A method for measuring critical flow pressure and critical flow saturation of condensate oil by combining one-dimensional and two-dimensional nuclear magnetic resonance techniques

Abstract

The present invention provides a method for measuring the critical flow pressure and critical flow saturation of condensate oil by combining one-dimensional and two-dimensional nuclear magnetic resonance technology, which belongs to the technical field of oil and gas reservoir development. The method solves the technical problems that the measurement accuracy of the critical flow pressure and critical flow saturation of condensate oil in the process of condensate gas reservoir development is low and the test method is complicated. The present invention combines one-dimensional nuclear magnetic resonance T2 spectrum testing technology with two-dimensional nuclear magnetic resonance T 1‑ The T2 spectrum testing technology detects the signal amount of the hydrogen-containing fluid inside the core, optimizes the artificial zoning method through the improved difference spectrum method, avoids the error introduced by the artificial signal area division, greatly improves the condensate oil signal recognition accuracy, and measures the critical flow pressure and critical flow saturation of condensate oil more quickly and accurately, so as to better study the flow law of condensate oil and gas phases in the formation, which is conducive to helping gas reservoirs formulate more reasonable development plans and provide theoretical guidance for improving condensate oil recovery.

Description

Technical Field

[0001] The invention belongs to the technical field of oil and gas reservoir development, and in particular relates to a method for determining the critical flow pressure and critical flow saturation of condensate oil. Background Art

[0002] Condensate gas reservoirs have huge geological reserves and high economic value, and occupy an important position in the exploration and utilization of global oil and gas resources. Condensate gas reservoirs have dual characteristics of oil and gas and complex phase changes. When the formation pressure drops below the dew point pressure, the condensate gas will undergo retrograde condensation, causing the heavy components to precipitate in liquid form, forming a gas-liquid two-phase. Only when the formation pressure is exhausted to the critical flow pressure, that is, when the condensate oil saturation reaches the critical flow saturation, the condensate oil will begin to flow. Therefore, the critical flow saturation of condensate oil is an important parameter reflecting the flow capacity of condensate oil. Determining the critical flow pressure and critical flow saturation is conducive to helping gas reservoirs formulate more reasonable development plans and provide theoretical guidance for improving condensate oil recovery. Therefore, how to determine the critical flow pressure and critical flow saturation has always been a hot issue in the development research of condensate gas reservoirs at home and abroad.

[0003] Currently, one-dimensional nuclear magnetic difference spectroscopy has been widely used, mainly for porosity or bound phase porosity determination, and does not have the ability to identify fluids. In order to distinguish oil and gas signals, the present invention adopts a two-dimensional nuclear magnetic difference spectrum method. Currently, there are few studies on two-dimensional nuclear magnetic difference spectrum. The existing two-dimensional nuclear magnetic difference spectrum patents are as follows: "A method for calibrating the two-dimensional nuclear magnetic distribution of fractures in carbonate light oil reservoirs" (CN 116953012 A), "A method for calibrating the two-dimensional nuclear magnetic T1-T2 distribution of fractures in shale medium oil reservoirs" (CN117110352 B) and "A method for calibrating the two-dimensional nuclear magnetic T1-T2 distribution of multi-scale fractures in laminated shale" (CN117269223 A). These three patents are to conduct two-dimensional nuclear magnetic scanning experiments on saturated oil rock samples and gas-driven oil rock samples respectively, to obtain the corresponding overall two-dimensional nuclear magnetic T1-T2 spectrum of pores and fractures and two-dimensional nuclear magnetic T1-T2 spectrum of matrix pores, and then obtain the two-dimensional nuclear magnetic T1-T2 spectrum of fractures based on the difference spectrum method, and finally cross-calibrate with the fracture aperture bar chart obtained by CT to obtain a two-dimensional nuclear magnetic fracture and matrix porosity distribution plate. The improved two-dimensional nuclear magnetic resonance difference spectrum method used in the present invention is to obtain the baseline and two-dimensional spectra of different saturation states through a targeted calibration method, and process the two-dimensional nuclear magnetic resonance T1-T2 spectrum through the improved improved difference spectrum method, so as to accurately measure the saturation of condensate oil at different pressures. It can be seen that the two methods are essentially different.

[0004] The present invention proposes to use an improved two-dimensional nuclear magnetic difference spectrum method to identify and measure condensate oil. The existing patent "A condensate oil saturation test method based on two-dimensional nuclear magnetic resonance" (CN 115266800 A) uses an artificial zoning method to identify oil and gas and measure condensate oil saturation. Its disadvantage is that the oil signal in the divided area is dominant, and the gas signal in the gas area is dominant, but the oil area will have a gas signal, and vice versa. Therefore, the artificial division of oil and gas areas leads to inaccurate measurement, and the measured condensate oil saturation has deviations. The two-dimensional nuclear magnetic improved difference spectrum method used in the present invention is an improvement on the artificial zoning method, which avoids the error introduced by the artificial signal area division and can more accurately measure the condensate oil saturation.

[0005] The present invention uses a one-dimensional nuclear magnetic resonance combined with a two-dimensional nuclear magnetic resonance test method, and the two methods are mutually verified. The method can accurately measure the rate at which hydrogen-containing substances leave the core, accurately determine the critical flow pressure point of condensate oil, and accurately determine the critical flow saturation of condensate oil by combining the condensate oil saturation obtained by the new two-dimensional nuclear magnetic resonance difference spectrum. The patent "A condensate oil saturation test method based on two-dimensional nuclear magnetic resonance" (CN 115266800 A) can identify the saturation of condensate oil within a certain error range, but does not have the ability to identify the critical flow pressure and critical flow saturation. Some scholars have also used a long core experimental device with a high-temperature and high-pressure observation window to directly observe the first drop of oil flowing out of the core to determine the critical flow pressure. Subsequently, the changes in the produced components were analyzed by a chromatograph and combined with the material balance equation and the phase equilibrium equation to infer the critical flow saturation of condensate oil in the core under this pressure (Tang Yong, Sun Lei. Long core experimental determination method for critical flow saturation of condensate oil [J]. Natural Gas Industry, 2006, 26(7): 100-102.). This method is an indirect determination method and has errors. The present invention combines one-dimensional and two-dimensional nuclear magnetic resonance signals to determine the rate at which hydrogen-containing fluid leaves the porous medium, thereby achieving direct determination of the critical flow pressure of condensate oil during condensate gas depletion, which greatly improves the test accuracy of the critical flow pressure compared to the existing indirect calculation method. The condensate saturation was accurately measured using the improved 2D NMR difference spectrum method. The 2D NMR T1-T2 spectrum calibration was completed through experimental design, and the gas signal in the oil zone and the oil signal in the gas zone were subtracted or increased. The traditional artificial zoning identification of oil and gas was improved, which greatly improved the condensate signal identification accuracy and accurately measured the condensate saturation. Combining the critical flow pressure identification and condensate saturation measurement, a more representative condensate critical flow saturation was obtained. Summary of the invention

[0006] The purpose of this section is to summarize some aspects of embodiments of the present invention and briefly introduce some preferred embodiments. Some simplifications or omissions may be made in this section and the specification abstract and the invention title of this application to avoid blurring the purpose of this section, the specification abstract and the invention title, and such simplifications or omissions cannot be used to limit the scope of the present invention.

[0007] The present invention provides a method for measuring the critical flow pressure and critical flow saturation of condensate oil based on nuclear magnetic resonance technology, which combines the one-dimensional nuclear magnetic resonance T2 testing technology with the two-dimensional nuclear magnetic resonance T 1- The principle of T2 testing technology is to use the change of the total signal amount of the fluid inside the core during the depletion process to judge the time when the condensate oil starts to flow. Nuclear magnetic resonance can detect the signal amount of the hydrogen-containing fluid inside the core. Since the hydrogen index of condensate oil is much higher than that of condensate gas, when condensate oil begins to flow out of the core in the form of liquid, the rate of decrease of the total signal amount of the fluid in the core will increase significantly. The sudden drop point of this signal amount represents that the condensate oil in the core begins to flow continuously under this pressure. The sudden drop point of the signal amount is measured by nuclear magnetic resonance to determine the critical flow pressure of condensate oil, and then combined with the improved difference spectrum method, the critical flow saturation corresponding to the critical flow pressure can be calculated, avoiding the inaccurate measurement caused by the artificial division of oil and gas areas, and the deviation of the measured condensate oil saturation; the present invention creatively adopts the two-dimensional nuclear magnetic improved difference spectrum method to improve the artificial zoning method, avoids the error introduced by the artificial signal area division, and can more accurately measure the condensate oil saturation.

[0008] In order to achieve the above technical purpose, the present invention adopts the following technical solution, including the following steps:

[0009] Step S1: performing a calibration experiment on the condensate oil signal amount and saturation to obtain a relationship curve between the signal amount and the condensate oil saturation;

[0010] Step S2: performing a depletion experiment on the core to obtain a one-dimensional nuclear magnetic resonance T2 spectrum and a two-dimensional nuclear magnetic resonance T1-T2 spectrum at each pressure point;

[0011] Step S3: Calculating the condensate oil saturation by using the original data of the two-dimensional nuclear magnetic resonance T1-T2 spectrum obtained in step S2 using the improved difference spectrum method;

[0012] Step S4: Plot the relationship curves between the total signal amount and pressure of the one-dimensional nuclear magnetic resonance T2 spectrum and the two-dimensional nuclear magnetic resonance T1-T2 spectrum obtained from the depletion experiment, and obtain the pressure point corresponding to the sudden drop point of the curve, which is the critical flow pressure point; then calculate the critical flow saturation corresponding to the critical flow pressure based on the pressure and condensate oil saturation relationship curve obtained by the improved difference spectrum method; the improved difference spectrum method can realize the subtraction or addition of gas signals in the oil zone and oil signals in the gas zone, optimize the traditional artificial zoning identification of oil and gas, greatly improve the recognition accuracy of condensate oil signals, and accurately measure the condensate oil saturation.

[0013] Step S5: Using long core failure experiments and numerical simulations to verify the reliability of the critical flow pressure and the critical flow saturation.

[0014] As a preferred solution of the method for measuring the critical flow pressure and critical flow saturation of condensate oil by combining one-dimensional and two-dimensional nuclear magnetic resonance techniques according to the present invention, the specific operation flow of step S1 includes:

[0015] Step S1.1: cleaning and drying the core;

[0016] Step S1.2: After soaking the core with the condensate oil sample, the core is weighed to obtain the condensate oil saturation, and then the soaked core is subjected to a two-dimensional nuclear magnetic resonance T1-T2 test to obtain the two-dimensional nuclear magnetic resonance T1-T2 spectrum of the core sample, that is, the two-dimensional nuclear magnetic resonance T1-T2 spectrum signal amount of the core is obtained;

[0017] Step S1.3: changing the condensate saturation by changing the degree of condensate oil soaking into the core, and measuring the corresponding two-dimensional nuclear magnetic resonance T1-T2 spectrum;

[0018] Step S1.4: in the above experiment, five different saturations are obtained and their corresponding two-dimensional nuclear magnetic resonance T1-T2 spectra are measured, and a relationship curve between the signal amount and the condensate saturation is drawn according to the signal amounts of the two-dimensional nuclear magnetic resonance T1-T2 spectra of the five different condensate saturations.

[0019] As a preferred solution of the method for measuring the critical flow pressure and critical flow saturation of condensate oil by combining one-dimensional and two-dimensional nuclear magnetic resonance techniques according to the present invention, the specific operation flow of step S2 includes:

[0020] Step S2.1: cleaning the core and then drying it;

[0021] Step S2.2: Place the core in the nuclear magnetic resonance holder, use pure methane to establish the core internal pressure, circulate the fluorinated liquid to apply a higher confining pressure than the core internal pressure, and control the back pressure pump to apply a higher back pressure than the core internal pressure. Start the circulating heating device and heat it to the experimental temperature. Continue to build pressure after the temperature is raised. During the methane pressure building process, perform a methane two-dimensional nuclear magnetic resonance T1-T2 spectrum test every 3 MPa until the maximum experimental pressure is reached.

[0022] Step S2.3: Inject the prepared high-pressure condensate gas into the core to displace methane; inject 10PV of condensate gas to complete condensate gas saturation, and the outlet gas-oil ratio should reach the initial gas-oil ratio, at which point condensate gas saturation is completed;

[0023] Step S2.4: The back pressure is reduced by controlling the back pressure pump, and the back pressure is reduced by 3 MPa each time to simulate the exhaustion process. The confining pressure is reduced synchronously. After each pressure point is stabilized for 2 hours, a one-dimensional nuclear magnetic resonance T2 spectrum test and a two-dimensional nuclear magnetic resonance T1-T2 spectrum test are carried out to obtain a one-dimensional nuclear magnetic resonance T2 spectrum and a two-dimensional nuclear magnetic resonance T1-T2 spectrum at each pressure point;

[0024] Step S2.5: After the experiment is completed, turn off the circulating heating device, remove the experimental pipeline and clean the experimental device to end the experiment.

[0025] As a preferred embodiment of the method for measuring the critical flow pressure and critical flow saturation of condensate oil by combining one-dimensional and two-dimensional nuclear magnetic resonance techniques described in the present invention, the circulating fluorinated liquid in step S2.2 applies a confining pressure 3 MPa higher than the core internal pressure.

[0026] As a preferred solution of the method for measuring the critical flow pressure and critical flow saturation of condensate oil by combining one-dimensional and two-dimensional nuclear magnetic resonance techniques described in the present invention, the back pressure is ensured to be 2 MPa higher than the core pressure during the pressure building process in step S2.2.

[0027] As a preferred solution of the method for measuring the critical flow pressure and critical flow saturation of condensate oil by combining one-dimensional and two-dimensional nuclear magnetic resonance techniques according to the present invention, the specific steps of the improved difference spectrum method in step S3 include:

[0028] Step S3.1: First, subtract the two-dimensional nuclear magnetic resonance T1-T2 spectrum of the dry rock sample from all the two-dimensional nuclear magnetic resonance T1-T2 spectra required;

[0029] Step S3.2: Divide the signal of the two-dimensional nuclear magnetic resonance T1-T2 spectrum scanned at a certain pressure point during the pressure building process by the signal of the two-dimensional nuclear magnetic resonance T1-T2 spectrum from the pressure building process to the highest experimental pressure point to obtain a series of ratios of each pressure point during the pressure building process to the highest experimental pressure point. This ratio is named the condensate gas correction coefficient. , as shown below;

[0030] Two-dimensional NMR T1-T2 spectrum of a certain pressure point: 2D NMR T1-T2 spectrum at the highest experimental pressure point: Step S3.3: Multiply the overall spectrum of the two-dimensional nuclear magnetic resonance T1-T2 spectrum of the saturated condensate gas at the highest experimental pressure point by the correction coefficient in step S3.2 , a new calibrated two-dimensional nuclear magnetic resonance T1-T2 spectrum is obtained. The actual meaning of this spectrum is the two-dimensional nuclear magnetic resonance T1-T2 spectrum of pure condensate gas at a certain pressure point, as shown below;

[0031] Two-dimensional NMR T1-T2 spectrum of saturated condensate gas at the highest experimental pressure point: Two-dimensional NMR T1-T2 spectrum of actual condensate gas at a certain pressure point: Two-dimensional NMR T1-T2 spectrum of pure condensate gas at a certain pressure point: Step S3.4: subtract the two-dimensional nuclear magnetic resonance T1-T2 spectrum of the pure condensate gas at a certain pressure point obtained in step S3.3 from the two-dimensional nuclear magnetic resonance T1-T2 spectrum of the actual condensate gas at a certain pressure point in the depletion process, and make an overall difference between the two-dimensional nuclear magnetic resonance T1-T2 spectra. The positive part of the obtained new spectrum represents the precipitated condensate oil, and the sum of the positive parts is the signal amount occupied by the precipitated condensate oil. Substituting the signal amount into the relationship curve between the signal amount and the condensate oil saturation in step S1, the condensate oil saturation at the certain pressure point can be obtained, as shown below;

[0032] Two-dimensional NMR T1-T2 spectrum of actual condensate gas at a certain pressure point - Two-dimensional NMR T1-T2 spectrum of pure condensate gas at a certain pressure point: Among them, the signal amount occupied by the condensate oil is the sum of all positive numbers in the matrix: Among them, i represents the row index, j represents the column index, and the matrix size is n×n; is the indicator function, when The value is 1 when it is, otherwise it is 0;

[0033] Step S3.5: Repeat steps S3.2 to S3.4 to calculate the signal amount of the condensate oil precipitated at each other pressure, so as to obtain the condensate oil saturation corresponding to each pressure point, and finally obtain the relationship curve between pressure and condensate oil saturation.

[0034] As a preferred solution of the method for measuring the critical flow pressure and critical flow saturation of condensate oil by combining one-dimensional and two-dimensional nuclear magnetic resonance technology according to the present invention, the specific operation flow of step S5 includes:

[0035] Step S5.1: Load the core into a long core holder, build up pressure with methane, and then displace methane with condensate gas to establish reservoir conditions;

[0036] Step S5.2: reducing the pressure step by step at a certain speed, and observing the phenomenon in the high-voltage visual window at the same time;

[0037] Step S5.3: When the first drop of oil appears in the visual window, the condensate saturation in the corresponding core is the critical flow saturation, and the corresponding long core holder outlet pressure P0 is recorded;

[0038] Step S5.4: Based on the long core physical parameters, a long core numerical simulation model is established to simulate the long core failure experiment process. According to the critical flow pressure P0 determined by the long core experiment, the critical flow saturation of condensate gas in the long core failure experiment process is finally determined.

[0039] Compared with the existing methods, this invention has the following beneficial effects:

[0040] During the development of condensate gas reservoirs, the pressure reduction will cause condensate oil to precipitate and block the oil and gas flow channel. However, when the condensate oil saturation increases to a certain level, the condensate oil begins to flow continuously. The continuous flow of condensate oil can greatly reduce the blockage of the flow channel by condensate oil. Therefore, it is particularly important to determine the critical flow pressure and critical flow saturation of condensate oil for the development of condensate gas reservoirs. The present invention uses the method of nuclear magnetic resonance and creatively adopts the two-dimensional nuclear magnetic modified difference spectrum method to improve the artificial zoning method, avoiding the error introduced by the artificial signal area division, and can more accurately measure the critical flow pressure and critical flow saturation of condensate oil, better study the flow law of condensate oil and gas two-phase in the formation, which is conducive to helping gas reservoirs formulate more reasonable development plans and provide theoretical guidance for improving condensate oil recovery.

[0041] 1. The present invention directly measures the condensate oil saturation inside the core, rather than the traditional calculation method of inferring the saturation inside the core by analyzing the output material. The measurement result of the present invention is more intuitive and reliable.

[0042] 2. The existing two-dimensional nuclear magnetic resonance difference spectrum method is mainly used to measure porosity or bound phase porosity, and does not have the ability to identify fluids. The present invention creatively proposes a new method for identifying and measuring condensate oil saturation using a two-dimensional nuclear magnetic resonance improved difference spectrum method, rather than the traditional method of calculating condensate oil saturation based on artificial zoning. The disadvantage of artificial zoning is that some oil signals in the divided area will be counted into the gas area, resulting in a deviation in the measured condensate oil saturation. The present method improves the artificial zoning method, avoids the error introduced by artificial signal area division, greatly improves the accuracy of condensate oil signal identification, and more accurately measures condensate oil saturation.

[0043] 3. The existing critical flow pressure is mostly determined through long core experiments. The critical flow pressure is determined when the first drop of oil flows out of the core in the high temperature and high pressure observation window, but the dead volume in the pipeline will have a certain impact on the accuracy of the experiment. The present invention uses a one-dimensional nuclear magnetic resonance combined with a two-dimensional nuclear magnetic resonance test method, and the two verify each other. Starting from the change in the nuclear magnetic signal inside the core, the rate at which hydrogen-containing substances leave the core is accurately measured, and the critical flow pressure of condensate oil can be determined more accurately, avoiding the impact of the dead volume of the pipeline on the measurement.

[0044] 4. This invention combines nuclear magnetic resonance experiments with core numerical simulations. The two results jointly verify the accuracy of the critical flow pressure and critical flow saturation, ensuring the reliability of the invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for describing the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative labor. Among them:

[0046] Figure 1 The original 2D NMR T1-T2 images corresponding to different condensate saturations during the calibration process;

[0047] Figure 2 To calibrate the relationship curve between condensate oil saturation and signal quantity;

[0048] Figure 3 This is a diagram of the nuclear magnetic resonance condensate gas depletion experimental device;

[0049] Figure 3 In: 1, 2, 3, 4-valves; 5-displacement pump; 6-methane sampler; 7-condensate gas sampler; 8-magnet; 9-nuclear magnetic core holder; 10-heating belt; 11-circulating heating device; 12-confining pressure pump; 13-back pressure valve; 14-back pressure pump; 15-gas-liquid separator; 16-gas meter; 17-signal receiving device;

[0050] Figure 4 It is the two-dimensional NMR T1-T2 spectrum under different pressures after being processed by the difference spectrum method;

[0051] Figure 5 This is the relationship curve between pressure and condensate oil saturation during condensate gas depletion process;

[0052] Figure 6 The relationship between the total signal of the two-dimensional nuclear magnetic resonance T1-T2 spectrum and pressure;

[0053] Figure 7This is the relationship between the total signal of one-dimensional NMR T2 spectrum and pressure. DETAILED DESCRIPTION

[0054] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the specific implementation methods of the present invention are described in detail below in conjunction with the accompanying drawings.

[0055] In the following description, many specific details are set forth to facilitate a full understanding of the present invention, but the present invention may also be implemented in other ways different from those described herein, and those skilled in the art may make similar generalizations without violating the connotation of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0056] Secondly, the term "one embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The term "in one embodiment" that appears in different places in this specification does not necessarily refer to the same embodiment, nor does it refer to a separate or selective embodiment that is mutually exclusive with other embodiments.

[0057] In order to achieve the above technical purpose, the present invention adopts the following technical solution, including the following steps:

[0058] Step S1: Calibration experiment is performed on the condensate oil signal and saturation to obtain the relationship curve between the signal and condensate oil saturation; in the two-dimensional nuclear magnetic resonance T1-T2 spectrum, only the relevant information of the signal can be obtained, and the specific value of the saturation cannot be directly obtained. Therefore, it is necessary to first perform a calibration experiment on the condensate oil signal and saturation. The specific operation process is as follows:

[0059] Step S1.1: cleaning and drying the core;

[0060] Step S1.2: After soaking the core with the condensate oil sample, the core is weighed to obtain the condensate oil saturation, and then the soaked core is subjected to a two-dimensional nuclear magnetic resonance T1-T2 test to obtain the two-dimensional nuclear magnetic resonance T1-T2 spectrum of the core sample, that is, the two-dimensional nuclear magnetic resonance T1-T2 spectrum signal amount of the core is obtained;

[0061] Step S1.3: changing the condensate saturation by changing the degree of condensate oil soaking into the core, and measuring the corresponding two-dimensional nuclear magnetic resonance T1-T2 spectrum;

[0062] Step S1.4: in the above experiment, five different saturations are obtained and their corresponding two-dimensional nuclear magnetic resonance T1-T2 spectra are measured, and a relationship curve between the signal amount and the condensate saturation is drawn according to the signal amounts of the two-dimensional nuclear magnetic resonance T1-T2 spectra of the five different condensate saturations.

[0063] Step S2: Perform a depletion experiment on the core to obtain a one-dimensional nuclear magnetic resonance T2 spectrum and a two-dimensional nuclear magnetic resonance T1-T2 spectrum at each pressure point; the specific operation process is as follows:

[0064] Step S2.1: cleaning the core and then drying it;

[0065] Step S2.2: Place the core in the nuclear magnetic holder, use pure methane to establish the core internal pressure, and circulate the fluorinated liquid to apply a confining pressure 3 MPa higher than the core internal pressure, and at the same time control the back pressure pump to apply a back pressure 2 MPa higher than the core internal pressure, start the circulating heating device, and heat it to the experimental temperature; continue to build pressure after the temperature is raised, and perform a methane two-dimensional nuclear magnetic resonance T1-T2 spectrum test every 3 MPa during the methane pressure building process until the maximum experimental pressure is reached;

[0066] Step S2.3: Inject the prepared high-pressure condensate gas into the core to displace methane; inject 10PV of condensate gas to complete condensate gas saturation, and the outlet gas-oil ratio should reach the initial gas-oil ratio, at which point condensate gas saturation is completed;

[0067] Step S2.4: The back pressure is reduced by controlling the back pressure pump, and the back pressure is reduced by 3 MPa each time to simulate the exhaustion process. The confining pressure is reduced synchronously. After each pressure point is stabilized for 2 hours, a one-dimensional nuclear magnetic resonance T2 spectrum test and a two-dimensional nuclear magnetic resonance T1-T2 spectrum test are carried out to obtain a one-dimensional nuclear magnetic resonance T2 spectrum and a two-dimensional nuclear magnetic resonance T1-T2 spectrum at each pressure point;

[0068] Step S2.5: After the experiment is completed, turn off the circulating heating device, remove the experimental pipeline and clean the experimental device to end the experiment.

[0069] Step S3: using the original data of the two-dimensional nuclear magnetic resonance T1-T2 spectrum obtained in step S2, the condensate oil saturation is calculated using the improved difference spectrum method, and the two-dimensional nuclear magnetic resonance T1-T2 spectrum calibration is completed through experimental design, so as to realize the subtraction or addition of the oil zone gas and gas zone oil signals, improve the traditional artificial zoning identification of oil and gas, greatly improve the condensate oil signal identification accuracy, accurately measure the condensate oil saturation, realize the improvement of the artificial zoning method, avoid the error introduced by the artificial signal area division, greatly improve the condensate oil signal identification accuracy, and more accurately measure the condensate oil saturation; wherein, the specific steps of the improved difference spectrum method are as follows:

[0070] Step S3.1: First, subtract the two-dimensional nuclear magnetic resonance T1-T2 spectrum of the dry rock sample from all the two-dimensional nuclear magnetic resonance T1-T2 spectra required to be used. This step is to ensure that the original hydrogen-containing substances in the core will not affect the subsequent calculation of condensate saturation;

[0071] Step S3.2: Divide the signal of the two-dimensional nuclear magnetic resonance T1-T2 spectrum scanned at a certain pressure point during the pressure building process by the signal of the two-dimensional nuclear magnetic resonance T1-T2 spectrum from the pressure building process to the highest experimental pressure point to obtain a series of ratios of each pressure point during the pressure building process to the highest experimental pressure point. This ratio is named the condensate gas correction coefficient. , as shown below;

[0072] Two-dimensional NMR T1-T2 spectrum of a certain pressure point: 2D NMR T1-T2 spectrum at the highest experimental pressure point: Step S3.3: Multiply the overall spectrum of the two-dimensional nuclear magnetic resonance T1-T2 spectrum of the saturated condensate gas at the highest experimental pressure point by the correction coefficient in step S3.2 , a new calibrated two-dimensional nuclear magnetic resonance T1-T2 spectrum is obtained. The actual meaning of this spectrum is the two-dimensional nuclear magnetic resonance T1-T2 spectrum of pure condensate gas (without condensate oil precipitation) at a certain pressure point, as shown below;

[0073] Two-dimensional NMR T1-T2 spectrum of saturated condensate gas at the highest experimental pressure point: Two-dimensional NMR T1-T2 spectrum of actual condensate gas at a certain pressure point: Two-dimensional NMR T1-T2 spectrum of pure condensate gas at a certain pressure point: Step S3.4: subtract the two-dimensional nuclear magnetic resonance T1-T2 spectrum of the actual condensate gas at a certain pressure point in the depletion process (with condensate oil precipitation) from the two-dimensional nuclear magnetic resonance T1-T2 spectrum of the pure condensate gas at a certain pressure point obtained in step S3.3 (without condensate oil precipitation), and make an overall difference between the two two-dimensional nuclear magnetic resonance T1-T2 spectra. The positive part of the obtained new spectrum represents the precipitated condensate oil, and the sum of the positive parts is the signal amount occupied by the precipitated condensate oil. Substituting the signal amount into the relationship curve between the signal amount and the condensate oil saturation in step S1, the condensate oil saturation at the certain pressure point can be obtained, as shown below;

[0074] Two-dimensional NMR T1-T2 spectrum of actual condensate gas at a certain pressure point - Two-dimensional NMR T1-T2 spectrum of pure condensate gas at a certain pressure point: Among them, the signal amount occupied by the condensate oil is the sum of all positive numbers in the matrix: Among them, i represents the row index, j represents the column index, and the matrix size is n×n; is the indicator function, when The value is 1 when it is, otherwise it is 0;

[0075] Step S3.5: Repeat steps S3.2 to S3.4 to calculate the signal amount of the condensate oil precipitated at each other pressure, so as to obtain the condensate oil saturation corresponding to each pressure point, and finally obtain the relationship curve between pressure and condensate oil saturation.

[0076] Step S4: Plot the relationship curves between the total signal amount and pressure of the one-dimensional nuclear magnetic resonance T2 spectrum and the two-dimensional nuclear magnetic resonance T1-T2 spectrum obtained in the depletion experiment, and obtain the pressure point corresponding to the sudden drop point of the curve, which is the critical flow pressure point; then calculate the critical flow saturation corresponding to the critical flow pressure based on the pressure and condensate oil saturation relationship curve obtained by the improved difference spectrum method.

[0077] Step S5: Verify the reliability of critical flow pressure and critical flow saturation by long core failure experiment and numerical simulation; the specific operation process is as follows:

[0078] Step S5.1: Load the core into a long core holder, build up pressure with methane, and then displace methane with condensate gas to establish reservoir conditions;

[0079] Step S5.2: reducing the pressure step by step at a certain speed, and observing the phenomenon in the high-voltage visual window at the same time;

[0080] Step S5.3: When the first drop of oil appears in the visual window, the condensate saturation in the corresponding core is the critical flow saturation, and the corresponding long core holder outlet pressure P0 is recorded;

[0081] Step S5.4: Based on the long core physical parameters, a long core numerical simulation model is established to simulate the long core failure experiment process. According to the critical flow pressure P0 determined by the long core experiment, the critical flow saturation of condensate gas in the long core failure experiment process is finally determined.

[0082] Implementation Cases:

[0083] The core selected in this implementation is an actual sandstone reservoir plunger core sample of a condensate gas reservoir. The specific parameters are shown in Table 1:

[0084]

[0085] The composition of the condensate gas fluid sample selected in this embodiment is shown in Table 2:

[0086]

[0087] The first step is to conduct a calibration experiment on the condensate oil signal and saturation:

[0088] (1) The core was dried and weighed to 69.6 g.

[0089] (2) The core is soaked in condensate oil sample (10% hexane + 90% decane) and weighed to obtain the condensate oil saturation. The 2D NMR T1-T2 spectrum of the core sample is obtained by performing a 2D NMR T1-T2 test, and the signal quantity of the 2D NMR T1-T2 spectrum of the core can be obtained.

[0090] (3) Then, the condensate saturation was changed by changing the degree of condensate oil soaking in the core, and the corresponding two-dimensional nuclear magnetic resonance T1-T2 spectrum was measured at the same time. The original image is shown in Figure 1. The experimental data of the condensate saturation and the two-dimensional nuclear magnetic resonance T1-T2 spectrum signal quantity were measured five times in succession and are shown in Table 3.

[0091]

[0092] (4) The condensate oil saturation in the core increases with the increase of the signal of the two-dimensional nuclear magnetic resonance T1-T2 spectrum. The regression formula is y=0.0204x-1.5111. The calibration curve is shown in Figure 2. The calibration curve has a good linear relationship. This formula is a calibration formula for quantitatively determining the condensate oil signal and saturation during the condensate gas depletion process. Subsequently, the condensate oil signal in the two-dimensional nuclear magnetic resonance T1-T2 spectrum collected at different pressures during the depletion process is identified and calculated, and the obtained condensate oil signal is substituted into the calibration formula to quantitatively obtain the condensate oil saturation corresponding to each pressure point during the core depletion process.

[0093] The second step is to conduct exhaustion experiment: the experimental device diagram is shown in Figure 3, and the specific operation steps are as follows:

[0094] (1) Clean the core and dry it for 24 hours.

[0095] (2) Place the core in the nuclear magnetic core holder, open valves 1 and 4, and use a displacement pump to inject methane into the core at a rate of 0.125 mL / min to establish internal pressure. Use a back pressure pump to continuously apply a back pressure that is always 2 MPa higher than the internal pressure. At the same time, open valve 3, and circulate the fluorinated liquid to apply a confining pressure that is always 3 MPa higher than the internal pressure. When the internal pressure reaches 5 MPa, start the circulating heating device and raise the temperature to 70 °C. After the temperature is raised, continue to build pressure. During the pressure building process, scan the two-dimensional nuclear magnetic resonance T1-T2 spectrum of methane every 3 MPa until the highest experimental pressure point is 30 MPa, then close valve 1.

[0096] (3) Open valve 2 and inject the prepared 30 MPa high-pressure condensate gas into the core at 0.125 mL / min to displace methane. Inject 10 PV of condensate gas to complete condensate gas saturation. The outlet gas-oil ratio reaches the initial gas-oil ratio of 1454 m 3 / m 3 At this time, the saturation of the condensate gas is completed and valve 2 is closed.

[0097] (4) The back pressure was reduced by controlling the back pressure pump, and the back pressure was reduced by 3 MPa each time to simulate the exhaustion process. The confining pressure was reduced synchronously. After each pressure point was stable for 2 h, a one-dimensional NMR T2 test and a two-dimensional NMR T1-T2 test were carried out to obtain a one-dimensional NMR T1 spectrum and a two-dimensional NMR T1-T2 spectrum for each stable pressure point until the pressure was reduced to the waste pressure.

[0098] (5) After the experiment is completed, turn off the circulating heating device, remove the experimental pipelines, clean the experimental device, and end the experiment.

[0099] The third step is data processing: After the depletion experiment is completed, the experimental data is processed. Since it is difficult to directly divide the signal of condensate oil from the original two-dimensional nuclear magnetic resonance T1-T2 spectrum obtained by nuclear magnetic scanning, it is necessary to use the improved difference spectrum method to process the original two-dimensional nuclear magnetic resonance T1-T2 spectrum. Take the difference spectrum method to process the two-dimensional nuclear magnetic resonance T1-T2 spectrum of 23.4MPa as an example:

[0100] (1) Subtract the two-dimensional NMR T1-T2 spectrum of the dry rock sample from all required two-dimensional NMR T1-T2 spectra;

[0101] (2) Divide the signal of the 2D NMR T1-T2 spectrum of methane at 23.4 MPa (2871) by the signal of the 3D NMR T1-T2 spectrum of methane at 30 MPa (3149) to obtain the condensate correction factor: ;

[0102] (3) Multiply the overall spectrum of the 2D NMR T1-T2 spectrum of 30 MPa saturated condensate gas by the correction factor A new calibrated two-dimensional nuclear magnetic resonance T1-T2 spectrum is obtained, which is a two-dimensional nuclear magnetic resonance T1-T2 spectrum of pure condensate gas at 23.4 MPa (without condensate oil precipitation);

[0103] (4) The two-dimensional NMR T1-T2 spectrum of the actual condensate gas at 23.4 MPa (condensate oil has been precipitated) is subtracted from the two-dimensional NMR T1-T2 spectrum of the pure condensate gas at 23.4 MPa after correction obtained in step (3). The two-dimensional NMR T1-T2 spectra are subtracted as a whole. The positive part of the new T1-T2 spectrum is the condensate oil that has been precipitated. The sum of all positive parts 209 is the signal amount occupied by the condensate oil precipitated at 23.4 MPa. The signal amount is substituted into the calibration formula y = 0.0204x - 1.5111, and the condensate oil saturation at 23.4 MPa is obtained to be 2.75%.

[0104] (5) The T1-T2 spectrum obtained after differential spectrum processing is shown in Figure 4, where the red part is the condensate oil that has been precipitated. Repeat steps (2) to (4) to calculate the signal amount occupied by the condensate oil that has been precipitated at each other pressure, and substitute them into the calibration formula of condensate oil signal and saturation y=0.024x-1.5111, respectively. The results of condensate oil saturation at different pressures are shown in Table 4. The relationship curve between condensate oil saturation and pressure is shown in Figure 5.

[0105]

[0106] The fourth step is to calculate the critical flow pressure and critical flow saturation:

[0107] (1) The relationship between the total signal amount and pressure of the two-dimensional nuclear magnetic resonance T1-T2 spectrum during the exhaustion process is shown in FIG6 ; the relationship between the total signal amount and pressure of the one-dimensional nuclear magnetic resonance T2 spectrum is shown in FIG7 .

[0108] (2) It can be concluded that the relationship curves of the total signal amount and pressure of the one-dimensional nuclear magnetic resonance T2 spectrum and the two-dimensional nuclear magnetic resonance T1-T2 spectrum of high-content condensate oil both have obvious sudden drop points near 17.93 MPa, indicating that the condensate oil begins to flow continuously and leaves the core in large quantities under this pressure. Therefore, 17.93 MPa is the critical flow pressure of the condensate oil in this embodiment. According to the relationship curve between pressure and condensate oil saturation during the condensate gas depletion process in Figure 5, it is concluded that the critical flow saturation reached at this time is 8.51%.

[0109] The fifth step is to verify the experimental results:

[0110] The core is loaded into a long core holder and after pressure build-up with methane, the methane is displaced with condensate gas to establish reservoir conditions.

[0111] The pressure was gradually reduced at a depletion rate of 3 MPa / h. When a drop of oil was observed in the viewing window, the outlet pressure of the long core holder was recorded as 18.15 MPa.

[0112] Based on the physical property parameters of the long core, a long core numerical simulation model was established to simulate the long core failure process. According to the critical flow pressure of 18.15 MPa determined in step (2), the critical flow saturation of condensate gas in the long core failure process was determined to be 8.68%.

[0113] Among them, the relative error of the critical flow pressure calculated by the two experimental methods is 1.21%, and the relative error of the critical flow saturation is 2.00%.

[0114] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the spirit and scope of the technical solutions of the present invention, which should all be included in the scope of the claims of the present invention.

Claims

1. A method for measuring the critical flow pressure and critical flow saturation of condensate oil by combining one-dimensional and two-dimensional nuclear magnetic resonance techniques, characterized in that: The steps include: Step S1: performing a calibration experiment on the condensate oil signal amount and saturation to obtain a relationship curve between the signal amount and the condensate oil saturation; Step S2: performing a depletion experiment on the core to obtain a one-dimensional nuclear magnetic resonance T2 spectrum and a two-dimensional nuclear magnetic resonance T1-T2 spectrum at each pressure point; Step S3: Calculating the condensate oil saturation by using the original data of the two-dimensional nuclear magnetic resonance T1-T2 spectrum obtained in step S2 using the improved difference spectrum method; The specific steps of the improved difference spectrum method include: Step S3.1: First, subtract the two-dimensional nuclear magnetic resonance T1-T2 spectrum of the dry rock sample from all the two-dimensional nuclear magnetic resonance T1-T2 spectra required; Step S3.2: Divide the signal of the two-dimensional nuclear magnetic resonance T1-T2 spectrum scanned at a certain pressure point during the pressure building process by the signal of the two-dimensional nuclear magnetic resonance T1-T2 spectrum from the pressure building process to the highest experimental pressure point to obtain a series of ratios of each pressure point during the pressure building process to the highest experimental pressure point. This ratio is named the condensate gas correction coefficient. , as shown below; Two-dimensional NMR T1-T2 spectrum of a certain pressure point: 2D NMR T1-T2 spectrum at the highest experimental pressure point: Step S3.3: Multiply the overall spectrum of the two-dimensional nuclear magnetic resonance T1-T2 spectrum of the saturated condensate gas at the highest experimental pressure point by the correction coefficient in step S3.2 , a new calibrated two-dimensional nuclear magnetic resonance T1-T2 spectrum is obtained. The actual meaning of this spectrum is the two-dimensional nuclear magnetic resonance T1-T2 spectrum of pure condensate gas at a certain pressure point, as shown below; Two-dimensional NMR T1-T2 spectrum of saturated condensate gas at the highest experimental pressure point: Two-dimensional NMR T1-T2 spectrum of actual condensate gas at a certain pressure point: Two-dimensional NMR T1-T2 spectrum of pure condensate gas at a certain pressure point: Step S3.4: subtract the two-dimensional nuclear magnetic resonance T1-T2 spectrum of the pure condensate gas at a certain pressure point obtained in step S3.3 from the two-dimensional nuclear magnetic resonance T1-T2 spectrum of the actual condensate gas at a certain pressure point in the depletion process, and make an overall difference between the two-dimensional nuclear magnetic resonance T1-T2 spectra. The positive part of the obtained new spectrum represents the precipitated condensate oil, and the sum of the positive parts is the signal amount occupied by the precipitated condensate oil. Substituting the signal amount into the relationship curve between the signal amount and the condensate oil saturation in step S1, the condensate oil saturation at the certain pressure point can be obtained, as shown below; Two-dimensional NMR T1-T2 spectrum of actual condensate gas at a certain pressure point - Two-dimensional NMR T1-T2 spectrum of pure condensate gas at a certain pressure point: Among them, the signal amount occupied by the condensate oil is the sum of all positive numbers in the matrix: Among them, i represents the row index, j represents the column index, and the matrix size is n×n; is the indicator function, when The value is 1 when it is, otherwise it is 0; Step S3.5: Repeat steps S3.2 to S3.4 to calculate the signal amount of the condensate oil precipitated at each other pressure, so as to obtain the condensate oil saturation corresponding to each pressure point, and finally obtain the relationship curve between pressure and condensate oil saturation; Step S4: Plotting the relationship curves between the total signal amount and pressure of the one-dimensional nuclear magnetic resonance T2 spectrum and the two-dimensional nuclear magnetic resonance T1-T2 spectrum obtained from the depletion experiment, respectively, to obtain the pressure point corresponding to the sudden drop point of the curve, which is the critical flow pressure point; then, calculating the critical flow saturation corresponding to the critical flow pressure based on the pressure and condensate oil saturation relationship curve obtained by the improved difference spectrum method; Step S5: Using long core failure experiments and numerical simulations to verify the reliability of the critical flow pressure and the critical flow saturation.

2. The method for measuring the critical flow pressure and critical flow saturation of condensate oil by combining one-dimensional and two-dimensional nuclear magnetic resonance techniques according to claim 1, characterized in that: The specific operation process of step S1 includes: Step S1.1: cleaning and drying the core; Step S1.2: After soaking the core with the condensate oil sample, the core is weighed to obtain the condensate oil saturation, and then the soaked core is subjected to a two-dimensional nuclear magnetic resonance T1-T2 test to obtain the two-dimensional nuclear magnetic resonance T1-T2 spectrum of the core sample, that is, the two-dimensional nuclear magnetic resonance T1-T2 spectrum signal amount of the core is obtained; Step S1.3: changing the condensate saturation by changing the degree of condensate oil soaking into the core, and measuring the corresponding two-dimensional nuclear magnetic resonance T1-T2 spectrum; Step S1.4: in the above experiment, five different saturations are obtained and their corresponding two-dimensional nuclear magnetic resonance T1-T2 spectra are measured, and a relationship curve between the signal amount and the condensate saturation is drawn according to the signal amounts of the two-dimensional nuclear magnetic resonance T1-T2 spectra of the five different condensate saturations.

3. The method for measuring the critical flow pressure and critical flow saturation of condensate oil by combining one-dimensional and two-dimensional nuclear magnetic resonance techniques according to claim 1, characterized in that: The specific operation process of step S2 includes: Step S2.1: cleaning the core and then drying it; Step S2.2: Place the core in the nuclear magnetic resonance holder, use pure methane to establish the core internal pressure, circulate the fluorinated liquid to apply a higher confining pressure than the core internal pressure, and control the back pressure pump to apply a higher back pressure than the core internal pressure. Start the circulating heating device and heat it to the experimental temperature. Continue to build pressure after the temperature is raised. During the methane pressure building process, perform a methane two-dimensional nuclear magnetic resonance T1-T2 spectrum test every 3 MPa until the maximum experimental pressure is reached. Step S2.3: Inject the prepared high-pressure condensate gas into the core to displace methane; inject 10PV of condensate gas to complete condensate gas saturation, and the outlet gas-oil ratio should reach the initial gas-oil ratio, at which point condensate gas saturation is completed; Step S2.4: The back pressure is reduced by controlling the back pressure pump, and the back pressure is reduced by 3 MPa each time to simulate the exhaustion process. The confining pressure is reduced synchronously. After each pressure point is stabilized for 2 hours, a one-dimensional nuclear magnetic resonance T2 spectrum test and a two-dimensional nuclear magnetic resonance T1-T2 spectrum test are carried out to obtain a one-dimensional nuclear magnetic resonance T2 spectrum and a two-dimensional nuclear magnetic resonance T1-T2 spectrum at each pressure point; Step S2.5: After the experiment is completed, turn off the circulating heating device, remove the experimental pipeline and clean the experimental device to end the experiment.

4. The method for measuring the critical flow pressure and critical flow saturation of condensate oil by combining one-dimensional and two-dimensional nuclear magnetic resonance techniques according to claim 3, characterized in that: The circulating fluorinated liquid in step S2.2 applies a confining pressure that is 3 MPa higher than the core internal pressure.

5. The method for measuring the critical flow pressure and critical flow saturation of condensate oil by combining one-dimensional and two-dimensional nuclear magnetic resonance techniques according to claim 4, characterized in that: During the pressure building process in step S2.2, the back pressure is ensured to be 2 MPa higher than the core internal pressure.

6. The method for measuring the critical flow pressure and critical flow saturation of condensate oil by combining one-dimensional and two-dimensional nuclear magnetic resonance techniques according to claim 1, characterized in that: The specific operation process of step S5 includes: Step S5.1: Load the core into a long core holder, build up pressure with methane, and then displace methane with condensate gas to establish reservoir conditions; Step S5.2: reducing the pressure step by step at a certain speed, and observing the phenomenon in the high-voltage visual window at the same time; Step S5.3: When the first drop of oil appears in the visual window, the condensate saturation in the corresponding core is the critical flow saturation, and the corresponding long core holder outlet pressure P0 is recorded; Step S5.4: Based on the long core physical parameters, a long core numerical simulation model is established to simulate the long core failure experiment process. According to the critical flow pressure P0 determined by the long core experiment, the critical flow saturation of condensate gas in the long core failure experiment process is finally determined.

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