Visualized experimental apparatus and method for simulating proppant transport in fractures

Abstract

The application discloses a kind of visualized experimental equipment and method for simulating the migration law of proppant in fracture, belong to core splitting test equipment technical field, equipment includes black box and main reaction system and pressure applying mechanism in it, main reaction system includes transparent simulation assembly and light source, two rough surfaces of simulation assembly are formed by 3D printing: calibration board with different height steps is laser ranging in black box, and chart is formulated;The surface of the core after splitting is treated with reflective paint, and the laser is emitted in the black box, the laser data reflected by the rock surface is collected, and the core surface is digitized;Pressure applying mechanism is arranged at the top of simulation assembly, and the fracture is filled with liquid to simulate the fracture after fracturing;The migration process of proppant in the fracture is monitored by image acquisition equipment, and the size, distribution and distribution form of accurate particle size are obtained, and the influence of fracture roughness on proppant movement is analyzed on the basis of visualization.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of core splitting test equipment, and particularly relates to a visual experiment equipment and method for simulating the migration law of proppants in a fracture. BACKGROUND

[0002] In recent years, a series of major breakthroughs have been made in unconventional oil and gas exploration and development. As an important means of unconventional oil and gas development, the main purpose of hydraulic fracturing technology is to effectively improve the flow capacity of underground oil and gas. Hydraulic fracturing is to use a high-pressure pump group on the ground to inject a high-viscosity fracturing fluid into the formation. When the injection speed of the fracturing fluid exceeds the absorption speed of the formation, a high pressure will be formed in the formation. When the pressure exceeds the breakdown pressure of the formation, the formation will be pressed open. Then, a sand-carrying fluid containing proppants is injected into the formation, so that the proppants are left in the fractured cracks to support the cracks, so that the cracks remain open, forming cracks with high conductivity. Therefore, after the hydraulic fracture is formed, proppants are needed to support the cracks to prevent the cracks from closing automatically after the fracturing is completed, resulting in poor fracturing effect.

[0003] Due to the roughness, complex shape and large length of the fracture wall formed by fracturing, the migration of proppants in the fracture is difficult, so the migration state of the proppants in the fracture is a factor affecting the success or failure of fracturing, which largely determines the final oil and gas production effect. Therefore, accurately detecting the placement position and migration law of the proppants in the fracturing crack is crucial for unconventional oil and gas development, and the migration law is worth exploring. At present, the research equipment in this aspect is mostly invisible, and a visual device and simulation method for studying the time distribution law of proppants under precise fracture specifications are needed to study the migration law. SUMMARY

[0004] The purpose of the present application is to provide a visual experiment equipment and method for simulating the migration law of proppants in a fracture, aiming to solve the technical problem that there is no visual device for studying the time distribution and migration law of proppants in a fracture in the prior art.

[0005] To solve the above technical problems, the technical solution adopted by the present application is:

[0006] A visual experiment equipment for simulating the migration law of proppants in a fracture, comprising a black box and a main reaction system and a pressure applying mechanism inside the black box, the main reaction system comprising a transparent simulation component with a fracture inside and a light source at the bottom thereof, the pressure applying mechanism being arranged at the top of the simulation component for applying pressure to the simulation component; the fracture in the simulation component being capable of filling a liquid simulating proppant for simulating the fracture after fracturing of the formation; the fracture in the simulation component being in communication with a conveying component for inputting a liquid at a target temperature into the fracture in the simulation component;

[0007] The laser emitter and the image acquisition device are arranged inside the black box to monitor the migration of the proppant in the fracture.

[0008] Preferably, the simulation assembly comprises two rectangular simulation plates made of transparent material, and the opposite sides of the two simulation plates are rough surfaces for simulating the fracture surface of the core.

[0009] Preferably, the bottom of the simulation assembly is provided with a base, and the base is provided with a light source arranged below the simulation assembly for irradiating the fracture in the simulation assembly.

[0010] Preferably, the top of the base is provided with a rectangular mounting groove matched with the simulation assembly, and the two simulation plates are arranged side by side, and the four peripheral edges of the two simulation plates are sealingly fitted with the inner wall of the mounting groove through a sealing member.

[0011] Preferably, the bottom surface of the mounting groove of the base is provided with a plurality of grooves, and each of the grooves is provided with a bulb for illuminating the fracture in the simulation assembly.

[0012] Preferably, the conveying assembly comprises a liquid storage cylinder, a conveying pump and a liquid storage tank capable of adjusting temperature, the liquid storage tank is communicated with the fracture in the simulation assembly through a liquid inlet pipe and the liquid storage cylinder is communicated with the fracture in the simulation assembly through a liquid outlet pipe, the conveying pump is arranged on the liquid inlet pipe, and the liquid inlet pipe between the conveying pump and the simulation assembly is further provided with an electric control valve, a pressure gauge and a check valve, and the liquid inlet pipe and the liquid outlet pipe are connected with the channel on the sealing member through the side wall of the black box and the side wall of the mounting groove of the base.

[0013] Preferably, the liquid storage tank is provided with a temperature controller for adjusting the temperature of the liquid in the liquid storage tank.

[0014] Preferably, the pressure applying mechanism comprises pressure claws and a pressure applying device at the top of the pressure claws, the pressure claws are arranged in a diffusion manner on a connecting seat, the lower end of the pressure claws can abut against the top of the simulation assembly, and the connecting seat is connected with the pressure applying device.

[0015] Preferably, the base is provided with two support columns, and the two support columns are rotatably connected with the two side surfaces of the base through connecting rods, and the fracture in the simulation assembly can be adjusted to be in a horizontal state or a vertical state by rotating the base.

[0016] Preferably, the base is provided with buckles on the two sides for fixing the relative position of the connecting rods and the base.

[0017] This invention also provides a visualization experimental method for simulating proppant migration within fractures, using the aforementioned experimental equipment and comprising the following steps:

[0018] Inside the black box, a laser rangefinder is used to emit lasers at calibration plates with steps of different heights to determine the laser parameters corresponding to different heights and to create a chart.

[0019] Reflective paint is sprayed onto the rough surface of the simulation board that makes up the simulation component. The board is placed in a black box, and a laser is emitted toward the surface of the split rock core. The laser data reflected back from the surface of the rock core is collected, and the split surface of the rock core is digitized.

[0020] The surface of the rock core fracture surface is replicated by 3D printing technology using a transparent simulation plate to simulate the fracture surface of the rock core.

[0021] The main reaction system and pressure application mechanism are placed inside a black box. A laser emitter inside the black box is used to scan the cracks in the simulated component to monitor the migration process of the simulated proppant liquid in the cracks of the simulated component throughout the entire experiment.

[0022] The beneficial effects of adopting the above technical solution are as follows: Compared with the prior art, the present invention determines the laser parameters corresponding to different heights by emitting lasers to calibration plates at different heights using a laser rangefinder installed in a black box, and then creating a map; the surface of the split rock core is treated with reflective paint, then placed in a black box, and lasers are emitted again to collect the laser data reflected back from the rock surface, thus digitizing the rock core; the surface of the split rock core is replicated using transparent materials and 3D printing technology; the proppant migration process is monitored throughout the experiment using laser scanning technology, obtaining accurate particle size and distribution as well as the final distribution morphology, and the influence of crack roughness on proppant movement is analyzed based on visualization. Attached Figure Description

[0023] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0024] Figure 1 This is a schematic diagram of the structure of a visualization experimental device for simulating the proppant migration law within a crack, provided in an embodiment of the present invention.

[0025] Figure 2 yes Figure 1 Schematic diagram of the main reaction system in the middle;

[0026] Figure 3 yes Figure 2 Cross-sectional view of the main reaction system;

[0027] Figure 4 yes Figure 3 Schematic diagram of the middle base;

[0028] Figure 5 is Figure 3 the structure diagram of the sealing element;

[0029] Figure 6 is Figure 5 the sectional view of the sealing element;

[0030] Figure 7 is Figure 3 the installation diagram of the pressure claw;

[0031] Figure 8 is the structure diagram of the calibration plate in the embodiment of the present application;

[0032] Figure 9 is the front view of the calibration plate in the embodiment of the present application;

[0033] In the figure: 1 - support column, 2 - base, 3 - pressure claw, 4 - simulation assembly, 40 - simulation plate; 5 - sealing element, 6 - liquid storage cylinder, 7 - check valve, 8 - pressure gauge, 9 - electric control valve, 10 - delivery pump, 11 - liquid storage tank; 12 - black box, 13 - calibration plate, 14 - crack, 15 - installation groove, 16 - channel, 17 - groove, 18 - connecting seat, 19 - connecting rod, 20 - through hole, 21 - step. DETAILED DESCRIPTION

[0034] In order to make the technical problems to be solved by the present application, the technical solutions and beneficial effects more clearly understood, the following will be further described in detail in combination with the drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application, and are not used to limit the present application. In the following detailed description of the present application, some specific details are described in detail. However, those skilled in the art can also fully understand the present application without the detailed description of the part.

[0035] In addition, those skilled in the art should understand that the provided drawings are only for the purpose of illustrating the purpose, features and advantages of the present application, and the drawings are not actually drawn according to the proportion.

[0036] At the same time, unless the context clearly requires, the "includes", "contains" and similar words in the entire specification and claims should be interpreted as containing meaning rather than exclusive or exhaustive meaning; that is, "including but not limited to" meaning.

[0037] Referring to Figure 1The application provides a visual experiment device for simulating proppant migration law in a fracture, which comprises a black box 12 and a main reaction system and a pressure applying mechanism in the black box 12, the main reaction system comprises a transparent simulation assembly 4 provided with a fracture 14 and a light source at the bottom of the simulation assembly 4, and the pressure applying mechanism is arranged at the top of the simulation assembly 4 and used for applying pressure to the simulation assembly 4; the fracture in the simulation assembly 4 can be filled with liquid simulation proppant and is used for simulating a fracture after formation fracturing; and the fracture of the simulation assembly is communicated with a conveying assembly and used for inputting liquid at a target temperature into the fracture of the simulation assembly.

[0038] The device further comprises a laser emitter and an image acquisition device arranged at the top of the black box 12, the laser emitter is used for scanning the fracture of the simulation assembly, and the image acquisition device is used for monitoring the migration process of the proppant in the fracture. During the experiment, the laser emitter can be combined with a laser particle size analyzer to obtain the accurate particle size and distribution of the proppant and the final distribution form. The scheme can solve the problem that the current research on the proppant migration distribution law mainly uses the motion characteristics of the proppant in the fracture (between two smooth glass plates under extrusion) and cannot analyze the influence of the roughness of the fracture on the motion of the proppant.

[0039] As a preferred structure, as shown in Figures 1-3 The simulation assembly 4 comprises two rectangular simulation plates 40 made of transparent material, and the opposite sides of the two simulation plates 40 are rough surfaces and are used for simulating a core split surface; the rough surfaces of the simulation plates 40 are engraved on the surfaces of the transparent material by using a 3D printing technology, and the data of the rough surfaces of the simulation plates 40 are obtained by using a laser range finder and a calibration plate 13.

[0040] The laser range finder is used for emitting laser to the calibration plate 13 with different height steps in the black box 12, the laser parameters corresponding to different heights are determined, and a graph is drawn; the split core surface is treated by spraying reflective paint, and then is placed in the black box 12, and then laser is emitted again, the laser data reflected by the rock surface is collected, and the core surface is digitized. The simulation plate simulating the core split surface is prepared by using the 3D printing technology, the geometric shape of the fracture is accurately described by using a rock splitting experiment or a triaxial fracturing experiment, the fracture is photographed and digitized, and the artificial simulation core split surface with the precise concave-convex shape of the fracture is prepared by using different gray scales, 3D printing and three-dimensional reconstruction. The scheme facilitates the analysis of the influence of the roughness of the fracture on the motion of the proppant on the basis of visualization.

[0041] In one specific embodiment of the application, as Figures 2-6As shown in the figure, the bottom of the simulation assembly 4 is provided with a base 2, the base 2 is provided with a light source, and the light source is arranged below the simulation assembly 4 for irradiating the crack 14 in the simulation assembly 4. Wherein, the top of the base 2 is provided with a rectangular mounting groove 15 matched with the simulation assembly 4; two simulation boards 40 are arranged side by side, and the four peripheral edges of the two simulation boards 40 are sealingly matched with the inner wall of the mounting groove 15 through the sealing element 5; as shown in Figure 5 、 6 the figure, the sealing element 5 is provided with a channel 16 communicated with the conveying assembly, and the sealing element 5 is provided with an extension part extending between the upper and lower simulation boards, the channel 16 horizontally penetrates the extension part, and the upper and lower simulation boards are pressed on the upper and lower extension parts to realize the sealing of the crack; meanwhile, the side wall of the mounting groove 15 of the base 2 is provided with a through hole 20 communicated with the channel 16 and the conveying assembly. The structure realizes the sealing of the crack in the simulation assembly, and avoids leakage during the conveying of the liquid.

[0042] As shown in the figure, Figure 4 the bottom surface of the mounting groove 15 of the base 2 is provided with a plurality of grooves 17, and the simulation assembly 4 is arranged on the top of the grooves 17 and in the inside of the mounting groove 15; a plurality of bulbs are arranged in the grooves 17 for illuminating the crack 14 in the simulation assembly 4. The structure provides illumination for the crack of the simulation assembly, and through the laser scanning technology, the distribution law of the proppant under the precise crack specification with time can be directly observed.

[0043] In one embodiment of the present application, as shown in the figure, Figure 1 the conveying assembly comprises a liquid storage cylinder 6, a conveying pump 10 and a liquid storage tank 11 capable of adjusting temperature, the liquid storage tank 11 is communicated with the crack 14 in the simulation assembly 4 through a liquid inlet pipe, the liquid storage cylinder 6 is communicated with the crack 14 in the simulation assembly 4 through a liquid outlet pipe, the conveying pump 10 is arranged on the liquid inlet pipe, and the liquid inlet pipe between the conveying pump 10 and the simulation assembly 4 is further provided with an electric control valve 9, a pressure gauge 8 and a check valve 7; the liquid inlet pipe and the liquid outlet pipe are respectively connected with the channel 16 on the sealing element 5 through the side wall of the black box 12 and the side wall of the mounting groove 15 of the base 2. Wherein, the liquid storage tank 11 is provided with a temperature controller for adjusting the temperature of the liquid in the liquid storage tank 11. The conveying pump is used to convey the liquid reaching the target temperature into the crack of the simulation assembly.

[0044] In one embodiment of the present application, as shown in the figure, Figure 1 、 2As shown in FIGS. 7, the pressure applying mechanism includes pressure claws 3 and pressure applying devices on the top of the pressure claws, the pressure claws are 3, the plurality of pressure claws 3 are arranged in a diffusion manner on a connecting seat 18, and the lower end of the pressure claw 3 can abut against the top of the simulation assembly 4, and the connecting seat 18 is connected with the pressure applying device; the image acquisition device is arranged on the bottom of the connecting seat 18. In this embodiment, the pressure claws 3 are 4 and are divided into two groups, and are symmetrically arranged at the two ends of the simulation plate 40. The structure is used to apply pressure to the simulation assembly 4, so that a high-pressure environment is formed, and different production pressures are simulated.

[0045] Further optimization of the above technical solutions, such as Figure 1 、 2 As shown in FIGS. 7, the pressure applying mechanism includes pressure claws 3 and pressure applying devices on the top of the pressure claws, the pressure claws are 3, the plurality of pressure claws 3 are arranged in a diffusion manner on a connecting seat 18, and the lower end of the pressure claw 3 can abut against the top of the simulation assembly 4, and the connecting seat 18 is connected with the pressure applying device; the image acquisition device is arranged on the bottom of the connecting seat 18. In this embodiment, the pressure claws 3 are 4 and are divided into two groups, and are symmetrically arranged at the two ends of the simulation plate 40. The structure is used to apply pressure to the simulation assembly 4, so that a high-pressure environment is formed, and different production pressures are simulated.

[0046] In specific manufacturing, buckles (not shown in the figure) are arranged on the two sides of the base 2, which are used to fix the relative position of the connecting rod 19 and the base 2. The buckles can be installed on the side wall of the base or the connecting rod, the base is rotated by opening the buckle, and the buckle is used to fix the connecting rod after the angle of the base is adjusted to the right position, so that the position of the base is prevented from changing during the experiment.

[0047] The application also provides a visual experiment method for simulating the migration rule of proppants in a fracture, which uses the above experiment device to perform experiments, and includes the following steps.

[0048] In the black box 12, a laser range finder installed on the top of the black box is used to emit laser to the calibration plate 13 (as shown in FIGS. 7) with different height steps 21, to determine the laser parameters corresponding to different heights, and to make a graph. Figure 8 、 9 The laser range finder and the calibration plate are used to determine the return laser parameters corresponding to different heights, so that the graph is easily made.

[0049] The rough surface of the simulation plate 40 constituting the simulation assembly is sprayed with reflective paint, and is placed in the black box 12, the laser range finder is started to emit laser to the surface of the fractured core, and the laser data reflected by the surface of the core is collected, so that the fracture surface of the core is digitized.

[0050] The surface of the simulation plate 40 of transparent material is engraved by using 3D printing technology, and the convex and concave degree of the fracture surface of the core is copied, which is used to simulate the fracture surface of the core; after the two simulation plates are combined, the precise fracture of the simulation core fracture surface can be formed between the two rough surfaces opposite to each other.

[0051] The main reaction system and pressure application mechanism were placed inside a black box. A laser emitter mounted on the top of the black box was used to scan the simulated components below, monitoring the migration of the simulated proppant liquid within the cracks of the simulated components throughout the experiment.

[0052] A black box was used to ensure that the light source conditions were unique during the experiment, and the proppant migration in the cracks of the simulated components was observed.

[0053] In practical operation, inside the black box, the formula for calculating the distance from the laser of the laser rangefinder to the crack measurement surface is as follows:

[0054] D = ct / 2

[0055] In the formula: D -- the distance between the object being measured and the photoelectric element on the top of the black box;

[0056] c -- laser propagation speed;

[0057] t -- The time required for the laser to travel back and forth between the crack being measured and the laser rangefinder on top of the black box once.

[0058] In summary, this invention offers advantages such as simple structure, easy operation, low manufacturing cost, high repeatability, stable operation, and low energy consumption. It not only visualizes the entire process but also allows for analysis of the impact of fracture roughness on proppant movement. Utilizing 3D printing technology to create transparent simulation plates, the surface roughness of split rock cores can be replicated, enabling visualization and analysis of the influence of fracture roughness on proppant movement. Laser scanning of the simulated component's fractures allows for direct observation of the proppant's distribution and movement over time under precise fracture specifications, clearly showing the formation process and final morphology of the proppant-formed sand bed. By creating artificial core splitting surfaces with precise fracture morphology, the impact of fracture roughness on proppant movement can be analyzed, addressing the lack of visibility of proppant migration and the inaccuracy of artificial core surface morphology. This invention is comprehensive, capable of simulating proppant migration and placement patterns under various experimental conditions to further analyze problems encountered during construction and optimize subsequent proppant migration design. Therefore, accurately detecting the placement and migration patterns of proppant within hydraulic fractures is crucial for unconventional oil and gas development, and its migration patterns warrant further investigation.

[0059] Many specific details have been set forth in the foregoing description in order to provide a full understanding of the present invention. However, the present invention may also be implemented in other ways different from those described herein. Those skilled in the art can make similar extensions without departing from the spirit of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed above.

Claims

1. A visualization experimental device for simulating proppant transport within fractures, characterized in that: The black box comprises a main body reaction system and a pressure applying mechanism, the main body reaction system comprises a transparent simulation assembly with a crack and a light source at the bottom of the simulation assembly, and the pressure applying mechanism is arranged at the top of the simulation assembly and is used for applying pressure to the simulation assembly; the crack of the simulation assembly can be filled with a liquid simulating proppant to simulate a fracture after formation fracturing; the crack of the simulation assembly is communicated with a delivery assembly and is used for inputting a liquid at a target temperature into the crack of the simulation assembly; The black box further comprises a laser emitter and an image acquisition device arranged in the black box, which are used for monitoring the migration process of the proppant in the crack. The pressure applying mechanism comprises pressure claws and a pressure applying device at the top of the pressure claws, the pressure claws are arranged in a diffusion mode on a connecting seat, the lower ends of the pressure claws are capable of abutting against the top of the simulation assembly, and the connecting seat is connected with the pressure applying device; the image acquisition device is arranged at the bottom of the connecting seat. The simulation assembly comprises two rectangular simulation plates made of transparent materials, and the opposite sides of the two simulation plates are rough surfaces used for simulating a core splitting surface; the rough surfaces of the simulation plates are engraved on the surfaces of the transparent materials by using a 3D printing technology; the data of the rough surfaces of the simulation plates are obtained by using a laser range finder and a calibration plate. In the black box, the laser range finder is used to emit laser to the calibration plate with different height steps to determine the laser parameters corresponding to different heights and to make a graph; the surface of the split core is treated by spraying reflective paint, and then is placed in the black box, and laser is emitted again to collect the laser data reflected by the surface of the rock, so that the surface of the core is digitized; the simulation plate simulating the core splitting surface is made by using the 3D printing technology, the geometry of the crack is accurately described by a rock splitting experiment or a triaxial fracturing experiment, the crack is photographed, digitized, 3D printed and reconstructed, and the artificial simulation core splitting surface with the concave-convex morphology of the crack is made.

2. The visualized experimental apparatus for simulating proppant transport in fractures of claim 1, wherein: The bottom of the simulation assembly is provided with a base, the base is provided with a light source, and the light source is arranged below the simulation assembly and is used for irradiating the crack in the simulation assembly; a plurality of grooves are arranged on the bottom surface of the mounting groove of the base, and a bulb is arranged in each groove.

3. The visualized experimental apparatus for simulating proppant transport in fractures of claim 2, wherein: The top of the base is provided with a rectangular mounting groove matched with the simulation assembly; the two simulation plates are arranged in parallel, and the periphery of the two simulation plates is sealingly connected with the inner wall of the mounting groove through a sealing member; the sealing member is provided with a channel communicated with the delivery assembly.

4. The visualized experimental apparatus for simulating proppant transport in fractures of claim 1, wherein: The delivery assembly comprises a liquid storage cylinder, a delivery pump and a liquid storage tank capable of adjusting temperature, the liquid storage tank is communicated with the crack in the simulation assembly through a liquid inlet pipe and the liquid storage cylinder is communicated with the crack in the simulation assembly through a liquid outlet pipe, the delivery pump is arranged on the liquid inlet pipe, and an electric control valve, a pressure gauge and a check valve are further arranged on the liquid inlet pipe between the delivery pump and the simulation assembly; the liquid inlet pipe and the liquid outlet pipe are connected with the channel on the sealing member through the side wall of the black box and the side wall of the mounting groove of the base.

5. The visualized experimental apparatus for simulating proppant transport in fractures of claim 4, wherein: A temperature controller is arranged in the liquid storage tank and is used for adjusting the temperature of the liquid in the liquid storage tank.

6. The visualized experimental apparatus for simulating proppant transport in fractures of claim 2, wherein: Supporting columns are arranged on the two sides of the base, the two supporting columns are rotatably connected with the two side surfaces of the base through connecting rods, and the crack in the simulation assembly is adjusted to be in a horizontal state or a vertical state by rotating the base.

7. The visualized experimental apparatus for simulating proppant transport in fractures of claim 2, wherein: The base is provided with buckles on both sides for fixing the relative position of the connecting rod and the base.

8. A visualization experimental method for simulating proppant migration within fractures, comprising using the visualization experimental equipment for simulating proppant migration within fractures as described in any one of claims 1-7, characterized in that: comprising the steps of In the black box, the laser range finder is used to emit laser to the calibration plate with different height steps, to determine the corresponding laser parameters of different heights, and to make a graph; Spray reflective paint on the rough surface of the simulation plate which constitutes the simulation assembly, place it in the black box, emit laser to the surface of the split core, collect the laser data reflected by the core surface, and digitize the core split surface; Carve the convex and concave degree of the core split surface on the surface of the simulation plate with transparent material by using 3D printing technology, for simulating the core split surface; Place the main reaction system and the pressure applying mechanism in the black box, and scan the crack in the simulation assembly by the laser emitter in the black box, to monitor the migration process of the liquid simulation proppant in the crack in the simulation assembly during the whole experiment process; In the black box, the distance from the laser of the laser range finder to the crack measurement surface is calculated according to the following formula: D=ct / 2 In the formula: D is the distance between the measured object and the two points of the photoelectric element at the top of the black box; c is the laser propagation speed; t is the time required for the laser to go back and forth between the measured crack and the laser range finder at the top of the black box once. ​ ​

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