CN105004746B - A visual evaluation method for the plugging performance of gel in rock microchannels - Google Patents

Abstract

The invention relates to a visual evaluation method for the plugging performance of gel in rock microchannels, and belongs to the fields of petroleum engineering enhanced oil recovery technology and experimental fluid mechanics. The invention adopts the low-field nuclear magnetic resonance imaging technology to study the distribution of the gel in the micropore, and can directly observe the distribution of the gel in the porous medium microchannel in the core displacement experiment through the nuclear magnetic resonance imaging technology and the T2 spectrum analysis technology. Gel images were then extracted from the MRI images to obtain parameters such as gel area, gel leading edge, gel trailing edge, and gel center. Finally, through the calculation of the gel plugging area ratio and plugging height ratio, the plugging performance of the gel in the core is quantitatively characterized. The method of the invention enables the plugging characteristics of the gel in the porous medium to be described intuitively.

Description

A visual evaluation method for the plugging performance of gel in rock microchannels

technical field

The invention relates to a visual evaluation method for the plugging performance of gel in rock microchannels, and belongs to the fields of petroleum engineering enhanced oil recovery technology and experimental fluid mechanics.

Background technique

The core displacement experiment in the laboratory is a recognized method for studying the fluid flow inside the core, and is widely used in petroleum development and environmental science. However, due to the invisible characteristics of the core, the fluid flow in the core can only be controlled by pressure It can be expressed indirectly by the two indexes of flow rate and pressure, or it can be represented indirectly by numerical simulation method according to the relationship between pressure and flow rate. Low-field nuclear magnetic resonance is a high-tech that has developed rapidly in recent years, and has been well applied in the medical and food industries. How to make the core displacement process in the laboratory more direct and even visible has become a goal pursued and an important work carried out in the field of petroleum engineering technology. In petroleum engineering, gel injection is often used to block the large pores in porous media, adjust the flow channel of the injected liquid, and improve the swept area and oil displacement efficiency. However, the current traditional core flow experiments can only use the macroscopic parameters of the end or boundary (such as outlet pressure, inlet pressure, flow velocity, etc.) to describe or deduce the flow status and displacement effect of the fluid inside the core. For researchers, the rock core is equivalent to a black box, and it is impossible to accurately understand the fluid movement inside it.

Contents of the invention

Aiming at the current problem that the plugging performance of gel in rock core is difficult to characterize, the purpose of this invention is to provide a visual evaluation method for the plugging performance of gel in rock microchannels, which combines core flow experiment and nuclear magnetic resonance detection technology. Combined organically, the plugging characteristics of gels in porous media can be described intuitively.

To achieve the above object, the present invention adopts the following technical solutions:

A method for visually evaluating the plugging properties of gel in rock microchannels, comprising the steps of:

(1) Turn on the nuclear magnetic resonance equipment, put the water-saturated core into the magnet, and determine the position of the core through nuclear magnetic imaging, so that it is in the center of the magnetic field; set the slice thickness, slice interval and slice position of the sagittal plane and cross section of the core , using low-field nuclear magnetic resonance equipment to obtain fluid signals in rock microchannels in core displacement experiments.

(2) Grayscale image extraction. The grayscale image represents the strength of the signal at different positions inside the core through light and dark (the stronger the signal, the brighter, and the weaker the signal, the darker).

(3) Region of interest (ROI) extraction. The NMR image acquisition signal window is 100mm×100mm, the sagittal plane of the core is a rectangle of 90mm×25mm, and the cross section is a circle with a diameter of 25mm, so it is necessary to extract the effective area of the grayscale image.

(4) Unified mapping. Each NMR grayscale image is displayed bright and dark according to its own signal strength. In order to compare the signal strength in different images, the MRI images of the same slice must be displayed bright and dark according to a uniform standard.

(5) Extract the core skeleton. In order to determine the skeleton structure of the core, binarize the nuclear magnetic image of the saturated core to separate the skeleton and fluid information of the core.

(6) Add pseudo-color. For the core with unified gray scale, add JET pseudo-color to make the distinction between gel and water more obvious.

(7) Extract the gel image. Gel images were extracted from the NMR images to obtain parameters such as gel area, gel leading edge, gel trailing edge, and gel center.

(8) Characterization of gel plugging height ratio: Displacing water with a certain pore volume, the ratio of the vertical plugging height of the gel after water flooding to the vertical plugging height of the gel after waiting to solidify is taken as the gel plugging height ratio, first calculate the plugging height ratio of each sagittal plane, and then take the average value and record it as the plugging height ratio of the gel in the whole core:

In the formula: r ₁ is the section height of the core, mm; r ₂ is the gel plugging height, mm; F ₁ is the plugging height ratio, dimensionless.

(9) Characterization of gel plugging area ratio: In step (8), at the same time, the ratio of the gel area to the total pore area of the gel in the lateral direction after water flooding is used as the gel plugging area ratio; Then take the average value and record it as the plugging area ratio of the gel in the whole core:

In the formula: S ₁ is the cross-sectional area of the core, mm ² ; S ₂ is the plugging area of the gel, mm ² ; F ₂ is the plugging area ratio, dimensionless.

(10) Measure the change of gel plugging performance by plugging height ratio and plugging area ratio: the larger the plugging height ratio and plugging area ratio, the stronger the plugging performance of the gel; the plugging height ratio and plugging area ratio The smaller the area ratio, the worse the gel plugging performance.

The size range of the rock core is 25 mm in diameter and 60 mm to 180 mm in length.

The core permeability ranges from 500mD to 5000mD.

The displacement pressure in step 8) ranges from 0.01MPa to 20MPa.

The displacement flow rate in step 8) ranges from 0.1ml/min to 5ml/min.

Compared with the prior art, the present invention has the following prominent substantive features and remarkable advantages:

The present invention uses low-field nuclear magnetic resonance imaging technology to study the plugging properties of gel in micropores, and combines the core flow experiment and nuclear magnetic resonance detection technology organically, so that the plugging characteristics of gel in porous media can be more intuitively described .

Description of drawings

Figure 1 is a schematic diagram of the plugging height ratio of the gel.

Fig. 2 is a schematic diagram of gel plugging area ratio.

Figure 3 is the NMR distribution image of the gel in the sagittal plane in the core.

Figure 4 is the cross-sectional NMR image of the gel in the core.

Figure 5 shows the characterization results of gel plugging height ratio.

Fig. 6 is the characterization result of gel plugging area ratio.

Figure 7 is a table of gel characteristic parameters.

detailed description

Now the specific embodiment of the present invention in conjunction with accompanying drawing is described in the following.

A kind of visual evaluation method of the plugging property of gel in the rock microchannel of the present embodiment, the test procedure is:

(1) Turn on the nuclear magnetic resonance equipment, put the water-saturated core into the magnet, and determine the position of the core through nuclear magnetic imaging, so that it is in the center of the magnetic field; set the slice thickness of the sagittal plane and cross section of the core to 0.7cm, and set the slice interval Set for 0.1cm and slice position, and use low-field nuclear magnetic resonance equipment to obtain fluid signals in rock microchannels in core displacement experiments;

(2) Grayscale image extraction. Grayscale images of the gel in the core were extracted by NMR.

(3) Region of interest (ROI) extraction. Extract a 90mm×25mm rectangle as the effective area of the grayscale image.

(4) Unified mapping. All sagittal planes are uniformly mapped in the range of 1 to 100,000 dimensionless equivalent luminance units.

(5) Extract the core skeleton. The nuclear magnetic image of the saturated core is binarized, and the separation threshold of the core skeleton and fluid is defined as 15,000 dimensionless equivalent brightness units.

(6) Adding false color: For the core with uniform gray scale, add JET false color to make the distinction between gel and water more obvious.

(7) Extract the gel image. The gel images were extracted from the NMR images, and the extraction results are shown in Figures 3 and 4. Parameters such as gel area, gel leading edge, gel trailing edge, and gel center are obtained. The extraction results are shown in Figure 7.

(8) Characterization of gel plugging height ratio: Displacing water with a certain pore volume, the ratio of the vertical plugging height of the gel after water flooding to the vertical plugging height of the gel after waiting to solidify is taken as the gel plugging height As shown in Figure 1, the plugging height ratio of each sagittal plane is first calculated, and then the average value is recorded as the plugging height ratio of the gel in the whole core, and the calculation results are drawn, and the drawing results are shown in Figure 5:

In the formula: r ₁ is the section height of the core, mm; r ₂ is the gel plugging height, mm; F ₁ is the plugging height ratio, dimensionless.

(9) Characterization of gel plugging area ratio: In the previous step, the ratio of the gel area to the total pore area of the gel in the lateral direction after water flooding was used as the gel plugging area ratio, as shown in Figure 2. First, Calculate the plugging area ratio of each cross-section, then take the average value and record it as the plugging area ratio of the gel in the entire rock, and draw the calculation results. The drawing results are shown in Figure 6:

In the formula: S ₁ is the cross-sectional area of the core, mm ² ; S ₂ is the plugging area of the gel, mm ² ; F ₂ is the plugging area ratio, dimensionless.

(10) Through the characterization of the plugging height ratio and plugging area ratio, it can be obtained that the plugging height ratio does not change much during the migration process of the gel, and the plugging area ratio first remains unchanged and then gradually decreases.

Claims (5)

1. a visual evaluation method of the plugging property of gel in rock microchannel, is characterized in that, comprises the steps: 1) Turn on the nuclear magnetic resonance equipment, put the water-saturated core into the magnet, determine the position of the core through nuclear magnetic imaging, so that it is in the center of the magnetic field; set the slice thickness, slice interval and slice position of the sagittal plane and cross section of the core, Using low-field nuclear magnetic resonance equipment to obtain fluid signals in rock microchannels in core displacement experiments; 2) Grayscale image extraction: the grayscale image represents the strength of fluid signals at different positions inside the core through light and dark, the stronger the signal, the brighter, and the weaker the signal, the darker; 3) Region of interest extraction: Extract the effective area of the grayscale image in the nuclear magnetic image acquisition signal window, wherein the nuclear magnetic image acquisition signal window is a window of 100mm × 100mm, wherein the sagittal plane of the rock core is a rectangle of 90mm × 25mm, and the cross section is a diameter of 25mm circle; 4) Unified mapping: NMR images of the same slice are displayed brightly and darkly according to a unified standard; 5) Extracting the core skeleton: binarize the nuclear magnetic image of the saturated rock core, separate the core skeleton and fluid information, and determine the skeleton structure of the rock core; 6) Add false color: add JET false color to the core after the gray level is unified, so that the distinction between gel and water is more obvious; 7) Extracting the gel image: extracting the gel image according to the nuclear magnetic resonance image, obtaining the gel area, gel front edge, gel trailing edge and gel center parameters; 8) Characterization of the gel plugging height ratio: after displacing water with a certain pore volume, the ratio of the gel plugging height to the core diameter in the longitudinal direction after coagulation is used as the gel plugging height ratio. The plugging height ratio, and then take the average value and record it as the plugging height ratio of the gel in the whole core: ${\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; r</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}{{\mathrm{<span\; class="notranslate">\; r</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}$ In the formula: r ₁ is the diameter of the core, mm; r ₂ is the plugging height of the gel after solidification, mm; F ₁ is the plugging height ratio, dimensionless; 9) Characterization of gel plugging area ratio: Displacing water with a certain pore volume, the ratio of the gel plugging area on the cross-section of the core after water flooding to the area of the core cross-section is taken as the gel plugging area ratio; first calculate each The plugging area ratio of the cross-section, and then take the average value and record it as the plugging area ratio of the gel in the whole core: ${\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; S</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}{{\mathrm{<span\; class="notranslate">\; S</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}$ In the formula: S ₁ is the cross-sectional area of the core, mm ² ; S ₂ is the plugging area of the gel on the cross-section of the core, mm ² ; F ₂ is the plugging area ratio, dimensionless; 10) Measure the change of gel plugging performance by plugging height ratio and plugging area ratio: the larger the plugging height ratio and plugging area ratio, the stronger the gel plugging performance; the plugging height ratio and plugging area The smaller the ratio, the poorer the plugging performance of the gel. 2. The visual evaluation method of the plugging property of gel in rock microchannels according to claim 1, characterized in that, the size range of the rock core is 25 mm in diameter and 60 mm to 180 mm in length. 3. The visual evaluation method of the plugging property of gel in rock microchannels according to claim 1, characterized in that, the permeability range of the rock core is 500mD～5000mD. 4. The visual evaluation method of the plugging property of gel in rock microchannels according to claim 1, characterized in that, the displacement pressure range in the step 8) is 0.01MPa～20MPa. 5. the visual evaluation method of the plugging property of gel in rock microchannel according to claim 1, it is characterized in that, described step 8) in the flow change range of displacement is 0.1ml/min～5ml/min .