Crack width characterization method based on low-field nuclear magnetic resonance and CT

Abstract

The invention discloses a crack width characterization method based on low-field nuclear magnetic resonance and CT, and the method comprises the steps: enabling a crack to be saturated with saline water, measuring the spectrogram results of a crack body under different stress changes through a low-field nuclear magnetic resonance instrument, obtaining the T2 spectrogram of the crack body under each stress, carrying out the statistics of the peak area of each T2 spectrogram, and calculating the peak area of each T2 spectrogram; drawing a curve graph of the peak area changing along with stress; after the fracture core models are combined, firstly, model signals are scanned layer by layer through CT, the change rule of the shape and size of a three-dimensional fracture body along with the stress effect is obtained, and equivalent width data of the fracture under different stresses are obtained through three-dimensional fracture body shape analysis; and combining crack equivalent width data obtained by CT with a peak area change curve obtained by nuclear magnetism along with stress, so as to accurately characterize the crack width of the rock core scale under the action of stress. Therefore, the use of micron CT can be reduced in the later period, the nuclear magnetic experiment efficiency is improved, and the experiment cost and the experiment time are saved.

Description

Technical Field

[0001] This invention belongs to the field of oil and gas extraction technology, and in particular relates to a method for characterizing fracture width based on low-field nuclear magnetic resonance and CT. Background Technology

[0002] While conventional oil and gas resources are relatively easy to extract, they only account for 20% of the global total, while unconventional oil and gas resources, such as shale gas, shale oil, tight gas, and tight oil, account for 80%. As conventional oil and gas resources gradually fail to meet extraction needs, development strategies are increasingly shifting towards unconventional resources. Unconventional oil and gas reservoirs are characterized by low porosity and low permeability, exhibiting poor physical properties. Therefore, hydraulic fracturing is typically required, and it has become an essential means for the efficient development of unconventional oil and gas reservoirs. Its purpose is to create a large-scale fracture network in unconventional oil and gas reservoirs, increasing the fluid seepage volume. The conductivity of the fractures formed by fracturing is a key factor in evaluating the effectiveness of fracturing operations. The greater the conductivity of the fractures, the lower the resistance to fluid passage, making it easier for oil and gas to flow into the wellbore, generally resulting in higher well production.

[0003] The width of the crack is the most critical indicator for measuring its conductivity. The larger the crack width, the larger the crack space volume, and the larger the cross-sectional area that allows fluid to pass through the crack. When the crack width reaches a certain level, it can change the flow state of the fluid in the crack, transforming it from seepage to a state close to pipe flow, thereby greatly improving the conductivity of the crack.

[0004] Measuring the width of artificial fractures formed by fracturing in unconventional oil and gas development has always been a difficult problem, especially when fractured cores are placed in core holders and confined under pressure. This makes it impossible to observe the fracture state, and the fracture width is difficult to measure effectively because it changes constantly with the confining pressure. Micron-CT scanners are analytical instruments used in mechanics and physics, capable of high-precision scanning of the three-dimensional pore structure of cores. Without damaging the specimen, they can simultaneously acquire information on the external morphology and internal structure of the sample from all angles, and can achieve three-dimensional visualization of morphology and internal structure. Micron-CT technology can observe 3D images of fracture morphology changing in real time with confining pressure, and the equivalent fracture width can be analyzed from the images. Nuclear magnetic resonance (NMR) technology can measure the hydrogen signal distribution in the core. After filling the fractured core model with water, scanning the T2 spectrum of the fractured core shows that shorter relaxation times on the horizontal axis correspond to smaller pore volumes, and longer relaxation times correspond to larger pore volumes. The relationship between fracture and matrix pore volume can be obtained by analyzing the peak area. Based on this, the present invention provides a crack width characterization method based on low-field nuclear magnetic resonance and CT, which effectively solves the problem of dynamic characterization of crack width under stress. Summary of the Invention

[0005] The purpose of this invention is to overcome the difficulty in measuring fracture width under applied stress. It discloses a fracture width characterization method based on low-field nuclear magnetic resonance (NMR) and CT. This invention involves cutting a complete long core into two identical parts. First, one part of the core matrix and fractures are evacuated and saturated with water. The T2 spectra of the fractured core under different stresses are then scanned. The relationship between fracture volume and matrix pore volume is obtained by analyzing the T2 peak areas corresponding to the fracture space and the matrix pore space. The absolute value of the matrix pore volume, obtained from the core porosity, is then used to calculate the absolute value of the fracture volume. Dividing this by the fracture length and height yields the fracture width. Second, the other part of the core is fractured, and micron-sized CT scans are used to obtain 3D images of the fractured core model under different stresses. The fracture width is then analyzed. Finally, the fracture widths obtained from NMR under different stresses are fitted with those obtained from micron-sized CT, which efficiently characterizes fracture width and corrects NMR data errors. This reduces the need for micron-sized CT scans, improves NMR experimental efficiency, and saves experimental costs and time.

[0006] The objective of this invention is achieved through the following technical solution:

[0007] A crack width characterization method based on low-field nuclear magnetic resonance and CT, the method comprising:

[0008] After evacuating the crack to saturated salt water, the crack volume spectrum under different stress changes was measured using a low-field nuclear magnetic resonance instrument. The T2 spectrum of the crack volume under each stress was obtained, the peak area of ​​each T2 spectrum was counted, and a curve of peak area change with stress was plotted.

[0009] After combining the fracture core models, the model signals were first scanned layer by layer by CT to obtain the variation law of the three-dimensional fracture morphology and size with stress. The equivalent width data of the fracture under different stresses were obtained by analyzing the three-dimensional fracture morphology.

[0010] By combining the equivalent crack width data obtained from CT scans with the peak area variation curves obtained from NMR, the core-scale crack width under stress can be accurately characterized.

[0011] According to a preferred embodiment, the core rock type includes, but is not limited to, sandstone, shale, and carbonate rock.

[0012] According to a preferred embodiment, the fracture core model is a self-supporting fracture.

[0013] According to a preferred embodiment, the fracture core model is a single-through fracture core model.

[0014] According to a preferred embodiment, the stress variation range is 0-70 MPa.

[0015] According to a preferred embodiment, the crack width characterization method specifically includes:

[0016] S1: Take a long core and cut it in half, dividing it into two identical segments;

[0017] S2: Take a portion of the core and test its porosity using a helium porosity tester;

[0018] S3: Make a fractured core model from the core, vacuum the matrix of the fractured core model and fully saturate it with water;

[0019] S4: Assemble the cores into a fracture model and place them in the core holder of the low-field nuclear magnetic resonance testing device. Vacuum the fractures with water from the outlet end and scan the T2 spectrum of the fracture cores under different stress conditions.

[0020] S5: By using the area of ​​the right T2 peak corresponding to the crack space and the area of ​​the left T2 peak corresponding to the matrix pore space in the figure, the relationship between the crack volume and the matrix pore volume can be obtained.

[0021] S6: The absolute value of the matrix pore volume obtained from the core porosity is used to calculate the absolute value of the fracture volume. Divide this value by the fracture length and fracture height to obtain the fracture width.

[0022] S7: After fracturing another part of the core, 3D images of the fractured core model under different stresses were obtained by CT scanning, and the fracture width value was analyzed.

[0023] S8: Fit the equivalent crack width maps obtained by CT under different stresses with the crack width variation maps obtained by NMR to characterize the equivalent crack width data under different stresses.

[0024] According to a preferred embodiment, in step S3, the fracture core model matrix is ​​vacuum-saturated with water for 24 hours.

[0025] The aforementioned main solution of the present invention and its various further alternative solutions can be freely combined to form multiple solutions, all of which are solutions that can be adopted and are claimed by the present invention. Those skilled in the art, after understanding the solution of the present invention, will realize that there are many combinations based on existing technology and common knowledge, all of which are technical solutions to be protected by the present invention, and will not be exhaustively listed here.

[0026] The beneficial effects of this invention: Currently, after making a fracture core model, the more conventional methods for testing the fracture width are: direct measurement with vernier calipers, fracture saturated water volume method, low-field nuclear magnetic resonance testing method, etc., but traditional methods all have serious defects.

[0027] The vernier caliper measurement method can only test the crack width under initial unpressured conditions. Once the core is placed in the holder and confining pressure is applied for the experiment, this method cannot be implemented. Furthermore, direct measurement with calipers is limited by human effort and has a large margin of error. The data obtained can only serve as a reference.

[0028] The fracture saturated water volume method involves filling the fracture with water, measuring the mass difference before and after filling, calculating the fracture volume using the mass of water, dividing it by the fracture length and height, and finally obtaining the fracture width. However, this method has significant drawbacks. First, saturating the fracture with water is difficult to perform in practice. Second, after filling the fracture with water, the water in the fracture will spontaneously seep into the core pores, making it difficult to guarantee that the mass difference before and after weighing is the mass of the water filling the fracture. Finally, this method also has the limitation of not being able to test the fracture width of the model placed in the core holder.

[0029] Low-field nuclear magnetic resonance (NMR) is more reliable than the previous two methods. First, the porosity of the core is tested and the pore volume is calculated. Then, the fractured core model is evacuated and saturated with water to fully fill the matrix. The model is then placed in a low-field NMR instrument, and a certain confining pressure is applied to the model. A vacuum is drawn at the outlet end and water is injected at the inlet end to fill the fracture with water. The T2 spectrum is scanned at intervals until the signal remains constant. After the T2 signal stabilizes, two signal peaks can be clearly observed on the signal spectrum. The left signal peak is considered to correspond to the matrix pores, and the right signal peak corresponds to the fracture space and the water film signal on the outer surface of the sample. The ratio of the peak area is used to obtain the volume ratio of the fracture space to the matrix pore space. The fracture volume is then calculated based on the measured pore volume. This method solves the problem that the first two methods cannot test the crack width when stress is applied. However, it still has the problem that for low-permeability cores, even with the vacuum saturation method, it is difficult to ensure that the matrix pores are filled with water. Secondly, when scanning the T2 spectrum of the core model with NMR equipment, the presence of water film on the outer surface of the core and the crack wall will cause the peak area of ​​the corresponding crack space on the right to be larger, which will ultimately lead to the test result of crack width being larger.

[0030] This invention provides a novel method for testing fracture width that combines nuclear magnetic resonance (NMR) technology with micron-scale CT (micron-CT) technology. The precise and effective results are ensured through matching and fitting the two techniques. A core sample is divided into two identical segments. First, a fracture core model is created from the first segment. After evacuating the core matrix and fractures to a vacuum and saturating them with water, the fracture width as a function of stress is obtained using low-field NMR. Then, a fracture core model is created from the second segment, and micron-scale CT is used to scan the model layer by layer to obtain the changes in the three-dimensional fracture morphology and size under stress. Finally, the fracture width data measured by CT is fitted with the fracture width data measured by NMR to accurately characterize the equivalent fracture width under different stresses. This method effectively solves the problem of difficulty in testing the fracture width of fracture cores under applied stress. Furthermore, the reliability of the obtained data is ensured by the precise measurement of micron-scale CT combined with the verification of low-field NMR data. The accurate fracture width obtained has significant reference value for laboratory experiments in oil and gas field development engineering. Attached Figure Description

[0031] Figure 1 This is a flowchart of the crack width testing process of this invention;

[0032] Figure 2 This is a graph showing the test results of Example 1;

[0033] Figure 3 This is a graph showing the test results of Example 2. Detailed Implementation

[0034] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that, unless otherwise specified, the following embodiments and features described therein can be combined with each other.

[0035] refer to Figure 1 As shown, this invention discloses a crack width characterization method based on low-field nuclear magnetic resonance and CT, the method comprising:

[0036] After saturating the fracture with brine, the fracture volume spectrum under different stress changes was measured using a low-field nuclear magnetic resonance instrument. The T2 spectrum of the fracture volume under each stress was obtained, the peak area of ​​each T2 spectrum was counted, and a curve of peak area change with stress was plotted.

[0037] After combining the fracture core models, the model signals were first scanned layer by layer by CT to obtain the variation law of the three-dimensional fracture morphology and size with stress. The equivalent width data of the fracture under different stresses were obtained by analyzing the three-dimensional fracture morphology.

[0038] By combining the equivalent crack width data obtained from CT scans with the peak area variation curves obtained from NMR, the core-scale crack width under stress can be accurately characterized.

[0039] Specifically, the crack width characterization method includes:

[0040] S1: Take a long core and cut it in half, dividing it into two identical segments;

[0041] S2: Take a portion of the core and test its porosity using a helium porosity tester;

[0042] S3: Make a fractured core model from the core, vacuum the matrix of the fractured core model and fully saturate it with water;

[0043] S4: Assemble the core samples into a fracture model. After accurately measuring the fracture length and height with a high-precision digital display vernier caliper, place the model into the core holder of the low-field nuclear magnetic resonance testing device. Vacuum the fracture from the outlet end to saturate it with water and scan the T2 spectrum of the fracture core samples under different stress conditions.

[0044] S5: By using the area of ​​the right T2 peak corresponding to the crack space and the area of ​​the left T2 peak corresponding to the matrix pore space in the figure, the relationship between the crack volume and the matrix pore volume can be obtained.

[0045] S6: The absolute value of the matrix pore volume obtained through core porosity is used to calculate the absolute value of the fracture volume. Divide this value by the fracture length and fracture height to obtain the equivalent fracture width.

[0046] S7: After fracturing another part of the core, 3D images of the fractured core model under different stresses were obtained by CT scanning, and the fracture width value was analyzed.

[0047] S8: Fit the equivalent crack width maps obtained by CT under different stresses with the crack width variation maps obtained by NMR to characterize the equivalent crack width data under different stresses.

[0048] Preferably, the core rock types include, but are not limited to, sandstone, shale, and carbonate rock.

[0049] Preferably, the fracture core model is a self-supporting fracture.

[0050] Preferably, the fracture core model is a single continuous fracture.

[0051] Preferably, the stress variation range is 0-70 MPa.

[0052] Preferably, in step S3, the matrix of the fractured core model is vacuum-saturated with water for 24 hours.

[0053] Example 1

[0054] The steps for testing crack width are as follows:

[0055] (1) Take a core of Changqing dense sandstone outcrop with a diameter of 2.5cm and a length of 10cm, cut it in the middle and divide it into two identical segments, each 5cm long.

[0056] (2) The porosity of the first core section was tested to be 8.9%, and the matrix pore volume was obtained by multiplying it by the core volume to be 2.18 cm3;

[0057] (3) Make it into a fracture core model and vacuum its matrix to fully saturate it with water for 24 hours;

[0058] (4) The core samples were assembled into a fracture model and placed in the core holder of the low-field nuclear magnetic resonance testing device. Vacuum was drawn from the outlet end to saturate the fracture with water. The T2 spectra of the fracture core samples were obtained under stresses of 2-4-6-8-10MPa.

[0059] (5) The ratio of crack volume to matrix pore volume under each stress condition was calculated from the areas of the left and right peaks as 1:0.67, 1:0.52, 1:0.44, 1:0.36, and 1:0.24, respectively.

[0060] (6) The absolute values ​​of the crack volume under each stress were calculated from the absolute value of the matrix pore volume. Divided by the crack length and crack height, the crack width values ​​were obtained as 116.7 μm, 91.1 μm, 76.8 μm, 63.1 μm and 59.6 μm, respectively.

[0061] (7) After fracturing another part of the core, 3D images of the fractured core model under different stresses were obtained by micron-CT scanning. The fracture width values ​​under different stresses were analyzed and found to be 105μm, 81.5μm, 65.7μm, 55.1μm and 49.2μm respectively.

[0062] (8) Fit the equivalent width map of cracks under different stresses obtained by CT with the crack width as a function of stress obtained by NMR to accurately characterize the equivalent width data of cracks under different stresses.

[0063] Test results are as follows Figure 2 As shown, the test results of the two methods fit well. By using CT-measured suture width data and MRI data for correction, the corrected MRI results can be directly used to test the suture width in subsequent tests, reducing CT test costs.

[0064] Example 2

[0065] The steps for testing crack width are as follows:

[0066] (1) Take a shale core from the Luzhou block with a diameter of 2.5cm and a length of 6cm, cut it in the middle, and divide it into two identical segments, each 3cm long.

[0067] (2) The porosity of the first core section was tested to be 5.6%, and the matrix pore volume was obtained by multiplying it by the core volume to be 0.824 cm3;

[0068] (3) Make it into a fracture core model and vacuum its matrix to fully saturate it with water for 24 hours;

[0069] (4) The core samples were assembled into a fracture model and placed in the core holder of the low-field nuclear magnetic resonance testing device. The fracture was saturated with water by drawing a vacuum from the outlet end. The T2 spectra of the fracture core samples were obtained under stresses of 10-20-30-40-50-60-70 MPa.

[0070] (5) The ratio of crack volume to matrix pore volume under each stress condition was calculated from the area of ​​the two peaks on the left and right as 1:0.76, 1:0.56, 1:0.45, 1:0.35, 1:0.27, 1:0.25, and 1:0.24, respectively.

[0071] (6) The absolute values ​​of the crack volume under each stress were calculated from the absolute value of the matrix pore volume. Divided by the crack length and crack height, the crack width values ​​were obtained as 83.6 μm, 61.5 μm, 49.5 μm, 38.3 μm, 29.4 μm, 27.2 μm, and 26.5 μm, respectively.

[0072] (7) After fracturing another part of the core, 3D images of the fractured core model under different stresses were obtained by micron-CT scanning. The fracture width values ​​under different stresses were analyzed and found to be 72μm, 55.9μm, 38.7μm, 26.9μm, 20.8μm, 15.9μm, and 12.7μm, respectively.

[0073] (8) Fit the equivalent width map of cracks under different stresses obtained by CT with the crack width as a function of stress obtained by NMR to accurately characterize the equivalent width data of cracks under different stresses.

[0074] Test results are as follows Figure 3 As shown, the test results of the two methods have a good fit.

[0075] Comparative Example 1

[0076] The difference between this comparative example and the embodiment is that the crack width was tested by directly measuring it with vernier calipers.

[0077] (1) Fix the fractured core model with tape;

[0078] (2) Secure the inner diameter measuring jaws of a vernier caliper with a graduation of 0.02 mm at both ends of the crack wall surface;

[0079] (3) The readings show that the crack width is approximately 640 μm.

[0080] The measurement results show that the crack width is significantly larger than expected, indicating a large measurement error.

[0081] Comparative Example 2

[0082] The difference between this comparative example and the embodiment is that the crack width is tested using the crack saturated water volume method.

[0083] (1) Weigh the mass of the quartz sand laid in the crack and calculate the volume of the quartz sand by converting the density of the sand body;

[0084] (2) After vacuuming the matrix of the fractured core model, it is fully saturated with water and assembled into a fractured core model.

[0085] (3) Vacuum the crack and inject water into the crack. After the air bubbles are expelled, weigh the mass of the model before and after water injection. Calculate the volume of water in the crack by the difference between the two masses.

[0086] (4) Calculate the crack width using equation (1).

[0087]

[0088] In the formula: W f m1 is the equivalent width of the crack, in cm; m2 is the total mass of the crack before water filling, in g; m3 is the total mass of the crack after water filling, in g; ρ w The density of water in the crack is g / cm³. 3 ;m p For the mass of quartz sand, g; ρ p Density of quartz sand, g / cm³ 3 L f H represents the longitudinal length of the crack, in cm. f The height of the crack is in cm.

[0089] The test results of this method are shown in Table 1. Typically, a certain degree of net stress is applied to the core in fracture core model displacement experiments. However, no stress was applied to the fracture model in this test, resulting in a significantly larger fracture width. Therefore, the reference value of these results for the analysis of other related experimental data is relatively limited.

[0090] Table 1. Test Results of Comparative Example 2

[0091] Quartz sand mass / g <![CDATA[Quartz sand density / (g / cm 3 )]]> <![CDATA[Volume of quartz sand / cm 3 > Total mass before filling with water / g 0.50 2.65 0.19 59.56 Total mass after filling with water / g <![CDATA[Volume of water in the seam / / cm 3 > <![CDATA[Total crack volume / / cm 3 > Crack equivalent width / μm 59.92 0.36 0.55 440

[0092] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for characterizing crack width based on low-field nuclear magnetic resonance and CT, characterized in that, The method includes: After evacuating the crack to saturated salt water, the crack volume spectrum under different stress changes was measured using a low-field nuclear magnetic resonance instrument. The T2 spectrum of the crack volume under each stress was obtained, the peak area of ​​each T2 spectrum was counted, and a curve of peak area change with stress was plotted. After combining the fracture core models, the model signals were first scanned layer by layer by CT to obtain the variation law of the three-dimensional fracture morphology and size with stress. The equivalent width data of the fracture under different stresses were obtained by analyzing the three-dimensional fracture morphology. By combining the data on the equivalent width of the crack obtained by CT as a function of stress with the peak area obtained by NMR as a function of stress, the core-scale crack width under stress can be accurately characterized.

2. The crack width characterization method as described in claim 1, characterized in that, The types of rock cores include, but are not limited to, sandstone, shale, and carbonate rocks.

3. The crack width characterization method as described in claim 1, characterized in that, The fractured core model is a self-supporting fracture.

4. The crack width characterization method as described in claim 1, characterized in that, The fracture core model is a single-through fracture core model.

5. The crack width characterization method as described in claim 1, characterized in that, The stress variation range is 0-70 MPa.

6. The crack width characterization method as described in claim 1, characterized in that, The crack width characterization method specifically includes: S1: Take a long core and cut it in half, dividing it into two identical segments; S2: Take a portion of the core and test its porosity using a helium porosity tester; S3: Make a fractured core model from the core, vacuum the matrix of the fractured core model and fully saturate it with water; S4: Assemble the core samples into a fracture model. After accurately measuring the fracture length and height with a high-precision digital display vernier caliper, place the model into the core holder of the low-field nuclear magnetic resonance testing device. Vacuum the fracture from the outlet end to saturate it with water and scan the T2 spectrum of the fracture core samples under different stress conditions. S5: By using the area of ​​the right T2 peak corresponding to the crack space and the area of ​​the left T2 peak corresponding to the matrix pore space in the figure, the relationship between the crack volume and the matrix pore volume can be obtained. S6: The absolute value of the matrix pore volume obtained from the core porosity is used to calculate the absolute value of the fracture volume. Divide this value by the fracture length and fracture height to obtain the fracture width. S7: After fracturing another part of the core, 3D images of the fractured core model under different stresses were obtained by CT scanning, and the equivalent width of the fracture was analyzed. S8: Fit the equivalent crack width maps obtained by CT under different stresses with the crack width variation maps obtained by NMR to characterize the equivalent crack width data under different stresses.

7. The crack width characterization method as described in claim 6, characterized in that, In step S3, the matrix of the fractured core model is vacuum-saturated with water for 24 hours.

8. The crack width characterization method as described in claim 6, characterized in that, In steps S4 and S7, the stress variation range is 0-70 MPa.

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