Low molecular weight polymer fracturing fluid

Abstract

The invention discloses a polymer fracturing fluid, and relates to the technical field of fracturing fluid. The polymer fracturing fluid comprises, by mass percentage, a thickener, a small molecule alcohol, and the thickener comprises a polyether and a cationic surfactant; the polyether uses chitosan oligosaccharide as an initiator, ethylene oxide and propylene oxide as main agents, and amino acids as end-capping agents for end-capping. The polymer fracturing fluid of the invention can automatically break gel under certain temperature conditions, and has low viscosity after breaking gel.

Description

Technical Field

[0001] The invention relates to the technical field of fracturing fluids, and in particular to a low-molecular polymer fracturing fluid. Background Art

[0002] With the in-depth development of tight reservoirs and shale reservoirs, fracturing operations have received more and more attention. The main component of fracturing fluid is the thickener. At present, conventional thickeners include natural thickeners and synthetic thickeners. Among them, natural guar gum thickeners and synthetic polyacrylamide thickeners are the most widely used. However, for these high-molecular thickeners, when they need to be broken, external breakers are usually required. However, the situation in the formation is complicated, and it is difficult for the external breakers to completely break the gel, resulting in a large amount of residual gel being retained in the bottom layer, resulting in a decrease in reservoir permeability, making the fracturing effect lower than expected. Therefore, the current research on clean fracturing fluids with low residue or even no residue is a hot topic.

[0003] At present, most of the clean fracturing fluids are surfactants with relatively high viscosity, such as Chinese patents CN201610685924.9 and CN201210077974.0, which use surfactants with relatively high viscosity to make viscoelastic active water fracturing fluids. These surfactants have low molecular weight and basically do not affect the permeability of the formation. However, for fracturing fluids, the fracturing fluid needs to be flowed back after fracturing to avoid the fracturing fluid being retained in the formation, and the lower the viscosity, the easier it is to flow back. The current clean fracturing fluids do not take this into account. Summary of the invention

[0004] In view of the above technical problems, the purpose of the present invention is to provide a polymer fracturing fluid for addressing the defects of the prior art, which can automatically break the gel under certain temperature conditions and has low viscosity after breaking the gel.

[0005] The present invention adopts the following technical scheme: a low molecular polymer fracturing fluid, which includes 0.5-1.2% of a thickener and 1-2.5% of a small molecule alcohol in terms of mass percentage, wherein the thickener includes a polyether and a cationic surfactant in a mass ratio of 1:0.3-0.5; the polyether uses chitosan oligosaccharide as an initiator, ethylene oxide and propylene oxide as main agents, and amino acids as capping agents for end-capping, and the mass ratio of the initiator, ethylene oxide, propylene oxide and amino acids is 10:50-80:20-30:1-10.

[0006] An embodiment of the present invention is that the small molecule alcohol is one of methanol and ethanol.

[0007] One embodiment of the present invention is that the cationic surfactant is a quaternary ammonium salt surfactant.

[0008] In one embodiment of the present invention, the amino acid is at least one of arginine, cystine, cysteine, histidine, lysine, methionine, serine and threonine.

[0009] One embodiment of the present invention is that the preparation method of the polyether comprises the following steps: adding an initiator catalyst into a reaction kettle and adding water to dissolve it, introducing an inert gas to remove oxygen, adding one-quarter to one-third of ethylene oxide and heating to 100-150° C. to react until the system pressure does not decrease for 20 minutes; then adding one-third to two-thirds of propylene oxide and continuing to react until the system pressure does not decrease for 20 minutes; then adding one-third to one-half of ethylene oxide and continuing to react until the system pressure does not decrease for 20 minutes; then adding the remaining propylene oxide and continuing to react until the system pressure does not decrease for 20 minutes; then adding the remaining ethylene oxide and reacting until the system pressure does not decrease for 20 minutes; then adjusting the temperature to 50-80° C., adding a capping agent and reacting under reduced pressure for 6-10 hours.

[0010] One embodiment of the present invention is that the preparation method of the polyether comprises the following steps: adding an initiator and a catalyst into a reaction kettle and adding water to dissolve them, introducing an inert gas to remove oxygen, adding one-third to two-thirds of propylene oxide and heating to 100-150° C. to react until the system pressure does not decrease for 20 minutes, then adding the remaining propylene oxide and one-third to two-thirds of ethylene oxide to react until the system pressure does not decrease for 20 minutes, then adding the remaining ethylene oxide to react until the system pressure does not decrease for 20 minutes, then adjusting the temperature to 50-80° C., adding a capping agent and reacting under reduced pressure for 6-10 hours to obtain the polyether.

[0011] Furthermore, the catalyst is an organic base or an inorganic base, and the amount of the catalyst added is 0.5 to 1.2% of the total mass of the final product.

[0012] One embodiment of the present invention is that it further comprises a clay stabilizer.

[0013] The beneficial effects of the present invention are as follows: the polymer fracturing fluid of the present invention is a clean fracturing fluid, which can automatically break gel under certain temperature conditions and has low viscosity after breaking gel, and therefore has good promotion and application value. DETAILED DESCRIPTION

[0014] In order to have a clearer understanding of the technical features, purposes and beneficial effects of the present invention, the technical scheme of the present invention is described in detail below in conjunction with embodiments, but it should not be construed as limiting the applicable scope of the present invention.

[0015] A low molecular polymer fracturing fluid comprises, by mass percentage, 0.5-1.2% of a thickener and 1-2.5% of a small molecule alcohol, wherein the thickener comprises a polyether and a cationic surfactant in a mass ratio of 1:0.3-0.5; the polyether uses chitosan oligosaccharide as an initiator, ethylene oxide and propylene oxide as main agents, and is capped with an amino acid as an end-capping agent, and the mass ratio of the initiator, ethylene oxide, propylene oxide and amino acid is 10:50-80:20-30:1-10.

[0016] Specifically, in the present invention, the thickener is the main component of the fracturing fluid, and the small molecule alcohol can further increase its viscosity so that its viscosity meets the actual fracturing requirements. The small molecule alcohol generally refers to one of methanol, ethanol, and ethylene glycol.

[0017] The reason why the corresponding cationic surfactant is added is that after compounding with the polyether, it can not only increase the viscosity of the system, but also act as a drainage aid. At the same time, after compounding with the existing anti-collapse plugging agent, it can play a better anti-collapse plugging role. The cationic surfactant is a quaternary ammonium salt cationic surfactant, such as the common trimethyl octadecyl ammonium bromide, trimethyl hexadecyl ammonium bromide, etc.

[0018] For thickeners, polyether is the main thickener. The reason why chitosan oligosaccharide is used instead of chitosan as the initiator is that the inventors found that when chitosan is used as the initiator, its solubility is poor, resulting in more by-products produced in the final reaction, so the performance is relatively poor. At the same time, the final product basically has no self-degradation performance, which is difficult to meet the flowback requirements of fracturing fluids. When chitosan oligosaccharide is used, firstly, the degree of polymerization of chitosan oligosaccharide is low, the water solubility is good, and the performance of the product obtained by the final reaction is relatively good; secondly, the final product still has a certain self-degradation performance under long-term high temperature conditions, which can better meet the actual use requirements.

[0019] Generally speaking, the viscosity of chitosan is low, and the viscosity of poly PO-EO is not very high. Even if chitosan is used as an initiator in the present invention to increase the molecular weight, its viscosity is still difficult to meet the actual use requirements; after a large number of experiments, the inventors found that after using amino acids as end-capping agents, the viscosity of the system can be better increased, so that it can meet the actual use requirements. The amino acid is at least one of arginine, cystine, cysteine, histidine, lysine, methionine, serine, and threonine. These amino acids have multiple active functional groups (at least three, including carboxyl, amino, hydroxyl, sulfhydryl and thioether). Multiple active functional groups make it have more active sites and ultimately produce higher viscosity; at the same time, amino acids containing multiple amino groups such as arginine and amino acids containing sulfur atoms such as cystine have a certain promoting effect on clay stability, and amino acids containing hydroxyl groups such as serine are more likely to produce intermolecular hydrogen bonds, making their viscosity higher. Moreover, these amino acids with multiple active functional groups have better effects than conventional amino acids with only two active functional groups, such as alanine, glycine, leucine, etc.

[0020] As for the preparation process of polyether, it can be the same as the preparation method of conventional polyether, and the current polyether is divided into random copolymer polyether and block polyether, both of which can be applied to the present invention, but from the actual effect point of view, the block polyether has a better effect.

[0021] As described above, although the prior art has provided corresponding methods for preparing polyethers, in order to facilitate the understanding of those skilled in the art, the present invention still proposes two different methods for preparing polyethers, which have better effects than conventional block polyethers.

[0022] Method 1: Add the initiator and catalyst into a reactor and add water to dissolve them, introduce inert gas to remove oxygen, add one-quarter to one-third of ethylene oxide and heat to 100-150°C to react until the system pressure does not drop for 20 minutes; then add one-third to two-thirds of propylene oxide and continue to react until the system pressure does not drop for 20 minutes, then add one-third to one-half of ethylene oxide and continue to react until the system pressure does not drop for 20 minutes, then add the remaining propylene oxide and continue to react until the system pressure does not drop for 20 minutes, then add the remaining ethylene oxide and react until the system pressure does not drop for 20 minutes, then adjust the temperature to 50-80°C, add a capping agent and react under reduced pressure for 6-10 hours.

[0023] One embodiment of the present invention is that the preparation method of the polyether comprises the following steps: adding an initiator and a catalyst into a reaction kettle and adding water to dissolve them, introducing an inert gas to remove oxygen, adding one-third to two-thirds of propylene oxide and heating to 100-150° C. to react until the system pressure does not decrease for 20 minutes, then adding the remaining propylene oxide and one-third to two-thirds of ethylene oxide to react until the system pressure does not decrease for 20 minutes, then adding the remaining ethylene oxide to react until the system pressure does not decrease for 20 minutes, then adjusting the temperature to 50-80° C., adding a capping agent and reacting under reduced pressure for 6-10 hours to obtain the polyether.

[0024] The catalyst is an organic base or an inorganic base, such as sodium hydroxide, potassium hydroxide, triethylamine, etc., which can be used as a catalyst. The amount of the catalyst added is 0.5-1.2% of the total mass of the final product.

[0025] The polyethers prepared by the above two methods have better effects than the traditional block polyethers of initiator-EO-PO-EO-capping agent and random copolymer polyethers.

[0026] At the same time, the conventional fracturing fluid in the art can also be added with common fracturing fluid additives such as clay stabilizers and fungicides. At the same time, since it contains cationic surfactants, when the polyether is hydrolyzed, it is also a good non-ionic surfactant. After the two are compounded, they have a good drainage effect, so there is no need to add additional drainage agents.

[0027] In the following embodiments, unless otherwise specified, the operations described are routine operations in the art.

[0028] In the following examples, unless otherwise specified, all raw materials used are conventional commercial products in the art.

[0029] In the following examples, unless otherwise specified, the concentrations are all mass concentrations.

[0030] Example 1

[0031] Preparation of polyether: 100g of chitosan oligosaccharide is dissolved in 150ml of water and then added to a reactor, triethylamine is added at the same time, nitrogen is introduced for 30 minutes to remove oxygen in the reactor, 200g of ethylene oxide is then added to the reactor, and the temperature is raised to 110°C for reaction until the pressure in the reactor does not decrease for 20 minutes; then 120g of propylene oxide is added to continue the reaction until the pressure in the reactor does not decrease for 20 minutes; then 300g of ethylene oxide is added to continue the reaction until the pressure in the reactor does not decrease for 20 minutes; then 130g of propylene oxide is added to continue the reaction until the pressure in the reactor does not decrease for 20 minutes; then 200g of ethylene oxide is added to continue the reaction until the pressure in the reactor does not decrease for 20 minutes; then the temperature is adjusted to 70°C, 30g of arginine and 10g of serine are prepared into an aqueous solution, and the amino acid solution is added to the reactor for reaction for 8 hours. After the reaction is completed, the solvent is removed by distillation under reduced pressure to obtain the obtained product.

[0032] Preparation of fracturing fluid: polyether and trimethyl hexadecyl ammonium bromide are added to water with concentrations of 0.7% and 0.35% respectively, stirred to completely dissolve, then 0.5% ethylenediamine and 1.5% ethanol are added, stirred to completely dissolve.

[0033] The fracturing fluid has a viscosity of 84.6 mPa·s and a sand-carrying ratio of 48%. After aging at 90°C for 17 days, the fracturing fluid breaks the gel, and the viscosity after breaking the gel is 6.8 mPa·s. After aging at 120°C for 12 days, the fracturing fluid breaks the gel, and the viscosity after breaking the gel is 7.8 mPa·s. The anti-swelling rate for shale is 68.9%.

[0034] Example 2

[0035] Preparation of polyether: Take 100g of chitosan oligosaccharide, dissolve it in 150ml of water, and add it to a reactor. At the same time, add triethylamine, pass nitrogen for 30 minutes to remove the oxygen in the reactor, add 130g of propylene oxide and heat it to 120°C and react until the pressure in the reactor does not decrease for 20 minutes; then add 120g of propylene oxide and 350g of ethylene oxide and continue to react until the pressure in the reactor does not decrease for 20 minutes; then add 350g of ethylene oxide and continue to react until the pressure in the reactor does not decrease for 20 minutes; then adjust the temperature to 80°C, take 20g of arginine, 10g of cysteine, and 10g of serine to prepare an aqueous solution, and add the amino acid solution to the reactor and react for 7 hours. After the reaction is completed, remove the solvent by reduced pressure distillation.

[0036] The preparation of the fracturing fluid is the same as that in Example 1.

[0037] The fracturing fluid has a viscosity of 87.2 mPa·s and a sand-carrying ratio of 44%. After aging at 90°C for 21 days, the fracturing fluid breaks the gel, and the viscosity after breaking the gel is 8.2 mPa·s. After aging at 120°C for 13 days, the fracturing fluid breaks the gel, and the viscosity after breaking the gel is 9.4 mPa·s. The anti-swelling rate for shale is 74.5%.

[0038] Example 3

[0039] Preparation of polyether: Take 100g of chitosan oligosaccharide, dissolve it in 150ml of water, and then add it to a reactor. At the same time, add triethylamine, pass nitrogen for 30 minutes to remove the oxygen in the reactor, add 350g of ethylene oxide and heat it to 120°C and react until the pressure in the reactor does not decrease for 20 minutes; then add 250g of propylene oxide and continue to react until the pressure in the reactor does not decrease for 20 minutes; then add 350g of ethylene oxide and continue to react until the pressure in the reactor does not decrease for 20 minutes; then adjust the temperature to 60°C, take 30g of arginine and 10g of serine to prepare an aqueous solution, and add the amino acid solution to the reactor to react for 10 hours. After the reaction is completed, remove the solvent by reduced pressure distillation.

[0040] In the preparation of the fracturing fluid, the concentrations of the polyether and trimethyl hexadecyl ammonium bromide are 0.6% and 0.3% respectively, and the rest are the same.

[0041] The fracturing fluid has a viscosity of 75.6 mPa·s and a sand-carrying ratio of 37%. After aging at 90°C for 16 days, the fracturing fluid breaks the gel, and the viscosity after breaking the gel is 7.1 mPa·s. After aging at 120°C for 11 days, the fracturing fluid breaks the gel, and the viscosity after breaking the gel is 8.2 mPa·s. The anti-swelling rate for shale is 64.9%.

[0042] Example 4

[0043] Preparation of polyether: Take 100g of chitosan oligosaccharide, dissolve it in 150ml of water and add it to the reactor, add triethylamine at the same time, pass nitrogen for 30 minutes to remove the oxygen in the reactor, add 700g of ethylene oxide and 250g of propylene oxide and raise the temperature to 120°C for reaction until the pressure in the reactor does not decrease for 20 minutes; then adjust the temperature to 70°C, take 30g of arginine and 10g of serine to prepare an aqueous solution, and add the amino acid solution to the reactor to react for 8 hours. After the reaction is completed, remove the solvent by distillation under reduced pressure.

[0044] The preparation of the fracturing fluid is the same as that in Example 1.

[0045] The fracturing fluid has a viscosity of 54.2 mPa·s and a sand-carrying ratio of 31%. After aging at 90°C for 18 days, the fracturing fluid breaks the gel, and the viscosity after breaking the gel is 7.6 mPa·s. After aging at 120°C for 12 days, the fracturing fluid breaks the gel, and the viscosity after breaking the gel is 8.7 mPa·s. The anti-swelling rate for shale is 68.1%.

[0046] Example 5

[0047] Preparation of polyether: The difference from Example 1 is that 180 g of ethylene oxide is added for the first time, 100 g of propylene oxide is added for the first time, 250 g of ethylene oxide is added for the second time, 120 g of propylene oxide is added for the second time, and 170 g of ethylene oxide is added for the third time; the amino acids are 40 g of lysine, 30 g of cystine, and 10 g of serine. The rest are the same.

[0048] The preparation of the fracturing fluid is the same as that in Example 1.

[0049] The fracturing fluid has a viscosity of 74.7 mPa·s and a sand-carrying ratio of 42%. After aging at 90°C for 18 days, the fracturing fluid breaks the gel, and the viscosity after breaking the gel is 9.3 mPa·s. After aging at 120°C for 13 days, the fracturing fluid breaks the gel, and the viscosity after breaking the gel is 11.5 mPa·s. The anti-swelling rate for shale is 74.9%.

[0050] Example 6

[0051] Preparation of polyether: The difference from Example 1 is that the amino acid added is 40 g of arginine, and the rest is the same.

[0052] In the preparation of the fracturing fluid, the concentrations of the polyether and trimethylhexadecyl ammonium bromide are 0.4 to 0.15 respectively, and the rest are the same.

[0053] The fracturing fluid has a viscosity of 64.7 mPa·s and a sand-carrying ratio of 35%. After aging at 90°C for 18 days, the fracturing fluid breaks the gel, and the viscosity after breaking the gel is 9.2 mPa·s. After aging at 120°C for 12 days, the fracturing fluid breaks the gel, and the viscosity after breaking the gel is 10.9 mPa·s. The anti-swelling rate for shale is 66.4%.

[0054] Comparative Example 1

[0055] The difference from Example 1 is that no amino acid is added during the preparation of the polyether, and the rest is the same.

[0056] The fracturing fluid has a viscosity of 41.7 mPa·s and a sand-carrying ratio of 26%. After aging at 90°C for 17 days, the fracturing fluid breaks the gel, and the viscosity after breaking the gel is 6.3 mPa·s. After aging at 120°C for 10 days, the fracturing fluid breaks the gel, and the viscosity after breaking the gel is 6.9 mPa·s. The anti-swelling rate for shale is 58.4%.

[0057] Comparative Example 2

[0058] The difference from Example 1 is that, in the preparation process of the polyether, chitosan oligosaccharide is replaced by chitosan (with a deacetylation degree of 95%), and the rest are the same.

[0059] The fracturing fluid has a viscosity of 245.8 mPa·s and a sand carrying ratio of 55%. It does not break after aging at 90°C for 30 days, and does not break after aging at 120°C for 30 days. The anti-swelling rate for shale is 65.5%.

[0060] Comparative Example 3

[0061] The difference from Example 1 is that in the preparation process of the polyether, 200 g of propylene oxide is added first, then 120 g of ethylene oxide is added, then 300 g of propylene oxide is added, then 130 g of ethylene oxide is added, and then 200 g of propylene oxide is added, and the rest are the same.

[0062] The fracturing fluid has a viscosity of 34.2 mPa·s and a sand-carrying ratio of 21%. After aging at 90°C for 15 days, the fracturing fluid breaks the gel, and the viscosity after breaking the gel is 4.3 mPa·s. After aging at 120°C for 10 days, the fracturing fluid breaks the gel, and the viscosity after breaking the gel is 5.8 mPa·s. The anti-swelling rate for shale is 61.7%.

[0063] Comparative Example 4

[0064] The difference from Example 1 is that during the preparation of the polyether, 40 g of glycine is added as the amino acid, and the rest are the same.

[0065] The fracturing fluid has a viscosity of 64.3 mPa·s and a sand-carrying ratio of 31%. After aging at 90°C for 17 days, the fracturing fluid breaks the gel, and the viscosity after breaking the gel is 5.3 mPa·s. After aging at 120°C for 11 days, the fracturing fluid breaks the gel, and the viscosity after breaking the gel is 7.9 mPa·s. The anti-swelling rate for shale is 58.7%.

[0066] In summary, the fracturing fluid of the embodiment of the present invention can be used in an environment of 80-120° C., and can automatically break the gel, and the viscosity is low after breaking the gel.

[0067] The above is only a preferred specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by a person skilled in the art within the technical scope disclosed in the embodiments of the present invention should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention should be based on the protection scope of the claims.

Claims

1. A low molecular weight polymer fracturing fluid, characterized in that: The invention comprises 0.5-1.2% thickener and 1-2.5% small molecule alcohol in mass percentage, wherein the thickener comprises polyether and cationic surfactant in a mass ratio of 1:0.3-0.5; the polyether uses chitosan oligosaccharide as the initiator, ethylene oxide and propylene oxide as the main agents, and amino acids as the end-capping agents for end-capping, and the mass ratio of the initiator, ethylene oxide, propylene oxide and amino acids is 10:50-80:20-30:1-10.

2. The fracturing fluid according to claim 1, characterized in that: The small molecule alcohol is one of methanol and ethanol.

3. The fracturing fluid according to claim 1, characterized in that: The cationic surfactant is a quaternary ammonium salt surfactant.

4. The fracturing fluid according to claim 1, characterized in that: The amino acid is at least one of arginine, cystine, cysteine, histidine, lysine, methionine, serine and threonine.

5. The fracturing fluid according to claim 1, characterized in that: The preparation method of the polyether comprises the following steps: adding an initiator catalyst into a reaction kettle and adding water to dissolve it, introducing an inert gas to remove oxygen, adding one-quarter to one-third of ethylene oxide and heating to 100-150° C. to react until the system pressure does not drop for 20 minutes; then adding one-third to two-thirds of propylene oxide and continuing to react until the system pressure does not drop for 20 minutes; then adding one-third to one-half of ethylene oxide and continuing to react until the system pressure does not drop for 20 minutes; then adding the remaining propylene oxide and continuing to react until the system pressure does not drop for 20 minutes; then adding the remaining ethylene oxide and reacting until the system pressure does not drop for 20 minutes; then adjusting the temperature to 50-80° C., adding a capping agent and reacting under reduced pressure for 6-10 hours to obtain the polyether.

6. The fracturing fluid according to claim 1, characterized in that: The preparation method of the polyether comprises the following steps: adding an initiator and a catalyst into a reaction kettle and adding water to dissolve them, introducing an inert gas to remove oxygen, adding one-third to two-thirds of propylene oxide and heating to 100-150° C. to react until the system pressure does not drop for 20 minutes, then adding the remaining propylene oxide and one-third to two-thirds of ethylene oxide to react until the system pressure does not drop for 20 minutes, then adding the remaining ethylene oxide to react until the system pressure does not drop for 20 minutes, then adjusting the temperature to 50-80° C., adding a capping agent and reacting for 6-10 hours under reduced pressure to obtain the polyether.

7. The fracturing fluid according to claim 5 or 6, characterized in that: The catalyst is an organic base or an inorganic base, and the amount of the catalyst added is 0.5-1.2% of the total mass of the final product.

8. The fracturing fluid according to claim 1, characterized in that: Clay stabilizers are also included.