Method and system for evaluating production efficiency and production lower limit of mixed-phase fluid in tight reservoir

Abstract

The invention relates to a method and a system for evaluating the utilization efficiency and the utilization lower limit of a mixed-phase fluid in a tight reservoir, and belongs to the field of unconventional oil-gas exploration and development. Comprising the following steps: 1, establishing mixed-phase fluid rock core samples containing different oil and water saturations based on nuclear magnetic detection of saturated and centrifuged rock core samples; 2, determining nuclear magnetic spectrum change characteristics of fluids with different properties before and after exhaustion based on 2D nuclear magnetic monitoring in a fluid energization-exhaustion mode; 3, respectively extracting nuclear magnetic spectrums of fluids with different properties before and after the elastic energy is released, and calibrating volume change characteristics of the fluids with different properties; 4, evaluating the utilization efficiency of the fluids with different properties based on an experimental result, and establishing a quantitative model of the utilization efficiency of the fluids with different properties in the tight reservoir; and 5, determining the saturation lower limit and the physical property lower limit of the fluid with different properties in the energy failure mode. According to the method, the relationship between the elastic energy oil discharge efficiency / water discharge efficiency of the saturated mixed-phase fluid sample and the reservoir physical property and the oil saturation / water saturation is determined.

Description

Technical Field

[0001] The invention relates to a method and system for evaluating the production efficiency and production lower limit of a mixed phase fluid in a tight reservoir, and belongs to the field of unconventional oil and gas exploration and development. Background Art

[0002] With the continuous advancement of oil and gas exploration and development technology and the increasing demand for energy, the goal of global oil and gas exploration and development has gradually shifted from conventional structural trap oil and gas reservoirs to tight oil and gas and shale oil and gas in unconventional fields, making tight reservoirs a new focus in oil and gas geological research. In recent years, domestic and foreign scholars have conducted more extensive and in-depth research on the storage space of tight reservoirs, and have realized that the large number of micro-nano pores developed in tight reservoirs are important storage spaces. There are two main methods for characterizing micro-nano pores: direct observation and fluid injection. Direct observation methods include optical microscopes and scanning electron microscopes. The main difference is the magnification and field size of the observation results. The former has a low magnification and a wider field of view, while the latter has a high magnification and can see nanopores, but the field of view is very small and lacks representativeness. Fluid injection methods include high-pressure / constant-rate mercury injection, nitrogen adsorption, nuclear magnetic resonance and other methods. The main difference between these methods is the range of pore sizes that can be characterized. The mercury injection method can characterize pores of 10nm and above, but it will damage the sample. The nitrogen adsorption method is only advantageous in characterizing pores at the level of a few nanometers to tens of nanometers. The nuclear magnetic resonance method can characterize pores of all sizes, but it needs to be converted through formulas to obtain pore information.

[0003] In the field of oil and gas development, nuclear magnetic resonance methods mostly obtain information about hydrogen-containing substances in rocks through the disturbance of hydrogen nuclei. Hydrogen atoms generally exist in pore fluids, and can directly measure information about pore fluids. It does not require damage to the core during measurement, so it is a non-destructive measurement technology that is widely used in reservoir interpretation and evaluation. In downhole logging and laboratory research, nuclear magnetic resonance has been proven to be effective in characterizing the composition of rock pore fluids. It is mainly used to measure total porosity, quantify kerogen volume, identify fluids such as water or hydrocarbons, and determine closed environments such as organic kerogen pores or inorganic mineral pores, thereby determining the volume of bound fluids and movable fluids, and evaluating reservoir fluid saturation and oil content in oil and gas reservoirs.

[0004] At present, although the one-dimensional nuclear magnetic resonance method with T2 spectrum as output has the above-mentioned advantages of accuracy, non-destructiveness and rapidity, the T2 spectrum alone cannot confirm the properties of pore fluids, such as the difference between oil and water. Two-dimensional nuclear magnetic resonance obtains a two-dimensional T1-T2 spectrum by simultaneously measuring the transverse relaxation time (T2) and longitudinal relaxation time (T1) properties of the fluid in the pores. Based on the differences in T1 and T2 of different fluids, the distribution position of the fluid on the two-dimensional spectrum can relatively effectively identify and quantitatively evaluate shale reservoir fluids. Dividing the two-dimensional T1-T2 spectrum into several regions can correspond to the identification of fluids of different properties. Combined with other experiments, it can achieve quantitative characterization of the changes of mixed phase fluids in tight reservoirs under development conditions. Summary of the invention

[0005] In order to solve the current problems faced in the tight oil depletion development mode, the present invention provides a method for quantitatively characterizing the efficiency of each phase of fluid production by combining 2D nuclear magnetic resonance with the process of fluid drainage of oil-water miscible tight reservoirs under formation conditions through indoor experiments. It is mainly based on 2D nuclear magnetic resonance to distinguish the types of miscible fluids and calculate the volume. Through the relationship between porosity, permeability, oil saturation, water saturation and depletion development efficiency, the lower limit of physical properties and oil / water content of each phase of fluid production in the elastic energy depletion mode is determined, thereby establishing a method for evaluating the efficiency and lower limit of miscible fluid production in tight reservoirs.

[0006] The present invention also provides a system for evaluating the production efficiency and production lower limit of mixed phase fluid in tight reservoirs.

[0007] The technical solution of the present invention is: Evaluation methods for the production efficiency and lower limit of miscible fluid in tight reservoirs include: Step 1: Based on the nuclear magnetic resonance detection of saturated and centrifuged core samples, establish core samples of mixed fluids with different oil and water saturations; Step 2: Based on 2D NMR monitoring in the fluid energization-depletion mode, determine the NMR spectrum change characteristics of fluids with different properties before and after depletion; Step 3: Extract the NMR spectra of fluids with different properties before and after the elastic energy is released, and calibrate the volume change characteristics of fluids with different properties; Step 4: Based on the experimental results, evaluate the production efficiency of fluids with different properties and establish a quantitative model for the production efficiency of fluids with different properties in tight reservoirs; Step 5: Based on the experimental results and the quantitative model of the utilization efficiency of fluids with different properties in tight reservoirs, determine the lower limits of saturation and physical properties for the utilization of fluids with different properties under the energy depletion model.

[0008] Furthermore, in step 1, centrifugation and saturation-NMR combined testing are carried out to establish plunger core samples containing different bound water contents, the lithological samples containing bound water are further saturated with crude oil to establish core samples containing different oil and water saturations, and 2D NMR testing is carried out.

[0009] Furthermore, in step 2, using the core samples of mixed fluids with different oil and water saturations established in step 1, an elastic energy depletion nuclear magnetic resonance measurement experiment is carried out, including: Step 2.1: Place the core samples containing mixed fluids with different oil and water saturations into the high-pressure displacement experimental device and apply confining pressure. Both ends of the core are sealed with plugs with flow guide tubes. Step 2.2: Fill the flow tubes connected to the two ends of the core column with fluorine liquid until the high-pressure pump system shows a pore pressure of 32-38 MPa, and implement online NMR monitoring until the morphology of the NMR 1D NMR spectrum is stable. Close the valves at both ends to cut off the fluid in the flow tube, and stew the core sample for 3-48 hours; Step 2.3: Open the valve of the fluid injection system to release the fluid, simulate the elastic energy exhaustion and discharge, and implement 2D NMR online monitoring until the NMR signal is stable and unchanged, and the discharge process is completed. Record the 2D NMR T1-T2 spectrum and NMR signal in this state, which are recorded as the characteristics after elastic energy exhaustion; Step 2.4: Compare the NMR spectra before and after elastic energy exhaustion, including the T1-T2 range of the 2D NMR signal distribution and the total signal volume before and after exhaustion, and the change in the total oil and water signal volume.

[0010] Furthermore, in step 3, based on the experimental results of step 2, the extraction of oil and water nuclear magnetic signals and the calculation of oil and water volumes before and after elastic energy release are realized, including: Step 3.1: Based on 2D NMR T 1- T 2. The distribution position of oil and water in the spectrum, extract the oil and water signals in the 2D NMR T1-T2 spectrum of the experimental sample before and after elastic energy failure drainage; Step 3.2: Prepare oil and water samples of different volumes, and use 2D NMR monitoring to obtain the total signals of oil and water of different volumes. The conversion formulas of oil and water volume and NMR signal are as follows: (1); (2); in, , They are the oil and water NMR signals extracted from 2D NMR, in units of au; , are the volumes of oil and water respectively, in ml;

[0011] Step 3.3: According to the relationship between the nuclear magnetic resonance signals of oil and water of different volumes, i.e., equation (1) and equation (2), the nuclear magnetic resonance signals of oil and water before and after elastic energy exhaustion drainage are converted into the volumes of oil and water.

[0012] Furthermore, in step 4, based on the volumes of oil and water before and after the elastic energy failure drainage calculated in step 3, the production efficiency of oil and water is calculated, the relationship between the production efficiency and the oil content and reservoir physical properties is analyzed, and a quantitative evaluation model for fluid production is established; including: Step 4.1: Based on the changes in the oil and water volumes before and after the elastic energy failure drainage, the oil and water utilization efficiency of each core sample during elastic energy drainage at different oil saturations is calculated using the following formula: (3); (4); in, is the volume of crude oil before discharge due to elastic energy failure, ml; is the volume of crude oil after elastic energy exhaustion and drainage, ml; is the utilization efficiency of crude oil after the elastic energy exhaustion and drainage, %; is the volume of water before elastic energy failure and drainage, ml; is the volume of water after the elastic energy is exhausted and the fluid is discharged, ml; is the water utilization efficiency after elastic energy exhaustion and drainage, %; Step 4.2: Regress the production efficiency of samples at different oil saturations with the physical properties of the samples. The results show the relationship between the production efficiency of oil and water and the physical properties of the samples in tight reservoirs at different oil saturations. They are all logarithmically positively correlated, as shown in formula (5): (5); In the formula, is the tight sandstone fluid production efficiency, %, k is the core sample permeability, mD; is the porosity of the core sample, %; regression factors A and B are linearly correlated, so it can be obtained from formula (5): (6); (7); Where: and are the production efficiency of tight sandstone oil and water, respectively; and is the regression factor corresponding to oil and water, dimensionless; Step 4.3: Fit the regression factor A with the oil saturation. The results are as follows: (8); (9); Where: is the oil saturation in the sample, %; Step 4.4: Substitute equations (8) and (9) into equations (6) and (7) respectively to obtain the quantitative model of the utilization efficiency of fluids with different properties in tight reservoirs, as shown in equations (10) and (11): (10); Similarly, the water utilization efficiency in tight sandstone is related to oil saturation. The relationship is: (11).

[0013] Further, in step 5, according to the oil-water producing efficiency model in step 4, the lower limits of physical properties and oil content are obtained by calculating the zero point of the producing efficiency model curve at each oil saturation; including: Step 5.1: According to formula (9), when =21.55 or =0.0057, =0, which is the lower limit of oil content and physical property for oil production in tight sandstone. The production lower limit includes the lower limit of oil content and physical property. Step 5.2: According to formula (10), when =76.17 or =0.0073, =0, which is the lower limit of oil content and physical property for water mobilization in tight sandstone.

[0014] The evaluation system of the efficiency and lower limit of miscible fluid production in tight reservoirs includes: The miscible fluid core sample establishment module is configured to: establish miscible fluid core samples containing different oil and water saturations based on centrifugation and saturation-NMR measurement; The change feature acquisition module is configured to: determine the change features of the nuclear magnetic spectrum of fluids with different properties before and after exhaustion based on 2D nuclear magnetic monitoring in the fluid energy enhancement-exhaustion mode; extract the nuclear magnetic spectrum of fluids with different properties before and after the elastic energy release, and calibrate the volume change features of fluids with different properties; The module for establishing a quantitative model of the production efficiency of fluids of different properties in tight reservoirs is configured to: evaluate the production efficiency of fluids of different properties based on experimental results, and establish a quantitative model of the production efficiency of fluids of different properties in tight reservoirs; The module for determining the lower limit of saturation and the lower limit of physical property is configured to determine the lower limit of saturation and the lower limit of physical property for the production of fluids of different properties under the energy depletion mode based on experimental results and a quantitative model for the production efficiency of fluids of different properties in tight reservoirs.

[0015] The beneficial effects of the present invention are: The present invention realizes the determination of the relationship between the elastic energy oil drainage efficiency / drainage efficiency of saturated mixed phase fluid samples and reservoir physical properties, oil saturation / water saturation. The physical property lower limit of oil mobilization, the oil saturation lower limit and the water saturation lower limit in the tight reservoir of the study area are determined. The method and results are highly reliable, well-founded and easy to operate, and have important guiding significance for the exploration and development of tight oil. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 This is a comparison diagram of the #20 tight sandstone sample in the 1D NMR monitoring, the saturated water state and the 30% bound water state. Figure 2 This is a schematic diagram comparing the 30% bound water state and the 30% water + 70% oil state of the #20 tight sandstone sample in 1D NMR monitoring.

[0017] Figure 3 This is the 2D NMR T1-T2 spectrum of the #20 tight sandstone sample in a water-saturated state.

[0018] Figure 4 This is the 2D NMR T1-T2 spectrum of the #20 tight sandstone sample in the 30% bound water state.

[0019] Figure 5 This is the 2D NMR T1-T2 spectrum of the #20 tight sandstone sample in the state of 30% water + 70% oil.

[0020] Figure 6 This is a schematic diagram of the high-pressure displacement experimental device in the elastic energy depletion nuclear magnetic resonance measurement experiment and its connection with the nuclear magnetic resonance device.

[0021] Figure 7 It is 2D NMR T 1- T 2 Schematic diagram of the fluid identification plate of the spectrum.

[0022] Figure 8 This is the T1-T2 spectrum of tight sandstone sample #20 before elastic energy exhaustion at oil saturation of 70%.

[0023] Fig. 9 This is the T1-T2 spectrum of tight sandstone sample #20 after elastic energy exhaustion at oil saturation of 70%.

[0024] Fig.10 This is the T1-T2 spectrum of tight sandstone sample #20 before elastic energy exhaustion at oil saturation of 55%.

[0025] Fig.11 This is the T1-T2 spectrum of tight sandstone sample #20 after elastic energy exhaustion at oil saturation of 55%.

[0026] Fig.12 This is the T1-T2 spectrum of tight sandstone sample #20 before elastic energy exhaustion at oil saturation of 40%.

[0027] Fig.13 This is the T1-T2 spectrum of tight sandstone sample #20 after elastic energy exhaustion at oil saturation of 40%.

[0028] Fig.14 This is the T1-T2 spectrum of tight sandstone sample #20 before elastic energy exhaustion at oil saturation of 25%.

[0029] Fig.15 This is the T1-T2 spectrum of tight sandstone sample #20 after elastic energy exhaustion at an oil saturation of 25%.

[0030] Fig.16 This is the T2 spectrum of the tight sandstone sample #20 before and after the elastic energy exhaustion at the oil saturation of 70%.

[0031] Fig.17 This is the T2 spectrum of water in tight sandstone sample #20 before and after elastic energy exhaustion at 70% oil saturation.

[0032] Fig.18 This is the T2 spectrum of the tight sandstone sample #20 before and after the elastic energy exhaustion at the oil saturation of 55%.

[0033] Fig.19 This is the T2 spectrum of water in tight sandstone sample #20 before and after elastic energy exhaustion at 55% oil saturation.

[0034] Fig. 20 This is the T2 spectrum of the tight sandstone sample #20 before and after the elastic energy exhaustion at the oil saturation of 40%.

[0035] Fig.21 This is the T2 spectrum of water in tight sandstone sample #20 before and after elastic energy exhaustion at 40% oil saturation.

[0036] Fig. 22 This is the T2 spectrum of the tight sandstone sample #20 before and after the elastic energy exhaustion at the oil saturation of 25%.

[0037] Fig.23 This is the T2 spectrum of water in tight sandstone sample #20 before and after elastic energy exhaustion at 25% oil saturation.

[0038] Fig.24 This is a schematic diagram of the fitting relationship between the total 2D NMR signal and the fluid volume. It is used to calculate the oil-water volume-2D NMR signal conversion coefficient.

[0039] Fig.25 is the permeability of the sample per unit porosity under different oil content conditions ( ) and the fitting curve of oil utilization efficiency in pores and the corresponding relationship.

[0040] Fig.26 is the permeability of the sample per unit porosity under different oil content conditions ( ) and the fitting curve and corresponding relationship between the water utilization efficiency in the pores.

[0041] Fig. 27 is the regression parameter in equations (6) and (7) A and B The fitting relationship.

[0042] Fig.28 is the regression parameter of oil and water in equations (8) and (9) A Relationship with oil saturation.

[0043] Fig.29 It is a flow chart of the method for evaluating the utilization efficiency and utilization lower limit of a miscible fluid in a tight reservoir according to the present invention. DETAILED DESCRIPTION

[0044] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0045] Example 1 Methods for evaluating the efficiency and lower limit of miscible fluid production in tight reservoirs, such as Fig.29 As shown, including: Step 1: Based on the nuclear magnetic resonance detection of saturated and centrifuged core samples, establish core samples of mixed fluids with different oil and water saturations; Step 2: Based on 2D NMR monitoring in the fluid energization-depletion mode, determine the NMR spectrum change characteristics of fluids with different properties before and after depletion; Step 3: Extract the NMR spectra of fluids with different properties before and after the elastic energy is released, and calibrate the volume change characteristics of fluids with different properties; Step 4: Based on the experimental results, evaluate the production efficiency of fluids with different properties and establish a quantitative model for the production efficiency of fluids with different properties in tight reservoirs; Step 5: Based on the experimental results and the quantitative model of the utilization efficiency of fluids with different properties in tight reservoirs, determine the lower limits of saturation and physical properties for the utilization of fluids with different properties under the energy depletion model.

[0046] The present invention is based on indoor fluid mobility simulation-nuclear magnetic resonance joint measurement experiments to establish the utilization characteristics of crude oil and pore water of core samples with different physical properties at different oil / water saturations, to establish a quantitative evaluation model of crude oil and pore water with respect to reservoir physical properties and oil saturation, to determine the physical property lower limit and oil content lower limit of oil and water utilization, and to serve the evaluation of sweet spots and production capacity assessment of tight oil reservoirs.

[0047] The present invention mainly proposes a quantitative characterization method for the utilization characteristics of miscible fluids in tight reservoirs under the depletion development mode based on 2D nuclear magnetic resonance, so as to determine the utilization characteristics of miscible fluids in tight reservoirs, and then quantitatively characterize the fluid mobility of oil / water in tight reservoirs, thereby determining the physical properties and oil-containing utilization limits of oil and water in tight reservoirs, and helping to optimize the tight oil geological sweet spot area.

[0048] Example 2 The difference between the method for evaluating the utilization efficiency and utilization lower limit of a miscible fluid in a tight reservoir described in Example 1 is that: At 25℃ and normal pressure, the results of water-saturated core nuclear magnetic resonance monitoring show that the saturated water signals of different samples are highly linearly positively correlated with the porosity. Since the samples used are all plunger samples with a diameter of 2.5cm and a length of 5cm, the relationship between porosity and nuclear magnetic resonance signals under 1D nuclear magnetic resonance monitoring, as well as the relationship between pore water volume and nuclear magnetic resonance signals can be calculated. In addition, a fixed volume oil sample was made using a glass bottle and a syringe, and the volume-signal conversion coefficient of oil under 2D nuclear magnetic resonance monitoring can be calculated based on this. The same steps were used to calculate the volume-signal conversion coefficient of water to oil.

[0049] In step 1, centrifugation, saturation-NMR joint testing are carried out to establish plunger core samples containing different bound water (water saturation is 30%, 45%, 60%, and 75%, respectively). The lithology samples containing bound water are further saturated with crude oil to establish core samples with different oil and water saturations, and 2D NMR testing is carried out. Including:

[0050] Step 1.1: Use a vacuum pressurized saturation device to fully saturate the core after oil washing and drying with water. At this time, the water saturation of the core is considered to be 100%. Measure the nuclear magnetic one-dimensional (1D) T2 spectrum at this time ( Figure 1 ) and two-dimensional (2D) T1-T2 spectra and NMR signal volume ( Figure 3 ).

[0051] Step 1.2: Use a high-speed centrifuge to perform dehydration at a fixed time and centrifugal force (e.g., 10,000 rpm, 1 hour), and measure the NMR 1D and 2D NMR spectrum signals after each dehydration.

[0052] Step 1.3: Compare the nuclear magnetic resonance signals of the core after centrifugation and the core under saturated water state. When the total nuclear magnetic resonance signal after a certain centrifugation reaches 30% of the total nuclear magnetic resonance signal under saturated state, it is considered that a core with bound water and a water saturation of 30% has been established ( Figure 1 , Figure 4 ).

[0053] Step 1.4: The sample with 30% water saturation is saturated with oil again using a vacuum pressurized saturation device to obtain a 30% water + 70% oil experimental sample. Measure its 2D NMR T1-T2 spectrum and total signal ( Figure 2 , Figure 5 ).

[0054] Step 1.5: Repeat the above process to create lithology samples with different oil and water saturations, such as 45% water + 55% oil, 60% water + 40% oil, and 75% water + 25% oil. The 2D NMR data under these conditions are recorded as pre-depletion NMR data (e.g. Figure 8 , 10 , 12, 14).

[0055] In step 2, using the core samples of mixed fluids with different oil and water saturations established in step 1, an elastic energy depletion nuclear magnetic resonance measurement experiment is carried out, including: Step 2.1: Place the core samples containing mixed fluids with different oil and water saturations into the holder of the high-pressure displacement experimental device, and apply confining pressure (the confining pressure is 3-5 MPa higher than the pore pressure, so that the core is wrapped by the rubber sleeve and there is no gap around it, and the confining pressure can be transmitted to the core), and the two ends of the core are sealed with plugs with guide tubes; like Figure 6 As shown in the figure, the high-pressure displacement experimental device is an experimental device used to simulate the fluid displacement process under high-pressure environment of underground oil and gas reservoirs. It is mainly used to study the flow law of multiphase fluids (such as oil and water) in porous media (such as rocks) during oil and gas development, displacement efficiency and other key parameters. It includes the following specific devices: high-pressure pump system (provides and controls driving pressure, confining pressure and realizes pressure visualization); core clamp (fixes core samples to simulate the temperature, pressure and fluid environment of the reservoir where the rock is located); fluid injection system (including liquid storage containers and flow guide tubes, which can inject different fluids such as crude oil and water into the core).

[0056] Step 2.2: Fill the flow tube connecting the two ends of the core column with fluorine liquid (fluorine liquid does not contain hydrogen, so the signal of nuclear magnetic detection is still only the signal of oil and water in the core), until the high-pressure pump system shows that the pore pressure is 32~38Mpa (35Mpa) (the pore pressure in the core can be designed according to the actual formation pore pressure in each region), and implement nuclear magnetic resonance online monitoring until the nuclear magnetic 1D nuclear magnetic spectrum morphology is stable, close the valves at both ends to cut off the fluid in the flow tube, and stew the core sample for 3~48h; Nuclear magnetic resonance online monitoring refers to the uninterrupted 1D monitoring by the nuclear magnetic resonance detection device during the whole process from sample installation to the end of the experiment in the elastic performance experiment. The purpose is to select and process the original experimental data according to the changes in the 1D nuclear magnetic spectrum. The process of 2D nuclear magnetic online monitoring is the same, except that the T1-T2 spectrum, that is, the 2D nuclear magnetic spectrum, is used in the process. The processing means include removing noise signals and redrawing, which is a part of the standard processing process of nuclear magnetic resonance data.

[0057] Step 2.3: Open the valve of the fluid injection system to release the fluid, simulate the elastic energy exhaustion and discharge, and implement 2D NMR online monitoring until the NMR signal is stable and unchanged, and the discharge process is completed. Record the 2D NMR T1-T2 spectrum and NMR signal in this state, which are recorded as the characteristics after elastic energy exhaustion; Fig. 9 , Fig.11 , Fig.13 , Fig.15 shown. Fig.10 This is the T1-T2 spectrum of tight sandstone sample #20 before elastic energy exhaustion at oil saturation of 55%. Fig.12 This is the T1-T2 spectrum of tight sandstone sample #20 before elastic energy exhaustion at oil saturation of 40%. Fig.14 This is the T1-T2 spectrum of tight sandstone sample #20 before elastic energy exhaustion at oil saturation of 25%.

[0058] Step 2.4: Compare the NMR spectra before and after elastic energy exhaustion, including the T1-T2 range of the 2D NMR signal distribution and the total signal volume before and after exhaustion, and the total oil-water signal volume change. Fig.16 , Fig.17 , Fig.18 , Fig.19 , Fig. 20 , Fig.21 , Fig. 22 , Fig.23 shown.

[0059] In step 3, based on the experimental results of step 2, the oil and water NMR signals are extracted and the volumes of oil and water before and after the elastic energy is released are calculated, including: Step 3.1: Based on 2D NMR T 1- T 2 Distribution of oil and water in the map ( Figure 7 According to Marc, 2015), the oil and water signals in the 2D NMR T1-T2 spectra of the experimental samples before and after elastic energy failure drainage were extracted ( Figure 6 ); The specific method is based on the plate ( Figure 4 ), the signal values ​​of the oil and water corresponding regions in the 2D NMR spectrum under each state are summed up, and the total signals of oil and water are calculated respectively.

[0060] Step 3.2: Prepare oil and water samples of different volumes (for example, 0.1 ml, 0.2 ml, 0.4 ml, 0.6 ml, and 1.0 ml), and use 2D NMR monitoring to obtain the total signals of oil and water of different volumes. The conversion formulas of oil and water volume and NMR signal are as follows: (1); (2); in, , They are the oil and water NMR signals extracted from 2D NMR, in units of au; , They are the volumes of oil and water respectively, in ml.

[0061] Step 3.3: According to the relationship between the NMR signals of oil and water of different volumes, namely equation (1) and equation (2) ( Fig.24 ), the nuclear magnetic signals of oil and water before and after elastic energy failure drainage are converted into the volumes of oil and water.

[0062] In step 4, based on the volumes of oil and water before and after the elastic energy failure drainage calculated in step 3, the production efficiency of oil and water is calculated, the relationship between the production efficiency and the oil content and reservoir physical properties is analyzed, and a quantitative evaluation model for fluid production is established; including: Step 4.1: Based on the changes in the oil and water volumes before and after the elastic energy failure drainage, the oil and water utilization efficiency of each core sample during elastic energy drainage at different oil saturations is calculated using the following formula: (3); (4); in, is the volume of crude oil before discharge due to elastic energy failure, ml; is the volume of crude oil after elastic energy exhaustion and drainage, ml; is the utilization efficiency of crude oil after the elastic energy exhaustion and drainage, %; is the volume of water before elastic energy failure and drainage, ml; is the volume of water after the elastic energy is exhausted and the fluid is discharged, ml; is the water utilization efficiency after elastic energy exhaustion and drainage, %; Step 4.2: Regress the production efficiency of samples at different oil saturations with the physical properties of the samples. The results show the relationship between the production efficiency of oil and water and the physical properties of the samples in tight reservoirs at different oil saturations. They are all logarithmically positively correlated. The difference in the regression formula of samples under different oil saturation is that the regression factors A and B are different ( Fig.25 , Fig.26 ), its general expression is shown in formula (5): (5); In the formula, is the tight sandstone fluid production efficiency, %, k is the core sample permeability, mD; is the porosity of the core sample, %; regression factors A and B show a good linear correlation ( Fig. 27 ), so from formula (5) we get: (6); (7); Where: and are the production efficiency of tight sandstone oil and water, respectively; and is the regression factor corresponding to oil and water, dimensionless; Step 4.3: Due to the difference in regression factors caused by different oil saturations, regression factor A is fitted with oil saturation ( Fig.28 ), and the results are as follows: (8); (9); Where: is the oil saturation in the sample, %; Step 4.4: Substitute equations (8) and (9) into equations (6) and (7) respectively to obtain the quantitative model of the utilization efficiency of fluids with different properties in tight reservoirs, as shown in equations (10) and (11): (10); Similarly, the water utilization efficiency in tight sandstone is related to oil saturation. The relationship is: (11).

[0063] In step 5, according to the oil-water producing efficiency model in step 4, the lower limits of physical properties and oil content are obtained by calculating the zero point of the producing efficiency model curve at each oil saturation; including: Step 5.1: According to formula (9), when =21.55 or =0.0057, =0, which is the lower limit of oil content and physical property for oil production in tight sandstone. The production lower limit includes the lower limit of oil content and physical property. Step 5.2: According to formula (10), when =76.17 or =0.0073, =0, which is the lower limit of oil content and physical property for water mobilization in tight sandstone.

[0064] In addition, the lower limit obtained in this part only represents the results of this experiment, and its purpose is to obtain the dual variables in the form of formula (9) and formula (10) ( , S o ) model. After the model is derived, the physical properties and oil content lower limits of fluid utilization efficiency can be calculated continuously, which means that all situations can be covered.

[0065] Example 3 The evaluation system of the efficiency and lower limit of miscible fluid production in tight reservoirs includes: The miscible fluid core sample establishment module is configured to: establish miscible fluid core samples containing different oil and water saturations based on centrifugation and saturation-NMR measurement; The change feature acquisition module is configured to: determine the change features of the nuclear magnetic spectrum of fluids with different properties before and after exhaustion based on 2D nuclear magnetic monitoring in the fluid energy enhancement-exhaustion mode; extract the nuclear magnetic spectrum of fluids with different properties before and after the elastic energy release, and calibrate the volume change features of fluids with different properties; The module for establishing a quantitative model of the production efficiency of fluids of different properties in tight reservoirs is configured to: evaluate the production efficiency of fluids of different properties based on experimental results, and establish a quantitative model of the production efficiency of fluids of different properties in tight reservoirs; The module for determining the lower limit of saturation and the lower limit of physical property is configured to determine the lower limit of saturation and the lower limit of physical property for the production of fluids of different properties under the energy depletion mode based on experimental results and a quantitative model for the production efficiency of fluids of different properties in tight reservoirs.

Claims

1. A method for evaluating the production efficiency and production lower limit of miscible fluid in tight reservoirs, characterized in that: include: Step 1: Based on the nuclear magnetic resonance detection of saturated and centrifuged core samples, establish core samples of mixed fluids with different oil and water saturations; Step 2: Based on 2D NMR monitoring in the fluid energization-depletion mode, determine the NMR spectrum change characteristics of fluids with different properties before and after depletion; Step 3: Extract the NMR spectra of fluids with different properties before and after the elastic energy is released, and calibrate the volume change characteristics of fluids with different properties; Step 4: Based on the experimental results, evaluate the production efficiency of fluids with different properties and establish a quantitative model for the production efficiency of fluids with different properties in tight reservoirs; Step 5: Based on the experimental results and the quantitative model of the utilization efficiency of fluids with different properties in tight reservoirs, determine the lower limits of saturation and physical properties for the utilization of fluids with different properties under the energy depletion model.

2. The method for evaluating the efficiency and lower limit of producing a miscible fluid in a tight reservoir according to claim 1, characterized in that: In step 1, centrifugation and saturation-NMR measurement are carried out to establish plunger core samples containing different bound water. The lithological samples containing bound water are further saturated with crude oil to establish core samples containing different oil and water saturations, and 2D NMR detection is carried out.

3. The method for evaluating the efficiency and lower limit of producing a miscible fluid in a tight reservoir according to claim 1, characterized in that: In step 2, using the core samples of mixed fluids with different oil and water saturations established in step 1, an elastic energy depletion nuclear magnetic resonance measurement experiment is carried out, including: Step 2.1: Place the core samples containing mixed fluids with different oil and water saturations into the high-pressure displacement experimental device and apply confining pressure. Both ends of the core are sealed with plugs with flow guide tubes. Step 2.2: Fill the flow tubes connected to the two ends of the core column with fluorine liquid until the high-pressure pump system shows a pore pressure of 32-38 MPa, and implement online NMR monitoring until the morphology of the NMR 1D NMR spectrum is stable. Close the valves at both ends to cut off the fluid in the flow tube, and stew the core sample for 3-48 hours; Step 2.3: Open the valve of the fluid injection system to release the fluid, simulate the elastic energy exhaustion and discharge, and implement 2D NMR online monitoring until the NMR signal is stable and unchanged, and the discharge process is completed. Record the 2D NMR T1-T2 spectrum and NMR signal in this state, which are recorded as the characteristics after elastic energy exhaustion; Step 2.4: Compare the NMR spectra before and after elastic energy exhaustion, including the T1-T2 range of the 2D NMR signal distribution and the total signal volume before and after exhaustion, and the change in the total oil and water signal volume.

4. The method for evaluating the efficiency and lower limit of producing a miscible fluid in a tight reservoir according to claim 1, characterized in that: In step 3, based on the experimental results of step 2, the oil and water NMR signals are extracted and the volumes of oil and water before and after the elastic energy is released are calculated, including: Step 3.1: Based on 2D NMR T 1- T 2. The distribution position of oil and water in the spectrum, extract the oil and water signals in the 2D NMR T1-T2 spectrum of the experimental sample before and after elastic energy failure drainage; Step 3.2: Prepare oil and water samples of different volumes, and use 2D NMR monitoring to obtain the total signals of oil and water of different volumes. The conversion formulas of oil and water volume and NMR signal are as follows: （1）； （2）； in, , They are the oil and water NMR signals extracted from 2D NMR, in units of au; , are the volumes of oil and water respectively, in ml; Step 3.3: According to the relationship between the nuclear magnetic resonance signals of oil and water of different volumes, i.e., equation (1) and equation (2), the nuclear magnetic resonance signals of oil and water before and after elastic energy exhaustion drainage are converted into the volumes of oil and water.

5. The method for evaluating the efficiency and lower limit of producing a miscible fluid in a tight reservoir according to claim 1, characterized in that: In step 4, based on the volumes of oil and water before and after the elastic energy failure drainage calculated in step 3, the production efficiency of oil and water is calculated, the relationship between the production efficiency and the oil content and reservoir physical properties is analyzed, and a quantitative evaluation model for fluid production is established; including: Step 4.1: Based on the changes in oil and water volumes before and after elastic energy failure drainage, calculate the oil and water utilization efficiency of each core sample during elastic energy drainage at different oil saturations. The formula is as follows: （3）； （4）； in, is the volume of crude oil before discharge due to elastic energy failure, ml; is the volume of crude oil after elastic energy exhaustion and drainage, ml; is the utilization efficiency of crude oil after the elastic energy exhaustion and drainage, %; is the volume of water before elastic energy failure and drainage, ml; is the volume of water after the elastic energy is exhausted and the fluid is discharged, ml; is the water utilization efficiency after elastic energy exhaustion and drainage, %; Step 4.2: Regress the production efficiency of samples at different oil saturations with the physical properties of the samples. The results show the relationship between the production efficiency of oil and water and the physical properties of the samples in tight reservoirs at different oil saturations. They are all logarithmically positively correlated, as shown in formula (5): （5）； In the formula, is the tight sandstone fluid production efficiency, %, k is the core sample permeability, mD; is the porosity of the core sample, %; regression factors A and B are linearly correlated, so it can be obtained from formula (5): （6）； （7）； Where: and are the production efficiency of tight sandstone oil and water, respectively; and is the regression factor corresponding to oil and water, dimensionless; Step 4.3: Fit the regression factor A with the oil saturation. The results are as follows: （8）； （9）； Where: is the oil saturation in the sample, %; Step 4.4: Substitute equations (8) and (9) into equations (6) and (7) respectively to obtain the quantitative model of the utilization efficiency of fluids with different properties in tight reservoirs, as shown in equations (10) and (11): （10）； Water utilization efficiency and oil saturation in tight sandstone The relationship is: （11）。 6. The method for evaluating the efficiency and lower limit of producing a miscible fluid in a tight reservoir according to claim 5, characterized in that: In step 5, according to the oil-water producing efficiency model in step 4, the lower limits of physical properties and oil content are obtained by calculating the zero point of the producing efficiency model curve at each oil saturation; include: Step 5.1: According to formula (9), when =21.55 or =0.0057, =0, which is the lower limit of oil content and physical property for oil production in tight sandstone. The production lower limit includes the lower limit of oil content and physical property. Step 5.2: According to formula (10), when =76.17 or =0.0073, =0, which is the lower limit of oil content and physical property for water mobilization in tight sandstone.

7. A system for evaluating the producing efficiency and lower limit of miscible fluid in a tight reservoir, used to implement the method for evaluating the producing efficiency and lower limit of miscible fluid in a tight reservoir as claimed in any one of claims 1 to 6, characterized in that: include: The miscible fluid core sample establishment module is configured to: establish miscible fluid core samples containing different oil and water saturations based on centrifugation and saturation-NMR measurement; The change feature acquisition module is configured to: determine the change features of the nuclear magnetic spectrum of fluids with different properties before and after depletion based on 2D nuclear magnetic monitoring in the fluid energization-depletion mode; Extract the NMR spectra of fluids with different properties before and after the elastic energy is released, and calibrate the volume change characteristics of fluids with different properties; The module for establishing a quantitative model of the production efficiency of fluids of different properties in tight reservoirs is configured to: evaluate the production efficiency of fluids of different properties based on experimental results, and establish a quantitative model of the production efficiency of fluids of different properties in tight reservoirs; The module for determining the lower limit of saturation and the lower limit of physical property is configured to determine the lower limit of saturation and the lower limit of physical property for the production of fluids of different properties under the energy depletion mode based on experimental results and a quantitative model for the production efficiency of fluids of different properties in tight reservoirs.

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