CN109446706B - Method for determining laying form of pulse fiber sand fracturing proppant cluster - Google Patents
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Abstract
The invention discloses a method for determining the laying form of pulse fiber sand fracturing proppant clusters, which comprises the following steps: 1) determining the geometrical structure parameters of the hydraulic fracture; 2) measuring rheological parameters of the fracturing fluid and the fiber fracturing fluid, and correcting to obtain the rheological parameters of the mixed liquid of the fracturing fluid, the fibers and the proppant; 3) establishing a hydraulic fracture geometric model, outputting a data file in an iges format, performing flow field calculation domain grid division, and outputting a grid file in an msh format; 4) respectively treating fracturing fluid and a mixed solution of the fracturing fluid, fiber and a propping agent which are alternately injected by pulses as two continuous fluid phases, and establishing a CFD model with the two phases flowing in a fracture; 5) simulating the flow of fracturing fluid and mixed liquid pulse injected into the hydraulic fracture to obtain the laying shape of the proppant cluster. The method can quantitatively describe the laying form of the proppant clusters in the fracture, and provides technical support and theoretical basis for researching the yield increase mechanism of pulse fiber sand fracturing and optimizing construction parameters.
Description
Technical Field
The invention relates to the field of oil and gas field exploration and development, in particular to a method for determining a proppant laying form of a hydraulic fracturing fracture in a yield-increasing transformation process of an oil and gas reservoir.
Background
With the depth of oil and gas field exploration and development, the development of low-permeability and ultra-low-permeability oil and gas resources plays an increasingly important role in the China petroleum industry and becomes a main battlefield for the future China petroleum exploration and development. The low-permeability reservoir and the ultra-low permeability reservoir have poor physical properties, are difficult to put into production naturally, and can be economically developed only by hydraulic fracturing.
Through decades of development of the traditional hydraulic fracturing technology, the traditional hydraulic fracturing technology is improved in the aspects of fracturing materials including a propping agent, fracturing fluid and the like and fracturing construction technology, the propping agent can effectively prop the hydraulic fracture, and the flow conductivity of the propped fracture is optimized to the greatest extent after the construction is finished. In order to break through the limitation of the traditional hydraulic fracturing technology on the diversion capacity of the propped Fracture, in recent years, various scientific research institutions and oil companies successively put forward concepts of channel fracturing technology (M R Gillard, O ome, P R Hosein, et al. a New Approach to Generating Fracture Conductivity [ C ]. SPE annular Technical Conference and inhibition, Florence, Italy,2010.SPE135034), high diversion fracturing technology (temperature celebration, romance, liufeng, etc.. a fracturing process [ P ] CN201310279118,2013 for realizing ultrahigh diversion capacity, pulse fiber sand fracturing technology (southwestern oil and gas branch of china). The essence of the technologies is that pure fracturing fluid and fracturing fluid containing proppant and fibers are injected in a pulse mode, so that columnar support clusters are formed in the fracture, the flowing mode in the closed fracture is converted from seepage among proppant particles into pipe flow among the proppant clusters, and the flow conductivity in the closed fracture is greatly improved. In the technology, the fibers are wound to form a net structure, so that the integrity of the proppant cluster column can be maintained in the process of conveying and sedimentation, the realization of the columnar supporting form in the final fracture is ensured, and the method has very important significance.
With respect to the Effect of fiber addition on fracturing fluid performance, researchers at home and abroad (J Guo, J Ma, Z ZHao, et.. Effect of fiber on the hydraulic property of free flowing [ J ]. Journal of Natural Gas Science & Engineering,2015,23(21):356-362) studied the rheological properties of the mixture, such as viscous modulus and elastic modulus, to indicate that fiber addition increases viscous and elastic modulus, and that the increase in viscous and elastic modulus of the mixture increases with increasing fiber length and concentration. Also, researchers (R Elgaddafi, RAhmed, M George, et al. Settling Behavior of biological Particles in Fiber-containing Drilling Fluids [ J ]. Journal of Petroleum Science & Engineering,2012,84:20-28) studied the Settling Behavior of single Particles of proppant after Fiber addition and found that the rate of particle Settling was significantly reduced and affected by Fiber concentration and length. Studies have been conducted by researchers on static settling of proppant clusters in plates (A VMedvedev, C Kraemer, A Pena, et al. on the Mechanisms of Channel Fracturing [ C ]. SPE Hydraulic Fracturing Technology Conference, the Woodlands, Texas, USA,2013.SPE 163836) and have found that fibers play an important role in maintaining the integrity of the proppant cluster settling process. Regarding the research on the laying form of fiber and proppant clusters in the fracture, schlumberger originally proposed a conceptual diagram of the laying form and an experimental result diagram of the distribution of two proppant clusters in a flat plate experiment, and wenqing et al (wenqing zhi, gaojinjian, yellow wave, etc.. research on the distribution rule of channel fracturing sand bank [ J ]. special oil and gas reservoirs, 2014,21(4):89-92) proposed that the laying form of the proppant in the fracture is represented by the channel rate, and other researchers simplified the proppant clusters into a columnar supporting structure to conduct research on the aspects of flow conductivity and the like. In general, there is no clear calculation method for the distribution of the proppant clusters in the fracture.
So far, the fracturing technology is widely applied to the traditional oil-gas reservoir and the compact oil-gas reservoir at home and abroad, and also produces a certain yield increasing effect, but is still greatly different from the theoretical expectation. At present, although many researchers and field engineers have understood the main ideas of this type of technique, the laying characteristics of the proppant clusters in the fracture are not clear, and it is difficult to efficiently optimize the construction parameters of the technique.
Disclosure of Invention
The invention aims to provide a method for determining the laying form of a pulse fiber sand fracturing propping agent cluster, which overcomes the defects that the laying form of the propping agent cluster in a fracture cannot be quantitatively described due to the complex flow rule of the traditional hydraulic fracturing technology characterized by injecting fracturing fluid, fiber and propping agent mixed liquid by pulse, can intuitively and simply quantitatively describe the laying form of the propping agent cluster in the fracture, and provides technical support and theoretical basis for researching the yield-increasing mechanism of pulse fiber sand fracturing and optimizing construction parameters.
In order to achieve the above technical objects, the present invention provides the following technical solutions.
The method comprises the steps of treating a fracturing fluid, a fracturing fluid + fiber + proppant three-phase mixed solution which are injected in a pulse mode in fracturing construction as a power law fluid phase independently, measuring physical properties (density rho) and rheological parameters (rheological index n and consistency coefficient K) of the fracturing fluid and the fiber-containing fracturing fluid in an experiment, and further correcting by adopting an empirical model to obtain the rheological parameters of the three-substance mixed fluid of the fracturing fluid + fiber + proppant. And establishing a control equation system of two-phase flow in the crack, and dispersing by using FVM (Finite Volume Method) to obtain an algebraic equation system. And (3) establishing a crack geometric model by adopting CAD software, discretely obtaining a grid model in the grid software, and specifying boundary conditions of a speed inlet and a pressure outlet. And introducing the grid model into CFD (Computational Fluid dynamics) software, establishing a two-phase Fluid medium of fracturing Fluid and mixed liquid according to rheological parameters, and performing two-phase flow calculation setting according to a setting method of a VOF (Volume of Fluid) model. And changing the inlet boundary fluid speed and the volume fraction of each phase according to a construction pumping program designed by fracturing construction to perform simulation calculation, and displaying the mixture phase distribution in the fracture after the calculation is finished to obtain the laying form of the proppant clusters.
A method for determining the laying form of pulse fiber sand fracturing proppant clusters sequentially comprises the following steps:
1) determining geometrical parameters of hydraulic fractures
The construction parameter design is carried out according to geological characteristics of a construction well, reservoir rock mechanical parameters (Poisson ratio sigma and Young modulus E) and logging information, the number n of perforation holes, perforation positions, hole opening sizes (diameter D) and intervals are given, the type and concentration of a needed fracturing fluid and a propping agent and the type and concentration of fibers are determined, a pulse fiber sand fracturing pump injection program (comprising discharge Q and pulse time delta T) is designed, and fracture design software (such as FracpropT) is adopted to simulate and obtain the height H, the width W and the length L of a constructed hydraulic fracture (K Tsai, E Fonseca, S degalesan. advanced composite model of fracturing in water reactors for fracture production [ J ]. SPE 151607,2012).
2) Determining rheological parameters of pulse injection fluid, namely independently treating the three-phase mixed liquid of fracturing fluid, fiber and proppant which are injected by pulse as power law fluid phase, measuring the rheological parameters of the fracturing fluid and the fiber fracturing fluid in an experiment, and correcting to obtain the rheological parameters of the three-phase mixed liquid of the fracturing fluid, the fiber and the proppant
(1) Testing and fitting to obtain rheological parameters of the fracturing fluid
Preparing fracturing fluid required by construction according to the requirements of field design, heating the fracturing fluid to the required temperature in a water bath, and testing the fracturing fluid at different shear rates by adopting a Haake star rotor rheometer
The shear stress tau is obtained by using the fracturing fluid as a power law type non-Newtonian fluid, and the rheological equation is as follows (Chen Xiao Yu et al. engineering hydrodynamics [ M)]Beijing: petroleum industry publishers, 2015: 318):
wherein tau is shear stress, Pa;
shear rate of fracturing fluid, s-1；n1The fracturing fluid rheological index is dimensionless; k1Is the consistency factor, Pa.sn。
Fitting the obtained experimental data to obtain K1And n1Apparent viscosity of fracturing fluid
(2) Testing and correcting the rheological parameters of the mixed solution of the fracturing fluid, the fiber and the proppant
And adding the fiber amount determined by the fracturing construction design into the fracturing fluid, and then fully stirring and mixing to obtain the fiber fracturing fluid. Heating the mixture to a desired temperature in a water bath, and testing the shear rate of the mixture by using a Haake Star rotor rheometer
Fitting the shear stress tau by using a power law type non-Newtonian fluid rheological equation to obtain a consistency coefficient K of the fiber fracturing fluid2And rheological index n2The rheological equation is as follows:
apparent viscosity of fiber fracturing fluid
The required amount of the propping agent is calculated according to the concentration of the propping agent designed according to the construction parameters, the propping agent and the fiber fracturing fluid are fully stirred and mixed to obtain a mixed solution (mixed solution for short) of the fracturing fluid, the fiber and the propping agent, and the density of the mixed solution of the three substances is as follows:
where rhomIs the density of the mixed solution, kg/m3(ii) a i ═ 1,2,3 represent fracturing fluids, fibers and proppants, respectively, ρiAnd αiRespectively the apparent density and the occupied volume fraction of the three.
The mixed solution is also described by a power law type non-Newtonian fluid rheological equation, and the consistency coefficient is set to be K3Rheological index of n3Then apparent viscosity μ before and after proppant additionlAnd muslThe ratio of (A) can be corrected according to the following relationship (D.Eskin, M.J.Miller.A model of non-Newtonian slurry flow in a fraction [ J.].Powder Technology,2008,182:313–322)：
Formula (III) αpIs the volume fraction of the added proppant, and is dimensionless;
and
respectively, the apparent viscosity of the mixed liquid and the fiber fracturing fluid is Pa.s.
The proppant concentration correction factor, which can be derived from the above relationship, is:
at a known fiber fracturing fluid thickness coefficient K2Rheological index n2And a concentration correction factor f (α) after proppant additionp) Under the condition of (1), the rheological property correction of the mixed liquid of the fracturing fluid, the fiber and the proppant can respectively and independently correct the consistency coefficient K3Or the rheological index n3It is also possible to correct the consistency factor K at the same time3And rheological index n3. In the invention, only the consistency coefficient K is corrected3By letting the rheological index n be3＝n2Coefficient of thickness K3＝f(αp)K2(D.Eskin,M.J.Miller.A model of non-Newtonian slurry flow in a fracture[J]Powder Technology,2008,182: 313-322). Therefore, the rheological equation of the fracturing fluid + fiber + proppant mixture is:
3) establishing hydraulic fracture geometry and grid model
According to the geometric structure seam width w, the seam length L and the seam height H of the hydraulic fracture determined in the step 1), and the design given perforation position, the orifice size and the interval, a geometric model of the hydraulic fracture is established by computer aided design software (such as AutoCAD and ProE) or CFD pretreatment software (such as Gambit and ICEM), and a data file in an iges format is output. In order to enable later numerical calculation to have better efficiency and convergence, geometric features with special shapes, such as crack tips and the like, are simplified (rounded or truncated) in the modeling process.
And importing the iges format data file into meshing software (such as Gambit, ICEM and the like) to perform flow field computational domain meshing. In order to obtain higher calculation accuracy, boundary layer grids can be added on the wall surfaces of the cracks during division, and then the rest space is divided into hexahedral grids (ANSYS Fluent masking User's Guide15.0.ANSYS, inc., 2013). After the grid division is completed, the quality of the grid is detected by using the function of software, and the requirements that the negative volume of the grid cannot exist, the ratio (edge ratio) of the longest edge to the shortest edge of the grid is not more than 5, and the distortion degree (Equisize Skew) of the grid is not more than 0.4 are required. Once the above condition is not satisfied, the meshing needs to be performed again until the above condition is satisfied.
And (3) defining the perforation position of the hydraulic fracture geometric model as a speed inlet boundary and the fracture tail end as a pressure outlet boundary condition in meshing software, and outputting a mesh file in an msh format.
4) Establishment of CFD (computational fluid dynamics) model based on two-phase flow in crack of VOF (Voltage induced fluorescence)
The method comprises the steps of treating a fracturing Fluid and a fracturing Fluid + fiber + proppant mixed solution which are alternately injected by pulses as two independent continuous Fluid phases of a fracturing liquid phase and a mixed liquid phase, and establishing a CFD model with two phases flowing in a fracture based on VOF (Volume of Fluid).
The VOF model only requires solving one flow control equation set (s.s.lafmejani, a.c.olesen, s.k) in the simulation solving process by averaging the two fluid phases.
VOF modelling of gas-liquidflow in PEM water electrolysis cell micro-channels[J]International journal of hydrogen energy,2017,42: 16333-. Assuming that the densities, volume fractions and apparent viscosities of a fracturing liquid phase and a mixed liquid phase in the fracture are respectively rhof、βf、μfAnd ρm、βm、μslWherein βf+βmWhen 1, the control equation set is as follows:
the average density and viscosity of the flow field fluid in the fracture is:
volume fraction transport equation for two phases:
wherein i ═ f, m; f, m respectively represents a fracturing liquid phase and a mixed liquid phase; v is the two-phase shared velocity vector, m/s.
Two-phase shared momentum equation:
wherein p is pressure, Pa; g is the acceleration of gravity, m/s2(ii) a F is the volume force converted from the surface tension of the two-phase interface, N/m3。
Because both phases have higher viscosity and present a laminar flow state when flowing in a crack, the control equations (7) - (9) of the VOF model can be dispersed by adopting a finite volume method to obtain an algebraic equation set (bolt (version 2) [ M ] numerical heat transfer, Sian: Sian university of transportation, 2001: 207) and further can be directly solved on a commercial Computational Fluid Dynamics (CFD) software platform.
5) Simulating the flow of fracturing fluid and mixed liquid pulse injected into hydraulic fracture to obtain the laying form of proppant cluster
And reading the msh format grid file by using CFD software (such as Fluent, CFX and the like), and sequentially setting according to a setting method of the software on the VOF multiphase flow simulation flow field and physical property parameters (ANSYS Fluent User's guide15.0.ANSYS, Inc.2013: 1273-.
The method comprises the steps of ① grid inspection, ② selection of a VOF multiphase flow model, ③ flow state selection of laminar flow, ④ definition of two power law type non-Newtonian fluids, and consistency coefficients K of input fracturing fluid and mixed fluid of the two power law type non-Newtonian fluids1、K3And rheological index n1、n3And fracturing fluid density ρfAnd density of mixed liquid ρm⑤ setting the fracturing fluid in two phases as main phase and the mixed fluid as second phase, ⑥ setting the pressure outlet boundary relative pressure as atmospheric pressure, ⑦ selecting discrete format higher than second order and less than 10-4⑧ and hooking the inter-phase contribution.
According to the designed pumping program, the mixed liquid inlet speed is respectively input into the software at the starting time point of each pulse time period
And volume fraction βm(fracturing fluid stage β during pulse injectionmMixed liquid segment β when equal to 0m1), setting a simulation time step dt to carry out iterative solution on the flow equation, and calculating the speed, the pressure and the two-phase concentration distribution value in the flow field at any moment by software.
After the designed pump injection program simulation calculation is completed, a distribution cloud picture of a mixed liquid phase in a flow field is displayed (the process can be carried out in a post-processing module of CFD software, and can also be carried out in post-processing software such as TECPLOT and the like), and the distribution form of the proppant clusters at any position in the fracture is obtained.
Drawings
FIG. 1 is a schematic view of a flat panel crack.
FIG. 2 is a graph of a slab fracture grid and boundary conditions.
FIG. 3 is a distribution plot of proppant cluster placement contours in a slab fracture.
Detailed Description
The invention is further illustrated by the following figures and examples.
Examples
A large-scale flat plate crack physical model simulating proppant conveying in a laboratory is used as a research object (Guojianchun and the like, experimental research on flow channel form influence factors in channel fracturing [ J ]. oil and gas geology and recovery ratio, 2017,24 (5:): 115-. The inlet of the left end of the flat plate is fed with liquid through 3 evenly distributed perforation orifices (the direction of the fluid is from left to right), and the top of the right end is provided with an outlet for discharging the liquid. The method is implemented by taking the flat fracture as an object, and the laying characteristics of the proppant clusters in the fracture are obtained.
A geometric model shown in figure 1 is built in CFD pretreatment software Gambit according to the size of the flat plate crack, 3 small rectangles uniformly distributed at the left end of the model are orifices for simulating perforation and are inlets of injected fluid objects, the height dimension of the crack is 50.0mm, and the width of the crack is the same as that of the flat plate crack. The upper side of the right end is an outlet boundary, the outlet direction is upward, the size of the outlet boundary in the slit length direction is 200mm, and the width of the outlet boundary is the same as that of a flat plate slit.
The geometric model in fig. 1 is further subjected to grid division in the Gambit software, the grid cells are hexahedral cells, the length of the grid cells in the slit width direction is 2mm, the slit height direction is 5mm, and the slit length direction is 10mm, so that a grid map as shown in fig. 2 is obtained, and the total number of grids is 73140. And setting a speed inlet boundary at a left side perforation and a pressure outlet boundary at a right upper side in the graph, and outputting the grid file.
In order to carry out the present invention, it is necessary to determine physical parameters and boundary condition parameters of the fluid. Combining construction parameters of a fracturing construction site of a Tibet tight sandstone gas well, selecting a hydroxypropyl guar gum crosslinking fracturing fluid with the fluid at 60 ℃, wherein the density value is 1000kg/m3Determining the consistency coefficient K of the fracturing fluid according to experimental data15.4, rheological index n10.45. The fiber added into the fiber fracturing fluid is polyester fiber commonly used in field construction, and the fiber density is 1300kg/m3The length was 6mm and the volume fraction (mass of fiber/volume of liquid) was taken to be 0.2%. The consistency coefficient K of the fiber fracturing fluid is obtained through testing26.2, rheological index n2＝0.42。
The proppant 20/40-mesh quartz sand is added into the fiber fracturing fluid, and the apparent density of the proppant is 2650kg/m3The volume concentration thereof was 15%. Therefore, the density of the fiber-fracturing fluid-proppant mixture is calculated according to formula (3) as:
and further obtaining the ratio of the apparent viscosity of the mixed solution after the proppant is added to the apparent viscosity of the fiber fracturing fluid before the proppant is added according to the formula (4):
and (3) importing the grid file into a 3D module of computational fluid dynamics software FLUENT 14.5, checking the quality of the grid without errors, and setting the length unit as mm. Creating two power law type non-Newtonian fluids (fracturing fluid and mixed liquid), setting the density and rheological parameters of the fluids according to the test and calculation results, selecting a VOF model in multiphase flow and selecting the flow state as laminar flow, selecting the fracturing fluid as phase 1 and the mixed liquid as phase 2, and simultaneously selecting the interaction items between the phases. The solution method selects the SIMPLE algorithm to realize the coupling of pressure and speed, the gradient term in the space dispersion selects the Least Square Cell Based, and the pressure term selects PRESTO! The momentum item is selected from Second Order Upwind, and the volume fraction item is selected from Geo-Reconstruct format.
Firstly, performing steady state calculation, setting an inlet in the graph 2 as a pure fracturing liquid phase, and setting the speed to be 0.5 m/s; the outlet is a pressure outlet and the relative pressure is set to 0. And after the steady-state calculation is completed, the flow field is filled with fracturing fluid in a flowing state. And setting the volume fraction of the phase 2 in the monitoring flat plate crack in the calculation process, switching the calculation mode into an unsteady state, dispersing the unsteady state item by adopting Second order Implict, and carrying out simulation according to the inlet boundary conditions given in the following table 1. As shown in the table, the pulse period 2 Δ T was 5.0s when different fluids were injected at 2.5s intervals.
TABLE 1 Inlet boundary conditions for pulse injection
| Number of stages | 1 | 2 | 3 | 4 | 5 | …… | 23 | 24 |
| Time(s) | 0-2.5 | 2.5-5.0 | 5.0-7.5 | 7.5-10.0 | 10.0-12.5 | …… | 55.0-57.5 | 57.5-60.0 |
| Phase 1 volume fraction | 0.0 | 1.0 | 0.0 | 1.0 | 0.0 | …… | 1.0 | 1.0 |
| Phase 2 volume fraction | 1.0 | 0.0 | 1.0 | 0.0 | 1.0 | …… | 0.0 | 0.0 |
| Speed (m/s) | 0.5 | 0.5 | 0.5 | 0.5 | 0.5 | …… | 0.5 | 0.5 |
The calculation time step dt is 0.1s, and after 12 cycles (60s) of calculation, the distribution of the phase 2 in the flow field is shown, that is, the distribution of the proppant clusters in the fracture is the laying shape, as shown in fig. 3.
Figure 3 shows the contour distribution plots of proppant-containing mixed liquor (referred to as "proppant clusters") and pure fracturing fluid (referred to as "channels") within the fracture, i.e., the zone bounded by contours with mixed liquor volume fraction above 0.95 is the proppant cluster distribution zone and the zone bounded by contours with mixed liquor volume fraction below 0.05 is the channel distribution zone. As can be seen from the figure, the proppant cluster column-segment plug distribution characteristics are obvious in the area near the inlet on the left side, although the proppant clusters from different perforation holes are connected with each other, the proppant clusters are basically one segment of the proppant cluster and one segment of the channel, and the fracturing fluid pushes the mixed liquor to move forwards.
As the position continues to the right, i.e., toward the mid-posterior region of the fracture, it can be seen that this slug feature is progressively broken, as the proppant clusters become interconnected in the direction of flow, while the channels are progressively interconnected in the direction of flow. This bedding characteristic is due to the greater density of proppant clusters, which tend to settle in the direction of flow. In addition, the apparent viscosity of the pure fracturing fluid is smaller than that of the mixed liquid, the resistance in the flowing process is smaller, the fingering phenomenon can be generated in the flowing process, and the proppant clusters are broken through, so that a continuous flowing channel is formed. Comparing the experimental results in the related documents (Guojianchun et al. experimental study on the morphological influence factors of the flow channel in channel fracturing [ J ]. oil and gas geology and recovery ratio, 2017,24 (5:): 115-.
Claims (5)
1. A method for determining the laying form of pulse fiber sand fracturing proppant clusters sequentially comprises the following steps:
1) determining the geometric structure parameters of the hydraulic fracture, and obtaining the height H, width W and length L of the constructed hydraulic fracture;
2) independently treating the fracturing fluid, the mixed liquid of the fracturing fluid, the fibers and the proppant which are injected by the pulse as power law fluid phases, measuring rheological parameters of the fracturing fluid and the fiber fracturing fluid in an experiment, and correcting to obtain the rheological parameters of the mixed liquid of the fracturing fluid, the fibers and the proppant;
3) establishing a geometric model of the hydraulic fracture according to the fracture width w, the fracture length L and the fracture height H of the hydraulic fracture determined in the step 1) and the design of a given perforation position, orifice size and interval, and outputting a data file in an iges format; importing the iges format data file into meshing software to perform flow field computational domain meshing, defining the perforation position of a hydraulic fracture geometric model as a speed inlet boundary, defining the fracture tail end as a pressure outlet boundary condition, and outputting a msh format mesh file;
4) respectively treating fracturing fluid and fracturing fluid + fiber + proppant mixed solution which are alternately injected by pulses as two independent continuous fluid phases of a fracturing liquid phase and a mixed liquid phase, and establishing a CFD model with two phases flowing in a fracture, wherein the process is as follows:
assuming that the densities, volume fractions and apparent viscosities of a fracturing liquid phase and a mixed liquid phase in the fracture are respectively rhof、βf、μfAnd ρm、βm、μslWherein βf+βmThe average density and viscosity of the flow field fluid in the fracture is 1:
volume fraction transport equation for two phases:
wherein i ═ f, m; f, m respectively represents a fracturing liquid phase and a mixed liquid phase; v is a two-phase shared velocity vector, m/s;
two-phase shared momentum equation:
wherein p is pressure, Pa; g is the acceleration of gravity, m/s2(ii) a F is the volume force converted from the surface tension of the two-phase interface, N/m3(ii) a Adopting a finite volume method to carry out dispersion, obtaining an algebraic equation set and solving the algebraic equation set;
5) simulating the flow of fracturing fluid and mixed liquid pulse injected into the hydraulic fracture to obtain the laying form of the proppant cluster, namely reading in msh-format grid files by CFD software and sequentially setting by combining physical property parameters; inputting mixed liquor inlet speed v in software at the beginning time point of each pulse time periodinAnd volume fraction βmAnd setting a simulation time step dt to carry out iterative solution on a flow equation to obtain the velocity, pressure and two-phase concentration distribution values in the flow field at any moment, and obtaining the distribution form of the proppant clusters at any position in the fracture by displaying a distribution cloud chart of the mixed liquid phase in the flow field.
2. The method for determining the pulse fiber sand fracturing proppant cluster laying morphology as claimed in claim 1, wherein the step 1) is that: the construction parameter design is carried out according to the geological characteristics of a construction well, the rock mechanical parameters of a reservoir stratum and well logging information, the number n of perforation, the perforation position, the orifice size and the interval are given, the type and the concentration of the needed fracturing fluid and propping agent and the type and the concentration of fiber are determined, a pulse fiber sand fracturing pump injection program is designed, and fracture height H, fracture width W and fracture length L of the constructed hydraulic fracture are obtained through simulation of fracturing design software.
3. The method for determining the pulse fiber sand fracturing proppant cluster placement morphology as claimed in claim 1, wherein said step 2) comprises:
the fracturing fluid was prepared, heated to the desired temperature in a water bath, and tested at various shear rates
The following shear stress tau is taken as a power law type non-Newtonian fluid of the fracturing fluid, and the rheological equation is as follows:
fitting the experimental data to obtain a fracturing fluid viscosity coefficient K1Rheological index n1And apparent viscosity
Adding fiber into the fracturing fluid, mixing to obtain fiber fracturing fluid, and obtaining the consistency coefficient K of the fiber fracturing fluid by the same method2Rheological index n2And apparent viscosity
Mixing a propping agent and a fiber fracturing fluid to obtain a fracturing fluid + fiber + propping agent mixed solution, wherein the consistency coefficient of the mixed solution is K3Rheological index of n3The apparent viscosity μ before and after addition of the proppant was determined by the following formulalAnd muslRatio f (α)p)：
Formula (III) αpIs the volume fraction of the added proppant, and is dimensionless;
and
respectively obtaining apparent viscosity of mixed liquor and fiber fracturing fluid, Pa.s;
let n be3＝n2The consistency coefficient K of the mixed solution3＝f(αp)K2。
4. The method for determining the laying form of the pulse fiber sand fracturing propping agent cluster as claimed in claim 1, wherein in step 3), a geometric model of a hydraulic fracture is established, a data file in an iges format is output, the data file in the iges format is introduced into meshing software for carrying out flow field computational domain meshing, boundary layer meshes are added on the wall surface of the fracture during the meshing, the rest space is divided by hexahedral meshes, the quality of the meshes is detected after the meshing is finished, the negative volume of the meshes cannot exist, the ratio of the longest side to the shortest side of the meshes is not more than 5, and the mesh skewness is not more than 0.4.
5. The method for determining the paving form of the pulse fiber sand fracturing propping agent mass as claimed in claim 1, wherein in the step 5), msh format grid files are read in by CFD software, and physical property parameters are sequentially set, and the steps comprise ① grid check, ② selection of VOF multiphase flow model, ③ flow state selection of laminar flow, ④ definition of two power law type non-Newtonian fluids, and the consistency coefficients K of the fracturing fluid and the mixed fluid are respectively input for the fluids1、K3And rheological index n1、n3And fracturing fluid density ρfAnd density of mixed liquid ρm⑤ setting the fracturing fluid in two phases as main phase and the mixed fluid as second phase, ⑥ setting the pressure outlet boundary relative pressure as atmospheric pressure, ⑦ selecting discrete format higher than second order and less than 10-4⑧ and hooking the inter-phase contribution.
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