Method and system for physically simulating in-seam backflow of proppant

Abstract

The invention belongs to the technical field of oil and gas field development, and discloses a physical simulation method and system for in-crack backflow of a proppant, and the simulation method comprises the steps: determining the displacement of experimental fluid input into a crack simulation device according to the actual displacement of fluid on a fracturing site; acquiring a first image set of migration of the fracturing fluid and the experimental proppant in the simulated fracturing process; after the propping agent filling layer in the crack is stable, a second image set of migration of the fracturing fluid and the experimental propping agent in the flowback process is obtained; and according to the first image set and the second image set, quantitative description is carried out on proppant backflow. According to the method, migration and sedimentation of the propping agent in a crack in the fracturing construction process and starting and migration of the propping agent in the flowback process are simulated, the backflow state and the motion law of the propping agent are observed and finely described, the backflow process of the propping agent is quantitatively described through multi-dimensional characterization parameters, and the backflow motion law of the propping agent is revealed.

Description

Technical Field

[0001] The invention belongs to the technical field of oil and gas field development, and particularly relates to a proppant seam internal reflux physical simulation method and system. Background Art

[0002] Hydraulic fracturing is an important technology for oil and gas field development and an important means of increasing the production of oil and gas reservoirs. Post-fracturing flowback is an important part of hydraulic fracturing construction and has an important impact on the overall fracturing effect. According to the current on-site flowback of tight gas wells, proppant backflow is a common phenomenon. According to incomplete statistics, the sand production of tight gas horizontal wells in the JQ block after fracturing is 5 to 239 tons, and the sand production rate is 2.3% to 6.9%. Proppant backflow will produce a 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 area, reducing the effective support area of ​​the fracture and the conductivity of the supporting fracture, thereby affecting the gas well production capacity. Related studies have shown that the average gas production decline rate of sand-producing gas wells is more than three times that of normal production wells. In addition, proppant reflux accumulates at the bottom of the well to bury the gas layer or sand erodes the surface pipeline, causing sand jams in downhole tools and valve blockage, which brings safety hazards in construction and production. Therefore, the study of proppant reflux is of great significance to the development and production of gas wells.

[0003] The existing physical simulation experimental methods for proppant reflux mainly include the following:

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

[0005] (2) Slot model: This experimental device mainly consists of a fluid inlet and outlet, a proppant-filled slot, and a sand collector. The experimental method is to first fill the middle of the slot with proppant, then adjust the slot width to simulate the field fracture morphology while applying closing pressure to the proppant filling layer. Then, the sand-carrying fluid is pumped in through a three-cylinder pump and the liquid displacement is increased step by step until proppant appears in the sand collector. The characterization parameter for proppant reflux is the fluid flow rate corresponding to the appearance of proppant in the sand collector, which is defined as the proppant reflux 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 flow meter, a diversion chamber, a hydraulic press, etc. The experimental method is to first fill the diversion chamber with proppant, apply closing pressure to the proppant filling layer through the hydraulic press, and the sand-carrying fluid saturates the proppant filling layer through the hydraulic device, and then increase the flow rate of the sand-carrying fluid step by step until sand is observed in the diversion chamber. The characterization parameter for proppant reflux is the fluid flow rate corresponding to sand production in the diversion chamber, which is defined as the critical flow rate of proppant reflux (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: This experimental device mainly consists of a diversion chamber (working area size 13.34 cm × 13.34 cm), a hydraulic press water pump equipped with a heating system, a sensor group (thermocouple, flow meter and pressure gauge), a data acquisition system and a computer. The experimental method is to fill the diversion chamber with proppant and lay it evenly, apply closing pressure to the set value through the hydraulic press, and then saturate the proppant filling layer with preheated sand-carrying fluid. After stabilization, the sand-carrying fluid flow rate is increased step by step to simulate the backflow process until sand comes out of the diversion chamber. The characterization parameter of the proppant backflow is the fluid flow rate corresponding to the sand coming out of the diversion chamber during the experiment, which is defined as the critical flow rate of proppant backflow (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 physical simulation experimental methods for proppant return are to fill the flow chamber with proppant to form a stable and continuous proppant filling layer, ignoring the influence of the sand spreading process and the shape of the sand bank on the proppant return. The shape of the sand bank formed during the actual fracturing process has a significant impact on the fluidization area of ​​the proppant filling layer. Secondly, the existing experimental methods cannot observe the proppant return during the backflow process, and cannot finely characterize the state and law of the proppant return. Third, the existing experimental scale is quite different from the actual fracture geometry on site, and the proppant return characteristics reflected are somewhat different from the actual on-site. In addition, the proppant return characterization parameters under the existing experimental methods are only the liquid flow rate corresponding to the sand discharge from the flow chamber, that is, the critical flow rate of the proppant return. The characterization parameters are too simple and single, and it is impossible to finely characterize the proppant movement state (proppant start-up, rolling and sedimentation) and various parameters (fluid velocity, particle velocity and velocity gradient) during the backflow process. Summary of the invention

[0009] In view of the above problems, the present invention provides a physical simulation method and system for proppant fracture reflux, which can realize the quantitative description of proppant migration in fractures during the whole process of fracturing and flowback under large-scale (4×0.3m) conditions, and reveal the movement law of proppant reflux through multi-dimensional characterization parameters.

[0010] A method for simulating physical backflow in a proppant fracture comprises the following steps: determining the experimental fluid displacement input into a simulated fracture device according to the actual fluid displacement at the fracturing site; wherein the experimental fluid displacement includes the sand-carrying fluid displacement of the simulated fracturing process and the debonding fluid displacement of the backflow process; delivering the sand-carrying fluid to the simulated fracture device according to the determined displacement, simulating the flow of fracturing fluid and proppant in the fracture during the fracturing process, and obtaining a first image set of the migration of the fracturing fluid and the experimental proppant during the simulated fracturing process; after the proppant filling layer in the fracture is stable, reversely injecting the debonding fluid into the simulated fracture device according to the determined displacement to flush the proppant filling layer, simulating the migration of the fracturing fluid and the experimental proppant during the backflow process, and obtaining a second image set of the migration of the fracturing fluid and the experimental proppant during the backflow process; and quantitatively describing the proppant backflow according to the first image set and the second image set.

[0011] Further, the experimental fluid displacement input into the simulated fracture device is determined according to the actual fluid displacement at the fracturing site, including the following steps:

[0012] 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;

[0013] 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.

[0014] Furthermore, 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, the tracer particles are mixed with the experimental proppant and the fracturing fluid to obtain the sand-carrying fluid, which includes the following steps:

[0016] The fracturing fluid is transported to a sand mixing tank, and at the same time, 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

[0017] The fracturing fluid is transported to the sand mixing tank, and at the same time, 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.

[0018] Furthermore, preparing the tracer particles comprises the following steps:

[0019] The proppant particles are cleaned and dried, and then the proppant particles are mixed and dyed with fluorescent reflective paint, and then ground, dispersed and screened;

[0020] The dyed proppant particles are then dried under set temperature conditions to obtain tracer particles.

[0021] Further, the first image set includes images of the fracturing fluid flow field in the fracture during the simulated fracturing process, images of experimental proppant particle movement, and images of the sand bank morphology during the experimental proppant settling process in the simulated fracture;

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

[0023] Further, obtaining a flow field image of the fracturing fluid in the fracture during the simulated fracturing process or the simulated flowback process includes the following steps:

[0024] 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 liquid 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.

[0025] The CCD camera is used to obtain N fracturing fluid flow field images at intervals of Δt, or the CCD camera is used to obtain N flow field images of the flowback fluid at intervals of Δt.

[0026] Further, obtaining experimental proppant particle movement images in the fracture during the simulated fracturing process or the simulated flowback process includes the following steps:

[0027] Arranging an illumination light source on one side of the simulated crack device, wherein the illumination light source irradiates the simulated crack device from top to bottom;

[0028] On the other side of the simulated fracture device, a CCD camera is used to obtain experimental proppant particle movement images at intervals Δt during the fracturing process, and M fracturing experimental proppant particle movement images are obtained. Alternatively, a CCD camera is used to obtain experimental proppant particle movement images at intervals Δt during the flowback process on the other side of the simulated fracture device, and M flowback experimental proppant particle movement images are obtained.

[0029] Furthermore, obtaining a picture of the sand bank morphology during proppant settling in a simulated fracture experiment includes the following steps:

[0030] After stopping the pump, the system 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 morphology of the sand bank around the simulated fracture device, and multiple pictures of the morphology of the fracturing sand bank were obtained.

[0031] Furthermore, obtaining the sand bank morphology image during the simulated flowback process includes the following steps:

[0032] During the simulated flowback process, four CCD cameras were used to record the changes in the morphology of the sand bank around the simulated fracture device, and multiple pictures of the morphology of the flowback sand bank were obtained.

[0033] Further, quantitatively describing the proppant flowback according to the first image set and the second image set includes the following steps:

[0034] 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;

[0035] According to the first image set and the second image set, a quantitative description of the sand bank change process is performed and a sand bank profile is determined, and the proppant flowback capacity under different working conditions is characterized by the sand bank profile;

[0036] Based on the fracturing fluid flow field, the angle velocity and number of tracer particles are extracted through the position information and time difference of tracer particles to characterize the velocity change of fracturing fluid during the flowback process; based on the proppant velocity field, the angle, velocity and number of experimental proppant particles are extracted through the position information and time difference of experimental proppant particles to characterize the velocity change of proppant particles during the flowback process;

[0037] According to 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.

[0038] Further, the following steps are included:

[0039] 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 the experimental proppant particle pixels from the tracer particle pixels;

[0040] 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;

[0041] The proppant velocity field is obtained by calculating the coordinates of the proppant pixel points in M ​​fracturing experiment proppant particle motion pictures and M flowback experiment proppant particle motion pictures using the PTV algorithm.

[0042] Furthermore, 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.

[0043] The present invention also provides a proppant seam internal reflux physical simulation system, comprising:

[0044] A 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; wherein the experimental fluid displacement includes the sand-carrying fluid displacement of the simulated fracturing process and the gel-breaking fluid displacement of the flowback process;

[0045] A second fluid supply pump is used to deliver the sand-carrying fluid to the simulated fracture device according to a determined displacement;

[0046] A simulated fracture device is used to simulate the flow of fracturing fluid and proppant in the fracture during the fracturing process;

[0047] A CCD camera is used to obtain a first set of images of fracturing fluid and experimental proppant migration during simulated fracturing;

[0048] The first liquid supply pump is used to reversely inject the debonding liquid into the simulated fracture device at a determined displacement to flush the proppant filling layer after the proppant filling layer in the fracture is stabilized;

[0049] The simulated fracture device is also used to simulate the migration of fracturing fluid and experimental proppant during the flowback process;

[0050] The CCD camera is also used to obtain a second set of images of the migration of the fracturing fluid and experimental proppant during the flowback process;

[0051] The computer is used to quantitatively describe the proppant flowback based on the first image set and the second image set.

[0052] Beneficial effects of the present invention:

[0053] The present invention uses a fracture experimental device to simulate the migration and sedimentation of proppants in fractures during fracturing construction, as well as the startup and migration of proppants during flowback, so as to observe and finely characterize the proppant reflux state and movement law under the conditions of sand bank shape and proppant laying law. The proppant reflux process is quantitatively described through multi-dimensional characterization parameters, revealing the proppant reflux movement law.

[0054] Other features and advantages of the present invention will be described in the following description, and partly become obvious from the description, or be understood by implementing the present invention. The purpose and other advantages of the present invention can be realized and obtained by the structures pointed out in the description and the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0055] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0056] Figure 1 A schematic flow chart of a method for physical simulation of proppant seam backflow according to an embodiment of the present invention is shown;

[0057] Figure 2 A picture of 70 / 140 quartz sand selected for use in a fracturing field according to an embodiment of the present invention is shown;

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

[0059] Figure 4 It shows a schematic structural diagram of a circulating pump injection system according to an embodiment of the present invention;

[0060] Figure 5 A schematic diagram showing the position of capturing images of a simulated fracture device and a simulated experiment of a fracturing flowback process according to an embodiment of the present invention is shown;

[0061] Figure 6 A schematic diagram of a process of calculating a picture using a PIV algorithm and a PTV algorithm to obtain a fracturing fluid velocity field and a proppant velocity field according to an embodiment of the present invention is shown;

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

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

[0064] Fig. 9 The local morphology of the sand bank according to an embodiment of the present invention is shown, and the height of the sand bank changes with time as recorded by CCD photos;

[0065] Fig.10 A schematic diagram of a fracturing fluid flow field according to an embodiment of the present invention is shown;

[0066] Fig.11 A schematic diagram of a proppant particle field for an experiment according to an embodiment of the present invention is shown;

[0067] Fig.12 A schematic diagram of a flow field along the longitudinal direction of a crack wall according to an embodiment of the present invention is shown;

[0068] Fig.13 A schematic diagram showing the longitudinal velocity distribution of a fluid along the longitudinal direction of a crack wall according to an embodiment of the present invention is shown;

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

[0070] In the figure: 1. First liquid dispensing tank; 2. Second liquid dispensing tank; 3. Sand mixing tank; 4. Adjusting sand adding auger; 5. First liquid supply pump; 6. Second liquid supply pump; 7. CCD camera. DETAILED DESCRIPTION

[0071] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

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

[0073] The embodiment of the present invention proposes a physical simulation method and system for proppant backflow in fractures. With the help of a complex fracture experimental device that can measure proppant migration in the entire flow field and visualize the complex fractures, the migration and sedimentation of proppant in fractures during fracturing construction and the start-up and migration of proppant during backflow 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 backflow state and movement law can be observed and finely characterized under the conditions of sand bank shape and proppant laying law. The proppant backflow process is quantitatively described through multi-dimensional characterization parameters (sand bank laying morphology, local sand bank morphology, fracturing fluid flow field and proppant particle field, vertical fluid velocity and velocity gradient), revealing the proppant backflow movement law.

[0074] like Figure 1 As shown, a physical simulation method for proppant internal reflux includes the following steps:

[0075] S1. Determine the experimental fluid displacement input into the simulated fracture device according to the actual fluid displacement at the fracturing site, wherein the determined experimental fluid displacement includes the sand-carrying fluid displacement of the simulated fracturing process and the gel-breaking fluid displacement of the flowback process, as follows:

[0076] S11. 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 by the Reynolds similarity principle, specifically:

[0077]

[0078] Where: Q F is the actual fluid displacement at the fracturing site; W E is the simulated crack width in the simulated crack device; H EH is the height of the simulated crack in the simulated crack device; F is the actual fracture height at the fracturing site; W F is the actual crack width at the fracturing site; Q E is the experimental fluid displacement.

[0079] S12, draw a correspondence table between the actual displacement of the fluid at the fracturing site and the displacement of the experimental fluid, as shown in Table 1, and determine the experimental fluid displacement according to the actual displacement of the fluid at the fracturing site based on the correspondence table between the actual displacement of the fluid at the site and the displacement of the experimental fluid.

[0080] The proppant migration during the simulated fracturing operation and the liquid displacement of the proppant reflux during the flowback process were set according to the experimental fluid displacement in Table 1.

[0081] Table 1 Correspondence between actual displacement of on-site fluid and displacement of sand-carrying fluid in experimental fluid

[0082]

[0083] This step realizes the mutual conversion between the actual fluid displacement at the fracturing site and the fluid displacement in the indoor experiment. For example, according to the field fracturing knowledge of a certain horizontal well with tight gas, the construction displacement during the fracturing process can reach 16m 3 / min, according to the later drainage rules, the liquid flow rate during the return flow can reach 2m 3 / min. According to Table 1, the appropriate laboratory displacements for fracturing and flowback processes are 3.6L / min and 0.45L / min respectively.

[0084] For example, according to the on-site fracturing experience of a certain horizontal well with consistent gas, the operation displacement during the fracturing process can reach 14m 3 / min. According to the later drainage rules, the liquid flow rate during backflow can reach 1-2m 3 / min. According to Table 1, the appropriate laboratory displacements for fracturing and flowback processes are 3.15L / min and 0.23-0.45L / min respectively.

[0085] S2. Prepare tracer particles as follows:

[0086] S21. Wash and dry the proppant particles, then fully mix and dye the proppant particles with fluorescent reflective paint, control the dyeing concentration within 10%, and grind, disperse and screen. During the screening process, the mesh size of the screen must be greater than 140 meshes.

[0087] S22, then drying the dyed proppant particles under set temperature conditions to obtain tracer particles, wherein the set temperature may be 50° C. and the drying time may be 48 hours.

[0088] In the embodiment of the present invention, Figure 4As shown, the circulating pumping system is used to prepare the experimental fluid and deliver the experimental fluid to the simulated fracture device, for example Figure 4 As shown, the circulating pumping system includes a first liquid dispensing tank 1, a second liquid dispensing 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] Among them, the outlet of the first liquid distribution tank 1 is connected to the inlet of the first liquid supply pump 5 through the first pipe, and the outlet of the second liquid distribution tank 2 is connected to the first pipe through the second pipe; valves are provided at the front and rear positions of the first pipe connected to the second pipe; the outlet of the first liquid supply pump 5 is connected to the sand mixing tank 3, and the inlet of the second liquid supply pump 6 is connected to the outlet of the sand mixing tank 3, and the regulating sand adding auger 4 is arranged above the sand mixing tank 3.

[0090] S3. Preparation of sand-carrying fluid: Mixing the tracer particles with the experimental proppant and the fracturing fluid to obtain the sand-carrying fluid, as follows:

[0091] S31. Mix the prepared tracer particles with the experimental proppant and place them in a sand tank.

[0092] The proppant used in the experiment is the proppant used in the fracturing field, for example 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 coated sand to quartz sand is 2:8.

[0093] S32, take the fracturing fluid used in the fracturing site, put it into the 300L first liquid distribution tank 1 with a stirrer, pump the fracturing fluid into the sand mixing tank 3 through the first liquid supply pump 5, and adjust the sand adding auger 4 to add the tracer particles and the experimental proppant into the sand mixing tank 3 at a certain rate (to ensure that the proppant concentration is increased step by step, and the simulated field proppant concentration in the fracture is 120kg / m 3 、360kg / m 3 , 480kg / m 3 ) to fully mix it with the fracturing fluid to obtain the sand-carrying fluid.

[0094] The viscosity of the fracturing fluid is 30 mPa·s, and the viscosity after gel breaking is 5 mPa·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. 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 also includes fibers, wherein the fibers are selected from 3 mm fibers used on site, and the mass of the fibers is 0.4% of the mass of the experimental proppant. When the sand-carrying fluid contains fibers, the fracturing fluid is pumped into the sand mixing tank 3 by the first fluid supply pump 5, and the sand adding auger 4 is adjusted to add the tracer particles, the experimental proppant and the fibers into the sand mixing tank 3 at a certain rate (ensuring that the proppant concentration is increased stepwise, and the simulated field proppant concentrations in the fractures are 120 kg / m 3 、360kg / m 3 , 480kg / m 3 ) to fully mix it 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 according to the determined displacement through 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 on-site fracturing experience of a certain horizontal well with consistent gas, the operation displacement during fracturing can reach 16m 3 / min, and the laboratory displacement of the appropriate fracturing process selected in Table 1 is 3.6L / min.

[0098] For example, according to the on-site fracturing experience of a certain horizontal well with consistent gas, the operation displacement during the fracturing process can reach 14m 3 / min, and the appropriate laboratory displacement for the fracturing process is 3.15L / min according to Table 1.

[0099] The first image set includes images of the fracturing fluid flow field in the fracture during the simulated fracturing process, images of the experimental proppant particle movement, and images of the sand bank morphology during the experimental proppant sedimentation process in the simulated fracture.

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

[0101] S41. A laser sheet light source is arranged 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. The CCD camera 7 is used to obtain fracturing fluid flow field images at an interval of Δt to obtain N fracturing fluid flow field images.

[0102] Specifically, the CCD camera 7 acquires the fracturing fluid flow field image by receiving the laser reflected by the tracer particles in the flow field in the fracture, thereby obtaining the moving image of the tracer particles, and characterizing the fracturing fluid flow field through the movement of the tracer particles.

[0103] For example, obtaining an experimental picture of proppant particle motion within a fracture during simulated fracturing involves the following steps:

[0104] S42. Arrange an illumination light source on one side of the simulated fracture device to increase the contrast of the experimental proppant for the convenience of image post-processing, wherein the illumination light source is located 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 a motion picture of the experimental proppant particles at an interval of Δt during the fracturing process, and M pictures of the motion of the proppant particles in the fracturing experiment are obtained.

[0105] The positions where the CCD camera 7 obtains the fracturing fluid flow field images in the fracture during the fracturing process and the experimental proppant particle movement images are the fracture mouth and the fracture front flow field, and the selected positions can reflect different flow field characteristics.

[0106] For example, obtaining an image of the sand bank morphology during proppant settling in a simulated fracture experiment involves the following steps:

[0107] S43, after stopping the pump, let it stand for 12 to 24 hours to simulate the proppant settling process in the fracture. 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, respectively, to obtain a plurality of fracturing sand bank morphology pictures.

[0108] S5, simulated flowback fracturing process: after the proppant filling layer in the fracture is stabilized, the degelling liquid in the second liquid distribution tank 2 is reversely injected into the simulated fracture device at a determined displacement through the first liquid supply pump 5 to flush the proppant filling layer, simulate the migration of the fracturing fluid and the experimental proppant during the flowback process, and 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 on-site fracturing experience of a certain horizontal well with consistent gas, the operation displacement during fracturing can reach 16m 3 / min, according to the later drainage rules, the liquid flow rate during the return flow can reach 2m 3 / min, the appropriate laboratory displacement of the flowback process is selected according to Table 1 as 0.45L / min, that is, the displacement of the debonding liquid in the flowback process is 0.45L / min.

[0110] For example, according to the on-site fracturing experience of a certain horizontal well with consistent gas, the operation displacement during the fracturing process can reach 14m 3 / min. According to the later drainage rules, the liquid flow rate during backflow can reach 1-2m 3 / min, the appropriate laboratory displacement of the flowback process is selected according to Table 1 as 0.23-0.45 L / min, that is, the displacement of the debonding liquid in the flowback process is 0.23-0.45 L / min.

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

[0112] For example, obtaining a picture of the flow field of the fracturing fluid in a fracture during simulated flowback includes the following steps:

[0113] S51. A laser sheet light source is arranged on one side of the simulated crack device. The laser sheet 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. The flow field images of the return fluid at an interval time Δt are obtained through the CCD camera 7 to obtain N return fluid flow field images.

[0114] For example, obtaining an experimental picture 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 for the convenience of image post-processing, wherein the illumination light source is located 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 a motion picture of the experimental proppant particles at an interval time Δt during the backflow process, and M motion pictures of the proppant particles in the backflow experiment are obtained.

[0116] The positions where the CCD camera 7 obtains the flow field images of the fracturing fluid in the fracture during the flowback process and the experimental proppant particle movement images are the flow fields at the fracture mouth and the front end of the fracture. The selected positions can reflect different flow field characteristics.

[0117] For example, obtaining an image of the sand bank morphology during simulated flowback includes 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 morphology of the return sand bank 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 reversely pump the gel breaking liquid through the circulating pumping system to flush the proppant filling layer, and the displacement is set to 0.45L / min.

[0120] S6. The computer quantitatively describes the proppant flowback according to 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 a fracturing fluid velocity field and a proppant velocity field, such as Figure 6 As shown, the following steps are included:

[0122] S611. Analyze and calculate the properties (exposure value) and pixel group characteristics (exposure area size) of each pixel point 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 between experimental proppant particle pixels and tracer particle pixels.

[0123] S612, using the PIV algorithm, the coordinates of the tracer particle pixels in N fracturing fluid flow field images and N flowback fluid flow field images are calculated to obtain the fracturing fluid velocity field, such as Fig.10 shown.

[0124] S613, using the PTV algorithm, the coordinates of the proppant pixel points in M ​​fracturing experiment proppant particle motion pictures and M flowback experiment proppant particle motion pictures are calculated to obtain the proppant velocity field, such as Fig.11 shown.

[0125] S62, quantitatively describe the change process of the sand bank and determine the sand bank profile according to the first image set and the second image set, and characterize the proppant flowback capacity under different working conditions through the sand bank profile, such as Figure 7 , Figure 8 and Fig. 9 As shown, the following steps are included:

[0126] S621. Quantitatively describe the sand bank change process based on multiple fracturing sand bank morphology pictures and multiple flowback sand bank morphology pictures.

[0127] S622. Multiple fracturing sand bank morphology images and multiple flowback sand bank morphology images are spliced ​​together to obtain a sand bank contour, and the sand bank contour is used to characterize the proppant flowback capacity under different working conditions.

[0128] S63. Determine the change value of the sand bank height within a certain period of time according to the first image set and the second image set, and characterize the scouring of the sand bank by the return fluid within a certain period of time through the change value of the sand bank height; and characterize the critical flow velocity of the return fluid that affects the sand bank height according to the relationship between the change of the sand bank height value and time.

[0129] For example, four CCD cameras 7 are used to respectively monitor the change in the height of the sand bank in an area of ​​197 mm × 171 mm around the simulated fracture device, thereby reflecting the scouring of the sand bank by the return liquid in a short period of time, and recording the change in the height of the sand bank over time, thereby characterizing the critical flow rate that affects the height of the sand bank.

[0130] S64, such as Fig.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, which characterizes 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, which characterizes the velocity change of the proppant particles during the backflow process.

[0131] S65, such as Fig.13 and Fig.14 As shown, according to 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, the embodiment of the present invention further provides a proppant fracture reflow physical simulation system, including a computing 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 degelling liquid 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 stable; 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 substitutions for some of the technical features therein; and these modifications or substitutions do not make the essence of the corresponding technical solutions 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 reflux, characterized in that: The following steps are involved: The experimental fluid displacement input into the simulated fracture device is determined according to the actual fluid displacement at the fracturing site; wherein the experimental fluid displacement includes the sand-carrying fluid displacement of the simulated fracturing process and the gel-breaking fluid displacement of the flowback process; The sand-carrying fluid is delivered to the simulated fracture device according to the determined displacement, 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; After the proppant filling layer in the fracture is stable, the degelling liquid is reversely injected into the simulated fracture device at a determined displacement to flush the proppant filling layer, simulate the migration of the fracturing fluid and the experimental proppant during the flowback process, and obtain a second image set of the migration of the fracturing fluid and the experimental proppant during the flowback process; The proppant flowback is quantitatively described based on the first image set and the second image set.

2. The method for physical simulation of proppant reflux in cracks according to claim 1, characterized in that: The experimental fluid displacement input into the simulated fracture device is determined according to the actual fluid displacement at the fracturing site, including 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 method for physical simulation of proppant reflux in cracks according to claim 1, characterized in that: The following steps are also included: The tracer particles are mixed with the experimental proppant and the fracturing fluid to obtain the sand-carrying fluid.

4. The method for physical simulation of proppant reflux in cracks according to claim 3, characterized in that: The tracer particles are mixed with the experimental proppant and the fracturing fluid to obtain the sand-carrying fluid, including the following steps: The fracturing fluid is transported to a sand mixing tank, and at the same time, 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 at the same time, 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.

5. The method for physical simulation of proppant reflux in cracks according to claim 3, characterized in that: The preparation of tracer particles includes the following steps: The proppant particles are cleaned and dried, and then the proppant particles are 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.

6. The method for physical simulation of proppant reflux in cracks according to claim 3, characterized in that: The first image set includes images of the fracturing fluid flow field in the fracture during the simulated fracturing process, images of experimental proppant particle movement, and images of the sand bank morphology during the experimental proppant settling process in the simulated fracture; The second image set includes images of the fracturing fluid flow field in the fracture during simulated flowback, experimental proppant particle movement images, and sand bank morphology images.

7. The method for physical simulation of proppant reflux in cracks according to claim 6, characterized in that: Obtaining a picture of the flow field of the fracturing fluid in the 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 liquid 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. The CCD camera is used to obtain N fracturing fluid flow field images at intervals of Δt, or the CCD camera is used to obtain N flow field images of the flowback fluid at intervals of Δt.

8. The method for physical simulation of proppant reflux in cracks according to claim 6, characterized in that: Acquiring an experimental proppant particle motion image 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 irradiates the simulated crack device from top to bottom; On the other side of the simulated fracture device, a CCD camera is used to obtain experimental proppant particle movement images at intervals Δt during the fracturing process, and M fracturing experimental proppant particle movement images are obtained. Alternatively, a CCD camera is used to obtain experimental proppant particle movement images at intervals Δt during the flowback process on the other side of the simulated fracture device, and M flowback experimental proppant particle movement images are obtained.

9. The method for physical simulation of proppant reflux in cracks according to claim 6, characterized in that: The following steps are involved in obtaining images of the sand bank morphology during proppant settling in simulated fractures: After stopping the pump, the system 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 morphology of the sand bank around the simulated fracture device, and multiple pictures of the morphology of the fracturing sand bank were obtained.

10. The method for physical simulation of proppant reflux in cracks according to claim 6, 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 morphology of the sand bank around the simulated fracture device, and multiple pictures of the morphology of the flowback sand bank were obtained.

11. The method for physical simulation of proppant reflux in cracks according to any one of claims 6 to 10, characterized in that: The method of quantitatively describing proppant flowback based on the first image set and the second image set comprises 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; According to the first image set and the second image set, a quantitative description of the sand bank change process is performed and a sand bank profile is determined, and the proppant flowback capacity under different working conditions is characterized by the sand bank profile; Based on the fracturing fluid flow field, the angle velocity and number of tracer particles are extracted through the position information and time difference of tracer particles to characterize the velocity change of fracturing fluid during the flowback process; based on the proppant velocity field, the angle, velocity and number of experimental proppant particles are extracted through the position information and time difference of experimental proppant particles to characterize the velocity change of proppant particles during the flowback process; According to 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.

12. The method for physical simulation of proppant reflux in cracks according to claim 11, 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 comprises 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 the experimental proppant particle pixels from the 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 pictures and M flowback experiment proppant particle motion pictures using the PTV algorithm.

13. The method for physical simulation of proppant reflux in cracks according to claim 4, 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.

14. A proppant seam internal reflux physical simulation system, characterized in that: include: A 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; wherein the experimental fluid displacement includes the sand-carrying fluid displacement of the simulated fracturing process and the gel-breaking fluid displacement of the flowback process; A 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 fracturing fluid and experimental proppant migration during simulated fracturing; The first liquid supply pump is used to reversely inject the debonding liquid into the simulated fracture device at a 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 flowback process; The CCD camera is also used to obtain a second set of images of the migration of the fracturing fluid and experimental proppant during the flowback process; The computer is used for quantitatively describing the proppant flowback based on the first image set and the second image set.

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