Experimental description:

Experimental preparation: two different sizes of glass beads, small glass beads with a diameter of um, large glass beads with a diameter of mm; tap water; copper sulfate pentahydrate.

Experimental instrument: core magnetic resonance imaging analyzer, frequency is about 22MHz, probe coil diameter is 15mm, experimental temperature is controlled at 31.99-32.00 °C;

Porosity and permeability measurement using nuclear magnetic resonance core analysis and measurement software;

玻璃珠堆积孔隙模型核磁共振成像与弛豫图谱测试实验

Using a magnetic resonance imaging software, the spin echo sequence is used in its entirety to complete the T2-weighted image;

Analysis and results:

Porosity determination

Due to the different pore sizes formed by the two types of glass beads, the relaxation time of the water in the pores after the water is saturated (the pores are completely filled with water) is different. When the calibration is performed, pure glass is used as the large glass bead calibration line. Small glass beads after water are used as calibration lines for small glass beads. The calibration data is as follows:

玻璃珠堆积孔隙模型核磁共振成像与弛豫图谱测试实验

Saturation, permeability measurement

Water is continuously dripped into the dried glass beads until it is saturated. Three time points (transient) of the dripping process were taken for porosity determination, saturation = (transient porosity / saturated porosity) * (transient total volume / total volume of saturated water). Permeability calculation According to the SDR model, the model parameter a=1. The test results are as follows:

玻璃珠堆积孔隙模型核磁共振成像与弛豫图谱测试实验

Saturation calculation is based on porosity, so its accuracy is closely related to the accuracy of porosity. The permeability increases with increasing water content, and at the same level of saturation, the permeability within the large glass beads is greater than that of the small glass beads.

Magnetic resonance imaging

Small glass bead imaging

Left: saturation when saturated, 100%; right: saturation = 14.31%

The brighter part above the image on the left is an aqueous solution, and the darker area below is a small glass bead that is saturated with water. The image on the right is a non-saturated sample. It can be seen from the image that the moisture distribution inside the sample is not uniform.

Large glass bead imaging

玻璃珠堆积孔隙模型核磁共振成像与弛豫图谱测试实验

Left and right are the same sample. After obtaining the left image, the sample was agitated and the right image was taken to observe the change.

Pore ​​size distribution

When the liquid in the pore is water and the magnetic field gradient is approximately zero, T2 is only related to the pore structure in the porous medium system. Under the condition of known T2 distribution, the pore distribution can be obtained according to the set model.

玻璃珠堆积孔隙模型核磁共振成像与弛豫图谱测试实验

Left: Pore distribution of large glass beads, saturation is 1:32.35%; 2:71.13%, 3:94.08%; 4:100%. Right: small glass beads pore distribution, saturation is 1:19.25%; 2: 40.57%, 3: 60.26%; 4: 100%. In the figure, the abscissa is T2 and the ordinate is the signal strength. For the same T2 profile, the ratio of the individual peak areas to the total area represents the proportion of the corresponding pores.

As the saturation of the large and small glass beads increases, the signal intensity increases gradually, and the proportional relationship between the peaks is relatively fixed, indicating that the pore distribution is less affected by the saturation.

Nuclear Magnetic Resonance Knowledge:

At present, nuclear magnetic resonance has become a powerful tool for oil exploration. It can accurately analyze the core pore structure and provide more comprehensive information on pore structure such as porosity, pore distribution, permeability, bound fluid saturation, and free fluid saturation. Secondly, NMR is also the only powerful tool to accurately analyze the properties and state of fluids in pores. Compared with conventional neutron, density and acoustic measurement methods, NMR methods can provide rock formation information that is not available in the following three conventional methods:

(1) Information on the amount of fluid in the pores;

(2) Information on fluid properties;

(3) Information on the pore size of fluids.

The content of low-field NMR research is mainly the relaxation characteristics of matter, that is, the decay of the nuclear magnetic signal of matter. Usually, we use relaxation time to express the speed of material relaxation. The shorter the relaxation time, the faster the material nuclear magnetic signal decays. The core is a relatively typical porous medium, and the following three kinds of relaxation phenomena often occur in the core:

1. Body relaxation

Bulk relaxation refers to the inherent relaxation mechanism of liquids in the pores of porous media. This is the same as the relaxation mechanism of most liquid systems. The relaxation speed is determined by the physical properties of the liquid (such as viscosity, material structure, etc.). It is decided that it is independent of the pore itself and usually occurs at macropores or core cracks with pore sizes above 50 nm.

2, surface relaxation

Surface relaxation is a relaxation mechanism related to the pore size of a porous medium. The relaxation occurs at the interface between liquid and solid. The relaxation speed is determined by the pore size. The smaller the pore, the relaxation. The faster the speed, usually occurs inside the pores between 2 nm and 50 nm in size.

3. Diffusion relaxation

Diffusion relaxation is a relaxation mechanism that is affected by the self-diffusion of the liquid itself in the pore. Diffusion only affects the transverse relaxation time of the system without affecting the longitudinal relaxation of the system. Therefore, only lateral diffusion relaxation exists. .

玻璃珠堆积孔隙模型核磁共振成像与弛豫图谱测试实验

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