阅读说明：本技术 一种土壤薄膜水厚度的测试方法 (Method for testing water thickness of soil film ) 是由 张志权 黄月 明心凯 井彦林 温馨 张旭彬 赵庄 于 2019-10-22 设计创作，主要内容包括：本发明公开了一种土壤薄膜水厚度的测试方法,本发明通过土壤的核磁共振试验得到其T<Sub>2</Sub>谱曲线,结合相关的数值模型,反演土壤的孔隙分布,结合离心试验确定T<Sub>2</Sub>截止值,从而得到岩土材料的孔隙分布、孔隙结构、流体性质等数据,利用相关的模型计算出土壤薄膜水厚度。本发明的有益效果是,公开了一种土壤薄膜水厚度的测试方法,为土壤薄膜水厚度的确定提供了一条新途径,可广泛应用于各类土壤的测试。(The invention discloses a method for testing the water thickness of a soil film, which obtains the T of the soil film through a nuclear magnetic resonance test of the soil 2 The spectral curve is combined with a relevant numerical model to invert the pore distribution of the soil, and the T is determined by combining a centrifugal test 2 And (4) cutting off the value so as to obtain data such as pore distribution, pore structure, fluid property and the like of the rock-soil material, and calculating the water thickness of the soil film by using a related model. The invention has the beneficial effects that the invention discloses a method for testing the water thickness of the soil film, provides a new way for determining the water thickness of the soil film, and can be widely applied to the test of various soils.)

1. A method for testing the water thickness of a soil film is characterized by comprising the following steps:

step one, calibrating a nuclear magnetic resonance instrument, and determining the central frequency of the nuclear magnetic resonance instrument;

placing the undisturbed soil sample to be detected into a nuclear magnetic resonance instrument for a first nuclear magnetic resonance test;

setting a pulse sequence of the hard pulse CPMG, and performing repeated sampling;

step four, inverting the sampling result to obtain a first time T₂A spectrum;

step five, wrapping the undisturbed soil sample by using a heat-shrinkable tube, forming a plurality of small holes on the surface of the heat-shrinkable tube, and putting the heat-shrinkable tube into kerosene until the heat-shrinkable tube is saturated;

sixthly, placing the saturated soil sample into a nuclear magnetic resonance instrument for a second nuclear magnetic resonance test, and performing inversion to obtain a second T₂A spectrum;

step seven, centrifuging the saturated soil sample to reach a water binding state;

step eight, placing the centrifuged soil sample into a nuclear magnetic resonance instrument for a third nuclear magnetic resonance test, and performing inversion to obtain a third T₂A spectrum;

step nine, carrying out T for the first time₂Spectrum, second time T₂Spectra and third time T₂Spectrum inversion to determine T₂Obtaining the pore distribution, nuclear magnetism porosity and irreducible water saturation of the sample according to the accumulation curve;

step ten, determining the specific surface area of the undisturbed soil sample to be detected;

step eleven, obtaining the water thickness of the soil film through pore distribution, nuclear magnetism porosity and specific surface area of binding degree of bound water saturation.

2. The method for testing the water thickness of the soil film according to claim 1, wherein in the first step, when the nuclear magnetic resonance instrument is calibrated, the soil sample with the measured density is placed in the nuclear magnetic resonance instrument for testing.

3. The method for testing water thickness of a soil film according to claim 1, wherein T is T₂The spectra are shown below:

wherein, T_2BIs T from the pore fluid itself₂A relaxation time; t is_2ST being the surface of solid particles₂A relaxation time; t is_2DT for molecular diffusion under the action of gradient magnetic field₂A relaxation time; rho₂For transverse relaxation time, and determined by the soil particle properties, S and V are the surface area and volume of the pore where water is located, respectively.

4. The method for testing water thickness of soil film according to claim 1, wherein T of soil sample₂The relationship between the spectral values and the pore structure of the soil body is as follows:

if the pore shape in the soil sample is columnar, the T of the soil sample₂The relation between the spectrum value and the soil pore structure is as follows:

where ρ is the density, S is the surface area, V is the volume, and R is the radius.

5. The method for testing the water thickness of the soil film according to claim 1, wherein in the seventh step, the centrifugal force is 0.25MPa and the soil film is centrifuged for 2 hours.

6. The method for testing the water thickness of the soil film according to claim 1, wherein in the step ten, the specific surface area of the undisturbed soil sample to be detected is measured by a nitrogen adsorption method.

Technical Field

The invention belongs to the field of soil microstructure and permeability test thereof, and particularly relates to a method for testing the water thickness of a soil film.

Background

The film water is weak binding water on the surface of soil particles, the thickness distribution of the film water is an important physical characteristic of soil, the film water has great influence on the evaluation of underground water resources, the water environment, the strength and stability of soil bodies, the water holding performance of the soil, agricultural production and the like, and the film water is an important factor influencing the soil permeability according to the migration rule of the underground water in the soil, the infiltration rule of surface water (including rainwater), the supply of underground soil, the buried water permeation condition of underground water level and the like. Therefore, the research on the thickness of the soil film water has important scientific practical value and effect on the research on the seepage of the soil. The water thickness of the film is extremely small, and the film belongs to a micro scale, and the testing method is always a difficult problem.

With the development of scientific technology, instruments and equipment are more advanced, and the nuclear magnetic resonance technology is gradually developed from the beginning to be used for medical diagnosis and applied to the fields of oil exploration and geotechnical engineering. The nuclear magnetic resonance test has the advantages of rapidness, economy, nondestructive detection, large scale and the like, so that the nuclear magnetic resonance technology shows remarkable superiority in the aspect of testing of rock and soil bodies.

Disclosure of Invention

The invention aims to overcome the defects and provide a method for testing the water thickness of the soil film, which applies a nuclear magnetic resonance test means to the thickness test of the soil film water for the first time.

In order to achieve the above object, the present invention comprises the steps of:

step one, calibrating a nuclear magnetic resonance instrument, and determining the central frequency of the nuclear magnetic resonance instrument;

placing the undisturbed soil sample to be detected into a nuclear magnetic resonance instrument for a first nuclear magnetic resonance test;

setting a pulse sequence of the hard pulse CPMG, and performing repeated sampling;

step four, inverting the sampling result to obtain a first time T₂A spectrum;

step five, wrapping the undisturbed soil sample by using a heat-shrinkable tube, forming a plurality of small holes on the surface of the heat-shrinkable tube, and putting the heat-shrinkable tube into kerosene until the heat-shrinkable tube is saturated;

sixthly, placing the saturated soil sample into a nuclear magnetic resonance instrument for a second nuclear magnetic resonance test, and performing inversion to obtain a second T₂A spectrum;

step seven, centrifuging the saturated soil sample to reach a water binding state;

step eight, placing the centrifuged soil sample into a nuclear magnetic resonance instrument for a third nuclear magnetic resonance test, and performing inversion to obtain a third T₂A spectrum;

step nine, carrying out T for the first time₂Spectrum, second time T₂Spectra and third time T₂Spectrum inversion to determine T₂Obtaining the pore distribution, nuclear magnetism porosity and irreducible water saturation of the sample according to the accumulation curve;

step ten, determining the specific surface area of the undisturbed soil sample to be detected;

step eleven, obtaining the water thickness of the soil film through pore distribution, nuclear magnetism porosity and specific surface area of binding degree of bound water saturation.

In the first step, when the nuclear magnetic resonance instrument is calibrated, the soil sample with the measured density is placed in the nuclear magnetic resonance instrument for testing.

T₂The spectra are shown below:

wherein, T_2BIs T from the pore fluid itself₂A relaxation time; t is_2ST being the surface of solid particles₂A relaxation time; t is_2DT for molecular diffusion under the action of gradient magnetic field₂A relaxation time; rho₂For transverse relaxation time, and determined by the soil particle properties, S and V are the surface area and volume of the pore where water is located, respectively.

T of soil sample₂The relationship between the spectral values and the pore structure of the soil body is as follows:

if the pore shape in the soil sample is columnar, the T of the soil sample₂The relation between the spectrum value and the soil pore structure is as follows:

where ρ is the density, S is the surface area, V is the volume, and R is the radius.

And seventhly, centrifuging for 2 hours at the centrifugal force of 0.25 MPa.

And step ten, determining the specific surface area of the undisturbed soil sample to be detected by a nitrogen adsorption method.

Compared with the prior art, the method obtains the T through the nuclear magnetic resonance test of the soil₂The spectral curve is combined with a relevant numerical model to invert the pore distribution of the soil, and the T is determined by combining a centrifugal test₂And (4) cutting off the value so as to obtain data such as pore distribution, pore structure, fluid property and the like of the rock-soil material, and calculating the water thickness of the soil film by using a related model. The invention has the beneficial effects that the invention discloses a method for testing the water thickness of the soil film, provides a new way for determining the water thickness of the soil film, and can be widely applied to the test of various soils.

Drawings

FIG. 1 shows the nuclear magnetic resonance T in example 1 of the present invention₂Distribution of the spectra.

Detailed Description

The invention is further described below with reference to the accompanying drawings.

The invention comprises the following steps:

placing a soil sample with measured density in a nuclear magnetic resonance instrument for testing, calibrating the nuclear magnetic resonance instrument, and determining the central frequency of the nuclear magnetic resonance instrument;

placing the undisturbed soil sample to be detected into a nuclear magnetic resonance instrument for a first nuclear magnetic resonance test;

setting a pulse sequence of the hard pulse CPMG, and performing repeated sampling;

step four, inverting the sampling result to obtain a first time T₂A spectrum;

step five, wrapping the undisturbed soil sample by using a heat-shrinkable tube, forming a plurality of small holes on the surface of the heat-shrinkable tube, and putting the heat-shrinkable tube into kerosene until the heat-shrinkable tube is saturated;

sixthly, placing the saturated soil sample into a nuclear magnetic resonance instrument for a second nuclear magnetic resonance test, and performing inversion to obtain a second T₂A spectrum;

seventhly, putting the saturated soil sample into a CSC-12 super core high-speed refrigerated centrifuge for a gas-water centrifugal test, and centrifuging for 2 hours by using a centrifugal force of 0.25MPa until all kerosene in the sample is centrifuged out to reach a water binding state;

step eight, placing the centrifuged soil sample into a nuclear magnetic resonance instrument for a third nuclear magnetic resonance test, and performing inversion to obtain a third T₂A spectrum;

step nine, carrying out T for the first time₂Spectrum, second time T₂Spectra and third time T₂Spectrum inversion to determine T₂Obtaining the pore distribution, nuclear magnetism porosity and irreducible water saturation of the sample according to the accumulation curve;

step ten, determining the specific surface area of the undisturbed soil sample to be detected by using a HYA2010-B1 nitrogen adsorption instrument through a nitrogen adsorption method;

step eleven, obtaining the water thickness of the soil film through pore distribution, nuclear magnetism porosity and specific surface area of binding degree of bound water saturation.

Nuclear magnetic resonance refers to a process in which a macroscopic magnetization vector of protons having spin magnetic moments cannot maintain balance under the action of a uniform magnetic field (main magnetic field) and a radio frequency magnetic field, and a proton group returns to an equilibrium state from an unbalanced state after radio frequency stops. The time-dependent change curve of the nuclear magnetic signal in the process can be abbreviated as an FID curve, and the information of the pore water distribution and the content thereof is contained by the FID curve. The initial point on the FID curve can yield information that the initial nuclear magnetic signal is proportional to the water content of the soil sample being measured.Therefore, the water content of the soil sample can be determined by using the initial point of the FID, and the feasibility and the precision of the method are verified by a large number of experiments at present. Meanwhile, the form of the FID curve is subjected to pore water T in the soil sample₂Value influence is carried out, and T of the soil sample can be obtained through Pulse Fourier Transform (PFT)₂The water content of the spectrum and the soil sample refers to the lower part of the curve and the T₂Peak area around the axis. T is₂The spectra are shown below:

wherein, T_2BIs T from the pore fluid itself₂A relaxation time; t is_2ST being the surface of solid particles₂A relaxation time; t is_2DT for molecular diffusion under the action of gradient magnetic field₂A relaxation time; rho₂For transverse relaxation time, and determined by the soil particle properties, S and V are the surface area and volume of the pore where water is located, respectively.

T in liquid water₂In relaxation time, T_2BCompared with T_2S，T_2SMuch smaller than T_2BThus 1/T_2BFor T₂The effect of (a) is almost negligible; in a uniformly distributed magnetic field, T_2DValue infinity, for T₂The value has little influence, i.e. 1/T_2DAlmost zero. In fact, the nuclear magnetic resonance test of loess can satisfy the above conditions, and thus the T obtained by the loess test₂The value is related to the pore structure of the soil body, T of the soil sample₂The relationship between the spectral values and the pore structure of the soil body is as follows:

if the pore shape in the soil sample is columnar, the T of the soil sample₂The relation between the spectrum value and the soil pore structure is as follows:

where ρ is the density, S is the surface area, V is the volume, and R is the radius.

The above formula shows T at each time₂Value is proportional to the pore radius R, T corresponding to different pore sizes₂The values are different. According to this theory, T of the soil sample₂The time distribution curve can correspond to the pore water distribution in the rock-soil material. The area of the peak (equivalent to the initial nuclear magnetic signal) enclosed under the curve is the T corresponding to the curve₂The water content is in a range, so the technology can be used for measuring the pore water related information in the rock-soil material. Wherein the key to the determination of the water content of the bound water is T₂The cutoff value is determined such that the left side of the cutoff value represents bound water and the right side represents free water.

The calculation principle of the water thickness of the film adopts a theoretical formula for calculating the water film thickness of the surface of the soil particles in the summary of the soil science such as sunny exposure, the thickness represents the average water film thickness, and the formula is as follows:

in the formula: h is the film water thickness in nm;

Φ is porosity in%;

S_wiin terms of bound water saturation;

a is the specific surface area of the soil sample and the unit is m²/g；

Rho is the density of the soil sample and the unit is g/cm³。