A method for in-situ synthesizing mullite-silicon carbide composite proppant by microwave sintering

Abstract

The invention discloses a method for in-situ synthesizing mullite-silicon carbide composite proppants by microwave sintering, belonging to the technical field of oil and gas field development. The steps are as follows: low-grade bauxite, SiC, CaO, and MnO2 are taken and mixed and ball-milled to obtain granulated powder materials; the granulated powder materials are granulated into balls, and a binder is sprayed during the granulation process to obtain spherical proppant green bodies; the optimal sintering position of the sample is calculated, and the proppant green bodies are in-situ sintered by microwave radiation under air conditions, and after cooling, screening is carried out to obtain mullite-silicon carbide composite ceramic proppants. The proppants prepared by the invention have a shorter sintering time and higher strength, and use low-grade bauxite as a raw material, which is beneficial to improving the resource utilization efficiency and reducing the cost of fracturing construction.

Description

Technical Field

[0001] The present invention belongs to the technical field of oil and gas field development, and particularly relates to a method for in-situ synthesizing mullite-silicon carbide composite proppants by microwave sintering. Background Art

[0002] Hydraulic sand fracturing is an important stimulation measure for low-permeability and tight oil and gas reservoirs. Proppants are carried by fracturing fluid into fractures, laid to form a proppant filling layer, and then form a proppant-filled artificial fracture with diversion ability, that is, a sand-filled fracture. The diversion ability of the sand-filled fracture is a key factor affecting the stimulation effect after fracturing, which is closely related to the migration and laying characteristics of proppants in the fracture and the diversion characteristics of the proppant filling layer under the fracture closure pressure. Therefore, proppants are one of the key materials affecting the effect of hydraulic fracturing. Natural quartz sand and sintered ceramsite are two major mainstream proppant products widely used in China at present. Quartz sand begins to break after the formation closure stress exceeds 20 MPa and is suitable for shallow formations. For reservoirs with high closure stress, especially for the fracturing of current deep shale gas wells, ceramsite proppants are still mainly selected.

[0003] Currently, the widely used high-grade bauxite-based proppants are mainly composed of high-strength corundum phases, with high compressive strength of proppants, but large density and high production cost. Reducing the production cost of high-strength proppants and improving the economic benefits of fracturing construction have become an important issue. China has rich reserves of low-grade bauxite and low prices. Developing high-performance low-grade bauxite-based proppants based on such raw materials is beneficial to reducing the cost of fracturing construction. The low-grade bauxite-based proppants are mainly composed of mullite phases and glass phases, and their compositions are completely different from those of high-grade bauxite-based proppants. To improve the performance of low-grade bauxite-based proppants, new formulas and technologies suitable for preparing high-performance low-grade bauxite-based proppants need to be developed.

[0004] In industry, the high-temperature sintering and forming of ceramsite proppants are mainly achieved through tunnel kilns and rotary kilns, both of which are based on flue gas heating for calcination, with high energy consumption and long sintering time.

[0005] Patent application 201910735343.5 discloses a method for preparing low-density proppants using oil-based mud waste as raw materials. Proppants with a particle size of 0.212 - 0.425 mm are obtained by this method, and their apparent density is 2.3 g / cm 3 , but their compressive strength is only 52 MPa.

[0006] Patent application 201910905037.1 discloses a fracturing proppant sintered from oil-based drill cuttings pyrolysis residues and its preparation method. Proppants with a particle size of 0.212 - 0.425 mm are obtained by this method, and the compressive strength of the proppants reaches 86 MPa, but their apparent density is much greater than 2.5 g / cm 3 .

[0007] Patent Application 202010299091.9 discloses a high-strength and low-density ceramsite proppant and its preparation method. By this method, a ceramsite proppant with a conventional oxide phase is prepared. The apparent density of this proppant is 2.61 - 2.68 g / cm3, and the crushing rate is less than 5% under a closing pressure of 52 Mpa. Its product has a large density and low strength.

[0008] Therefore, providing a method for in-situ synthesizing mullite-silicon carbide composite proppant by microwave sintering is a technical problem that needs to be urgently solved by those skilled in the art. Summary of the Invention

[0009] In view of this, the present invention provides a method for in-situ synthesizing mullite-silicon carbide composite proppant by microwave sintering.

[0010] To achieve the above object, the present invention adopts the following technical solutions:

[0011] A method for in-situ synthesizing mullite-silicon carbide composite proppant by microwave sintering, characterized by comprising the following steps:

[0012] (1) Take low-grade bauxite, SiC, CaO, and MnO2, mix and ball-mill them to obtain granulating powder.

[0013] (2) Granulate the granulating powder into spheres, and spray a binder during the granulation process to obtain spherical proppant green bodies.

[0014] (3) Calculate the optimal sintering position of the sample, and use microwave radiation to in-situ sinter the proppant green body under air conditions. After cooling, screen to obtain mullite-silicon carbide composite ceramsite proppant.

[0015] In the mullite-silicon carbide composite proppant prepared by the present invention, under the induction of CaO and MnO2, A12O3 and SiO2 generate a structural framework of mullite whiskers bridging SiC particles.

[0016] Furthermore, the composition raw materials of the granulating powder in step (1) include: 80 - 100 parts of low-grade bauxite, 10 - 15 parts of SiC, 1 - 3 parts of CaO, and 1 - 4 parts of MnO2.

[0017] Even further, the mass fraction of alumina in the low-grade bauxite is less than 50%.

[0018] Furthermore, step (1) further includes: drying the material after ball-milling at 80°C - 110°C to remove moisture and then grinding it into a powder to obtain granulating powder with a powder particle size below 30 um.

[0019] Furthermore, the binder in step (2) is any one of carboxymethyl cellulose, polyvinyl alcohol, water glass, and water.

[0020] Further, the method for calculating the optimal sintering position of the specimen in step (3) is as follows: measure the real part and imaginary part of the dielectric constant, density, thermal conductivity, electrical conductivity, and specific heat capacity of the proppant green body as input parameters, establish a simulation model, and after finite element solution, select, through the temperature difference coefficient, the positions where the electromagnetic field distribution is the most uniform and the electromagnetic field strength gradient is relatively small among the three cross-sections passing through the center of the specimen as the suitable specimen sintering positions conducive to uniform heating.

[0021] Even further, the steps for establishing the simulation are as follows:

[0022] S1: Establish a finite element geometric physical model of the microwave heating furnace, and determine the waveguide position and model for electromagnetic emission according to the design of the microwave heating furnace; the waveguide is a rectangular waveguide.

[0023] S2: Establish a mathematical model of the electromagnetic field controlled by Maxwell's equations:

[0024]

[0025]

[0026]

[0027]

[0028] H represents the magnetic field strength, A / m; D represents the electric flux density, C / m 2 ; J represents the current density, A / m 2 ; E represents the electric field strength, V / m; B represents the magnetic flux density, Wb / m 2 ; ρ represents the charge density, C / m 3 .

[0029] S3: Establish a temperature field data model controlled by Fourier's heat conduction equation:

[0030]

[0031]

[0032] T represents the absolute temperature, K; C p represents the constant pressure heat capacity, J / (kg·K); (ρC p ) eff represents the effective volume heat capacity under constant pressure; keff represents the effective thermal conductivity, W / (m·K); q represents the conductive heat flux;

[0033] S4: Establish a multi-physics field equation for electromagnetic and heat transfer coupling:

[0034]

[0035] Q e is the electromagnetic power loss density, W / m 3 ; Re represents the operation of taking the real part of the complex quantity in the corresponding content, J is the current density, A / m 2 ; B represents the magnetic flux density, Wb / m 2 ; H is the magnetic field strength, A / m; the superscript * represents the conjugate. E represents the electric field strength, V / m; ω represents the angular frequency, rad / s;

[0036] S5: Use the finite element method to solve the above mathematical equations to obtain the electric field, magnetic field and temperature distributions of the proppant particles at different positions and powers; according to the temperature difference coefficient, optimize the sintering position of the proppant in the microwave equipment. The calculation method of the temperature difference coefficient is:

[0037]

[0038] COV temperature difference coefficient; Average temperature; T i represents the temperature at each point; n identifies the total number selected; i represents the i-th point.

[0039] Furthermore, in step (3), the microwave heating power is 200 - 1200 W, the sintering temperature is 1200 - 1600 °C, the heat preservation time is 30 - 120 min, and the average heating rate is 15 - 30 °C / min.

[0040] Even further, in step (3), the method of in-situ microwave radiation sintering is to first rapidly heat the sample to the target temperature with medium-high power, and then adjust to medium-low power for heat preservation for a predetermined time.

[0041] Even further, the medium-high power is 600 - 1200 W, and the medium-low power is 200 - 600 W.

[0042] The beneficial effects of the present invention are as follows:

[0043] 1) The microwave method adopted by the present invention is efficient and energy-saving. The coupling effect between the microwave and the material converts the electromagnetic energy of the microwave into heat energy, and the electromagnetic energy absorbed by the material is directly used for its own heating, with less energy loss and high utilization rate;

[0044] 2) The method of the present invention has a fast sintering speed and a short cycle. When the temperature of the dielectric material is higher than the critical temperature, the dielectric loss increases rapidly, and the coupling effect between the material and the microwave enhances, causing the temperature to rise rapidly. Microwave sintering can effectively reduce the activation energy required for the reaction, further reduce the sintering temperature, accelerate the sintering process, and shorten the sintering time.

[0045] 3) The method of the present invention is easy to control, green, clean, and has little pollution. The microwave sintering technology controls the rise and fall of temperature by controlling the incident power of the input device. Only the heated materials in the sealed furnace cavity can absorb microwaves to generate heat, and the furnace body hardly heats up, with relatively small thermal inertia. Even computerized automated control can be achieved, which is convenient and safe.

[0046] The proppant prepared by the present invention has a shorter sintering time and higher strength, and uses low-grade bauxite as the raw material, which is beneficial to improving the resource utilization efficiency and reducing the cost of fracturing construction. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Figure 1 is the process flow chart of the microwave sintering in-situ synthesis of mullite-silicon carbide composite proppant of the present invention;

[0048] Figure 2 is the multi-particle morphology diagram of the mullite-silicon carbide composite ceramic proppant of the present invention;

[0049] Figure 3 is the XRD spectrum of the proppant sintered at different times under 1000W microwave power of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0050] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0051] Example 1

[0052] A method for microwave synthesis of mullite-silicon carbide composite proppant considering non-uniform heating:

[0053] (1) Set the ball milling speed to 350 r / min, weigh 100 g of low-grade bauxite, 12 g of silicon carbide powder, 2 g of CaO, and 2 g of MnO2. Dry the ball-milled material at 100 °C for 5 h, and then grind it with a rapid grinder to obtain granulated powder.

[0054] (2) Granulate the granulated powder into spheres in a disk granulator, and spray the binder polyvinyl alcohol solution during the granulation process to obtain spherical proppant green bodies.

[0055] (3) Dry the proppant green body, test its real and imaginary parts of dielectric constant, density, thermal conductivity, electrical conductivity and specific heat capacity as input parameters, establish a finite element geometric physical model of the microwave heating furnace, and determine the waveguide position and model of electromagnetic emission according to the design of the microwave heating furnace. Simulate the electromagnetic field distribution in the microwave cavity through the simulation software Comsol to obtain the temperature variation coefficient at each point (as shown in Table 1), and select the point with the smallest temperature difference coefficient, that is, X3, as the best sintering position of the sample;

[0056] Table 1 Average temperature and temperature variation coefficient of X1 - X6

[0057]

[0058] (4) In an air environment, first heat up to 1250 °C at a power of 900 W, and then keep the temperature for 120 min at a power of 200 W. Cool and screen to obtain particles with a particle size of 0.320 - 0.890 mm, which are the mullite-silicon carbide composite proppant.

[0059] Example 2

[0060] A method for microwave synthesis of mullite-silicon carbide composite proppant considering non-uniform heating:

[0061] (1) Set the ball milling speed to 350 r / min, weigh 80 g of low-grade bauxite, 10 g of silicon carbide powder, 1 g of CaO, and 2 g of MnO2. Dry the ball-milled material at 100 °C for 5 h, and then grind it using a rapid grinder to obtain granulated powder;

[0062] (2) Granulate the granulated powder into spheres in a disk granulator, and spray the binder water glass during the granulation process to obtain spherical proppant green bodies.

[0063] (3) Dry the proppant green body, test its real and imaginary parts of dielectric constant, density, thermal conductivity, electrical conductivity and specific heat capacity as input parameters, establish a finite element geometric physical model of the microwave heating furnace, and determine the waveguide position and model of electromagnetic emission according to the design of the microwave heating furnace. Simulate the electromagnetic field distribution in the microwave cavity through the simulation software Comsol to obtain the temperature variation coefficient at each point (the results are shown in Table 2), and select the point with the smallest temperature difference coefficient, that is, select Y3 as the best sintering position of the sample;

[0064] Table 2 Average temperature and temperature variation coefficient of Y1 - Y6

[0065]

[0066] (4) In an air environment, first heat up to 1300 °C at a power of 800 W, and then keep the temperature constant at 200 W for 100 min; cool and screen to obtain particles with a particle size of 0.290 - 0.850 mm, which are the mullite-silicon carbide composite proppants.

[0067] Example 3

[0068] A method for microwave synthesis of mullite-silicon carbide composite proppants considering non-uniform heating:

[0069] (1) Set the ball milling speed to 350 r / min, weigh 90 g of low-grade bauxite, 12 g of silicon carbide powder, 1.5 g of CaO, and 2 g of MnO2. Dry the ball-milled material at 100 °C for 5 h, and then grind it using a rapid grinder to obtain granulation powder.

[0070] (2) Granulate the granulation powder into spheres in a disk granulator, and spray the binder carboxymethyl cellulose solution during the granulation process to obtain spherical proppant green bodies.

[0071] (3) Dry the proppant green bodies, measure the real and imaginary parts of the dielectric constant, density, thermal conductivity, electrical conductivity, and specific heat capacity as input parameters, establish a finite element geometric physical model of the microwave heating furnace, and determine the waveguide position and model of electromagnetic emission according to the design of the microwave heating furnace. Simulate the electromagnetic field distribution in the microwave cavity using the simulation software Comsol to obtain the temperature variation coefficient at each point (the results are shown in Table 3), and select the point with the smallest temperature difference coefficient, that is, select Z3 as the best sintering position for the specimen;

[0072] Table 3 Average temperature and temperature variation coefficient of Z1 - Z6

[0073]

[0074] (4) In an air environment, first heat up to 1350 °C at a power of 1000 W, and then keep the temperature constant at 300 W for 45 min; cool and screen to obtain particles with a particle size of 0.290 - 0.850 mm, which are the mullite-silicon carbide composite proppants.

[0075] Performance test

[0076] Apparent density is a density that characterizes the pore volume between proppants. Usually, a low-viscosity liquid is used to measure the apparent density, and the liquid wets the particle surface, including the pore volume that cannot be reached by the liquid.

[0077] The breakage rate refers to the percentage of proppant broken under specified closure pressure conditions. The specific test conditions are as follows: Weigh the proppant, record its mass as m1, pour it into the crushing chamber, place the crushing chamber at the center of the automatic press test bench, select the value to be tested (such as 69 MPa), start the program. After the automatic test is completed, take out the crushing chamber, use the sieve with the lower limit of the particle size of the corresponding proppant to screen out the crushed fine powder and weigh it, record it as m2, and m2 / m1 is the breakage rate.

[0078] Fracture conductivity generally refers to the ability of the proppant filling layer to pass fluid, which is expressed by the product of the permeability of the proppant filling layer and the width of the support fracture, and can be used to evaluate and screen proppants.

[0079] According to the petroleum and natural gas industry standard of the People's Republic of China "SY / T5108 - 2014", particles with a particle size of 0.425 - 0.850 mm are screened and selected for testing apparent density and breakage rate. According to the petroleum and natural gas industry standard of the People's Republic of China "SY / T6302 - 2009", particles with a particle size of 0.212 - 0.425 mm are screened and selected for testing fracture conductivity.

[0080] The test results are as follows:

[0081]

[0082] Comparative Example 1: A comparative example lacking or violating the key steps of the present invention

[0083] Taking Example 3 as an example, in addition to the positions of Z1 - Z6, arbitrarily select another position point, and do not use the operation of changing the microwave power. Other parameters and operations remain the same. The performance test results of the obtained proppant samples are as follows:

[0084]

[0085] The present invention includes a method for microwave sintering in-situ synthesis of mullite - silicon carbide composite ceramic proppant, which uses low-grade bauxite as raw material, SiC particles as microwave hot spots, introduces CaO - MnO2 as a composite sintering aid, and is fired by microwave electromagnetic heating method, having a mullite - silicon carbide composite phase structure. Different from the existing conventional ceramic proppant sintering process, the present invention introduces SiC as microwave hot spots and optimizes the sintering position, achieving the purpose of preparing mullite-based ceramic proppant based on low-grade bauxite by microwave sintering technology, shortening the production time required, saving energy and improving production efficiency; at the same time, by controlling the addition amount of CaO - MnO2, mullite - silicon carbide composite ceramic proppant is prepared. Compared with the "aluminum oxide - mullite - quartz" mineral phase structure of conventional ceramic proppants, the proppant prepared by the present invention has higher strength.

[0086] Although the embodiments of the present invention have been shown and described above, it can be understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those of ordinary skill in the art can make changes, modifications, substitutions, and variations to the above embodiments within the scope of the present invention.

Claims

1. A method for in-situ synthesizing mullite-silicon carbide composite proppant by microwave sintering, characterized in that It includes the following steps: (1) Take low-grade bauxite, SiC, CaO, and MnO₂, mix and ball-mill them to obtain granulating powder materials; The composition raw materials of the granulating powder materials include: 80 - 100 parts of low-grade bauxite, 10 - 15 parts of SiC, 1 - 3 parts of CaO, and 1 - 4 parts of MnO₂; (2) Granulate the granulating powder materials into balls, and spray a binder during the granulation process to obtain spherical proppant green bodies; (3) Calculate the optimal sintering position of the sample, and in an air condition, use microwave radiation to in-situ sinter the proppant green bodies, and after cooling, screen to obtain mullite-silicon carbide composite ceramic proppants; The method for calculating the optimal sintering position of the sample is: Test the real part and imaginary part of the dielectric constant, density, thermal conductivity, electrical conductivity, and specific heat capacity of the proppant green body as input parameters, establish a simulation model, after finite element solution, select through the temperature difference coefficient the three cross-sections passing through the center of the sample where the electromagnetic field distribution is the most uniform and the electromagnetic field strength gradient is smaller as the suitable sample sintering positions conducive to uniform heating.

2. The method for in-situ synthesizing mullite-silicon carbide composite proppant by microwave sintering according to claim 1, wherein The mass fraction of alumina in the low-grade bauxite is less than 50%.

3. The method for in-situ synthesizing mullite-silicon carbide composite proppant by microwave sintering according to claim 1, wherein Step (1) also includes: Dry the material after ball-milling at 80°C - 110°C to remove moisture and then grind it into a powder to obtain granulating powder materials with a powder particle size below 30um.

4. The method for in-situ synthesizing mullite-silicon carbide composite proppant by microwave sintering according to claim 1, characterized in that In step (2), the binder is any one of carboxymethyl cellulose, polyvinyl alcohol, water glass, and water.

5. The method for in-situ synthesizing mullite-silicon carbide composite proppant by microwave sintering according to claim 1, characterized in that In step (3), the microwave heating power is 200 - 1200W, the sintering temperature is 1200 - 1600°C, the heat preservation time is 30 - 120min, and the average heating rate is 15 - 30°C / min.

6. The method for in-situ synthesizing mullite-silicon carbide composite proppant by microwave sintering according to claim 5, characterized in that The method of microwave radiation in-situ sintering in step (3) is to first quickly heat the sample to the target temperature with medium-high power, and then adjust to medium-low power to keep warm for a preset time.

7. The method for in-situ synthesizing mullite-silicon carbide composite proppant by microwave sintering according to claim 6, characterized in that, The medium-high power is 600 - 1200W, and the medium-low power is 200 - 600W.

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