Shale oil carbon dioxide huff and puff graded pressure drop mining experiment method

Abstract

The invention discloses a shale oil carbon dioxide huff and puff graded pressure drop mining experiment method. The method comprises the following steps that S1, the crude oil recovery degrees under different huff and puff experiment requirements are obtained through experiments; the method is suitable for the technical field of oil and gas field development, the CO2 utilization rate and the recovery percentage of each stage in the shale oil carbon dioxide huff and puff graded pressure drop exploitation process can be accurately measured, a database is established in combination with influence factors of different injection conditions, and the reliability of the shale oil carbon dioxide huff and puff graded pressure drop exploitation process is improved. And on the basis of experimental results, weight analysis of shale oil carbon dioxide huff and puff graded pressure drop mining is carried out on each factor, so that main influence factors of shale oil carbon dioxide huff and puff graded pressure drop mining can be screened out, which is of great significance to subsequent shale oil carbon dioxide huff and puff mining adjustment.

Description

Technical Field

[0001] The present invention belongs to the technical field of oil and gas field development, and particularly relates to an experimental method for staged pressure drop exploitation of shale oil by CO₂ huff and puff Background Art

[0002] With the gradual increase in the consumption of global conventional energy, the call for developing clean and efficient unconventional energy has become increasingly strong. As a new type of energy, shale oil has come onto the historical stage. It has the characteristics of stable distribution, large thickness, and wide distribution range, and has become a hot topic for research in various countries around the world.

[0003] So far, the shale oil revolution in the United States has achieved remarkable results. Countries such as Canada and Australia have successively carried out commercial development of shale oil. In the future, it is expected to become an important energy connection link. China has a high dependence on foreign crude oil and relatively limited resources. Therefore, the efficient development of unconventional natural gas mainly based on shale oil is one of the most effective ways to alleviate the energy shortage situation and improve the energy structure in China.

[0004] China's shale oil reserves are very rich. According to incomplete statistics, its total recoverable reserves are 5.0×10⁹ t, ranking among the top in the world. In recent years, after years of exploration and development practice, significant breakthroughs have been made in shale oil pilot tests in demonstration areas such as Tarim, Bohai Bay, and Songliao. However, due to the limitations of relevant technical conditions, no key breakthroughs have been achieved in shale oil extraction methods and development technologies, and there is an obvious gap compared with the development means of developed countries such as the United States. Accelerating the large-scale industrial development of domestic shale oil and achieving a revolutionary strategic replacement is the dream of the Chinese continental shale oil revolution.

[0005] So far, oil production engineering has developed into three major series for enhancing oil recovery, namely chemical flooding, miscible flooding, and thermal oil recovery. In recent years, methods for enhancing oil recovery using microorganisms and principles such as sound, light, electricity, and magnetism are also in development. The gas injection huff and puff method is the most potential way to enhance the recovery of shale oil. The injected gas mainly includes hydrocarbon gas, CO 2 and N 2 .

[0006] Yu et al. explored the effects of soaking time and production time consumption on fractured matrix shale oil reservoirs in huff and puff experiments with different gases.

[0007] Lambert et al. found through comparing miscible and immiscible processes that when the injection pressure is lower than the MMP, the oil production coefficient gradually increases with the increase in injection pressure.

[0008] Among them, injecting CO₂ is one of the most important ways of gas injection development. The reason why CO 2 injection development can be promoted and utilized globally mainly has two reasons. One is that injecting CO 2It can effectively reduce the viscosity of crude oil and has an obvious production increase effect. Second, the injection of CO 2 The demand for its dosage is large in development, which can, to a certain extent, help alleviate the global greenhouse effect. CO 2 Huff and puff is an important way to improve oil recovery. It injects CO 2 at an injection pressure lower than the formation fracture pressure into the corresponding reservoir, soaks the well for a certain period of time, and makes CO 2 fully dissolve in the reservoir crude oil, reduce the viscosity of the crude oil, improve the flow capacity of the crude oil, and then achieve the goal of improving oil recovery. 2

[0009] Currently, CO 2 huff and puff has been successfully used to improve the production degree under various different reservoir conditions. Compared with other enhanced oil recovery measures to improve oil recovery, it has the advantages of less investment, quick effect, less gas consumption, early income and low technical requirements, and has obvious economic advantages, so it is favored by each oilfield. In addition, even though the oil recovery rate is the highest under MMP, it is still lower than that of crude oil under miscible state.

[0010] Zhan et al. conducted a series of CO 2 huff and puff experiments, investigated the effects of pressure, injection rate, and soaking time on huff and puff performance, and obtained an oil recovery rate of more than 30% after 3 huff and puff cycles.

[0011] However, in the current existing technologies, there is no experimental method to accurately quantify the oil production degree of crude oil under different pressure drops during the CO 2 huff and puff backflow process of shale oil, and in the analysis of different factors, there is no analysis method to calculate the main controlling factors of the oil production degree during the staged pressure reduction production process.

[0012] In summary, there is an urgent need for an experimental method for staged pressure reduction production of shale oil carbon dioxide huff and puff, which not only needs to accurately measure the oil production degree during production under different pressure drops, but also needs to analyze the main controlling factors and influence weights among different injection-production influence parameters, which is of great significance for guiding the adjustment of production conditions. Summary of the Invention

[0013] The purpose of the present invention is to overcome the defects of the existing technologies and provide an experimental method for staged pressure reduction production of shale oil carbon dioxide huff and puff.

[0014] To achieve the above purpose, the present invention adopts the following technical solutions:

[0015] An experimental method for staged pressure reduction production of shale oil carbon dioxide huff and puff, comprising the following steps:

[0016] S1 Obtain the oil production degree under different huff and puff experimental requirements through experiments;

[0017] ​Analysis of the main controlling factors for the effect of CO₂ huff and puff

[0018] Preferably, in step S1, the oil production degree under different huff and puff experiment requirements is obtained through experiments, specifically including:

[0019] S11 Select a core, calculate the pore volume of the core, and prepare it into a core sample;

[0020] S12 Obtain a given pressure environment through the experimental device; 2

[0021] S13 Inject CO₂ into the core holder, calculate the injected volume of CO₂ and the number of moles of CO₂ in the core holder; 2 2 2

[0022] S14 Obtain the weight of the drying tube for every 5 MPa pressure drop, and calculate the final mole fraction of CO₂; 2

[0023] S15 Repeat steps S12 - S14, record the weight of the drying tube for each huff and puff cycle, and calculate the oil production degree for every 5 MPa pressure drop.

[0024] Preferably, in step S11, select a core, wash it with oil, dry it and weigh it, denoted as w₁, then measure the basic parameters such as the core length L, diameter D, permeability K, and porosity Φ, and calculate the pore volume of the core; a

[0025] Subsequently, evacuate the core, pressurize it and saturate it with formation oil. After the weight is stable, take out the core sample, wipe it dry and weigh it again, denoted as w₂. b

[0026] Preferably, in step S11, the calculation formula for the pore volume of the core is:

[0027]

[0028] Preferably, in step S12, it specifically includes the following steps:

[0029] Open the first two-way valve, use an air compressor to add the gas in the CO₂ gas cylinder to the intermediate container and pressurize it to the required pressure, then close the first two-way valve; 2

[0030] Put the core into the core holder, connect all pipelines, open the ninth two-way valve, use a hand pump to increase the confining pressure and then close the ninth two-way valve; open the second two-way valve and the third two-way valve, set the high-pressure displacement pump to a constant pressure mode, use the high-pressure displacement pump to add CO₂ at a constant pressure, and then close the second two-way valve and the third two-way valve; 2

[0031] Open the No. 9 two-way valve, use a hand pump to apply confining pressure 5 MPa higher than the given pressure, and record the amount of CO added through the high-pressure displacement pump as V; close the No. 9 two-way valve; open the No. 2 two-way valve and the No. 3 two-way valve, and use the high-pressure displacement pump to add CO to the given pressure, then close the No. 2 two-way valve and the No. 3 two-way valve. 2 Open the No. 9 two-way valve, use a hand pump to apply confining pressure 5 MPa higher than the given pressure, and record the amount of CO added through the high-pressure displacement pump as V; close the No. 9 two-way valve; open the No. 2 two-way valve and the No. 3 two-way valve, and use the high-pressure displacement pump to add CO to the given pressure, then close the No. 2 two-way valve and the No. 3 two-way valve. 2 to the given pressure, and then close the No. 2 two-way valve and the No. 3 two-way valve.

[0032] Preferably, in step S13, the following steps are specifically included:

[0033] Place the experimental device in an incubator and control the experimental temperature, i.e., the actual formation temperature of the block, as T;

[0034] Open the No. 6 two-way valve, use a pressure sensor to record the pressure at the start of well shut-in as P, and record the pressure change during well shut-in. Close all other valves to allow the rock sample in the core holder to fully contact with CO, let CO diffuse in the rock sample, conduct well shut-in, and record the pressure as P 2 contact, and let CO 2 diffuse in the rock sample, conduct well shut-in, and record the pressure as P 1 at this time, and the temperature is T 1 ;

[0035] Calculate the volume V 2 of the injected CO 1 at this time and the number of moles of CO 2 in the core holder.

[0036] Preferably, in step S13, the volume calculation formula for injecting CO 2 is as follows:

[0037]

[0038] The calculation formula for the number of moles of CO 2 in the core holder is as follows:

[0039]

[0040] In the formula: Z 1 ——Compression factor at P 1 ;

[0041] R——Gas constant, 8.314 J / (mol·K).

[0042] Preferably, in step S14, the specific steps include:

[0043] Weigh the initial weight of the drying tube and record it as m 0 , and install the drying tube on the pipeline;

[0044] Set the backpressure pump pressure to the production pressure, open the No. 7 two-way valve and the No. 8 two-way valve. After the pressure stabilizes, close the No. 7 two-way valve and the No. 8 two-way valve, remove the drying tube, weigh it, and record the weight as m 1 , reinstall the drying tube to the pipeline again; adjust the backpressure pump pressure to reduce it by 5 MPa, and record the pressure at this time as P 2 ;

[0045] Open the No. 7 two-way valve and the No. 8 two-way valve. After the pressure stabilizes, close the No. 7 two-way valve and the No. 8 two-way valve, remove the drying tube, weigh it, and record the weight as m 2 , use a wet gas flowmeter to record the collected CO 2 volume as V 2 ’, calculate the volume V of this volume at P 2 pressure 2 ;

[0046] Calculate the remaining and discharged CO 2 moles in the core holder, calculate the CO 2 moles n of CO that enters the core at this pressure 2 and the utilization rate of CO 4 at this pressure 2 .

[0047] Preferably, in the step S14, the calculation formula for the remaining and discharged CO 2 moles in the core holder is:

[0048]

[0049]

[0050] In the formula: Z 2 —— Compression factor at P 2 ;

[0051] T 2 —— Temperature of the thermostat at this time

[0052] Preferably, in the step S14, the CO 2 moles n of CO that enters the core at this pressure 2 and the utilization rate calculation formula of CO 4 at this pressure are: 2

[0053] n 4 =n 1 -n 2 +n 3 (6);

[0054] In the formula: n 1 —— CO injected into the core holder 2 ​Number of moles;

[0055] n 2 ——CO in the intermediate container after the pressure is reduced 2 Number of moles;

[0056] n 3 ——CO is discharged after the pressure is reduced 2 The number of moles;

[0057] n 4 ——CO 2 The number of moles interacting with crude oil;

[0058]

[0059] Preferably, the step S14 further includes:

[0060] Repeatedly reduce the pressure by 5 MPa and record the weight of the drying tube as m 3 、m 4 ... until the pressure drops to 5 MPa, and calculate the final CO 2 Mole fraction n ＝ During this period, online nuclear magnetic resonance can be used to observe the occurrence and displacement of crude oil in the core.

[0061] Preferably, the step S15 specifically includes:

[0062] According to different throughput test requirements, steps S12 to S14 are repeated to start a new round of throughput. After each throughput cycle, the weight of the drying tube should be recorded in time to calculate the crude oil recovery degree for every 5 MPa pressure drop. The calculation formula is:

[0063]

[0064] Preferably, the step S1 further includes:

[0065] After the S16 experiment was completed, the pipeline was cleaned by displacement with alcohol and petroleum ether, and the instrument was disassembled and properly placed.

[0066] Preferably, the step S2 specifically includes the following steps:

[0067] S21 divides the value of each influencing factor into n evaluation values ​​according to its value range, thereby establishing an experimental result data table, an experimental result analysis database, an average effect database of each factor level, and a standard deviation database of each factor interference;

[0068] S22 builds shale oil CO 2 The judgment matrix of the two-to-two comparison of the factors affecting the throughput and production degree coefficient;

[0069] S23 Shale Oil CO2 Calculation of the weight of influencing factors for the production degree of huff and puff

[0070] S24 Consistency check.

[0071] Preferably, in the step S21, the influencing factors include static parameters: permeability, porosity; dynamic parameters: injection pressure, shut-in time.

[0072] Preferably, in the step S21, the value ranges of the influencing factors are as follows:

[0073] Permeability, mD: 0.001 - 0.5;

[0074] Porosity, %: 2 - 12;

[0075] Injection pressure, MPa: 20 - 50;

[0076] Shut-in time, h: 1 - 12.

[0077] Preferably, in the step S22, it specifically includes:

[0078] The standard deviation results of each factor on the shale oil CO 2 production degree obtained by calculating the standard deviation, so as to quantitatively characterize the influence degree of each factor on the shale oil CO 2 production degree;

[0079] The influencing factors of the shale oil CO 2 production degree are compared pairwise for their importance to the shale oil CO 2 production degree coefficient, that is, the standard deviations b i and b j of two factors are taken each time, and a ij is used to quantitatively characterize the relative importance degree of the influence of b i and b j on the shale oil CO 2 production degree, and the result is represented by the matrix A = (a ij ) n×n ;

[0080] Among them, the matrix A is the judgment matrix of each factor influencing the shale oil CO 2 production degree.

[0081] Preferably, in the step S23, the maximum eigenvalue λ max of the judgment matrix and the eigenvector are calculated, and the calculation formula is:

[0082] AW = λ max W (5);

[0083] Among them, the maximum eigenvalue λ maxThe solution method is to find the roots of the characteristic equation det(λI - A) = 0 of matrix A. Multiple roots λ are the eigenvalues of matrix A. Take the root with the largest numerical value as the largest eigenvalue λ of matrix A. max ;

[0084] Among them, I is the identity matrix with the same dimension as matrix A;

[0085] Substitute the obtained largest eigenvalue λ max into the formula AW = λ max W, and the eigenvector W corresponding to the largest eigenvalue of matrix A can be obtained; after normalizing the eigenvector W, the relative weight coefficients of each influencing factor of shale oil CO 2 huff and puff can be obtained.

[0086] Preferably, in step S24, calculate the consistency ratio of the judgment matrix, and its index is CI = (λ max - n) / (n - 1), the CI value obtained by calculation; among them, when CI = 0, there is complete consistency; when CI is close to 0, there is satisfactory consistency; the larger CI is, the more serious the inconsistency is.

[0087] Preferably, in step S24, in order to measure the magnitude of CI, introduce the random consistency index RI.; according to the saaty table, RI. can be calculated; the consistency ratio CR is calculated by CI / RI.; if the consistency ratio meets the design requirements, the weight coefficients obtained above are the final weights of each production influencing factor, and the weight database of each factor is obtained, otherwise, reconstruct the judgment matrix.

[0088] In summary, due to the adoption of the above technical solutions, the beneficial effects of the present invention are:

[0089] In the present invention, it is possible to accurately measure the CO 2 utilization rate and recovery degree of each stage during the shale oil carbon dioxide huff and puff staged pressure drop production process, establish a database in combination with the influencing factors of different injection conditions, and conduct a weight analysis of shale oil carbon dioxide huff and puff staged pressure drop production for each factor based on the experimental results, so as to screen out the main influencing factors of shale oil carbon dioxide huff and puff staged pressure drop production, which is of great significance for the subsequent adjustment of shale oil carbon dioxide huff and puff production. BRIEF DESCRIPTION OF THE DRAWINGS

[0090] Figure 1 is a schematic diagram of the carbon dioxide huff and puff experimental device in the present invention;

[0091] Figure 2 is a schematic diagram of the recovery degree of different rounds after soaking for 12h in the embodiment of the present invention;

[0092] Figure 3It is a schematic diagram of the production degree per 5 MPa pressure difference at 30 MPa in the embodiment of the present invention.

[0093] Reference numerals: 1, air compressor; 2, CO 2 gas cylinder; 3, displacement pump; 4, first two-way valve; 5, second two-way valve; 6, intermediate container; 7, third two-way valve; 8, fourth two-way valve; 9, intermediate container filled with water; 10, fifth two-way valve; 11, pressure sensor; 12, sixth two-way valve; 13, core holder; 14, sleeve of on-line NMR; 15, seventh two-way valve; 16, back pressure pump; 17, eighth two-way valve; 18, drying tube; 19, ninth two-way valve; 20, hand-operated pressure pump; 21, pipeline; 22, wet gas flowmeter. Detailed implementation manners

[0094] The following further describes the detailed implementation manners of an experimental method for staged pressure drop exploitation of shale oil carbon dioxide huff and puff in combination with the attached Figures 1-3 , and is not limited to the description of the following embodiments for an experimental method for staged pressure drop exploitation of shale oil carbon dioxide huff and puff of the present invention.

[0095] During the process of shale oil carbon dioxide huff and puff, the pressure generally does not directly drop to atmospheric pressure when the well is opened for production. There are obvious differences in the amount of carbon dioxide produced each time the pressure is reduced, and the amount of carbon dioxide produced is affected by factors such as injection pressure and shut-in time. This process has a great impact on the productivity prediction, measure adjustment, and injection-production optimization of the shale oil carbon dioxide huff and puff development mode. Therefore, it is necessary to establish an experimental method to quantitatively characterize the staged pressure drop exploitation of shale oil carbon dioxide huff and puff.

[0096] 1. Experimental principle and process

[0097] This experimental device can measure the pressure and production degree during the staged pressure drop exploitation of shale oil carbon dioxide huff and puff. The structure is simple, and only manual adjustment of the change in exploitation pressure is required during the experiment. The device has high precision, and the precision of the hand pump is 2.7×10 -3 ml, and the manufacturing and maintenance costs are low.

[0098] As Figure 1 shown, it is a schematic diagram of the carbon dioxide huff and puff experimental device in the present invention. Among them, 1 is an air compressor; 2 is a CO 2 gas cylinder; 3 is a displacement pump; 4 is a first two-way valve; 5 is a second two-way valve; 6 is an intermediate container; 7 is a third two-way valve; 8 is a fourth two-way valve; 9 is an intermediate container filled with water; 10 is a fifth two-way valve; 11 is a pressure sensor; 12 is a sixth two-way valve; 13 is a core holder; 14 is a sleeve of on-line NMR; 15 is a seventh two-way valve; 16 is a back pressure pump; 17 is an eighth two-way valve; 18 is a drying tube; 19 is a ninth two-way valve; 20 is a hand-operated pressure pump; 21 is the pipeline used in the experiment; 22 is a wet gas flowmeter.

[0099] The experimental procedure is as follows:

[0100] (1) Select a core, wash it with oil, dry it, and weigh it, denoted as w a , then measure the basic parameters of the core such as length L, diameter D, permeability K, and porosity Φ, and then calculate the pore volume PV of the core using Equation (1);

[0101]

[0102] Subsequently, evacuate the core, apply pressure and saturate it with formation oil. After the weight is stable, take out the core sample, dry it, and weigh it again, denoted as w b .

[0103] (2) Open the first two-way valve 4, and use the air compressor 1 to add the gas in the CO 2 cylinder 2 into the intermediate container 6 and pressurize it to the required pressure, then close the first two-way valve 4. Place the core in the core holder 13, connect all pipelines, open the ninth two-way valve 19, use the hand pump 20 to increase the confining pressure, and then close the ninth two-way valve 19. Open the second two-way valve 5 and the third two-way valve 7, set the high-pressure displacement pump 3 to the constant pressure mode, use the high-pressure displacement pump 3 to add CO 2 at a constant pressure, and then close the second two-way valve 5 and the third two-way valve 7. Open the ninth two-way valve 19, use the hand pump 20 to increase the confining pressure by 5 MPa higher than the given pressure, and record the amount of CO 2 added through the high-pressure displacement pump 3 as V. Close the ninth two-way valve 19; open the second two-way valve 5 and the third two-way valve 7, use the high-pressure displacement pump 3 to add CO 2 to the given pressure, and then close the second two-way valve 5 and the third two-way valve 7;

[0104] (3) Place the entire set of equipment in an incubator, control the experimental temperature, i.e., the actual formation temperature of the block, as T. Open the sixth two-way valve 12, use the pressure sensor to record the pressure at the start of the shut-in well as P, and record the pressure change during the shut-in well. Close all other valves to allow the core sample in the core holder 13 to fully contact with CO 2 , and let CO 2 diffuse in the core sample. Shut in the well and record the pressure at this time as P 1 , and the temperature as T 1 . Then, calculate the volume V 1 of CO2 injected at this time and the number of moles n 2 of CO 1 in the core holder 13 using Equations 2 and 3.

[0105]

[0106]

[0107] Where: Z 1 —— Compression factor at P 1 ;

[0108] R —— Gas constant, 8.314 J / (mol·K).

[0109] (4) Weigh the initial weight of the drying tube 18 and record it as m 0 , install the drying tube 18 to the pipeline. Set the pressure of the back-pressure pump 16 to the production pressure, open the No. 7 two-way valve 15 and the No. 8 two-way valve 17. After the pressure is stable, close the No. 7 two-way valve 15 and the No. 8 two-way valve 17, remove the drying tube 18 and weigh it, record the weight as m1, and install the drying tube 18 to the pipeline again; adjust the pressure of the back-pressure pump 16 to reduce the pressure by 5 MPa, record the pressure at this time as P 2 , open the No. 7 two-way valve 15 and the No. 8 two-way valve 17. After the pressure is stable, close the No. 7 two-way valve 15 and the No. 8 two-way valve 17, remove the drying tube 18 and weigh it, record the weight as m 2 , use the wet gas flowmeter 22 to record the volume of the collected CO2 as V 2 ’, from which the volume V 2 at the pressure of P 2 can be calculated, and the remaining and discharged CO 2 moles in the core holder 13 can be calculated by Equations 4 and 5, which are n 2 and n 3 respectively. Thus, the moles of CO 2 entering the core at this pressure, n 2 , and the utilization rate ζ of CO 4 at this pressure can be calculated according to Equations 6 and 7. 2

[0110]

[0111]

[0112] Where: Z 2 —— Compression factor at P 2 ;

[0113] T 2 —— Temperature of the thermostat at this time.

[0114] n 4 = n 1 - n 2 + n 3 (6);

[0115] Where: n 1 —— Moles of CO 2 injected into the core holder;

[0116] n 2 ​—— Moles of CO in the intermediate container after pressure reduction 2 Moles;

[0117] n 3 —— Moles of CO discharged after pressure reduction 2 ;

[0118] n 4 —— Moles of CO 2 reacting with crude oil.

[0119]

[0120] Record the weight of the drying tube 18 as m 3 , m 4 … until the pressure drops to 5 MPa. And calculate the final CO2 mole fraction n 总 , and during this period, online NMR 14 can be used to observe the occurrence and displacement of crude oil in the core.

[0121] (5) According to the requirements of different huff and puff experiments, repeat steps (2)-(4) to start a new round of huff and puff; record the weight of the drying tube in time after each huff and puff cycle. Calculate the oil production degree per 5 MPa pressure drop through Equation 8

[0122]

[0123] As Figure 2 shown, it is a schematic diagram of the oil production degree of different rounds after soaking the well for 12 h.

[0124]

[0125] As Figure 3 shown, it is a schematic diagram of the oil production degree per 5 MPa pressure difference at 30 MPa.

[0126] (6) After the experiment, use alcohol and petroleum ether for displacement to clean the pipeline, then disassemble the instrument device and place it properly.

[0127] 2. Analysis of the main controlling factors of CO2 huff and puff effect

[0128] (1) Static parameters

[0129] Permeability: Select the development effect of CO 2 huff and puff for 5 cycles when the average permeability is 0.01 times (i.e., the average permeability is 0.001 mD) to 10 times (i.e., the average permeability is 1 mD) of the typical permeability. Plot the cumulative oil production degree curve of permeability for CO 2 huff and puff for 5 rounds.

[0130] Porosity: The development effect of 5 cycles of huff and puff was selected when the average porosity was 0.5 times (i.e., the average porosity was 2%) to 3 times (i.e., the average porosity was 12%) of the typical porosity. The porosity was plotted against the cumulative oil production increment curve of CO for 5 cycles of huff and puff. 2 The development effect of 5 cycles of huff and puff was selected. The porosity was plotted against the cumulative oil production increment curve of CO for 5 cycles of huff and puff. 2 Cumulative oil production increment curve for 5 cycles of huff and puff.

[0131] (2) Dynamic parameters

[0132] Injection pressure: The influence of different injection pressures on the development effect was experimented, and the injection pressure was plotted against the cumulative oil production increment curve of CO for 5 cycles of huff and puff. 2 Cumulative oil production increment curve for 5 cycles of huff and puff.

[0133] Soaking time: The influence of different soaking times on the development effect was experimented, and the soaking time was plotted against the cumulative oil production increment curve of CO for 5 cycles of huff and puff. 2 Cumulative oil production increment curve for 5 cycles of huff and puff.

[0134] (3) Analysis of main controlling factors

[0135] In order to evaluate the static and dynamic parameters of the oil production increase effect of CO huff and puff in shale oil, a quantitative evaluation and factor weight analysis method for CO huff and puff in shale oil in the invention includes the following steps. 2 A quantitative evaluation and factor weight analysis method for CO huff and puff in shale oil, including the following steps. 2 Including the following steps.

[0136] Step 1: Divide the numerical values of the influencing factors into n evaluation values according to their value ranges, and establish an experimental result data table, an experimental result analysis database, an average effect database of each factor level, and an interference standard deviation database of each factor. The value range tables of various influencing factors are as follows:

[0137] Table 1 Value range tables of various influencing factors

[0138]

[0139]

[0140] Step 2: Construct a pairwise comparison judgment matrix of the influencing factors of the CO huff and puff oil production degree coefficient in shale oil. For the conventional analytic hierarchy process method, generally the expert evaluation method is adopted. According to the empirical understanding, the importance degree of the influencing factors of the CO huff and puff oil production degree coefficient in shale oil is quantitatively scaled from 1 to 9, representing the importance degree of different factors, that is, the larger the numerical value, the greater the importance degree; that is, take the ratio c of the hierarchical scales of any two factors 2 And c 2 The importance degree of the influencing factors of the CO huff and puff oil production degree coefficient in shale oil is quantitatively scaled from 1 to 9, representing the importance degree of different factors, that is, the larger the numerical value, the greater the importance degree; that is, take the ratio c of the hierarchical scales of any two factors i And c j , Take d ij =c i / c j Quantitatively characterize c i And cj Degree of relative importance to production. This method is greatly affected by human factors. To avoid the influence of human factors, here we calculate the standard deviation of each factor on shale oil CO 2 The standard deviation results of the production degree of huff and puff are used to quantitatively characterize the influence degree of each factor on shale oil CO 2 Degree of influence on the production degree of huff and puff. For shale oil CO 2 The influencing factors of the production degree of huff and puff on shale oil CO 2 The importance of the influencing factors of the production degree of huff and puff on the coefficient of shale oil CO i is compared pairwise, that is, the standard deviations b j and b ij are taken each time, and a i is used to quantitatively characterize b j and b 2 The relative importance degree of the influence on the production degree of shale oil CO ij ) n×n is expressed, where the matrix A is the judgment matrix of each factor affecting the production degree of shale oil CO 2 .

[0141] Step 3: Calculate the weights of the influencing factors of the production degree of shale oil CO 2 ;

[0142] Calculate the maximum eigenvalue λ max of the judgment matrix and the eigenvector. For

[0143] AW = λ max W (5)

[0144] In the above equation, the method for solving the maximum eigenvalue λ max is to solve the roots of the characteristic equation det(λI - A) = 0 of the matrix A. Its multiple roots λ are the eigenvalues of the matrix A. Take the root with the largest numerical value among them as the maximum eigenvalue λ max of the matrix A. Where I is the identity matrix with the same dimension as the matrix A. Substitute the obtained maximum eigenvalue λ max into the equation AW = λ max W, and the eigenvector W corresponding to the largest eigenvalue of the matrix A can be obtained; after normalizing the eigenvector W, the relative weight coefficients of the influencing factors of shale oil CO 2 huff and puff are obtained;

[0145] Step 4: Consistency test;

[0146] Calculate the consistency ratio of the judgment matrix, and its index is CI = (λ max-n) / (n - 1), the CI value obtained through calculation. CI = 0 indicates perfect consistency; CI close to 0 indicates satisfactory consistency; the larger the CI, the more serious the inconsistency; to measure the size of CI, the random consistency index RI is introduced. According to the Saaty table, RI can be calculated; the consistency ratio CR is calculated by CI / RI; if the consistency ratio meets the design requirements, the weight coefficients calculated above are the final weights of each production influencing factor, and a weight database for each factor is obtained; otherwise, the judgment matrix is reconstructed.

[0147] Example:

[0148] Design a core huff and puff experiment based on the existing production data of typical wells in the actual block of the oilfield:

[0149]

[0150] Taking the specific implementation steps of Example 2 as an example, the steps of other examples are similar to those of Example 2.

[0151] The experimental procedure is as follows:

[0152] (1) Select a core, wash it with oil, dry it and weigh it, denoted as w a , then measure the basic parameters of the core such as length L, diameter D, permeability K, and porosity Φ, and then calculate the pore volume PV of the core using Equation (1);

[0153]

[0154] Subsequently, evacuate the core, pressurize it and saturate it with formation oil. After the weight is stable, take out the core sample, dry it and weigh it again, denoted as w b .

[0155] (2) Open the first two-way valve 4, and use the air compressor 1 to add the gas in the CO 2 cylinder 2 into the intermediate container 6 and pressurize it to the required pressure, then close the first two-way valve 4. Place the core in the core holder 13, connect all pipelines, open the ninth two-way valve 19, use the hand pump 20 to increase the confining pressure and then close the ninth two-way valve 19. Open the second two-way valve 5 and the third two-way valve 7, set the high-pressure displacement pump 3 to the constant pressure mode, use the high-pressure displacement pump 3 to add constant-pressure CO 2 , and close the second two-way valve 5 and the third two-way valve 7. Open the ninth two-way valve 19, use the hand pump 20 to increase the confining pressure by 5 MPa higher than the given pressure, and record the amount of CO 2 added through the high-pressure displacement pump 3 as V. Close the ninth two-way valve 19; open the second two-way valve 5 and the third two-way valve 7, use the high-pressure displacement pump 3 to add CO 2 to the given pressure, and close the second two-way valve 5 and the third two-way valve 7;

[0156] (3) Place the entire set of equipment in an incubator, control the experimental temperature, i.e., the actual formation temperature of the block, to be T. Open the No. 6 two-way valve 12, use a pressure sensor to record the pressure at the start of well shut-in as P, and record the pressure changes during well shut-in. Close all other valves to allow the rock sample in the core holder 13 to fully contact with CO 2 , and let CO 2 diffuse in the rock sample, shut in the well, and record the pressure as P 1 and the temperature as T 1 . Then, calculate the volume V of CO2 injected at this time and the number of moles n of CO 1 in the core holder 13 according to Equations 2 and 3 2 . 1 .

[0157]

[0158]

[0159] Where: Z 1 —— Compression factor at P 1 ;

[0160] R —— Gas constant, 8.314 J / (mol·K).

[0161] (4) Weigh the initial weight of the drying tube 18 and record it as m 0 . Install the drying tube 18 onto the pipeline. Set the pressure of the backpressure pump 16 to the production pressure, open the No. 7 two-way valve 15 and the No. 8 two-way valve 17. After the pressure stabilizes, close the No. 7 two-way valve 15 and the No. 8 two-way valve 17, remove the drying tube 18 and weigh it, record the weight as m1. Install the drying tube 18 onto the pipeline again; adjust the pressure of the backpressure pump 16 to reduce the pressure by 5 MPa, record the pressure at this time as P 2 , open the No. 7 two-way valve 15 and the No. 8 two-way valve 17. After the pressure stabilizes, close the No. 7 two-way valve 15 and the No. 8 two-way valve 17, remove the drying tube 18 and weigh it, record the weight as m 2 . Use the wet gas flowmeter 22 to record the volume of CO2 collected as V 2 '. From this, calculate the volume V of this volume at the pressure P 2 , and calculate the remaining and discharged numbers of moles of CO 2 in the core holder 13 as n 2 and n 2 respectively according to Equations 4 and 5. Then, calculate the number of moles n of CO 3 entering the core and the utilization rate ζ of CO 2 at this pressure according to Equations 6 and 7 2 . 4 and the utilization rate ζ of CO 2 at this pressure.

[0162]

[0163]

[0164] Where: Z 2 —— Compression factor at P 2 ;

[0165] T 2 —— Temperature of the thermostat at this time.

[0166] n 4 = n 1 - n 2 + n 3 (6);

[0167] Where: n 1 —— Moles of CO 2 injected into the core holder;

[0168] n 2 —— Moles of CO 2 in the intermediate container after pressure reduction;

[0169] n 3 —— Moles of CO 2 discharged after pressure reduction;

[0170] n 4 —— Moles of CO 2 reacting with the crude oil.

[0171]

[0172] Repeat the pressure reduction by 5 MPa and record the weight of the drying tube 18 as m 3 , m 4 …, until the pressure drops to 5 MPa. And calculate the final CO2 mole fraction n 总 , and during this period, online NMR 14 can be used to observe the occurrence and displacement of crude oil in the core.

[0173] (5) According to the requirements of different huff and puff experiments, repeat steps (2)-(4) to start a new round of huff and puff; the weight of the drying tube should be recorded in time after each huff and puff cycle. Calculate the oil production degree for every 5 MPa pressure drop through Equation 8

[0174]

[0175] (6) After the experiment, use alcohol and petroleum ether for displacement to clean the pipeline, then disassemble the instrument device and place it properly.

[0176] Record the oil production degree of staged pressure reduction production under this scheme:

[0177] Table 3 Changes in the degree of recovery of shale oil CO2 during each stage of depressurization

[0178]

[0179] Based on the experimental results, the incremental curve of the recovery degree of staged depressurization was drawn, and the difference in the recovery degree of each 5MPa staged depressurization under different injection conditions was analyzed. The sensitivity index of various parameters was calculated according to the formula, and then the main influencing factors affecting the shale oil carbon dioxide staged depressurization throughput were analyzed.

[0180] The results of each experimental scheme are summarized in the following table:

[0181] Table 4 Experimental results

[0182] Example Permeability / mD Porosity Injection Pressure / Mpa Soaking Time / h Recovery Factor / % Example 1 0.1 2 20 3 37.5 Example 2 0.1 2 25 3 35.0 Example 3 0.1 2 30 3 30.6 Example 4 0.1 2 40 (miscible) 3 50.5 Example 5 0.1 2 50 (miscible) 3 55.0 Example 6 0.1 2 25 1 30.5 Example 7 0.1 2 25 3 35.0 Example 8 0.1 2 25 12 37.4 Example 9 0.1 2 25 3 35.0 Example 10 0.1 6 25 3 42.1 Example 11 0.1 9 25 3 48.7 Example 12 0.1 12 25 3 50.3 Example 13 0.001 2 25 3 10.6 Example 14 0.01 2 25 3 22.3 Example 15 0.1 2 25 3 35.0 Example 16 0.5 2 25 3 36.5

[0183] In order to 2 The static parameters and dynamic parameters of the huff and puff oil increase effect are used to evaluate the effect. 2 The quantitative evaluation and factor weight analysis method in throughput includes the following steps.

[0184] Step 1: Divide the values ​​of the influencing factors into 3 evaluation values ​​according to their size distribution range, as shown in the following table:

[0185] Table 5 Analysis and evaluation value table

[0186] Evaluation Value Permeability Porosity Injection Pressure Soaking Time 1 0.001-0.01 2-6 20-30 1 2 0.01-0.1 6-9 30-40 3 3 0.1-0.5 9-12 40-50 12

[0187] According to the evaluation value table, the results of each experimental scheme are summarized as follows:

[0188] Table 6 Experimental results analysis database

[0189]

[0190]

[0191] The average effect of different values ​​of each dynamic parameter and static parameter is calculated based on the analysis database, and the results are shown in the following table. Take the permeability level of 3 as an example:

[0192]

[0193] Table 7 Average effect database of each factor evaluation

[0194] Evaluation Value Permeability Porosity Injection Pressure Soaking Time 1 10.6 35.2 34.8 30.5 2 22.3 48.7 50.5 37.4 3 39.9 50.3 55.0 37.4

[0195] The standard deviation of the average effect of different evaluation values ​​of each factor is calculated based on the data in the average effect database of each factor evaluation to obtain the effect of different influencing factors on shale oil CO 2The standard deviation database of the throughput recovery degree, that is, the standard deviation database of each factor interference, is as follows:

[0196] Table 8 Standard deviation database of each factor interference

[0197] Permeability Porosity Injection Pressure Soaking Time 12.04 6.77 8.66 3.25

[0198] Step 2, construct the pairwise comparison judgment matrix of the factors affecting the coefficient of shale oil CO 2 For the conventional analytic hierarchy process, the expert evaluation method is generally adopted. Based on empirical understanding, for shale oil CO 2 The importance degree of the factors affecting the coefficient of throughput recovery degree adopts a hierarchical quantitative scale of 1-9 to characterize the importance degree of different factors, that is, the larger the numerical value, the greater the importance degree; that is, take the ratio c i and c j , and use d ij =c i / c j to quantitatively characterize the relative importance degree of c i and c j on the production. This method is greatly affected by human factors. To avoid the influence of human factors, here we use the standard deviation results of each factor on the throughput recovery degree of shale oil CO 2 calculated by the standard deviation to quantitatively characterize the influence degree of each factor on the throughput recovery degree of shale oil CO 2 . Compare the importance of the factors affecting the throughput recovery degree of shale oil CO 2 to the coefficient of throughput recovery degree of shale oil CO 2 pairwise, that is, each time take the standard deviations b i and b j , and use a ij to quantitatively characterize the relative importance degree of b i and b j on the throughput recovery degree of shale oil CO 2 . The result is represented by the matrix A=(a ij ) n×n , where the matrix A is the judgment matrix of the factors affecting the throughput recovery degree of shale oil CO 2 . As follows:

[0199] Table 9 Judgment matrix of the factors affecting the throughput recovery degree of shale oil CO 2

[0200] Permeability Porosity Injection Pressure Soaking Time Permeability 1 1.78 1.39 3.70 Porosity 0.56 1 0.78 2.08 Injection Pressure 0.72 1.28 1 2.66 Soaking Time 0.27 0.48 0.38 1

[0201] Step 3, calculate the weights of the factors affecting the throughput recovery degree of shale oil CO 2 ; ​

[0202] Calculate the maximum eigenvalue λ of the judgment matrix max and the eigenvector. The calculation formula is as follows

[0203] AW = λ max W (5);

[0204] In the above equation, the maximum eigenvalue λ max is solved by finding the roots of the characteristic equation det(λI - A) = 0 of matrix A. Its multiple roots λ are the eigenvalues of matrix A. Take the root with the largest numerical value among them as the maximum eigenvalue λ of matrix A max . Where I is the identity matrix with the same dimension as matrix A. Substitute the obtained maximum eigenvalue λ max into the equation AW = λ max W, and the eigenvector W corresponding to the maximum eigenvalue of matrix A can be obtained; after normalizing the eigenvector W, the relative weight coefficients of each influencing factor of shale oil CO 2 huff and puff are as follows in the table:

[0205] Table 10 Relative weight coefficients of each influencing factor of shale oil CO 2 huff and puff

[0206] Permeability Porosity Injection Pressure Soaking Time 0.3919 0.2204 0.2818 0.1059

[0207] Step 4, consistency check;

[0208] Calculate the consistency ratio of the judgment matrix. The index is CI = (λ max -n) / (n - 1), which is the CI value obtained through calculation. When CI = 0, there is perfect consistency; when CI is close to 0, there is satisfactory consistency; the larger CI is, the more serious the inconsistency is; in order to measure the size of CI, the random consistency index RI is introduced. According to the saaty table, RI can be calculated; the consistency ratio CR is calculated by CI / RI; if the consistency ratio meets the design requirements, the weight coefficients calculated above are the final weights of each production influencing factor, and the weight database of each factor is obtained, otherwise the judgment matrix is reconstructed

[0209] Here CI = 2.8348e -11 , CR = 3.1498e -11 ; the discrimination matrix A passes the consistency check. It can be seen from this that the factors affecting the recovery degree of CO 2 huff and puff are ranked from largest to smallest as: permeability > injection pressure > porosity > shut-in time

[0210] The above content is a further detailed description of the present invention in combination with specific preferred embodiments. It cannot be determined that the specific implementation of the present invention is only limited to these descriptions. For those of ordinary skill in the technical field to which the present invention pertains, without departing from the concept of the present invention, several simple deductions or substitutions can still be made, which should all be regarded as belonging to the protection scope of the present invention.

Claims

1. A method for experimental exploitation of shale oil by carbon dioxide huff and puff with graded pressure drop, characterized in that, it includes the following steps: S1 Obtain the degree of crude oil recovery under different huff and puff experimental requirements through experiments; S2 Analyze the main controlling factors of the carbon dioxide huff and puff effect.

2. The method for experimental exploitation of shale oil by carbon dioxide huff and puff with graded pressure drop according to claim 1, characterized in that, in the step S1, obtaining the degree of crude oil recovery under different huff and puff experimental requirements through experiments specifically includes: S11 Select a core, calculate the pore volume of the core, and prepare it into a rock sample; S12 obtains CO through an experimental device 2 a given pressure environment; S13 Inject CO into the core holder 2 , calculate the injected volume of CO 2 and the number of moles of CO in the core holder 2 ; S14 Obtain the weight of the drying tube for every 5 MPa pressure drop and calculate the final CO 2 mole fraction; S15 Repeat steps S12 - S14, record the weight of the drying tube in each huff and puff cycle, and calculate the degree of crude oil recovery for every 5 MPa pressure drop.

3. The method for experimental exploitation of shale oil by carbon dioxide huff and puff with graded pressure drop according to claim 2, characterized in that, In the step S11, a core is selected, washed with oil, dried and weighed, denoted as w a , and then basic parameters such as the core length L, diameter D, permeability K, and porosity Φ are measured, and the pore volume of the core is calculated; Subsequently, the core is evacuated, pressurized and saturated with formation oil. After the weight is stable, the rock sample is taken out, dried and weighed again, denoted as w b .

4. The method for experimental exploitation of shale oil by carbon dioxide huff and puff with graded pressure drop according to claim 3, characterized in that, in the step S11, the formula for calculating the pore volume of the core is:

5. The method for experimental exploitation of shale oil by carbon dioxide huff and puff with graded pressure drop according to claim 2, characterized in that, in the step S12, it specifically includes the following steps: Open the No. 1 two-way valve, and use an air compressor to add the gas in the CO 2 cylinder into the intermediate container and pressurize it to the required pressure, then close the No. 1 two-way valve; Place the core in the core holder, connect all pipelines, open the No. 9 two-way valve, increase the confining pressure using a hand pump and then close the No. 9 two-way valve; open the No. 2 two-way valve and the No. 3 two-way valve, set the high-pressure displacement pump to the constant pressure mode, and use the high-pressure displacement pump to add CO at a constant pressure 2 , and close the No. 2 two-way valve and the No. 3 two-way valve; Open the No. 9 two-way valve, use a hand pump to add confining pressure 5 MPa higher than the given pressure, and record the amount of CO 2 added as V through a high-pressure displacement pump; close the No. 9 two-way valve; open the No. 2 two-way valve and the No. 3 two-way valve, and use the high-pressure displacement pump to add CO 2 to the given pressure, and then close the No. 2 two-way valve and the No. 3 two-way valve.

6. The method for experimental exploitation of shale oil by carbon dioxide huff and puff with graded pressure drop according to claim 2, characterized in that, in the step S13, it specifically includes the following steps: Place the experimental device in an incubator, and control the experimental temperature, i.e., the actual formation temperature of the block, to be T; Open the No. 6 two-way valve, use the pressure sensor to record the pressure at the start of the well shut-in as P, and record the pressure change during the well shut-in process. Close all other valves to allow the rock sample in the core holder to fully contact with CO 2 , and let CO 2 diffuse in the rock sample, shut in the well, and record the pressure as P 1 at this time and the temperature as T 1 ; Calculate the volume V of CO 2 injected at this time 1 and the number of moles of CO 2 in the core holder.

7. The method for experimental exploitation of shale oil by carbon dioxide huff and puff with graded pressure drop according to claim 6, characterized in that, In the step S13, the volume calculation formula for injecting CO 2 is as follows: CO in the core holder 2 The calculation formula for the number of moles is as follows: Where: Z 1 —— P 1 Compression factor at R - gas constant, 8.314 J / (mol·K).

8. The method for experimental exploitation of shale oil by carbon dioxide huff and puff with graded pressure drop according to claim 2, characterized in that, in the step S14, the specific steps include: The initial weight of the drying tube is weighed and recorded as m 0 , install the drying tube to the pipeline; Set the backpressure pump pressure to the production pressure, open the No. 7 two-way valve and the No. 8 two-way valve. After the pressure stabilizes, close the No. 7 two-way valve and the No. 8 two-way valve, remove the drying tube and weigh it, and record the weight as m 1 , reinstall the drying tube to the pipeline again; adjust the backpressure pump pressure to reduce it by 5 MPa, and record the pressure at this time as P 2 ; Open the No. 7 two-way valve and the No. 8 two-way valve. After the pressure stabilizes, close the No. 7 two-way valve and the No. 8 two-way valve, remove the drying tube, weigh it, and record the weight as m 2 , use a wet gas meter to record the collected CO 2 volume as V 2 ’. Calculate the volume V of this volume under P 2 pressure 2 ; Calculate the remaining and discharged CO in the core holder 2 moles, and calculate the CO 2 moles that enter the core at this pressure 2 n 4 and the utilization rate of CO at this pressure 2 ​ 9. The method for experimental exploitation of shale oil by carbon dioxide huff and puff with graded pressure drop according to claim 8, characterized in that, In the step S14, the remaining and discharged CO in the core holder 2 The calculation formula for the number of moles is as follows: Where: Z 2 ——P 2 Compressibility factor at T 2 —— The temperature of the incubator at this time.

10. The method for experimental exploitation of shale oil by carbon dioxide huff and puff with graded pressure drop according to claim 8, characterized in that, In the step S14, CO 2 enters the core under this pressure, and the 2 number of moles n 4 of CO and the utilization rate calculation formula of CO under this pressure are as follows: 2 ​ n 4 = n 1 -n 2 +n 3 (6); where: n 1 —— the number of moles of CO 2 injected into the core holder; n 2 —— Moles of CO in the intermediate container after the pressure reduction 2 ; n 3 —— Moles of CO discharged after pressure reduction 2 ; n 4 —— the number of moles reacting with crude oil 2 —— the number of moles reacting with crude oil 11. The method for experimental exploitation of shale oil by carbon dioxide huff and puff with graded pressure drop according to claim 8, characterized in that, in the step S14, it further includes: Record the weight of the drying tube as m after repeated pressure reduction to 5 MPa 3 、m 4 …, until the pressure drops to 5 MPa, and calculate the final CO 2 mole fraction n ＝ , and during this period, online nuclear magnetic resonance can be used to observe the occurrence and displacement of crude oil in the core.

12. The method for experimental exploitation of shale oil by carbon dioxide huff and puff with graded pressure drop according to claim 2, characterized in that, in the step S15, it specifically includes: According to different huff and puff experimental requirements, repeat steps S12 - S14 to start a new round of huff and puff; after each huff and puff cycle, record the weight of the drying tube in time and calculate the degree of crude oil recovery for every 5 MPa pressure drop. The calculation formula is:

13. The method for experimental exploitation of shale oil by carbon dioxide huff and puff with graded pressure drop according to claim 2, characterized in that, in the step S1, it further includes: S16 After the experiment is completed, use alcohol and petroleum ether for displacement to clean the pipeline, then disassemble the instrument device and place it properly.

14. The method for experimental exploitation of shale oil by carbon dioxide huff and puff with graded pressure drop according to claim 1, characterized in that, in the step S2, it specifically includes the following steps: S21 divides the numerical values of each influencing factor into n evaluation values according to their value ranges, thereby establishing an experimental result data table, an experimental result analysis database, an average effect database of each factor level, and a standard deviation database of each factor interference; S22 constructs shale oil CO 2 The pairwise comparison judgment matrix of the influencing factors of the throughput recovery degree coefficient; S23 Shale Oil CO 2 Calculation of the weight of influencing factors for the production degree of huff and puff S24 Consistency test.

15. A method for experimental exploitation of shale oil carbon dioxide huff and puff with staged pressure drop as claimed in claim 14, characterized in that, in the step S21, the influencing factors include static parameters: permeability, porosity; dynamic parameters: injection pressure, shut-in time.

16. A method for experimental exploitation of shale oil carbon dioxide huff and puff with staged pressure drop as claimed in claim 15, characterized in that, in the step S21, the value ranges of each influencing factor are: Permeability, mD: 0.001 - 0.5; Porosity, %: 2 - 12; Injection pressure, MPa: 20 - 50; Shut-in time, h: 1 - 12.

17. A method for experimental exploitation of shale oil carbon dioxide huff and puff with staged pressure drop as claimed in claim 14, characterized in that, in the step S22, it specifically includes: The standard deviation results of the factors affecting the CO of shale oil obtained by calculating the standard deviation are used to quantitatively characterize the degree of influence of each factor on the CO 2 of the production degree of huff and puff, so as to quantitatively characterize the influence degree of each factor on the CO 2 of the production degree of huff and puff; Regarding the shale oil CO 2 The influencing factors of the production degree of huff and puff on shale oil CO 2 Make pairwise comparisons of the importance of the production degree coefficients of huff and puff, that is, take the standard deviations b i and b j , and use a ij to quantitatively characterize b i and b j 's relative importance in influencing the production degree of shale oil CO 2 by huff and puff. The results are represented by the matrix A=(a ij ) n×n ; Among them, matrix A is the judgment matrix of various factors affecting the recovery degree of shale oil CO 2 huff and puff production.

18. A method for experimental exploitation of shale oil carbon dioxide huff and puff with staged pressure drop as claimed in claim 14, characterized in that, In the step S23, calculate the maximum eigenvalue λ of the judgment matrix max and the eigenvector, and the calculation formula is as follows: AW = λ max W (5); Among them, the largest eigenvalue λ max is obtained by solving the roots of the characteristic equation det(λI - A) = 0 of matrix A. These multiple roots λ are the eigenvalues of matrix A. Take the root with the largest numerical value among them as the largest eigenvalue λ of matrix A max ; where I is an identity matrix of the same dimension as matrix A; Substitute the obtained maximum eigenvalue λ max into the equation AW = λ max W, and the eigenvector W corresponding to the maximum eigenvalue of matrix A can be obtained; after normalizing the eigenvector W, the relative weight coefficients of each influencing factor for shale oil CO 2 huff and puff can be obtained.

19. A method for experimental exploitation of shale oil carbon dioxide huff and puff with staged pressure drop as claimed in claim 14, characterized in that, In the step S24, the consistency ratio of the judgment matrix is calculated, and its index is CI = (λ max - n) / (n - 1), which is the CI value obtained through calculation; among them, when CI = 0, there is complete consistency; when CI is close to 0, there is satisfactory consistency; the larger the CI, the more serious the inconsistency.

20. A method for experimental exploitation of shale oil carbon dioxide huff and puff with staged pressure drop as claimed in claim 19, characterized in that, in the step S24, in order to measure the magnitude of CI, a random consistency index RI is introduced; according to the saaty table, RI can be calculated; the consistency ratio CR is calculated by CI / RI; if the consistency ratio meets the design requirements, the weight coefficients calculated above are the final weights of each production influencing factor, and a weight database of each factor is obtained, otherwise, the judgment matrix is reconstructed.

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