A nano calcium carbide plugging agent for gas channeling in low oil saturation reservoirs, a preparation method and applications thereof

Abstract

This invention relates to the field of plugging agent preparation technology, specifically to a nano-calcium carbide plugging agent for gas channeling in low-oil-saturation reservoirs, its preparation method, and its application. The method includes the following steps: S1. Preparation of modified nano-calcium particles; S2. Laying the modified nano-calcium particles on a graphene surface and heating to obtain nano-calcium carbide particles; S3. Dispersing the nano-calcium carbide particles in a modified coating agent, maintaining the temperature at room temperature in a vacuum drying oven for 3-5 hours, then circulating back to atmospheric pressure, repeating this process 2-3 times to obtain coated modified nano-calcium carbide particles; S4. Placing the particles in a Wurster fluidized bed, atomizing and spraying a modified ethyl cellulose solution upwards, allowing it to encounter the particles and form a film on the surface, thus obtaining the nano-calcium carbide plugging agent. This invention uses a plugging agent in the form of nano-solid capsules, which is injected into the formation along with liquid carbon dioxide during carbon dioxide gas-driven production, eliminating the need for well shut-in and production shutdown. It achieves plugging of micro-fractures in the formation, causing no harm to the reservoir environment and allowing for easy unplugging later.

Description

Technical Field

[0001] This invention relates to the field of plugging agent preparation technology, specifically to a nano-calcium carbide plugging agent for gas channeling in low oil saturation reservoirs, its preparation method, and its application. Background Technology

[0002] Low oil saturation reservoirs are characterized by low crude oil content and high water saturation. These reservoirs often suffer from a high formation water content, which, when combined with external fluids, leads to clay swelling, decomposition, and migration, resulting in a significant decrease in reservoir permeability. Therefore, when choosing water injection development for low oil saturation reservoirs, it is crucial to determine the appropriate injected water salinity and suitable anti-swelling agents. Furthermore, determining the optimal well spacing, well pattern, and injection timing is also essential. This process is complex, costly, and may not even achieve the desired results after implementation.

[0003] Carbon dioxide flooding, as one of the most popular oil recovery (EOR) technologies, boasts unique advantages in oil displacement efficiency and environmental protection, and has received widespread attention both domestically and internationally in recent years. However, due to the low mobility of carbon dioxide gas, its application in fractured or heterogeneous formations with significant permeability differences carries a greater risk of early gas breakthrough or gas channeling than water injection. This leads to a decrease in swept volume, leaving a large amount of remaining oil unrecovered and drastically reducing oil recovery. Therefore, preventing gas channeling is crucial for successful carbon dioxide flooding in these reservoirs.

[0004] Currently, the main methods for preventing gas channeling in carbon dioxide flooding are water-gas alternation technology and polymer, gel, and foam plugging technology. Water-gas alternation technology is simple to operate and economical. This method introduces another phase fluid to change the relative permeability of the gas, increasing the viscosity of the injected gas and improving the mobility ratio, thereby reducing channeling. It is a widely used control measure. However, this method is less effective in water-sensitive reservoirs and fractured reservoirs. Furthermore, the injection of water can easily cause problems such as reservoir clay swelling and migration, causing some damage to the reservoir and significantly impacting the oil displacement effect. Polymer, gel, and foam plugging technology mainly aims to achieve relative homogenization of heterogeneous reservoirs by plugging local high-permeability areas. It is an effective means of deep profile control with good plugging effects. However, this technology is often applied after severe gas channeling has occurred in the reservoir. Configuring the corresponding polymer, gel, and foam systems requires high costs and necessitates well shutdown and production stoppage, severely disrupting the economic benefits of the oilfield.

[0005] The invention patent with publication number CN117126653A discloses a nanofluid for selectively blocking gas channeling, its preparation method and application. The method uses a nanofluid system as a plugging agent, which dehydrates under the action of gas to form a solid substance, thereby reducing the gas phase permeability. However, this method requires the nano system to be injected into the formation to form a gel. After the gel is formed, the fluid viscosity will increase significantly, the flow resistance will increase, and it is easy to form a blockage in the near-wellbore area. It is difficult to block advantageous channels such as microfractures in deep formations. Chinese patent CN110483684A discloses a gel particle and its preparation method for preventing gas channeling in continuous carbon dioxide injection oil displacement processes. This method uses water-soluble reactive monomers to undergo free radical copolymerization under the action of two crosslinking agents (a stable crosslinking agent and an unstable crosslinking agent) to obtain gel particles. These gel particles require initial partial swelling in injection water or formation water with a pH of 5-9, followed by secondary swelling after high-pressure carbon dioxide injection when the formation water pH is ≤4, to achieve significant blocking. However, the formation water conditions in most low-oil-saturation reservoirs cannot meet the pH requirement of 5-9, and injecting water with a pH of 5-9 can easily trigger reservoir gas channeling. Clay swelling and particle migration can damage reservoir properties. Patent CN115073663A discloses a CO2-triggered anti-gas channeling plugging agent, its preparation method, and its application. This method uses hydroxyethyl acrylate, a monomer containing terminal double bonds, and KH-570 silane coupling agent as raw materials to produce oligomers of the reactant monomers through polymerization. These oligomers have low viscosity, high fluidity, and good injectability, enabling effective plugging in formations. However, hydroxyethyl acrylate (also known as 2-hydroxyethyl acrylate) used in this preparation is a toxic and environmentally hazardous substance; large-scale use poses a certain risk to the reservoir environment.

[0006] Therefore, it is urgent to develop a gas channeling blocker that can effectively block gas channeling in low-oil-saturation reservoirs during the production process, with good blocking performance and no environmental hazards. Summary of the Invention

[0007] To address the shortcomings of existing technologies, the present invention aims to provide a nano-calcium carbide plugging agent for gas channeling in low-oil-saturation reservoirs, its preparation method, and its application. The nano-calcium carbide plugging agent for gas channeling in low-oil-saturation reservoirs is injected into the formation along with liquid carbon dioxide. Due to the heterogeneity of permeability in underground reservoirs, carbon dioxide carries the nano-calcium carbide plugging agent preferentially into dominant channels such as microfractures and large pores. As the injection distance of carbon dioxide increases, the carbon dioxide flow rate gradually decreases, and the nano-calcium carbide plugging agent begins to settle in these dominant channels. After settling, the capsule shell of the nano-calcium carbide plugging agent gradually opens under the influence of formation water, exposing the nano-calcium carbide particles inside. These particles react with water and carbon dioxide in the formation to form calcium carbonate precipitate. This calcium carbonate precipitate, following the flow of carbon dioxide, blocks the reservoir until a certain amount of calcium carbonate precipitate accumulates, thus sealing the dominant channels such as microfractures in the reservoir.

[0008] To achieve the above objectives, the present invention provides the following technical solution: A method for preparing a nano-calcium carbide plugging agent for gas channeling in low-oil-saturation reservoirs includes the following preparation steps: S1. Preparation of modified calcium nanoparticles: S11. Dissolve the calcium-containing metal salt in ethylene glycol to form a calcium salt ethylene glycol solution with a concentration of 0.1-0.2 mL / L; S12. Dissolve the polyvinylpyrrolidone-polyvinyl alcohol copolymer in ethylene glycol to form an ethylene glycol etherified PVP-VA solution with a concentration of 0.2-0.375 mL / L; S13. Place ethylene glycol in a reaction vessel and heat it to 120-170℃ and keep it at that temperature for 25-30 minutes. Inject the calcium salt ethylene glycol solution and the ethylene glycol etherified PVP-VA solution into the reaction vessel using a syringe and keep it at 120-200℃ for 30-40 minutes. S14. Pour the reaction solution obtained in step S13 into a chloroform solution, separate and wash it in a centrifuge at a speed of 5000-6000 rpm to obtain modified calcium nanoparticles with a particle size of 50-100 nm. S2. The modified calcium nanoparticles obtained in step S1 are laid on the surface of graphene and placed in a vacuum chamber. A heating device is set under the graphene and heated at 1950-2000℃ to obtain calcium carbide nanoparticles. S3. By mass, take 90-100 parts of nano-calcium carbide particles and disperse them in 100-110 parts of modified coating agent. Disperse them ultrasonically for 13-15 minutes, keep them at room temperature in a vacuum drying oven for 3-5 hours, and then circulate them back to atmospheric pressure. Repeat this process 2-3 times to obtain coated modified nano-calcium carbide particles (aggregation will occur when pH≤5). S4. Place the coated modified nano-calcium carbide particles into a Wurster fluidized bed, atomize 95-105 parts of modified ethyl cellulose solution and spray it upwards, so that it can meet the coated nano-calcium carbide particles and form a film on the surface. After controlling the particle size to reach 200-300nm, the nano-calcium carbide blocking agent is obtained.

[0009] Preferably, the preparation of the modified coating agent includes the following steps: S21. By weight, add 1.6-2 parts benzotriazole, 4.5-5 parts citric acid, and 40-55 parts anhydrous copper chloride to 70-80 parts methanol, and stir at 300-500 r / min until completely dissolved to obtain a preliminary mixture. S22. Add 1.6-2 parts sodium dodecyl sulfate, 3.5-4 parts polyvinyl alcohol 1788 and 30-40 parts methanol to the preliminary mixture obtained in step S21, and stir at 300-400 r / min until it is evenly dissolved to obtain the secondary mixture. S23. After letting the secondary mixture obtained in step S22 stand at room temperature for 18-20 minutes, filter it with a 0.45μm organic filter membrane to finally obtain the modified coating agent.

[0010] Preferably, the preparation of the modified ethyl cellulose solution includes the following steps: S31. By mass, slowly add 35-40 parts of ethyl cellulose to 130-140 parts of anhydrous ethanol, and stir in a sealed container at a speed of 400-500 r / min for 25-30 min to obtain an ethyl cellulose solution. S32. Add 4.5-5 parts of diethyl phthalate and 2.5-3 parts of polyethylene glycol 400 to the ethyl cellulose solution obtained in step S31, and continue stirring for 10-15 minutes; S33. Take 0.8-1 parts of nano-silica and 10-12 parts of anhydrous ethanol, mix them, and then ultrasonically disperse them at a frequency of 40kHz for 8-10 minutes. Mix them with the solution obtained in step S32, stir them at a speed of 400-600r / min for 8-10 minutes, and let them stand in a vacuum dryer for 10-15 minutes to obtain a modified ethyl cellulose solution.

[0011] Preferably, the calcium-containing metal salt is selected from one of calcium acetate, calcium formate, calcium nitrate, and calcium chloride.

[0012] Preferably, in step S13, the mass ratio of the calcium salt ethylene glycol solution to the ethylene glycol etherified PVP-VA solution is 1:1.

[0013] Preferably, the mass ratio of the reaction solution to the chloroform solution in step S14 is 1:4-5.

[0014] Preferably, the frequency of ultrasonic dispersion in step S3 is 40 kHz.

[0015] Preferably, the Wurster fluidized bed has a fluidization pressure of 20-30 kPa, an atomization pressure of 80-100 kPa, an inlet air temperature of 38-40℃, a material temperature of 29-31℃, a liquid feed rate of 4-6 r / min, a spray gun orifice diameter of 0.8 mm, and a setting time of 4-5 min.

[0016] A nano-calcium carbide plugging agent for gas channeling in low-oil-saturation reservoirs is prepared by the above-mentioned method.

[0017] An application of a nano-calcium carbide plugging agent for gas channeling in low-oil-saturation reservoirs during gas drive production.

[0018] Compared with the prior art, the beneficial effects of the present invention are: 1. The present invention uses a plugging agent in the form of nano-solid capsules, which is injected into the formation along with liquid carbon dioxide during the carbon dioxide gas drive production process. It does not require shutting down the well and stopping production, and can seal micro-fractures in the formation without harming the reservoir environment. It can also be easily unblocked later, thereby improving the economic benefits of the oilfield.

[0019] 2. This invention ensures uniform coating by utilizing the synergistic effect of modified nano-calcium particles, modified coating agent, and modified ethyl cellulose solution. It also ensures that the plugging agent can be transported to deep micro-fractures with carbon dioxide before initiating the plugging reaction, thus avoiding premature blockage in the near-wellbore area. This achieves precise plugging of dominant channels and significantly improves the transport efficiency and plugging success rate of the plugging agent in complex reservoir environments. Attached Figure Description

[0020] Figure 1 This is a process flow diagram of the preparation of the nano-calcium carbide plugging agent for gas channeling in low oil saturation reservoirs according to the present invention. Figure 2 This is a flow chart of the preparation process of the modified coating agent of the present invention; Figure 3 This is a flow chart illustrating the preparation process of the modified ethyl cellulose solution of this invention. Detailed Implementation

[0021] The present invention will now be clearly and completely described in conjunction with embodiments thereof. Obviously, the described embodiments are merely some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0022] Please see Figures 1-3 The present invention provides a technical solution: Example 1 A method for preparing a nano-calcium carbide plugging agent for gas channeling in low-oil-saturation reservoirs: Before preparing the nano-calcium carbide plugging agent for gas channeling in low-oil-saturation reservoirs, the following steps are taken: Preparation of the modified coating agent and the modified ethyl cellulose solution are performed: The preparation of the modified coating agent includes the following steps: S21. Add 1.6g benzotriazole, 4.5g citric acid, and 40g anhydrous copper chloride to 70g methanol and stir at 300r / min until completely dissolved to obtain a preliminary mixture. S22. Add 1.6g sodium dodecyl sulfate, 3.5g polyvinyl alcohol 1788 and 30g methanol to the preliminary mixture obtained in step S21, and stir at 300r / min until it is evenly dissolved to obtain the second step mixture; S23. After the secondary mixture obtained in step S22 is allowed to stand at room temperature for 18 minutes, it is filtered through a 0.45 μm organic filter membrane to finally obtain the modified coating agent.

[0023] The preparation of the modified ethyl cellulose solution includes the following steps: S31. Slowly add 35g of ethyl cellulose to 130g of anhydrous ethanol, and stir in a sealed container at 400r / min for 25min to obtain an ethyl cellulose solution. S32. Add 4.5g of diethyl phthalate and 2.5g of polyethylene glycol 400 to the ethyl cellulose solution obtained in step S31, and continue stirring for 10 minutes; S33. Take 0.8g of nano silica and 10g of anhydrous ethanol, mix them, and then ultrasonically disperse them at a frequency of 40kHz for 8min. Mix them with the solution obtained in step S32, stir at a speed of 400r / min for 8min, and let them stand in a vacuum dryer for 10min to obtain a modified ethyl cellulose solution.

[0024] S1. Preparation of modified calcium nanoparticles: S11. Dissolve calcium acetate in ethylene glycol to form a calcium salt ethylene glycol solution with a concentration of 0.1 mL / L; S12. Dissolve the polyvinylpyrrolidone-polyvinyl alcohol copolymer in ethylene glycol to form a 0.2 mL / L ethylene glycol etherified PVP-VA solution; S13. Place ethylene glycol in a reaction vessel and heat it to 120°C and keep it at that temperature for 25 min. Inject the calcium salt ethylene glycol solution and the ethylene glycol etherified PVP-VA solution into the reaction vessel through a syringe at a mass ratio of 1:1 and keep it at 120°C for 30 min. S14. Pour the reaction solution obtained in step S13 into the chloroform solution. The mass ratio of the reaction solution to the chloroform solution is 1:5. Separate and wash the mixture in a centrifuge at a speed of 5000 rpm to obtain modified calcium nanoparticles. S2. The modified calcium nanoparticles obtained in step S1 are laid on the surface of graphene and placed in a vacuum chamber. A heating device is set under the graphene and heated at 1950°C to obtain calcium nanoparticles. S3. Take 90g of nano-calcium carbide particles and disperse them in 100g of modified coating agent. After ultrasonic dispersion at a frequency of 40kHz for 13min, keep them at room temperature in a vacuum drying oven for 3h and then circulate them back to atmospheric pressure. Repeat this process twice to obtain coated modified nano-calcium carbide particles. S4. Place the coated modified nano-calcium carbide particles into a Wurster fluidized bed (fluidization pressure 20 kPa, atomization pressure 80 kPa, inlet air temperature 38℃, material temperature 29℃, liquid feed rate 4 r / min, spray gun orifice diameter 0.8 mm, set time 4 min), atomize 95 g of modified ethyl cellulose solution and spray it upwards, so that it can meet the coated nano-calcium carbide particles and form a film on the surface. After controlling the particle size to reach 200 nm, the nano-calcium carbide sealing agent is obtained.

[0025] Example 2 A method for preparing a nano-calcium carbide plugging agent for gas channeling in low-oil-saturation reservoirs: Before preparing the nano-calcium carbide plugging agent for gas channeling in low-oil-saturation reservoirs, the following steps are taken: Preparation of the modified coating agent and the modified ethyl cellulose solution are performed: The preparation of the modified coating agent includes the following steps: S21. Add 2g benzotriazole, 5g citric acid, and 55g anhydrous copper chloride to 80g methanol and stir at 500r / min until completely dissolved to obtain a preliminary mixture. S22. Add 2g sodium dodecyl sulfate, 4g polyvinyl alcohol 1788 and 40g methanol to the preliminary mixture obtained in step S21, and stir at 400r / min until it is evenly dissolved to obtain the second step mixture; S23. After letting the secondary mixture obtained in step S22 stand at room temperature for 20 minutes, filter it with a 0.45 μm organic filter membrane to finally obtain the modified coating agent.

[0026] The preparation of the modified ethyl cellulose solution includes the following steps: S31. Slowly add 40g of ethyl cellulose to 140g of anhydrous ethanol, and stir in a sealed container at 500r / min for 30min to obtain an ethyl cellulose solution. S32. Add 5g of diethyl phthalate and 3g of polyethylene glycol 400 to the ethyl cellulose solution obtained in step S31, and continue stirring for 15min; S33. Take 1g of nano silica and 12g of anhydrous ethanol, mix them, and then ultrasonically disperse them at a frequency of 40kHz for 10min. Mix them with the solution obtained in step S32, stir at a speed of 600r / min for 10min, and let them stand in a vacuum dryer for 15min to obtain a modified ethyl cellulose solution.

[0027] S1. Preparation of modified calcium nanoparticles: S11. Dissolve calcium nitrate in ethylene glycol to form a calcium salt ethylene glycol solution with a concentration of 0.2 mL / L; S12. Dissolve the polyvinylpyrrolidone-polyvinyl alcohol copolymer in ethylene glycol to form a ethylene glycol etherified PVP-VA solution with a concentration of 0.375 mL / L; S13. Place ethylene glycol in a reaction vessel and heat it to 170°C and keep it at that temperature for 30 min. Inject the calcium salt ethylene glycol solution and the ethylene glycol etherified PVP-VA solution into the reaction vessel through a syringe at a mass ratio of 1:1 and keep it at 200°C for 40 min. S14. Pour the reaction solution obtained in step S13 into the chloroform solution. The mass ratio of the reaction solution to the chloroform solution is 1:4. Separate and wash the mixture in a centrifuge at a speed of 6000 rpm to obtain modified calcium nanoparticles. S2. The modified calcium nanoparticles obtained in step S1 are laid on the surface of graphene and placed in a vacuum chamber. A heating device is set under the graphene and heated at 2000℃ to obtain calcium nanoparticles. S3. Take 100g of nano-calcium carbide particles and disperse them in 110g of modified coating agent. After ultrasonic dispersion at a frequency of 40kHz for 15min, keep them at room temperature in a vacuum drying oven for 5h and then circulate them back to atmospheric pressure. Repeat this process 3 times to obtain coated modified nano-calcium carbide particles. S4. Place the coated modified nano-calcium carbide particles into a Wurster fluidized bed (fluidization pressure 30 kPa, atomization pressure 100 kPa, inlet air temperature 40℃, material temperature 31℃, liquid feed rate 6 r / min, spray gun orifice diameter 0.8 mm, set time 5 min), atomize 105 g of modified ethyl cellulose solution and spray it upwards, so that it can meet the coated nano-calcium carbide particles and form a film on the surface. After controlling the particle size to reach 300 nm, the nano-calcium carbide sealing agent is obtained.

[0028] Example 3 A method for preparing a nano-calcium carbide plugging agent for gas channeling in low-oil-saturation reservoirs: Before preparing the nano-calcium carbide plugging agent for gas channeling in low-oil-saturation reservoirs, the following steps are taken: Preparation of the modified coating agent and the modified ethyl cellulose solution are performed: The preparation of the modified coating agent includes the following steps: S21. Add 1.8g benzotriazole, 4.8g citric acid, and 45g anhydrous copper chloride to 75g methanol and stir at 400r / min until completely dissolved to obtain a preliminary mixture. S22. Add 1.8g sodium dodecyl sulfate, 3.8g polyvinyl alcohol 1788 and 32g methanol to the preliminary mixture obtained in step S21, and stir at 320r / min until it is evenly dissolved to obtain the second step mixture; S23. After the secondary mixture obtained in step S22 is allowed to stand at room temperature for 19 min, it is filtered through a 0.45 μm organic filter membrane to finally obtain the modified coating agent.

[0029] The preparation of the modified ethyl cellulose solution includes the following steps: S31. Slowly add 40g of ethyl cellulose to 140g of anhydrous ethanol, and stir in a sealed container at 500r / min for 30min to obtain an ethyl cellulose solution. S32. Add 5g of diethyl phthalate and 3g of polyethylene glycol 400 to the ethyl cellulose solution obtained in step S31, and continue stirring for 15min; S33. Take 1g of nano silica and 12g of anhydrous ethanol, mix them, and then ultrasonically disperse them at a frequency of 40kHz for 10min. Mix them with the solution obtained in step S32, stir at a speed of 600r / min for 10min, and let them stand in a vacuum dryer for 15min to obtain a modified ethyl cellulose solution.

[0030] S1. Preparation of modified calcium nanoparticles: S11. Dissolve calcium formate in ethylene glycol to form a calcium salt ethylene glycol solution with a concentration of 0.15 mL / L; S12. Dissolve the polyvinylpyrrolidone-polyvinyl alcohol copolymer in ethylene glycol to form a 0.25 mL / L ethylene glycol etherified PVP-VA solution; S13. Place ethylene glycol in a reaction vessel and heat it to 140°C and keep it at that temperature for 28 min. Inject the calcium salt ethylene glycol solution and the ethylene glycol etherified PVP-VA solution into the reaction vessel through a syringe at a mass ratio of 1:1 and keep it at 150°C for 35 min. S14. Pour the reaction solution obtained in step S13 into the chloroform solution. The mass ratio of the reaction solution to the chloroform solution is 1:4.5. Separate and wash the mixture in a centrifuge at a speed of 5500 rpm to obtain modified calcium nanoparticles with a particle size of 80 nm. S2. The modified calcium nanoparticles obtained in step S1 are laid on the surface of graphene and placed in a vacuum chamber. A heating device is set under the graphene and heated at 1980°C to obtain calcium nanoparticles. S3. Take 95g of nano-calcium carbide particles and disperse them in 105g of modified coating agent. After ultrasonic dispersion at a frequency of 40kHz for 14min, keep them at room temperature in a vacuum drying oven for 4h and then circulate them back to atmospheric pressure. Repeat this process twice to obtain coated modified nano-calcium carbide particles. S4. Place the coated modified nano-calcium carbide particles into a Wurster fluidized bed (fluidization pressure 25 kPa, atomization pressure 90 kPa, inlet air temperature 39℃, material temperature 30℃, liquid feed rate 5 r / min, spray gun orifice diameter 0.8 mm, set time 4.5 min), atomize 100 g of modified ethyl cellulose solution and spray it upwards, so that it can meet the coated nano-calcium carbide particles and form a film on the surface. After controlling the particle size to reach 250 nm, the nano-calcium carbide sealing agent is obtained.

[0031] Comparative Example 1 The only difference between Comparative Example 1 and Example 1 is that no modified coating agent was added in this comparative example; the other steps are exactly the same in Comparative Example 1 and Example 1.

[0032] Comparative Example 2 The only difference between Comparative Example 2 and Example 1 is that the modified ethyl cellulose solution is replaced with a 25% ethyl cellulose solution. The remaining steps are exactly the same in Comparative Example 2 and Example 1.

[0033] Comparative Example 3 The only difference between Comparative Example 3 and Example 1 is that the modified calcium nanoparticles are replaced with ordinary calcium nanoparticles in this comparative example. The remaining steps are exactly the same in Comparative Example 3 and Example 1.

[0034] Performance testing: Calcium carbide microcapsule blocking effect test Based on the geological characteristics of the target reservoir, cores with permeability ranges (0.1-400 mD) were selected for calcium carbide microcapsule plugging experiments. To reproduce the long-term underground migration and plugging situation as much as possible, large-size physical simulation cores with dimensions of 4.5×4.5×30cm were used for the experiment. The experimental steps are as follows: (1) The core was placed in an oven to dry, and the weight of the core was measured continuously during the process. When the weight of the core no longer changed, the first permeability measurement was performed. (2) Cut the core along the long side and make artificial cracks. Set the crack width to 2.5 mm and the crack length to 100 mm. After the cracks are made, merge the cores and make a second permeability measurement. (3) ISCO pumps were used to saturate the formation water, and heating devices were used to heat the core to simulate the formation temperature of the target oil reservoir; (4) Displacement was carried out by injecting carbon dioxide at a constant rate of 0.04 mL / min. After injecting 1 PV of carbon dioxide, the same concentration of nano-calcium carbide plugging agent prepared in Examples 1-3 and Comparative Examples 1-3 was added to the carbon dioxide. The carbon dioxide and nano-calcium carbide plugging agent were injected into the core together at a constant rate of 0.04 mL / min. The experiment was stopped after a total of 3 PV was injected, and the third permeability measurement was carried out. Table 1. Changes in plugging rate after gas-driven plugging agent As can be seen from the data in Table 1, the nano-calcium carbide plugging agents prepared according to Examples 1-3 have an average core plugging effect of over 92%, with Example 1 showing the best plugging effect, achieving a core plugging rate as high as 94.6%. The nano-calcium carbide plugging agents obtained in the comparative examples have a poorer plugging effect.

[0035] Calcium carbide microcapsule release test at different pH values The experimental steps are as follows: (1) Prepare 1L of NaCl solution (mass fraction 3.5%) with different pH values ​​of 2, 3, 5, 6.5 and 7 respectively. (2) Take 20 mg of the calcium carbide microcapsules obtained in Example 1 and disperse them in the above solution. After standing for 10 min, take the supernatant and examine it with UV-Vis. 2+ Ultraviolet spectroscopy was performed, with a scanning wavelength of 200-500 nm, to measure Ca. 2+ Absorbance; (3) Determine Ca by absorbance 2+ The maximum absorption wavelength is 420 nm. Ca was prepared at concentrations of 1 mg / L, 2 mg / L, 5 mg / L, 10 mg / L, and 20 mg / L, respectively. 2+ Aqueous solutions were used to measure the absorbance of each solution at the wavelength of maximum absorption. (4) Plot absorbance against concentration and obtain the Ca concentration by linear fitting. 2+ UV absorbance versus concentration standard curve; (5) Calculate Ca at different pH values 2+ The release rate was analyzed to assess the release behavior of calcium carbide microcapsules at different pH values.

[0036] Table 2. Blocking agent release experiments at different pH values. As shown in Table 2, the effectiveness of nano-calcium carbide plugging agents in sealing formation fractures varies significantly under different pH values. At pH 7, the nano-calcium carbide plugging agent releases almost no nano-calcium carbide particles. When pH ≤ 5, the coated modified nano-calcium carbide particles in the nano-calcium carbide plugging agent aggregate under acidic conditions, releasing nano-calcium carbide particles and Ca. 2+ The release rate is as high as 98.6%.

[0037] Experiment on the plugging effect of calcium carbide microcapsules under different gas driving times Based on the geological characteristics of the target reservoir, cores with permeability ranges (0.1-400 mD) were selected for calcium carbide microcapsule plugging experiments. In order to reproduce the long-term underground migration and plugging situation as much as possible, large-size physical simulation cores with dimensions of 4.5×4.5×30cm were used to carry out the experiment. The experimental steps are as follows: (1) The core was placed in an oven to dry, and the weight of the core was measured continuously during the process. When the weight of the core no longer changed, the first permeability measurement was performed. (2) Cut the core along the long side and make artificial cracks. Set the crack width to 2.5 mm and the crack length to 100 mm. After the cracks are made, merge the cores and make a second permeability measurement. (3) ISCO pumps were used to saturate the formation water, and heating devices were used to heat the core to simulate the formation temperature of the target oil reservoir; (4) Displacement was carried out by injecting carbon dioxide at a constant rate of 0.04 mL / min. After injecting 1 PV of carbon dioxide, the nano-calcium carbide plugging agent (size about 220 nm) prepared in Example 3 was added to the carbon dioxide at a constant rate of 20 g / L. The carbon dioxide and the nano-calcium carbide plugging agent were injected into the core together at a constant rate of 0.04 mL / min. The experiment was stopped after a total of 3 PV was injected, and the third permeability measurement was carried out. (5) Continue to inject different PV carbon dioxide for gas drive, and perform a fourth permeability measurement after the experiment; Table 3. Changes in plugging rate of the plugging agent after gas-driven operation at different gas-driven times. As can be seen from the data in Table 3, the use of nano-calcium carbide plugging agent can effectively seal the fractures in the formation. Nano-calcium carbide generates calcium carbonate precipitate in the fractures to seal them. Among them, the calcium carbonate precipitate has good stability in the fractures and strong erosion resistance. Even when the subsequent gas drive reaches 10 PV, it can still maintain a sealing rate of 91.4%.

[0038] This invention addresses the issue of gas channeling in low-oil-saturation reservoirs by injecting a nano-calcium carbide plugging agent into the formation along with liquid carbon dioxide. The carbon dioxide carries the nano-calcium carbide plugging agent preferentially into dominant channels such as microfractures and large pores. Under the influence of formation water, the nano-calcium carbide particles encapsulated within the plugging agent are exposed. These particles react with water and carbon dioxide in the formation to form calcium carbonate precipitates. These precipitates then block the reservoir as they flow with the carbon dioxide, until a certain amount of calcium carbonate precipitate accumulates in the reservoir, thus sealing off the dominant channels such as microfractures. In particular, the comparison of data from the examples and comparative examples in Table 1 further illustrates that the present invention avoids the traditional preparation method of plugging agents by using the synergistic effect of modified nano-calcium particles, modified coating agents, and modified ethyl cellulose solutions to prepare nano-calcium carbide particles into nano-solid capsules. This significantly improves the success rate of gas channeling plugging in low oil saturation reservoirs, effectively extends the gas drive production time, reduces damage to the reservoir environment, and improves the economic benefits of the oilfield. It is of great significance for the subsequent gas drive development of low oil saturation reservoirs.

[0039] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a nano-calcium carbide plugging agent for gas channeling in low-oil-saturation reservoirs, characterized in that, The preparation steps include the following: S1. Preparation of modified calcium nanoparticles: S11. Dissolve the calcium-containing metal salt in ethylene glycol to form a calcium salt ethylene glycol solution with a concentration of 0.1-0.2 mL / L; S12. Dissolve the polyvinylpyrrolidone-polyvinyl alcohol copolymer in ethylene glycol to form an ethylene glycol etherified PVP-VA solution with a concentration of 0.2-0.375 mL / L; S13. Place ethylene glycol in a reaction vessel and heat it to 120-170℃ and keep it at that temperature for 25-30 minutes. Inject the calcium salt ethylene glycol solution and the ethylene glycol etherified PVP-VA solution into the reaction vessel using a syringe and keep it at 120-200℃ for 30-40 minutes. S14. Pour the reaction solution obtained in step S13 into chloroform solution, separate and wash it in a centrifuge at a speed of 5000-6000 rpm to obtain modified nano-calcium particles. S2. The modified calcium nanoparticles obtained in step S1 are laid on the surface of graphene and placed in a vacuum chamber. A heating device is set under the graphene and heated at 1950-2000℃ to obtain calcium carbide nanoparticles. S3. By mass, take 90-100 parts of nano-calcium carbide particles and disperse them in 100-110 parts of modified coating agent. Disperse them ultrasonically for 13-15 minutes, keep them at room temperature in a vacuum drying oven for 3-5 hours, and then circulate them back to atmospheric pressure. Repeat this process 2-3 times to obtain coated modified nano-calcium carbide particles. S4. Place the coated modified nano-calcium carbide particles into a Wurster fluidized bed, atomize 95-105 parts of modified ethyl cellulose solution and spray it upwards, so that it can meet the coated nano-calcium carbide particles and form a film on the surface. After controlling the particle size to reach 200-300nm, the nano-calcium carbide blocking agent is obtained.

2. The method for preparing a nano-calcium carbide plugging agent for gas channeling in low-oil-saturation reservoirs according to claim 1, characterized in that, The preparation of the modified coating agent includes the following steps: S21. By weight, add 1.6-2 parts benzotriazole, 4.5-5 parts citric acid, and 40-55 parts anhydrous copper chloride to 70-80 parts methanol, and stir at 300-500 r / min until completely dissolved to obtain a preliminary mixture. S22. Add 1.6-2 parts sodium dodecyl sulfate, 3.5-4 parts polyvinyl alcohol 1788 and 30-40 parts methanol to the preliminary mixture obtained in step S21, and stir at 300-400 r / min until it is evenly dissolved to obtain the secondary mixture. S23. After letting the secondary mixture obtained in step S22 stand at room temperature for 18-20 minutes, filter it with a 0.45μm organic filter membrane to finally obtain the modified coating agent.

3. The method for preparing a nano-calcium carbide plugging agent for gas channeling in low-oil-saturation reservoirs according to claim 1, characterized in that, The preparation of the modified ethyl cellulose solution includes the following steps: S31. By mass, slowly add 35-40 parts of ethyl cellulose to 130-140 parts of anhydrous ethanol, and stir in a sealed container at a speed of 400-500 r / min for 25-30 min to obtain an ethyl cellulose solution. S32. Add 4.5-5 parts of diethyl phthalate and 2.5-3 parts of polyethylene glycol 400 to the ethyl cellulose solution obtained in step S31, and continue stirring for 10-15 minutes; S33. Take 0.8-1 parts of nano-silica and 10-12 parts of anhydrous ethanol, mix them, and then ultrasonically disperse them at a frequency of 40kHz for 8-10 minutes. Mix them with the solution obtained in step S32, stir them at a speed of 400-600r / min for 8-10 minutes, and let them stand in a vacuum dryer for 10-15 minutes to obtain a modified ethyl cellulose solution.

4. The method for preparing a nano-calcium carbide plugging agent for gas channeling in low-oil-saturation reservoirs according to claim 1, characterized in that, The calcium-containing metal salt is selected from one of calcium acetate, calcium formate, calcium nitrate, and calcium chloride.

5. The method for preparing a nano-calcium carbide plugging agent for gas channeling in low-oil-saturation reservoirs according to claim 1, characterized in that, In step S13, the mass ratio of the calcium salt ethylene glycol solution to the ethylene glycol etherified PVP-VA solution is 1:

1.

6. The method for preparing a nano-calcium carbide plugging agent for gas channeling in low-oil-saturation reservoirs according to claim 1, characterized in that, In step S14, the mass ratio of the reaction solution to the chloroform solution is 1:4-5.

7. The method for preparing a nano-calcium carbide plugging agent for gas channeling in low-oil-saturation reservoirs according to claim 1, characterized in that, In step S3, the frequency of ultrasonic dispersion is 40 kHz.

8. The method for preparing a nano-calcium carbide plugging agent for gas channeling in low-oil-saturation reservoirs according to claim 1, characterized in that, The Wurster fluidized bed has a fluidization pressure of 20-30 kPa, an atomization pressure of 80-100 kPa, an inlet air temperature of 38-40℃, a material temperature of 29-31℃, a liquid feed rate of 4-6 r / min, a spray gun orifice diameter of 0.8 mm, and a set time of 4-5 min.

9. A nano-calcium carbide plugging agent for gas channeling in low-oil-saturation reservoirs, characterized in that, It is prepared by the preparation method described in any one of claims 1-8.

10. The application of the nano-calcium carbide plugging agent for gas channeling in low-oil-saturation reservoirs according to claim 9 in the gas drive production process for plugging gas channeling in low-oil-saturation reservoirs.

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