Clean fracturing fluid for deep coal bed gas and preparation method of clean fracturing fluid

Abstract

The invention belongs to the technical field of petroleum and natural gas extraction, and discloses a clean fracturing fluid for deep coal bed gas, the clean fracturing fluid comprises the following raw materials by weight: 2.5%-7.5% of a surfactant, 1.2%-3.5% of a thickener, 0.6%-2% of a cleanup additive, 0.2%-0.45% of a gel breaker, 0.05%-0.3% of a bactericide, and the balance of water, the surfactant is a compound of a zwitterionic surfactant and a cationic surfactant. The invention also discloses a preparation method of the fracturing fluid. The fracturing fluid is good in temperature resistance, good in shear resistance, low in interfacial tension and capable of meeting the deep coal bed gas fracturing requirement, and the preparation process is high in stability.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of oil and natural gas exploitation, and particularly relates to a clean fracturing fluid for deep coalbed methane and a preparation method thereof. BACKGROUND

[0002] Deep coalbed methane is a clean energy, and its efficient exploitation is of great significance to energy structure adjustment and environmental protection. Fracturing technology is one of the key means to improve the production of deep coalbed methane, and the performance of fracturing fluid directly affects the fracturing effect. Traditional fracturing fluid has problems such as high residue content and great damage to the formation, which is not conducive to long-term and efficient exploitation of deep coalbed methane. Clean fracturing fluid has a broad application prospect in deep coalbed methane fracturing due to its advantages such as low damage and easy gel breaking. However, the clean fracturing fluid currently used for deep coalbed methane still has some deficiencies in performance, such as the need to improve the sand carrying capacity and limited adaptability to different deep coalbed methane geological conditions. Therefore, it is of great practical significance to develop a clean fracturing fluid with excellent performance and suitable for various deep coalbed methane conditions. SUMMARY

[0003] In view of the defects of the prior art, the application provides a clean fracturing fluid for deep coalbed methane and a preparation method thereof, which has good temperature resistance, good shear resistance and low interfacial tension, and can meet the fracturing requirements of deep coalbed methane.

[0004] The application is realized by the following technical solutions.

[0005] A clean fracturing fluid for deep coalbed methane: the clean fracturing fluid is composed of the following raw materials in a weight content of 100%: a surfactant 2.5%-7.5%, a thickening agent 1.2%-3.5%, a cleanup agent 0.6%-2%, a gel breaker 0.2%-0.45%, a bactericide 0.05%-0.3%, and the balance is water; the surfactant is a complex of a zwitterionic surfactant and a cationic surfactant.

[0006] Preferably, the mass ratio of the zwitterionic surfactant to the cationic surfactant is (1.5-3.5):1.

[0007] Preferably, the zwitterionic surfactant is alkyl betaine or sulfobetaine; and the cationic surfactant is a quaternary ammonium salt surfactant. Preferably, the alkyl betaine is dodecyl dimethyl betaine; the sulfobetaine is dodecyl dimethyl sulfopropyl betaine; and the quaternary ammonium salt surfactant is a long-chain alkyl quaternary ammonium salt.

[0008] Preferably, the thickening agent is a complex of xanthan gum and polyacrylamide.

[0009] Preferably, the thickening agent is xanthan gum and polyacrylamide with a mass ratio of (6-7.5):1.

[0010] Preferably, the cleanup agent is one of fluorocarbon surfactant or silicone surfactant.

[0011] Preferably, the fluorocarbon surfactant is one of potassium perfluorooctyl sulfonate or ammonium perfluorooctanoate; the silicone surfactant is amino polyether modified silicone oil.

[0012] Preferably, the gel breaker is one of ammonium persulfate or sodium persulfate; the bactericide is isothiazolinone bactericide.

[0013] More preferably, the isothiazolinone bactericide is one of 2-methyl-4-isothiazolin-3-one or 5-chloro-2-methyl-4-isothiazolin-3-one.

[0014] The preparation method of the clean fracturing fluid for deep coalbed methane gas, comprising the following steps: (1) raw material pretreatment: the thickening agent is crushed to a particle size of ≤100 μm to improve the dissolution rate and avoid agglomeration; the bactericide is diluted with propylene glycol to a bactericide mother liquor of 20wt% to enhance the compatibility with water; (2) control the heating rate to be 5-8℃ / min, heat the water to 50-60℃, at the same time, start stirring, control the stirring rate to be 80-120r / min; under the conditions of 50-60℃ and stirring rate of 80-120r / min, add the zwitterionic surfactant thereto, the adding rate is controlled to be 0.5-1.0kg / min, continue stirring until the solution is uniform and transparent; then add the cationic surfactant, keep the temperature at 50-60℃ and the stirring rate at 100-150r / min, continue stirring; when the viscosity of the system is 30-50 mPa・s, keep the temperature at 50-60℃ and the stirring rate at 150-200r / min, add the thickening agent, the feeding rate is 0.2-0.5kg / min, continue stirring; when the viscosity of the system under the shear rate of 170s -1 -1 is 80-120mPa・s, control the stirring rate to be 120-150r / min, add the cleanup agent, continue stirring for 15-20min; keep the stirring rate at 100-120r / min, add the gel breaker, stir for 10-15min; add the bactericide mother liquor, stir for 20-30min; stop heating, control the stirring rate to be 80-100r / min, continue stirring for 30-45min for curing.

[0015] The performance index requirements of the clean fracturing fluid are as follows: ① viscosity: 170s -1Shear rate, viscosity retention rate ≥ 50% at 80℃; ②Interfacial tension: interfacial tension with deep coalbed methane formation water ≤ 5 × 10 -4 mN / m; ③Bacteriostasis: 24h sterilization rate of SRB (sulfate-reducing bacteria) ≥ 99.5%.

[0016] The amphoteric ionic surfactant in the clean fracturing fluid can reduce the surface tension of the clean fracturing fluid, enhance the wettability, and be beneficial to the penetration and diffusion of the fracturing fluid in the deep coalbed methane; the cationic surfactant can help to improve the sand carrying capacity of the fracturing fluid, so that the proppant can be uniformly distributed in the fracture, and the fracture closure can be effectively prevented; the thickening agent is a compound of xanthan gum and polyacrylamide, has good thickening performance and temperature resistance, can improve the viscosity of the fracturing fluid, enhance the sand carrying capacity and sand suspension stability, and ensure that the proppant can be smoothly conveyed to the deep part of the fracture during the fracturing process; the cleanup agent can reduce the interfacial tension between the fracturing fluid and the surface of the deep coalbed methane rock, so that the fracturing fluid can be quickly discharged from the deep coalbed methane after fracturing, and the damage to the permeability of the deep coalbed methane is reduced; the gel breaker can rapidly reduce the viscosity of the fracturing fluid under certain conditions, realize gel breaking, and facilitate the discharge of the fracturing fluid from the deep coalbed methane; and the bactericide can effectively inhibit the growth and reproduction of microorganisms in the fracturing fluid, prevent the deterioration of the fracturing fluid during storage and use, and ensure the stable performance of the fracturing fluid.

[0017] Advantages of the present application: (1) Strong process stability: through stepwise feeding, precise temperature control (50-60℃) and stirring rate adjustment, component aggregation or decomposition is avoided, the prepared fracturing fluid has small viscosity uniformity deviation and good batch reproducibility; (2) Performance adaptation to deep coalbed methane: the prepared fracturing fluid has a viscosity of 170s -1 -1 under shear rate, a viscosity retention rate ≥ 50% at 80℃, an interfacial tension with deep coalbed methane formation water ≤ 5 × 10 -4 -1 / m, a sterilization rate of SRB ≥ 99.5%, and a clay swelling rate ≤ 4%, meeting the fracturing requirements of deep coalbed methane; (3) Environmental protection and economic optimization: the water in the present application can use formation recycled water to reduce fresh water consumption; the component compounding is reasonable, the bactericide dosage is small, and the cost is reduced compared with conventional systems. BRIEF DESCRIPTION OF DRAWINGS

[0018] Figure 1 The appearance of the fracturing fluid described in Example 1; Figure 2 The viscosity change of the fracturing fluid described in Example 1 under 170s -1 -1 shear rate; Figure 3 The appearance of the fracturing fluid described in Example 2; Figure 4 The viscosity change of the fracturing fluid described in Example 2 at 170s -1 The viscosity change at the shear rate; Figure 5 The appearance of the fracturing fluid described in Example 3; Figure 6 The viscosity change of the fracturing fluid described in Example 3 at 170s -1 The viscosity change at the shear rate. DETAILED DESCRIPTION

[0019] The reaction kettle used in the embodiment of the present application is a stainless steel reaction kettle (material 316L, temperature resistance ≥ 300℃, pressure resistance ≥ 1.0MPa), equipped with an anchor type stirrer (stirring rate adjustable range: 50-300r / min), a jacket heating system (temperature control accuracy: ±2℃), an online viscometer (measurement range: 1-1000mPa・s) and a pH meter (measurement accuracy: ±0.1).

[0020] The amino polyether modified silicone oil used in the embodiment of the present application is the product of Qingdao Zhongbao Silicon Material Technology Co., Ltd. with the drug type number 207, and the long chain alkyl quaternary ammonium salt used is octadecyl benzyl dimethyl ammonium chloride.

[0021] Example 1 A clean fracturing fluid for deep coalbed methane, with a total amount of 100kg, the components and contents are as follows: Surfactant 4.8kg: composed of dodecyl dimethyl betaine and long chain alkyl quaternary ammonium salt with a mass ratio of 3:1; Thickening agent 2.4kg: composed of xanthan gum and polyacrylamide mixed with a mass ratio of 7:1; Drainage aid 1.2kg: potassium perfluorooctyl sulfonate; Gel breaker 0.3kg: ammonium persulfate; Bactericide 0.1kg: 2-methyl-4-isothiazolin-3-one; Deionized water 91.2kg; The preparation method of the clean fracturing fluid for deep coalbed methane is as follows: (1) Raw material pretreatment: the thickening agent is pulverized to a particle size ≤100μm to improve the dissolution rate and avoid agglomeration; the bactericide is diluted with propylene glycol to a 20wt% bactericide stock solution to enhance the compatibility with water; (2) Open the reaction kettle feed valve, add deionized water to the reaction kettle, and close the feed valve. Start the jacket heating system, raise the temperature in the reaction kettle to 55℃, and control the temperature rising rate at 6℃ / min to avoid local overheating leading to decomposition of subsequent components; at the same time, start the anchor type stirrer and set the stirring rate to 100r / min to form a stable vortex to provide power for the dissolution of subsequent components; At 55℃, stirring rate 100r / min, through the feed port at the top of the reactor, first slowly add 0.8kg / min of dodecyl dimethyl betaine, to avoid the local concentration of high once added to form micellar aggregation, after the completion of the addition, continue to stir for 18min, through the reactor sight observation, the solution is uniform and transparent; Then add long chain alkyl quaternary ammonium salt, keep the temperature 55℃, stirring rate 120r / min, stirring for 25min, through the online viscometer monitoring, the system viscosity rises to 40mPa・s, considered as mixing qualified, at this time the two kinds of surfactants fully associated to form a complex micelle; Keep the reactor temperature 55℃, stirring rate 180r / min, increase the shear force to promote the dissolution of thickener, add the compound of xanthan gum and polyacrylamide through the screw feeder at a rate of 0.4kg / min, to prevent the thickener from forming "fish eye" aggregation on the liquid surface, after the completion of the thickener addition, continue to stir for 35min, through the online viscometer real-time monitoring, the system viscosity is stable at 100mPa・s(170s -1 Shear rate), considered as the complete dissolution of thickener; Reduce the stirring rate to 130r / min, add potassium perfluorooctyl sulfonate, stir for 18min, the cleanup agent is uniformly dispersed in the micellar system, to avoid local enrichment affecting the interfacial tension; Keep the stirring rate 110r / min, add ammonium persulfate, stir for 13min, to ensure the delayed gel breaking effect; Finally add the bactericide mother liquor, continue to stir for 25min, the components are fully mixed and uniform; Turn off the jacket heating system, keep the stirring rate 90r / min, mature the material in the reactor for 40min, to make the micellar structure stable and the viscosity further uniform.

[0022] The appearance of the prepared fracturing fluid is shown in Figure 1 , it can be seen that the appearance is uniform; Take samples for quality testing: ①viscosity: at 170s -1 Shear rate, 80℃, the final viscosity retention rate is greater than 58%(compared with the original viscosity), see Figure 2 ; ②interfacial tension: the interfacial tension with deep coalbed methane formation water is 4×10 -4 mN / m; ③bacteriostasis: the 24h sterilization rate of SRB is 99.7%.

[0023] Example 2 A kind of clean fracturing fluid for deep coalbed methane, the total amount is 100kg, the components and contents are as follows: Surfactant 4.2 kg: composed of dodecyl dimethyl sulfopropyl betaine and long chain alkyl quaternary ammonium salt with a mass ratio of 2:1; Thickening agent 2.1 kg: composed of xanthan gum and polyacrylamide with a mass ratio of 6:1; Flowback aid 0.6 kg: amino polyether modified silicone oil; Gel breaker 0.25 kg: sodium persulfate; Bactericide 0.05 kg: 5-chloro-2-methyl-4-isothiazolin-3-one; Deionized water 92.8 kg; The preparation method of the clean fracturing fluid for deep coalbed methane is as follows: (1) Raw material pretreatment: crush the thickening agent to a particle size of ≤100 μm to improve the dissolution rate and avoid agglomeration; dilute the bactericide with propylene glycol to a bactericide mother liquor of 20 wt% to enhance the compatibility with water; (2) Open the feed valve of the reaction kettle, add deionized water to the reaction kettle, and close the feed valve. Start the jacket heating system, and raise the temperature in the reaction kettle to 50°C at a rate of 5°C / min to avoid local overheating leading to decomposition of subsequent components; at the same time, start the anchor stirrer and set the stirring rate to 80 r / min to form a stable vortex to provide power for the dissolution of subsequent components; Under the conditions of 50°C and a stirring rate of 80 r / min, slowly add dodecyl dimethyl sulfopropyl betaine through the feed port at the top of the reaction kettle at a rate of 0.5 kg / min, continue stirring for 18 min after the addition is complete, and the solution is uniform and transparent; then add the long chain alkyl quaternary ammonium salt, maintain the temperature at 50°C and the stirring rate at 100 r / min, and stir for 20 min. The viscosity of the system rises to 30 mPa・s, which is considered to be a qualified mixture. At this time, the two surfactants are fully associated to form a complex micelle; Maintain the temperature of the reaction kettle at 50°C and the stirring rate at 150 r / min, slowly add the compound of xanthan gum and polyacrylamide through a screw feeder at a rate of 0.2 kg / min, continue stirring for 30 min after the addition of the thickening agent is complete, and monitor the viscosity of the system in real time through an online viscometer. The viscosity of the system is stable at 80 mPa・s (170 s -1 under shear rate), which is considered to be complete dissolution of the thickening agent; Reduce the stirring rate to 120 r / min, add amino polyether modified silicone oil, and stir for 15 min to uniformly disperse the flowback aid; Maintain the stirring rate at 100 r / min, add sodium persulfate, and stir for 10 min; finally, add the bactericide mother liquor, and continue stirring for 20 min to fully mix and evenly disperse the components; Close the jacket heating system, keep the stirring rate 80 r / min, and mature the material in the reactor for 30 min to stabilize the micelle structure and further homogenize the viscosity.

[0024] The appearance of the prepared fracturing fluid is shown in Figure 3 , and it can be seen that the appearance is uniform. Sampling for quality testing: ①Viscosity: at 170 s -1 Shear rate, 80℃, viscosity retention rate is greater than 51% (compared with the original viscosity), see Figure 4 ; ②Interfacial tension: the interfacial tension with deep coalbed methane formation water is 4.5×10 -4 mN / m; ③Bacteriostatic property: 24h bactericidal rate on SRB is 99.6%.

[0025] Example 3 A clean fracturing fluid for deep coalbed methane, the total amount is 100 kg, and the components and contents are as follows: Surfactant 6.3 kg: composed of dodecyl dimethyl betaine and long-chain alkyl quaternary ammonium salt with a mass ratio of 3.5:1; Thickening agent 2.975 kg: composed of xanthan gum and polyacrylamide with a mass ratio of 7.5:1; Fluxing agent 1.9 kg: ammonium perfluorooctanoate; Gel breaker 0.45 kg: ammonium persulfate; Bactericide 0.225 kg: 2-methyl-4-isothiazolin-3-one; Deionized water 88.15 kg; The preparation method of the clean fracturing fluid for deep coalbed methane is as follows: (1) Raw material pretreatment: crush the thickening agent to a particle size of ≤100 μm to improve the dissolution rate and avoid agglomeration; dilute the bactericide with propylene glycol to a bactericide mother liquor of 20wt% to enhance the compatibility with water; (2) Open the reactor feed valve, add deionized water to the reactor, and close the feed valve. Start the jacket heating system, raise the temperature in the reactor to 60℃, and control the heating rate at 8℃ / min to avoid local overheating leading to decomposition of subsequent components; at the same time, start the anchor stirrer and set the stirring rate to 120 r / min to form a stable vortex and provide power for the dissolution of subsequent components; At 60℃, stirring rate 120r / min, through the feed port at the top of the reactor, first slowly add dodecyl dimethyl betaine at a rate of 1.0kg / min, after the addition is completed, continue to stir for 20min, the solution is uniform and transparent; then add long chain alkyl quaternary ammonium salt, keep the temperature at 60℃, stirring rate 150r / min, stir for 30min, through the online viscometer monitoring, the system viscosity rises to 50mPa・s, considered as mixing qualified, at this time the two surfactants fully associated to form complex micelles; Keep the temperature of the reactor at 60℃, stirring rate 200r / min, slowly add the compound of xanthan gum and polyacrylamide through the screw feeder at a rate of 0.5kg / min, after the thickener is added, continue to stir for 40min, through the online viscometer real-time monitoring, the system viscosity is stable at 120mPa・s(170s -1 under shear rate), considered as the thickener is completely dissolved; Reduce the stirring rate to 150r / min, add ammonium perfluorooctanoate, stir for 20min, the cleanup agent is uniformly dispersed; Keep the stirring rate at 120r / min, add ammonium persulfate, stir for 15min; finally add the bactericide mother liquor, continue to stir for 30min, the components are fully mixed and uniform; Turn off the jacket heating system, keep the stirring rate at 100r / min, mature the material in the reactor for 45min, make the micelle structure stable, and the viscosity is further uniform.

[0026] The appearance of the prepared fracturing fluid is shown in Figure 5 , it can be seen that the appearance is uniform; Take samples for quality testing: ①viscosity: at 170s -1 under shear rate, 80℃, the final viscosity retention rate is greater than 62%(compared with the original viscosity), see Figure 6 ; ②interfacial tension: the interfacial tension with deep coalbed methane formation water is 3.8×10 -4 mN / m; ③bacteriostasis: the 24h bactericidal rate on SRB is 99.9%.

[0027] Example 4 A kind of clean fracturing fluid for deep coalbed methane, the total amount is 100kg, the components and contents are as follows: Surfactant 2.5kg: composed of dodecyl dimethyl betaine and long chain alkyl quaternary ammonium salt with a mass ratio of 1.5:1; Thickener 3.5kg: composed of xanthan gum and polyacrylamide with a mass ratio of 7:1; Cleanup agent 2.0kg: potassium perfluorooctyl sulfonate; Gel breaker 0.2kg: ammonium persulfate; Bactericide 0.3 kg: 2-methyl-4-isothiazolin-3-one; Deionized water 91.5 kg; The preparation method is the same as that of Example 1.

[0028] Example 5 A clean fracturing fluid for deep coalbed methane, with a total amount of 100 kg, the components and contents are as follows: Surfactant 7.5 kg: composed of dodecyl dimethyl betaine and long chain alkyl quaternary ammonium salt with a mass ratio of 2:1; Thickening agent 1.2 kg: composed of xanthan gum and polyacrylamide with a mass ratio of 7:1; Fluxing agent 1.0 kg: potassium perfluorooctyl sulfonate; Glue breaker 0.2 kg: ammonium persulfate; Bactericide 0.3 kg: 2-methyl-4-isothiazolin-3-one; Deionized water 89.8 kg; The preparation method is the same as that of Example 1.

Claims

1. A clean fracturing fluid for deep coalbed methane, characterized in that: The clean fracturing fluid is composed of the following raw materials by weight percentage (100%): surfactant 2.5%-7.5%, thickener 1.2%-3.5%, flow aid 0.6%-2%, breaker 0.2%-0.45%, bactericide 0.05%-0.3%, with the balance being water; the surfactant is a compound of zwitterionic and cationic surfactants.

2. The clean fracturing fluid for deep coalbed methane according to claim 1, characterized in that: The mass ratio of the zwitterionic surfactant to the cationic surfactant is (1.5-3.5):

1.

3. The clean fracturing fluid for deep coalbed methane according to claim 1, characterized in that: The zwitterionic surfactant is alkyl betaine or sulfobetaine; the cationic surfactant is a quaternary ammonium salt surfactant.

4. The clean fracturing fluid for deep coalbed methane according to claim 3, characterized in that: The alkyl betaine is dodecyl dimethyl betaine; the sulfobetaine is dodecyl dimethyl sulfopropyl betaine; and the quaternary ammonium salt surfactant is a long-chain alkyl quaternary ammonium salt.

5. The clean fracturing fluid for deep coalbed methane according to claim 1, characterized in that: The thickener is a compound of xanthan gum and polyacrylamide.

6. The clean fracturing fluid for deep coalbed methane according to claim 5, characterized in that: The thickener is xanthan gum and polyacrylamide in a mass ratio of (6-7.5):

1.

7. The clean fracturing fluid for deep coalbed methane according to claim 1, characterized in that: The drainage aid is one of fluorocarbon surfactants or organosilicon surfactants.

8. The clean fracturing fluid for deep coalbed methane according to claim 6, characterized in that: The fluorocarbon surfactant is one of potassium perfluorooctyl sulfonate or ammonium perfluorooctanoate; the organosilicon surfactant is amino polyether modified silicone oil.

9. The clean fracturing fluid for deep coalbed methane according to claim 1, characterized in that: The de-gelling agent is either ammonium persulfate or sodium persulfate; the bactericide is an isothiazolinone bactericide.

10. A method for preparing a clean fracturing fluid for deep coalbed methane as described in claim 1, comprising the following steps: (1) Raw material pretreatment: The thickener is crushed to a particle size ≤100μm; The bactericide was diluted with propylene glycol to prepare a 20 wt% bactericide stock solution; (2) Control the heating rate to 5-8℃ / min, raise the water temperature to 50-60℃, and at the same time, start stirring and control the stirring rate to 80-120r / min; add amphoteric surfactant at 50-60℃ and stirring rate of 80-120r / min, with the addition rate controlled at 0.5-1.0kg / min, and continue stirring until the solution is homogeneous and transparent; then add cationic surfactant, maintain the temperature at 50-60℃ and stirring rate of 100-150r / min, and continue stirring; when the system viscosity is 30-50mPa·s, maintain the temperature at 50-60℃ and stirring rate of 150-200r / min, add thickener at a feed rate of 0.2-0.5kg / min, and continue stirring; when the system viscosity is 170s -1 When the viscosity at the shear rate is 80-120 mPa·s, control the stirring speed at 120-150 r / min, add the drainage aid, and continue stirring for 15-20 min; maintain the stirring speed at 100-120 r / min, add the desiccant, and stir for 10-15 min; add the bactericide stock solution, and stir for 20-30 min; stop heating, control the stirring speed at 80-100 r / min, and continue stirring for 30-45 min for maturation.

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