Evaluation method and system for production efficiency and production lower limit of miscible fluid in tight reservoirs

Abstract

The present invention relates to a method and system for evaluating the production efficiency and production lower limit of miscible fluids in tight reservoirs, belonging to the field of unconventional oil and gas exploration and development. It includes: Step 1: Based on the nuclear magnetic detection of core samples in the saturated state and after centrifugation, establish miscible fluid core samples with different oil and water saturations; Step 2: Based on the 2D nuclear magnetic monitoring under the fluid energy enhancement - depletion mode, determine the changes in the nuclear magnetic spectra of different property fluids before and after depletion; Step 3: Extract the nuclear magnetic spectra of different property fluids before and after the release of elastic energy respectively, and calibrate the volume change characteristics of different property fluids; Step 4: Based on the experimental results, evaluate the production efficiency of different property fluids and establish a quantitative model for the production efficiency of different property fluids in tight reservoirs; Step 5: Determine the saturation lower limit and physical property lower limit for the production of different property fluids under the energy depletion mode. The present invention realizes the determination of the relationship between the elastic energy oil drainage efficiency / water drainage efficiency of saturated miscible fluid samples and reservoir physical properties, oil saturation / water saturation.

Description

Technical Field

[0001] The present invention relates to a method and system for evaluating the utilization efficiency and lower limit of miscible fluids in tight reservoirs, and belongs to the field of unconventional oil and gas exploration and development. Background Art

[0002] With the continuous progress of oil and gas exploration and development technologies and the increasing energy demand, the global oil and gas exploration and development targets have gradually shifted from conventional structural trap oil and gas reservoirs to tight oil and gas, shale oil and gas in the unconventional field, which has made tight reservoirs become new targets of concern in oil and gas geological research. In recent years, scholars at home and abroad have studied the reservoir spaces in tight reservoirs more extensively and deeply, and have recognized that a large number of micro-nano scale pores developed in tight reservoirs are important reservoir spaces. The characterization methods of micro-nano pores mainly include two types: direct observation method and fluid injection method. The direct observation method includes optical microscopy and scanning electron microscopy. The main difference between them is the magnification and field of view of the observation results. The former has a low magnification and a wide field of view, while the latter has a high magnification and can see nano-scale pores, but the field of view is very small and lacks representativeness. The fluid injection method includes high-pressure / constant-rate mercury injection, nitrogen adsorption, nuclear magnetic resonance, etc. The main difference between these methods is the pore size range that can be characterized. The mercury injection method can characterize pores of 10 nm and above, but it will damage the sample. The nitrogen adsorption method only has advantages in characterizing pores in the range of a few nanometers to dozens of nanometers. The nuclear magnetic resonance method can characterize pores of all sizes, but it needs to be converted by a formula to obtain pore information.

[0003] In the field of oil and gas development, the nuclear magnetic resonance method mostly obtains information on hydrogen-containing substances in rocks through the perturbation of hydrogen nuclei. Hydrogen atoms generally exist in pore fluids, and the information of pore fluids can be directly measured. At the same time, the measurement does not damage the core. It is a non-destructive measurement technology and 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 for the measurement of total porosity, quantification of kerogen volume, identification of fluids such as water or hydrocarbons, determination of closed environments such as organic kerogen pores or inorganic mineral pores, so as to determine the volumes of bound fluids and movable fluids, and evaluate the reservoir fluid saturation and oiliness of oil and gas reservoirs.

[0004] At present, although the one-dimensional nuclear magnetic resonance method with the T2 spectrum as the output result has the above-mentioned advantages of accuracy, non-destructiveness, and rapidity, the nature of the pore fluid, such as the difference between oil and water, cannot be confirmed solely based on the T2 spectrum. 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 the shale reservoir fluid. By dividing the two-dimensional T1-T2 spectrum into several regions, different types of fluids can be identified respectively, and combined with other experiments, quantitative characterization of the changes of miscible fluids in tight reservoirs under development conditions can be achieved. Summary of the Invention

[0005] In order to solve the problems faced in the current depletion development mode of tight oil, the present invention provides a method for quantitatively characterizing the displacement efficiency of each phase fluid by simulating the liquid drainage process of an oil-water miscible tight reservoir under formation conditions through laboratory experiments and combining 2D nuclear magnetic resonance. It is mainly based on 2D nuclear magnetic resonance to distinguish the types of miscible fluids and calculate their volumes, and through the relationships between porosity, permeability, oil saturation, water saturation and the depletion development displacement efficiency, the physical property lower limits and oil / water content lower limits for the start of displacement of each phase fluid are determined under the elastic energy depletion mode, thereby establishing an evaluation method for the displacement efficiency and displacement lower limit of miscible fluids in tight reservoirs.

[0006] The present invention also provides an evaluation system for the displacement efficiency and displacement lower limit of miscible fluids in tight reservoirs.

[0007] The technical solution of the present invention is as follows:

[0008] An evaluation method for the displacement efficiency and displacement lower limit of miscible fluids in tight reservoirs, comprising:

[0009] Step 1: Based on the nuclear magnetic resonance detection of core samples in the saturated state and after centrifugation, establish miscible fluid core samples with different oil and water saturations;

[0010] Step 2: Based on 2D nuclear magnetic resonance monitoring under the fluid energy increase - depletion mode, determine the change characteristics of the nuclear magnetic resonance spectra of different fluids before and after depletion;

[0011] Step 3: Extract the nuclear magnetic resonance spectra of different fluids before and after the release of elastic energy respectively, and calibrate the volume change characteristics of different fluids;

[0012] Step 4: Based on the experimental results, evaluate the displacement efficiency of different fluids, and establish a quantitative model for the displacement efficiency of different fluids in tight reservoirs;

[0013] Step 5: Based on the experimental results and the quantitative model for the displacement efficiency of different fluids in tight reservoirs, determine the saturation lower limit and physical property lower limit for the displacement of different fluids under the energy depletion mode.

[0014] Further, in Step 1, centrifugation and saturation-NMR joint measurement are carried out to establish plug core samples containing different amounts of bound water. Further, the lithologic samples containing bound water are saturated with crude oil to establish core samples with different oil-water saturations, and 2D NMR detection is carried out.

[0015] Further, in Step 2, using the miscible fluid core samples with different oil and water saturations established in Step 1, an elastic energy depletion NMR joint measurement experiment is carried out, including:

[0016] Step 2.1: Place the miscible fluid core samples with different oil and water saturations into a high-pressure displacement experimental device, apply confining pressure, and seal both ends of the core with plugs equipped with flow guide tubes.

[0017] Step 2.2: Inject fluorinated liquid into the flow guide tubes connected to both ends of the core column until the pore pressure shown by the high-pressure pump system is 32 - 38 Mpa, and implement on-line NMR monitoring until the 1D NMR spectrum form remains stable. Then close the valves at both ends to cut off the fluid in the flow guide tubes, and leave the core sample to stand for 3 - 48 h.

[0018] Step 2.3: Open the valve of the fluid injection system to release the fluid, simulate elastic energy depletion drainage, and implement 2D NMR on-line monitoring until the NMR signal quantity remains stable and the drainage process ends. Record the 2D NMR T1-T2 map and the NMR signal quantity in this state, which is denoted as the characteristics after elastic energy depletion.

[0019] Step 2.4: Compare the NMR spectra before and after elastic energy depletion, including the T1-T2 range of the 2D NMR signal distribution and the total signal quantity before and after depletion, as well as the change in the total oil and water signal quantity.

[0020] Further, in Step 3, based on the experimental results of Step 2, the extraction of oil and water NMR signals and the calculation of the volumes of oil and water before and after the release of elastic energy are realized, including:

[0021] Step 3.1: Based on the 2D NMR T 1- T Extract the oil and water signals in the 2D NMR T1-T2 maps of the experimental samples before and after elastic energy depletion drainage according to the distribution positions of oil and water in the 2-

[0022] Step 3.2: Prepare oil-water samples with different volumes, and use 2D nuclear magnetic resonance monitoring to obtain the total signals of oil and water with different volumes. The conversion formulas for the volume and NMR signal quantity of oil and water are as follows:

[0023] (1);

[0024] (2);

[0025] Among them, and are the nuclear magnetic resonance signal amounts of oil and water extracted from 2D nuclear magnetic resonance, with the unit of a.u.; and are the volumes of oil and water respectively, with the unit of ml;

[0026] Step 3.3: According to the nuclear magnetic resonance signal relationship of oil and water with different volumes, namely Equation (1) and Equation (2), convert the nuclear magnetic resonance signal amounts of oil and water before and after the elastic energy depletion drainage into the volumes of oil and water.

[0027] Furthermore, in Step 4, based on the volumes of oil and water before and after the elastic energy depletion drainage calculated in Step 3, calculate the recovery efficiencies of oil and water, analyze the relationship between the recovery efficiency and oiliness and reservoir physical properties, and establish a quantitative evaluation model for fluid recovery; including:

[0028] Step 4.1: Based on the change amounts of the volumes of oil and water before and after the elastic energy depletion drainage, calculate the recovery efficiencies of oil and water during the elastic energy drainage for each core sample at different oil saturations. The formula is as follows:

[0029] (3);

[0030] (4);

[0031] Among them, is the volume of crude oil before the elastic energy depletion drainage, in ml; is the volume of crude oil after the elastic energy depletion drainage, in ml; is the recovery efficiency of crude oil after the elastic energy depletion drainage, in %; is the volume of water before the elastic energy depletion drainage, in ml; is the volume of water after the elastic energy depletion drainage, in ml; is the recovery efficiency of water after the elastic energy depletion drainage, in %;

[0032] Step 4.2: Regress the recovery efficiencies of samples at different oil saturations with the physical properties of the samples. The results show that the recovery efficiencies of oil and water in tight reservoirs with different oiliness are logarithmically positively correlated with the physical property parameters as shown in Equation (5):

[0033] (5);

[0034] In the formula, is the recovery efficiency of tight sandstone fluid, in %, k is the permeability of the core sample, in mD; is the porosity of the core sample, %; Since the regression factors A and B are linearly correlated, they are obtained from Equation (5) as follows:

[0035] (6);

[0036] (7);

[0037] In the formula: and are the production efficiencies of tight sandstone oil and water respectively, %; and are the regression factors corresponding to oil and water, dimensionless;

[0038] Step 4.3: Fit the regression factor A with the oil saturation, and the results are as follows:

[0039] (8);

[0040] (9);

[0041] In the formula: is the oil saturation in the sample, %;

[0042] Step 4.4: Substitute Equations (8) and (9) into Equations (6) and (7) respectively, and the quantitative models of the production efficiencies of different property fluids in the tight reservoir can be obtained, as shown in Equations (10) and (11):

[0043] (10);

[0044] Similarly, the relationship between the production efficiency of water in tight sandstone and the oil saturation is:

[0045] (11).

[0046] Furthermore, in Step 5, according to the oil-water production efficiency models in Step 4, the physical property lower limit and the oiliness lower limit are obtained by calculating the zero points of the production efficiency model curves at each oil saturation; including:

[0047] Step 5.1: From Equation (9), when = 21.55 or = 0.0057, = 0, which is the oiliness lower limit and the physical property lower limit for the production of oil in tight sandstone. The production lower limit includes the oiliness lower limit and the physical property lower limit;

[0048] Step 5.2: From Equation (10), when = 76.17 or = 0.0073, = 0, which is the lower limit of oil-bearing property and physical property for water utilization in tight sandstone.

[0049] The evaluation system for the utilization efficiency and lower limit of miscible fluids in tight reservoirs includes:

[0050] The miscible fluid core sample establishment module is configured to: establish miscible fluid core samples with different oil and water saturations based on centrifugation and saturation-NMR joint measurement;

[0051] The variation characteristic acquisition module is configured to: determine the NMR spectrum variation characteristics of different property fluids before and after depletion based on 2D NMR monitoring in the fluid energy enhancement-depletion mode; extract the NMR spectra of different property fluids before and after the release of elastic energy respectively, and calibrate the volume variation characteristics of different property fluids;

[0052] The quantitative model establishment module for the utilization efficiency of different property fluids in tight reservoirs is configured to: evaluate the utilization efficiency of different property fluids based on experimental results and establish a quantitative model for the utilization efficiency of different property fluids in tight reservoirs;

[0053] The saturation lower limit and physical property lower limit determination module is configured to: determine the saturation lower limit and physical property lower limit for the utilization of different property fluids in the energy depletion mode based on experimental results and the quantitative model for the utilization efficiency of different property fluids in tight reservoirs.

[0054] The beneficial effects of the present invention are:

[0055] The present invention realizes the determination of the relationship between the elastic energy oil displacement efficiency / water displacement efficiency of saturated miscible fluid samples and reservoir physical properties, oil saturation / water saturation. The lower limit of physical property, the lower limit of oil saturation and the lower limit of water saturation for oil utilization in the tight reservoirs 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. Description of the Drawings

[0056] Figure 1 It is a comparison schematic diagram of the #20 tight sandstone sample in the saturated water state and the 30% irreducible water state in 1D NMR monitoring

[0057] Figure 2 It is a comparison schematic diagram of the #20 tight sandstone sample in the 30% irreducible water state and the 30% water + 70% oil state in 1D NMR monitoring.

[0058] Figure 3 It is the 2D NMR T1-T2 spectrum of the #20 tight sandstone sample in the saturated water state.

[0059] Figure 4 It is the 2D NMR T1-T2 spectrum of the #20 tight sandstone sample in the 30% irreducible water state.

[0060] Figure 5 It is the 2D NMR T1-T2 spectrum of the #20 tight sandstone sample in the state of 30% water + 70% oil.

[0061] Figure 6 It is the schematic diagram of the high-pressure displacement experimental device and its connection with the NMR device in the nuclear magnetic resonance combined measurement experiment of elastic energy depletion.

[0062] Figure 7 It is 2D NMR T 1- T Schematic diagram of the fluid identification chart of the 2 spectrum.

[0063] Figure 8 It is the T1-T2 spectrum of the #20 tight sandstone sample before the elastic energy depletion when the oil saturation is 70%.

[0064] Figure 9 It is the T1-T2 spectrum of the #20 tight sandstone sample after the elastic energy depletion when the oil saturation is 70%.

[0065] Figure 10 It is the T1-T2 spectrum of the #20 tight sandstone sample before the elastic energy depletion when the oil saturation is 55%.

[0066] Figure 11 It is the T1-T2 spectrum of the #20 tight sandstone sample after the elastic energy depletion when the oil saturation is 55%.

[0067] Figure 12 It is the T1-T2 spectrum of the #20 tight sandstone sample before the elastic energy depletion when the oil saturation is 40%.

[0068] Figure 13 It is the T1-T2 spectrum of the #20 tight sandstone sample after the elastic energy depletion when the oil saturation is 40%.

[0069] Figure 14 It is the T1-T2 spectrum of the #20 tight sandstone sample before the elastic energy depletion when the oil saturation is 25%.

[0070] Figure 15 It is the T1-T2 spectrum of the #20 tight sandstone sample after the elastic energy depletion when the oil saturation is 25%.

[0071] Figure 16 It is the T2 spectrum of oil before and after the elastic energy depletion of the #20 tight sandstone sample when the oil saturation is 70%.

[0072] Figure 17 It is the T2 spectrum of water before and after the elastic energy depletion of the #20 tight sandstone sample when the oil saturation is 70%.

[0073] Figure 18It is the T2 spectrum of oil before and after the elastic energy depletion of tight sandstone sample #20 at an oil saturation of 55%.

[0074] Figure 19 It is the T2 spectrum of water before and after the elastic energy depletion of tight sandstone sample #20 at an oil saturation of 55%.

[0075] Figure 20 It is the T2 spectrum of oil before and after the elastic energy depletion of tight sandstone sample #20 at an oil saturation of 40%.

[0076] Figure 21 It is the T2 spectrum of water before and after the elastic energy depletion of tight sandstone sample #20 at an oil saturation of 40%.

[0077] Figure 22 It is the T2 spectrum of oil before and after the elastic energy depletion of tight sandstone sample #20 at an oil saturation of 25%.

[0078] Figure 23 It is the T2 spectrum of water before and after the elastic energy depletion of tight sandstone sample #20 at an oil saturation of 25%.

[0079] Figure 24 It is a schematic diagram of the fitting relationship between the 2D NMR total signal and the fluid volume. It is used to calculate the oil-water volume-2D NMR signal conversion coefficient.

[0080] Figure 25 It is the permeability per unit porosity of the sample ( ) and the fitting curve and corresponding relationship of the oil utilization efficiency in the pores.

[0081] Figure 26 It is the permeability per unit porosity of the sample ( ) and the fitting curve and corresponding relationship of the water utilization efficiency in the pores.

[0082] Figure 27 It is the fitting relationship between the regression parameters A and B in equations (6) and (7).

[0083] Figure 28 It is the relationship between the oil and water regression parameters A and the oil saturation in equations (8) and (9).

[0084] Figure 29 It is a schematic diagram of the process of the evaluation method for the utilization efficiency and utilization lower limit of the miscible fluid in the tight reservoir of the present invention. Detailed implementation manners

[0085] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Apparently, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0086] Example 1

[0087] A method for evaluating the production efficiency and production lower limit of miscible fluids in a tight reservoir, as Figure 29 shown, includes:

[0088] Step 1: Based on the nuclear magnetic detection of core samples in the saturated state and after centrifugation, establish miscible fluid core samples with different oil and water saturations;

[0089] Step 2: Based on the 2D nuclear magnetic monitoring under the fluid energy enhancement - depletion mode, determine the changes in the nuclear magnetic spectra of fluids with different properties before and after depletion;

[0090] Step 3: Extract the nuclear magnetic spectra of fluids with different properties before and after the release of elastic energy respectively, and calibrate the volume change characteristics of fluids with different properties;

[0091] 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 a tight reservoir;

[0092] Step 5: Based on the experimental results and the quantitative model for the production efficiency of fluids with different properties in a tight reservoir, determine the saturation lower limit and physical property lower limit for the production of fluids with different properties under the energy depletion mode.

[0093] The present invention is based on indoor fluid mobility simulation - nuclear magnetic resonance joint measurement experiments to establish the production characteristics of crude oil and pore water in core samples with different physical properties under different oil / water saturations, establish a quantitative evaluation model for crude oil and pore water regarding reservoir physical properties and oil saturation, and determine the physical property lower limit and oiliness lower limit for the production of oil and water, serving for the evaluation of sweet spots and production capacity assessment in tight oil reservoirs.

[0094] The present invention mainly proposes a method for quantitatively characterizing the production characteristics of miscible fluids in a tight reservoir under the depletion development mode based on 2D nuclear magnetic resonance, so as to determine the production characteristics of miscible fluids in the tight reservoir, and further quantitatively characterize the fluid mobility of oil / water in the tight reservoir, thereby determining the physical property and oiliness production boundaries of oil and water in the tight reservoir, and helping to optimize the geological sweet spot areas of tight oil.

[0095] Example 2

[0096] The method for evaluating the utilization efficiency and lower limit of miscible fluids in tight reservoirs according to Embodiment 1 is different in that:

[0097] At 25 °C and normal pressure, the nuclear magnetic resonance (NMR) monitoring results of water-saturated cores show that the water-saturated signals of different samples are highly linearly positively correlated with porosity. Since the samples used are all plug samples with a diameter of 2.5 cm and a length of 5 cm, the relationships between porosity and NMR signal, and between pore water volume and NMR signal can be calculated under 1D NMR monitoring. In addition, using a glass bottle and a syringe, fixed-volume oil samples are made, and based on this, the volume-signal conversion coefficient of oil under 2D NMR monitoring can be calculated. The volume-signal conversion coefficient of water in oil is calculated again using the same steps.

[0098] In Step 1, centrifugation and saturation-NMR joint measurement are carried out to establish plug core samples containing different amounts of irreducible water (with water saturations of 30%, 45%, 60%, and 75% respectively). Further, the lithologic samples containing irreducible water are saturated with crude oil to establish core samples with different oil-water saturations, and 2D NMR detection is carried out. It includes:

[0099] Step 1.1: Use a vacuum pressure saturation device to fully saturate the core after washing with oil and drying with water. At this time, the water saturation of the core is regarded as 100%, and the nuclear magnetic resonance one-dimensional (1D) T2 spectrum ( Figure 1 ), two-dimensional (2D) T1-T2 spectrum, and the amount of NMR signal ( Figure 3 ) are measured.

[0100] Step 1.2: Use a high-speed centrifuge to dehydrate at a fixed time and centrifugal force (such as 10,000 revolutions per minute for 1 hour), and measure the 1D and 2D NMR spectral signal amounts after each dehydration.

[0101] Step 1.3: Compare the NMR signal amounts of the core after centrifugal dehydration and the core in the water-saturated state. When the total NMR signal after a certain centrifugation reaches 30% of the total NMR signal in the saturated state, it is considered that a core containing irreducible water with a water saturation of 30% has been established ( Figure 1 、 Figure 4 ).

[0102] Step 1.4: Use a vacuum pressure saturation device to fully saturate the sample with 30% water saturation with oil again, and an experimental sample of 30% water + 70% oil can be obtained. Measure its 2D NMR T1-T2 spectrum and total signal ( Figure 2 、 Figure 5 ).

[0103] Step 1.5: Repeat the above process to establish lithology samples with different oil and water saturations, such as 45% water + 55% oil, 60% water + 40% oil, 75% water + 25% oil. The 2D NMR detection data in these states are recorded as the pre-depletion NMR data (such as Figure 8 , 10 , 12, 14).

[0104] In Step 2, using the miscible fluid core samples with different oil and water saturations established in Step 1, conduct an elastic energy depletion NMR joint measurement experiment, including:

[0105] Step 2.1: Place the miscible fluid core samples with different oil and water saturations into the holder in the high-pressure displacement experiment device, and apply confining pressure (the confining pressure should be 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 surrounding of the core). Plug the two ends of the core with plugs equipped with diversion tubes;

[0106] As Figure 6 shown, the high-pressure displacement experiment device is an experimental equipment used to simulate the fluid displacement process under the high-pressure environment of underground oil and gas reservoirs, mainly used to study key parameters such as the flow law and displacement efficiency of multiphase fluids (such as oil and water) in porous media (such as rocks) during the oil and gas development process. It includes the following specific devices: a high-pressure pump system (providing and controlling the displacement pressure, confining pressure and realizing pressure visualization); a core holder (fixing the core sample and simulating the temperature, pressure and fluid environment of the reservoir where the rock is located); a fluid injection system (including a liquid storage container and a diversion tube, which can inject different fluids such as crude oil and water into the core).

[0107] Step 2.2: Inject fluorinated liquid into the diversion tubes connected to both ends of the core column (the fluorinated liquid does not contain hydrogen, so the signals detected by NMR are still only the signals of oil and water in the core) until the pore pressure shown by the high-pressure pump system is 32 - 38 Mpa (35 Mpa) (the pore pressure in the core can be designed according to the actual formation pore pressure in each area), and conduct on-line nuclear magnetic resonance monitoring until the 1D NMR spectrum shape remains stable. Then close the valves at both ends to cut off the fluid in the diversion tubes, and leave the core sample to stand for 3 - 48 h; On-line nuclear magnetic resonance monitoring refers to the whole process from installing the sample to the end of the experiment in the elastic energy experiment, during which 1D monitoring is continuously carried out by the nuclear magnetic resonance detection device. The purpose is to select and process the original experimental data according to the changes of the 1D NMR spectrum. The process of 2D NMR on-line monitoring is the same, the difference is that the T1-T2 spectrum, that is, the 2D NMR spectrum, is used during the process. The processing means include removing miscellaneous signals and redrawing the graph, which is a part of the standard processing process of nuclear magnetic resonance data.

[0108] Step 2.3: Open the valve of the fluid injection system to release the fluid, simulate the drainage during elastic energy depletion, and implement 2D nuclear magnetic resonance (NMR) on-line monitoring until the NMR signal quantity becomes stable and unchanged, at which point the drainage process ends. Record the 2D NMR T1-T2 spectrum and the NMR signal quantity in this state, which is denoted as the characteristics after elastic energy depletion. If Figure 9 , Figure 11 , Figure 13 , Figure 15 shown. Figure 10 Figure Figure 12 is the T1-T2 spectrum of tight sandstone sample #20 before elastic energy depletion at an oil saturation of 55%. Figure 14 is the T1-T2 spectrum of tight sandstone sample #20 before elastic energy depletion at an oil saturation of 40%.

[0109] Step 2.4: Compare the NMR spectra before and after elastic energy depletion, including the T1-T2 range of the 2D NMR signal distribution and the total signal quantity before and after depletion, as well as the change in the total oil and water signal quantity. As Figure 16 , Figure 17 , Figure 18 , Figure 19 , Figure 20 , Figure 21 , Figure 22 , Figure 23 shown.

[0110] In Step 3, based on the experimental results in Step 2, the extraction of the NMR signals of oil and water and the calculation of the volumes of oil and water before and after the release of elastic energy are realized, including:

[0111] Step 3.1: Based on the distribution positions of oil and water in the 2D NMR T 1- T 2 spectrum ( Figure 7 , according to Marc, 2015), extract the oil and water signals in the 2D NMR T1-T2 spectra of the experimental sample before and after the drainage during elastic energy depletion ( Figure 6 ); The specific method is to sum up the signal values in the corresponding regions of oil and water in the 2D NMR spectra in each state according to the graph plate ( Figure 4 ) to calculate the total signals of oil and water respectively.

[0112] Step 3.2: Prepare oil-water samples with different volumes (such as 0.1 ml, 0.2 ml, 0.4 ml, 0.6 ml, 1.0 ml), and use 2D nuclear magnetic resonance to monitor the total signals of oil and water with different volumes. The conversion formulas for the volume and NMR signal quantity of oil and water are as follows:

[0113] (1);

[0114] (2);

[0115] Wherein, and are the amounts of oil and water nuclear magnetic resonance signals extracted from 2D nuclear magnetic resonance, with the unit of a.u.; and are the volumes of oil and water respectively, with the unit of ml.

[0116] Step 3.3: According to the relationship between the nuclear magnetic resonance signals of oil and water with different volumes, namely Equation (1) and Equation (2) ( Figure 24 ), convert the amounts of nuclear magnetic resonance signals of oil and water before and after the elastic energy depletion drainage into the volumes of oil and water.

[0117] In Step 4, based on the volumes of oil and water before and after the elastic energy depletion drainage calculated in Step 3, calculate the recovery efficiencies of oil and water, analyze the relationship between the recovery efficiency and oiliness and reservoir physical properties, and establish a quantitative evaluation model for fluid recovery; including:

[0118] Step 4.1: Based on the change amounts of the volumes of oil and water before and after the elastic energy depletion drainage, calculate the recovery efficiencies of oil and water during the elastic energy drainage for each core sample under different oil saturations. The formula is as follows:

[0119] (3);

[0120] (4);

[0121] Wherein, is the volume of crude oil before the elastic energy depletion drainage, ml; is the volume of crude oil after the elastic energy depletion drainage, ml; is the recovery efficiency of crude oil after the elastic energy depletion drainage, %; is the volume of water before the elastic energy depletion drainage, ml; is the volume of water after the elastic energy depletion drainage, ml; is the recovery efficiency of water after the elastic energy depletion drainage, %;

[0122] Step 4.2: Regress the recovery efficiencies of samples under different oil saturations with the physical properties of the samples. The results show that the recovery efficiencies of oil and water in tight reservoirs with different oiliness are logarithmically positively correlated with the physical property parameters Both show a logarithmic positive correlation. The difference in the regression formulas of samples under different oil saturations lies in the different regression factors A and B ( Figure 25 and Figure 26 ), and its general expression is as shown in Equation (5):

[0123] (5);

[0124] In the formula, is the fluid utilization efficiency of tight sandstone, %, k is the permeability of the core sample, mD; is the porosity of the core sample, %; The regression factors A and B show a good linear correlation ( Figure 27 ), so from Equation (5), we get:

[0125] (6);

[0126] (7);

[0127] In the formula: and are the fluid utilization efficiencies of tight sandstone oil and water respectively, %; and are the regression factors corresponding to oil and water, dimensionless;

[0128] Step 4.3: Due to the difference in regression factors caused by different oil saturation, fit the regression factor A with the oil saturation ( Figure 28 ), and the results are as follows:

[0129] (8);

[0130] (9);

[0131] In the formula: is the oil saturation in the sample, %;

[0132] Step 4.4: Substitute Equations (8) and (9) into Equations (6) and (7) respectively, and then the quantitative models of fluid utilization efficiency of different properties in tight reservoirs can be obtained, as shown in Equations (10) and (11):

[0133] (10);

[0134] Similarly, the relationship between the water utilization efficiency in tight sandstone and the oil saturation is:

[0135] (11).

[0136] In Step 5, according to the oil-water utilization efficiency models in Step 4, the physical property lower limit and oil-bearing property lower limit are both obtained by calculating the zero points of the utilization efficiency model curves at each oil saturation; including:

[0137] Step 5.1: From Equation (9), when = 21.55 or = 0.0057, = 0, which is the lower limit of oil-bearing property and physical property for the mobilization of oil in tight sandstone. The lower limit of mobilization includes the lower limit of oil-bearing property and the lower limit of physical property;

[0138] Step 5.2: From Equation (10), when = 76.17 or = 0.0073, = 0, which is the lower limit of oil-bearing property and physical property for the mobilization of water in tight sandstone.

[0139] In addition, the lower limits obtained in this part only represent the results of this experiment. The purpose is to obtain a bivariate ( , S o ) model in the form of Equation (9) and Equation (10). After the model is obtained, the physical property and the lower limit of oil-bearing property of the fluid mobilization efficiency can be continuously calculated, that is, all situations can be covered.

[0140] Example 3

[0141] The evaluation system for the mobilization efficiency and the lower limit of mobilization of miscible fluids in tight reservoirs includes:

[0142] The miscible fluid core sample establishment module is configured to: establish miscible fluid core samples with different oil and water saturations based on centrifugation and combined saturation-NMR measurement;

[0143] The variation characteristic acquisition module is configured to: determine the variation characteristics of the NMR spectra of different property fluids before and after depletion based on 2D NMR monitoring in the fluid energy enhancement-depletion mode; extract the NMR spectra of different property fluids before and after the release of elastic energy respectively, and calibrate the volume variation characteristics of different property fluids;

[0144] The quantitative model establishment module for the mobilization efficiency of different property fluids in tight reservoirs is configured to: evaluate the mobilization efficiency of different property fluids based on the experimental results and establish a quantitative model for the mobilization efficiency of different property fluids in tight reservoirs;

[0145] The saturation lower limit and physical property lower limit determination module is configured to: determine the saturation lower limit and physical property lower limit for the mobilization of different property fluids in the energy depletion mode based on the experimental results and the quantitative model for the mobilization efficiency of different property fluids 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 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）； 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 described 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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