A proppant in-situ flowback physical modeling method and system

Abstract

The present application belongs to the technical field of oil and gas field development, and discloses a proppant in-fracture flowback physical simulation method and system, wherein the simulation method comprises the following steps: determining the experimental fluid discharge in the input simulation fracture device according to the actual discharge of the fracturing site fluid; obtaining a first image set of the fracturing fluid and the experimental proppant migration in the simulation fracturing process; after the proppant filling layer in the fracture is stabilized, obtaining a second image set of the fracturing fluid and the experimental proppant migration in the flowback process; and quantitatively describing the proppant flowback according to the first image set and the second image set. The present application simulates the migration and settlement of the proppant in the fracture during the fracturing construction process, and the start and migration of the proppant during the flowback, realizes the observation and fine description of the proppant flowback state and motion law, quantitatively describes the proppant flowback process through multi-dimensional characterization parameters, and reveals the proppant flowback motion law.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of oil and gas field development, and particularly relates to a proppant in-fracture flowback physical simulation method and system. BACKGROUND

[0002] Hydraulic fracturing is an important technology for oil and gas field development, and is an important means for increasing production and transforming oil and gas reservoirs. Flowback after fracturing is an important link in hydraulic fracturing construction, and has an important influence on the overall fracturing effect. According to the current field flowback of tight gas wells, the phenomenon of proppant flowback is common. According to incomplete statistics, the amount of sand production after fracturing in the JQ block tight gas horizontal well is 5-239 t, and the sand production rate is 2.3%-6.9%. Proppant flowback can produce fracture choke skin effect (Romero DJ. Optimization of the Productivity Index and the Fracture Geometry of a Stimulated Well With Fracture Face and Choke Skins [J]. SPE Production & Facilities, 2003, 18(01): 57-64.), forming a fracture damage zone in the near-wellbore region, reducing the effective propped fracture area and the propped fracture conductivity, and thus affecting the gas well productivity. Related studies have shown that the average gas production decline rate of sand-producing gas wells is more than 3 times that of normal production wells. In addition, proppant flowback can bury the gas layer at the bottom of the well, erode the surface pipeline, cause sand sticking of downhole tools, block valves, and other problems, and bring safety hazards to construction and production. Therefore, the research on proppant flowback is of great significance to the development and production of gas wells.

[0003] The existing proppant flowback physical simulation experiment methods mainly include the following:

[0004] (1) Circular pipe-perforation model: The experimental device mainly consists of fluid inlet and outlet, sand collector and proppant filling layer. The experimental method is to first fill the circular pipe with proppant, then saturate it with sand-carrying fluid, and continuously increase the fluid pumping rate until proppant flows out with the sand-carrying fluid in the circular pipe. Install a sand collector at the outlet end, and stop the experiment when proppant is found in the sand collector. The characterization parameter of proppant flowback is the fluid flow rate corresponding to the appearance of proppant in the sand collector, which is defined as the proppant flowback critical flow rate (Mark P. Understanding Proppant Flow-back [C]. SPE Annual Technical Conference and Exhibition. 1999.).

[0005] (2) Slot model: This experimental device is mainly composed of fluid inlet and outlet, proppant filling slot and sand collector. The experimental method is to fill proppant in the middle of the slot first, then adjust the slot width to simulate the fracture morphology on site while applying closure pressure to the proppant packing layer. Then pump in the sand-carrying fluid through the three-cylinder pump and increase the liquid displacement in stages until the proppant appears in the sand collector. Its characterization parameter for proppant flowback is the fluid flow rate when proppant appears in the sand collector, defined as the proppant flowback critical flow rate (Goel N. Experimental Investigation of Proppant Flow-back Phenomena Using a Large Scale Fracturing Simulator [C]. SPE Annual Technical Conference and Exhibition. 1999.).

[0006] (3) API linear flow core holder: This experimental device is mainly composed of a hydraulic device, a flowmeter, a flow guide chamber, a hydraulic machine and the like. The experimental method is to fill the proppant in the flow guide chamber first, apply closure pressure to the proppant packing layer through the hydraulic machine, saturate the proppant packing layer with sand-carrying fluid through the hydraulic device, then increase the sand-carrying fluid flow rate in stages until sand is observed to flow out of the flow guide chamber. Its characterization parameter for proppant flowback is the fluid flow rate when sand flows out of the flow guide chamber, defined as the proppant flowback critical flow rate (Canon J. Avoiding Proppant Flow-back in Tight-Gas Completions with Improved Fracture Design [C]. SPE Annual Technical Conference and Exhibition. 2003.).

[0007] (4) Large-Scale Flow-back Apparatus: The experimental device is mainly composed of a flow guide chamber (the working area size is 13.34 cm x 13.34 cm, ) a hydraulic pump with a heating system, a sensor group (thermocouple, flow meter and pressure gauge), data acquisition system and computer, etc. The experimental method is to fill the proppant into the flow guide chamber and uniformly lay it, apply a closed pressure to the set value by the hydraulic machine, then saturate the proppant packing layer with preheated sand-carrying liquid, and after stabilization, increase the sand-carrying liquid flow rate in steps to simulate the flow-back process until the proppant flows out of the flow guide chamber. The characterization parameter of proppant flow-back is the fluid flow rate corresponding to the proppant flow-back critical flow rate when the proppant flows out of the flow guide chamber during the experiment (Dimitry C. Proppant Flow-back: Can We Mitigate the Risk? [C]. SPE Hydraulic Fracturing Technology Conference and Exhibition held in The Woodlands, Texas, USA, 2020.).

[0008] In summary, the existing proppant flow-back physical simulation experiment method is to fill the proppant into the flow guide chamber to form a stable and continuous proppant packing layer, ignoring the influence of the sand laying process and the sand dike shape on the proppant flow-back. The sand dike shape formed in the actual fracturing process has a major impact on the fluidization area of the proppant packing layer. Secondly, the existing experimental method cannot observe the proppant flow-back during the flow-back process, and cannot finely describe the proppant flow-back state and law. Thirdly, the existing experimental scale is quite different from the actual fracture geometry size, and the reflected proppant flow-back characteristics have certain differences from the actual field. In addition, the proppant flow-back characterization parameter under the existing experimental method is only the liquid flow rate corresponding to the proppant flow-back critical flow rate when the proppant flows out of the flow guide chamber, which is too simple and single, and cannot finely describe the proppant motion state (proppant start, roll and settlement) during the flow-back process and various parameters (fluid velocity, particle velocity and velocity gradient) in the motion process. SUMMARY

[0009] In view of the above problems, the present application provides a proppant in-slit flow-back physical simulation method and system, which can realize quantitative description of proppant in-slit migration during the whole process of fracturing-flow-back under large-scale (4 x 0.3 m) conditions, and reveal the proppant flow-back motion law through multi-dimensional characterization parameters.

[0010] A proppant in-situ flowback physical simulation method, comprising the following steps: determining an experimental fluid discharge in an input simulation fracture device according to an actual discharge of a fracturing site fluid; wherein the experimental fluid discharge comprises a sand-carrying fluid discharge simulating a fracturing process and a gel-breaking fluid discharge simulating a flowback process; delivering the sand-carrying fluid to the simulation fracture device according to the determined discharge, simulating the flow of the fracturing fluid and the proppant in the fracture in the fracturing process, and obtaining a first image set of the fracturing fluid and the experimental proppant migration in the simulation fracturing process; after the proppant filling layer in the fracture is stabilized, injecting the gel-breaking fluid into the simulation fracture device in a reverse direction according to the determined discharge to flush the proppant filling layer, simulating the migration of the fracturing fluid and the experimental proppant in the flowback process, and obtaining a second image set of the fracturing fluid and the experimental proppant migration in the flowback process; and quantitatively describing the proppant flowback according to the first image set and the second image set.

[0011] Further, determining the experimental fluid discharge in the input simulation fracture device according to the actual discharge of the fracturing site fluid comprises the following steps:

[0012] Based on the actual fracture size of the fracturing site and the simulation fracture size in the simulation fracture device, the actual discharge of the site fluid is converted into the experimental fluid discharge through the Reynolds similarity principle.

[0013] Based on the corresponding table of the actual discharge of the fracturing site fluid and the experimental fluid discharge, the experimental fluid discharge is determined according to the actual discharge of the fracturing site fluid.

[0014] Further, the method further comprises the following steps: mixing the tracer particles with the experimental proppant and the fracturing fluid to obtain the sand-carrying fluid.

[0015] Further, mixing the tracer particles with the experimental proppant and the fracturing fluid to obtain the sand-carrying fluid comprises the following steps:

[0016] delivering the fracturing fluid to a sand mixing tank, and simultaneously adding the tracer particles and the experimental proppant into the sand mixing tank at a certain rate to fully mix them with the fracturing fluid to obtain the sand-carrying fluid; or

[0017] delivering the fracturing fluid to a sand mixing tank, and simultaneously adding the tracer particles, the experimental proppant and the fiber into the sand mixing tank at a certain rate to fully mix them with the fracturing fluid to obtain the sand-carrying fluid.

[0018] Further, the preparation of the tracer particles comprises the following steps:

[0019] cleaning and drying the proppant particles, then stirring and dyeing the proppant particles with fluorescent reflective paint, and performing grinding, dispersing and screening;

[0020] Then, the dyed proppant particles are dried under a set temperature condition to obtain the tracer particles.

[0021] Further, the first image set includes pictures of the fracturing fluid flow field in the fracture during the fracturing simulation, pictures of the experimental proppant particle movement, and pictures of the sand bank shape during the experimental proppant settlement in the fracture;

[0022] The second image set includes pictures of the fracturing fluid flow field in the fracture during the flowback simulation, pictures of the experimental proppant particle movement, and pictures of the sand bank shape.

[0023] Further, the pictures of the fracturing fluid flow field in the fracture during the fracturing simulation or the flowback simulation are obtained by the following steps:

[0024] A laser sheet light source is arranged on one side of the simulated fracture device, the laser sheet light source is parallel to the sand-carrying fluid flow direction in the fracture in the simulated fracture device, and the laser sheet light source is injected into the fracture from the organic glass strip at the top of the simulated fracture device;

[0025] The fracturing fluid flow field pictures at the interval time Δt are obtained by the CCD camera, and N fracturing fluid flow field pictures are obtained, or the flowback fluid flow field pictures at the interval time Δt are obtained by the CCD camera, and N flowback fluid flow field pictures are obtained.

[0026] Further, the pictures of the experimental proppant particle movement in the fracture during the fracturing simulation or the flowback simulation are obtained by the following steps:

[0027] An illumination light source is arranged on one side of the simulated fracture device, wherein the illumination light source irradiates the simulated fracture device from top to bottom;

[0028] The experimental proppant particle movement pictures at the interval time Δt during the fracturing process are obtained by the CCD camera on the other side of the simulated fracture device, and M fracturing experimental proppant particle movement pictures are obtained, or the experimental proppant particle movement pictures at the interval time Δt during the flowback process are obtained by the CCD camera on the other side of the simulated fracture device, and M flowback experimental proppant particle movement pictures are obtained.

[0029] Further, the pictures of the sand bank shape during the experimental proppant settlement in the simulated fracture are obtained by the following steps:

[0030] After the pump is stopped, the proppant settlement process in the simulated fracture is simulated, and the changes in the sand bank shape around the simulated fracture device are recorded by four CCD cameras respectively, and a plurality of fracturing sand bank shape pictures are obtained.

[0031] Further, the pictures of the sand bank shape during the flowback simulation are obtained by the following steps:

[0032] During the flowback simulation, the changes in the sand bank shape around the simulated fracture device are recorded by four CCD cameras respectively, and a plurality of flowback sand bank shape pictures are obtained.

[0033] Further, the proppant flowback is quantitatively described according to the first image set and the second image set, including the following steps:

[0034] The first image set and the second image set are subjected to image processing to obtain a fracturing fluid velocity field and a proppant velocity field;

[0035] The sand bank change process is quantitatively described and the sand bank profile is determined according to the first image set and the second image set, and the proppant flowback capacity under different working conditions is represented by the sand bank profile;

[0036] Based on the fracturing fluid flow field, the angular velocity and the number of tracer particles are extracted through the position information and the time difference of the tracer particles, and the velocity change of the fracturing fluid in the flowback process is represented; based on the proppant velocity field, the angular velocity, the velocity and the number of experimental proppant particles are extracted through the position information and the time difference of the experimental proppant particles, and the velocity change of the proppant particles in the flowback process is represented;

[0037] According to the fracturing fluid flow field and the proppant velocity field, the velocity and the velocity gradient of the fracturing fluid and the experimental proppant particles in the longitudinal direction are calculated, and the shear force of the flowback fluid on the sand bank surface is represented.

[0038] Further, the following steps are included:

[0039] The exposure value and the pixel exposure area size of each pixel point in the N fracturing fluid flow field pictures, the N flowback fluid flow field pictures, the M fracturing experimental proppant particle motion pictures and the M flowback experimental proppant particle motion pictures are analyzed and calculated, and the experimental proppant particle pixel points and the tracer particle pixel points are distinguished;

[0040] The tracer particle pixel point coordinates in the N fracturing fluid flow field pictures and the N flowback fluid flow field pictures are calculated by the PIV algorithm to obtain a fracturing fluid velocity field;

[0041] The proppant pixel point coordinates in the M fracturing experimental proppant particle motion pictures and the M flowback experimental proppant particle motion pictures are calculated by the PTV algorithm to obtain a proppant velocity field.

[0042] Further, the mass of the tracer particle is not greater than 5% of the mass of the sand-carrying fluid and not greater than 10% of the mass of the experimental proppant; the mass of the fiber is 0.4% of the mass of the experimental proppant.

[0043] The application also provides a proppant in-slit flowback physical simulation system, which comprises:

[0044] The calculation module is used for determining the experimental fluid discharge in the input simulation fracture device according to the actual discharge of the fracturing site fluid; wherein the experimental fluid discharge includes the sand-carrying fluid discharge in the simulation fracturing process and the gel-breaking fluid discharge in the flowback process;

[0045] a second liquid supply pump configured to deliver the sand-carrying liquid into the simulated fracture device at a determined displacement;

[0046] a simulated fracture device configured to simulate the flow of the fracturing fluid and the experimental proppant in the fracture during the fracturing process;

[0047] a CCD camera configured to acquire a first image set of the flow of the fracturing fluid and the experimental proppant during the simulated fracturing process;

[0048] a first liquid supply pump configured to inject a gel-breaking liquid into the simulated fracture device in a reverse direction at a determined displacement after the proppant filling layer in the fracture is stabilized, so as to flush the proppant filling layer;

[0049] the simulated fracture device is further configured to simulate the flow of the fracturing fluid and the experimental proppant during the flowback process;

[0050] the CCD camera is further configured to acquire a second image set of the flow of the fracturing fluid and the experimental proppant during the flowback process;

[0051] a computer configured to quantitatively describe the proppant flowback according to the first image set and the second image set.

[0052] The present application has the following beneficial effects:

[0053] The present application simulates the flow and settlement of the proppant in the fracture during the fracturing process and the flow and movement of the proppant during the flowback process by means of the fracture experiment device, so as to observe and finely depict the flowback state and movement law of the proppant under the condition of considering the sandbank shape and the proppant laying law, quantitatively describe the proppant flowback process through multi-dimensional characterization parameters, and reveal the proppant flowback movement law.

[0054] Other features and advantages of the present application will be set forth in the following description, and in part will become apparent to those skilled in the art from the description, or can be learned by practice of the present application. The objects and other advantages of the present application will be realized and attained by the structure particularly pointed out in the written description and claims hereof. BRIEF DESCRIPTION OF DRAWINGS

[0055] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiments or the prior art description. Obviously, the drawings in the following description are some embodiments of the present application, and for those skilled in the art, other drawings can also be obtained from these drawings without any creative effort.

[0056] Figure 1 a flowchart of a proppant flowback physical simulation method according to an embodiment of the present application is shown;

[0057] a flowchart of a proppant flowback physical simulation method according to an embodiment of the present application is shown;Figure 2 A picture of 70 / 140 quartz sand selected for fracturing site according to an embodiment of the present application is shown;

[0058] Figure 3 A picture of 40 / 70 coated sand selected for fracturing site according to an embodiment of the present application is shown;

[0059] Figure 4 A structural schematic diagram of a circulating pump injection system according to an embodiment of the present application is shown;

[0060] Figure 5 A schematic diagram of a picture capture position of a simulated fracture device and fracturing flowback process simulation experiment according to an embodiment of the present application is shown;

[0061] Figure 6 A flowchart of obtaining fracturing fluid velocity field and proppant velocity field by calculating pictures through PIV algorithm and PTV algorithm according to an embodiment of the present application is shown;

[0062] Figure 7 A schematic diagram of sand bank placement profile before and after flowback according to an embodiment of the present application is shown;

[0063] Figure 8 A graph of height variation trend of sand bank at different positions before and after flowback according to an embodiment of the present application is shown;

[0064] Figure 9 A sand bank local form according to an embodiment of the present application is shown, and a graph of sand bank height change with time recorded by CCD picture is shown;

[0065] Figure 10 A schematic diagram of fracturing fluid liquid flow field according to an embodiment of the present application is shown;

[0066] Figure 11 A schematic diagram of proppant particle field for experiment according to an embodiment of the present application is shown;

[0067] Figure 12 A schematic diagram of flow field along the longitudinal direction of the fracture wall surface according to an embodiment of the present application is shown;

[0068] Figure 13 A schematic diagram of longitudinal velocity distribution of fluid along the longitudinal direction of the fracture wall surface according to an embodiment of the present application is shown;

[0069] Figure 14 A schematic diagram of velocity gradient of fluid along the longitudinal direction of the fracture wall surface according to an embodiment of the present application is shown.

[0070] In the figure: 1, first liquid preparation tank; 2, second liquid preparation tank; 3, sand mixing tank; 4, sand adjusting and adding screw; 5, first liquid supply pump; 6, second liquid supply pump; 7, CCD camera. DETAILED DESCRIPTION

[0071] In order to make the purposes, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are some but not all of the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative efforts should fall into the scope of the present application.

[0072] It should be noted that the terms "first", "second" and the like in the present application are used to distinguish similar objects, and do not necessarily indicate a specific order or a chronological sequence.

[0073] The embodiments of the present application provide a proppant in-fracture flowback physical simulation method and system. With the aid of a flow field global measurable proppant migration visualization complex fracture experimental device, the migration and settlement of proppants in fractures during the fracturing construction process and the start and migration of proppants during flowback are simulated. Through a quantitative laser test module combined with an ion separation algorithm, a cross-correlation principle PIV (Particle Image Velocimetry) algorithm and a PTV (Particle Tracking Velocimetry) algorithm, the proppant flowback state and motion law can be observed and finely described under the condition of considering the sand dike shape and proppant placement law. The proppant flowback process is quantitatively described through multi-dimensional characterization parameters (sand dike placement morphology, sand dike local morphology, fracturing fluid flow field and proppant particle field, fluid velocity and velocity gradient in the longitudinal direction), and the proppant flowback motion law is revealed.

[0074] As shown in Figure 1 A proppant in-fracture flowback physical simulation method, comprising the following steps:

[0075] S1, determining an experimental fluid discharge in a simulation fracture device according to an actual fluid discharge in a fracturing site, wherein the determined experimental fluid discharge includes a sand-carrying fluid discharge in a simulation fracturing process and a gel-breaking fluid discharge in a flowback process, and specifically as follows:

[0076] S11, converting the actual fluid discharge in the fracturing site into the experimental fluid discharge through the Reynolds similarity principle based on an actual fracture size in the fracturing site and a simulation fracture size in the simulation fracture device, and specifically as follows:

[0077]

[0078] In the formula, Q F is the actual fluid discharge in the fracturing site; W E is a simulation fracture width in the simulation fracture device; H EH is the simulated fracture height in the fracture simulation device; F W is the actual fracture height in the fracturing site; F Q is the actual fracture width in the fracturing site; E Q is the actual fracture width in the fracturing site;

[0079] S12, a corresponding table of the actual fluid displacement in the fracturing site and the experimental fluid displacement is drawn, as shown in Table 1, and based on the corresponding table of the actual fluid displacement in the fracturing site and the experimental fluid displacement, the experimental fluid displacement is determined according to the actual fluid displacement in the fracturing site.

[0080] The liquid displacement of the proppant migration in the fracturing construction process and the proppant backflow in the flowback process is set according to the experimental fluid displacement in Table 1.

[0081] Table 1 corresponding table of the actual fluid displacement in the fracturing site and the experimental fluid displacement

[0082]

[0083] This step realizes the mutual conversion of the actual fluid displacement in the fracturing site and the laboratory fluid displacement, for example, according to the understanding of the field fracturing of a certain tight gas horizontal well, the construction displacement in the fracturing process can reach 16 m 3 / min, according to the late liquid discharge rule, the liquid flow rate during the flowback can reach 2 m 3 / min, and according to Table 1, the appropriate laboratory displacement in the fracturing and flowback processes is respectively 3.6 L / min and 0.45 L / min.

[0084] For example, according to the understanding of the field fracturing of a certain tight gas horizontal well, the construction displacement in the fracturing process can reach 14 m 3 / min, according to the late liquid discharge rule, the liquid flow rate during the flowback can reach 1-2 m 3 / min, and according to Table 1, the appropriate laboratory displacement in the fracturing and flowback processes is respectively 3.15 L / min and 0.23-0.45 L / min.

[0085] S2, the tracer particles are prepared, and the preparation is specifically as follows:

[0086] S21, the proppant particles are washed, dried, and then the proppant particles are fully stirred and dyed with fluorescent reflective paint, the dyeing concentration is controlled to be within 10%, and grinding, dispersion and screening are performed, and the mesh number of the screen in the screening process must be greater than 140 meshes.

[0087] S22, then the dyed proppant particles are dried under a set temperature condition to obtain the tracer particles, wherein the set temperature can be 50 DEG C, and the drying time is 48 hours.

[0088] In the embodiment of the present application, as Figure 4As shown, the circulating pumping system is used to prepare experimental fluid and deliver the experimental fluid to the simulated fracture device, for example, as shown in Figure 4 As shown, the circulating pumping system includes a first liquid preparation tank 1, a second liquid preparation tank 2, a sand mixing tank 3, an adjusting sand adding auger 4, a first liquid supply pump 5 and a second liquid supply pump 6.

[0089] The outlet of the first liquid preparation tank 1 is communicated with the inlet of the first liquid supply pump 5 through a first pipeline, and the outlet of the second liquid preparation tank 2 is communicated with the first pipeline through a second pipeline; the first pipeline is provided with a valve at the front and rear positions connected with the second pipeline; the outlet of the first liquid supply pump 5 is communicated with the sand mixing tank 3, and the inlet of the second liquid supply pump 6 is communicated with the outlet of the sand mixing tank 3, and the adjusting sand adding auger 4 is arranged above the sand mixing tank 3.

[0090] S3, preparing sand-carrying fluid: mixing tracer particles with experimental proppant and fracturing fluid to obtain sand-carrying fluid, specifically as follows:

[0091] S31, mixing the prepared tracer particles with experimental proppant and placing them in a sand tank.

[0092] The experimental proppant is the proppant used in fracturing site, for example, as shown in Figure 2 and Figure 3 As shown, the experimental proppant includes 70 / 140 quartz sand and 40 / 70 coated sand, wherein the mass ratio of the coated sand to the quartz sand is 2:8.

[0093] S32, taking fracturing fluid used in fracturing site and placing it in a 300L first liquid preparation tank 1 with a stirrer, pumping the fracturing fluid into the sand mixing tank 3 through the first liquid supply pump 5, and at the same time, adjusting the sand adding auger 4 to add the tracer particles and experimental proppant into the sand mixing tank 3 at a certain rate (to ensure the stepwise increase of the proppant concentration, and the simulated site proppant concentration in the fracture is 120kg / m 3 , 360kg / m 3 , 480kg / m 3 , respectively), so that they are fully mixed with the fracturing fluid to obtain sand-carrying fluid.

[0094] The viscosity of the fracturing fluid is 30mPa·s, and the viscosity after gel breaking is 5mPa·s. The mass of the tracer particles is not more than 5% of the mass of the sand-carrying fluid, and not more than 10% of the mass of the experimental proppant, and the mass of the experimental proppant should be more than 90% of the mass of the tracer particles.

[0095] For example, the sand-carrying fluid further comprises fibers, wherein the fibers are selected from 3mm fibers on site, and the mass of the fibers is 0.4% of the mass of the experimental proppant. When the sand-carrying fluid comprises fibers, the fracturing fluid is pumped into the sand mixing tank 3 by the first fluid supply pump 5, and the tracer particles, the experimental proppant and the fibers are added into the sand mixing tank 3 at a certain rate by adjusting the sand auger 4 (to ensure the proppant concentration step-up, the simulated on-site proppant concentration in the fracture is 120kg / m 3 , 360kg / m 3 , 480kg / m 3 , respectively), so as to be fully mixed with the fracturing fluid to obtain the sand-carrying fluid.

[0096] S4, simulate the fracturing process: the sand-carrying fluid is delivered to the simulated fracture device at a determined displacement by the second fluid supply pump 6, the flow of the fracturing fluid and the proppant in the fracture during the simulated fracturing process is simulated, and a first image set of the migration of the fracturing fluid and the experimental proppant during the simulated fracturing process is obtained, and the first image set is transmitted to the computer in real time.

[0097] For example, according to the understanding of the fracturing of a certain tight gas horizontal well, the construction displacement during the fracturing process can reach 16m 3 / min, and the appropriate laboratory displacement of the fracturing process is 3.6L / min according to Table 1.

[0098] For example, according to the understanding of the fracturing of a certain tight gas horizontal well, the construction displacement during the fracturing process can reach 14m 3 / min, and the appropriate laboratory displacement of the fracturing process is 3.15L / min according to Table 1.

[0099] The first image set comprises a picture of the flow field of the fracturing fluid in the simulated fracture, a picture of the movement of the experimental proppant particles, and a picture of the sand bank morphology during the settling process of the experimental proppant in the simulated fracture.

[0100] For example, obtaining the picture of the flow field of the fracturing fluid in the simulated fracture comprises the following steps:

[0101] S41, arranging a laser sheet light source on one side of the simulated fracture device, the laser sheet light source being parallel to the flow direction of the sand-carrying fluid in the fracture in the simulated fracture device, and the laser sheet light source being emitted into the fracture from the organic glass strip at the top of the simulated fracture device, and obtaining the picture of the flow field of the fracturing fluid at an interval time Δt by the CCD camera 7 to obtain N pictures of the flow field of the fracturing fluid.

[0102] The CCD camera 7 obtains the picture of the flow field of the fracturing fluid by receiving the laser reflected by the tracer particles in the flow field in the fracture, thereby obtaining the movement image of the tracer particles, and the movement of the tracer particles is used to characterize the flow field of the fracturing fluid.

[0103] For example, the steps for obtaining the pictures of the experimental proppant particles in the fracture during the simulated fracturing process include the following steps:

[0104] S42, arranging a light source on one side of the simulated fracture device to increase the contrast of the experimental proppant, facilitating the post-processing of the image, wherein the light source is arranged to irradiate from top to bottom at a distance of 1 m from the visualized simulated fracture device, and the pictures of the experimental proppant particles at intervals Δt during the fracturing process are obtained by the CCD camera 7 on the other side of the simulated fracture device, thereby obtaining M pictures of the experimental proppant particles during the fracturing process.

[0105] The positions of the pictures of the fracturing fluid flow field and the pictures of the experimental proppant particles in the fracture during the fracturing process obtained by the CCD camera 7 are the fracture mouth and the front end of the fracture, and the positions are selected to reflect different flow field characteristics.

[0106] For example, the steps for obtaining the pictures of the sand bank shape during the experimental proppant settling process in the simulated fracture include the following steps:

[0107] S43, after the pump is stopped, the experimental proppant is allowed to settle for 12-24 hours, and the proppant settling process in the simulated fracture is simulated, as shown in FIG. 5, wherein the changes in the sand bank shape around the simulated fracture device are recorded by four CCD cameras 7, and a plurality of pictures of the fracturing sand bank shape are obtained. Figure 5

[0108] S5, simulate the fracturing process: after the proppant packing layer in the fracture is stabilized, the gel breaking fluid in the second liquid preparation tank 2 is injected into the simulated fracture device in the reverse direction at a determined displacement by the first liquid supply pump 5 to flush the proppant packing layer, simulate the migration of the fracturing fluid and the experimental proppant during the flowback process, obtain a second image set of the migration of the fracturing fluid and the experimental proppant during the flowback process, and transmit the second image set to the computer in real time.

[0109] For example, according to the field fracturing of a certain tight gas horizontal well, the construction displacement during the fracturing process can reach 16 m 3 / min, according to the late liquid discharge rule, the liquid flow rate during the flowback process can reach 2 m 3 / min, and according to Table 1, the appropriate laboratory displacement during the flowback process is 0.45 L / min, i.e., the displacement of the gel breaking fluid during the flowback process is 0.45 L / min.

[0110] For example, according to the field fracturing of a certain tight gas horizontal well, the construction displacement during the fracturing process can reach 14 m 3 / min, according to the late liquid discharge rule, the liquid flow rate during the flowback process can reach 1-2 m 3 / min, and according to Table 1, the appropriate laboratory displacement during the flowback process is 0.23-0.45 L / min, i.e., the displacement of the gel breaking fluid during the flowback process is 0.23-0.45 L / min. ​

[0111] The second image set includes images of the fracturing fluid flow field in the fracture during the simulated flowback process, images of the experimental proppant particle movement, and images of the sand bank morphology.

[0112] For example, obtaining an image of the fracturing fluid flow field within a fracture during a simulated flowback process involves the following steps:

[0113] S51. Arrange a laser sheet light source on one side of the simulated fracture device. The laser sheet light source is parallel to the flow direction of the sand-carrying fluid in the fracture of the simulated fracture device and is emitted into the fracture from the organic glass strip on the top of the simulated fracture device. Use CCD camera 7 to obtain flow field images of the return fluid at intervals of time Δt, and obtain N flow field images of the return fluid.

[0114] For example, obtaining an experimental image of proppant particle motion within a fracture during simulated flowback involves the following steps:

[0115] S52. Arrange an illumination light source on one side of the simulated fracture device to increase the contrast of the experimental proppant and facilitate post-processing of the image. The illumination light source is positioned 1 m away from the visualized simulated fracture device and irradiates from top to bottom. On the other side of the simulated fracture device, a CCD camera 7 is used to obtain an image of the experimental proppant particle movement at an interval Δt during the flowback process, and M images of the proppant particle movement in the flowback experiment are obtained.

[0116] The CCD camera 7 acquires the fracturing fluid flow field image in the fracture during the flowback process and the experimental proppant particle movement image at the fracture mouth and the fracture front flow field. The selected positions can reflect different flow field characteristics.

[0117] For example, obtaining images of sand bank morphology during simulated flowback involves the following steps:

[0118] S53, such as Figure 5 As shown, four CCD cameras 7 are used to record the changes in the morphology of the sand bank around the simulated fracture device, and multiple pictures of the flowback sand bank morphology are obtained.

[0119] For example, the simulated flowback process is as follows: wait for 12 hours until the proppant filling layer is completely stable, and then reversely pump the gel breaking liquid through the circulating pumping system to flush the proppant filling layer. The displacement is set to 0.45L / min.

[0120] S6. The computer quantitatively describes the proppant flowback based on the first image set and the second image set, as follows:

[0121] S61, performing image processing on the first image set and the second image set to obtain the fracturing fluid velocity field and the proppant velocity field, such as Figure 6 As shown, the following steps are included:

[0122] S611, analyze and operate the property (exposure value) of each pixel point itself and the pixel point group feature (exposure area size) in N fracturing fluid flow field pictures, N flowback fluid flow field pictures, M fracturing experimental proppant particle motion pictures and M flowback experimental proppant particle motion pictures, and distinguish the experimental proppant particle pixel points and the tracer particle pixel points.

[0123] S612, calculate the tracer particle pixel point coordinates in N fracturing fluid flow field pictures and N flowback fluid flow field pictures by PIV algorithm to obtain the fracturing fluid velocity field, as shown in Figure 10 .

[0124] S613, calculate the proppant pixel point coordinates in M fracturing experimental proppant particle motion pictures and M flowback experimental proppant particle motion pictures by PTV algorithm to obtain the proppant velocity field, as shown in Figure 11 .

[0125] S62, according to the first image set and the second image set, quantitatively describe the sand bank change process and determine the sand bank profile, and the sand bank profile represents the proppant flowback capacity under different working conditions, as shown in Figure 7 , Figure 8 and Figure 9 , including the following steps:

[0126] S621, quantitatively describe the sand bank change process according to the multiple fracturing sand bank shape pictures and the multiple flowback sand bank shape pictures.

[0127] S622, splice the multiple fracturing sand bank shape pictures and the multiple flowback sand bank shape pictures to obtain the sand bank profile, and the sand bank profile represents the proppant flowback capacity under different working conditions.

[0128] S63, according to the first image set and the second image set, determine the sand bank height change value within a certain time, and the sand bank height change value represents the scouring of the flowback fluid on the sand bank within a certain time; according to the relationship between the sand bank height value and the time, the critical flow rate of the flowback fluid affecting the sand bank height is represented.

[0129] For example, the sand bank height change in the area of 197mm x 171mm around the simulated fracture device is recorded by four CCD cameras 7 respectively, so as to reflect the scouring of the flowback fluid on the sand bank within a short time, and the sand bank height change with time is recorded, so as to represent the critical flow rate affecting the sand bank height.

[0130] S64, as Figure 12As shown, based on the fracturing fluid flow field, the angle velocity and number of tracer particles are extracted through the tracer particle position information and time difference to characterize the velocity change of the fracturing fluid during the backflow process; based on the proppant velocity field, the angle, velocity and number of experimental proppant particles are extracted through the experimental proppant particle position information and time difference to characterize the velocity change of the proppant particles during the backflow process.

[0131] S65, such as Figure 13 and Figure 14 As shown, based on the fracturing fluid flow field and proppant velocity field, the vertical fracturing fluid and experimental proppant particle velocity and velocity gradient are calculated to characterize the shear force of the return fluid on the sand bank surface and reflect the erosion effect of the return fluid on the proppant filling layer.

[0132] Based on the above-mentioned proppant fracture reflow physical simulation method, an embodiment of the present invention further provides a proppant fracture reflow physical simulation system, including a calculation module, a liquid supply pump, a simulated fracture device, a CCD camera 7, a circulation pumping system and a computer.

[0133] Among them, the calculation module is used to determine the experimental fluid displacement input into the simulated fracture device according to the actual fluid displacement at the fracturing site; the fluid supply pump is used to transport the sand-carrying fluid to the simulated fracture device according to the determined displacement; the simulated fracture device is used to simulate the flow of fracturing fluid and proppant in the fracture during the fracturing process; the CCD camera 7 is used to obtain a first image set of the migration of fracturing fluid and experimental proppant during the simulated fracturing process; the circulating pumping system is used to reversely inject the breaker fluid into the simulated fracture device according to the determined displacement to flush the proppant filling layer after the proppant filling layer in the fracture is stabilized; the simulated fracture device is also used to simulate the migration of fracturing fluid and experimental proppant during the backflow process; the CCD camera 7 is also used to obtain a second image set of the migration of fracturing fluid and experimental proppant during the backflow process; the computer is used to quantitatively describe the proppant reflux based on the first image set and the second image set.

[0134] Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein; and these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A physical simulation method for proppant fracture backflow, characterized in that: The following steps are involved: The experimental fluid displacement input into the simulated fracture device is determined based on the actual fluid displacement at the fracturing site; wherein the experimental fluid displacement includes the sand-carrying fluid displacement during the simulated fracturing process and the gel-breaking fluid displacement during the flowback process; The tracer particles are mixed with the experimental proppant and the fracturing fluid to obtain a sand-carrying fluid; delivering the sand-carrying fluid to the simulated fracture device at a determined displacement rate, simulating the flow of the fracturing fluid and proppant in the fracture during the fracturing process, and obtaining a first set of images of the migration of the fracturing fluid and the experimental proppant during the simulated fracturing process; the first set of images includes images of the fracturing fluid flow field within the fracture during the simulated fracturing process, images of the movement of the experimental proppant particles, and images of the sand bank morphology during the settling of the experimental proppant within the simulated fracture; After the proppant filling layer in the fracture stabilizes, the gel-breaking fluid is reversely injected into the simulated fracture device at a predetermined flow rate to flush the proppant filling layer, simulating the migration of the fracturing fluid and experimental proppant during the flowback process. A second image set of the migration of the fracturing fluid and experimental proppant during the flowback process is obtained. The second image set includes images of the fracturing fluid flow field in the fracture during the simulated flowback process, images of the movement of experimental proppant particles, and images of the sand bank morphology. Based on the first image set and the second image set, a quantitative description of proppant return is performed, including the following steps: performing image processing on the first image set and the second image set to obtain a fracturing fluid velocity field and a proppant velocity field; based on the first image set and the second image set, a quantitative description of the sand bank change process is performed and the sand bank contour is determined, and the sand bank contour is used to characterize the proppant return capacity under different working conditions; based on the fracturing fluid flow field, the angle, velocity and number of tracer particles are extracted through the tracer particle position information and time difference, and the velocity change of the fracturing fluid during the return process is characterized; based on the proppant velocity field, the angle, velocity and number of experimental proppant particles are extracted through the experimental proppant particle position information and time difference, and the velocity change of the proppant particles during the return process is characterized; based on the fracturing fluid flow field and the proppant velocity field, the longitudinal fracturing fluid and experimental proppant particle velocities and velocity gradient are calculated to characterize the shear force of the return fluid on the sand bank surface.

2. The physical simulation method for proppant reflux according to claim 1, characterized in that: The method of determining the experimental fluid displacement input into the simulated fracture device according to the actual fluid displacement at the fracturing site includes the following steps: Based on the actual fracture size at the fracturing site and the simulated fracture size in the simulated fracture device, the actual fluid displacement at the site is converted into the experimental fluid displacement through the Reynolds similarity principle; Based on the correspondence table between the actual fluid displacement at the fracturing site and the experimental fluid displacement, the experimental fluid displacement is determined according to the actual fluid displacement at the fracturing site.

3. The physical simulation method for proppant reflux in cracks according to claim 1, characterized in that: Mixing the tracer particles with the experimental proppant and the fracturing fluid to obtain the sand-carrying fluid includes the following steps: The fracturing fluid is transported to a sand mixing tank, and tracer particles and experimental proppant are added to the sand mixing tank at a certain rate to fully mix with the fracturing fluid to obtain a sand-carrying fluid; or The fracturing fluid is transported to the sand mixing tank, and tracer particles, experimental proppants and fibers are added to the sand mixing tank at a certain rate to fully mix them with the fracturing fluid to obtain sand-carrying fluid.

4. The physical simulation method for proppant reflux in cracks according to claim 1, characterized in that: The preparation of tracer particles includes the following steps: The proppant particles are cleaned and dried, then mixed and dyed with fluorescent reflective paint, and then ground, dispersed and screened; The dyed proppant particles are then dried under set temperature conditions to obtain tracer particles.

5. The physical simulation method for proppant reflux in cracks according to claim 1, characterized in that: Acquiring images of the fracturing fluid flow field within a fracture during a simulated fracturing process or a simulated flowback process includes the following steps: A laser light source is arranged on one side of the simulated crack device. The laser light source is parallel to the flow direction of the sand-carrying fluid in the crack of the simulated crack device and is emitted into the crack from the organic glass strip on the top of the simulated crack device. Capture interval time with CCD camera Δt Alternatively, a CCD camera is used to obtain flow field images of the flowback fluid at intervals of Δt to obtain N flow field images of the flowback fluid.

6. The physical simulation method for proppant reflux in cracks according to claim 1, characterized in that: Acquiring an image of experimental proppant particle motion within a fracture during a simulated fracturing process or a simulated flowback process includes the following steps: Arranging an illumination light source on one side of the simulated crack device, wherein the illumination light source illuminates the simulated crack device from top to bottom; The time interval during the fracturing process is captured by a CCD camera on the other side of the simulated fracture device. Δt Experimental proppant particle movement pictures, obtain M pictures of proppant particle movement in fracturing experiments, or obtain the interval time of the backflow process through a CCD camera on the other side of the simulated fracture device Δt Experimental proppant particle movement pictures, obtain M pictures of proppant particle movement in the backflow experiment.

7. The physical simulation method for proppant reflux in cracks according to claim 1, characterized in that: The following steps are involved in obtaining images of the sand bank morphology during proppant settling in a simulated fracture: After stopping the pump, the device was left to stand for 12 to 24 hours to simulate the proppant sedimentation process in the fracture. Four CCD cameras were used to record the changes in the sand bank morphology around the simulated fracture device, and multiple pictures of the fracturing sand bank morphology were obtained.

8. The physical simulation method for proppant reflux in cracks according to claim 1, characterized in that: The following steps are involved in obtaining images of the sand bank morphology during simulated flowback: During the simulated flowback process, four CCD cameras were used to record the changes in the sand bank morphology around the simulated fracture device, and multiple images of the flowback sand bank morphology were obtained.

9. The method for physical simulation of proppant reflux in cracks according to claim 1, characterized in that: Performing image processing on the first image set and the second image set to obtain a fracturing fluid velocity field and a proppant velocity field includes the following steps: Analyze and calculate the exposure value and exposure area of ​​each pixel in N fracturing fluid flow field images, N flowback fluid flow field images, M fracturing experiment proppant particle movement images, and M flowback experiment proppant particle movement images, and distinguish experimental proppant particle pixels from tracer particle pixels; The fracturing fluid velocity field is obtained by calculating the coordinates of the tracer particle pixels in N fracturing fluid flow field images and N flowback fluid flow field images using the PIV algorithm; The proppant velocity field is obtained by calculating the coordinates of the proppant pixel points in M ​​fracturing experiment proppant particle motion images and M flowback experiment proppant particle motion images using the PTV algorithm.

10. The physical simulation method for proppant reflux in cracks according to claim 3, characterized in that: The mass of the tracer particles is no more than 5% of the mass of the sand-carrying fluid and no more than 10% of the mass of the experimental proppant; the mass of the fiber is 0.4% of the mass of the experimental proppant.

11. A proppant fracture reflux physical simulation system, characterized in that: The method for performing physical simulation of proppant fracture backflow according to any one of claims 1 to 10 comprises: A calculation module is used to determine the experimental fluid displacement input into the simulated fracture device based on the actual fluid displacement at the fracturing site; wherein the experimental fluid displacement includes the sand-carrying fluid displacement during the simulated fracturing process and the gel-breaking fluid displacement during the flowback process; The second fluid supply pump is used to deliver the sand-carrying fluid to the simulated fracture device according to a determined displacement; A simulated fracture device is used to simulate the flow of fracturing fluid and proppant in the fracture during the fracturing process; A CCD camera is used to obtain a first set of images of the migration of fracturing fluid and experimental proppant during the simulated fracturing process; The first liquid supply pump is used to reversely inject the gel breaking liquid into the simulated fracture device at a determined displacement after the proppant filling layer in the fracture is stabilized to flush the proppant filling layer; The simulated fracture device is also used to simulate the migration of fracturing fluid and experimental proppant during the flowback process; The CCD camera is also used to obtain a second set of images of the migration of fracturing fluid and experimental proppant during the flowback process; The computer is configured to quantitatively describe proppant flowback based on the first image set and the second image set.

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