Supercritical CO2 fracturing fluid in-situ rheology and visual crack sand-carrying experiment system and method

Abstract

The invention relates to the technical field of indoor evaluation systems and evaluation methods of a fracturing transformation technology, and discloses a supercritical CO2 fracturing fluid in-situ rheology and visual crack sand-carrying experiment system and method. The experimental system comprises a supercritical CO2 high-pressure phase balance unit, a high-pressure capillary viscosity testing unit, a high-pressure visual dynamic sand-carrying testing unit and a data acquisition and analysis unit. The experimental method comprises capillary viscosity testing and dynamic sand-carrying performance testing. According to the scheme, the sand-carrying experiment and the viscosity experiment are combined, the sand-carrying experiment is used for testing the migration conditions of the supercritical CO2 fracturing fluid obtained by different formulas under different simulated temperatures and pressures, and the viscosity experiment is used for testing the viscosity of the supercritical CO2 fracturing fluid obtained by different formulas, evaluating the friction resistance of the supercritical CO2 fracturing fluid and optimizing the rheological property of the fracturing fluid, so that the friction resistance is reduced, and the fracturing efficiency is improved. Energy consumption is reduced, and operation efficiency is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of indoor evaluation systems and evaluation methods for fracturing transformation technology, and in particular to an in-situ rheology and visible fracture sand carrying experiment system and method for supercritical CO2 fracturing fluid. Background Art

[0002] Fracturing technology is the main measure to increase the production of unconventional oil and gas, and has been widely used at home and abroad. Compared with traditional hydraulic fracturing technology, CO2 fracturing has developed into a new generation of fracturing technology for increasing the production of unconventional reservoirs. It has the advantages of low damage, easy flowback, replacement of methane gas to increase gas production, and storage of CO2. It has good application prospects for tapping the potential of unconventional oil and gas reservoirs in my country and promoting the continuous, efficient and green development of unconventional oil and gas fields in my country.

[0003] However, CO2 fracturing fluid has a complex phase change (forming a supercritical state) during fracturing, has low viscosity, and poor sand carrying performance, and is in urgent need of tackling key problems in engineering applications. At present, by adding an efficient additive suitable for supercritical CO2 fracturing fluid, it is an effective method to improve the key performances of supercritical CO2 fracturing system rheology, sand carrying, etc. As prior art CN115166138A discloses an evaluation system and evaluation method for dynamic sand carrying of a high and low temperature full phase closed cycle visualization crack for anhydrous CO2 fracturing, although it can detect the sand carrying capacity of anhydrous CO2 fracturing fluid system in cracks under the conditions of high and low temperature changes, pressure changes, and thickener addition, however, its evaluation method detection index for supercritical CO2 fracturing fluid performance is still relatively one-sided, and the performance of supercritical CO2 fracturing fluid cannot be accurately evaluated, and it is difficult to accurately guide the optimization and engineering application of supercritical CO2 fracturing fluid system.

[0004] The patent of this invention proposes an in-situ rheology and visible fracture sand carrying experimental system and method for supercritical CO2 fracturing fluid, which not only effectively makes up for the shortcomings of the existing technology, but also provides support for the standardized evaluation method and system optimization of the key properties of supercritical CO2 fracturing fluid, which is of great significance to the innovation of CO2 fracturing technology. Summary of the invention

[0005] The present invention aims to provide an in-situ rheology and visible fracture sand carrying experimental system and method for supercritical CO2 fracturing fluid, so as to solve the technical problem that the existing experimental system or experimental method is too simple to accurately evaluate the performance of supercritical CO2 fracturing fluid.

[0006] To achieve the above-mentioned purpose, the present invention adopts the following technical scheme: an in-situ rheology and visible fracture sand carrying experimental system of supercritical CO2 fracturing fluid, comprising a supercritical CO2 high-pressure phase equilibrium unit, a high-pressure capillary viscosity test unit, a high-pressure visualized dynamic sand carrying test unit and a data acquisition and analysis unit.

[0007] The principles and advantages of this solution are as follows:

[0008] Compared with the existing technology that can only detect some properties of supercritical CO2 fracturing fluid through sand carrying test, this scheme combines sand carrying test and viscosity test. The sand carrying test tests the migration of supercritical CO2 fracturing fluid obtained from different formulas under simulated different temperatures and pressures, and the viscosity test is used to test the viscosity of supercritical CO2 fracturing fluid obtained from different formulas, evaluate its friction resistance, and optimize the rheology of fracturing fluid, thereby reducing friction resistance, reducing energy consumption, and improving operating efficiency. In addition, this scheme combines the viscosity test results and the sand carrying test results to not only accurately evaluate the performance of supercritical CO2 fluid, but also reversely adjust the formula of supercritical CO2 fracturing fluid according to the experimental results, effectively ensuring that the proppant can be effectively transported to the depth of the crack, preventing the crack from closing, and improving the continuity of fracturing.

[0009] In addition, this scheme effectively forms supercritical CO2 in phase equilibrium by setting up a supercritical CO2 high-pressure phase equilibrium unit, providing experimental conditions for sand carrying experiments and viscosity experiments. By setting up a data acquisition and analysis unit, the experimental process can be effectively controlled and adjusted, and the collected data can be analyzed. The results can be used to reversely adjust and optimize the experimental design, so as to obtain a better supercritical CO2 fracturing fluid formula, thereby effectively improving the performance of supercritical CO2 fluid, increasing gas production and production efficiency.

[0010] Preferably, as an improvement, the data acquisition and analysis unit is electrically connected to the supercritical CO2 high-pressure phase equilibrium unit, the high-pressure capillary viscosity test unit, and the high-pressure visual dynamic sand carrying test unit; the data acquisition and analysis unit includes a controller and a data analysis module, the controller controls the working state of the entire system, adjusts the working parameters and collects data, and the data analysis module is used to analyze the collected experimental data.

[0011] Preferably, as an improvement, the supercritical CO2 high-pressure phase equilibrium unit includes a CO2 gas source, a refrigeration unit, a first power pump, a first high-pressure phase balancer, a phase equilibrium CO2 storage tank, a first temperature control cabinet and a first exhaust pipeline connected in sequence by pipelines, and the first high-pressure balancer is also connected to a plurality of dosing components, and the dosing components include a first dosing pump and an additive storage tank connected in sequence.

[0012] Preferably, as an improvement, the additive includes a thickener and / or a co-solvent.

[0013] Technical effect: This scheme adopts the above-mentioned setting, which is convenient for fully mixing additives and supercritical CO2 to obtain supercritical CO2 fracturing fluid required for different experiments. Specifically, after the liquid CO2 is converted into supercritical CO2 under the conditions of temperature and pressure in the first high-pressure balancer, the additive is weighed or measured according to the formula and pumped into the first high-pressure balancer, and stirred and mixed into supercritical CO2 fracturing fluid; at this time, by adjusting the type and amount of additives and solvents, any proportion of additives and supercritical CO2 can be quickly mixed, so as to quickly obtain the supercritical CO2 fracturing fluid required for the experiment.

[0014] Preferably, as an improvement, the high-pressure capillary viscosity testing unit comprises a first intermediate container for CO2 fracturing fluid, a second power pump, a capillary, a CO2 flowmeter, a second intermediate container for CO2 fracturing fluid and a second drain line which are sequentially connected by pipelines, the capillary is located in a circulating constant temperature water bath, and the inlet and outlet of the capillary are connected to both ends of the differential pressure sensor; the first intermediate container for CO2 fracturing fluid is connected to the first drain line pipeline.

[0015] Technical effect: This scheme adopts the above-mentioned setting, which makes it convenient for the high-pressure capillary viscosity test unit to test the viscosity of the supercritical CO2 fracturing fluid. On the one hand, it can effectively evaluate its sand carrying performance and ensure that the proppant can be effectively transported to the deep of the crack to prevent the crack from closing; on the other hand, it can reversely optimize the rheology and formulation of the fracturing fluid, thereby reducing friction, reducing energy consumption and improving operating efficiency.

[0016] Preferably, as an improvement, the high-pressure visualized dynamic sand-carrying test unit comprises a third intermediate container for CO2 fracturing fluid, a third power pump, a second high-pressure phase balancer, a pressure chamber, and a second temperature control cabinet which are sequentially connected by pipelines; the second high-pressure phase balancer is also sequentially connected by pipelines to a second dosing pump and a proppant storage tank, and a visualized high-pressure flat plate crack is provided in the pressure chamber; the outlet of the pressure chamber is connected to an emptying pipeline and a circulating recovery pipeline, and a filter is provided on the circulating recovery pipeline; the third intermediate container for CO2 fracturing fluid is pipeline-connected to the first emptying pipeline.

[0017] Technical effect: This scheme adopts the above settings, which is convenient for stable sand carrying experiments and improves the accuracy of test results. Specifically, this scheme effectively ensures the uniformity of the mixing of the proppant and the supercritical CO2 fracturing fluid in the second high-pressure phase balancer and then pumps them into the pressure chamber to simulate the sand carrying experiment, thereby accurately simulating the state of the fracturing fluid in the formation, avoiding the influence of migration caused by uneven mixing from interfering with the accurate evaluation of the results of the sand carrying experiment by the experimenters, thereby effectively improving the accuracy of the experimental results. In this scheme, a visualized high-pressure flat plate fracture is set in the pressure chamber. On the one hand, it is convenient for the supercritical CO2 fracturing fluid mixed with the proppant to carry out the sand carrying experiment after entering the visualized high-pressure flat plate fracture. On the other hand, by adjusting the temperature and pressure conditions of the pressure chamber, the temperature and pressure of the visualized high-pressure flat plate fracture can be ensured to be stable, and it is also convenient to maintain the stability of the experimental conditions in the fracture during the sand carrying experiment, thereby improving the accuracy of the test results. Finally, this scheme sets an emptying pipeline and a circulating recovery pipeline at the outlet of the pressure chamber, which is convenient for processing materials in the sand carrying experiment according to different experimental requirements, and is more conducive to meeting various personalized experimental requirements. Specifically, this solution can recover most of the proppant through the filter, and the recovered proppant can be reused in the next experiment; and the emptying pipeline is used to clean the equipment after the experiment and remove the small amount of proppant remaining in the gap of the plate. The CO2 fracturing fluid through the recycling pipeline can be circulated multiple times in the same experiment, and then by increasing the amount of additives, the system performance under different amounts can be obtained without disassembling the equipment.

[0018] Preferably, as an improvement, the outlet of the circulation recovery pipeline is connected to the supercritical CO2 high-pressure phase equilibrium unit pipeline.

[0019] Beneficial effect: This scheme adopts the above-mentioned setting, which is convenient for recovering CO2 after the proppant is removed by the filter. The recovered CO2 can be reused after being converted into liquid CO2 by the supercritical CO2 high-pressure phase equilibrium unit, thereby reducing the experimental cost.

[0020] Preferably, as an improvement, one side wall of the visual high-pressure flat plate crack is a glass window.

[0021] Beneficial effect: This scheme adopts the above-mentioned setting, which is convenient for real-time observation and recording of the sand carrying experiment process and results.

[0022] Preferably, as an improvement, the first high-pressure phase balancer and the second high-pressure phase balancer are both provided with visualization windows.

[0023] Beneficial effect: This scheme adopts the above-mentioned setting, which makes it easy to observe whether the mixture of supercritical CO2 and additives or proppants is mixed evenly and reaches a phase equilibrium state before the experiment, thereby effectively ensuring that the experimental materials or conditions are infinitely close to actual production, thereby improving the accuracy and practicality of the test results.

[0024] Preferably, as an improvement, the present solution also provides an in-situ rheology and visible fracture sand carrying experimental method of supercritical CO2 fracturing fluid, comprising the following steps:

[0025] Step 1: Preparation of supercritical CO2 fracturing fluid;

[0026] Step 2: Equipment debugging;

[0027] Step 3: Capillary viscosity test: Start the second power pump to pump the supercritical CO2 fracturing fluid mixed with proppant into the capillary, and detect the pressure difference and flow rate at both ends of the capillary;

[0028] Step 4: Dynamic sand carrying performance test: start the third power pump, pump the supercritical CO2 fracturing fluid mixed with proppant into the second high-pressure phase balancer, add proppant in proportion and then pump it into the pressure chamber, observe and record the migration of fracturing fluid carrying sand in the visualized high-pressure flat plate crack;

[0029] Step 5: Post-processing of experimental materials: After the experiment, filter the test materials, recover the proppant, and evacuate or recover the CO2.

[0030] Beneficial effect: This scheme adopts the above settings to facilitate testing of the viscosity and sand-carrying performance of the fracturing fluid. This scheme can not only accurately evaluate the performance of the supercritical CO2 fluid by combining the viscosity test results and the sand-carrying test results, but also reversely adjust the formula of the supercritical CO2 fracturing fluid according to the experimental results, effectively ensuring that the proppant can be effectively transported to the depth of the fracture, preventing the fracture from closing and improving the continuity of the fracturing.

[0031] Preferably, as an improvement, in step three, the pressure difference ΔP and flow rate Q at both ends of the capillary are recorded, and the capillary viscosity at the temperature, pressure and shear rate is calculated according to formula (1);

[0032]

[0033] Where: μ——viscosity of liquid / supercritical CO2 fracturing fluid, in millipascals·seconds (mPa·s);

[0034] R——capillary radius, in meters (m);

[0035] ΔP——capillary inlet and outlet pressure difference, in millipascals (mPa);

[0036] Q——flow rate, in cubic meters per second (m3·s-1);

[0037] L——capillary length, in meters (m);

[0038] The conversion relationship between pump flow rate and shear rate:

[0039] For Newtonian fluids, the shear rate of the fluid at the capillary wall is calculated according to formula (2):

[0040]

[0041] Where: Q – fluid volume flow rate;

[0042] For non-Newtonian fluids, the shear rate of the fluid at the capillary wall is calculated according to formula (3):

[0043]

[0044] Where: σ is the shear stress of the fluid at the capillary wall.

[0045] Beneficial effect: This scheme adopts the above-mentioned setting, which is convenient for calculating the capillary viscosity of the CO2 fracturing fluid according to the experimental record of the pressure difference ΔP and flow rate Q at both ends of the capillary, evaluating its friction resistance, and then optimizing the rheology of the fracturing fluid, reducing friction resistance, reducing energy consumption, and improving operating efficiency.

[0046] Preferably, as an improvement, in step 4, the longest distance that the proppant migrates in the flat plate seam is recorded, and the maximum sand-carrying migration distance ratio is calculated according to formula (4):

[0047]

[0048] Where: υ——the maximum sand-carrying migration distance ratio of supercritical CO2 fracturing fluid, expressed as a percentage;

[0049] lmax is the maximum distance that the proppant migrates in the flat plate fracture, in centimeters (cm);

[0050] l0——Length of flat plate seam, in centimeters (cm).

[0051] In summary, this scheme controls the in-situ feed, prepares different supercritical CO2 fracturing fluid systems, and tests the high-pressure rheology and high-pressure fracture dynamic sand-carrying performance of supercritical CO2 fracturing fluid in an integrated manner, and provides quantitative index parameters (especially the characterization of the sand-carrying effect by migration distance, which is easy to operate and comparable in results), providing support for additive optimization, system optimization and process parameter specification. The scheme also provides an experimental scheme for the integrated multi-module experimental system and quantitative results of the in-situ rheology and visible fracture sand-carrying of supercritical CO2 fracturing fluid, which is convenient for comprehensive evaluation of fracturing fluid performance and reverse adjustment and optimization of the fracturing fluid formula, in order to achieve the best technical effect. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] Figure 1This is a schematic diagram of the structure of the supercritical CO2 visible crack sand carrying experimental system in Example 1 of the present invention.

[0053] Figure 2 This is a schematic diagram of the structure of the supercritical CO2 high-pressure phase equilibrium unit in Example 1 of the present invention.

[0054] Figure 3 Schematic diagram of the structure of the high pressure capillary viscosity testing unit in Example 1 of the present invention.

[0055] Figure 4 This is a schematic structural diagram of the high-pressure visual dynamic sand carrying test unit in Example 1 of the present invention.

[0056] Figure 5 This is a schematic structural diagram of visualizing high-pressure flat plate cracks in the high-pressure visualized dynamic sand-carrying test unit in Example 1 of the present invention (the left figure is a front view, and the right figure is an AA cross-sectional view of the left figure).

[0057] Figure 6 This is a graph showing the relationship between the viscosity of the fracturing fluid and time when the thickener is 1% of the modified phosphate ester HHCH-14 in Experimental Example 1 of the present invention.

[0058] Figure 7 This is a graph showing the relationship between the viscosity of the fracturing fluid and time when the thickener is 2% ester ether polymer in Experimental Example 1 of the present invention.

[0059] Figure 8 This is a relationship diagram between the viscosity of the fracturing fluid and the amount of the thickener added when the thickener is 1% styrene fluorinated acrylic in Experimental Example 1 of the present invention.

[0060] Fig. 9 The sand-carrying migration of the fracturing fluid when the thickener is 1% styrene fluorinated acrylic acid in Experimental Example 2 of the present invention (the left side is the sand-carrying migration without adding the thickener, and the right side is the sand-carrying migration after adding the thickener). DETAILED DESCRIPTION

[0061] The present invention is further described in detail below in conjunction with examples, but the embodiments of the present invention are not limited thereto. Unless otherwise specified, the technical means used in the following examples and experimental examples are conventional means well known to those skilled in the art, and the materials, reagents, etc. used can be obtained from commercial sources.

[0062] Explanation of the reference numerals: CO2 gas source 11, refrigeration unit 12, first power pump 13, additive storage tank 14, first dosing pump 15, first high-pressure phase balancer 16, phase-balanced CO2 storage tank 17, first temperature-controlled cabinet 18, first drain pipeline 19, first intermediate container 21 for CO2 fracturing fluid, second power pump 22, capillary 23, differential pressure sensor 24, circulating constant temperature water bath 25, CO2 flowmeter 26, second intermediate container 27 for CO2 fracturing fluid, second drain pipeline 28, third intermediate container 31 for CO2 fracturing fluid, third power pump 32, second high-pressure phase balancer 33, proppant storage tank 34, second dosing pump 35, pressure chamber 36, visualized high-pressure flat plate crack 361, crack width d, glass window 362, adjustment plate 363, second temperature-controlled cabinet 37, filter 38, drain pipeline 391, circulating recovery pipeline 392.

[0063] Example 1

[0064] This scheme provides an in-situ rheological and visible fracture sand carrying experimental system for supercritical CO2 fracturing fluid. Figures 1 to 5 As shown, it includes a supercritical CO2 high-pressure phase equilibrium unit, a high-pressure capillary viscosity test unit, a high-pressure visual dynamic sand carrying test unit and a data acquisition and analysis unit.

[0065] The data acquisition and analysis unit is electrically connected to the supercritical CO2 high-pressure phase equilibrium unit, the high-pressure capillary 23 viscosity test unit, and the high-pressure visual dynamic sand carrying test unit; the data acquisition and analysis unit includes a controller and a data analysis module. The controller controls the working state of the entire system, adjusts the working parameters and collects data. The data analysis module is used to analyze the collected experimental data. This part is the existing technology and will not be elaborated here.

[0066] The supercritical CO2 high-pressure phase balance unit includes a CO2 gas source 11, a refrigeration unit 12, a first power pump 13, a first high-pressure phase balancer 16, a phase balance CO2 storage tank, a first temperature control cabinet 18 and a first exhaust pipeline 19 connected in sequence by pipelines. The first high-pressure balancer is also connected to a plurality of dosing components, and the dosing components include a first dosing pump 15 and an additive storage tank 14 connected; wherein the additive includes a thickener and / or a cosolvent. As a reference, the first high-pressure phase balancer 16, the phase balance CO2 storage tank, the first dosing pump 15 and the additive storage tank 14 are all located in the first temperature control cabinet 18, which is convenient for centralized temperature control. In order to facilitate the observation of the state of the fracturing fluid, the first high-pressure phase balancer 16 in this scheme is provided with a visualization window.

[0067] The high-pressure capillary 23 viscosity test unit includes a first intermediate container 21 of CO2 fracturing fluid, a second power pump 22, a capillary 23, a CO2 flowmeter 26, a second intermediate container 27 of CO2 fracturing fluid and a second drain line 28 which are sequentially connected by pipelines. The capillary 23 is located in a circulating constant temperature water bath 25. The inlet and outlet of the capillary 23 are connected to both ends of a differential pressure sensor 24. The first intermediate container 21 of CO2 fracturing fluid is connected to the first drain line 19 by pipeline. The high-pressure capillary 23 viscosity test unit provided in this scheme can test the change in the capillary 23 viscosity of the supercritical CO2 fracturing fluid after adding different thickeners through in-situ feeding, and establish the relationship between the rheology and sand carrying performance of supercritical CO2 fracturing fluid through an integrated sand carrying performance test, so as to better provide experimental support for the optimization of key treatment agents and process parameters.

[0068] The high-pressure visualized dynamic sand-carrying test unit includes a third intermediate container 31 of CO2 fracturing fluid, a third power pump 32, a second high-pressure phase balancer 33, a pressure chamber 36, and a second temperature control cabinet 37 which are sequentially connected by pipelines; the second high-pressure phase balancer 33 is also sequentially connected by pipelines to a second dosing pump 35 and a proppant storage tank 34, and a visualized high-pressure flat plate crack 361 is arranged in the pressure chamber 36, and a side wall of the visualized high-pressure flat plate crack 361 is a glass window 362, as shown in FIG. Figure 5 As shown, by adjusting the position of the adjustment plate 363, the width d of the visualized high-pressure flat plate crack 361 can also be adjusted according to actual experimental needs. The second high-pressure phase balancer 33 is also provided with a visualization window, which is convenient for real-time observation of the state of the fracturing fluid in the equipment. The outlet of the pressure chamber 36 is connected to the drain line 391 and the circulation recovery line 392, and the circulation recovery line 392 is provided with a filter 38; the third intermediate container 31 of the CO2 fracturing fluid is connected to the first drain line 19 pipeline. In order to facilitate the unified adjustment of the experimental temperature, as a reference, in this scheme, the second high-pressure phase balancer 33, the pressure chamber 36, the second dosing pump 35 and the proppant storage tank 34 are all located in the second temperature control cabinet 37. For specific physical drawings, please see Figure 5 .

[0069] As a reference, a lightweight high-pressure visualized dynamic sand-carrying test unit is used, with a crack model length of 300mm, a height of 100mm, and a crack width of 10mm (different crack widths d can be replaced as needed). The visualized high-pressure flat plate crack 361 is placed in a pressure chamber 36, and the pressure chamber 36 is used to generate a high-temperature and high-pressure environment so that CO2 reaches and maintains a supercritical state. At the same time, since one side of the crack is an open outlet, the pressure inside and outside the crack is connected to the pressure inside the chamber. Therefore, the forces inside and outside the visible glass plate are balanced and are not affected by pressure changes, and can remain intact under high fluid pressure. This design idea breaks through the limitations of the pressure-bearing capacity of conventional crack wall materials, and thus realizes the process of simulating the migration of sand in supercritical CO2 cracks under high-temperature and high-pressure conditions.

[0070] In order to facilitate the recovery of CO2 resources in the fracturing fluid, as a reference, the outlet of the circulating recovery pipeline 392 in this scheme is connected to the supercritical CO2 high-pressure phase equilibrium unit pipeline, so that the CO2 from which the proppant is removed by the filter 38 can re-enter the supercritical CO2 high-pressure phase equilibrium unit and can be reused after being converted into liquid CO2, thereby reducing the experimental cost.

[0071] In some other embodiments, the phase equilibrium CO2 storage tank 17 and the first intermediate container 21 and the third intermediate container 31 of the CO2 fracturing fluid are the same equipment, and the phase equilibrium CO2 storage tank 17 may have a heating and heat preservation function (as a reference, it may be a hot box with a coil center) to ensure that the fracturing fluid maintains a supercritical state. That is, the supercritical CO2 fracturing fluid produced by the supercritical CO2 high-pressure phase equilibrium unit is stored in the phase equilibrium CO2 storage tank 17, and the fracturing fluid is pumped into the subsequent unit for viscosity test or sand carrying test by starting the second power pump 22 or the third power pump 32 respectively.

[0072] According to the method provided by the present invention, a plurality of valves, tees, high-pressure pipelines, pressure gauges, differential pressure gauges and other components may be arranged as required; each inlet or outlet may be cross-connected, and the pipeline closing and opening may be realized through the pipeline and valve arrangement. The present invention has no special requirements for this and will not be elaborated herein.

[0073] This scheme also provides a supercritical CO2 fracturing fluid in-situ rheology and visible fracture sand carrying experimental method, which is based on the above-mentioned supercritical CO2 visible fracture sand carrying experimental system and includes the following steps:

[0074] Step 1: Preparation of supercritical CO2 fracturing fluid: According to experimental requirements, control the CO2 temperature to obtain supercritical CO2 in the temperature range of 5 to 100°C; then according to experimental requirements, add appropriate amounts of additives to the supercritical CO2 and mix well to obtain supercritical CO2 fracturing fluid.

[0075] Specifically, as a reference, the steps include:

[0076] ①Introduce a certain amount of CO2 gas into the high-pressure phase equilibrium module, cool it to 0-5°C, and pressurize it to 4-7MPa to generate liquid CO2.

[0077] ② The generated liquid CO2 is pressurized to above 7.38MPa and the temperature is controlled to above 31.1°C to generate supercritical CO2 in a high-pressure phase equilibrium reactor.

[0078] ③ Weigh or measure the supercritical CO2 fracturing fluid additive according to the formula and pump it into the high-pressure phase equilibrium kettle, such as Figure 2 shown.

[0079] ④ Start stirring, keep the temperature and pressure stable, and observe the dissolution of the additives.

[0080] ⑤ Adjust the type and amount of additives and solvents, repeat steps ③-④, obtain different supercritical CO2 fracturing fluids, and transfer them into the phase equilibrium CO2 storage tank 17 for standby use.

[0081] Step 2: Equipment debugging: Debug the equipment according to industry specifications to ensure its sealing and normal operation;

[0082] Step 3: Capillary viscosity test: Start the second power pump, pump the supercritical CO2 fracturing fluid mixed with proppant into the capillary, and detect the pressure difference and flow rate at both ends of the capillary. Specifically, start the second power pump, pump the supercritical CO2 fracturing fluid in the first intermediate container of CO2 fracturing fluid into the capillary at a certain pumping flow rate, the differential pressure sensor records the pressure difference between the inlet and outlet of the capillary, and the CO2 flowmeter records the flow rate. After the rheological test of the supercritical CO2 fracturing fluid is completed, the supercritical CO2 fracturing fluid is discharged in sequence along the second intermediate container of CO2 fracturing fluid and the second exhaust pipeline.

[0083] After recording the pressure difference ΔP and flow rate Q at both ends of the capillary, the capillary viscosity at the temperature, pressure and shear rate is calculated according to formula (1);

[0084]

[0085] Where: μ——viscosity of liquid / supercritical CO2 fracturing fluid, in millipascals·seconds (mPa·s);

[0086] R——capillary radius, in meters (m);

[0087] ΔP——capillary inlet and outlet pressure difference, in millipascals (mPa);

[0088] Q——flow rate, in cubic meters per second (m3·s-1);

[0089] L——capillary length, in meters (m);

[0090] The conversion relationship between pump flow rate and shear rate:

[0091] For Newtonian fluids, the shear rate of the fluid at the capillary wall is calculated according to formula (2):

[0092]

[0093] Where: Q – fluid volume flow rate;

[0094] For non-Newtonian fluids, the shear rate of the fluid at the capillary wall is calculated according to formula (3):

[0095]

[0096] Where: σ is the shear stress of the fluid at the capillary wall.

[0097] Step 4: Dynamic sand carrying performance test, start the third power pump, pump the supercritical CO2 fracturing fluid mixed with proppant into the second high-pressure phase balancer, add proppant in proportion and pump it into the pressure chamber, observe and record the migration of fracturing fluid carrying sand in the visualized high-pressure flat plate crack.

[0098] Specifically, start the third power pump, pump the supercritical CO2 fracturing fluid mixed with proppant into the second high-pressure phase balancer at a certain flow rate, start the second dosing pump to add proppant according to the formula, observe that the proppant and fracturing fluid in the second high-pressure phase balancer are evenly mixed and then pumped into the pressure chamber, adjust the second temperature control cabinet so that the temperature and pressure in the pressure chamber meet the experimental requirements, observe and record the migration of fracturing fluid sand in the visualized high-pressure flat plate cracks, and complete the test of the dynamic sand carrying performance of the supercritical CO2 fracturing fluid.

[0099] Record the maximum distance that the proppant migrates in the flat plate fracture, and calculate the maximum sand-carrying migration distance ratio according to formula (4):

[0100]

[0101] Where: υ——the maximum sand-carrying migration distance ratio of supercritical CO2 fracturing fluid, expressed as a percentage;

[0102] lmax is the maximum distance that the proppant migrates in the flat plate fracture, in centimeters (cm);

[0103] l0——Length of flat plate seam, in centimeters (cm).

[0104] Step 5: Post-processing of experimental materials: After the experiment, filter the test materials, recover the proppant, and evacuate or recover the CO2.

[0105] Experimental Example 1: Capillary Viscosity Test

[0106] The viscosity of the fracturing fluid was tested using the method in the above scheme, and the experimental group was designed as follows:

[0107] Experimental group 1: The thickener is 1% modified phosphate ester HHCH-14, which dissolves in supercritical CO2 for 2-3 minutes. After adding the thickener, the system becomes milky white and the viscosity increases. The viscosity of the fracturing fluid system under supercritical conditions is 2-3mPa·s (shear rate 170s-1, temperature 60°C). The relationship between the viscosity and time is shown in detail. Figure 6 .

[0108] Experimental group 2: The thickener is 2% ester ether polymer, the dissolution time in supercritical CO2 is 3min, the viscosity of the fracturing fluid system under supercritical state is 18.0mPa.s (shear rate 170s-1, temperature 60℃), the relationship between the viscosity and time is shown in detail. Figure 7 .

[0109] Experimental group 3: The thickener is 1% styrene fluorinated acrylic acid, the dissolution time in supercritical CO2 is 4-5min, the viscosity of the fracturing fluid system under supercritical state is about 11mPa.s (shear rate 100s-1, temperature 60℃), and the thickener addition concentration is used as a variable. The relationship between the viscosity and the amount of thickener added is detailed in Figure 8 .

[0110] Experimental Example 2: High-pressure visual dynamic sand carrying test

[0111] The sand carrying performance of the fracturing fluid was tested using the method in the above scheme, and the experimental group was designed as follows:

[0112] Add 1% styrene fluorinated acrylic thickener and set the flow rate in the seam to 0.75m / s. For detailed experimental results, see Fig. 9 .

[0113] The experimental results show that the maximum sand transport distance of supercritical CO2 fracturing fluid is increased by about 40% compared with that without thickener. Fig. 9 (Left) shows the sand transport without adding thickener. Fig. 9 (Right) shows the sand transport after adding thickener. By comparison, the sand bank transport distance increased from 2 windows to 2.8 windows.

[0114] Specifically, Fig. 9 The migration and placement of proppants in the fractures in the experiment are shown. As can be seen from the figure, after the supercritical CO2 sand-carrying fluid without thickener is injected into the fracture, the proppant quickly accumulates near the entrance to form sand dunes. This is because the density difference between the proppant particles and the supercritical CO2 fluid is large, causing the proppant particles to separate quickly from the sand-carrying fluid. In addition, after the sand-carrying fluid enters the fracture from the injection hole, the flow channel area increases sharply, and the fluid velocity decreases rapidly, resulting in the particles being unable to obtain sufficient lifting force from the fluid and causing gravity sedimentation.

[0115] After adding the thickener, it can be clearly seen that as the dune grows, the dune front begins to be eroded by the injected fluid, and a blank area appears near the injection point. In addition, the dune front obtains a stronger fluid force in the vertical direction and can move deeper into the crack, and the dune laying distance is significantly increased.

[0116] In summary, this scheme controls the in-situ feed, prepares different supercritical CO2 fracturing fluid systems, and tests the high-pressure rheology and high-pressure fracture dynamic sand-carrying performance of supercritical CO2 fracturing fluid in an integrated manner, and provides quantitative index parameters (especially the characterization of the sand-carrying effect by migration distance, which is easy to operate and comparable in results), providing support for additive optimization, system optimization and process parameter specification. The scheme also provides an experimental scheme for the integrated multi-module experimental system and quantitative results of the in-situ rheology and visible fracture sand-carrying of supercritical CO2 fracturing fluid, which is convenient for comprehensive evaluation of fracturing fluid performance and reverse adjustment and optimization of the fracturing fluid formula, in order to achieve the best technical effect.

[0117] The above is only an embodiment of the present invention, and the common knowledge such as the known specific technical solutions and / or characteristics in the solution is not described in detail here. It should be pointed out that for those skilled in the art, several modifications and improvements can be made without departing from the technical solution of the present invention, which should also be regarded as the protection scope of the present invention, and these will not affect the effect of the implementation of the present invention and the practicality of the patent. The scope of protection required by this application shall be based on the content of its claims, and the specific implementation methods and other records in the specification can be used to interpret the content of the claims.

Claims

1. A supercritical CO2 fracturing fluid in-situ rheology and visible fracture sand carrying experimental system, characterized by: It includes a supercritical CO2 high-pressure phase equilibrium unit, a high-pressure capillary viscosity test unit, a high-pressure visual dynamic sand carrying test unit and a data acquisition and analysis unit.

2. A supercritical CO2 fracturing fluid in-situ rheology and visible fracture sand carrying experimental system according to claim 1, characterized in that: The data acquisition and analysis unit is electrically connected to the supercritical CO2 high-pressure phase equilibrium unit, the high-pressure capillary viscosity test unit, and the high-pressure visualized dynamic sand carrying test unit; the data acquisition and analysis unit includes a controller and a data analysis module, the controller controls the working state of the entire system, adjusts working parameters and collects data, and the data analysis module is used to analyze the collected experimental data.

3. A supercritical CO2 fracturing fluid in-situ rheology and visible fracture sand carrying experimental system according to claim 2, characterized in that: The supercritical CO2 high-pressure phase balance unit includes a CO2 gas source, a refrigeration unit, a first power pump, a first high-pressure phase balancer, a phase balance CO2 storage tank, a first temperature control cabinet and a first exhaust pipeline connected in sequence by pipelines. The first high-pressure balancer is also connected to a plurality of dosing components, and the dosing components include a first dosing pump and an additive storage tank that are connected.

4. A supercritical CO2 fracturing fluid in-situ rheology and visible fracture sand carrying experimental system according to claim 3, characterized in that: The additives include thickeners and / or co-solvents.

5. A supercritical CO2 fracturing fluid in-situ rheology and visible fracture sand carrying experimental system according to claim 2, characterized in that: The high-pressure capillary viscosity testing unit includes a first intermediate container for CO2 fracturing fluid, a second power pump, a capillary, a CO2 flowmeter, a second intermediate container for CO2 fracturing fluid and a second drain line which are sequentially connected by pipelines; the capillary is located in a circulating constant temperature water bath, and the inlet and outlet of the capillary are connected to both ends of the differential pressure sensor; the first intermediate container for CO2 fracturing fluid is connected to the first drain line pipeline.

6. A supercritical CO2 fracturing fluid in-situ rheology and visible fracture sand carrying experimental system according to claim 2, characterized in that: The high-pressure visualized dynamic sand-carrying test unit includes a third intermediate container for CO2 fracturing fluid, a third power pump, a second high-pressure phase balancer, a pressure chamber, and a second temperature control cabinet, which are sequentially connected by pipelines; the second high-pressure phase balancer is also sequentially connected by pipelines to a second dosing pump and a proppant storage tank, and a visualized high-pressure flat plate crack is provided in the pressure chamber; the outlet of the pressure chamber is connected to an emptying pipeline and a circulating recovery pipeline, and a filter is provided on the circulating recovery pipeline; the third intermediate container for CO2 fracturing fluid is connected to the first emptying pipeline.

7. A supercritical CO2 fracturing fluid in-situ rheology and visible fracture sand carrying experimental system according to claim 6, characterized in that: The outlet of the circulation recovery pipeline is connected to the supercritical CO2 high-pressure phase balance unit pipeline; one side wall of the visualized high-pressure flat plate crack is a glass window, and the first high-pressure phase balancer and the second high-pressure phase balancer are both provided with visualization windows.

8. A supercritical CO2 fracturing fluid in-situ rheology and visible fracture sand carrying experimental method, based on a supercritical CO2 fracturing fluid in-situ rheology and visible fracture sand carrying experimental system according to any one of claims 1 to 7, characterized in that: The steps include: Step 1: Preparation of supercritical CO2 fracturing fluid; Step 2: Equipment debugging; Step 3: Capillary viscosity test: Start the second power pump to pump the supercritical CO2 fracturing fluid mixed with proppant into the capillary, and detect the pressure difference and flow rate at both ends of the capillary; Step 4: Dynamic sand carrying performance test: start the third power pump, pump the supercritical CO2 fracturing fluid mixed with proppant into the second high-pressure phase balancer, add proppant in proportion and then pump it into the pressure chamber, observe and record the migration of fracturing fluid carrying sand in the visualized high-pressure flat plate crack; Step 5: Post-processing of experimental materials: After the experiment, filter the test materials, recover the proppant, and evacuate or recover the CO2.

9. The in-situ rheology and visible fracture sand carrying experimental method of supercritical CO2 fracturing fluid according to claim 8, characterized in that: In step three, the pressure difference ΔP and flow rate Q at both ends of the capillary are recorded, and the capillary viscosity at the temperature, pressure and shear rate is calculated according to formula (1); Where: μ——viscosity of liquid / supercritical CO2 fracturing fluid, in millipascals·seconds (mPa·s); R——capillary radius, in meters (m); ΔP——capillary inlet and outlet pressure difference, in millipascals (mPa); Q——flow rate, in cubic meters per second (m3·s-1); L——capillary length, in meters (m); The conversion relationship between pump flow rate and shear rate: For Newtonian fluids, the shear rate of the fluid at the capillary wall is calculated according to formula (2): Where: Q – fluid volume flow rate; For non-Newtonian fluids, the shear rate of the fluid at the capillary wall is calculated according to formula (3): Where: σ is the shear stress of the fluid at the capillary wall.

10. The in-situ rheology and visible fracture sand carrying experimental method of supercritical CO2 fracturing fluid according to claim 8, characterized in that: In step 4, the maximum distance that the proppant migrates in the flat plate seam is recorded, and the maximum sand-carrying migration distance ratio is calculated according to formula (4): Where: υ——the maximum sand-carrying migration distance ratio of supercritical CO2 fracturing fluid, expressed as a percentage; lmax is the maximum distance that the proppant migrates in the flat plate fracture, in centimeters (cm); l0——Length of flat plate seam, in centimeters (cm).

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