Quantitative evaluation method for shale oil reservoir proppant reflux

Abstract

The invention relates to a shale oil reservoir propping agent reflux quantitative evaluation method, which comprises the following steps of: calculating the width, height and length of a full-size coring core crack, and converting the field fracturing fluid flow-back displacement and shale oil daily output into laboratory simulation displacement and output; preparing simulated shale oil and fracturing fluid; filling a propping agent; vacuumizing is conducted; filling the artificial fracture with fracturing fluid; the empty measuring cylinder is filled with fracturing fluid, and the volume is recorded; the simulated shale oil is injected according to the yield, and the production process is simulated; taking down the measuring cylinder which collects the fracturing fluid and simulates the shale oil and the proppant every 24 hours; the fracturing fluid is slowly added into the measuring cylinder which is taken down at the ith time and filled with the fracturing fluid, the simulated shale oil and the proppant until the simulated shale oil is completely discharged, the volume V < branch i > of the proppant flowing back within 24 hours at the ith time is calculated, and the ratio of the V < branch i > to the volume of the proppant filled in the crack before the experiment is the proppant backflow rate at the moment. According to the invention, the proppant backflow condition under the real shale reservoir condition can be evaluated.

Description

Technical field:

[0001] The invention relates to the technical field of shale oil and gas reservoir development, and in particular to a shale oil reservoir proppant reflux quantitative evaluation method. Background technology:

[0002] Hydraulic fracturing is an effective measure to increase the production capacity of shale oil and gas reservoirs. The ground high-pressure pump group injects fracturing fluid into the well, and the high-pressure liquid expands the cracks in the formation or creates new cracks. The proppant is carried into the formation by the fracturing fluid and is used to support the hydraulic fractures after fracturing, so that artificial fractures with certain conductivity are formed in the formation, providing seepage channels for oil and gas extraction. As a key material, proppant directly affects the effect of fracturing transformation. However, after some oil and gas wells implement fracturing to increase production, proppant reflux occurs during the return of fracturing fluid and normal production.

[0003] Proppant backflow can cause many problems. First, it can reduce the width of the fracture or even close it, making it impossible to ensure long-term fracture conductivity. Second, it can cause bottleneck fractures to form near the wellbore, making the fracturing effect worse. Third, for wells with severe backflow and sand production, sand flushing and other well repair operations are often required, which affects the normal production of oil and gas wells and increases operating costs. In addition, the produced sand will also corrode the equipment downhole and on the ground, reducing their service life. Therefore, it is necessary to quantitatively evaluate the proppant backflow in shale oil reservoirs.

[0004] The prior art (CN117514121A, CN116201518A, CN116771319A, CN118128512A) introduces similar testing methods. However, the above test only simulates the backflow process of the proppant in the crack, and does not provide a method for accurately calculating the backflow rate of the proppant. All of the above technologies use glass plates or steel plates to perform backflow tests on the proppant, but do not place the proppant in the cracks of the reservoir rock. The glass plates and steel plates cannot reflect the rock properties of the real reservoir. In addition, the reservoir closure stress acts on the proppant through the rock, and the proppant will be embedded in the reservoir rock to varying degrees. The contact area and stress state between the proppant and the rock in the reservoir are quite different from those in the glass plates and steel plates. Among them, the existing technologies (CN117514121A, CN116201518A, CN116771319A) all use proppants at room temperature, while the working environment of the proppants is in the underground reservoir, and its reflux also occurs under the high temperature and high pressure conditions of the reservoir, so the above method cannot truly reflect the reflux of the proppant under the reservoir conditions. The existing technology (CN111060284A) provides a test device and method for simulating the reflux of proppant after crack closure using a diversion chamber, but does not provide a specific method for calculating the reflux rate of proppant, and because shale develops a large number of lamellae fractures and rock plates are difficult to make, this technology is suitable for conventional reservoir proppant reflux testing, and is no longer applicable to shale reservoirs. In addition, the above methods all require the proppant to be separated and its mass measured. In this process, it is difficult to collect fine broken proppant particles and it is difficult to remove the fracturing fluid residue attached to the proppant surface after drying, which brings errors to the test results.

[0005] Therefore, it is urgent to design a real-time quantitative evaluation method for proppant return rate after fracturing under shale oil reservoir conditions, so as to provide evaluation methods and experimental data support for proppant optimization, performance evaluation, and drainage scheme design during fracturing construction design. Summary of the invention:

[0006] The purpose of the present invention is to provide a quantitative evaluation method for proppant reflux in shale oil reservoirs. The quantitative evaluation method for proppant reflux in shale oil reservoirs is used to solve the problems of low accuracy of proppant reflux rate measurement methods in the prior art, inability to measure the reflux rate in real time, and inability to truly reflect reservoir temperature and pressure conditions and proppant embedding.

[0007] The technical solution adopted by the present invention to solve the technical problem is: the quantitative evaluation method for shale oil reservoir proppant reflux comprises the following steps:

[0008] Step 1: Calculate the full-scale coring core fracture width W f , high H f , calculate the full-scale coring core fracture length L f, and then the on-site fracturing fluid flowback volume Q F and daily shale oil production Q O Converted into laboratory simulation displacement q f and the output q o ;

[0009]

[0010] Where W F is the crack width, H F is the crack height, N is the number of cracks, Q F is the flowback volume of on-site fracturing fluid, Q O is the daily production of shale oil;

[0011] Step 2, preparing simulated shale oil and fracturing fluid;

[0012] Step 3, prepare core and proppant;

[0013] Step 4, filling with proppant, placing the core filled with proppant into the proppant reflux test system;

[0014] Step 5, vacuuming: Open the constant pressure pump of the confining pressure system, set its pressure to 2 MPa, and turn on the vacuum pump to vacuum the artificial fracture 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 artificial fractures are filled with fracturing fluid; set the temperature of the fluid preheater of the confining pressure system and the pressure of the constant pressure pump according to the temperature and horizontal minimum principal stress of the shale oil reservoir;

[0016] Step 7: Place an empty measuring cylinder in the fluid collection container. The mass of the empty measuring cylinder is m0. Fill the empty measuring cylinder with fracturing fluid and record the volume V of the fracturing fluid at this time. 压 ;

[0017] Step 8: Set the temperature of the fluid preheater of the fluid injection system according to the reservoir temperature, and use a hand pump to set the pressure of the back pressure valve of the fluid collection system according to the bottom hole flow pressure; open the fluid collection system and allow the measuring cylinder to start collecting the fluid flowing out of the full-size core holder; and set the pressure of the back pressure valve of the fluid collection system according to the laboratory simulation fracturing fluid displacement q f Set the flow rate of the constant speed and constant pressure pump, determine the fracturing fluid injection time of the fluid injection system according to the on-site fracturing fluid return time, and then use the production q o Reinject simulated shale oil to simulate the production process;

[0018] Step nine, every 24 hours, remove the measuring cylinder that has collected the fracturing fluid, simulated shale oil, and proppant, and place another measuring cylinder filled with fracturing fluid in the fluid collection container;

[0019] Step 10, slowly add fracturing fluid to the graduated cylinder containing fracturing fluid, simulated shale oil, and proppant taken out for the i-th time in step 9 until all the simulated shale oil is discharged, and measure the total mass of the graduated cylinder, fracturing fluid, and proppant at this time; the total mass of the graduated cylinder, fracturing fluid, and proppant measured for the i-th time is recorded as m i ; Calculate the volume V of the proppant refluxed within the i-th time within 24 hours 支i , V 支i The ratio of the volume V1-V2 of the proppant filled in the fracture before the experiment is the proppant reflux rate n at this moment. i ;

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

[0021] Where V 支i is the volume of the refluxed proppant in the i-th time period, ρ 支 is the true density of the proppant,

[0022] ρ 压 is the density of the fracturing fluid, m i is the total mass of the fracturing fluid and proppant dosage cylinder filled at the i-th moment;

[0023]

[0024] Where n i is the reflux rate of the proppant at the i-th moment;

[0025] Step eleven, taking out the measuring cylinders filled with fracturing fluid, simulated shale oil and proppant each time in step nine, repeat step ten in chronological order, so as to realize real-time and accurate measurement of the proppant reflux situation under the conditions of simulated shale oil reservoir, and obtain the proppant reflux amount in different time periods and the proppant reflux rate at different times.

[0026] The full-scale coring core fracture width W is calculated in the above scheme. f , high H f , calculate the full-scale coring core fracture length L f Method: Get the full-size coring core diameter D C , crack half length L F , Width W F , high H F , number of cracks N, on-site fracturing fluid flowback volume Q F 、Daily production of shale oil Q O , calculate the full-scale coring core fracture width W f , high H fAccording to the similarity ratio principle, the full-scale coring core fracture length L is calculated. f :

[0027] W f =W F

[0028] H f =D C

[0029]

[0030] The specific method of step 2 in the above scheme is: according to the reservoir temperature conditions, simulated shale oil is prepared using shale oil produced on the ground and kerosene to make it have the viscosity under reservoir conditions; fracturing fluid is prepared according to the on-site fracturing construction data; the prepared fracturing fluid and simulated shale oil are placed in two intermediate containers respectively.

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

[0032] The specific method of filling the proppant in step 4 of the above scheme is as follows: pre-place a supporting gasket in the artificial fracture, the thickness of the supporting gasket is the fracture width, tightly wrap the core with a thermoplastic film, slowly withdraw the supporting gasket, and simultaneously fill the fracture with proppant to obtain an artificial fracture of the shale core filled with proppant; after the proppant filling is completed, measure the volume V2 of the remaining proppant, then the volume of the proppant filled in the core fracture is V1-V2; place the core filled with proppant in a full-size core holder, when placing the core, the fracture is vertical to the horizontal plane, and fix the core in the middle position of the core holder using the clamp plug.

[0033] In the above scheme, the proppant reflux test 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 core filled with proppant is placed in the full-size core holder, the fluid injection system is connected to the inlet of the core holder through a pipeline, the core holder is provided with a temperature sensor, and the outlet of the core holder is connected to the fluid collection system and the vacuum system through a pipeline; the confining pressure maintenance system connects the two ends of the core holder to form a circulation loop.

[0034] In the above scheme, the fluid injection system 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, and the fluid injection system is connected to the inlet of the core clamp.

[0035] In the above scheme, the fluid collection system includes a valve, a back pressure valve, a hand pump, a fluid collection container, a measuring cylinder, and a balance, and the fluid collection system is connected to the outlet of the core holder.

[0036] In the above scheme, the confining pressure maintaining system includes valves, fluid preheaters, and constant pressure pumps, which are connected to the clamp temperature and pressure maintaining system by pipelines and connected to both sides of the core clamp.

[0037] In the above scheme, the vacuum system includes a valve and a vacuum pump, which are connected by a pipeline, and the vacuum system is connected to the outlet of the core holder. The vacuum system is used to vacuum the artificial fractures and shale cores filled with proppants, and the vacuum pump can be disassembled after the vacuum is completed.

[0038] Beneficial effects:

[0039] 1. In the test process of the present invention, actual coring shale cores, on-site fracturing fluid, and shale oil are used. The reservoir closure stress acts on the proppant through the shale rock, which can accurately reflect the stress state and embedding condition of the proppant in the reservoir rock; based on the principle of similarity ratio and equal flow rate (equal Reynolds number), the geometric dimensions of the fractures and the displacement of the produced fluid at the fracturing site are converted into corresponding values ​​under laboratory test conditions, which can evaluate the proppant reflux under the real reservoir conditions of shale.

[0040] 2. The present invention can heat and pressurize the full-size core holder, can truly simulate the high temperature and high pressure environment of the reservoir, and can evaluate the proppant reflux rate under different closing pressure conditions during the mining process;

[0041] 3. The test process of the present invention does not need to separate the proppant from the test fluid, thus avoiding the error caused by the attachment of fracturing fluid components; there is no need to filter or transfer proppant particles through a sieve to determine the reflux rate, and all refluxed proppant, including broken proppant fine particles, is collected. There is no proppant loss during the test process, and the evaluation result is highly accurate; in addition, the proppant reflux rate can be monitored in real time at different times, and the relationship between the proppant reflux rate and time can be determined. Description of the drawings:

[0042] Figure 1 This is a schematic diagram of the proppant flowback test system;

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

[0044] In the figure: 1 constant speed constant 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 proppant. Specific implementation method:

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

[0046] Combination Figure 1-Figure 2 As shown, this method for quantitatively evaluating proppant flowback in shale oil reservoirs uses a full-size core with pre-set fractures and a full-size core holder to simulate the proppant flowback process under shale oil reservoir conditions and accurately determine the flowback 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 flow rate of on-site fracturing fluid Q F =80m 3 / d, daily shale oil production Q O =20m 3 / d, and the full-scale coring core fracture width W is calculated according to formula 1 and formula 2 f =5mm, height H f = 10.0 cm. According to the similarity ratio principle, the full-scale coring core fracture length L is calculated using formula 3. f =33.3cm, and then according to the principle of equal flow rate (equal Reynolds number), use Formula 4 and Formula 5 to calculate the on-site fracturing fluid return flow volume Q F and daily shale oil production Q O Converted into laboratory simulation displacement q f =1.03ml / min and output q o =0.26ml / min;

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

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

[0050]

[0051]

[0052]

[0053] Where D C =10.0cm, which is the full-size coring core diameter.

[0054] Step 2, preparation of simulated shale oil and fracturing fluid: simulated shale oil is prepared using shale oil produced from the ground and kerosene, so that its viscosity is 0.595cp under reservoir temperature of 110°C; fracturing fluid is prepared according to on-site fracturing construction 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: Preparation of core and proppant: Drill a full-size coring core with a length of 33.3 cm and a diameter of 10 cm from the shale oil reservoir, split the core axially for artificial seam creation; select 70 / 140 mesh quartz sand proppant according to the proppant specifications used in on-site fracturing, and take a volume of V1 = 200 cm 3 The proppant 24 is used for flowback evaluation test;

[0056] Step 4, proppant filling: pre-place a proppant pad in the artificial fracture, the thickness of the proppant pad is 5 mm, tightly wrap the core 15 with a thermoplastic film, slowly withdraw the proppant pad, and simultaneously fill the fracture with proppant 24 to obtain a shale core artificial fracture filled with proppant. After the proppant 24 is filled, measure the volume of the remaining proppant V2 = 67.5 cm 3 , then the volume of proppant filling the core fracture is V1-V2=132.5cm 3 The core filled with proppant is placed in the full-size core holder 14, and the fracture is vertical to the horizontal plane ( Figure 2 ), fix the core in the middle position of the core holder 14 using the holder plug 13, and then adjust all valves to the closed state;

[0057] Step 5, vacuuming: open the fifth valve 18 and the constant pressure pump 20, the fifth valve 18 is a 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 fracture filled with proppant for 2h, and close the sixth valve 21 and the vacuum pump 22 after completion;

[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 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 fractures. Set the temperature of the second fluid preheater 19 in the confining pressure system to 110°C and the pressure of the constant pressure pump 20 to 46 MPa according to the temperature and horizontal minimum principal stress of the shale oil reservoir.

[0059] Step 7: Place an empty measuring cylinder 11 in the fluid collection container 10. The mass of the empty measuring cylinder is m0 = 26.5724 g. Fill the empty measuring 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°C according to the reservoir temperature, and set the pressure of the back pressure valve 9 of the fluid collection system to 20MPa using the hand pump 23 according to the bottom hole flow pressure. Open the fourth valve 8 in the fluid collection system, and according to the laboratory simulated fracturing fluid displacement q determined in step 1, f Set the flow rate of constant speed and pressure pump 1 to 1.03 ml / min, determine the fracturing fluid injection time of the fluid injection system according to the on-site fracturing fluid return time, close the first valve 2 after 48 hours, open the second valve 3, and calculate the laboratory simulated shale oil production q determined in step 1. o The flow rate of the constant speed and constant pressure pump 1 was set to 0.26 ml / min; simulated shale oil was injected to simulate the shale oil production process;

[0061] Step nine, every 24 hours, remove the measuring cylinder 11 that has collected the fracturing fluid, simulated shale oil, and proppant, and place another measuring cylinder filled with fracturing fluid in 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 in step 9 until all the simulated shale oil is discharged. Measure the total mass of the measuring cylinder, fracturing fluid and proppant at this time, which is recorded as m1=65.8912g. Calculate the volume V of the proppant refluxed within 24 hours using formula 6: 支1 =6.44cm 3 . V 支1 The volume of proppant filled in the fracture before the experiment is V1-V2=132.5cm 3 The ratio is the proppant reflux rate n1=4.86% at this moment, which can be calculated by formula 7. Repeat the above steps to achieve real-time and accurate measurement of the proppant reflux under simulated shale oil reservoir conditions. Then, the proppant reflux amount in different time periods and the proppant reflux rate at different moments are obtained (see Table 1 for details).

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

[0064] Where V 支i is the volume of the refluxed proppant in the i-th time period, ρ支 =2.63g / cm 3 is the density of the proppant, ρ 压 =1.01g / cm 3 is the density of the fracturing fluid, m i is the total mass of the cylinder filled with fracturing fluid and proppant at the i-th moment.

[0065]

[0066] Where n i is the reflux rate of the proppant at the i-th moment.

[0067] Table 1 Proppant reflux volume at different time periods and proppant reflux rate at different times

[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 test system of the present 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 core 15 filled with proppant is placed in the full-size core holder 14, the fluid injection system is connected to the inlet of the core holder through a pipeline 17, the core holder is provided with a temperature sensor 16, the core holder outlet is connected to the fluid collection system and the vacuum system through a pipeline 17, and 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, and the fluid injection system is connected to the inlet of the full-size core clamp 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 the full-size core holder 14 .

[0072] The confining pressure maintaining 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 pumping system includes a sixth valve 21 and a vacuum pump 22. The sixth valve 21 and the vacuum pump 22 are connected by a pipeline, and the vacuum pumping system is connected to the outlet of the core holder. The vacuum pumping system is used to vacuum the artificial fractures and shale cores filled with proppant. After the vacuum pumping is completed, the vacuum pump can be disassembled.

[0074] The present invention firstly conducts a proppant reflux rate measurement experiment under the conditions of shale oil reservoir temperature and effective stress on the proppant, simulating the reflux process of the proppant in the core fracture of the reservoir, and then calculates the proppant reflux rate through data processing, thereby obtaining the change law of the reflux of the proppant in the fracture under the conditions of shale oil reservoir over time. This method is not only applicable to the quantitative evaluation of proppant reflux in shale oil reservoirs, but also to conventional oil and gas reservoirs. The measurement results of this method are highly accurate and can truly reflect the reflux of the proppant in the core fracture of the reservoir under the conditions of shale oil reservoirs.

[0075] The purpose of step 1 in the present invention is to convert the actual fracturing conditions on site into laboratory simulation conditions.

[0076] The purpose of step 2 is to obtain fracturing fluid and simulated shale oil for field use.

[0077] The purpose of steps 3 and 4 is to prepare for the establishment of a fracture-proppant simulation system under shale oil reservoir conditions. In step 3, the core can be artificially fractured by splitting or by conventional wire cutting. In step 4, proppants of different particle sizes and types can be filled, and different proppant laying methods and different sand laying concentrations (achieved by using proppant pads of different thicknesses) can be used to study the effects of different factors on the proppant reflux rate in fractures under shale oil reservoir conditions.

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

[0079] In step six, different pressures of the constant pressure pump can be set, that is, different effective stresses can be applied to the proppant to study the effects of different fracture closure stresses on the proppant return rate.

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

[0081] The purpose of step 10 is to accurately calculate the proppant return rate. The fluid collection system uses a high-precision online balance to achieve real-time measurement of the flow through the fracture. The measurement results can truly reflect the return of the proppant in the fracture under formation conditions. In addition, the change of the return of the proppant in the fracture over time under different fracture closure stress conditions can be studied.

[0082] In summary, the real-time determination method of proppant return rate under shale oil reservoir conditions provided by the present application can achieve real-time determination of proppant return rate under simulated shale oil reservoir temperature, pressure and rock physical property conditions, truly reflect the return of proppant in fractures at different times under the effective stress of the reservoir, and study the influence of different factors on the return rate. The measurement process of this method is simple, and a method for determining proppant return rate under simulated shale oil reservoir conditions is provided, which improves the accuracy of determining proppant return rate under shale oil reservoir conditions.

Claims

1. A quantitative evaluation method for proppant reflux in shale oil reservoirs, characterized in that The following steps are involved: Step 1: Calculate the full-scale coring core fracture width W f , high H f , calculate the full-scale coring core fracture length L f , and then the on-site fracturing fluid flowback volume Q F and daily shale oil production Q O Converted into laboratory simulation displacement q f and the output q o ; Where W F is the crack width, H F is the crack height, N is the number of cracks, Q F is the flowback volume of on-site fracturing fluid, Q O is the daily production of shale oil; Step 2, preparing simulated shale oil and fracturing fluid; Step 3, prepare core and proppant; Step 4, filling with proppant, placing the core filled with proppant into the proppant reflux test system; Step 5, vacuuming: Open the constant pressure pump of the confining pressure system, set its pressure to 2 MPa, and turn on the vacuum pump to vacuum the artificial fracture 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 artificial fractures are filled with fracturing fluid; set the temperature of the fluid preheater of the confining pressure system and the pressure of the constant pressure pump according to the temperature and horizontal minimum principal stress of the shale oil reservoir; Step 7: Place an empty measuring cylinder in the fluid collection container. The mass of the empty measuring cylinder is m0. Fill the empty measuring cylinder with fracturing fluid and record the volume V of the fracturing fluid at this time. 压 ; Step 8, setting the temperature of the fluid preheater of the fluid injection system according to the reservoir temperature, and setting the pressure of the back pressure valve of the fluid collection system by using a hand pump according to the bottom hole flow pressure; Turn on the fluid collection system so that the graduated cylinder begins to collect fluid from the full-size core holder; According to the laboratory simulation fracturing fluid displacement q f Set the flow rate of the constant speed and constant pressure pump, determine the fracturing fluid injection time of the fluid injection system according to the on-site fracturing fluid return time, and then use the production q o Reinject simulated shale oil to simulate the production process; Step nine, every 24 hours, remove the measuring cylinder that has collected the fracturing fluid, simulated shale oil, and proppant, and place another measuring cylinder filled with fracturing fluid in the fluid collection container; Step 10, slowly add fracturing fluid to the graduated cylinder containing fracturing fluid, simulated shale oil, and proppant taken out for the i-th time in step 9 until all the simulated shale oil is discharged, and measure the total mass of the graduated cylinder, fracturing fluid, and proppant at this time; the total mass of the graduated cylinder, fracturing fluid, and proppant measured for the i-th time is recorded as m i ; Calculate the volume V of the proppant that flows back out during the ith 24-hour period 支i , V 支i The ratio of the volume of proppant filled in the fracture before the experiment V1-V2 is the proppant reflux rate n at this moment. i ; V 支 - ρ 支 +(V 压 -V 支i )·ρ 压 =m i -m0 Where V 支i is the volume of the refluxed proppant in the i-th time period, ρ 支 is the true density of the proppant, ρ 压 is the density of the fracturing fluid, m i is the total mass of the fracturing fluid and proppant dosage cylinder filled at the i-th moment; Where n i is the reflux rate of the proppant at the i-th moment; Step eleven, taking out the measuring cylinders filled with fracturing fluid, simulated shale oil and proppant each time in step nine, repeat step ten in chronological order, so as to realize real-time and accurate measurement of the proppant reflux situation under the conditions of simulated shale oil reservoir, and obtain the proppant reflux amount in different time periods and the proppant reflux rate at different times.

2. The method for quantitative evaluation of proppant reflux in shale oil reservoirs according to claim 1, characterized in that: The calculation of the full-scale coring core fracture width W f , high H f , calculate the full-scale coring core fracture length L f Method: Get the full-size coring core diameter D C , crack half length L F , Width W F , high H F , number of cracks N, on-site fracturing fluid flowback volume Q F 、Daily output of shale oil Q O , calculate the full-scale coring core fracture width W f , high H f According to the similarity ratio principle, the full-scale coring core fracture length L is 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 of step 2 is as follows: simulated shale oil is prepared using shale oil and kerosene produced on the ground according to reservoir temperature conditions so that it has the viscosity under reservoir conditions; fracturing fluid is prepared according to on-site fracturing construction data; and the prepared fracturing fluid and simulated shale oil are placed in two intermediate containers respectively.

4. The method for quantitatively evaluating reflux of proppant in shale oil reservoir according to claim 3, characterized in that: The specific method of step three is as follows: a full-size coring core is drilled from a shale oil reservoir, and the core is split axially to create artificial fractures; a proppant is selected according to the proppant specifications used for on-site fracturing, and a volume of V1 of proppant is taken for a reflux evaluation test.

5. The method for quantitatively evaluating reflux of proppant in shale oil reservoir according to claim 4, characterized in that: The specific method of filling the proppant in the step 4 is as follows: pre-place a supporting gasket in the artificial fracture, the thickness of the supporting gasket is the fracture width, tightly wrap the core with a thermoplastic film, slowly withdraw the supporting gasket, and simultaneously fill the fracture with proppant to obtain an artificial fracture of the shale core filled with proppant; after the proppant filling is completed, measure the volume V2 of the remaining proppant, and the volume of the proppant filled in the core fracture is V1-V2; place the core filled with proppant in a full-size core holder, and when placing the core, the fracture is vertical to the horizontal plane, and the core is fixed in the middle position of the core holder by using the clamp plug.

6. The method for quantitatively evaluating reflux of proppant in shale oil reservoir according to claim 5, characterized in that: The proppant reflux test system comprises a full-size core holder, a fluid injection system, a fluid collection system, a confining pressure maintaining system, and a vacuum pumping system. The core filled with proppant is placed in the full-size core holder. The fluid injection system is connected to the inlet of the core holder through a pipeline. The core holder is provided with a temperature sensor. The outlet of the core holder is connected to the fluid collection system and the vacuum pumping system through a pipeline; the confining pressure maintaining system connects the two ends of the core holder to form a circulation loop.

7. The method for quantitatively evaluating reflux of proppant in shale oil reservoir according to claim 6, characterized in that: The fluid injection system comprises 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, and the fluid injection system is connected to the inlet of the core holder.

8. The method for quantitatively evaluating reflux of proppant in shale oil reservoir according to claim 7, characterized in that: The fluid collection system comprises a valve, a back pressure valve, a hand pump, a fluid collection container, a measuring cylinder and a balance. The fluid collection system is connected to the outlet of the core holder.

9. The method for quantitatively evaluating reflux of proppant in shale oil reservoir according to claim 8, characterized in that: The confining pressure maintaining system comprises valves, fluid preheaters and constant pressure pumps, which are connected to the clamp by pipelines. The temperature and pressure maintaining system is connected to both sides of the core clamp.

10. The method for quantitatively evaluating proppant reflux in shale oil reservoirs according to claim 9, characterized in that: The vacuum pumping system comprises a valve and a vacuum pump, the valve and the vacuum pump are connected by a pipeline, and the vacuum pumping system is connected to the outlet of the core holder.

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