Carbon dioxide energized fracturing method

Abstract

The invention discloses a carbon dioxide energized fracturing method, and relates to the technical field of carbon dioxide fracturing, the method comprises the following steps: (1) dispersing guar gum in isopropanol, adding alkali liquor to adjust pH, adding 2-(3, 4-cyclohexene oxide) ethyltrimethoxysilane to react, and filtering to obtain modified guar gum; (2) dissolving the modified guar gum in 15-20vt% ethanol, and adding a cross-linking agent, potassium chloride, a gel breaker, a cleanup additive and a bactericide to obtain a fracturing fluid; and (3) mixing the fracturing fluid and the liquid carbon dioxide, and then carrying out sand fracturing construction. According to the method, guar gum is modified, so that the viscosity of the fracturing fluid is improved, the sand-carrying capacity is enhanced, the effect of modifying the rock surface can be achieved, the flowback rate of the fracturing fluid is increased, and the method has a relatively high application value.

Description

Technical Field

[0001] The invention relates to the technical field of carbon dioxide fracturing, and in particular to a method for carbon dioxide energized fracturing. Background Art

[0002] After the oil well production reaches a certain stage, the production capacity and permeability decrease. In order to enhance the oil drainage capacity, it is necessary to use fracturing to increase the oil well production. At present, hydraulic fracturing technology is very mature, and the effect of increasing oil well production is obvious. It has long become the preferred common technology. Hydraulic fracturing relies on the ground high-pressure pump truck group to inject fluid into the well at high speed. With the help of the high pressure at the bottom of the well, the oil layer rock is broken and cracks are generated. Sand with a density several times greater than the formation is injected into the formation fracture to support the fracture in an open state, so that the oil flow environment can be improved for a long time.

[0003] my country's shale gas is mostly distributed in water-scarce and arid areas with fragile environments. Existing hydraulic fracturing technology has extremely high water consumption, and the salinity of the backflow drainage is extremely high. It often carries underground radioactive species and is difficult to purify, causing serious secondary pollution. In sharp contrast, the carbon dioxide fracturing process is expected to reduce water consumption by more than 80% compared to hydraulic fracturing. Carbon dioxide has higher mobility than water, so the injection pressure at the wellhead can be better transmitted to the natural pores of the formation, which changes the mechanism of fracture initiation and forms a more complex fracture network, thereby improving the production efficiency of shale gas. If the carbon sequestration effect of this technology is further considered, the carbon dioxide fracturing process is expected to reduce greenhouse gas emissions exponentially. Compared with water, carbon dioxide fracturing greatly reduces process water consumption, reduces reservoir damage, and can provide a storage place for captured carbon dioxide. Therefore, carbon dioxide may be a more ideal fracturing fluid.

[0004] However, carbon dioxide fracturing technology currently faces problems. The viscosity of carbon dioxide injected into the formation is very low, and its proppant carrying capacity is weak. Proppants help keep the fractures open during shale gas extraction. How to increase the viscosity of carbon dioxide fracturing fluid is still an urgent problem to be solved in the current application of carbon dioxide fracturing technology. Summary of the invention

[0005] In order to solve the above technical problems, the purpose of the present invention is to provide a method for carbon dioxide energized fracturing, which improves the viscosity of the fracturing fluid and enhances the sand carrying capacity by modifying guar gum, and can also modify the rock surface, thereby improving the return rate of the fracturing fluid, and has great application value.

[0006] The technical solution of the present invention to solve the above technical problems is as follows: a method for carbon dioxide energized fracturing is provided, comprising the following steps:

[0007] (1) dispersing guar gum in isopropanol, then adding alkali solution to adjust the pH, adding 2-(3,4-epoxycyclohexane)ethyltrimethoxysilane to react, and filtering to obtain modified guar gum;

[0008] (2) dissolving the modified guar gum obtained in step (1) in 15-20 vt% ethanol, and then adding a cross-linking agent, potassium chloride, a gel breaker, a drainage agent and a bactericide to obtain a fracturing fluid;

[0009] (3) The fracturing fluid obtained in step (2) is mixed with liquid carbon dioxide and then subjected to sand fracturing construction.

[0010] Furthermore, in step (1), the molar volume ratio of guar gum, 2-(3,4-epoxycyclohexane)ethyltrimethoxysilane and isopropanol is 1 mol:3-5 mol:1L.

[0011] Furthermore, in step (1), the alkali solution is a sodium hydroxide solution; and the pH is adjusted to 9-11.

[0012] Furthermore, in step (1), the reaction is carried out at room temperature for 12-36 hours.

[0013] Furthermore, in step (2), the mass volume ratio of modified guar gum, cross-linking agent, potassium chloride, gel breaker, drainage aid, bactericide and 15-20vt% ethanol is 0.3-0.6g: 0.5-2g: 0.08-0.12g: 0.005-0.01g: 0.2-0.4g: 0.05-0.15g: 100mL.

[0014] Furthermore, the crosslinking agent is one of a chromium crosslinking agent, an organic boron crosslinking agent and an organic titanium crosslinking agent.

[0015] Furthermore, in step (2), the gel breaker is ammonium persulfate.

[0016] Furthermore, in step (3), the mass ratio of the fracturing fluid to the liquid carbon dioxide is 10-20:80-90.

[0017] Furthermore, in step (3), the foam mass is 30-52%.

[0018] The present invention has the following beneficial effects:

[0019] 1. The present invention first modifies the guar gum molecules (such as Figure 1As shown in the figure, under alkaline conditions, guar gum forms sodium hydroxyl, which reacts with the epoxy group in the 2-(3,4-epoxycyclohexane)ethyltrimethoxysilane molecule to form an etherification reaction. While maintaining the inherent advantages of guar gum, the performance of 2-(3,4-epoxycyclohexane)ethyltrimethoxysilane is introduced. On the one hand, the introduced Si-OC bonds and methoxy groups will interact with the hydroxyl groups in ethanol to produce hydrogen bonds, which will promote the solvent of guar gum, and due to the strong hydrogen bonds, these molecules will gradually polymerize into chain polymers, thereby increasing the viscosity of the mixture and enhancing the sand carrying capacity. On the other hand, as Figure 2 As shown in the figure, the silanol generated by the hydrolysis of (OCH3)3Si- groups can produce covalent bond chemical reactions with the hydroxyl groups on the rock surface, forming strongly hydrophobic and oleophobic R`-Si rock modification groups on the rock surface, which changes the surface properties of the rock, improves the conductivity of the fractures, and further reduces the filtration loss of the fracturing fluid, thereby improving the fracture creation effect and the return flow rate.

[0020] 2. While ethanol acts as a solvent, the hydrogen bonds it produces with modified guar gum can also increase the viscosity of the fracturing fluid system. In addition, ethanol can improve the fluidity of carbon dioxide and increase its diffusion capacity. In terms of geological storage, adding ethanol can change the solubility and diffusivity of carbon dioxide, making it easier to penetrate into geological layers, increasing storage efficiency, and thus reducing carbon dioxide emissions.

[0021] 3. Under the action of the gel breaker, the polymer gel in the fracturing fluid of the present invention is decomposed, and the viscosity and molecular weight are reduced, which is convenient for modifying the rock surface and is conducive to rapid flowback after the fracturing operation is completed, thereby achieving the effect of reducing the flowback time and improving the flowback rate. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 Schematic diagram of the modification principle of guar gum;

[0023] Figure 2 This is a schematic diagram of the rock modification principle;

[0024] Figure 3 This is the result of the fracturing fluid viscosity test;

[0025] Figure 4 This is the contact angle test result diagram after rock modification;

[0026] Figure 5 Schematic diagram of the principle that the concentration of rock modifiers causes the contact angle to decrease. DETAILED DESCRIPTION

[0027] The principles and features of the present invention are described below, and the examples are only used to explain the present invention and are not used to limit the scope of the present invention. If no specific conditions are specified in the embodiments, they are carried out according to normal conditions or conditions recommended by the manufacturer. If the manufacturer of the reagents or instruments is not specified, they are all conventional products that can be purchased commercially.

[0028] Example 1

[0029] A method for carbon dioxide energized fracturing, comprising the following steps:

[0030] (1) Dispersing guar gum in isopropanol, then adding sodium hydroxide solution to adjust the pH to 10, adding 2-(3,4-epoxycyclohexane)ethyltrimethoxysilane, reacting at room temperature for 24 hours, and filtering to obtain modified guar gum; the molar volume ratio of guar gum, 2-(3,4-epoxycyclohexane)ethyltrimethoxysilane and isopropanol is 1 mol:4.5 mol:1 L;

[0031] (2) dissolving the modified guar gum obtained in step (1) in 15-20 vt% ethanol, and then adding an organic boron crosslinking agent, potassium chloride, ammonium persulfate, TCF-1 drainage aid and CJSJ-3 fungicide to obtain a fracturing fluid; the mass volume ratio of the modified guar gum, the organic boron crosslinking agent, potassium chloride, ammonium persulfate, TCF-1 drainage aid, CJSJ-3 fungicide and 15-20 vt% ethanol is 0.5 g: 1.5 g: 0.01 g: 0.008 g: 0.3 g: 0.01 g: 100 mL;

[0032] (3) The fracturing fluid obtained in step (2) and liquid carbon dioxide are mixed in a mass ratio of 15:85 and then sand fracturing is performed.

[0033] Example 2

[0034] A method for carbon dioxide energized fracturing, comprising the following steps:

[0035] (1) Dispersing guar gum in isopropanol, then adding sodium hydroxide solution to adjust the pH to 9, adding 2-(3,4-epoxycyclohexane)ethyltrimethoxysilane, reacting at room temperature for 12 hours, and filtering to obtain modified guar gum; the molar volume ratio of guar gum, 2-(3,4-epoxycyclohexane)ethyltrimethoxysilane and isopropanol is 1 mol:3 mol:1 L;

[0036] (2) dissolving the modified guar gum obtained in step (1) in 15 vt% ethanol, and then adding a cross-linking agent, potassium chloride, ammonium persulfate, TCF-1 drainage aid and CJSJ-3 fungicide to obtain a fracturing fluid; the mass volume ratio of the modified guar gum, the cross-linking agent, potassium chloride, ammonium persulfate, TCF-1 drainage aid, CJSJ-3 fungicide and 15 vt% ethanol is 0.3 g: 0.5 g: 0.08 g: 0.005 g: 0.2 g: 0.05 g: 100 mL.

[0037] (3) The fracturing fluid obtained in step (2) and liquid carbon dioxide are mixed in a mass ratio of 20:80 and then sand fracturing is performed.

[0038] Example 3

[0039] A method for carbon dioxide energized fracturing, comprising the following steps:

[0040] (1) Dispersing guar gum in isopropanol, then adding sodium hydroxide solution to adjust the pH to 11, adding 2-(3,4-epoxycyclohexane)ethyltrimethoxysilane, reacting at room temperature for 36 hours, and filtering to obtain modified guar gum; the molar volume ratio of guar gum, 2-(3,4-epoxycyclohexane)ethyltrimethoxysilane and isopropanol is 1 mol:5 mol:1 L;

[0041] (2) dissolving the modified guar gum obtained in step (1) in 20 vt% ethanol, and then adding a cross-linking agent, potassium chloride, ammonium persulfate, TCF-1 drainage aid and CJSJ-3 bactericide to obtain a fracturing fluid; the mass volume ratio of the modified guar gum, the cross-linking agent, potassium chloride, ammonium persulfate, TCF-1 drainage aid, CJSJ-3 bactericide and 20 vt% ethanol is 0.6 g: 2 g: 0.12 g: 0.01 g: 0.4 g: 0.15 g: 100 mL.

[0042] (3) The fracturing fluid obtained in step (2) and liquid carbon dioxide are mixed in a mass ratio of 10:90 and then sand fracturing is performed.

[0043] Test Example 1 Viscosity

[0044] The modified guar gum obtained in step (1) of Example 1, boron crosslinking agent, potassium chloride, ammonium persulfate, TCF-1 drainage aid, CJSJ-3 fungicide and 15-20vt% ethanol were mixed in a mass volume ratio of 0.3g:1.5g:0.01g:0.008g:0.3g:0.01g:100mL to obtain a fracturing fluid, and the fracturing fluid was prepared by replacing it with hydroxypropyl guar gum for comparison and testing the viscosity. The result is as follows: Figure 3 shown.

[0045] Depend on Figure 3It can be seen that compared with hydroxypropyl guar gum, the viscosity of the fracturing fluid prepared with modified guar gum is effectively improved after cross-linking, and after 100-120 minutes of gel breaking, its viscosity is basically reduced to the same level, which shows that the viscosity of the fracturing fluid in the method of the present invention is effectively improved, the sand carrying capacity is stronger, and the gel breaking effect is good.

[0046] Test Example 2 Contact Angle

[0047] The modified guar gum and 2-(3,4-epoxycyclohexane)ethyltrimethoxysilane prepared in Example 1 were prepared into 0.05-0.4% aqueous solutions, and then the core pieces were soaked and evacuated respectively. The fracturing fluid was dripped onto the core pieces to measure the wetting contact angle. The results are as follows: Figure 4 shown.

[0048] Depend on Figure 4 It can be seen that under the same concentration conditions, the contact angle of the core piece modified by modified guar gum is larger than that of 2-(3,4-epoxycyclohexane)ethyltrimethoxysilane, indicating that it has better hydrophobicity. This is because the hydrophilic group hydroxyl group of guar gum is partially replaced during modification, and the methyl group on the sugar ring shows more hydrophobicity. In addition, it can be seen that the contact angle of 2-(3,4-epoxycyclohexane)ethyltrimethoxysilane reaches the maximum when the concentration is about 0.2%, and begins to decrease as the concentration increases. The main reasons for this phenomenon are as follows: Figure 5 As shown in the figure, after the concentration increases, too many molecules spread to the rock surface, the molecular chains become entangled, and the hydrophobic groups in the molecular structure are covered, resulting in enhanced fluid wetting. However, the modified guar gum has a larger molecular weight, and the cross-linking between molecules weakens the entanglement of the molecular chains, so that the concentration at which the maximum contact angle is achieved is higher, and it can be used at the conventional concentration for preparing fracturing fluid.

[0049] Test Example 3 Flowback Rate

[0050] Referring to the scheme of Example 1, carbon dioxide energized fracturing was carried out in six wells including the Changqing Oilfield gas reservoir, Puguang Gas Field and PetroChina Huabei Oilfield. The drainage effects are shown in Table 1.

[0051] Table 1 Carbon dioxide enhanced fracturing fluid discharge table

[0052]

[0053] As shown in Table 1, when the method of the present invention is used for carbon dioxide energized fracturing, the average flowback rate is improved. Therefore, this method can not only avoid the problem of large water consumption in conventional hydraulic fracturing, but also has a high flowback rate, and can also reduce the residual fracturing fluid in the geology and reduce geological damage.

[0054] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modification, equivalent substitution or improvement made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for carbon dioxide energized fracturing, characterized in that: The following steps are involved: (1) dispersing guar gum in isopropanol, then adding alkali solution to adjust the pH, adding 2-(3,4-epoxycyclohexane)ethyltrimethoxysilane to react, and filtering to obtain modified guar gum; (2) dissolving the modified guar gum obtained in step (1) in 15-20 vt% ethanol, and then adding a cross-linking agent, potassium chloride, a gel breaker, a drainage agent and a bactericide to obtain a fracturing fluid; (3) The fracturing fluid obtained in step (2) is mixed with liquid carbon dioxide and then subjected to sand fracturing construction.

2. The method for carbon dioxide energized fracturing according to claim 1, characterized in that: In step (1), the molar volume ratio of the guar gum, 2-(3,4-epoxycyclohexane)ethyltrimethoxysilane and isopropanol is 1 mol:3-5 mol:1L.

3. The method for carbon dioxide energized fracturing according to claim 1, characterized in that: In step (1), the alkali solution is a sodium hydroxide solution; and the pH is adjusted to 9-11.

4. The method for carbon dioxide energized fracturing according to claim 1, characterized in that: In step (1), the reaction is carried out at room temperature for 12-36 hours.

5. The method for carbon dioxide energized fracturing according to claim 1, characterized in that: In step (2), the mass volume ratio of the modified guar gum, cross-linking agent, potassium chloride, gel breaker, drainage aid, bactericide and 15-20vt% ethanol is 0.3-0.6g: 0.5-2g: 0.08-0.12g: 0.005-0.01g: 0.2-0.4g: 0.05-0.15g: 100mL.

6. The method for carbon dioxide energized fracturing according to claim 1, characterized in that: In step (2), the crosslinking agent is one of a chromium crosslinking agent, an organic boron crosslinking agent and an organic titanium crosslinking agent.

7. The method for carbon dioxide energized fracturing according to claim 1, characterized in that: In step (2), the breaker is ammonium persulfate.

8. The method for carbon dioxide energized fracturing according to claim 1, characterized in that: In step (3), the mass ratio of the fracturing fluid to liquid carbon dioxide is 10-20:80-90.

9. The method for carbon dioxide energized fracturing according to claim 1, characterized in that: In step (3), the foam mass is 30-52%.

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