Fracturing fluid preflush for strengthening in-situ self-borne proppant strength and morphology and application

Abstract

The application belongs to the technical field of oil fracturing reconstruction, and particularly relates to a fracturing fluid preflush for strengthening in-situ self-grown proppant strength and morphology and application, which is used to solve the problems of insufficient strength and irregular morphology of generated in-situ proppant crystals. The fracturing fluid preflush is suitable for calcium-rich reservoirs, and is composed of clean water, phosphate, and pH adjuster and Ca 2+ The content of the phosphate is 5-7 parts by weight, the volume molar concentration of the pH adjuster is 1-2 M, the volume molar concentration of the chelating agent is 0.001-1 M, and the content of the doping element is 1 part by weight, relative to 100 parts by weight of clean water. 2+ The application strengthens the in-situ self-grown proppant strength and optimizes the crystal morphology, improves the supporting effect of the in-situ self-grown proppant in the formation micro-cracks, improves the fracture conductivity, and improves the oil and gas fracturing stimulation effect.

Description

Technical Field

[0001] This invention belongs to the field of petroleum fracturing and production enhancement technology, and particularly relates to fracturing fluid pre-flush fluid and its application for enhancing the strength and morphology of in-situ self-generated proppant. Background Technology

[0002] Fracturing technology is one of the key methods for the efficient development of unconventional oil and gas reservoirs. It involves injecting high-pressure fluids to create artificial fractures in the reservoir, forming a fracture network and establishing highly conductive oil and gas flow channels to improve oil and gas recovery. Proppants, as a crucial material in the fracturing process, are used to support fractures, prevent them from closing, and maintain long-term conductivity. However, traditional proppants (such as ceramsite and quartz sand) have relatively large particle sizes, making it difficult for them to penetrate the microfractures (especially those ≤150μm) formed at the distal end of the fracturing process. Unsupported microfractures gradually close due to stress during production, severely impacting the fracturing effect.

[0003] To address the challenge of supporting microfractures at the distal end of fracturing, Chinese invention patent CN118562485B proposes an in-situ self-generated proppant for microfractures in calcium-rich reservoirs, its preparation method, and its application. This technology involves injecting a pure liquid phosphate system into the microfractures in the formation, utilizing the formation environment to generate a proppant through an in-situ hydrothermal reaction in the calcium-rich reservoir, effectively supporting the microfractures and improving the conductivity of the fractured reservoir rock after fracturing. However, despite the initial success in improving fracture conductivity, the generated in-situ proppant still faces several key challenges: firstly, the particle size is generally small, limiting the propping effect and application range; secondly, insufficient control over crystal strength and morphology affects long-term stability under high closure pressure. Existing technologies also introduce pH adjusters and Ca into the reaction system... 2+ Chelating agents and other methods have increased the particle size of in-situ self-generated proppant. However, the strength of proppant crystals generated in-situ using this technology is still relatively low, and they are prone to breakage under high closure pressure, blocking the pore channels. The crystal morphology is irregular, and it is easy to rotate and displace under stress, leading to local closure of cracks and negatively affecting the crack conductivity.

[0004] The strength of proppant particles is a key factor determining their resistance to fracturing and long-term conductivity stability, while crystal morphology (such as sphericity and particle uniformity) directly affects the proppant's deposition pattern, embedding resistance, and anti-closure ability in fractures, thus influencing the maintenance of fracture conductivity. Therefore, based on the in-situ self-generated proppant size optimization disclosed in Chinese Invention Patent No. CN120272185B, this paper further explores methods to enhance crystal strength and optimize crystal morphology, aiming to improve the applicability and stability of proppant under complex formation conditions and achieve efficient and stable propping of distal microfractures in hydraulic fracturing. Summary of the Invention

[0005] The purpose of this invention is to provide a fracturing fluid pre-flush fluid and its application that enhances the strength and morphology of in-situ self-generated proppant, in order to solve the problems of insufficient strength and irregular morphology of in-situ proppant crystals generated in the prior art.

[0006] To solve the above-mentioned technical problems, the technical solution adopted in this invention is: a fracturing fluid pre-flush fluid that enhances the strength and morphology of in-situ self-generated proppant, suitable for calcium-rich reservoirs, composed of water, phosphate, pH adjuster, and Ca. 2+ The composition includes chelating agents and doping elements, wherein, relative to 100 parts by weight of water, the phosphate content is 5 to 7 parts by weight, the pH adjuster has a volume molar concentration of 1M to 2M, and Ca... 2+ The volumetric molar concentration of the chelating agent is 0.001 M to 1 M, and the content of the dopant element is 1 part by weight. The phosphate is one or more of the following: diammonium hydrogen phosphate, diammonium dihydrogen phosphate, dipotassium hydrogen phosphate, potassium dihydrogen phosphate, disodium hydrogen phosphate, sodium dihydrogen phosphate, and sodium phosphate. The pH adjuster is one or a mixture of urea, ammonia, and propionamide; The Ca 2+ The chelating agent is one or more of the following: glutamic acid, ethylenediaminetetraacetic acid, disodium ethylenediaminetetraacetic acid, trisodium ethylenediaminetetraacetic acid, tetrasodium ethylenediaminetetraacetic acid, citric acid, monosodium citrate, disodium citrate, and trisodium citrate. The doping element is one or more of Zn, Mg, and Sr.

[0007] This invention also provides the application of fracturing fluid pre-flush fluid that enhances the strength and morphology of in-situ self-generated proppant, including the following steps: S1. Weigh out the phosphate and water according to the preset ratio, then add the phosphate to the water and stir until the phosphate is completely dissolved; S2. Add pH adjuster and Ca to the solution prepared in S1. 2+ Chelating agents; S3. Add compounds of different elements to the solution prepared in S2, stir until completely dissolved, and obtain fracturing fluid pre-fracturing fluid; S4. The fracturing fluid pre-flush fluid is injected into the microfractures of the calcium-rich reservoir and reacted for 12 to 48 hours. Hydroxyapatite crystals are synthesized hydrothermally using the formation temperature, and in-situ self-generated proppant is generated on the surface of the microfractures of the calcium-rich reservoir.

[0008] Compared with the prior art, the beneficial technical effects of the present invention are: This invention enhances the strength and optimizes the crystal morphology of in-situ self-generated proppant, thereby improving its propping effect in formation microfractures. This helps maintain fracture conductivity under high closure pressure and improves the effectiveness of oil and gas fracturing and production enhancement. By doping with different types of elements such as Zn, Mg, and Sr, which exist in ionic form in the system and enter the crystal lattice during the hydrothermal reaction, causing lattice distortion and reducing crystal defects, the strength of the crystal is improved. The dopant ions can selectively adsorb on specific crystal faces of hydroxyapatite crystals, altering the relative growth rates of these faces and thus changing the crystal growth habit to generate crystals with specific morphologies. For example, dopant ions can adsorb on the c-face of hydroxyapatite crystals, inhibiting c-face growth and making the growth rates of the a, b, and c faces comparable, thus making it easier for the crystal to grow into a spherical shape. Attached Figure Description

[0009] Figure 1 This is a particle size distribution diagram of the in-situ self-generated proppant particles prepared in Example 1 before and after strength testing; Figure 2 These are particle size distribution diagrams of the in-situ self-generated proppant particles prepared in Example 2 before and after strength testing; Figure 3 These are particle size distribution diagrams of the in-situ self-generated proppant particles prepared in Example 3 before and after strength testing; Figure 4 The particle size distribution diagrams of the in-situ self-generated proppant particles prepared in Comparative Example 1 before and after strength testing are shown. Figure 5 This is a 60x magnified microscopic image of the in-situ proppant particles generated on the surface of the crack after the reaction in Example 4; Figure 6 This is a 330x magnified microscopic image of the in-situ proppant particles generated on the surface of the crack after the reaction in Example 4; Figure 7 This is a 60x magnified microscopic image of the in-situ proppant particles generated on the surface of the crack after the reaction in Example 5; Figure 8 This is a 330x magnified microscopic image of the in-situ proppant particles generated on the surface of the crack after the reaction in Example 5. Figure 9 This is a 60x magnified microscopic image of the in-situ proppant particles generated on the surface of the crack after the reaction in Comparative Example 2. Figure 10 This is a 330x magnified microscopic image of the in-situ proppant particles generated on the surface of the crack after the reaction in Comparative Example 2. Figure 11 This is a graph showing the crack conductivity test under different closure pressures before and after the reaction (i.e. before and after the in-situ proppant is generated) in Example 4. Figure 12This is a graph showing the crack conductivity test curves under different closure pressures before and after the reaction (i.e. before and after the in-situ proppant is generated) in Example 5. Figure 13 This is a graph showing the test curves of the fracture conductivity under different closure pressures before and after the reaction (i.e. before and after the formation of in-situ proppant) in Comparative Example 2. Detailed Implementation

[0010] The present invention will be further illustrated below with reference to embodiments. These embodiments are for illustrative purposes only and are not intended to limit the invention in any way. It should be understood that the described embodiments are merely some, not all, of the embodiments described in this application. All other embodiments obtained by those skilled in the art based on the embodiments in this application without inventive effort are within the scope of protection of this application.

[0011] Example 1 Add 100 parts by weight of water, 5 parts by weight of sodium dihydrogen phosphate, and a pH adjuster urea with a volume molar concentration of 1.5M. Add Ca... 2+ The chelating agent glutamic acid was prepared with a volume molar concentration of 0.5 M, and 1 part by weight of SrCl2 (i.e., Sr element) was added to prepare the solution. Calcium carbonate was added to the prepared solution, stirred evenly, sealed with plastic wrap, and then placed in a 90℃ electric heating oven for 24 hours to react. After the reaction was completed, the unreacted calcium carbonate was separated and dried to obtain the in-situ proppant particles.

[0012] Because the in-situ proppant particles in this invention are small and mainly target micro-cracks at the distal end of fracturing, resulting in a limited quantity, a combination of laser particle size analyzer and pressure testing machine was used for strength testing. The specific steps are as follows: First, the particles are sieved, and then the particle size of the generated proppant particles is measured using a laser particle size analyzer. Subsequently, 0.5g of sample is placed in a test chamber with a diameter of 1.5cm and spread evenly. A constant pressure of 30MPa is applied using a pressure testing machine, maintained for 2 minutes, and then the pressure is released. The sample is then removed from the test chamber, and the particle size distribution of the sample is measured again using a laser particle size analyzer. The results are compared with those before the test to evaluate the strength of the proppant particles. Figure 1 The image shows the particle size distribution of the in-situ self-generated proppant particles prepared in Example 1 before and after strength testing.

[0013] Example 2 The difference between this embodiment and Example 1 is that: 5 parts by weight of sodium dihydrogen phosphate in Example 1 are replaced with 7 parts by weight of ammonium dihydrogen phosphate, 1.5M urea is replaced with 2M ammonia, 0.5M glutamic acid is replaced with 1M ethylenediaminetetraacetic acid, and 1 part by weight of SrCl2 is replaced with 1 part by weight of MgCl2 (i.e., Mg element). All other conditions are the same, and in-situ proppant particles are obtained. The strength of the proppant particles is evaluated under the same conditions as in Example 1. Figure 2 The image shows the particle size distribution of the in-situ self-generated proppant particles prepared in Example 2 before and after the strength test.

[0014] Example 3 The difference between this embodiment and Example 1 is that: 5 parts by weight of sodium dihydrogen phosphate in Example 1 are replaced with 7 parts by weight of potassium dihydrogen phosphate, 1.5M urea is replaced with 1M propionamide, 0.5M glutamic acid is replaced with 0.001M citric acid, and 1 part by weight of SrCl2 is replaced with 1 part by weight of ZnCl2 (i.e., Zn element). All other conditions are the same, and in-situ proppant particles are obtained. The strength of the proppant particles is evaluated under the same conditions as in Example 1. Figure 3 The image shows the particle size distribution of the in-situ self-generated proppant particles prepared in Example 3 before and after strength testing.

[0015] Example 4 Add 100 parts by weight of water and 5 parts by weight of sodium dihydrogen phosphate, with a pH adjuster urea concentration of 1.5M, and add Ca... 2+ A fracturing fluid pre-fracturing fluid system was prepared by adding 1 part by weight of SrCl2 to 0.5 M chelating agent glutamic acid. Carbonate cores with a calcium content exceeding 90% were selected, and artificial fracture-making devices were used to create fractures in the cores to simulate calcium-rich reservoirs. The prepared fracturing fluid pre-fracturing fluid was injected into the microfractures of the test cores and reacted at 80℃ for 24 hours. After the reaction, the cores were retrieved, and the fracture conductivity under different closure pressures before and after the reaction was tested to compare the changes in conductivity. The morphology of in-situ proppant particles formed on the fracture surface after the reaction was observed using confocal microscopy. Figure 5 and Figure 6 This is a microscopic image of in-situ proppant particles generated on the surface of the crack after the reaction in Example 4. Figure 11 The graph shows the crack conductivity test curves under different closure pressures before and after the reaction (i.e. before and after the in-situ proppant is generated) in Example 4.

[0016] Example 5 The difference between this embodiment and Embodiment 4 is that 1 part by weight of SrCl2 was replaced with 1 part by weight of ZnCl2, while the other conditions remained the same. The crack conductivity under different closure pressures before and after the reaction was tested under the same conditions, and the changes in conductivity before and after the reaction were compared. The morphology of the in-situ proppant particles formed on the crack surface after the reaction was observed using a confocal microscope. Figure 7 and Figure 8 This is a microscopic image of in-situ proppant particles generated on the surface of the crack after the reaction in Example 5. Figure 12 The graph shows the crack conductivity test curves under different closure pressures before and after the reaction (i.e. before and after the in-situ proppant is generated) in Example 5.

[0017] Comparative Example 1 This comparative example uses 100 parts by weight of water, 5 parts by weight of sodium dihydrogen phosphate, and urea as a pH adjuster (urea volume molar concentration of 1.5M). Ca is also added. 2+ Glutamic acid, a chelating agent, was prepared in a 0.5 M molar concentration solution. Calcium carbonate was added to the prepared solution, stirred thoroughly, sealed with plastic wrap, and then placed in a 90°C oven for 24 hours. After the reaction was complete, the unreacted calcium carbonate was separated and dried to obtain the in-situ proppant particles. The strength of the in-situ proppant was tested according to the method in Example 1. Figure 4 The particle size distribution diagrams are shown before and after the strength test of the in-situ self-generated proppant particles prepared in Comparative Example 1.

[0018] Comparative Example 2 This comparative example uses 100 parts by weight of water, 5 parts by weight of sodium dihydrogen phosphate, and adds urea as a pH adjuster. The urea has a volume molar concentration of 1.5M. Ca is added. 2+ A chelating agent, glutamic acid, was used to prepare a fracturing fluid pre-fracturing system at a volume molar concentration of 0.5 M. Carbonate cores with a calcium content exceeding 90% were selected, and fractures were created in the cores using an artificial fracture-making device. The carbonate cores were then used to simulate calcium-rich reservoirs. The prepared fracturing fluid pre-fracturing system was injected into the microfractures in the test cores and reacted at 80℃ for 24 hours. After the reaction, the cores were retrieved, and the fracture conductivity under different closure pressures before and after the reaction was tested to compare the changes in conductivity. The morphology of the in-situ proppant particles formed on the fracture surface after the reaction was observed using a confocal microscope. Figure 9 and Figure 10 This is a microscopic image of in-situ proppant particles generated on the surface of the crack after the reaction in Comparative Example 2. Figure 13 The graph shows the fracture conductivity test curves under different closure pressures before and after the reaction (i.e. before and after the formation of in-situ proppant) in Comparative Example 2.

[0019] The strength test results of Examples 1, 2, and 3 and Comparative Example 1 are as follows: Figure 1 , Figure 2 , Figure 3 , Figure 4 As shown in Table 1, using the reduction in median particle size before and after testing as the evaluation criterion, the reduction in median particle size of the in-situ proppant particles prepared in Examples 1, 2, and 3 was less than that in Comparative Example 1, indicating that the strength of the in-situ proppant particles prepared in Examples 1, 2, and 3 was improved. Among them, Example 3, which was doped with Zn, showed the greatest strength improvement after testing, with a median particle size reduction of only 31.39%, while Comparative Example 1, which was not doped, showed a median particle size reduction of 87.83% after testing.

[0020] Table 1 shows the change in median particle size of the proppant product before and after the strength test. .

[0021] Microscopic images of in-situ proppant particles generated on the surface of cracks after the reaction in Examples 4, 5, and Comparative Example 2 are shown below. Figure 5 , Figure 6 , Figure 7 , Figure 8 , Figure 9 and Figure 10 As shown. Figure 9 and Figure 10 As can be seen, in Comparative Example 2 without element doping, the proppant crystal particles formed on the crack surface exhibit elongated or needle-like morphologies. For example... Figure 5 and Figure 6 It can be seen that the proppant crystal particles formed on the crack surface in Example 4, which is doped with Sr, have a predominantly spherical morphology, with some exhibiting irregular shapes; such as Figure 7 and Figure 8 It can be seen that the proppant crystal particles generated on the crack surface in Example 5, which is doped with Zn, are almost completely independent spheres with a high degree of sphericity.

[0022] Figure 11 , Figure 12 and Figure 13 The figures show the test results of crack conductivity before and after the reaction in Examples 4, 5, and Comparative Example 2. From... Figure 13 It can be seen that, in Comparative Example 2 without doped elements, the crack conductivity after the reaction to generate in-situ proppant was increased by approximately 5 to 19 times compared to before the reaction, under a closure pressure of 5 MPa to 30 MPa; from Figure 11 It can be seen that in Example 4, which is doped with Sr, the crack conductivity after the in-situ proppant is generated under a closure pressure of 5 MPa to 30 MPa is increased by approximately 354 to 2866 times compared to before the reaction; from Figure 12 As can be seen, in Example 5 with Zn doping, the crack conductivity after the reaction to generate in-situ proppant was increased by about 130 to 1518 times compared with that before the reaction, under a closing pressure of 5 MPa to 30 MPa. The experimental results prove that the method of doping elements in this invention enhances the strength of in-situ proppant particles and optimizes the morphology of particles, which can further improve crack conductivity.

[0023] Of course, the above description is not intended to limit the present invention, and the present invention is not limited to the examples given above. Any changes, modifications, additions or substitutions made by those skilled in the art within the scope of the present invention should also fall within the protection scope of the present invention.

Claims

1. A fracturing fluid pre-flush fluid that enhances the strength and morphology of in-situ self-generated proppant, characterized in that, Suitable for calcium-rich reservoirs, consisting of water, phosphates, pH adjusters, and Ca. 2+ The compound consists of chelating agents and dopant elements, wherein, relative to 100 parts by weight of water, the phosphate content is 5 to 7 parts by weight, the pH adjuster has a volume molar concentration of 1M to 2M, and Ca... 2+ The volumetric molar concentration of the chelating agent is 0.001 M to 1 M, and the content of the compound containing the dopant element is 1 part by weight. The phosphate is one or more of the following: diammonium hydrogen phosphate, diammonium dihydrogen phosphate, dipotassium hydrogen phosphate, potassium dihydrogen phosphate, disodium hydrogen phosphate, sodium dihydrogen phosphate, and sodium phosphate. The pH adjuster is one or a mixture of urea, ammonia, and propionamide; The Ca 2+ The chelating agent is one or more of the following: glutamic acid, ethylenediaminetetraacetic acid, disodium ethylenediaminetetraacetic acid, trisodium ethylenediaminetetraacetic acid, tetrasodium ethylenediaminetetraacetic acid, citric acid, monosodium citrate, disodium citrate, and trisodium citrate. The compound containing the doping element is one or more of ZnCl2, MgCl2, and SrCl2.

2. The application of the fracturing fluid pre-flush fluid as described in claim 1, which enhances the strength and morphology of in-situ self-generated proppant, is characterized in that... Includes the following steps: S1. Weigh out the phosphate and water according to the preset ratio, then add the phosphate to the water and stir until the phosphate is completely dissolved; S2. Add pH adjuster and Ca to the solution prepared in S1. 2+ Chelating agents; S3. Add compounds of different elements to the solution prepared in S2, stir until completely dissolved, and obtain fracturing fluid pre-fracturing fluid; S4. The fracturing fluid pre-fracturing fluid prepared in S3 is injected into the microfractures of the calcium-rich reservoir and reacted for 12 to 48 hours. Hydroxyapatite crystals are synthesized hydrothermally using the formation temperature, and in-situ self-generated proppant is generated on the surface of the microfractures of the calcium-rich reservoir.

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