Fracturing fluid pre-fluid for optimizing in-situ autogenous proppant size and its application

Abstract

The present invention belongs to the technical field of petroleum fracturing production enhancement and modification, and specifically discloses a fracturing fluid pre-fluid for optimizing the size of in-situ autogenous proppant and its application, which is used to solve the problem of small particle size of proppant particles generated by hydrothermal reaction of formation minerals. The fracturing fluid pre-fluid is suitable for calcium-rich reservoirs and is composed of clean water, phosphate, pH regulator and Ca 2+ The chelating agent composition comprises 1 to 10 parts by weight of phosphate relative to 100 parts by weight of clean water, a volume molar concentration of the pH regulator is 1 to 5 mol / L, and Ca 2+ The molar concentration of the chelating agent is 0.001~1mol / L; the pH regulator is urea, ammonia, propionamide, etc. 2+ The chelating agent is EDTA, glutamic acid, etc. The present invention increases the average particle size of proppant particles generated by hydrothermal reaction of formation minerals, thereby improving the supporting effect thereof in micro-fractures of the formation.

Description

Technical Field

[0001] The invention belongs to the technical field of petroleum fracturing production enhancement and modification, and in particular relates to a fracturing fluid pre-fluid for optimizing the size of in-situ autogenous proppant and application thereof. Background Art

[0002] Hydraulic fracturing is a key technology in oil and gas field development and has been widely used in the development of low-permeability reservoirs. Hydraulic fracturing involves pumping a pre-fluid into the target formation to create and extend fractures. A sand-carrying fluid mixed with proppants is then injected. This fluid continues to extend the fractures, carrying the proppant deep into the fractures. This fluid then breaks down the fracturing fluid, degrading it into a low-viscosity fluid that flows to the bottom of the well, leaving behind a highly conductive channel. Proppants, a key material in the fracturing process, primarily support the fractures formed during fracturing, preventing them from closing and maintaining their conductivity, ensuring the smooth outflow of oil and gas.

[0003] Hydraulic fracturing is beneficial for oil and gas development and production, but current challenges include the closure of microfractures after fracturing, difficulties in proppant delivery, and poor effective support in the distal microfractures, severely limiting the conductivity of the fracture network. The small aperture of the microfractures prevents the penetration of traditional proppants such as expanded clay and quartz sand, leaving them without effective support. During production, these unsupported microfractures are susceptible to closure due to factors such as geostress and rock elastic recovery. This reduces the ability of the fracture network to communicate with natural fractures, restricting the efficiency and stability of hydraulic stimulation and causing a rapid decline in production from fractured wells. Research has shown that the majority of microfractures remain unsupported after fracturing, with unsupported fractures accounting for over 50% of the fracture volume.

[0004] Maintaining microfracture aperture is one of the keys to increasing and stabilizing reservoir production through fracturing. Using microproppants to support microfractures is the most direct method. However, microproppants are prone to agglomeration and difficult to disperse. Post-fracturing backflow poses the potential risk of blocking the sand diversion channel in the main fracture. The transport of microproppants to distal microfractures still presents significant obstacles, especially for microfractures smaller than 150 μm, which still requires further solutions.

[0005] To achieve support for microfractures in hydraulic fracturing, the applicant proposed, in Chinese invention patent publication number CN118562485A, an in-situ autogenous proppant for hydraulic fracturing microfractures in calcareous reservoirs, as well as a preparation method and application. This method involves injecting a phosphate system into the hydraulic fractures of the formation, utilizing the formation environment to hydrothermally synthesize the proppant in situ within the calcareous reservoir. This proppant can effectively improve the conductivity of rock fractures after hydraulic fracturing. However, the resulting proppant particles are too small, resulting in limited support effectiveness and limiting its scope of application. Therefore, increasing the size of hydroxyapatite crystals, an in-situ autogenous proppant synthesized hydrothermally from formation minerals, to further improve fracture conductivity is currently one of the key research directions for this technology. Summary of the Invention

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

[0007] In order to solve the above technical problems, the technical solution adopted by the present invention is: a fracturing fluid pre-fluid for optimizing the size of in-situ autogenous proppant, suitable for calcium-rich reservoirs, comprising the following raw materials: clean water, phosphate and pH regulator and / or Ca 2+ The chelating agent comprises 1 to 10 parts by weight of phosphate relative to 100 parts by weight of water, the volume molar concentration of the pH regulator is 1 to 5 mol / L, and the Ca 2+ The volume molar concentration of the chelating agent is 0.001 to 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 a mixture of one or more of urea, ammonia water, propionamide, sodium carbonate, and sodium bicarbonate; and the Ca 2+ The chelating agent is one or more mixtures of ethylenediaminetetraacetic acid (EDTA), EDTA disodium salt, EDTA trisodium salt, EDTA tetrasodium salt, citric acid, monosodium citrate, disodium citrate, trisodium citrate, glutamic acid, hydroxyethylidene diphosphonic acid, aminotrimethylenephosphonic acid, and diethylenetriaminepentaacetic acid.

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

[0010] Furthermore, the calcium content of the core minerals in the calcareous reservoir is greater than 50%.

[0011] Another object of the present invention is to provide an application of the fracturing fluid pre-fluid for optimizing the size of in-situ autogenous proppants as described in the above embodiment, comprising the following steps: S1, weighing phosphate and clean water according to a preset ratio, adding the phosphate to the clean water and stirring until the phosphate is completely dissolved; S2, adding a pH regulator and / or CaCl2 to the solution prepared in step S1; 2+ Chelating agent, to prepare fracturing fluid pre-fluid; S3, injecting the fracturing fluid pre-fluid into the micro-cracks of the calcium-rich reservoir to react for 12 to 48 hours, utilizing the formation temperature (generally the formation temperature of the shallow layer can reach 80 to 120°C) to hydrothermally synthesize hydroxyapatite crystals, and generating in-situ autogenous proppants on the surface of the micro-cracks of the calcium-rich reservoir.

[0012] Compared with the prior art, the beneficial technical effect of the present invention is that the present invention adds pH regulator and / or Ca 2+ Chelating agents and pH regulators can form a buffer system by decomposing or hydrolyzing under high temperature hydrothermal conditions, which slowly increases the pH value of the reaction system, inhibits the nucleation rate, and promotes the preferential deposition of ions on existing crystal nuclei, promoting crystal growth rather than nucleation, and ultimately forming large-sized crystals; Ca 2+ Chelating agents can bind to Ca 2+ Forms stable water-soluble chelates to reduce free Ca in the solution 2+ The concentration of Ca 2+ The concentration leads to 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 supporting effect of the in-situ autogenous proppant in the micro-cracks of the formation, is conducive to further improving the fracture conductivity and improving the oil and gas fracturing production enhancement effect. In addition, the method provided by the present invention is simple to operate and has a wide source of raw materials. BRIEF DESCRIPTION OF THE DRAWINGS

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

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

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

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

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

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

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

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

[0022] Figure 9 This is an electron microscopic image of the in-situ autogenous proppant particles in Example 1 magnified 5000 times.

[0023] Figure 10 This is an electron microscope image of the in-situ autogenous proppant particles in Comparative Example 1 after magnification 5000 times.

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

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

[0026] Figure 13 This is the X-ray diffraction pattern of the in-situ autogenous proppant obtained after separation in Example 1. JCPDS#86-0740 in the figure comes from the hydroxyapatite standard powder diffraction data of the International Diffraction Data Center.

[0027] Figure 14 This is the X-ray diffraction pattern of the in-situ autogenous proppant obtained after separation in Example 2. JCPDS#86-0740 in the figure comes from the hydroxyapatite standard powder diffraction data of the International Diffraction Data Center. DETAILED DESCRIPTION

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

[0029] A fracturing fluid pre-pad was prepared with 100 parts by weight of clean water, 7 parts by weight of sodium dihydrogen phosphate, and urea. Excess calcium carbonate was added to the prepared fracturing fluid pre-pad, stirred evenly, sealed with plastic wrap, and then placed in a 90°C electric oven to react for 24 hours. After the reaction was completed, the unreacted calcium carbonate was separated and dried to obtain in-situ self-generated proppant particles. The in-situ self-generated proppant obtained after separation (the product of Example 1) was subjected to X-ray diffraction analysis. Figure 13 It can be seen that the main component of the in-situ autogenous proppant is hydroxyapatite.

[0030] A specific method for separating unreacted calcium carbonate after the reaction is completed is described in detail in Chinese invention patent publication number CN118562485A as follows: Since the decomposition temperature of calcium carbonate is 825°C, while the decomposition temperature of in-situ self-generated proppant is 1200°C, the filtered solid mixture is placed in a muffle furnace at 900°C for high-temperature calcination to decompose the calcium carbonate in the solid mixture into calcium oxide and carbon dioxide, thereby obtaining a solid mixture of calcium oxide and proppant. Then, an NH4Cl solution having a volume concentration of 20% is prepared, and the NH4Cl solution is added to the solid mixture of calcium oxide and proppant, followed by reacting in a constant temperature water bath at 50°C for 0.5 to 1 hour. After the reaction is completed, the mixture is filtered and the reaction is repeated 3 to 5 times until no obvious irritating odor is emitted or the pH of the solution reaches 7, indicating that the reaction is complete. The reacted solution is filtered and dried to obtain in-situ self-generated proppant particles, thereby separating the unreacted calcium carbonate from the mixture.

[0031] Example 2: Based on Example 1, this example adds Ca2+ with a molar concentration of 1 mol / L. 2+ The chelating agent is EDTA, 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, the unreacted calcium carbonate is separated and dried to obtain in-situ self-generated proppant particles. The in-situ self-generated proppant obtained after separation (the product of Example 2) is subjected to X-ray diffraction analysis. Figure 14 It can be seen that the main component of the in-situ autogenous proppant is hydroxyapatite.

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

[0033] In this example, a solution was prepared with 100 parts by weight of water, 7 parts by weight of sodium dihydrogen phosphate, and 0.001 mol / L EDTA. An excess amount of calcium carbonate was added to the prepared solution, stirred evenly, sealed with plastic wrap, and then placed in a 90° C. electric oven to react for 24 hours. After the reaction was complete, the unreacted calcium carbonate was separated and dried to obtain in-situ autogenous proppant particles.

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

[0035] Example 5: The phosphate used in this example 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] A fracturing fluid pre-fluid was prepared according to 100 parts by weight of clean water, 7 parts by weight of sodium dihydrogen phosphate, 0.5 mol / L glutamic acid (7 parts by weight) and 1 mol / L urea. An excess amount of calcium carbonate was added to the prepared fracturing fluid pre-fluid, stirred evenly, sealed with plastic wrap, and then placed in a 90°C electric oven to react for 24 hours. After the reaction was completed, the unreacted calcium carbonate was separated and dried to obtain in-situ autogenous proppant particles.

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

[0038] Example 7: In this example, a fracturing fluid pre-fluid is prepared according to 100 parts by weight of clean water, 7 parts by weight of sodium dihydrogen phosphate and EDTA with a molar concentration of 0.001 mol / L, and a carbonate rock core with a calcium content of more than 90% is selected (in order to have obvious effects and facilitate observation and comparison in this example, a rock core with a calcium content of more than 90% is selected, but theoretically, as long as there are calcium minerals in the rock minerals, in-situ autogenous proppants can be generated, 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%). Micro-cracks in the core are prepared using a wire cutting process to simulate micro-cracks in the formation. The fracturing fluid pre-fluid is injected into the micro-cracks of the test core and reacted at 80°C for 24 hours to generate in-situ proppants on the surface of the micro-cracks in the core. The micro-crack conductivity under different confining pressures before and after the reaction is tested. The experimental results are shown in FIG. Figure 11 The results show that the conductivity of the core microfractures is greatly improved after the reaction. Under a high confining pressure of 20 MPa, the conductivity of the microfractures is about 146 times that before the reaction.

[0039] Comparative Example 1: In this comparative example, a fracturing fluid pre-fluid was prepared according to 100 parts by weight of clean water and 5 parts by weight of sodium dihydrogen phosphate; an excess amount of calcium carbonate was added to the prepared fracturing fluid pre-fluid, stirred evenly, sealed with plastic wrap, and then placed in a 90°C electric oven to react for 24 hours. After the reaction was completed, the unreacted calcium carbonate was separated and dried to obtain in-situ autogenous proppant particles.

[0040] Comparative Example 2: This comparative example differs from Comparative Example 1 in that the amount of sodium dihydrogen phosphate added is 7 parts by weight.

[0041] Comparative Example 3: In this comparative example, a fracturing fluid pre-fluid was prepared according to 100 parts by weight of clean water and 7 parts by weight of sodium dihydrogen phosphate. A carbonate rock core with a calcium content of more than 90% was also selected. The core micro-cracks were prepared using a wire cutting process to simulate the formation micro-cracks. The fracturing fluid pre-fluid was injected into the micro-cracks of the test core and reacted at 80°C for 24 hours to generate in-situ proppant on the surface of the core micro-cracks. The micro-crack conductivity 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 FIG. Figure 12 The results show that the conductivity of core microfractures does not change significantly before and after the reaction.

[0042] The particle size of the in-situ self-generated proppant particles of Examples 1-6 and Comparative Examples 1-2 was tested using a laser particle size analyzer. The test results are as follows: Figures 1-8 As shown in Table 1. The median particle size of the proppant particles prepared in each example is larger than that of the proppant particles prepared in Comparative Examples 1 and 2. The reason is that the pH adjuster can form a buffer system by decomposing or hydrolyzing under high-temperature hydrothermal conditions, which slowly increases the pH value of the reaction system, inhibits the nucleation rate, and promotes the preferential deposition of ions on existing crystal nuclei, thereby promoting crystal growth rather than nucleation, and ultimately forming large-sized crystals. Ca 2+ Chelating agents can bind to Ca 2 + Forms stable water-soluble chelates to reduce free Ca in the solution 2+ The concentration of Ca 2+ The concentration leads to a decrease in the ion supersaturation in the solution, inhibiting the nucleation rate and promoting the growth of crystals into larger particles.

[0043] Among them, Example 2, Example 5 and Example 6 (adding pH regulator and Ca 2+ The median particle size of the proppant particles prepared with the addition of a chelating agent (a chelating agent) was significantly improved, especially the median particle size of the proppant particles prepared in Examples 5 and 6, which achieved a qualitative improvement. The proppant particle size of Example 2 increased by approximately 621.53% compared to Comparative Example 1, and the proppant particle size of Example 2 increased by approximately 135.14% compared to Comparative Example 2; the proppant particle size of Example 5 increased by approximately 9323.44% compared to Comparative Example 1, and the proppant particle size of Example 5 increased by approximately 2971.03% compared to Comparative Example 2; the proppant particle size of Example 6 increased by approximately 7410.64% compared to Comparative Example 1, and the proppant particle size of Example 6 increased by approximately 2347.66% compared to Comparative Example 2. This indicates that the pH adjuster and Ca 2+ Chelating agents can synergistically extend the growth period of proppant crystals and promote the formation of large particles; and different types and concentrations of Ca 2+ Chelating agents can also affect proppant particle size.

[0044] Table 1 Median particle size of proppant particles in various examples and comparative examples

[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 as follows: Figure 9 and Figure 10 As shown, 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 with Comparative Example 3, it can be seen that the present invention adds pH regulator and / or Ca in the preparation process of the fracturing fluid pre-fluid of the in-situ self-generated proppant for fracturing micro-fractures of calcium-rich reservoirs provided by the Chinese invention patent with publication number CN118562485A. 2+ The chelating agent effectively increases the size of the in-situ proppant, enhancing its support effectiveness, thereby increasing fracture conductivity and long-term stability, and optimizing oil and gas recovery efficiency. It is worth noting that, unlike other proppants currently available on the market, this invention optimizes the size of proppants generated in situ through hydrothermal reactions 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 technicians in this technical field within the essential scope of the present invention should also fall within the scope of protection of the present invention.

Claims

1. A fracturing fluid pre-fluid for optimizing the size of in-situ autogenous proppants, characterized in that: Applicable to calcium-rich reservoirs, composed of clean water, phosphate, pH regulator and Ca 2+ The chelating agent composition comprises 1 to 10 parts by weight of phosphate relative to 100 parts by weight of clean water, a volume molar concentration of the pH regulator is 1 to 5 mol / L, and Ca 2+ The molar concentration of the chelating agent is 0.001 to 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 a mixture of one or more of urea, ammonia water, propionamide, sodium carbonate, and sodium bicarbonate; and the Ca 2+ The chelating agent is one or more mixtures of ethylenediaminetetraacetic acid, EDTA disodium salt, EDTA trisodium salt, EDTA tetrasodium salt, citric acid, monosodium citrate, disodium citrate, trisodium citrate, glutamic acid, hydroxyethylidene diphosphonic acid, aminotrimethylenephosphonic acid, and diethylenetriaminepentaacetic acid.

2. The fracturing fluid pre-pad for optimizing the size of in-situ autogenous proppant according to claim 1, characterized in that: The content of phosphate is 5 to 7 parts by weight relative to 100 parts by weight of clean water, the volume molar concentration of the pH regulator is 1 to 2 mol / L, and the Ca 2+ The volume molar concentration of the chelating agent is 0.001 to 1 mol / L.

3. The fracturing fluid pre-pad for optimizing the size of in-situ autogenous proppant according to claim 1, characterized in that: The calcium content of the core minerals in the calcareous reservoir is greater than 50%.

4. The use of the fracturing fluid pre-pad for optimizing the size of in-situ autogenous proppants according to any one of claims 1 to 3, characterized in that: The following steps are involved: S1. Weigh phosphate and water according to a preset ratio, add phosphate to the water and stir until the phosphate is completely dissolved; S2, add pH regulator and Ca to the solution prepared in step S1 2+ Chelating agent, to prepare fracturing fluid pre-fluid; S3. Injecting the fracturing fluid pre-fluid into the micro-fractures of the calcium-rich reservoir for a reaction of 12 to 48 hours, utilizing the formation temperature to hydrothermally synthesize hydroxyapatite crystals, and generating in-situ autogenous proppants on the surface of the micro-fractures of the calcium-rich reservoir.

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