Low-density high-strength ceramsite proppant as well as preparation process and application thereof

Abstract

The invention relates to the technical field of oil and gas exploitation, discloses a low-density high-strength ceramsite proppant as well as a preparation process and application thereof, and aims to solve the technical problem that the density and the strength of an existing ceramsite proppant are mutually restricted. The proppant has a core-shell structure with an inner core and an outer shell. The inner core is formed by sintering a first material containing bauxite, clay, a composite pore-forming agent and a carbon-silicon precursor and is used for forming a loose and porous low-density structure, and silicon carbide whiskers are generated in situ in the loose and porous low-density structure for reinforcement. The shell is formed by sintering a second material containing bauxite, clay and zircon sand and is used for forming a compact and high-strength protective layer and generating zirconium oxide particles in situ in the protective layer for toughening. The preparation process comprises the following steps: preparing core-shell structure green pellets by a two-step method, and adopting a double-atmosphere program temperature control sintering process of medium-temperature reduction and high-temperature oxidation. According to the invention, through the combination of the structure zoning and the zoning in-situ enhancement technology, the prepared proppant has ultralow density and excellent crushing resistance.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of oil and gas exploitation, and in particular to a low-density high-strength ceramic proppant, a preparation process thereof and an application thereof. BACKGROUND

[0002] In the process of oil and gas exploitation, hydraulic fracturing is a key stimulation technology. This technology forms artificial fractures in the reservoir rock by injecting high-pressure fracturing fluid into the formation, and fills the fractures with solid particulate material known as proppant to prevent the fractures from closing again after the fracturing fluid flows back due to the closure pressure of the formation, thereby constructing an oil and gas seepage channel with high conductivity.

[0003] As a mainstream artificial proppant, the performance of ceramic proppant directly affects the effect of fracturing reconstruction. An ideal proppant should have two core characteristics: one is high crushing strength to ensure that it remains intact under the huge closure pressure of the formation at a depth of thousands of meters underground, effectively supporting the fractures; the other is low density to facilitate its suspension and transportation in the fracturing fluid, so that it can be carried to the far end of the fracture to form a longer and wider effective support area.

[0004] However, in the existing ceramic proppant technology, there is an inherent technical contradiction between the two key performance indicators of density and strength. Conventional ceramic proppant is usually made of bauxite, clay and other raw materials through homogeneous mixing, granulation and high-temperature sintering, and its internal structure is uniform. In order to reduce the density of such proppant, pore-forming agents or lightweight fillers are usually added to the raw materials. Although this method can reduce the apparent density of the product to some extent, since the introduced pores or low-strength phases are uniformly distributed in the entire particle matrix, these pores will become stress concentration points when under pressure, greatly weakening the overall skeletal structure of the particle, thereby causing the crushing strength of the particle to decrease significantly. Conversely, if the strength is to be improved, a more dense internal structure must be pursued, but this inevitably leads to an increase in density. Therefore, for traditional ceramic proppants designed with a homogeneous structure, it is difficult to simultaneously achieve the performance combination of ultra-low density and high crushing strength, which limits its application effect in deep, ultra-deep and special density requirements of oil and gas reservoir fracturing operations. SUMMARY

[0005] The technical problem to be solved by the present application is to provide a low-density high-strength ceramic proppant that can overcome the inherent contradiction between density and strength of existing ceramic proppants, and to provide a preparation process and an application thereof.

[0006] To solve the above technical problems, the present application provides the following technical solutions: The present application provides a low-density high-strength ceramic proppant.

[0007] The proppant is a particle with a core-shell structure, which comprises an inner core and an outer shell covering the outer part of the inner core. This structure partitions the low-density function and the high-strength function in the particle interior.

[0008] The inner core is prepared by sintering the first material. The first material comprises, by mass percentage, bauxite 40-60%, hard clay 30-55%, spodumene 2-6%, hollow glass microspheres 1-5%, industrial starch 1-3%, carbon source 0.5-2%, and silicon source 0.5-2%. In this composition, the hollow glass microspheres, as a lightweight filler, and the industrial starch, as an organic pore-forming agent, work together to form a porous network structure in the inner core region after sintering, thereby significantly reducing the overall density of the particle.

[0009] The outer shell is prepared by sintering the second material. The second material comprises, by mass percentage, bauxite 50-70%, hard clay 25-40%, spodumene 6-12%, and zircon sand 1-4%. This composition does not contain an organic pore-forming agent or a lightweight filler, and is designed to form a dense and high-strength protective layer after sintering.

[0010] In a preferred embodiment, the carbon source in the first material is high-specific-surface-area carbon black, and the silicon source is nanoscale silicon dioxide. These two components act as reaction precursors, and their design purpose is to undergo in-situ chemical reactions under specific temperature and atmosphere conditions during the sintering process to generate silicon carbide whisker reinforcing phases on the porous framework of the inner core.

[0011] In another preferred embodiment, the zircon sand in the second material acts as a reaction precursor, and its design purpose is to undergo in-situ chemical reactions with other components in the material system under specific temperature and atmosphere conditions during the sintering process to generate dispersed zirconia particle reinforcing phases in the ceramic matrix of the outer shell.

[0012] The second aspect of the present application provides a preparation process for the low-density high-strength ceramic proppant as described above.

[0013] The preparation process comprises the following steps: S1, material preparation step: according to the chemical composition and mass percentage of the first material and the second material of the first aspect, the core material mixed powder and the shell material mixed powder are independently prepared by weighing, mixing, ball milling, etc.

[0014] S2, core-shell structure green ball forming step: firstly, the core material mixed powder is prepared into an inner core with a predetermined size; then, the shell material mixed powder is uniformly coated on the surface of the inner core to form a green ball with a core-shell structure. In a preferred embodiment, this step is specifically: the core material mixed powder is made into an inner core by spray granulation; then, the shell material mixed powder is applied to the surface of the inner core by rolling coating with the inner core as a crystal nucleus.

[0015] S3, partitioned reaction sintering step: the core-shell structure green ball is placed in a controllable atmosphere sintering furnace for program-controlled temperature sintering. The key of this step is that the sintering process is divided into different temperature zones and matched with different furnace atmospheres. Specifically, the sintering process includes a medium temperature stage and a high temperature stage, wherein a reducing atmosphere is used in the medium temperature stage, and an oxidizing atmosphere is switched in the subsequent high temperature stage.

[0016] In a preferred embodiment, the temperature range of the medium temperature stage is set to 400-900℃. Under this temperature range and reducing atmosphere, the carbon source and silicon source in the core material mixed powder undergo carbothermal reduction reaction, so that silicon carbide whiskers are in-situ grown in the inner core.

[0017] In another preferred embodiment, the temperature range of the high temperature stage is set to 900-1250℃. Under this temperature range and oxidizing atmosphere, the shell material mixed powder undergoes liquid phase sintering to form a dense shell structure, and at the same time, the durax undergoes chemical reaction to generate zirconia particles in-situ in the densified shell matrix.

[0018] The third aspect of the present application provides an application of the low-density high-strength ceramic proppant as described in the first aspect in the hydraulic fracturing process of oil and gas exploitation.

[0019] In summary, the present application has at least one of the following beneficial technical effects: 1. The present application realizes the separation of weight reduction function and strength enhancement function in a single proppant particle through core-shell structure design. The inner core forms a loose porous structure by introducing a composite pore former to undertake the main weight reduction task, and the outer shell is designed as a dense high-strength layer to resist external pressure. This structural innovation fundamentally solves the technical bottleneck that the mechanical strength is inevitably greatly sacrificed while pursuing low density in traditional proppants, and effectively unifies ultra-low density and high crushing strength.

[0020] 2.The application adopts a partition-differentiated in-situ strengthening technology, which significantly improves the mechanical properties of each part of the particle. In the core area, the silicon carbide whiskers grown in-situ through the carbon thermal reduction reaction form a micro-reinforced network in the porous skeleton, effectively supporting the pore structure and improving the anti-collapse ability of the core. Meanwhile, the dispersed zirconia particles generated in-situ in the shell area can effectively inhibit the initiation and propagation of micro-cracks through the phase transformation toughening mechanism, greatly improving the toughness and wear resistance of the shell.

[0021] 3.The application adopts a double-atmosphere programmed temperature sintering process of medium-temperature reduction and high-temperature oxidation to create necessary and independent reaction conditions for the in-situ growth of silicon carbide whiskers in the core and the in-situ generation of zirconia particles in the shell and the densification of the matrix, respectively. The process ensures that each designed function can be accurately realized, and the preparation of the proppant is improved from traditional mixing and sintering to the level of micro-controllable chemical synthesis and material design. DETAILED DESCRIPTION

[0022] The application will be further described in detail below in combination with examples, comparative examples and test examples.

[0023] The sources and specifications of the main raw materials and reagents used in the following examples and comparative examples are as follows. The reagents not specifically mentioned are commercially available analytical pure or higher grade products.

[0024] Lithium aluminosilicate: CAS No. 66057-55-4.

[0025] Carbon black: CAS No. 1333-86-4.

[0026] Nano-sized silicon dioxide: CAS No. 60676-86-0.

[0027] Polyvinyl alcohol (PVA): CAS No. 9002-89-5, industrial grade.

[0028] Hydrofluoric acid: CAS No. 7664-39-3, analytical pure.

[0029] Examples 1-3: Example 1: The present embodiment provides a preparation method of low-density high-strength ceramic proppant.

[0030] (1) Preparation of core material: The following components are weighed according to a total mass of 1000 g: high alumina bauxite 500 g (50%), hard clay 400 g (40%), spodumene 40 g (4%), hollow glass microspheres 25 g (2.5%), industrial starch 15 g (1.5%), high specific surface area carbon black 10 g (1%), and nano-sized silicon dioxide 10 g (1%). All the above components are placed in a ball mill and dry ball-milled for 4 hours to obtain a uniformly mixed core material mixture powder.

[0031] (2) Preparation of the shell material: The following components are weighed according to a total mass of 1000 g: high alumina bauxite 600 g (60%), hard clay 300 g (30%), spodumene 80 g (8%), and zirconite 20 g (2%). All the above components are placed in another ball mill and dry ball-milled for 4 hours to obtain a uniformly mixed shell material mixture powder.

[0032] (3) Preparation of the core-shell structure green ball The core material mixture powder prepared in step (1) is placed in a disc granulator with a diameter of 1000 mm and an inclination angle of 50°. The device is started, and a 4% mass fraction of polyvinyl alcohol (PVA) aqueous solution is uniformly sprayed into the powder as a binder through a spraying device to prepare an inner core with a particle size of about 0.5 mm. Then, the inner core is used as a crystal nucleus, and the shell material mixture powder prepared in step (2) is continuously added to the granulator, and a small amount of binder is intermittently sprayed to uniformly adhere the shell material powder to the surface of the inner core through rolling coating until the final particle size reaches 0.8 mm, thereby obtaining a core-shell structure green ball.

[0033] (4) Programmed temperature sintering: After the above core-shell structure green ball is dried at 80 °C for 8 hours, it is moved into a controllable atmosphere box furnace, and sintered according to the following program: ① From room temperature to 400 °C at a rate of 2.5 °C / min.

[0034] ② From 400 °C to 900 °C at a rate of 4 °C / min, in the process, nitrogen gas with a flow rate of 1.0 L / min is introduced into the furnace as a reducing atmosphere, and the temperature is maintained at 900 °C for 1.5 hours.

[0035] ③ Stop introducing nitrogen gas, and restore to air (oxidizing atmosphere), from 900 °C to 1220 °C at a rate of 4 °C / min, and maintain the temperature at 1220 °C for 2.5 hours.

[0036] ④ Cool down to room temperature at a rate of 4 °C / min. After sintering is completed, the sample is taken out, and the proppant product of Example 1 is obtained.

[0037] Example 2: The embodiment provides a preparation method of low-density high-strength ceramic proppant, which is different from the embodiment 1 in that the material composition of the core material and the shell material.

[0038] (1) Preparation of the core material: The following components are weighed according to the total mass of 1000g: bauxite 450g (45%), hard clay 420g (42%), spodumene 30g (3%), hollow glass microspheres 40g (4%), industrial starch 30g (3%), high specific surface area carbon black 15g (1.5%), and nano-sized silicon dioxide 15g (1.5%). The mixing mode is the same as that in the embodiment 1.

[0039] (2) Preparation of the shell material: The following components are weighed according to the total mass of 1000g: bauxite 650g (65%), hard clay 250g (25%), spodumene 70g (7%), and zircon sand 30g (3%). The mixing mode is the same as that in the embodiment 1.

[0040] (3) Preparation of the core-shell structure green ball and the preparation method and the process of the programmed temperature sintering of the core-shell structure green ball are all the same as those in the embodiment 1.

[0041] Embodiment 3: The embodiment provides a preparation method of low-density high-strength ceramic proppant, which is different from the embodiment 1 in that the process parameters of the programmed temperature sintering.

[0042] (1) Preparation of the core material and the shell material and preparation of the core-shell structure green ball: the material composition of the core material and the shell material, the mixing method and the preparation method of the core-shell structure green ball are all the same as those in the embodiment 1.

[0043] (2) Programmed temperature sintering: after the prepared core-shell structure green ball is dried, it is moved into a controllable atmosphere box furnace, and sintering is performed according to the following program: ①From room temperature to 400℃ at a rate of 3℃ / min.

[0044] ②From 400℃ to 900℃ at a rate of 5℃ / min, in the process, nitrogen gas with a flow rate of 1.0L / min is introduced into the furnace as a reducing atmosphere, and the temperature is kept at 900℃ for 1.5 hours.

[0045] ③Stop introducing nitrogen and restore to air (oxidizing atmosphere), from 900℃ to 1250℃ at a rate of 5℃ / min, and keep the temperature at 1250℃ for 2 hours.

[0046] ④Cool to room temperature at a rate of 5℃ / min. After sintering is completed, the sample is taken out, and the proppant product of the embodiment 3 is obtained.

[0047] Comparative Example 1-3: Comparative Example 1: Comparative Example 1-3:

[0048] Comparative Example 2: Comparative Example 2:

[0049] Comparative Example 3: Comparative Example 3:

[0050] Test Example 1-2: Test Example 1: Test Method: To evaluate the performance of the proppant particles prepared in the above examples and comparative examples, the following standard test methods were used.

[0051] Apparent Density Test: The test was performed according to the standard of API RP 19C. About 50 g of dried sample was accurately weighed as mass m. The sample was placed in a graduated pycnometer, and anhydrous kerosene was added to a certain scale line, and the volume V1 at this time was recorded. The sample was completely immersed in kerosene and shaken thoroughly to remove air bubbles, and the total volume V2 of the pycnometer at this time was recorded. The volume of the sample V = V2-V1. The apparent density of the sample p = m / V.

[0052] Resistance to Crushing Test: The test was performed according to API RP 19C standard. The sample was sieved to take the particles in the size range of 20-40 mesh (0.425mm-0.850mm), and dried at 110°C for 2 hours. A certain mass of the sample in the size range was accurately weighed and placed in a pressure cell with an inner diameter of 50.8mm. The test cell was placed on a hydraulic machine, and a uniform pressure was applied to 52MPa within 2 minutes, and maintained at this pressure for 2 minutes. After unloading the pressure, all the samples were carefully taken out and sieved on a 40 mesh standard sieve. The mass of the fine powder passing through the sieve was weighed. The calculation formula of the anti-crushing rate is: anti-crushing rate (%) = (mass of fine powder / initial total mass of sample) x 100%.

[0053] Acid solubility test: The test was performed according to API RP 19C standard. About 5g of the dried sample was accurately weighed for its initial mass m initial . The sample was placed in a beaker, and 100mL of mixed acid solution with a concentration of 12% hydrochloric acid (HCl) and 3% hydrofluoric acid (HF) was added. The beaker was placed in a constant temperature water bath at 65°C for heating for 0.5 hours, and continuous stirring was performed. After the reaction was completed, the sample was filtered, washed repeatedly with deionized water until neutral, and then placed in an oven at 110°C for drying to a constant weight. The final mass of the dried sample m_final was weighed. The calculation formula of the acid solubility is: acid solubility (%) = [(m initial -m final ) / m initial ] x 100%.

[0054] Test results: The support agent samples prepared in Examples 1-3 and Comparative Examples 1-3 were subjected to performance tests according to the above test methods, and the results obtained are recorded in Table 1 below.

[0055] Table 1: Performance test results of examples and comparative examples Summary of experiments: From the test results in Table 1, it can be seen that the support agent particles prepared by the technical scheme provided in the present application achieve a good balance between density and anti-crushing performance. The samples of Examples 1, 2 and 3 exhibit different performance combinations compared to the samples of Comparative Examples. By distinguishing the core and the shell in structure, and designing the material components of each region, the final product maintains a relatively low apparent density while obtaining a relatively high anti-crushing strength. The homogeneous structure of Comparative Example 1 has internal pores uniformly distributed as mechanical weak points, resulting in a significant decrease in anti-crushing performance.

[0056] Another benefit of the present application is derived from the in-situ differential strengthening technology. Comparing the data of Example 1 and Comparative Example 2, the anti-crushing rate of Example 1 is significantly lower than that of Comparative Example 2 under the condition of similar apparent density. This is because the reinforcing phase in Example 1 is generated by in-situ chemical reaction, and the silicon carbide whiskers formed in the core and the zirconia particles generated in the shell form a firm interfacial bond with the matrix. While in Comparative Example 2, the reinforcing phase powder is introduced by physical addition, and its bonding force with the matrix is weak, which is prone to interfacial debonding under pressure, so its mechanical strengthening effect is limited.

[0057] The controllability of the preparation process of the present application is the basis for achieving the expected performance. Comparing the data of Comparative Example 1 and Comparative Example 3, both use the same material formula and green ball structure, but due to different sintering processes, the performance shows significant difference. The dual-atmosphere sintering process of intermediate temperature reduction and high temperature oxidation used in Example 1 provides the necessary chemical environment for the carbothermal reduction reaction of silicon carbide whiskers in the core. While Comparative Example 3 is sintered in an oxidizing atmosphere all the time, the reaction cannot proceed, resulting in a lack of effective microstructure in the core, which is insufficient in mechanical support, and ultimately shows a higher anti-crushing rate.

[0058] Test Example 2: Anti-crushing performance evaluation test under gradient pressure

[0059] Test method: The test samples are the proppant particles prepared in Example 1, Example 2, and Comparative Examples 1, 2 and 3.

[0060] The specific test steps of anti-crushing rate are exactly the same as the "anti-crushing rate test" method described in the "Test Example 1" above. The difference is that this test example tests each group of samples under three different final closure pressure values of 35 MPa, 52 MPa and 69 MPa, respectively, to record their anti-crushing rates under low, medium and high pressure conditions.

[0061] Test results: The anti-crushing rate results of each sample under different pressures are recorded in Table 2 below.

[0062] Table 2: Anti-crushing rate (%) of each sample under different pressures Test samples Under 35MPa pressure Under 52MPa pressure Under 69MPa pressure Example 1 1.8 4.5 8.1 Example 2 2.1 4.9 8.7 Comparative Example 1 9.5 18.2 35.4 Comparative Example 2 5.8 11.5 21.3 Comparative Example 3 7.1 14.8 28.5 Experimental summary: From the results of the gradient pressure test in Table 2, it can be seen that the anti-crushing rates of all samples increase with the increase of the closure pressure, but the increasing amplitude is significantly different. The growth rate of the anti-crushing rate of the samples prepared in Example 1 and Example 2 is relatively flat with the increase of the pressure. In contrast, the anti-crushing rate of the samples of Comparative Examples 1, 2 and 3 deteriorates sharply when the pressure increases from 52 MPa to 69 MPa. This result shows that the core-shell structure and the partition in-situ strengthening mechanism constructed by the technical solution give the proppant particles excellent structural stability. The dense shell effectively resists the invasion of external stress, and the internally enhanced porous core provides stable mechanical support. This structural synergy delays the structural failure of the particles caused by defect propagation under high stress. Therefore, the proppant prepared by the technical solution can maintain higher integrity under high closure pressure conditions, and exhibits its applicability in high stress formation environments such as deep and ultra-deep wells.

[0063] Although embodiments of the present application have been shown and described, it is to be understood that various modifications, substitutions, replacements and changes can be made to these embodiments without departing from the principles and spirit of the present application, and the scope of the present application is defined by the appended claims and their equivalents.

Claims

1. A low-density, high-strength ceramsite proppant, characterized in that: The proppant particles include a core and a shell covering the core; The core is sintered from a first material, wherein the first material comprises, by mass percentage, 40%-60% bauxite, 30%-55% hard clay, 2%-6% spodumene, 1%-5% hollow glass microspheres, 1%-3% industrial starch, 0.5%-2% carbon source, and 0.5%-2% silicon source; The shell is sintered from a second material. Calculated by mass percentage, the second material comprises: 50%-70% bauxite, 25%-40% hard clay, 6%-12% spodumene, and 1%-4% zircon sand.

2. A low-density, high-strength ceramsite proppant according to claim 1, characterized in that: The carbon source in the first material is high specific surface area carbon black, and the silicon source is nano-scale silicon dioxide, and the two are used to react in situ during the sintering process to generate silicon carbide whiskers.

3. A low-density, high-strength ceramsite proppant according to claim 1, characterized in that: The zircon sand in the second material is used for in-situ reaction during the sintering process to generate dispersed zirconium oxide particles.

4. A process for preparing a low-density, high-strength ceramsite proppant according to any one of claims 1 to 3, characterized in that: The following steps are involved: S1. According to the ratio of the first material and the second material, prepare a core material mixed powder and a shell material mixed powder respectively; S2, preparing the core material mixed powder into a core, and then coating the shell material mixed powder on the surface of the core to form a core-shell structure green ball; S3. The core-shell structure green balls are subjected to programmed temperature-controlled sintering, wherein the sintering process includes adopting a reducing atmosphere in a medium temperature stage and an oxidizing atmosphere in a high temperature stage.

5. The process for preparing a low-density, high-strength ceramsite proppant according to claim 4, wherein: The S2 step is specifically as follows: The core material mixed powder is made into a core by spray granulation; Then, the core is used as a crystal nucleus, and the shell material mixed powder is coated on its surface by rolling coating.

6. The process for preparing a low-density, high-strength ceramsite proppant according to claim 4, characterized in that: The temperature range of the medium temperature stage in the S3 step is 400°C to 900°C.

7. The process for preparing a low-density, high-strength ceramsite proppant according to claim 4, characterized in that: The temperature of the high temperature stage in step S3 ranges from 900°C to 1250°C.

8. The process for preparing a low-density, high-strength ceramsite proppant according to claim 4, wherein: In step S3, under the reducing atmosphere of the medium temperature stage, the carbon source and the silicon source in the core material mixed powder undergo a carbothermal reduction reaction, and silicon carbide whiskers are grown in situ in the core.

9. The process for preparing a low-density, high-strength ceramsite proppant according to claim 4, wherein: In step S3, in the oxidizing atmosphere of the high temperature stage, the shell material mixed powder undergoes liquid phase sintering to form a dense shell, and simultaneously the zircon sand reacts to generate zirconium oxide particles in situ.

10. Use of the low-density, high-strength ceramsite proppant according to any one of claims 1 to 3 in a hydraulic fracturing process for oil and gas production.

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