CN115876825A - Method for determining self-absorption rule in nano-pores by utilizing nuclear magnetic resonance instrument and application - Google Patents
Method for determining self-absorption rule in nano-pores by utilizing nuclear magnetic resonance instrument and application Download PDFInfo
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Abstract
The invention provides a method for determining a self-absorption rule in a nanometer pore by using a nuclear magnetic resonance instrument and application thereof, belonging to the field of shale gas exploration and development. The method simulates a gas flow rule and a self-absorption rule in the nano-pores based on molecular dynamics, and quantitatively determines the pore volume of the displacement gas of water entering the nano-pores and the self-absorption amount of the shale rock core by applying a nuclear magnetic resonance instrument. The invention describes the self-absorption phenomenon of the shale nano-pores from the micro-flow mechanism, and uses a nuclear magnetic resonance instrument to measure the T of the core immersed in water at regular time 2 And obtaining a spectrum, namely obtaining a change rule of water displacement gas in the self-absorption process of the rock core, and finally measuring the total pore volume occupied by water after the self-absorption of the rock core occurs. The invention not only can quantitatively calculate the self-absorption quantity of the rock core, but also can obtain the self-absorption quantity through the test of a nuclear magnetic resonance instrumentThe sequence of the obtained water entering the shale pores plays an important role in researching the self-suction rule and the flowback rule of the shale reservoir and evaluating the productivity and the economic benefit of the shale gas well.
Description
Technical Field
The invention belongs to the field of shale gas exploration and development, and particularly relates to a method for determining a self-absorption rule in a nanometer pore by using a nuclear magnetic resonance instrument and application of the method.
Background
From the perspective of global energy development strategies, the development of unconventional gas reserves is changing the world's energy landscape and is the focus of global development. The total amount of shale gas resources worldwide is predicted to be about 456 x 10 12 m 3 Accounting for about 50% of the total resource of unconventional natural gas (coal bed gas, tight sandstone gas, shale gas). The permeability of shale reservoir pores is extremely low, single wells generally have no natural capacity or the natural capacity is lower than the lower limit of industrial gas flow, hydraulic fracturing is a key technology for exploiting unconventional shale oil gas and natural gas resources, and is a common technical means for improving the gas well capacity, in the hydraulic fracturing process, fracturing fluid can be filtered into the stratum, a large amount of field construction in foreign countries finds that the flowback rate of the fracturing fluid is only 35-62%, while the flowback rate of shale gas wells in China is lower, which indicates that a large amount of fracturing fluid is retained in the stratum, and for the conventional reservoir, the retention phenomenon can cause permeability reduction or water lock phenomenon, thereby damaging the reservoir and causing yield reduction. However, in-situ construction finds that after most shale gas reservoirs are shut in for a period of time, the permeability of the shale gas reservoirs is increased, the productivity is increased, and researches show that the shale gas reservoirs possibly cause the phenomenonThe reservoir has a self-priming phenomenon. On one hand, the phenomenon can cause the reduction of the flowback rate of the fracturing fluid, on the other hand, the change of the permeability of a reservoir can be caused in the imbibition process, and the change has great influence on the productivity of a gas well. The research on the self-absorption phenomenon is deepened, the understanding on the characteristics of the shale reservoir is realized, and the method has important significance for improving the yield of shale gas, improving the utilization rate of fracturing fluid and protecting the underground environment, so the research on the self-absorption of the shale reservoir is very important.
Many domestic scholars carry out researches on the aspects of experiments, numerical simulation and the like aiming at the self-priming phenomenon, the research on the self-priming effect of the compact sandstone is mature at home and abroad, the self-priming quantity of the sandstone obtained through research is in direct proportion to the square root of time, and as the self-priming quantity is increased in a sandstone reservoir, the permeability of the reservoir is reduced due to hydration expansion and particle migration to block a gas flow channel, so that the reservoir is damaged. On the basis of research on self-absorption of compact sandstone, research is carried out on the self-absorption of a shale reservoir, and two distinct recognitions exist: one is because of self-absorption, which causes shale particle powder migration and airflow obstruction, thereby reducing permeability; the other is due to mineral dissolution and the creation of induced microcracks that increase permeability.
At present, there are many patents for determining the pore structure of shale and quantitatively measuring the self-absorption of shale by applying nuclear magnetic resonance, and the self-absorption of shale is studied from different angles and methods, for example:
chinese patent publication No. CN113075102A discloses a method for establishing a mathematical model of the relation between the spontaneous imbibition amount of a porous medium and time, and a nuclear magnetic resonance instrument is applied to measure a rock core T 2 And (3) calculating the imbibition permeability, the average capillary pressure and the surface relaxation rate of the rock sample, and establishing a mathematical model which is based on the nuclear magnetic resonance principle and is suitable for the relationship between the imbibition amount and time in the spontaneous imbibition process of the porous medium.
Chinese patent publication No. CN112378943A discloses an evaluation model, an evaluation method and application of shale oil saturation, which invert the content of crude oil in pore throats according to sample nuclear magnetic signal quantity so as to obtain the oil saturation of a sample and the occurrence pore diameters of oil and water.
Chinese patent publication CN201810042413.4 discloses a quantitative calculation method for shale gas reservoir pore structure based on nuclear magnetic resonance, and the method applies water-saturated core nuclear magnetic resonance T 2 The spectrogram is compared with the saturation calculated by the pore size distribution obtained by other methods to obtain the accumulated saturation and T 2 So as to obtain the aperture corresponding T 2 A cross-plot;
chinese patent publication CN107014728B discloses a pore measurement method, which adopts a nuclear magnetic resonance nondestructive detection means and is based on a low-temperature variable entropy nuclear magnetic resonance test technology to test the nuclear magnetic resonance T of a shale reservoir at different temperatures 2 And spectrum, which is used as a test basis, realizes the measurement of micro-nano-scale micro-pore size distribution of the shale reservoir.
The Chinese published document 'self-priming characteristics of shale in reed grass ditch groups in three ponds, lakes and basins' (Xinjiang petroleum geology, 2020) is based on a static self-priming experiment, starts from the basic physical phenomenon of water absorption of shale, reflects the self-priming capability of shale through 2 self-priming characteristic parameters of self-priming saturation and self-priming rate, analyzes a drainage curve generated after field fracturing, and considers that an indoor experiment result has a certain prediction effect on the drainage generated after the shale fracturing.
The Chinese published literature, "the imbibition dynamic characteristics and the water lock release potential of shale gas reservoirs" (Chinese science: physics and mechanics astronomy, 2017) carries out experiments based on the imbibition problem of fracturing fluid of shale gas reservoirs, analyzes the interaction between water and shale and the dynamic effect caused by the interaction, and researches the relationship between the imbibition characteristics and the water lock release potential of the reservoirs. The water lock release potential evaluation method is provided with the initial water saturation, the imbibition capacity, the diffusion capacity, the water absorption induced micro-cracks, the clay chemical action and the like as main evaluation parameters, and the water lock release potential is evaluated by utilizing spontaneous imbibition, nuclear magnetic resonance and pulse permeability tests.
Therefore, the research on the self-absorption capability experiment of shale is urgently needed, and particularly, a set of complete self-absorption experiment test analysis method needs to be established to further research the influence of the self-absorption capability on the microstructure and physical properties of shale.
Disclosure of Invention
The invention aims to solve the problems in the prior art, and provides a method for determining a self-priming rule in a nano-pore by using a nuclear magnetic resonance instrument and application thereof, which not only can quantitatively calculate the total volume of water capable of being sucked after a rock core is self-primed, but also can observe the dynamic process of water-flooding displacement gas by performing timing test through the nuclear magnetic resonance instrument, obtain the change rule of a pore preferentially subjected to self-priming, and have important effects on researching the self-priming rule and the flowback rule of a shale reservoir and evaluating the productivity and the economic benefit of a shale gas well.
The invention is realized by the following technical scheme:
in a first aspect of the invention, a method for determining a self-absorption rule in a nanopore by using a nuclear magnetic resonance instrument is provided, the method simulates a gas flow rule and a self-absorption rule in the nanopore on the basis of molecular dynamics, and quantitatively determines a pore volume of displacement gas of water entering the nanopore and a self-absorption amount of a shale core by using the nuclear magnetic resonance instrument.
The invention is further improved in that:
the method comprises the following steps:
(1) Selecting an experimental core, and measuring the initial weight of the core;
(2) Measuring a base value of the core;
(3) Immersing the core into water, starting a core self-absorption experiment, and detecting the core for multiple times, wherein the weight and T of the core are obtained in each detection 2 Mapping until the self-absorption experiment of the rock core is finished;
(4) Acquiring the weight of water self-absorption obtained by each detection and drawing a total T 2 A map;
(5) Determining a self-absorption rule in the nano pores;
(6) And obtaining the pore volume of the displacement gas of the water entering the nano pores and the self-absorption quantity of the shale core.
The invention is further improved in that:
the operation of the step (2) comprises the following steps:
detecting rock core by using nuclear magnetic resonance instrument to obtain T of rock core 2 Map, the T 2 And the atlas is the base value of the rock core.
The invention is further improved in that:
the operation of the step (3) comprises the following steps:
placing the core in a container filled with clear water to ensure that the core is completely immersed in water, and setting test time;
taking out the core at each test time, wiping off water on the surface of the core, weighing the weight of the core, and detecting the core by using a nuclear magnetic resonance instrument to obtain the T of the core 2 A map;
and after each measurement is finished, putting the core back into the container to be continuously soaked in the water, and taking out the core for detection when the next test time is up.
The invention is further improved in that:
the operation of the step (4) comprises the following steps:
subtracting the initial weight from the weight of the core obtained by each detection to obtain the self-absorption weight of water;
t to be obtained for each test 2 Mapping on a chart to obtain total T 2 And (4) mapping.
The invention is further improved in that:
the operation of the step (5) comprises the following steps:
(51) T obtained by detection 2 And (3) obtaining the aperture ratio by map calculation, and drawing an aperture ratio map:
(52) And determining the self-absorption rule in the nano pores.
The invention is further improved in that:
the operation of step (51) comprises:
the pore size r of the pores is calculated using the formula:
r＝2ρ 2 T 2
ρ 2 is the relaxation intensity coefficient;
the pore size ratio for each pore size was calculated using the following formula:
pore diameter ratio = nuclear magnetic signal amplitude/(total pore volume-substrate total pore volume) of pore diameter
Wherein, the total pore volume is T obtained by the detection 2 The sum of the peak areas of the maps;
the substrate total pore volume is the sum of the peak areas in the substrate values;
and drawing an aperture ratio graph on a coordinate graph with the abscissa as the aperture and the ordinate as the aperture ratio.
The invention is further improved in that:
the operation of step (52) comprises:
and drawing a pore diameter ratio graph obtained by each detection on a graph, and obtaining a self-absorption rule in the nano pores from the graph.
The invention is further improved in that:
the operation of the step (6) comprises the following steps:
after the self-absorption experiment of the rock core is finished, weighing to obtain the weight of the rock core, and subtracting the initial weight from the weight to obtain the self-absorption amount of the shale rock core;
multiplying the self-priming capacity of the shale core by the density of the water yields the pore volume of the water-invading nanoporous displacement gas.
The invention is further improved in that:
in the step (1) and the step (6), the initial weight and the weight of the core after the saturated water are obtained by weighing respectively by using a high-precision electronic balance with the precision of 0.0001 g.
In a second aspect of the invention, the application of the method for determining the self-absorption rule in the nano-pores by using the nuclear magnetic resonance instrument in shale gas exploration is provided.
Compared with the prior art, the invention has the beneficial effects that:
the invention provides a method for determining a gas flow rule and a shale self-absorption rule in a shale nanometer pore, which is based on a molecular dynamics simulation method, simulates a dynamic process that methane gas in the nanometer pore in a shale clay mineral is displaced by water, describes a shale nanometer pore self-absorption phenomenon from a microcosmic flow mechanism, and regularly measures the T of a rock core immersed in water by applying a nuclear magnetic resonance instrument 2 Obtaining a spectrum of the change of water displacement gas in the self-absorption process of the rock coreAnd (4) regularly measuring the total pore volume occupied by water after the core is subjected to self-absorption.
The method can quantitatively calculate the self-absorption amount of the rock core, obtains the sequence of water entering the shale pores through the test of a nuclear magnetic resonance instrument, and has important effects on researching the self-absorption rule and the flowback rule of the shale reservoir and evaluating the productivity and the economic benefit of the shale gas well.
Drawings
FIG. 1T 2 Performing map inversion on a shale pore schematic diagram;
FIG. 2 is a schematic illustration of water flooding within nanopores in a shale clay mineral;
FIG. 3 shale core sample multiple testing T 2 A schematic diagram of a variation of the map;
FIG. 4 core sample substrate test T 2 A map spectrum;
FIG. 5 is a plot of the ratio of the pore size of the core sample substrate;
FIG. 6 is a graph of the change of pore diameter ratio during self-priming of a core sample;
fig. 7 is a block diagram of the steps of the method of the present invention.
Detailed Description
The invention is described in further detail below with reference to the accompanying drawings:
the new method for determining the gas-water flow rule and the shale self-absorption rule in the nano pores of the shale, which is provided by the invention, is different from the existing method. Starting from the flowing rule of gas molecules and water molecules in the nanopores of the shale, the invention simulates the dynamic process of displacing methane gas in the nanopores of the shale clay minerals by water based on a molecular dynamics simulation method, describes the self-absorption phenomenon of the nanopores of the shale from a micro-flowing mechanism, and regularly measures the T of a rock core immersed in water by applying a nuclear magnetic resonance instrument 2 And obtaining a spectrum, namely obtaining a change rule of water displacement gas in the self-absorption process of the rock core, and finally measuring the total pore volume occupied by water after the self-absorption of the rock core occurs. At present, no relevant technical report is seen at home and abroad.
The method is based on molecular dynamics to simulate the flow rule and self-absorption rule of air in the nano-pores, and a nuclear magnetic resonance instrument is applied to quantitatively determine the sequence of water entering the nano-pore displacement gas and the self-absorption amount of the shale core.
Nuclear magnetic resonance T 2 Shale aperture principle by spectrum calculation
Nuclear magnetic resonance refers to the response of the nucleus to radio frequency after it is magnetized by a magnetic field. When the radio frequency pulse impacts on the nuclei in the magnetic field, the nuclei in the low energy state will jump to the high energy state due to the continuous energy absorption, the number of the nuclei in the high energy state will increase continuously, the number of the nuclei in the low energy state will decrease, and finally the number of the nuclei in the two energy states will be equal. At this point, the system does not reabsorb rf energy, indicating that saturation has been reached. The process of releasing energy to the surroundings by nuclei in high energy states after the radio frequency stops is called relaxation, and the time required when the transitioning nuclei return to thermal equilibrium is called relaxation time. The relaxation times are further divided into longitudinal relaxation times (T) 1 ) Transverse relaxation time (T) 2 ) Surface relaxation, volume relaxation, and diffusion relaxation.
The relaxation process is a decay process, generally an exponential decay process, i.e., a process in which the transverse magnetization vector changes from a certain value to zero, and the time required to reduce the transverse magnetization vector Mxy from the maximum value to 37% (1/e) of the maximum value is called a transverse relaxation time.
Reservoir rock usually has a pore size distribution and often contains a plurality of fluid components, in which case there are a plurality of relaxation components in the pores, i.e. the transverse relaxation time is not a single value, but a distribution, i.e. T 2 And (4) mapping. Inversion of exponential decay curve to T using inversion method 2 The map generally adopts an inversion method such as a least square method, a singular value decomposition method, a transformation inversion algorithm and the like, and the curve of Mxy attenuation can be automatically inverted into T by applying the existing nuclear magnetic resonance experiment software 2 Time versus signal amplitude.
Measuring transverse relaxation time T 2 The measurement is more rapid, the rapid positioning of pore water in rock soil is facilitated, when liquid in pores is water and the magnetic field gradient is approximately zero, the transverse relaxation time of a porous medium system is only related to the pore structure of the porous medium and is mainly influenced by the surface relaxation mechanism of the systemResponse, and is approximately independent of the other two relaxation mechanisms, therefore, T 2 The calculation formula can be simplified as:
in the formula, T 2 -is the transverse relaxation time, ms;
ρ 2 - -is the transverse surface relaxation intensity coefficient of the rock, μm/ms;
s- -is pore surface area, μm 2 ；
V- -is pore volume, μm 3 。
The degree of freedom of the pore water in the material can be controlled by the transverse relaxation time T 2 To reflect it. The shorter the relaxation time, the tighter the bond between water and material is illustrated. If the sample is in a saturated state, the transverse relaxation time can also reflect the size of the pores, and the nuclear magnetic signal amplitude can reflect the number of the pores. Transverse relaxation time T according to equation (1) 2 The specific surface area of the rock is in positive correlation, and the pore radius is in negative correlation. Therefore, the contact degree of the rock pore fluid and the rock pore surface is measured by the saturated water rock nuclear magnetic resonance T 2 The spectrum is reflected. Cylindrical pores, S/V is simplified to 2/r. Substituting the commonly used cylindrical pore expression:
r＝2ρ 2 T 2 (2)
wherein r-is the pore radius, mu m.
In the porous material, T is found after inversion 2 The time distribution curve reflects the pore water distribution in the medium, each peak represents the water signal in the pore diameter within a certain range, and the peak area represents the water content, and can also represent the proportion of the pore diameter within the range.
The formula (2) is obtained by deducing the formula (1), and it can be seen that T can be used 2 The pore radius r, which reflects the radius of the pore in which the water molecule is located, is directly calculated, and in FIG. 1, the abscissa represents the transverse relaxation time T 2 The ordinate represents the amplitude of the nuclear magnetic signal, the larger the amplitude is, the relaxation time is representedThe more pores are corresponding to, the more equation (2) shows that the abscissa relaxation time is proportional to the pore radius, and the nuclear magnetic resonance T 2 The larger the spectrum is, the larger r is, so it is judged from fig. 1 that each peak of the curve represents that the corresponding pore proportion is more under the relaxation time (i.e. pore radius), and the large pore, the larger pore and the small pore are only one relative concept, and fig. 1 has three peaks, which correspond to the small, the larger and the large relaxation time respectively, and correspondingly represent the small pore, the larger pore and the large pore.
Air-water flow rule and self-suction phenomenon in nano pores
Self-priming refers to the process by which rock and fluid interact to cause fluid to invade the rock matrix by some mechanism, a process by which rock spontaneously inhales fluid. The shale reservoir has strong self-absorption capacity due to low water saturation and strong water-wet property of the shale reservoir and the rich clay minerals in the reservoir matrix. After the fluid enters the core, the density and the mass of the rock change, and the volume of the fluid sucked into the core is equal to the volume of the fluid displaced out of the core.
The invention starts from the flowing rule of gas molecules and water molecules in the nano holes of the shale, and researches the micro flowing mechanism in the nano holes and the self-absorption phenomenon of the nano holes of the shale. Based on a molecular dynamics simulation method, the dynamic process that methane gas in the nanopores in the shale clay minerals is displaced by water is simulated, and the flowing rule of the gas and the water in the nanopores is theoretically researched.
Fig. 2 shows the movement law of methane molecules and water molecules in nanopores under the action of molecular force. The molecular simulator firstly establishes kaolinite pores of 2nm and illite pore frameworks of 5nm, then arranges methane molecules into the pores, arranges water molecules into a larger crack space, and then simulates the motion rule of the water molecules and the methane molecules under the action of molecular force without the action of external force. The molecular simulation result shows that water molecules can enter the nano pores of the shale from the cracks to displace gas. After the gas is exhausted, the water in the micro-nano pores is difficult to discharge back. This is because the clay mineral surface is easily charged with a positively charged ion layer, and water molecules having a polarity and a strong dipole moment have a higher affinity with the clay mineral than non-polar methane molecules having a zero dipole moment. The water forms a water film on the round or square nanometer pore surface of the clay mineral, and the gas flows in the channel formed by the water film. The smaller the pores, the greater the proportion of gas that is displaced. Gas in illite is displaced more by water than kaolinite. The reason is that since the I Li Danfu contains potassium ions, the surface layer is charged and more hydrophilic. The molecular dynamics simulation result explains the phenomenon that water is absorbed by a shale stratum and shale gas is displaced out of pores, so that the water backflow rate is low, and the self-absorption phenomenon of a shale reservoir is shown.
Method for determining flow and self-absorption in nano-pore by nuclear magnetic resonance experiment
In order to verify the flow rule of water molecules in nanopores simulated by molecular dynamics, a method for determining the flow of the water and the gas in the nanopores by using a nuclear magnetic resonance instrument is designed, and the purpose is to know how the water molecules displace methane molecules in the nanopores under the action of molecular force in a shale gas reservoir without any external force, and determine the total volume of substitute gas entering the pores, wherein the volume can be approximately equal to the self-absorption capacity of a shale core.
The principle of application is as follows: the gas has no signal when the nuclear magnetic resonance instrument is used for measurement, so that the T obtained by testing a rock core containing shale gas obtained from a shale reservoir 2 The profile can be considered as a base value; then, the rock core is immersed in water, the water is used for displacing gas in the shale pores under the condition of no external force, the measurement time is set, and the T of the rock core is measured by a nuclear magnetic resonance instrument periodically 2 The atlas obtains the rock core pore change condition, observes the change rule of water displacement gas in-process pore, and after the experiment was enough long-time, guaranteed that water has replaced the gas in the pore completely, the pore volume that records this moment is that shale rock core takes place from inhaling the back, and water invades the total pore volume that accounts for, can observe that water preferentially gets into in the pore of which scope simultaneously.
(IV) Nuclear magnetic resonance Experimental procedures
As shown in fig. 7, an embodiment of the method of the present invention is as follows:
[ EXAMPLES one ]
The specific experimental procedure is as follows:
(1) Selecting an experimental core, and measuring the initial weight: selecting a core with the diameter of about 25mm in the shale gas reservoir, and weighing the weight of the core by using a high-precision electronic balance to obtain the initial weight of the core which is not immersed in water.
(2) Measurement of the substrate value: testing the rock core by using a nuclear magnetic resonance instrument to obtain the T of the rock core 2 Map, the T 2 The spectra are the base values for cores that were not immersed in water. Relevant parameters adopted in the nuclear magnetic resonance test in the invention are as follows: the waiting time Tw =3s, the echo interval TE =0.1ms, the number of echoes NECH =6000, and the number of scans n =64.
(3) The core is immersed in water, so that the core can perform self-absorption, namely, a self-absorption experiment is started, and the nuclear magnetic resonance detection is continuously performed in the immersion process until the self-absorption experiment is finished, wherein the self-absorption experiment comprises the following specific steps:
placing the core in a container filled with clear water to ensure that the core is completely immersed in the water, setting a test time, taking out the core at each test time as shown in table 1, wiping off the water on the surface of the core, weighing the weight of the core, and detecting T by using a nuclear magnetic resonance instrument 2 And (4) spectrum, after each measurement is finished, putting the core back into the container and continuously soaking in water, and taking out the core for testing when the next testing time is up.
The time for nuclear magnetic resonance detection of the shale core is set as shown in table 1, and the self-absorption amount of the shale core can be obtained after about 1 month of detection.
| Interval of time | Number of measurements | Number of days | Interval of time | Number of measurements | Number of |
| 2 hours (h) | 12 times of | 1 |
4 hours | 6 times of | 1 day |
| 8 hours | 6 times of | 2 days | 12 |
4 times (twice) | 2 |
| 24 |
4 times (twice) | 4 days | 48 |
4 times (twice) | 8 days |
| 120 |
3 times of | 15 days |
TABLE 1
(4) Before each nuclear magnetic resonance detection, firstly wiping off water on the surface of the rock core, then weighing the weight of the rock core, subtracting the initial weight of the rock core from the weight of the rock core to obtain the self-absorption weight of the water, and then carrying out the nuclear magnetic resonance detection to obtain the T after each immersion in the water 2 Map, T obtained from each test 2 The map is plotted on a graph, as shown in FIG. 3, each curve in FIG. 3 representing T at one detection time point 2 And (4) mapping.
(5) T obtained by detection 2 And (3) obtaining a core pore distribution diagram by spectrum calculation: and (3) calculating corresponding pore distribution by using the formula (2), so that the shale pores with certain sizes into which water firstly enters can be seen, the micro-flow rule of water-flooding displacement gas is obtained, the molecular dynamics simulation result is verified, and meanwhile, the self-absorption amount of the shale core can be obtained according to the final pore volume. The method comprises the following specific steps:
calculating radius r =2 ρ using equation (2) 2 T 2 Where coefficient of relaxation intensity p 2 Get 10,T 2 Values of (c) are obtained from the abscissa in the right hand graph of fig. 1, as follows: reading T in the range of small aperture 2 Radius r =2 × 10 × 0.2=4nm when =0.2, and T is read in the macropore range 2 When =200, the radius r =2 × 10 × 200=4000nm.
The pore diameter ratio is the ratio of each pore diameter (namely the pore radius) calculated, and the total pore volume can be obtained by utilizing the nuclear magnetic resonance experiment (the nuclear magnetic resonance instrument is used for carrying out the experiment, and the experimental result is provided with the T of each test 2 The sum of the peak areas of the maps, which is the total pore volume, is not described in detail herein for conventional methods. ) The pore size ratio for each pore size is obtained by dividing the nuclear magnetic signal amplitude for each pore size by the total pore volume. Fig. 5 is the ratio of pore diameters at each pore diameter thus calculated, wherein the ordinate is the ratio of pore diameters in units of%, and the abscissa is the pore radius in units of μm. FIG. 5 is the pore size of the core when not immersed in waterThe ratio of the components is.
Further, after plotting the historical pore distribution (fig. 6 is the ratio of pores from which the basis value has been subtracted, and thus the ratio of pores in fig. 6 is much smaller), as shown in fig. 6, it can be seen from fig. 6 that over time the ratio of pores of different sizes changes, peaking at the smaller pore locations initially, and peaking at the larger pores later, indicating that during micro-flow, water is entering the smaller pores first, then gradually entering the larger pores, and the second peaking gradually appearing, indicating that water is also gradually entering the larger pores. Therefore, the flowing rule of water in nanometer and micron pores is observed, and the result of molecular simulation is verified.
And after the core self-priming experiment is finished, weighing the obtained weight of the core, and subtracting the initial weight of the core from the weight to obtain the weight of water for self-priming of the core. The volume change is the pore volume obtained after the self-priming experiment of the core is finished, and the pore volume tested by the substrate is subtracted, so that the self-priming volume of the core is obtained.
Specifically, the method for obtaining the self-absorption pore volume of the core comprises the following steps: the weight of the core before immersion in water and the weight of the core after saturation (i.e., the weight after the last immersion in water and the weight after taking out the water outside the core and wiping it off) were weighed using a high-precision electronic balance with an accuracy of 0.0001g, and the weight of the obtained water was self-priming and the density of the water was 1g/cm 3 From this, the total volume for self-priming can be calculated.
In conclusion, the method of the present invention utilizes the above experimental method to obtain the micro flow law and the self-priming volume.
The examples to which the invention is applied are as follows:
[ example two ]
The application of the invention is illustrated by taking the analysis of Fuling shale core as an example. The procedure of the experiment was as follows:
(1) A standard core t-7 from the Fuling area, 25mm in diameter and 30mm in length, was taken and found to have an initial weight of 59.0554g.
(2) Putting the core into a glass bottle, and applying MacroMR12-150H-GPerforming T by using a dimensional nuclear magnetic resonance analyzer 2 And (3) performing spectrum test, wherein parameters adopted by the test are shown in table 2, and the T of the rock core is obtained through the test 2 The spectra are shown in FIG. 4 as the basis values for the experiments.
TABLE 2
(3) Calculating the corresponding pore size distribution using equation (2) wherein the relaxation intensity coefficient ρ 2 Taking 10, drawing a pore diameter distribution diagram according to a calculation result, wherein the abscissa is the radius of the pores and is calculated by using a formula (2), the ordinate is the ratio of pore diameters (namely the pore diameter distribution), and the ratio of pore diameters is calculated by using the following formula:
pore diameter ratio = nuclear magnetic signal amplitude/total pore volume of pore diameter
The total pore volume can be obtained by NMR experiment, and the ratio of pore diameter of each pore diameter is obtained by dividing the NMR signal amplitude obtained by measurement of each pore diameter by the total pore volume (the sum of the NMR signal amplitudes obtained by NMR instrument test, i.e. T plotted 2 Sum of areas of peaks in the map) as shown in the above formula. Fig. 5 is a calculated ratio of pore diameters at each pore radius in the core before flooding, i.e., pore size distribution, where the abscissa is the pore diameter in microns um.
The core is immersed in water to enable the core to generate self-absorption effect, and nuclear magnetic resonance detection is continuously carried out in the immersion process, wherein the detection time is selected as shown in table 3, 30 days are tested totally, and the test is carried out for 20 times.
(4) Nuclear magnetic T of each test 2 And converting the spectrum into the pore diameter of the core, calculating the pore volume occupied by each pore diameter, subtracting the substrate proportion, and drawing a distribution diagram of the pore diameter of the core in the self-absorption process, wherein the distribution diagram is shown in fig. 6. The method comprises the following specific steps:
in the calculation of the pore size ratio, the pore size ratio of each pore size is obtained by dividing the amplitude of the nuclear magnetic signal obtained by measuring each pore size by the total pore volume of the test minus the total pore volume of the substrate, as shown in the following formula:
pore size ratio = nuclear magnetic signal amplitude of pore size/(total pore volume-substrate total pore volume)
When the core is not immersed in water, the nuclear magnetic resonance signals the pore framework, and when no water is introduced into the pores, the signals are taken as the base values. When the core is immersed in water, the water slowly enters the pores of the rock, after the core is immersed for a period of time, the nuclear magnetic resonance test is carried out on the rock, the measured signal is calculated by using a first formula, the sum of the signal of the former rock skeleton and the signal of the immersed water is obtained, and when the sum is calculated by using a second formula, the base value is subtracted, so that only the value of the entered water is left, and the pore size range of which the water preferentially enters is shown.
After the previous plot of pore size fraction is plotted as shown in fig. 6, it can be seen that the pore size fraction for different pore sizes changes over time, with the initial peak at the smaller pore location and the subsequent peak of the curve moving slowly towards the larger pore, indicating that during the micro-flow process, water first enters the small pores and then gradually enters the larger pores, and the second peak gradually appears indicating that water also gradually enters the larger pores. Therefore, the flowing rule of water in nanometer and micron pores is observed, and the result of molecular simulation is verified.
And subtracting the initial weight of the core from the finally weighed weight of the core self-priming experiment to obtain the weight of the water self-priming of the core. And subtracting the pore volume tested by the substrate from the pore volume finally obtained in the core self-priming experiment to obtain the self-priming volume of the core. In this example, the weight of the core before water immersion is 59.0554g, the weight of the core after 720 hours of water immersion is 60.1802g, and the weight of the self-priming water is 1.1248g, since the density of water is 1g/cm 3 From this, the total pore volume of self-priming is calculated to be 1.1248cm 3 。
In this example, the core weight is measured once for each measurement of nmr, because the measurements are not normative and because the time intervals are short, there is an error. But the last measurement with the substrate value is measured by using a high-precision electronic balance, the sucked water quantity is large, and the accuracy and precision of the measurement are enough, so the self-sucking weight and the pore volume calculated by the method are reliable.
| Time of |
2 |
4 hours | 8 hours | 12 |
16 |
24 hours | 36 hours |
| Core weight g | 59.0554 | 59.1826 | 59.2455 | 59.3193 | 59.4268 | 59.5182 | 59.6055 |
| Time of measurement | 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 |
| Time of measurement | 336 hours | 384 hours | 432 hours | 480 |
600 hours | 720 hours | |
| Core weight g | 60.254 | 60.2836 | 60.3203 | 60.291 | 60.3237 | 60.1802 |
TABLE 3
The invention is based on a molecular dynamics simulation method, simulates the dynamic process that methane gas in nanopores in a shale clay mineral is displaced by water, describes the self-absorption phenomenon of the nanopores of the shale from a micro-flow mechanism, and regularly measures the T of a core immersed in water by applying a nuclear magnetic resonance instrument 2 And the spectrum quantitatively determines the sequence of water entering the nano-pores and displacing gas and the self-absorption amount of the shale core, obtains the change rule of the water displacing gas in the self-absorption process of the core, and finally measures the total pore volume occupied by water after the core is subjected to self-absorption. The method can quantitatively calculate the self-absorption amount of the rock core, and perform timing test through a nuclear magnetic resonance instrument to observe the dynamic process of water displacement gas and obtain the sequence of water entering the shale pores.
The invention can be applied to nuclear magnetic resonance experiments of any shale core, has important functions for researching the self-absorption rule and the flowback rule of the shale reservoir and evaluating the productivity and the economic benefit of the shale gas well, and also has wide application prospect.
Finally, it should be noted that the above-mentioned technical solution is only one embodiment of the present invention, and it will be apparent to those skilled in the art that various modifications and variations can be easily made based on the application method and principle of the present invention disclosed herein, and the method is not limited to the method described in the above-mentioned embodiment of the present invention, so that the above-mentioned embodiment is only preferred and not restrictive.
Claims (11)
1. A method for determining self-absorption rule in nanometer pore by utilizing nuclear magnetic resonance instrument is characterized in that: the method is based on molecular dynamics to simulate the flow rule and self-absorption rule of gas in the nano-pores, and a nuclear magnetic resonance instrument is applied to quantitatively determine the pore volume of displacement gas entering the nano-pores and the self-absorption amount of the shale core.
2. The method for determining self-absorption law in nanopores according to claim 1 using a nuclear magnetic resonance instrument, wherein: the method comprises the following steps:
(1) Selecting an experimental core, and measuring the initial weight of the core;
(2) Measuring a base value of the core;
(3) Immersing the core into water, starting a core self-absorption experiment, and detecting the core for multiple times, wherein the weight and T of the core are obtained in each detection 2 Mapping until the self-absorption experiment of the rock core is finished;
(4) Acquiring the weight of the water self-absorption obtained by each detection and drawing the total T 2 A map;
(5) Determining a self-absorption rule in the nano pores;
(6) And obtaining the pore volume of the water invading nano-pore displacement gas and the self-absorption amount of the shale core.
3. The method for determining self-absorption law in nanopores according to claim 2 using a nuclear magnetic resonance instrument, wherein: the operation of the step (2) comprises the following steps:
detecting the rock core by using a nuclear magnetic resonance instrument to obtain the T of the rock core 2 Map, the T 2 And the atlas is the base value of the rock core.
4. The method for determining self-absorption law in nanopores according to claim 3 using a nuclear magnetic resonance instrument, wherein: the operation of the step (3) comprises the following steps:
placing the core in a container filled with clear water to ensure that the core is completely immersed in water, and setting test time;
taking out the core at each test time, wiping off water on the surface of the core, weighing the weight of the core, and detecting the core by using a nuclear magnetic resonance instrument to obtain the T of the core 2 A map;
and after each measurement is finished, putting the core back into the container to be continuously soaked in the water, and taking out the core for detection when the next test time is up.
5. The method for determining self-absorption law in nanopores by using NMR instrument according to claim 4, wherein: the operation of the step (4) comprises the following steps:
subtracting the initial weight from the weight of the core obtained by each detection to obtain the self-absorption weight of water;
t to be obtained for each test 2 Mapping on a chart to obtain total T 2 And (4) mapping.
6. The method for determining self-absorption law in nanopores according to claim 5 using a nuclear magnetic resonance instrument, wherein: the operation of the step (5) comprises the following steps:
(51) T obtained by detection 2 And (3) obtaining the aperture ratio by spectrum calculation, and drawing an aperture ratio graph:
(52) And determining the self-absorption rule in the nano pores.
7. The method of claim 6, wherein the method comprises the steps of: the operation of step (51) comprises:
the pore size r of the pores is calculated using the formula:
r＝2ρ 2 T 2
ρ 2 is the relaxation intensity coefficient;
the pore size ratio for each pore size was calculated using the following formula:
pore size ratio = nuclear magnetic signal amplitude of pore size/(total pore volume-substrate total pore volume)
Wherein, the total pore volume is T obtained by the detection 2 Sum of peak areas of the maps;
the substrate total pore volume is the sum of the peak areas in the substrate values;
and drawing an aperture ratio graph on a coordinate graph with the abscissa as the aperture and the ordinate as the aperture ratio.
8. The method for determining self-absorption law in nanopores according to claim 7 using a nuclear magnetic resonance instrument, wherein: the operation of step (52) comprises:
and drawing a pore diameter ratio graph obtained by each detection on a graph, and obtaining a self-absorption rule in the nano pores from the graph.
9. The method for determining self-absorption in nanopores according to claim 8 using a nuclear magnetic resonance instrument, wherein: the operation of the step (6) comprises the following steps:
after the self-absorption experiment of the rock core is finished, weighing to obtain the weight of the rock core, and subtracting the initial weight from the weight to obtain the self-absorption amount of the shale rock core;
multiplying the self-absorption of the shale core by the density of the water to obtain the pore volume of the water entering the nanopores to displace the gas.
10. The method for determining self-absorption law in nanopores by using nmr apparatus according to claim 9, wherein: in the step (1) and the step (6), the initial weight and the weight of the core after the saturated water are obtained by weighing respectively by using a high-precision electronic balance with the precision of 0.0001 g.
11. Use of a method according to any one of claims 1 to 10 in shale gas exploration.
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