duration is tau; 6) Repeat steps 4) and 5) cyclically, repeating N times, the time interval of the cycle is 2*tau; the value range of N is 256-25600; 7) Sequentially collect N series of discrete spin echo signals on the reading channel, the time interval of each series of discrete signals is 2*tau, and the number of points of each series of discrete signals is SI; SI*N signal amplitude values are obtained by collecting, Form a set of echo train signals S_1(n, tau), where 1≤n≤N; close the reading gradient channel, and the single scan ends; 8) Circularly repeat steps 2)-7) to perform a single scan M times, and the value range of M is 1-32; a) In the read channel, M groups of echo train signals S_m(n, tau) are obtained, where 1≤m≤M; b) performing signal accumulation on M groups of echo train signals S_m(n, tau) to obtain accumulated echo train signals S(n, tau); 9) Perform nuclear magnetic resonance data processing on the echo train signal S(n, tau) obtained in step 8) to obtain echo train signals ρ(z, n*tau) of different slice selections, where z is the slice selection direction coordinate; ρ(z, n*tau) is the macroscopic magnetization signal obtained by the excitation of the H nuclear spins of the porous medium test sample, and is a function of the layer selection direction z and the n*tau of each acquisition time of the reading channel; then the layer selection T2 relaxation is obtained spectrum P(z, T ₂ ); Through the above steps, the rapid test of the nuclear magnetic resonance T2 relaxation spectrum of the layer selection of the porous medium is realized. 2 . The method for rapidly testing the layer-selective T2 relaxation spectrum of porous media according to claim 1 , wherein in step 2) the gradient of the constant amplitude is preset to an amplitude G in the range of 10-300 mT/m. 3 . 3. The method for rapidly testing layer selection T2 relaxation spectrum of porous media according to claim 1, wherein in step 2), the reading gradient channel specifically selects the axial direction of the porous media test sample to perform one-dimensional layer selection coding. 4 . The method for rapidly testing layer-selected T2 relaxation spectra of porous media according to claim 1 , wherein, in step 5), the range of DW is 1-40us; the range of SI is 16-256. 5 . 5 . The method for rapid testing of layer-selected T2 relaxation spectra of porous media according to claim 1 , wherein the porous media test sample adopts sandstone cores. 6 . 6. The method for fast measurement of layer-selective T2 relaxation spectra of porous media according to claim 1, wherein step 9) carries out nuclear magnetic resonance data processing, comprising the following steps: The echo train signal S(n, tau) is expressed as formula (1): S(n, tau)=∫ρ(z, n*tau)exp[i2π(kk ₀ )z]dz (1) in,

Through the inverse Fourier transform of equation (1), that is, equation (4), ρ(z, n*tau) is obtained: ρ(z, n*tau)=∫S(n, tau)exp[-i2π(kk ₀ )z]dk (4) Among them, z is the coordinate of the slice selection direction; k is the nuclear magnetic resonance K space wave vector; k ₀ is the additional wave vector; ρ(z, n*tau) is the echo train signal of different slice selection; γ is the gyromagnetic ratio; i is a complex number; t is the acquisition time; t is zero at the beginning of each acquisition in the READ acquisition channel, and the acquisition process duration is DW*SI; T is half of the maximum value of t. 7. The method for fast measurement of layer-selected T2 relaxation spectra of porous media according to claim 6, wherein the acquisition process of the signal S(n, tau) is discrete acquisition; the inverse Fourier transform in formula (4) Use the fast Fourier transform FFT algorithm to solve. 8. The method for rapidly testing the layer selection T2 relaxation spectrum of a porous medium according to claim 7, wherein the ρ(z, n*tau) obtained by FFT is used as a function of the layer selection direction z coordinate, which is a plurality of Layer selection; analyze the decay of each selection layer of ρ(z, n*tau) with time n*tau; use an exponential function to fit the decay of each point on the selection layer, expressed as formula (5):

Among them, P(z, T ₂ ) is the layer-selected T2 relaxation spectrum.

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