Quantitative Evaluation Method for Shale Oil Reservoir Propionage Recirculation

Abstract

This invention relates to a quantitative evaluation method for proppant backflow in shale oil reservoirs. It includes calculating the fracture width, height, and length of a full-size core sample; converting the on-site fracturing fluid backflow rate and daily shale oil production into simulated laboratory flow rate and production; preparing simulated shale oil and fracturing fluid; filling with proppant; evacuating; filling the artificial fractures with fracturing fluid; filling an empty graduated cylinder with fracturing fluid and recording the volume; injecting simulated shale oil at the production rate to simulate the production process; and every 24 hours, removing the graduated cylinder after collecting fracturing fluid, simulated shale oil, and proppant; and then injecting the simulated shale oil backflow rate into the next... i The fracturing fluid is slowly added to the graduated cylinder containing fracturing fluid, simulated shale oil, and proppant until all the simulated shale oil is discharged. The calculation is then performed. i The volume of proppant refluxing within the next 24 hours V 支i , V 支i The ratio of the volume of proppant in the fracture to the volume of proppant filled before the experiment represents the proppant backflow rate at that moment. This invention can evaluate the proppant backflow under actual shale reservoir conditions.

Description

Technical fields:

[0001] This invention relates to the field of shale oil and gas reservoir development technology, specifically to a quantitative evaluation method for proppant reflux in shale oil reservoirs. Background technology:

[0002] Hydraulic fracturing is an effective measure to improve the production capacity of shale oil and gas reservoirs. High-pressure pumps on the surface inject fracturing fluid into the well, and the high-pressure fluid expands existing fractures or creates new ones. Proppant, carried by the fracturing fluid, enters the formation and supports the hydraulically fractured structures, thus creating artificial fractures with a certain conductivity, providing seepage channels for oil and gas extraction. As a key material, proppant directly affects the fracturing effect. However, in some oil and gas wells, after implementing fracturing enhancement measures, proppant backflow occurs during fracturing fluid flowback and normal production.

[0003] Propionate backflow can cause numerous problems. First, it can reduce or even close fracture width, compromising long-term fracture conductivity. Second, it may lead to bottleneck fractures near the wellbore, reducing fracturing effectiveness. Third, for wells with severe sand production from backflow, frequent workover operations such as sand flushing are required, impacting normal oil and gas well production and increasing operating costs. Furthermore, the produced sand can corrode downhole and surface equipment, reducing its lifespan. Therefore, it is necessary to conduct a quantitative evaluation of proppant backflow in shale oil reservoirs.

[0004] Existing technologies (CN117514121A, CN116201518A, CN116771319A, CN118128512A) describe similar testing methods. However, these tests only simulate the proppant backflow process in fractures and do not provide a method for accurately calculating the proppant backflow rate. All of the above technologies use glass or steel plates for proppant backflow testing, without placing the proppant within the reservoir rock fractures. Glass and steel plates cannot reflect the actual rock properties of the reservoir. Furthermore, reservoir closure stress is exerted on the proppant through the rock, and the proppant will be embedded to varying degrees in the reservoir rock. The contact area and stress state between the proppant and the rock in the reservoir differ significantly from those in glass or steel plates. Existing technologies (CN117514121A, CN116201518A, CN116771319A) all use proppant at room temperature. However, the working environment of proppant is in the underground reservoir, and its backflow occurs under high temperature and high pressure conditions. Therefore, these methods cannot accurately reflect the proppant backflow under reservoir conditions. Existing technology (CN111060284A) provides a testing device and method for simulating proppant backflow after fracture closure using a flow chamber, but it does not provide a specific method for calculating the proppant backflow rate. Furthermore, due to the numerous foliation fractures in shale and the difficulty in fabricating rock plates, this technology is suitable for conventional reservoir proppant backflow testing but is no longer applicable to shale reservoirs. In addition, all the above methods require separating the proppant to measure its mass. During this process, it is difficult to collect fine, broken proppant particles and remove fracturing fluid residue adhering to the surface of the dried proppant, introducing errors into the test results.

[0005] Therefore, there is an urgent need to design a real-time quantitative evaluation method for proppant reflux rate after fracturing under shale oil reservoir conditions, so as to provide evaluation methods and experimental data support for the selection of proppant, performance evaluation, and drainage scheme design in the process of fracturing construction. Summary of the Invention:

[0006] The purpose of this invention is to provide a quantitative evaluation method for proppant reflux in shale oil reservoirs. This method addresses the problems in existing technologies, such as low accuracy of proppant reflux rate measurement, inability to measure reflux rate in real time, and failure to accurately reflect reservoir temperature and pressure conditions and proppant embedding.

[0007] The technical solution adopted by this invention to solve its technical problem is as follows: This quantitative evaluation method for proppant reflux in shale oil reservoirs includes the following steps:

[0008] Step 1: Calculate the fracture width W in the full-size core sample. f High H f The length L of the fracture in the full-size core sample was calculated. fThen, the on-site fracturing fluid return flow rate Q F and daily production of shale oil Q O Converted to laboratory simulated displacement q f and output q o ;

[0009]

[0010] In the formula, W F For crack width, H F Where N is the crack height, Q is the number of cracks, and N is the number of cracks. F For the flowback rate of fracturing fluid in the field, Q O This represents the daily production of shale oil.

[0011] Step two: Prepare simulated shale oil and fracturing fluid;

[0012] Step 3: Prepare the core and proppant;

[0013] Step 4: Fill with proppant and place the proppant-filled core into the proppant reflow test system;

[0014] Step 5, Vacuuming: Turn on the constant pressure pump of the confining pressure system, set its pressure to 2MPa, and turn on the vacuum pump to evacuate the artificial crack filled with proppant for 2 hours.

[0015] Step 6: Turn on the constant speed and pressure pump of the fluid injection system and set its flow rate to 1 ml / min until the fracturing fluid fills the artificial fracture; set the temperature of the confining pressure system fluid preheater and the pressure of the constant pressure pump according to the temperature of the shale oil reservoir and the minimum horizontal principal stress.

[0016] Step 7: Place an empty graduated cylinder (mass m0) in the fluid collection container, fill the empty graduated cylinder with fracturing fluid, and record the volume V of the fracturing fluid at this point. 压 ;

[0017] Step 8: Set the temperature of the fluid preheater in the fluid injection system according to the reservoir temperature; set the pressure of the back pressure valve in the fluid collection system using a hand pump according to the bottom hole flowing pressure; open the fluid collection system so that the measuring cylinder begins to collect the fluid flowing out of the full-size core holder; and set the fracturing fluid discharge rate q according to the laboratory simulation. f Set the flow rate of the constant speed and pressure pump, determine the fracturing fluid injection time of the fluid injection system based on the on-site fracturing fluid backflow time, and then use the production rate q as the basis. o Modify the injection of simulated shale oil to simulate the production process;

[0018] Step 9: Every 24 hours, remove the graduated cylinder that has collected fracturing fluid, simulated shale oil, and proppant, and place another graduated cylinder filled with fracturing fluid into the fluid collection container.

[0019] Step 10: Slowly add fracturing fluid to the graduated cylinder containing fracturing fluid, simulated shale oil, and proppant, which was removed in Step 9 for the first time, until all the simulated shale oil is discharged. Measure the total mass of the graduated cylinder, fracturing fluid, and proppant at this point. Record the total mass of the graduated cylinder, fracturing fluid, and proppant measured in the first step as m. i ; Calculate the volume V of proppant refluxed within the i-th 24-hour period. 支i V 支i The ratio of the volume of proppant in the crack before the experiment (V1-V2) to the volume of proppant filling the crack is the proppant backflow rate n at this moment. i ;

[0020] V 支i ·ρ 支 +(V 压 -V 支i )·ρ 压 =m i -m0

[0021] In the formula, V 支i Let ρ be the volume of the refluxed proppant during the i-th time period. 支 The true density of the proppant,

[0022] ρ 压 m is the density of the fracturing fluid. i The total mass of the fracturing fluid and proppant dosing cylinder at the i-th time step;

[0023]

[0024] In the formula, n i Let be the reflux rate of the proppant at time i;

[0025] Step 11: Repeat step 10 with the graduated cylinders containing fracturing fluid, simulated shale oil, and proppant removed in step 9 in chronological order. This allows for real-time and accurate measurement of proppant reflux under simulated shale oil reservoir conditions, obtaining proppant reflux flow rate and proppant reflux rate at different time points.

[0026] The above scheme calculates the fracture width W in full-size cored rock cores. f High H f The length L of the fracture in the full-size core sample was calculated. f Method: Obtain the diameter D of a full-size core sample. C Crack half length L F Width W F High H F Number of fractures N, in-situ fracturing fluid flowback rate Q F Shale oil daily production Q O The width W of the fracture in the full-size core sample was calculated. f High H fBased on the similarity ratio principle, the fracture length L of the full-size core sample was calculated. f :

[0027] W f =W F

[0028] H f =D C

[0029]

[0030] The specific method for step two in the above scheme is as follows: Based on the reservoir temperature conditions, simulated shale oil is prepared by mixing shale oil extracted from the ground with kerosene, so that it has the viscosity under reservoir conditions; fracturing fluid is prepared based on the field fracturing construction data; and the prepared fracturing fluid and simulated shale oil are placed into two intermediate containers respectively.

[0031] The specific method for step three in the above scheme is as follows: drill a full-size core from the shale oil reservoir, split the core along the axial direction to create artificial fractures; select the proppant according to the specifications of the proppant used for fracturing on site, and take a volume of proppant V1 for reflux evaluation testing.

[0032] The specific method for filling proppant in step four of the above scheme is as follows: a proppant pad is pre-placed in the artificial fracture, the thickness of which is the width of the fracture. The core is tightly wrapped with a thermoplastic film. The proppant pad is slowly pulled out while proppant is filled into the fracture, resulting in an artificial fracture of the shale core with proppant filling. After the proppant filling is completed, the volume of the remaining proppant, V2, is measured. The volume of proppant filling the core fracture is V1-V2. The proppant-filled core is placed in a full-size core holder. When placing the core, the fracture is perpendicular to the horizontal plane. The core is fixed in the middle position of the core holder using the holder plug.

[0033] The proppant recirculation testing system described above includes a full-size core holder, a fluid injection system, a fluid collection system, a confining pressure maintenance system, and a vacuum system. The proppant-filled core is placed inside the full-size core holder. The fluid injection system is connected to the inlet of the core holder via a pipeline. The core holder is equipped with a temperature sensor. The outlet of the core holder is connected to the fluid collection system and the vacuum system via a pipeline. The confining pressure maintenance system is connected to both ends of the core holder and forms a circulation loop.

[0034] The fluid injection system in the above scheme includes a constant speed and constant pressure pump, an intermediate container, and a fluid preheater. The constant speed and constant pressure pump, the intermediate container, and the fluid preheater are connected by pipelines. The fluid injection system is connected to the inlet of the core holder.

[0035] The fluid collection system in the above scheme includes valves, back pressure valves, hand pumps, fluid collection containers, measuring cylinders, and balances. The fluid collection system is connected to the outlet of the core holder.

[0036] The confining pressure maintaining system in the above scheme includes valves, a fluid preheater, and a constant pressure pump. The valves, fluid preheater, and constant pressure pump are connected to the core holder by pipelines. The temperature and pressure maintaining system is connected to both sides of the core holder.

[0037] The vacuum system described above includes valves and a vacuum pump, which are connected by pipelines. The vacuum system is connected to the outlet of the core holder. The function of the vacuum system is to evacuate the artificial fractures and shale cores containing proppant. The vacuum pump can be disassembled after the vacuuming process is complete.

[0038] Beneficial effects:

[0039] 1. The present invention uses actual cored shale cores, in-situ fracturing fluid, and shale oil in the testing process. The reservoir closure stress acts on the proppant through the shale rock, which can accurately reflect the stress state and embedding of the proppant in the reservoir rock. Based on the principles of similarity ratio and equal flow velocity (equal Reynolds number), the geometric dimensions of the fractures and the output fluid discharge in the fracturing field are converted into corresponding values ​​under laboratory test conditions, which can evaluate the proppant backflow under real shale reservoir conditions.

[0040] 2. This invention can heat and pressurize a full-size core holder, which can realistically simulate the high temperature and high pressure environment of the reservoir and evaluate the proppant return rate under different closure pressure conditions during the mining process.

[0041] 3. The testing process of this invention does not require separation of the proppant from the test fluid, thus avoiding errors caused by the adhesion of fracturing fluid components; there is no need to filter or transfer proppant particles to determine the return flow rate, as all returned proppant, including broken proppant particles, are collected, and there is no proppant loss during the test, resulting in high accuracy of the evaluation results; in addition, the proppant return rate can be monitored in real time at different times, and the relationship between the proppant return rate and time can be determined. Attached image description:

[0042] Figure 1 This is a schematic diagram of the proppant reflux testing system;

[0043] Figure 2 A schematic diagram of a full-size shale core with proppant filled into an artificial fracture, where: 2-1 shows the core placement orientation in the holder, with the fracture perpendicular to the horizontal plane; 2-2 shows the core along... Figure 2-1 Cross-sectional view.

[0044] In the diagram: 1 Constant speed and pressure pump, 2 First valve, 3 Second valve, 4 First intermediate container, 5 Second intermediate container, 6 Third valve, 7 First fluid preheater, 8 Fourth valve, 9 Back pressure valve, 10 Fluid collection container, 11 Measuring cylinder, 12 Balance, 13 Clamp plug, 14 Full-size core clamp, 15 Core, 16 Temperature sensor, 17 Pipeline, 18 Fifth valve, 19 Second fluid preheater, 20 Constant pressure pump, 21 Sixth valve, 22 Vacuum pump, 23 Hand pump, 24 Propionate. Detailed implementation method:

[0045] The present invention will be further described below:

[0046] Combination Figures 1-2 As shown, this quantitative evaluation method for proppant recirculation in shale oil reservoirs utilizes pre-fractured full-size cores and full-size core holders to simulate the proppant recirculation process under shale oil reservoir conditions and accurately determine the recirculation rate under reservoir temperature and pressure. The specific steps are as follows:

[0047] Step 1: Obtain the crack half-length L F =100m, width W F =5mm, height H F =30m, number of fractures N=180 and in-situ fracturing fluid flowback rate Q F =80m 3 / d, daily shale oil production Q O =20m 3 / d, calculate the fracture width W of the full-size core sample according to formulas 1 and 2. f =5mm, height H f =10.0cm. Based on the similarity ratio principle, the fracture length L of the full-size core sample is calculated using Formula 3. f =33.3cm, and then, based on the principle of equal flow velocity (equal Reynolds number), use formulas 4 and 5 to determine the on-site fracturing fluid return flow rate Q. F and daily production of shale oil Q O Converted to laboratory simulated displacement q f =1.03 ml / min and yield q o =0.26 ml / min;

[0048] W f =W F (Formula 1)

[0049] H f =D C (Formula 2)

[0050]

[0051]

[0052]

[0053] In the formula, D C =10.0cm, which is the diameter of the full-size core sample.

[0054] Step 2, preparation of simulated shale oil and fracturing fluid: simulated shale oil is prepared by mixing shale oil extracted from the ground with kerosene, so that its viscosity is 0.595cp under the condition of reservoir temperature of 110℃; fracturing fluid is prepared according to the field fracturing operation data; the prepared fracturing fluid and simulated shale oil are placed in the first intermediate container 4 and the second intermediate container 5 respectively.

[0055] Step 3, Core and Proppant Preparation: Drill a full-size core (33.3cm long, 10cm in diameter) from the shale oil reservoir. Split the core axially to create artificial fractures. Select 70 / 140 mesh silica sand proppant according to the specifications used for fracturing in the field, and take a volume of V1 = 200cm³. 3 Support 24 was used for reflux evaluation testing;

[0056] Step 4, proppant filling: Pre-place proppant pads with a thickness of 5mm in the artificial fracture. Tightly wrap the core 15 with thermoplastic film, slowly remove the proppant pads, and simultaneously fill the fracture with proppant 24, resulting in a proppant-filled artificial fracture in the shale core. After proppant 24 is filled, measure the remaining proppant volume V2 = 67.5cm³. 3 Therefore, the volume of proppant used to fill the core fracture is V1 - V2 = 132.5 cm³. 3 The proppant-filled core is placed inside the full-size core holder 14, with the fracture perpendicular to the horizontal plane when the core is placed. Figure 2 The core is fixed in the middle position of the core holder 14 using the clamp plug 13, and then all valves are turned to the closed position.

[0057] Step 5, vacuuming: Open the fifth valve 18 and the constant pressure pump 20. The fifth valve 18 is the confining pressure system valve. Set the pressure of the constant pressure pump 20 to 2MPa. Open the sixth valve 21 and the vacuum pump 22 to vacuum the artificial crack filled with proppant for 2 hours. After the vacuuming is completed, close the sixth valve 21 and the vacuum pump 22.

[0058] Step 6: Open the first valve 2, the third valve 6, and the constant speed and pressure pump 1 of the fluid injection system. The first valve 2 and the third valve 6 are the valves of the fluid injection system. Set the flow rate of the constant speed and pressure pump 1 to 1 ml / min until the fracturing fluid fills the artificial fracture. Set the temperature of the second fluid preheater 19 in the confining pressure system to 110℃ and the pressure of the constant pressure pump 20 to 46 MPa according to the temperature of the shale oil reservoir and the minimum horizontal principal stress.

[0059] Step 7: Place an empty graduated cylinder 11 in the fluid collection container 10. The mass of the empty graduated cylinder is m0 = 26.5724 g. Fill the empty graduated cylinder with fracturing fluid and record the volume V of the fracturing fluid at this time. 压 =28.6cm 3 ;

[0060] Step 8: Set the temperature of the first fluid preheater 7 in the fluid injection system to 110℃ based on the reservoir temperature, and set the pressure of the back pressure valve 9 in the fluid collection system to 20MPa using the hand pump 23 based on the bottom hole flowing pressure. Open the fourth valve 8 in the fluid collection system, and discharge the fracturing fluid q as determined in Step 1 (based on the laboratory simulation). f The flow rate of constant speed and pressure pump 1 is set to 1.03 ml / min. The fracturing fluid injection time of the fluid injection system is determined based on the on-site fracturing fluid backflow time. After 48 hours, the first valve 2 is closed and the second valve 3 is opened. The laboratory simulated shale oil production q is determined according to step one. o The flow rate of constant speed and pressure pump 1 was set to 0.26 ml / min; simulated shale oil was injected to simulate the shale oil production process;

[0061] Step 9: Every 24 hours, remove the measuring cylinder 11 that has collected fracturing fluid, simulated shale oil and proppant, and place another measuring cylinder filled with fracturing fluid into the fluid collection container 12.

[0062] Step 10: Slowly add fracturing fluid to the measuring cylinder 11 containing fracturing fluid, simulated shale oil, and proppant from Step 9 until all the simulated shale oil is discharged. Measure the total mass of the measuring cylinder, fracturing fluid, and proppant at this point, and record it as m1 = 65.8912 g. Calculate the volume V of the proppant that flows back out within 24 hours using Formula 6. 支1 =6.44cm 3 V 支1 The volume of proppant filled in the crack before the experiment, V1-V2 = 132.5 cm³. 3 The ratio is the proppant reflux rate n1 = 4.86% at this moment, which can be calculated using Formula 7. Repeat the above steps to achieve real-time and accurate measurement of proppant reflux under simulated shale oil reservoir conditions. This yields the proppant reflux flow rate and proppant reflux rate at different time periods (see Table 1 for details).

[0063] V 支i ·ρ 支 +(V 压 -V 支i )·ρ 压 =m i -m0 (Formula 6)

[0064] In the formula, V 支i Let ρ be the volume of the refluxed proppant during the i-th time period.支 =2.63g / cm 3 ρ is the density of the proppant. 压 =1.01g / cm 3 m is the density of the fracturing fluid. i Let be the total mass of the fracturing fluid and proppant dosing cylinder at the i-th time step.

[0065]

[0066] In the formula, n i Let be the reflux rate of the proppant at time i.

[0067] Table 1. Propionate reflux volume and proppant reflux rate at different time periods.

[0068] Serial Number Time, h <![CDATA[Reflux flow rate, c m 3 > Reflux rate, % 1 24 6.44 4.86％ 2 48 9.29 7.01％ 3 72 10.24 7.73％ 4 96 10.97 8.28％ 5 120 11.59 8.75％ 6 144 11.98 9.04％ 7 168 12.16 9.18％ 8 192 12.24 9.24％

[0069] The testing system of this invention includes a full-size core holder 14, a fluid injection system, a fluid collection system, a confining pressure maintenance system, and a vacuum system. A proppant-filled core 15 is placed inside the full-size core holder 14. The fluid injection system is connected to the inlet of the core holder via a pipeline 17. A temperature sensor 16 is installed in the core holder. The outlet of the core holder is connected to the fluid collection system and the vacuum system via pipeline 17. The confining pressure maintenance system and the full-size core holder form a circulation loop.

[0070] The fluid injection system includes a constant speed and constant pressure pump 1, a first valve 2, a second valve 3, a first intermediate container 4, a second intermediate container 5, a third valve 6, and a first fluid preheater 7. The constant speed and constant pressure pump 1, the first valve 2, the second valve 3, the first intermediate container 4, the second intermediate container 5, the third valve 6, and the fluid preheater 7 are connected by pipelines. The fluid injection system is connected to the inlet of the full-size core holder 14.

[0071] The fluid collection system includes a fourth valve 8, a back pressure valve 9, a hand pump 23, a fluid collection container 10, a measuring cylinder 11, and a balance 12. The fluid collection system is connected to the outlet of a full-size core holder 14.

[0072] The confining pressure holding system includes a fifth valve 18, a second fluid preheater 19, a constant pressure pump 20, and a pipeline 17. The fifth valve 18, the second fluid preheater 19, and the constant pressure pump 20 are connected to both sides of the full-size core holder 14 by the pipeline 17.

[0073] The vacuum system includes a sixth valve 21 and a vacuum pump 22, which are connected by pipelines. The vacuum system is connected to the outlet of the core holder. The function of the vacuum system is to evacuate the artificial fractures and shale cores containing proppant. The vacuum pump can be disassembled after the vacuuming is completed.

[0074] This invention first conducts experiments to measure the proppant backflow rate under shale oil reservoir temperature and effective stress conditions, simulating the backflow process of proppant within reservoir core fractures. Then, through data processing, the proppant backflow rate is calculated, thus revealing the time-varying pattern of proppant backflow in fractures under shale oil reservoir conditions. This method is not only applicable to the quantitative evaluation of proppant backflow in shale oil reservoirs but also suitable for conventional oil and gas reservoirs. The measurement results obtained using this method are highly accurate and can accurately reflect the proppant backflow situation in reservoir core fractures under shale oil reservoir conditions.

[0075] The purpose of step one in this invention is to convert the actual field fracturing conditions into laboratory simulation conditions.

[0076] The purpose of step two is to obtain fracturing fluid and simulated shale oil for field use.

[0077] Steps three and four aim to prepare for establishing a fracture-proppane simulation system under shale oil reservoir conditions. In step three, artificial fractures can be created in the core using either a splitting method or a conventional wire-cutting method. In step four, proppant of different particle sizes and types can be filled, and different proppant placement methods and proppant concentrations can be used (achieved by using proppant pads of varying thicknesses) to study the influence of different factors on the proppant backflow rate within fractures under shale oil reservoir conditions.

[0078] The purpose of step five is to vacuum the artificial cracks filled with proppant.

[0079] In step six, different constant pressure pumps can be set to apply different effective stresses to the proppant, and the effect of different crack closure stresses on the proppant return rate can be studied.

[0080] The purpose of step eight is to simulate the fracturing fluid flowback, shale oil production, and proppant recirculation processes under shale oil reservoir conditions. During the experiment, a confining pressure maintenance system is used to simulate the temperature and closure stress of the fracture-proppant system in the shale oil reservoir.

[0081] The purpose of step ten is to accurately calculate the proppant backflow rate. The fluid collection system utilizes a high-precision online balance to achieve real-time measurement of the flow rate through the fracture. The measurement results accurately reflect the proppant backflow within the fracture under formation conditions. Furthermore, the variation of proppant backflow within the fracture over time under different fracture closure stress conditions can be studied.

[0082] In summary, the real-time proppant backflow rate determination method provided in this application for shale oil reservoir conditions can achieve real-time measurement of proppant backflow rate under simulated shale oil reservoir temperature, pressure, and rock properties. This method realistically reflects the proppant backflow situation in fractures at different times under effective reservoir stress, and allows for the study of the influence of different factors on the backflow rate. The method is simple to implement and provides a simulated proppant backflow rate determination method under shale oil reservoir conditions, improving the accuracy of proppant backflow rate determination under these conditions.

Claims

1. A method for quantitative evaluation of proppant reflux in shale oil reservoirs, characterized in that... Includes the following steps: Step 1: Calculate the fracture width W in the full-size core sample. f High H f The length L of the fracture in the full-size core sample was calculated. f Then, the on-site fracturing fluid return flow rate Q F and daily production of shale oil Q O Converted to laboratory simulated displacement q f and output q o ; In the formula, W F For crack width, H F Where N is the crack height, Q is the number of cracks, and Q is the crack height. F For the flowback rate of fracturing fluid in the field, Q O This represents the daily production of shale oil. Step two: Prepare simulated shale oil and fracturing fluid; Step 3, prepare the core and proppant: Full-size core samples are drilled from the shale oil reservoir, and the cores are split along the axial direction to create artificial fractures. Propionage is selected according to the specifications of the proppant used in the field fracturing, and a volume of V1 of proppant is taken for reflux evaluation testing. Step 4: Fill with proppant and place the proppant-filled core into the proppant reflow testing system. Pre-place support pads in the artificial fractures, with the thickness of the support pads equal to the fracture width. Tightly wrap the core with a thermoplastic film, slowly remove the support pads, and simultaneously fill the fractures with proppant to obtain an artificial fracture in the shale core that has been propped in. After the proppant filling is completed, measure the volume of the remaining proppant, V2. The volume of proppant used to fill the core fractures is V1-V2. Place the propped-filled core in a full-size core holder, ensuring the fracture is perpendicular to the horizontal plane. Use the holder plug to fix the core in the middle position of the core holder. Step 5, Vacuuming: Turn on the constant pressure pump of the confining pressure system, set its pressure to 2MPa, and turn on the vacuum pump to evacuate the artificial crack filled with proppant for 2 hours. Step 6: Turn on the constant speed and pressure pump of the fluid injection system and set its flow rate to 1 ml / min until the fracturing fluid fills the artificial fracture; set the temperature of the confining pressure system fluid preheater and the pressure of the constant pressure pump according to the temperature of the shale oil reservoir and the minimum horizontal principal stress. Step 7: Place an empty graduated cylinder (mass m0) in the fluid collection container, fill the empty graduated cylinder with fracturing fluid, and record the volume V of the fracturing fluid at this point. 压 ; Step 8: Set the temperature of the fluid preheater in the fluid injection system according to the reservoir temperature, and set the pressure of the back pressure valve in the fluid collection system using a hand pump according to the bottom hole pressure. Turn on the fluid collection system so that the graduated cylinder begins to collect the fluid flowing out of the full-size core holder; Based on laboratory simulation of fracturing fluid displacement q f Set the flow rate of the constant speed and pressure pump, determine the fracturing fluid injection time of the fluid injection system based on the on-site fracturing fluid backflow time, and then use the production rate q as the basis. o Modify the injection of simulated shale oil to simulate the production process; Step 9: Every 24 hours, remove the graduated cylinder that has collected fracturing fluid, simulated shale oil, and proppant, and place another graduated cylinder filled with fracturing fluid into the fluid collection container. Step 10: Slowly add fracturing fluid to the graduated cylinder containing fracturing fluid, simulated shale oil, and proppant, which was removed in Step 9 for the first time, until all the simulated shale oil is discharged. Measure the total mass of the graduated cylinder, fracturing fluid, and proppant at this point. Record the total mass of the graduated cylinder, fracturing fluid, and proppant measured in the first step as m. i ; Calculate the volume V of the proppant refluxed within the i-th 24-hour period. 支i V 支i The ratio of the volume of proppant in the crack before the experiment (V1-V2) to the volume of proppant filling the crack is the proppant backflow rate n at this moment. i ; V 支i ·ρ 支 +(V 压 -V 支i )·ρ 压 =m i -m0 In the formula, V 支i Let ρ be the volume of the refluxed proppant during the i-th time period. 支 ρ is the true density of the proppant. 压 m is the density of the fracturing fluid. i The total mass of the fracturing fluid and proppant dosing cylinder at the i-th time step; In the formula, n i Let be the reflux rate of the proppant at time i; Step 11: Repeat step 10 with the graduated cylinders containing fracturing fluid, simulated shale oil, and proppant removed in step 9 in chronological order. This allows for real-time and accurate measurement of proppant reflux under simulated shale oil reservoir conditions, obtaining proppant reflux flow rate and proppant reflux rate at different time points.

2. The method for quantitative evaluation of shale oil reservoir proppant reflux according to claim 1, characterized in that: The calculation of the fracture width W in the full-size cored rock core is described above. f High H f The length L of the fracture in the full-size core sample was calculated. f Method: Obtain the diameter D of a full-size core sample. C Crack half length L F Width W F High H F Number of fractures N, in-situ fracturing fluid flowback rate Q F Shale oil daily production Q O The width W of the fracture in the full-size core sample was calculated. f High H f Based on the similarity ratio principle, the fracture length L of the full-size core sample was calculated. f : IN f =In F H f =D C 3. The method for quantitative evaluation of shale oil reservoir proppant reflux according to claim 2, characterized in that: The specific method for step two is as follows: based on the reservoir temperature conditions, simulated shale oil is prepared by mixing shale oil extracted from the surface with kerosene, so that it has the viscosity under reservoir conditions; fracturing fluid is prepared based on the field fracturing construction data; the prepared fracturing fluid and simulated shale oil are placed into two intermediate containers respectively.

4. The method for quantitative evaluation of shale oil reservoir proppant reflux according to claim 3, characterized in that: The proppant recirculation testing system includes a full-size core holder, a fluid injection system, a fluid collection system, a confining pressure maintenance system, and a vacuum system. The proppant-filled core is placed inside the full-size core holder. The fluid injection system is connected to the inlet of the core holder via a pipeline. The core holder is equipped with a temperature sensor. The outlet of the core holder is connected to the fluid collection system and the vacuum system via a pipeline. The confining pressure maintenance system is connected to both ends of the core holder and forms a circulation loop.

5. The method for quantitative evaluation of shale oil reservoir proppant reflux according to claim 4, characterized in that: The fluid injection system includes a constant speed and pressure pump, an intermediate container, and a fluid preheater. The constant speed and pressure pump, the intermediate container, and the fluid preheater are connected by pipelines. The fluid injection system is connected to the inlet of the core holder.

6. The method for quantitative evaluation of shale oil reservoir proppant reflux according to claim 5, characterized in that: The fluid collection system includes valves, back pressure valves, hand pumps, fluid collection containers, measuring cylinders, and balances. The fluid collection system is connected to the outlet of the core holder.

7. The method for quantitative evaluation of shale oil reservoir proppant reflux according to claim 6, characterized in that: The confining pressure maintaining system includes valves, a fluid preheater, and a constant pressure pump. The valves, fluid preheater, and constant pressure pump are connected to the core holder via pipelines. The temperature and pressure maintaining system is connected to both sides of the core holder.

8. The method for quantitative evaluation of shale oil reservoir proppant reflux according to claim 7, characterized in that: The vacuum system includes valves and a vacuum pump, which are connected by pipelines. The vacuum system is connected to the outlet of the core holder.

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