Fracturing fluid prepad fluid for optimizing size of in-situ authigenic proppant and application of fracturing fluid prepad fluid

Abstract

The invention belongs to the technical field of petroleum fracturing yield increase transformation, and particularly discloses fracturing fluid prepad fluid for optimizing the size of an in-situ authigenic proppant and application of the fracturing fluid prepad fluid, which are used for solving the problem of small particle size of proppant particles generated by a formation mineral hydrothermal reaction in the prior art. The fracturing fluid prepad fluid is suitable for a calcium-rich reservoir and is composed of clear water, phosphate, a pH regulator and a Ca < 2 + > chelating agent, relative to 100 parts by weight of clear water, the content of the phosphate is 1-10 parts by weight, the volume molar concentration of the pH regulator is 1-5 mol / L, and the volume molar concentration of the Ca < 2 + > chelating agent is 0.001-1 mol / L; the pH regulator is urea, ammonia water, propanamide and the like, and the Ca < 2 + > chelating agent is EDTA (Ethylene Diamine Tetraacetic Acid), glutamic acid and the like. The average particle size of the proppant particles generated by the formation mineral hydrothermal reaction is increased, and the supporting effect of the proppant particles in formation microfractures is improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of oil fracturing stimulation and transformation, and particularly relates to a fracturing fluid preflush for optimizing the size of in-situ self-generated proppants and its application. Background Technique

[0002] Fracturing technology is one of the important technologies for oil and gas field development and has been widely used in the development of low-permeability oil and gas reservoirs. Hydraulic fracturing is to pump a preflush into the target layer to form and extend fractures, and then pump a sand-carrying fluid mixed with proppants. The sand-carrying fluid continues to extend the fractures and carries the proppants deep into the fractures. Then, the fracturing fluid is broken and degraded into a low-viscosity fluid flowing towards the bottom hole, leaving a high-conductivity channel. As a key material in the process of oil and gas well fracturing, the main function of proppants is to support the fractures formed during the fracturing process, prevent them from closing, so as to maintain the conductivity of the fractures and ensure that oil and gas can flow smoothly.

[0003] Hydraulic fracturing is beneficial to oil and gas development and production. However, at present, there are problems such as easy closure of microfractures after fracturing, difficult proppant transportation and poor effective support in distal microfractures, which seriously limit the conductivity of the fracture network. The aperture of microfractures is small, and traditional proppants such as ceramsite and quartz sand cannot enter. Without effective support for microfractures, during the production process, affected by in-situ stress, rock elastic recovery, etc., the microfractures without effective support are easy to close, reducing the communication ability between the fracture network and natural fractures, restricting the efficiency and stability of fracturing stimulation and transformation, and resulting in a rapid decline in the production of fractured wells. Research shows that most of the microfractures after fracturing are in an unsupported state, and the volume of unsupported fractures accounts for more than 50% of the fracturing volume.

[0004] Maintaining the aperture of microfractures is one of the keys to increasing production and stabilizing production in reservoir fracturing stimulation. Using micro-proppants to support microfractures is the most direct method. However, micro-proppants are prone to agglomeration, difficult to disperse, and there is a potential risk of blocking the sand-carrying and conductivity channels of the main fracture during post-fracture backflow. There are still great obstacles to the transportation of micro-proppants for distal microfractures. In particular, the support for microfractures with a size of <150μm still needs to be further solved.

[0005] In order to achieve the support of fracturing microfractures, the applicant proposed a kind of in-situ self-generated proppant for fracturing microfractures in calcium-rich reservoirs, its preparation method and application in the Chinese invention patent with the publication number of CN118562485A. By injecting a phosphate system into the formation fracturing fractures and using the formation environment to carry out hydrothermal synthesis in-situ in calcium-rich reservoirs to generate in-situ self-generated proppants, it can effectively improve the conductivity of rock fractures after fracturing. However, the particle size of the generated proppants is too small and the support effect is limited, restricting its application range. Therefore, increasing the size of the in-situ self-generated proppant - hydroxyapatite crystals synthesized by using formation minerals through hydrothermal synthesis and further improving the fracture conductivity is one of the current key research directions of this technology. Summary of the Invention

[0006] One object of the present invention is to provide a fracturing fluid preflush for optimizing the size of in-situ self-generated proppants, effectively solving the problem that the particle size of proppant particles generated by the hydrothermal reaction of formation minerals in the prior art is small.

[0007] To solve the above technical problems, the technical solution adopted by the present invention is: a fracturing fluid preflush for optimizing the size of in-situ self-generated proppants, applicable to calcium-rich reservoirs, comprising the following raw materials: clear water, phosphate, and a pH regulator and / or Ca 2+ chelating agent, wherein, relative to 100 parts by weight of clear water, the content of phosphate is 1-10 parts by weight, the volume molar concentration of the pH regulator is 1-5 mol / L, and Ca 2+ the volume molar concentration of the chelating agent is 0.001-1 mol / L.

[0008] The phosphate is at least one of diammonium hydrogen phosphate, ammonium dihydrogen phosphate, dipotassium hydrogen phosphate, potassium dihydrogen phosphate, disodium hydrogen phosphate, sodium dihydrogen phosphate, and sodium phosphate. The pH regulator is one or a mixture of urea, ammonia water, propionamide, sodium carbonate, and sodium bicarbonate. The Ca 2+ chelating agent is one or a mixture of ethylenediaminetetraacetic acid (EDTA), disodium EDTA, trisodium EDTA, tetrasodium EDTA, citric acid, sodium citrate monohydrate, disodium citrate, trisodium citrate, glutamic acid, hydroxyethylidene diphosphonic acid, aminotrimethylene phosphonic acid, and diethylenetriamine pentaacetic acid.

[0009] Further, relative to 100 parts by weight of clear water, the content of phosphate is 5-7 parts by weight, the volume molar concentration of the pH regulator is 1-2 mol / L, and Ca 2+ the volume molar concentration of the chelating agent is 0.001-1 mol / L.

[0010] Further, the calcium content of the formation core minerals in the calcium-rich reservoir is greater than 50%.

[0011] Another object of the present invention is to provide an application of the fracturing fluid preflush for optimizing the size of in-situ self-generated proppants described in the above embodiments, comprising the following steps: S1. After weighing phosphate and clear water according to a preset ratio, add the phosphate to the clear water and stir until the phosphate is completely dissolved; S2. Add a pH regulator and / or Ca 2+ chelating agent to the solution prepared in step S1 to obtain a fracturing fluid preflush; S3. Inject the fracturing fluid preflush into the microfractures of the calcium-rich reservoir and react for 12-48 h, and use the formation temperature (generally, the formation temperature of the middle and shallow layers can reach 80-120 °C) for hydrothermal synthesis of hydroxyapatite crystals to generate in-situ self-generated proppants on the surface of the microfractures of the calcium-rich reservoir.

[0012] Compared with the prior art, the beneficial technical effects of the present invention are as follows: By adding a pH regulator and / or a Ca 2+ chelating agent to the phosphate system, the pH regulator can form a buffer system through decomposition or hydrolysis reactions under high-temperature hydrothermal conditions, causing the pH value of the reaction system to slowly increase, inhibiting the nucleation rate, promoting the preferential deposition of ions on the existing crystal nuclei, promoting crystal growth rather than nucleation, and finally forming large-sized crystals; the Ca 2+ chelating agent can form a stable water-soluble chelate with Ca 2+ , reducing the concentration of free Ca 2+ in the solution. The lower Ca 2+ concentration results in a decrease in the ion supersaturation in the solution, inhibiting the nucleation rate and promoting the growth of crystals into larger particles.

[0013] Therefore, the present invention increases the average particle size of the proppant particles generated by the hydrothermal reaction of formation minerals, improves the support effect of the in-situ generated proppant in the formation microfractures, is beneficial to further improving the fracture conductivity, improving the fracturing stimulation effect of oil and gas, and the method provided by the present invention is simple to operate and has a wide range of raw material sources. Description of the Drawings

[0014] Figure 1 is the particle size distribution diagram of the in-situ generated proppant particles in Example 1.

[0015] Figure 2 is the particle size distribution diagram of the in-situ generated proppant particles in Example 2.

[0016] Figure 3 is the particle size distribution diagram of the in-situ generated proppant particles in Example 3.

[0017] Figure 4 is the particle size distribution diagram of the in-situ generated proppant particles in Example 4.

[0018] Figure 5 is the particle size distribution diagram of the in-situ generated proppant particles in Example 5.

[0019] Figure 6 is the particle size distribution diagram of the in-situ generated proppant particles in Example 6.

[0020] Figure 7 is the particle size distribution diagram of the in-situ generated proppant particles in Comparative Example 1.

[0021] Figure 8 is the particle size distribution diagram of the in-situ generated proppant particles in Comparative Example 2.

[0022] Figure 9 is the electron micrograph of the in-situ generated proppant particles in Example 1 magnified 5000 times.

[0023] Figure 10 It is an electron micrograph of the in-situ self-generating proppant particles in Comparative Example 1 magnified 5000 times.

[0024] Figure 11 It is a comparison chart of the microfracture conductivity before and after the reaction in Example 7.

[0025] Figure 12 It is a comparison chart of the microfracture conductivity before and after the reaction in Comparative Example 3.

[0026] Figure 13 It is the X-ray diffraction pattern of the in-situ self-generating proppant obtained after separation in Example 1. In the figure, JCPDS#86-0740 comes from the standard powder diffraction data of hydroxyapatite of the International Centre for Diffraction Data.

[0027] Figure 14 It is the X-ray diffraction pattern of the in-situ self-generating proppant obtained after separation in Example 2. In the figure, JCPDS#86-0740 comes from the standard powder diffraction data of hydroxyapatite of the International Centre for Diffraction Data. Detailed implementation manners

[0028] Example 1: This example provides an application of a fracturing fluid preflush for optimizing the size of in-situ self-generating proppants. In this example, the phosphate used is sodium dihydrogen phosphate, and the pH regulator used is urea with a volume molar concentration of 1 mol / L.

[0029] Prepare the fracturing fluid preflush according to 100 parts by weight of clear water, 7 parts by weight of sodium dihydrogen phosphate and urea. Add excessive calcium carbonate to the prepared fracturing fluid preflush, stir evenly, seal it with plastic wrap, and then place it in an electric heating oven at 90 °C for 24 h. After the reaction is completed, separate the unreacted calcium carbonate and dry it to obtain in-situ self-generating proppant particles. Perform X-ray diffraction analysis on the in-situ self-generating proppant (the product of Example 1) obtained after separation. From Figure 13 It can be seen that the main component of the in-situ self-generating proppant is hydroxyapatite.

[0030] The specific method for separating unreacted calcium carbonate after the reaction is described in detail in the Chinese patent for invention with the publication number CN118562485A as follows: Since the decomposition temperature of calcium carbonate is 825 °C and the decomposition temperature of the in-situ self-supporting agent is 1200 °C, the filtered solid mixture is placed in a muffle furnace at 900 °C for high-temperature calcination treatment to decompose the calcium carbonate in the solid mixture into calcium oxide and carbon dioxide. After obtaining the solid mixture of calcium oxide and the supporting agent, an NH4Cl solution with a volume concentration of 20% is prepared, and this NH4Cl solution is added to the solid mixture of calcium oxide and the supporting agent and placed in a constant temperature water bath at 50 °C for reaction for 0.5 - 1 h. After the reaction is completed, filtration is carried out, and the reaction is repeated 3 - 5 times until no obvious pungent odor is emitted or the pH of the solution is 7, at which point it is considered that the reaction is complete. The reaction solution is filtered and dried to obtain in-situ self-supporting agent particles, thereby separating unreacted calcium carbonate from the mixture and obtaining in-situ self-supporting agent particles.

[0031] Example 2: On the basis of Example 1, a chelating agent - EDTA with a volume molar concentration of 1 mol / L is added, and the molar ratio of EDTA to calcium carbonate is 1:1. Other raw materials and reaction conditions remain unchanged. After the reaction is completed, unreacted calcium carbonate is separated and dried to obtain in-situ self-supporting agent particles. X-ray diffraction analysis is carried out on the in-situ self-supporting agent (the product of Example 2) obtained after separation. 2+ It can be seen that the main component of the in-situ self-supporting agent is hydroxyapatite. Figure 14

[0032] Example 3: In this example, the phosphate used is sodium dihydrogen phosphate, and the Ca 2+ chelating agent is EDTA with a volume molar concentration of 0.001 mol / L.

[0033] In this example, a solution is prepared according to 100 parts by weight of clear water, 7 parts by weight of sodium dihydrogen phosphate, and 0.001 mol / L EDTA. An excessive amount of calcium carbonate is added to the prepared solution, stirred evenly, sealed with plastic wrap, and then placed in an electric heating oven at 90 °C for reaction for 24 h. After the reaction is completed, unreacted calcium carbonate is separated and dried to obtain in-situ self-supporting agent particles.

[0034] Example 4: The difference between this example and Example 3 is that the Ca 2+ chelating agent used in this example is EDTA with a volume molar concentration of 0.003 mol / L.

[0035] Example 5: In this example, the phosphate used is sodium dihydrogen phosphate, the pH regulator used is urea with a volume molar concentration of 1 mol / L, and the Ca 2+ ​The chelating agent is glutamic acid with a molar concentration of 0.5 mol / L.

[0036] Prepare the preflush fluid of the fracturing fluid by mixing 100 parts by weight of clear water, 7 parts by weight of sodium dihydrogen phosphate, 0.5 mol / L glutamic acid (7 parts by weight), and 1 mol / L urea. Add excessive calcium carbonate to the prepared preflush fluid of the fracturing fluid, stir evenly, seal it with plastic wrap, and then place it in an electric heating oven at 90 °C for reaction for 24 h. After the reaction is completed, separate the unreacted calcium carbonate, and dry it to obtain in-situ self-generating proppant particles.

[0037] Example 6: The difference between this example and Example 5 is that: the Ca used in this example 2+ The chelating agent is glutamic acid with a molar concentration of 1 mol / L.

[0038] Example 7: In this example, prepare the preflush fluid of the fracturing fluid by mixing 100 parts by weight of clear water, 7 parts by weight of sodium dihydrogen phosphate, and EDTA with a molar concentration of 0.001 mol / L. Select a carbonate rock core with a calcium content exceeding 90% (in this example, in order to have obvious effects and facilitate observation and comparison, a core with a calcium content exceeding 90% is selected. However, theoretically, as long as there are calcium minerals in the rock minerals, in-situ self-generating proppants can be formed, but the effect depends on the content and distribution of calcium minerals in the rock minerals. For example, an ideal effect can be achieved when the mineral calcium content is greater than 50%). Use wire cutting technology to prepare microfractures in the core to simulate formation microfractures. Inject the preflush fluid of the fracturing fluid into the microfractures of the test core, and react for 24 h at 80 °C. In-situ proppants are generated on the surface of the microfractures of the core. Test the conductivity of the microfractures under different confining pressures before and after the reaction. The experimental results are shown in Figure 11 . The results show that: the conductivity of the microfractures of the core after the reaction has increased significantly. The conductivity of the microfractures under a high confining pressure of 20 MPa is about 146 times that before the reaction.

[0039] Comparative Example 1: In this comparative example, prepare the preflush fluid of the fracturing fluid by mixing 100 parts by weight of clear water and 5 parts by weight of sodium dihydrogen phosphate; add excessive calcium carbonate to the prepared preflush fluid of the fracturing fluid, stir evenly, seal it with plastic wrap, and then place it in an electric heating oven at 90 °C for reaction for 24 h. After the reaction is completed, separate the unreacted calcium carbonate, and dry it to obtain in-situ self-generating proppant particles.

[0040] Comparative Example 2: The difference between this comparative example and Comparative Example 1 is that: the addition amount of sodium dihydrogen phosphate is 7 parts by weight.

[0041] Comparative Example 3: In this comparative example, a fracturing fluid preflush was prepared by mixing 100 parts by weight of clear water and 7 parts by weight of sodium dihydrogen phosphate. Similarly, carbonate rock cores with a calcium content exceeding 90% were selected, and wire cutting technology was used to prepare microfractures in the cores to simulate formation microfractures. The fracturing fluid preflush was injected into the microfractures of the test cores, and the reaction was carried out at 80 °C for 24 h. In-situ proppants were formed on the surface of the core microfractures, and the conductivity of the microfractures under different confining pressures before and after the reaction was tested under the same conditions as in Example 7. The experimental results are shown in Figure 12 . The results show that the change in the conductivity of the core microfractures before and after the reaction is not obvious.

[0042] The particle sizes of the in-situ self-generated proppant particles in Examples 1-6 and Comparative Examples 1-2 were measured using a laser particle size analyzer. The test results are shown in Figures 1 - 8 and Table 1. The median particle sizes of the proppant particles prepared in each example are larger than those of the proppant particles prepared in Comparative Example 1 and Comparative Example 2. The reason is that the pH regulator can form a buffer system through decomposition or hydrolysis reactions under high-temperature hydrothermal conditions, slowly increase the pH value of the reaction system, inhibit the nucleation rate, promote the deposition of ions on the existing crystal nuclei preferentially, and promote crystal growth rather than nucleation, ultimately forming large-sized crystals. Ca 2+ chelating agent can form a stable water-soluble chelate with Ca 2 + , reduce the concentration of free Ca 2+ in the solution. The lower Ca 2+ concentration leads to a decrease in the ion supersaturation in the solution, inhibits the nucleation rate, and promotes crystal growth into larger particles.

[0043] Among them, the median particle sizes of the proppant particles prepared in Example 2, Example 5, and Example 6 (adding pH regulator and Ca 2+ chelating agent) increased significantly. In particular, the median particle sizes of the proppant particles prepared in Example 5 and Example 6 achieved a qualitative improvement. The size of the proppant particles in Example 2 increased by about 621.53% compared to those in Comparative Example 1, and the size of the proppant particles in Example 2 increased by about 135.14% compared to those in Comparative Example 2; the size of the proppant particles in Example 5 increased by about 9323.44% compared to those in Comparative Example 1, and the size of the proppant particles in Example 5 increased by about 2971.03% compared to those in Comparative Example 2; the size of the proppant particles in Example 6 increased by about 7410.64% compared to those in Comparative Example 1, and the size of the proppant particles in Example 6 increased by about 2347.66% compared to those in Comparative Example 2. This shows that the pH regulator and Ca 2+ chelating agent can synergistically extend the crystal growth cycle of the proppant and promote the formation of large particles; moreover, different types and concentrations of Ca 2+ chelating agents also affect the size of the proppant particles.

[0044] Table 1 Median particle size of proppant particles in each example and comparative example

[0045]

[0046] The proppant particles prepared in Example 1 and Comparative Example 1 were observed and tested using a scanning electron microscope (SEM). The test results are shown in Figure 9 and Figure 10 respectively. The microscopic size of the proppant particles prepared in Example 1 is significantly larger than that of the proppant particles prepared in Comparative Example 1 at the same magnification.

[0047] By comparing Example 7 and Comparative Example 3, it can be seen that in the preparation process of the preflush fluid of the fracturing fluid for in-situ self-generating proppants in calcium-rich reservoirs provided by the Chinese invention patent with the publication number CN118562485A, the present invention adds a pH regulator and / or a Ca 2+ chelating agent, effectively increasing the generation size of the in-situ proppants, improving the propping effect, thereby enhancing the fracture conductivity and long-term stability, and optimizing the oil and gas production efficiency. It should be noted that different from the proppants made by other methods on the market now, the present invention is an optimization of the size of the in-situ generated proppants using the hydrothermal reaction of formation minerals.

[0048] Of course, the above description is not a limitation of the present invention, and the present invention is not limited to the above examples. Changes, modifications, additions or substitutions made by those skilled in the art within the scope of the essence of the present invention should also fall within the protection scope of the present invention.

Claims

1. A fracturing fluid preflush for optimizing the size of in-situ generated proppants, characterized in that, Applicable to calcium-rich reservoirs, composed of fresh water, phosphate, pH regulator and Ca 2+ chelating agent. Among them, relative to 100 parts by weight of fresh water, the content of phosphate is 1-10 parts by weight, the molar concentration of the pH regulator is 1-5 mol / L, and Ca 2+ the molar concentration of the chelating agent is 0.001-1 mol / L; The phosphate is at least one of diammonium hydrogen phosphate, ammonium dihydrogen phosphate, dipotassium hydrogen phosphate, potassium dihydrogen phosphate, disodium hydrogen phosphate, sodium dihydrogen phosphate, and sodium phosphate. The pH regulator is one or a mixture of urea, ammonia water, propionamide, sodium carbonate, and sodium bicarbonate. The Ca 2+ chelating agent is one or a mixture of ethylenediaminetetraacetic acid, disodium EDTA, trisodium EDTA, tetrasodium EDTA, citric acid, sodium citrate, disodium citrate, trisodium citrate, glutamic acid, hydroxyethylidene diphosphonic acid, aminotrimethylenephosphonic acid, and diethylenetriaminepentaacetic acid.

2. The preflush fluid for fracturing fluid used to optimize the size of in-situ generated proppant according to claim 1, wherein With respect to 100 parts by weight of clear water, the content of phosphate is 5 to 7 parts by weight, the molar concentration of the pH regulator is 1 to 2 mol / L, and the molar concentration of the Ca 2+ chelating agent is 0.001 to 1 mol / L.

3. The preflush fluid for fracturing fluid used to optimize the size of in-situ generated proppants according to claim 1, characterized in that, The calcium content of the formation core minerals in the calcium-rich reservoir is greater than 50%.

4. Use of a fracturing fluid preflush for optimizing the size of in-situ generated proppants according to any one of claims 1-3, characterized in that It includes the following steps: S1. After weighing phosphate and clear water according to a preset ratio, add the phosphate to the clear water and stir until the phosphate is completely dissolved; S2. Add a pH regulator and Ca 2+ chelating agent to the solution prepared in step S1 to obtain a preflush fluid for fracturing fluid; S3. Inject the fracturing fluid preflush into the microfractures of the calcium-rich reservoir and react for 12 to 48 hours. Use the formation temperature to hydrothermally synthesize hydroxyapatite crystals and generate in-situ self-supporting agents on the surface of the microfractures in the calcium-rich reservoir.

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