Methods and applications for determining self-absorption behavior in nanopores using nuclear magnetic resonance (NMR) instruments

Abstract

This invention provides a method and application for determining the self-absorption behavior in nanopores using nuclear magnetic resonance (NMR) instruments, belonging to the field of shale gas exploration and development. This method is based on molecular dynamics simulations of gas-water flow and self-absorption behavior in nanopores, and uses NMR instruments to quantitatively determine the pore volume of water displacing gas as it enters the nanopores, as well as the amount of self-absorption in shale cores. This invention describes the self-absorption phenomenon in shale nanopores from a microscopic flow mechanism perspective, and uses NMR instruments to periodically measure the T2 spectrum of cores immersed in water to obtain the changes in the gas displaced by water during the self-absorption process, ultimately measuring the total pore volume occupied by water after self-absorption occurs in the core. This invention not only quantitatively calculates the amount of self-absorption in the core but also obtains the sequence of water entry into shale pores through NMR testing, which is of great importance for studying the self-absorption and flowback behavior of shale reservoirs and evaluating the productivity and economic benefits of shale gas wells.

Description

Technical Field

[0001] This invention belongs to the field of shale gas exploration and development, specifically relating to a method and its application for determining the self-absorption behavior in nanopores using nuclear magnetic resonance instruments. Background Technology

[0002] Shale reservoirs have extremely low porosity and permeability, with single wells generally having no natural production capacity or a natural production capacity below the lower limit of industrial gas flow. Hydraulic fracturing is a key technology for exploiting unconventional shale oil and gas resources and a common technique for improving gas well productivity. During hydraulic fracturing, fracturing fluid is lost into the formation. Extensive field operations abroad have revealed that fracturing fluid flowback rates are only between 35% and 62%, while the flowback rate in domestic shale gas wells is even lower, indicating that a large amount of fracturing fluid remains trapped in the formation. For conventional reservoirs, this retention leads to decreased permeability or water lock, thereby damaging the reservoir and causing a decrease in production. However, field operations have shown that in most shale gas reservoirs, after a period of shut-in, the permeability increases and productivity rises. Research suggests that this phenomenon may be due to the self-absorption phenomenon in shale reservoirs. This phenomenon, on the one hand, reduces the fracturing fluid flowback rate, and on the other hand, alters reservoir permeability during the seepage process, significantly impacting gas well productivity. Deepening the study of self-absorption phenomena is of great significance for improving the characteristics of shale reservoirs, increasing the utilization rate of fracturing fluid, and protecting the underground environment. Therefore, the study of self-absorption in shale reservoirs is very important.

[0003] Many domestic scholars have conducted experimental and numerical simulation studies on the phenomenon of self-absorption. Research on the self-absorption of tight sandstone is well-established both domestically and internationally. Studies have shown that the amount of self-absorption in sandstone is proportional to the square root of time. Furthermore, in sandstone reservoirs, as the amount of self-absorption increases, hydration expansion and particle migration obstruct gas flow channels, leading to a decrease in reservoir permeability and thus damaging the reservoir. Building upon the research on the self-absorption of tight sandstone, studies have been conducted on the self-absorption of shale reservoirs, revealing two diametrically opposed understandings: one is that self-absorption leads to the migration of shale particles and powder, and gas flow obstruction, thereby reducing permeability; the other is that mineral dissolution and the generation of induced microfractures increase permeability.

[0004] Currently, there are many patents applying nuclear magnetic resonance (NMR) to determine the pore structure of shale and quantitatively measure shale self-absorption, studying the self-absorption effect of shale from different angles and methods, for example:

[0005] Chinese patent publication CN113075102A discloses a method for establishing a mathematical model of the relationship between spontaneous permeation amount and time in porous media. It uses nuclear magnetic resonance (NMR) instruments to measure the T2 spectrum of rock cores, calculates the permeability, average capillary pressure, and surface relaxation rate of the rock sample, and establishes a mathematical model based on the principle of nuclear magnetic resonance applicable to the relationship between permeation amount and time in the spontaneous permeation process of porous media.

[0006] Chinese patent publication CN112378943A discloses a shale oil saturation evaluation model, evaluation method, and application. It uses the sample's nuclear magnetic resonance signal quantity to invert the crude oil content in the pore throat, thereby obtaining the sample's oil saturation and the pore size where oil and water are present.

[0007] Chinese patent publication CN201810042413.4 discloses a quantitative calculation method for pore structure of shale gas reservoirs based on nuclear magnetic resonance. It uses the T2 spectrum of nuclear magnetic resonance of water-saturated cores and compares it with the saturation calculated by pore size distribution obtained by other methods to obtain the cumulative saturation and T2 variation curve, and then obtains the T2 cross plot corresponding to the pore size.

[0008] Chinese patent publication CN107014728B discloses a porosity measurement method that uses nuclear magnetic resonance non-destructive testing. Based on low-temperature variable entropy nuclear magnetic resonance testing technology, it tests the nuclear magnetic resonance T2 spectrum of shale reservoirs at different temperatures. Using this spectrum as the basis, it realizes the measurement of the micro-nano scale micropore size distribution of shale reservoirs.

[0009] The Chinese open-source paper "Self-absorption characteristics of Lucaogou Formation shale in Santanghu Basin" (Xinjiang Petroleum Geology, 2020) is based on static self-absorption experiments. Starting from the basic water absorption physical phenomena of shale, it reflects the self-absorption capacity of shale through two self-absorption characteristic parameters: self-absorption saturation and self-absorption rate. It analyzes the production drainage curve after field fracturing and believes that the indoor experimental results have a certain predictive value for the production drainage of shale after fracturing.

[0010] The Chinese publicly available literature, "Permeation Kinetics and Water-Locking Relief Potential of Shale Gas Reservoirs" (Science in China: Physics, Mechanics and Astronomy, 2017), conducts experiments based on the permeation problem of fracturing fluids in shale gas reservoirs. It analyzes the interaction between water and shale and the resulting kinetic effects, and studies the relationship between permeation characteristics and the reservoir's water-locking relief potential. A method for evaluating water-locking relief potential is proposed, using initial water saturation, permeation capacity, diffusion capacity, water absorption-induced microfractures, and clay chemical effects as the main evaluation parameters. Spontaneous permeation, nuclear magnetic resonance, and pulsed permeability tests are used to evaluate the water-locking relief potential.

[0011] Therefore, there is an urgent need to conduct experimental research on the self-absorption capacity of shale, especially to establish a complete set of experimental testing and analysis methods for self-absorption, and to further study the impact of self-absorption capacity on the microstructure and physical properties of shale. Summary of the Invention

[0012] The purpose of this invention is to solve the problems existing in the prior art and provide a method and application for determining the self-absorption law in nanopores using nuclear magnetic resonance (NMR) instruments. This method can not only quantitatively calculate the total volume of water that can be absorbed after core self-absorption, but also observe the dynamic process of water displacing gas through timed tests using NMR instruments, and obtain the variation law of pores that preferentially undergo self-absorption. This method plays an important role in studying the self-absorption law and flowback law of shale reservoirs, and evaluating the productivity and economic benefits of shale gas wells.

[0013] This invention is achieved through the following technical solution:

[0014] In a first aspect, the present invention provides a method for determining the self-absorption behavior in nanopores using a nuclear magnetic resonance (NMR) instrument. The method is based on molecular dynamics simulations of gas-water flow and self-absorption behavior in nanopores, and uses an NMR instrument to quantitatively determine the pore volume of water displacing gas in nanopores, as well as the self-absorption amount in shale cores.

[0015] A further improvement of the present invention is that:

[0016] The method includes:

[0017] (1) Select experimental cores and determine the initial weight of the cores;

[0018] (2) Measure the basement value of the core;

[0019] (3) Immerse the core in water to start the core self-absorption experiment, and perform multiple tests on the core. Each test obtains the weight and T2 spectrum of the core until the core self-absorption experiment ends.

[0020] (4) Obtain the weight of water self-absorbed in each test and plot the total T2 spectrum;

[0021] (5) Determine the self-absorption behavior in nanopores;

[0022] (6) Obtain the pore volume of water entering the nanopores to displace the gas, and the self-absorption amount of shale core.

[0023] A further improvement of the present invention is that:

[0024] The operation of step (2) includes:

[0025] The T2 spectrum of the rock core was obtained by using a nuclear magnetic resonance instrument. This T2 spectrum is the base value of the rock core.

[0026] A further improvement of the present invention is that:

[0027] The operation of step (3) includes:

[0028] Place the core sample in a container filled with clean water, ensuring the core sample is completely submerged, and set the test time.

[0029] At each testing time, the core is taken out, the water on the surface of the core is wiped clean, the weight of the core is weighed first, and then the core is tested with a nuclear magnetic resonance instrument to obtain the T2 spectrum of the core.

[0030] After each measurement, the core is placed back into the container and soaked in water until the next testing time, at which point the core is removed for testing.

[0031] A further improvement of the present invention is that:

[0032] The operation of step (4) includes:

[0033] The weight of water self-absorption is obtained by subtracting the initial weight from the weight of the core obtained from each test.

[0034] The total T2 spectrum is obtained by plotting the T2 spectrum obtained from each test on a single graph.

[0035] A further improvement of the present invention is that:

[0036] The operation of step (5) includes:

[0037] (51) The pore size percentage is calculated from the T2 spectrum obtained by detection, and a pore size percentage diagram is plotted:

[0038] (52) Determine the self-absorption law in nanopores.

[0039] A further improvement of the present invention is that:

[0040] The operation of step (51) includes:

[0041] The pore diameter r is calculated using the following formula:

[0042] r=2ρ2T2

[0043] ρ2 is the relaxation strength coefficient;

[0044] The aperture ratio of each aperture is calculated using the following formula:

[0045] Pore ​​size ratio = NMR signal amplitude of pore size / (total pore volume - total pore volume of substrate)

[0046] The total pore volume is the sum of the peak areas of the T2 spectra obtained in this test;

[0047] The total pore volume of the substrate is the sum of the peak areas of the substrate values;

[0048] Draw an aperture percentage chart on a coordinate graph with aperture diameter on the horizontal axis and aperture percentage on the vertical axis.

[0049] A further improvement of the present invention is that:

[0050] The operation of step (52) includes:

[0051] The pore size ratio obtained from each test is plotted on a single graph, from which the self-absorption pattern in the nanopores is obtained.

[0052] A further improvement of the present invention is that:

[0053] The operation of step (6) includes:

[0054] After the core self-absorption experiment is completed, the weight of the core is obtained by weighing it. The initial weight is then subtracted from this weight to obtain the self-absorption amount of the shale core.

[0055] Multiplying the self-absorption amount of the shale core by the density of water yields the pore volume of water that displaces gas through the nanopores.

[0056] A further improvement of the present invention is that:

[0057] In steps (1) and (6), the initial weight and the core weight after saturation with water are obtained by weighing with a high-precision electronic balance with an accuracy of 0.0001g.

[0058] A second aspect of the present invention provides an application of a method for determining the self-absorption behavior in nanopores using nuclear magnetic resonance instruments in shale gas exploration.

[0059] Compared with the prior art, the beneficial effects of the present invention are:

[0060] This invention provides a method for determining the gas-water flow patterns and self-absorption patterns in shale nanopores. Based on molecular dynamics simulation, it simulates the dynamic process of methane gas being displaced by water within the nanopores of shale clay minerals. This method describes the self-absorption phenomenon in shale nanopores from a microscopic flow mechanism perspective. Furthermore, it uses nuclear magnetic resonance (NMR) to periodically measure the T2 spectrum of core samples immersed in water to obtain the changes in the gas displaced by water during the self-absorption process. Finally, it measures the total pore volume occupied by water after the self-absorption of the core sample.

[0061] This invention can not only quantitatively calculate the self-absorption of core samples, but also obtain the order in which water enters the pores of shale through nuclear magnetic resonance testing. This is of great significance for studying the self-absorption and flowback patterns of shale reservoirs and evaluating the productivity and economic benefits of shale gas wells. Attached Figure Description

[0062] Figure 1 Schematic diagram of shale porosity inversion from T2 map;

[0063] Figure 2 Schematic diagram of water-driven gas in nanopores of shale clay minerals;

[0064] Figure 3 Schematic diagram of the changes in T2 spectra of shale core samples after multiple tests;

[0065] Figure 4 T2 spectrum of core sample matrix analysis;

[0066] Figure 5 Diagram showing the percentage of basal pore size in core samples;

[0067] Figure 6 Graph showing the change in pore size ratio during the self-absorption process of core samples;

[0068] Figure 7 A flowchart illustrating the steps of the method of this invention. Detailed Implementation

[0069] The present invention will now be described in further detail with reference to the accompanying drawings:

[0070] This invention proposes a novel method for determining the gas-water flow patterns and self-absorption mechanisms in shale nanopores, which differs from existing methods. Starting with the flow patterns of gas and water molecules within shale nanopores, this invention uses molecular dynamics simulation to model the dynamic process of methane gas being displaced by water within the nanopores of shale clay minerals. It describes the self-absorption phenomenon in shale nanopores from a microscopic flow mechanism perspective. Using nuclear magnetic resonance (NMR) instruments, the T2 spectra of core samples immersed in water are measured at regular intervals to obtain the changes in the gas displaced by water during the self-absorption process. Finally, the total pore volume occupied by water after self-absorption is measured. Currently, no related technologies have been reported domestically or internationally.

[0071] This invention is based on molecular dynamics simulations of gas-water flow and self-absorption in nanopores. It uses nuclear magnetic resonance (NMR) instruments to quantitatively determine the order in which water enters nanopores to displace gas, as well as the amount of self-absorption in shale cores.

[0072] (I) Principle of Calculating Shale Pore Size Using Nuclear Magnetic Resonance T2 Spectra

[0073] Nuclear magnetic resonance (NMR) refers to the response of atomic nuclei to radio frequency (RF) after being magnetized by a magnetic field. When an RF pulse impacts nucleons in a magnetic field, nucleons in low-energy states absorb energy and transition to higher-energy states. The number of nucleons in high-energy states increases, while the number of nucleons in low-energy states decreases, eventually reaching a balance between the two energy states. At this point, the system no longer absorbs RF energy, indicating saturation. After the RF pulse stops, the process by which the high-energy nucleons release energy to their surroundings is called relaxation. The time required for the transitioned nucleons to return to thermal equilibrium is called the relaxation time. Relaxation time is further categorized into longitudinal relaxation time (T1), transverse relaxation time (T2), surface relaxation, volume relaxation, and diffusion relaxation.

[0074] The relaxation process is a decaying process, which generally decays exponentially, that is, the process by which the transverse magnetization vector changes from a certain value to zero. The time required for the transverse magnetization vector Mxy to decrease from its maximum value to 37% (1 / e) of its maximum value is called the transverse relaxation time.

[0075] Reservoir rocks typically exhibit a pore size distribution and often contain multiple fluid components. In this case, multiple relaxation components exist within the pores, meaning the transverse relaxation time is not a single value but a distribution, i.e., a T2 spectrum. Inversion methods are used to convert exponential decay curves into T2 spectra. Commonly used inversion methods include least squares, singular value decomposition, and transform inversion algorithms. Existing nuclear magnetic resonance experimental software can automatically convert the Mxy decay curve into a curve showing the relationship between T2 time and signal amplitude.

[0076] Measuring the transverse relaxation time T2 is faster and facilitates the rapid location of pore water in soil and rock. When the liquid within the pores is water and the magnetic field gradient is approximately zero, the transverse relaxation time of the porous media system is only related to the pore structure of the porous media and is mainly affected by the surface relaxation mechanism of the system, while being approximately independent of the other two types of relaxation mechanisms. Therefore, the formula for calculating T2 can be simplified to:

[0077]

[0078] In the formula, T2 is the transverse relaxation time, in milliseconds;

[0079] ρ2 -- the relaxation strength coefficient of the rock's transverse surface, μm / ms;

[0080] S -- Pore surface area, μm 2 ;

[0081] V -- pore volume, μm 3 .

[0082] The degree of freedom of pore water inside a material can be reflected by the transverse relaxation time T2. The shorter the relaxation time, the tighter the bond between water and the material. If the sample is saturated, the length of the transverse relaxation time can also reflect the size of the pores, and the amplitude of the nuclear magnetic resonance signal can reflect the number of pores. According to formula (1), the transverse relaxation time T2 is positively correlated with the specific surface area of ​​the rock and negatively correlated with the pore radius. Therefore, the degree of contact between the pore fluid and the surface of the rock pores will be reflected by the T2 spectrum of the nuclear magnetic resonance of saturated water rock. For cylindrical pores, S / V is simplified to 2 / r. Substituting the commonly used expression for cylindrical pores into the formula:

[0083] r=2ρ2T2 (2)

[0084] Where r -- Where is the pore radius, in μm.

[0085] In porous materials, after inversion, it was found that the T2 time distribution curve reflects the distribution of pore water in the medium. Each peak represents the water signal in the pore size within a certain range, and the peak area represents the water content, which can also represent the proportion of pore size in this range.

[0086] Formula (1) is derived into Formula (2). It can be seen that the pore radius r can be directly calculated from T2. This radius reflects the radius of the pore where the water molecule is located. Figure 1 In the figure, the horizontal axis represents the transverse relaxation time T2, and the vertical axis represents the amplitude of the NMR signal. The larger the amplitude, the more pores are corresponding to that relaxation time. Formula (2) shows that the relaxation time on the horizontal axis is proportional to the pore radius. The larger the NMR T2 spectrum, the larger r is. Therefore, from Figure 1 Based on the above judgment, each peak of the curve represents a relatively large proportion of pores at that relaxation time (i.e., pore radius). Large pores, relatively large pores, and small pores are only relative concepts. Figure 1 There are three peak values, which correspond to small, relatively large, and large relaxation times, respectively, representing small pores, relatively large pores, and large pores.

[0087] (II) Gas-water flow patterns and self-absorption phenomena in nanopores

[0088] Self-absorption refers to the process by which fluids interact with rocks, causing them to penetrate the rock matrix through a specific mechanism. It is a process in which rocks spontaneously absorb fluids. Due to the low water saturation and strong water permeability of shale reservoirs, as well as the abundance of clay minerals in the reservoir matrix, shale reservoirs possess a strong self-absorption capacity. After fluid enters the core, the density and mass of the rock change. At this point, the volume of fluid absorbed into the core equals the volume of fluid displaced from the core. Experiments based on this principle are called self-absorption experiments.

[0089] This invention starts with the flow patterns of gas and water molecules in shale nanopores, investigating the microscopic flow mechanism within nanopores and the self-absorption phenomenon in shale nanopores. Based on molecular dynamics simulations, the dynamic process of methane gas being displaced by water within the nanopores of shale clay minerals is simulated, theoretically studying the flow patterns of gas and water in nanopores.

[0090] Figure 2 This study demonstrates the movement of methane and water molecules within nanopores under the influence of molecular forces. A molecular simulator was first used to establish a 2 nm kaolinite pore framework and a 5 nm illite pore framework. Methane molecules were then placed within the pores, while water molecules were placed in larger crack spaces. The simulation then examined the movement of water and methane molecules under the influence of molecular forces alone, without any external forces. The simulation results show that water molecules can enter the shale nanopores through cracks, displacing gas. After the gas is expelled, the water in the micro- and nano-pores is difficult to expel. This is because clay minerals readily distribute a layer of positively charged ions on their surfaces. Compared to nonpolar methane molecules with zero dipole moment, polar water molecules with strong dipole moments have a greater affinity for clay minerals. Water forms a film on the surface of the circular or square nanopores of clay minerals, and gas flows through the channels formed by this film. The smaller the pore size, the greater the proportion of gas displaced. Compared to kaolinite, illite displaces more gas. The reason is that illite is rich in potassium ions, and its surface layer carries a charge, making it more hydrophilic. Molecular dynamics simulations explain the phenomenon of water being absorbed by the shale formation while simultaneously displacing shale gas from the pores, resulting in a low water backflow rate and demonstrating the existence of self-absorption in shale reservoirs.

[0091] (III) Methods for determining flow and self-absorption in nanopores using nuclear magnetic resonance experiments

[0092] To verify the flow patterns of gas and water molecules in nanopores as simulated by molecular dynamics, a method was designed to determine the flow of gas and water in nanopores using nuclear magnetic resonance (NMR) instruments. The aim was to understand how water molecules displace methane molecules in nanopores in shale gas reservoirs under the influence of molecular forces alone, without any external forces, and to determine the total volume of water entering the pores and replacing the gas. This volume can be approximately equal to the self-absorption of the shale core.

[0093] The principle of the application is as follows: gas does not produce a signal when measured by a nuclear magnetic resonance (NMR) instrument. Therefore, the T2 spectrum obtained by testing shale gas-bearing cores from shale reservoirs can be regarded as the baseline value. Then, the core is immersed in water, and without external force, the water displaces the gas in the shale pores. The measurement time is set, and the T2 spectrum of the core is measured periodically with an NMR instrument to obtain the changes in the core pores and observe the changes in pores during the water displacement of gas. After the experiment is conducted for a sufficiently long time, it is ensured that the water completely replaces the gas in the pores. The pore volume measured at this time is the total pore volume occupied by water after the shale core undergoes self-absorption. At the same time, it is possible to observe which ranges of pores the water preferentially enters.

[0094] (IV) Nuclear Magnetic Resonance Experiment Procedure

[0095] like Figure 7 As shown, embodiments of the method of the present invention are as follows:

[0096] Example 1

[0097] The specific experimental steps are as follows:

[0098] (1) Select experimental core and determine the initial weight: Select a core with a diameter of about 25 mm from the shale gas reservoir and weigh the core using a high-precision electronic balance as the initial weight before immersion in water.

[0099] (2) Measurement of baseline values: The T2 spectrum of the core was obtained by testing the core using a nuclear magnetic resonance (NMR) instrument. This T2 spectrum represents the baseline values ​​of the core when it was not immersed in water. The relevant parameters used in the NMR testing in this invention are: waiting time T... w＝ 3s, echo interval TE = 0.1ms, number of echoes NECH = 6000, number of scans n = 64.

[0100] (3) Immerse the core in water to induce self-absorption, thus initiating the self-absorption experiment. Continuously perform nuclear magnetic resonance (NMR) detection during the immersion process until the self-absorption experiment is completed, as detailed below:

[0101] Place the core sample in a container filled with clean water, ensuring it is completely submerged. Set a testing time as shown in Table 1. At each testing time, remove the core sample, wipe the water off its surface, weigh it, and then use a nuclear magnetic resonance instrument to detect the T2 spectrum. After each measurement, return the core sample to the container and continue immersing it in water until the next testing time.

[0102] The time settings for nuclear magnetic resonance (NMR) testing of shale cores are shown in Table 1. Generally, the amount of self-absorption of shale cores can be obtained after about one month of testing.

[0103] Interval time Number of measurements Days Interval time Number of measurements Days 2 hours 12 times 1 day 4 hours 6 times 1 day 8 hours 6 times 2 days 12 hours 4 times 2 days 24 hours 4 times 4 days 48 hours 4 times 8 days 120 hours 3 times 15 days

[0104] Table 1

[0105] (4) Before each NMR test, wipe the water off the core surface, weigh the core, subtract the initial weight of the core from the core weight to obtain the weight of the water absorbed, and then perform NMR testing to obtain the T2 spectrum after each immersion in water. Plot the T2 spectrum obtained from each test on a single graph, such as... Figure 3 As shown, Figure 3 Each curve in the graph represents the T2 spectrum at a detection time point.

[0106] (5) The core pore distribution map was obtained by calculating the T2 spectrum obtained from the detection: By applying formula (2) to calculate the corresponding pore distribution, it can be seen which sizes of shale pores water enters first, thus obtaining the microscopic flow law of water-displaced gas, verifying the molecular dynamics simulation results, and at the same time, the amount of self-absorption of shale core can also be obtained based on the final pore volume. The details are as follows:

[0107] Apply formula (2) to calculate the radius r = 2ρ²T², where the relaxation strength coefficient ρ² is taken as 10, and the value of T² is from... Figure 1 The horizontal axis in the right-hand figure shows that, for example, when reading within the small pore range T2 = 0.2, the radius r = 2 * 10 * 0.2 = 4 nm; when reading within the large pore range T2 = 200, the radius r = 2 * 10 * 200 = 4000 nm.

[0108] The pore size percentage is calculated as the proportion of each pore size (i.e., pore radius). The total pore volume is obtained using nuclear magnetic resonance (NMR) experiments (the results of NMR experiments include the sum of the peak areas of the T2 spectra for each test; this is the total pore volume, a conventional method and will not be elaborated further here). The pore size percentage of each pore size is obtained by dividing the NMR signal amplitude of each pore size by the total pore volume. Figure 5 This refers to the percentage of pore size obtained after such calculation, where the vertical axis represents the percentage of pore size in %, and the horizontal axis represents the pore radius in micrometers (µm). Figure 5 It represents the percentage of the core sample's pore size when it was not submerged in water.

[0109] Furthermore, after drawing the diagrams of the pore distribution over time ( Figure 6 The porosity percentage of the substrate value has already been subtracted, therefore Figure 6 The porosity is much smaller, such as Figure 6 As shown, from Figure 6As can be seen, the proportion of pores of different sizes changed over time. Initially, the peak value was located in smaller pores, but later the peak value of the curve gradually shifted towards larger pores. This indicates that during the microscopic flow process, water first enters the smaller pores and then gradually enters the larger pores. The second peak value gradually appears, indicating that water also gradually enters the larger pores. This allows us to observe the flow pattern of water in nano- and micro-sized pores, verifying the results of molecular simulations.

[0110] After the core self-absorption experiment, the weight of the obtained core was weighed, and this weight was subtracted from the initial weight of the core to obtain the weight of the water self-absorbed by the core. The volume change is the pore volume obtained after the core self-absorption experiment, minus the pore volume of the base test, which is the volume of the core self-absorbed.

[0111] Specifically, the method for obtaining the pore volume of the core by self-absorption is as follows: A high-precision electronic balance with an accuracy of 0.0001g is used to weigh the core before immersion in water and after saturation with water (i.e., after the final immersion, the core is removed, dried, and then weighed). This yields the weight of the self-absorbed water, with a density of 1 g / cm³. 3 Therefore, the total volume of self-absorption can be calculated.

[0112] In summary, the method of the present invention utilizes the above experimental methods to obtain the micro-flow law and the amount of self-absorption.

[0113] The following are examples of applications of this invention:

[0114]

Example 2

[0115] The application of this invention is illustrated using the analysis of a shale core sample from Fuling as an example. The experimental steps are as follows:

[0116] (1) Take a standard core t-7 from the Fuling area, with a diameter of 25 mm and a length of 30 mm, and measure its initial weight as 59.0554 g.

[0117] (2) The core was placed in a glass bottle, and T2 spectra were measured using a MacroMR12-150H-G multidimensional nuclear magnetic resonance analyzer. The parameters used for the test are shown in Table 2. The T2 spectra of the core obtained are as follows: Figure 4 As shown, these serve as the base values ​​for the experiment.

[0118]

[0119] Table 2

[0120] (3) Apply formula (2) to calculate the corresponding pore size distribution, where the relaxation strength coefficient ρ2 is taken as 10. Plot the pore size distribution diagram based on the calculation results. The horizontal axis is the pore radius, which is calculated using formula (2). The vertical axis is the pore size percentage (i.e., pore size distribution), which is calculated using the following formula:

[0121] Pore ​​size ratio = NMR signal amplitude of pore size / total pore volume

[0122] The total pore volume can be obtained through nuclear magnetic resonance experiments. The pore size percentage of each pore is obtained by dividing the amplitude of the nuclear magnetic signal measured for each pore by the total pore volume (the sum of the amplitudes of the nuclear magnetic signals measured by the nuclear magnetic resonance instrument, i.e., the sum of the areas of the peaks in the plotted T2 spectrum), as shown in the above formula. Figure 5 This is the pore diameter distribution obtained after calculation, where the horizontal axis represents the pore diameter in micrometers (µm).

[0123] The core samples were immersed in water to induce self-absorption, and nuclear magnetic resonance (NMR) tests were performed continuously during the immersion process. The testing time was selected as shown in Table 3. A total of 30 days and 20 tests were conducted.

[0124] (4) Convert the NMR T2 spectra from each test into core pore sizes, calculate the pore volume occupied by each pore size, subtract the basement percentage, and plot the core pore size distribution during self-absorption, as shown in the figure. Figure 6 As shown. Specifically:

[0125] In the calculation of pore size percentage, the pore size percentage of each pore size is obtained by dividing the amplitude of the NMR signal measured for each pore size by the total pore volume of this test and subtracting the total pore volume of the substrate, as shown in the following formula:

[0126] Pore ​​size ratio = NMR signal amplitude of pore size / (total pore volume - total pore volume of substrate)

[0127] When the core sample is not immersed in water, NMR measurements detect the signal of the pore framework. At this stage, no water has entered the pores, and this signal is taken as the baseline value. When the core sample is immersed in water, the water slowly enters the rock pores. After a period of immersion, NMR testing is performed on the rock again. The measured signal is calculated using the first formula, which yields the sum of the previous rock framework signal and the signal from the immersed water. When calculated using the second formula, the baseline value is subtracted, leaving only the value of the entered water, and it also shows which size range of pores the water preferentially enters.

[0128] like Figure 6As shown in the diagram, after plotting the pore size ratios over time, it can be seen that the pore size ratios of different sizes have changed. Initially, the peak value was located in smaller pores, and later the peak value of the curve gradually shifted towards larger pores. This indicates that in the microscopic flow process, water first enters the smaller pores and then gradually enters the larger pores. The second peak value gradually appears, indicating that water also gradually enters the larger pores. This allows us to observe the flow pattern of water in nano- and micro-sized pores, verifying the results of molecular simulations.

[0129] The weight of the water absorbed by the core is obtained by subtracting the initial weight of the core from the final weight of the core self-absorption experiment. The volume of water absorbed by the core is obtained by subtracting the pore volume from the base test from the final pore volume obtained from the core self-absorption experiment. In this example, the weight of the core before immersion in water was 59.0554 g, and the weight of the core after immersion in water for 720 hours was 60.1802 g. The weight of the self-absorbed water was 1.1248 g. Since the density of water is 1 g / cm³... 3 Therefore, the total pore volume of self-absorption is calculated to be 1.1248 cm³. 3 .

[0130] In this example, the core weight was measured each time an NMR measurement was performed. Due to non-standard measurement methods and short intervals between measurements, errors occurred. However, the final measurement, taken with a high-precision electronic balance and with a significant amount of water absorbed, provided sufficient accuracy and precision. Therefore, the self-absorbed weight and pore volume calculated using this method are reliable.

[0131] Testing Time 2 hours 4 hours 8 hours 12 hours 16 hours 24 hours 36 hours Core weight (g) 59.0554 59.1826 59.2455 59.3193 59.4268 59.5182 59.6055 Testing Time 48 hours 72 hours 96 hours 144 hours 196 hours 240 hours 288 hours Core weight (g) 59.7265 59.8513 59.9086 60.0458 60.0804 60.2598 60.1668 Testing Time 336 hours 384 hours 432 hours 480 hours 600 hours 720 hours Core weight (g) 60.254 60.2836 60.3203 60.291 60.3237 60.1802

[0132] Table 3

[0133] This invention, based on molecular dynamics simulation, simulates the dynamic process of methane gas being displaced by water within the nanopores of shale clay minerals. It describes the self-absorption phenomenon in shale nanopores from a microscopic flow mechanism perspective and uses nuclear magnetic resonance (NMR) to periodically measure the T2 spectrum of core samples immersed in water. This quantitatively determines the order in which water enters the nanopores and displaces the gas, as well as the amount of self-absorption in the shale core, obtaining the variation law of water displaced gas during the self-absorption process. Finally, it measures the total pore volume occupied by water after the self-absorption occurs in the core. This invention not only quantitatively calculates the amount of self-absorption in the core but also observes the dynamic process of water displaced gas through periodic NMR testing, obtaining the order in which water enters the shale pores.

[0134] This invention can be applied to nuclear magnetic resonance experiments on any shale core. It plays an important role in studying the self-absorption and flowback laws of shale reservoirs and evaluating the productivity and economic benefits of shale gas wells. It also has broad application prospects.

[0135] Finally, it should be noted that the above technical solution is only one embodiment of the present invention. For those skilled in the art, based on the application methods and principles disclosed in the present invention, it is easy to make various types of improvements or modifications, and not limited to the methods described in the above specific embodiments of the present invention. Therefore, the methods described above are only preferred and have no limiting significance.

Claims

1. A method for determining the self-absorption behavior in nanopores using nuclear magnetic resonance (NMR) instruments, characterized in that: The method is based on molecular dynamics simulation of gas-water flow and self-absorption in nanopores, and uses nuclear magnetic resonance to quantitatively determine the pore volume of water entering nanopores to displace gas, as well as the self-absorption amount of shale core. The method includes: (1) Select experimental cores and determine the initial weight of the cores; (2) Measure the basement value of the core; (3) Immerse the core in water to start the core self-absorption experiment, and perform multiple tests on the core. Each test obtains the weight and T2 spectrum of the core until the core self-absorption experiment ends. (4) Obtain the weight of water self-absorbed in each test and plot the total T2 spectrum; (5) Determine the self-absorption behavior in nanopores; (6) Obtain the pore volume of water intrusion into nanopores to displace gas, and the self-absorption amount of shale core. The operation of step (5) includes: (51) The pore size percentage is calculated from the T2 spectrum obtained by detection, and a pore size percentage diagram is plotted: (52) Determine the self-absorption behavior in nanopores; The operation of step (51) includes: The pore diameter r is calculated using the following formula: r=2ρ2T2 ρ2 is the relaxation strength coefficient; The aperture ratio of each aperture is calculated using the following formula: Pore ​​size ratio = NMR signal amplitude of pore size / (total pore volume - total pore volume of substrate) The total pore volume is the sum of the peak areas of the T2 spectra obtained in this test; The total pore volume of the substrate is the sum of the peak areas of the substrate values; Draw an aperture percentage chart on a coordinate graph with aperture diameter on the horizontal axis and aperture percentage on the vertical axis; The operation of step (52) includes: The pore size ratio obtained from each test is plotted on a single graph. As time goes by, the pore size ratio of different sizes changes. The peak of the curve moves from small pores to large pores, and the second peak gradually appears, thus obtaining the self-absorption law in nanopores: water first enters the small pores and then gradually enters the larger pores.

2. The method for determining the self-absorption behavior in nanopores using nuclear magnetic resonance instruments according to claim 1, characterized in that: The operation of step (2) includes: The T2 spectrum of the rock core was obtained by using a nuclear magnetic resonance instrument. This T2 spectrum is the base value of the rock core.

3. The method for determining the self-absorption behavior in nanopores using nuclear magnetic resonance instruments according to claim 2, characterized in that: The operation of step (3) includes: Place the core sample in a container filled with clean water, ensuring the core sample is completely submerged, and set the test time. At each testing time, the core is taken out, the water on the surface of the core is wiped clean, the weight of the core is weighed first, and then the core is tested with a nuclear magnetic resonance instrument to obtain the T2 spectrum of the core. After each measurement, the core is placed back into the container and soaked in water until the next testing time, at which point the core is removed for testing.

4. The method for determining the self-absorption behavior in nanopores using nuclear magnetic resonance instruments according to claim 3, characterized in that: The operation of step (4) includes: The weight of water self-absorption is obtained by subtracting the initial weight from the weight of the core obtained from each test. The total T2 spectrum is obtained by plotting the T2 spectrum obtained from each test on a single graph.

5. The method for determining the self-absorption behavior in nanopores using nuclear magnetic resonance instruments according to claim 4, characterized in that: The operation of step (6) includes: After the core self-absorption experiment is completed, the weight of the core is obtained by weighing it. The initial weight is then subtracted from this weight to obtain the self-absorption amount of the shale core. The pore volume of water entering the nanopores to displace gas was obtained based on the self-absorption capacity of shale cores and the density of water.

6. The method for determining the self-absorption behavior in nanopores using nuclear magnetic resonance instruments according to claim 5, characterized in that: In steps (1) and (6), the initial weight and the core weight after saturation with water are obtained by weighing with a high-precision electronic balance with an accuracy of 0.0001g.

7. The application of the method as described in any one of claims 1 to 6 in shale gas exploration.

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