Salt-tolerant clean fracturing fluid and preparation method thereof

Abstract

The invention discloses a salt-tolerant clean fracturing fluid and a preparation method thereof, and the salt-tolerant clean fracturing fluid comprises the following components: an anionic-nonionic surfactant NPEC and a cationic surfactant CTAB, the preparation method comprises the following steps: step a, uniformly mixing an anionic-nonionic surfactant NPEC and a cationic surfactant CTAB according to a ratio; step b, adding water, stirring until the materials are completely dissolved, adjusting the pH value, and performing cooling and aging treatment; and c, adjusting the viscosity and rheological property of the fracturing fluid according to actual requirements by adding NaCl with different concentrations. The fracturing fluid prepared by the invention can effectively cope with the influence of hypersalinity formation water in shallow oil and gas reservoirs; the stability of the fracturing fluid is improved, and performance reduction caused by salting-out is reduced; the method has high salt tolerance, is suitable for development of shallow oil and gas reservoirs, does not need high-temperature crosslinking, and has good economical efficiency and practicability.

Description

Technical Field

[0001] The present invention relates to the field of fracturing fluid preparation, and particularly to a salt-tolerant clean fracturing fluid and a preparation method thereof. Background Art

[0002] In the field of oil and gas exploitation, fracturing technology is an important means to improve the production of low-permeability oil and gas reservoirs. As a key working medium, the performance of fracturing fluid directly affects the fracturing effect and the reservoir stimulation effect. In recent years, with the rise of unconventional oil and gas resource development, shallow oil and gas reservoirs have received extensive attention and made certain progress due to their shallow burial depth and low production cost. However, the formation water salinity of shallow oil and gas reservoirs is usually relatively high (up to 10%-20%), which poses higher requirements for the salt tolerance of fracturing fluid.

[0003] Traditional fracturing fluid systems usually have the problem of insufficient salt tolerance. When used in high-salinity formations, problems such as viscosity reduction, poor stability, and sand-carrying capacity decline are likely to occur due to salting-out, resulting in poor fracturing effects.

[0004] The bioenzyme low-temperature fracturing gel-breaking technology disclosed in June 2023 uses bioenzymes to replace traditional oxidative gel-breaking agents to solve the problem of gel-breaking of fracturing fluid in shallow low-temperature oil reservoirs.

[0005] Its mechanism of action is as follows:

[0006] Bioenzymes (such as glycanase) specifically decompose the guar gum molecular chain, degrade it into monosaccharides and disaccharides, and the viscosity after gel-breaking is less than 2 mPa·s. The enzyme-catalyzed reaction remains active at low temperatures (20-50°C) and does not require high-temperature activation.

[0007] Its formulation composition is as follows:

[0008] Guar gum-based fracturing fluid (0.33%), organic boron crosslinking agent (0.3%), bioenzyme gel-breaking agent (0.01%).

[0009] However, it has the following disadvantages:

[0010] (1) Limited salt tolerance:

[0011] The activity of bioenzymes decreases significantly when the salinity > 10% (for example, the gel-breaking time is extended to 48 hours when the NaCl concentration is 15%), which cannot meet the requirements of shallow high-salinity oil reservoirs (salinity ≤ 20%).

[0012] (2) Gel-breaking depends on thickeners:

[0013] It is necessary to add guar gum (0.33%) to form the basic viscosity, resulting in a residue content of still 50-80 mg / L, which may block the formation pores.

[0014] (3) pH sensitivity:

[0015] The optimal working pH is 6 - 10. In acidic formations (pH < 5), the enzyme activity is lost and the gel-breaking failure rate > 30%.

[0016] Therefore, developing a clean fracturing fluid system with high salt tolerance and suitable for shallow oil and gas reservoirs is of great significance for improving the fracturing effect and reducing formation damage. Summary of the Invention

[0017] In view of this, the present invention provides a salt-tolerant clean fracturing fluid and its preparation method. This fracturing fluid has high salt tolerance, is suitable for the development of shallow oil and gas reservoirs, and can maintain good performance stability in high salinity formation water.

[0018] To solve the above technical problems, the present invention adopts the following technical solutions:

[0019] A salt-tolerant clean fracturing fluid, comprising the following components:

[0020] An anionic-nonionic surfactant NPEC and a cationic surfactant CTAB;

[0021] The mass percentage of the anionic-nonionic surfactant NPEC is 0.5% - 1.5%;

[0022] The mass percentage of the cationic surfactant CTAB is 1.5 - 2.5%.

[0023] A preparation method of a salt-tolerant clean fracturing fluid, comprising the following steps:

[0024] Step a: Mix the anionic-nonionic surfactant NPEC and the cationic surfactant CTAB evenly according to the ratio;

[0025] Step b: Add water, stir until completely dissolved, adjust the pH value, and perform cooling and aging treatment;

[0026] Step c: After the total concentration of the composite system is fixed, adjust the viscosity and rheological properties of the fracturing fluid according to actual needs by adding different concentrations of NaCl.

[0027] Preferably, in step a, the mixing ratio of the anionic-nonionic surfactant NPEC and the cationic surfactant CTAB is 1:4 - 1:2.

[0028] Preferably, in step b, the anionic-nonionic surfactant NPEC and the cationic surfactant CTAB are pre-mixed with water respectively and stirred at 500 rpm for 20 minutes under the condition of 40 - 45 °C.

[0029] Preferably, in step b, the pH is adjusted to 5.5 - 6.5 with citric acid.

[0030] Preferably, in step b, the cooling and aging treatment time is 24 h.

[0031] Preferably, in step c, the mass percentage of NaCl is 20 - 25%.

[0032] The present invention has achieved the following technical effects compared with the prior art:

[0033] (1) The salt - tolerant clean fracturing fluid prepared by the present invention has a salt tolerance of up to 20%, and can effectively cope with the influence of high - salinity formation water in shallow oil and gas reservoirs;

[0034] (2) Through the synergistic effect of the anionic - nonionic surfactant NPEC and the cationic surfactant CTAB, the present invention improves the stability of the fracturing fluid and reduces the performance decline caused by salting - out;

[0035] (3) The present invention is applicable to the development of shallow oil and gas reservoirs, does not require high - temperature cross - linking, and has good economic efficiency and practicability. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 It is a relationship curve graph between viscosity and NaCl concentration when the total concentration of the composite system of a salt - tolerant clean fracturing fluid and its preparation method of the present invention is 3.0%;

[0037] Figure 2 It is a steady - state rheological curve graph of the CTAB / NPEC composite system with a total concentration of 3.0% of a salt - tolerant clean fracturing fluid and its preparation method of the present invention after adding different concentrations of NaCl;

[0038] Figure 3 It is a viscosity change graph of the CTAB / NPEC composite system with a total concentration of 3.0% of a salt - tolerant clean fracturing fluid and its preparation method of the present invention under different shear times. DETAILED DESCRIPTION OF THE INVENTION

[0039] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0040] The present invention discloses a salt - tolerant clean fracturing fluid, which comprises the following components:

[0041] An anionic - nonionic surfactant NPEC and a cationic surfactant CTAB;

[0042] The mass percentage of the anionic-nonionic surfactant NPEC is 0.5%-1.5%;

[0043] The mass percentage of the cationic surfactant CTAB is 1.5-2.5%.

[0044] The present invention also discloses a preparation method of a salt-resistant clean fracturing fluid, comprising the following steps:

[0045] Step a: Mix the anionic-nonionic surfactant NPEC and the cationic surfactant CTAB evenly at a ratio of 1:4-1:2;

[0046] Step b: Pre-mix the anionic-nonionic surfactant NPEC and the cationic surfactant CTAB with water respectively, stir at 500 rpm for 20 minutes until completely dissolved at 40-45°C, adjust the pH value to 5.5-6.5, and perform a cooling and aging treatment for 24 h;

[0047] Step c: After the total concentration of the composite system is fixed, adjust the viscosity and rheological properties of the fracturing fluid according to actual needs by adding 20-25% of NaCl by mass.

[0048] Example 1:

[0049] A preparation method of a salt-resistant clean fracturing fluid, comprising the following steps:

[0050] Step a: Mix the anionic-nonionic surfactant NPEC and the cationic surfactant CTAB evenly at a ratio of 1:4;

[0051] Step b: Pre-mix the anionic-nonionic surfactant NPEC and the cationic surfactant CTAB with water respectively, stir at 500 rpm for 20 minutes until completely dissolved at 40°C, adjust the pH value to 5.5, and perform a cooling and aging treatment for 24 h;

[0052] Step c: After the total concentration of the composite system is fixed, adjust the viscosity and rheological properties of the fracturing fluid according to actual needs by adding 20% of NaCl by mass.

[0053] Example 2;

[0054] A preparation method of a salt-resistant clean fracturing fluid, comprising the following steps:

[0055] Step a: Mix the anionic-nonionic surfactant NPEC and the cationic surfactant CTAB evenly at a ratio of 1:3;

[0056] Step b: The anionic-nonionic surfactant NPEC and the cationic surfactant CTAB are pre-mixed with water respectively, stirred at 500 rpm for 20 minutes until completely dissolved at 43°C, the pH value is adjusted to 6.0, and then aged by cooling for 24 h;

[0057] Step c: After the total concentration of the composite system is fixed, the viscosity and rheological properties of the fracturing fluid are adjusted according to actual needs by adding 23% NaCl by mass percentage.

[0058] Example 3:

[0059] A preparation method of a salt-tolerant clean fracturing fluid, comprising the following steps:

[0060] Step a: Mix the anionic-nonionic surfactant NPEC and the cationic surfactant CTAB evenly at a ratio of 1:4;

[0061] Step b: The anionic-nonionic surfactant NPEC and the cationic surfactant CTAB are pre-mixed with water respectively, stirred at 500 rpm for 20 minutes until completely dissolved at 45°C, the pH value is adjusted to 6.5, and then aged by cooling for 24 h;

[0062] Step c: After the total concentration of the composite system is fixed, the viscosity and rheological properties of the fracturing fluid are adjusted according to actual needs by adding 25% NaCl by mass percentage.

[0063] Example 4:

[0064] A preparation method of a salt-tolerant clean fracturing fluid, comprising the following steps:

[0065] Step a: Mix the anionic-nonionic surfactant NPEC and the cationic surfactant CTAB evenly at a ratio of 1:3;

[0066] Step b: The anionic-nonionic surfactant NPEC and the cationic surfactant CTAB are pre-mixed with water respectively, stirred at 500 rpm for 20 minutes until completely dissolved at 40°C, the pH value is adjusted to 6.5, and then aged by cooling for 24 h;

[0067] Step c: After the total concentration of the composite system is fixed, the viscosity and rheological properties of the fracturing fluid are adjusted according to actual needs by adding 22% NaCl by mass percentage.

[0068] Example 5:

[0069] Salt tolerance performance test:

[0070] The viscosity test and rheological test were carried out for the present invention;

[0071] Static shear rheological experiments were carried out using a HAAKE rheometer (HAAKE RS600, Germany).

[0072] A cone-plate model with a disk diameter of 50.00 mm and a cone angle of 1° was selected; a sample lid was used to prevent the evaporation of the solution during long-term measurement, and the measurement temperature was 25 °C.

[0073] The experimental results are as Figure 1 and Figure 2 shown.

[0074] It can be seen that the salt tolerance is as high as 15%, and the viscosity > 50 mPa·s can be achieved at a salinity of 15% without additional thickeners.

[0075] Example 6:

[0076] Stability performance test:

[0077] As Figure 3 shown, it is the viscosity change diagram of the CTAB / NPEC composite system with a total concentration of 3.0% of the present invention at different shear times. Specifically, NaCl: 1%, shear rate: 170 s -1 ;

[0078] Before and after adding NaCl, the viscosity of the composite system decreased slightly with the increase of shear time, indicating that the system itself has good shear resistance; at the same time, after adding 1.0% (w / w) NaCl to the composite system, the viscosity of the composite system was always much higher than that of the salt-free composite system.

[0079] This is mainly because the addition of NaCl provides counterions for the composite system, thus generating an electrostatic shielding effect on the surfactant charged head groups of the micelle aggregates in the system, which is conducive to the formation of a more saturated and structured three-dimensional network structure, making the composite system have better viscoelasticity and stronger shear resistance.

[0080] It can be seen that after adding 1.0% (w / w) NaCl, the system has strong viscosity retention ability, good shear resistance and stable sand-carrying performance.

[0081] Among them, the sand-carrying performance measurement results of the composite system are shown in Table 1:

[0082] Table 1:

[0083]

[0084] The sand-carrying law of the viscoelastic surfactant composite system depends on the formation of aggregates and the stability of their network structure in the formulation system.

[0085] The addition of NaCl promoted the formation of aggregates such as worm-like micelles and the construction of a three-dimensional network structure in the composite system. This not only increased the viscosity, but also made the network structure more stable due to the entanglement of the aggregates with each other, enhancing the viscoelasticity of the system.

[0086] Only higher viscosity and viscoelasticity can enable the formulation system to have better proppant-carrying capacity.

[0087] Example 7:

[0088] Effects of stirring time and temperature on raw material dissolution and fracturing fluid performance:

[0089] ① Pour solid CTAB into a certain amount of ultrapure water, place the beaker on a magnetic stirrer and stir for 5 - 10 minutes. When the concentration is relatively high, CTAB dissolves slowly, and the beaker can be sealed and placed in a water bath at 40 - 45 °C for 3 - 5 minutes to be completely dissolved.

[0090] At the same time, place NPEC-5 in a beaker containing a certain amount of ultrapure water, and quickly stir with a magnetic stirrer for 20 minutes until it is completely dissolved in ultrapure water.

[0091] ② Dilute the CTAB mother liquor and NPEC-5 mother liquor to the required concentrations of 3.0% - 6.0% respectively, and then measure 50 milliliters of the two diluted solutions with a measuring cylinder and pour them into a round-bottom flask.

[0092] ③ Place it under continuous stirring at a temperature of 40 - 45 °C for 2 hours, and after complete dissolution, place it at room temperature.

[0093] ④ Finally, transfer the coolant to a wide-mouth bottle, seal and age for 24 hours for standby.

[0094] In subsequent experiments, a certain amount of salt (NaCl) needs to be added. The solution in the wide-mouth bottle can be transferred to a round-bottom flask, and step ③ is repeated until the solution is mixed evenly, and then step ④ is repeated. Composite systems with different composite ratios, different total concentrations, and different concentrations of NaCl are obtained.

[0095] The clean fracturing fluid of the present invention is applicable to the development of shallow oil and gas reservoirs with a salinity ≤ 20% and a formation temperature below 60 °C. For deep oil and gas reservoirs, due to the relatively high formation temperature, it may affect the stability of the fracturing fluid, so the fracturing fluid system of the present invention is not recommended for use.

[0096] By the above technical means, the present invention has achieved the following advantages:

[0097] (1) System self-thickening

[0098] ① Synergistic thickening: Utilize the supramolecular interaction between the anionic-nonionic surfactant NPEC and CTAB to form a worm-like micelle network, enabling the viscosity to be > 50 mPa·s at a salinity of 20% without the need for additional thickeners.

[0099] ② Salt response mechanism: Salt ions (Na+) compress the double electric layer of micelles, enhance the cross-linking strength of micelles, and achieve the "salt thickening" effect rather than the "salting out" effect.

[0100] (2) Low-cost gel breaking

[0101] ① Dilution gel breaking: After injecting into the formation, the micelle network is destroyed by dilution with fresh water (or the low-salt area of formation water), and the viscosity is reduced to below 5 mPa·s without the need for chemical gel breakers.

[0102] ② Environmental protection: Avoid the use of chemical gel breakers such as ammonium persulfate, and the degradation rate > 90%.

[0103] (3) Ultra-simple formulation design

[0104] ① It only contains three core components: NPEC, CTAB, and salt, eliminating other auxiliaries and additives, and reducing the cost.

[0105] ② The interfacial tension between oil and water is reduced to below 20 mN / m by self-assembly of surfactants, promoting the backflow.

[0106] As described above, it is only a preferred embodiment of the present invention, and does not impose any limitation on the technical scope of the present invention. Therefore, any minor modifications, equivalent changes, and decorations made to the above embodiments based on the technical essence of the present invention still fall within the scope of the technical solution of the present invention.

Claims

1. A salt-tolerant clean fracturing fluid, characterized in that, It includes the following components: Anionic-nonionic surfactant NPEC and cationic surfactant CTAB; The mass percentage of the anionic-nonionic surfactant NPEC is 0.5%-1.5%; The mass percentage of the cationic surfactant CTAB is 1.5-2.5%.

2. A preparation method of a salt-tolerant clean fracturing fluid, characterized in that, It includes the following steps: Step a: Mix the anionic-nonionic surfactant NPEC and the cationic surfactant CTAB evenly according to the ratio; Step b: Add water, stir until completely dissolved, adjust the pH value, and perform cooling and aging treatment; Step c: After the total concentration of the composite system is fixed, adjust the viscosity and rheological properties of the fracturing fluid according to actual needs by adding different concentrations of NaCl.

3. The preparation method of a salt-tolerant clean fracturing fluid according to claim 2, characterized in that, In the said step a, the mixing ratio of the anionic-nonionic surfactant NPEC and the cationic surfactant CTAB is 1:4-1:

2.

4. The preparation method of a salt-tolerant clean fracturing fluid according to claim 2, characterized in that, In the said step b, the anionic-nonionic surfactant NPEC and the cationic surfactant CTAB are respectively premixed with water, and stirred at 500 rpm for 20 minutes under the condition of 40-45 °C.

5. The preparation method of a salt-tolerant clean fracturing fluid according to claim 2, characterized in that, In the said step b, the pH value is adjusted to 5.5-6.5 with citric acid.

6. The preparation method of a salt-tolerant clean fracturing fluid according to claim 2, characterized in that In the said step b, the cooling and aging treatment time is 24 h.

7. The preparation method of a salt-tolerant clean fracturing fluid according to claim 2, wherein, In the said step c, the mass percentage of NaCl is 20-25%.

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