Fracturing fluid system for compact limestone as well as preparation method and application of fracturing fluid system

Abstract

The invention provides a fracturing fluid system for compact limestone as well as a preparation method and application of the fracturing fluid system. The fracturing fluid system comprises a high-viscosity gel fracturing fluid with the apparent viscosity of 500-1000 mPa.s, a low-viscosity slick water fracturing fluid with the apparent viscosity of 9-15 mPa.s and a medium-viscosity linear gel fracturing fluid with the apparent viscosity of 90-150 mPa.s; the polymer thickening agent is formed by inverse emulsion polymerization of polymer monomers, and the polymer monomers comprise acrylamide, dimethyl diallyl ammonium chloride, sodium p-styrenesulfonate and N-acryloylmorpholine. By adjusting the component content of the fracturing fluid system, fracturing fluids with different viscosities can be formed, and on-line free conversion among the fracturing fluids with different viscosities can be realized, so that the system barrier can be broken, on-line continuous mixing and seamless viscosity conversion can be realized, the construction effect of alternate fracturing can be achieved, the fluid does not need to be prepared in advance, sand is easy to add, and the construction pressure is moderate.

Description

Technical Field

[0001] The present invention relates to the technical field of oil and gas field development, and in particular to a fracturing fluid system for dense limestone, a preparation method and application thereof. Background Art

[0002] At present, the transformation of low-permeability sandstone oil and gas reservoirs at home and abroad mainly adopts the method of volume fracturing. Through fracturing, the reservoir matrix is ​​"broken" to form a network of cracks, so that the contact area between the crack wall and the reservoir matrix is ​​maximized, the seepage distance of oil and gas in the matrix to the crack is the shortest, and the seepage resistance of the matrix fluid to the crack is minimized, which greatly improves the overall seepage capacity of the reservoir, thereby realizing the "three-dimensional transformation" of the reservoir in the three-dimensional directions of length, width and height.

[0003] Low-viscosity liquids can be used as fracturing fluids to enter natural fractures and bedding in formations, forming network fractures. Low-viscosity slickwater fracturing is an effective means of achieving network fracturing, but the sand-carrying effect of low-viscosity slickwater is poor and cannot meet the requirements of high sand volume; although linear gel fracturing fluids have good sand-carrying and fracture-forming capabilities, their temperature resistance is insufficient; the cross-linking effect of cross-linked gel fracturing fluids is difficult to control, and there are many uncontrollable influencing factors, and there are system barriers among the three types of fracturing fluids. Summary of the invention

[0004] The main purpose of the present invention is to provide a fracturing fluid system for dense limestone, a preparation method and application thereof, so as to solve the problems in the prior art of requiring advance preparation of fluid, difficulty in adding sand and high construction pressure during the fracturing of dense limestone.

[0005] In order to achieve the above-mentioned object, according to one aspect of the present invention, a fracturing fluid system for dense limestone is provided, comprising a high-viscosity jelly fracturing fluid, a low-viscosity slickwater fracturing fluid and a medium-viscosity linear gel fracturing fluid; in terms of weight percentage, the high-viscosity jelly fracturing fluid comprises 0.5-1wt% of a polymer thickener, 0.1-0.3wt% of a cross-linking agent, the balance being water, and an apparent viscosity of 500-1000 mPa·s; the low-viscosity slickwater fracturing fluid comprises 0.01-0.1wt% of a polymer thickener, the balance being water, and an apparent viscosity of 9-15 mPa·s; the medium-viscosity linear gel fracturing fluid comprises 0.5-1wt% of a polymer thickener, the balance being water, and an apparent viscosity of 90-150 mPa·s; wherein the polymer thickener is formed by polymer monomers through inverse emulsion polymerization, and the polymer monomers comprise acrylamide, dimethyldiallylammonium chloride, sodium p-styrenesulfonate and N-acryloylmorpholine.

[0006] Furthermore, the polymer monomers include 20 to 25 parts of acrylamide, 1 to 8 parts of dimethyldiallylammonium chloride, 1 to 5 parts of sodium p-styrenesulfonate, and 1 to 10 parts of N-acryloylmorpholine, by weight; and / or the cross-linking agent includes one or more of a zirconium citrate cross-linking agent, a zirconium lactate cross-linking agent, and a zirconium acetate cross-linking agent.

[0007] Furthermore, the polymer thickener has a structure shown in general formula (I): Among them, * is the chemical bond connection position, x is 11000-28000, y is 200-500, z is 2000-4000, and w is 100-400.

[0008] According to another aspect of the present invention, a method for preparing the above-mentioned fracturing fluid system for dense limestone of the present invention is provided, comprising the following steps: step S1, subjecting polymer monomers to inverse emulsion polymerization to obtain a polymer thickener; the polymer monomers include acrylamide, dimethyldiallyl ammonium chloride, sodium p-styrene sulfonate and N-acryloylmorpholine; step S2, mixing the polymer thickener with an optional cross-linking agent and water, respectively, to obtain a high-viscosity jelly fracturing fluid, a low-viscosity slippery water fracturing fluid and a medium-viscosity linear gel fracturing fluid, thereby obtaining a fracturing fluid system.

[0009] Further, step S1 includes the following steps: step S11, mixing the solvent and the emulsifier, and stirring and heating to obtain a first solution; step S12, under an inert gas atmosphere, adding a polymer monomer to the first solution, and stirring and emulsifying to obtain a second solution; step S13, adding an initiator to the second solution, and stirring and initiating to obtain a third solution; step S14, under an air atmosphere, adding a phase transfer agent to the third solution, and performing phase inversion to obtain a polymer thickener.

[0010] Further, in step S11, the solvent includes white oil and water, preferably the weight ratio of the two is 1:(1.2-1.7); more preferably, the weight ratio of the solvent to the polymer monomer is (58-62):(23-48); and / or the emulsifier includes Span-80 and Tween-40, preferably the weight ratio of the two is (4-6):1; more preferably, the weight ratio of the emulsifier to the polymer monomer is (7-7.5):(23-48); and / or the stirring temperature is 20-30°C, the stirring rate is 1000-1500r / min, and the stirring time is 30-40min.

[0011] Furthermore, in step S12, the inert gas includes one or more of nitrogen, carbon dioxide and helium; and / or the stirring emulsification temperature is 20-30° C., the stirring rate is 1000-1500 r / min, and the stirring time is 30-40 min.

[0012] Further, in step S13, the initiator is a redox initiator; preferably, the redox initiator includes one or more of potassium persulfate-sodium bisulfite, ammonium persulfate-sodium bisulfite, potassium persulfate-ascorbic acid and ammonium persulfate-ascorbic acid; more preferably, the weight ratio of the initiator to the polymer monomer is (0.02-0.03):(23-48); preferably, after adding the initiator, the pH value of the feed solution is adjusted to 6.5-7.5, and then stirring initiation is carried out; preferably, the stirring initiation temperature is 38-45°C, the stirring rate is 100-150r / min, and the stirring time is 3.5-4.5h.

[0013] Further, in step S14, the phase transfer agent includes one or more of OP-10, NP-10 and polyoxyethylene ether; preferably, the weight ratio of the phase transfer agent to the polymer monomer is (0.3-0.4):(23-48).

[0014] According to another aspect of the present invention, a method for fracturing production of a dense limestone reservoir is provided, which is carried out using the above-mentioned fracturing fluid system for dense limestone of the present invention, firstly using a high-viscosity jelly fracturing fluid to press open at least one main fracture in the dense limestone reservoir, then using a low-viscosity slickwater fracturing fluid to complicate the main fracture network, and finally using a medium-viscosity linear gel fracturing fluid to carry a proppant to support the fracture network for production.

[0015] By applying the technical solution of the present invention, by adjusting the content of the components of the fracturing fluid system of the present invention, fracturing fluids of different viscosities can be formed, and online free conversion between fracturing fluids of different viscosities can be achieved, thereby breaking the system barriers, achieving online continuous mixing and seamless viscosity conversion, and achieving the construction effect of alternating fracturing, without the need for advance fluid preparation, easy sand addition, and moderate construction pressure. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The drawings constituting a part of the present application are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the drawings:

[0017] Figure 1 The reaction mechanism diagram of the preparation process of the polymer thickener according to Example 1 of the present invention is shown;

[0018] Figure 2 Shows a photograph of the appearance of the polymer thickener according to Example 2 of the present invention;

[0019] Figure 3 The drag reduction performance results of the low-viscosity slickwater fracturing fluid according to Example 3 of the present invention are shown;

[0020] Figure 4 The results of temperature and shear resistance of the medium-viscosity linear adhesive fracturing fluid according to Example 4 of the present invention are shown;

[0021] Figure 5 A photograph showing the degree of cross-linking of the high-viscosity jelly fracturing fluid according to Example 5 of the present invention is shown;

[0022] Figure 6 States of the low-viscosity 3-3 low-viscosity slippery water fracturing fluid according to Example 3 of the present invention, the medium-viscosity linear gel fracturing fluid (carrying sand) according to Example 4, and the high-viscosity jelly gel fracturing fluid according to Example 5 are shown. DETAILED DESCRIPTION

[0023] It should be noted that, in the absence of conflict, the embodiments and features in the embodiments of the present application can be combined with each other. The present invention will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0024] As described in the background of the present invention, the prior art has the technical problems of requiring advance preparation of fluid during the fracturing of dense limestone, difficulty in adding sand, and high construction pressure. In order to solve the above problems, in a typical embodiment of the present invention, a fracturing fluid system for dense limestone is provided, including high-viscosity jelly fracturing fluid, low-viscosity slippery water fracturing fluid and medium-viscosity linear gel fracturing fluid; in terms of weight percentage, the high-viscosity jelly fracturing fluid includes 0.5-1wt% of a polymer thickener, 0.1-0.3wt% of a cross-linking agent, and the balance is water, and the apparent viscosity is 500-1000mPa·s; the low-viscosity slippery water fracturing fluid includes 0.01-0.1wt% of a polymer thickener, and the balance is water, and the apparent viscosity is 9-15mPa·s; the medium-viscosity linear gel fracturing fluid includes 0.5-1wt% of a polymer thickener, and the balance is water, and the apparent viscosity is 90-150mPa·s; wherein the polymer thickener is formed by polymer monomers through inverse emulsion polymerization, and the polymer monomers include acrylamide, dimethyldiallyl ammonium chloride, sodium p-styrene sulfonate and N-acryloylmorpholine.

[0025] The present invention provides a variable viscosity fracturing fluid system suitable for various types of reservoirs, especially tight limestone reservoirs, based on the actual situation of the construction site, including a polymer thickener obtained by inverse emulsion polymerization and an optional cross-linking agent component. The polymer thickener formed by inverse emulsion polymerization of acrylamide, dimethyldiallylammonium chloride, sodium p-styrenesulfonate and N-acryloylmorpholine has the characteristics of high molecular weight, fast dissolution rate, and viscosity sensitive to changes in the mass percentage of the thickener, and is therefore more suitable for the variable viscosity fracturing fluid system of the present invention. Among them, the cross-linking agent can use conventional types in the field, and the weight percentage of the polymer thickener and the cross-linking agent can be adjusted according to the needs of the construction site, so that the viscosity of the fracturing fluid changes seamlessly within the range of 9 to 1000 mPa·s.

[0026] During the construction process, specifically, three different viscosity systems can be formed by changing the amount of polymer thickener and cross-linker used, wherein the high-viscosity gel fracturing fluid includes 0.5-1wt% polymer thickener, 0.1-0.3wt% cross-linker, and the balance is water, with an apparent viscosity of 500-1000mPa·s. A large amount of polymer thickener and a large amount of cross-linker form a gel system, which is responsive to temperature and does not cross-link at low temperatures. As the temperature rises, the acrylamide structure in the polymer thickener coordinates and cross-links with the cross-linker. As the temperature continues to increase, the N-acryloylmorpholine ring structure opens and cross-links with the cross-linker, thereby achieving a "secondary" cross-linking effect. The higher the temperature, the better the cross-linking effect. Temperature-controlled cross-linking helps reduce construction friction. After injection into the formation, it is conducive to the formation of main fractures, expanding the fracture width and fracture height, and facilitating the formation of secondary fractures in the subsequent low-viscosity stage to form a fracture network.

[0027] Among them, the low-viscosity slickwater fracturing fluid includes 0.01-0.1wt% polymer thickener, the balance is water, and the apparent viscosity is 8-15mPa·s. A small amount of polymer thickener can be quickly dissolved in water, and the polymer molecules make the water molecules arranged in order through hydrogen bonding, thereby inhibiting the generation of turbulence. The fluid has excellent drag reduction performance, which helps to achieve the complexity of the fracture network.

[0028] Among them, the medium viscosity linear gel fracturing fluid includes 0.5-1wt% polymer thickener, the balance is water, and the apparent viscosity is 90-150mPa·s. By increasing the amount of polymer thickener, the polymer molecules are entangled with each other to form a three-dimensional network space structure, and the liquid viscoelasticity is enhanced, so that the fracturing fluid has excellent temperature and shear resistance. It can be used as a sand-carrying fluid to carry proppants and support the main and secondary cracks opened by high-viscosity fracturing fluid and low-viscosity fracturing fluid.

[0029] That is, by adjusting the content of the components of the fracturing fluid system of the present invention, fracturing fluids of different viscosities can be formed, and online free conversion of fracturing fluids of different viscosities can be achieved, thereby breaking the system barriers, achieving online continuous mixing and seamless viscosity conversion, and achieving the construction effect of alternating fracturing, so that there is no need to prepare the fluid in advance, sand can be added easily, and the construction pressure is moderate.

[0030] In order to further improve the performance of the polymer thickener in the fracturing fluid system, so as to more conveniently achieve viscosity control and conversion of fracturing fluids of different viscosities, in a preferred embodiment, the polymer monomers include 20 to 25 parts of acrylamide, 1 to 8 parts of dimethyldiallyl ammonium chloride, 1 to 5 parts of sodium p-styrene sulfonate, and 1 to 10 parts of N-acryloylmorpholine, and the polymer thickener obtained within the above content range can be more suitable for fracturing production of tight limestone reservoirs.

[0031] To further facilitate the viscosity adjustment of the polymer thickener, the crosslinking agent includes one or more of a zirconium citrate crosslinking agent, a zirconium lactate crosslinking agent and a zirconium acetate crosslinking agent, which are more suitable for the crosslinking of the above-mentioned polymer thickener of the present invention, thereby further improving the crosslinking effect and accurately controlling the viscosity.

[0032] In a preferred embodiment, the polymer thickener has a structure shown in general formula (I):

[0033]

[0034] Among them, * is the chemical bond connection position, the acrylamide polymer segment x is 11000-28000, the N-acryloylmorpholine polymer segment y is 200-500, the dimethyldiallylammonium chloride polymer segment z is 2000-4000, and the sodium p-styrene sulfonate polymer segment w is 100-400. The above polymer thickener can better meet the requirements of the tight limestone reservoir fracturing fluid system.

[0035] In another typical embodiment of the present invention, a preparation method of the above-mentioned fracturing fluid system for dense limestone of the present invention is also provided, comprising the following steps: step S1, inverse emulsion polymerization of polymer monomers to obtain a polymer thickener; the polymer monomers include acrylamide, dimethyldiallyl ammonium chloride, sodium p-styrene sulfonate and N-acryloylmorpholine; step S2, respectively mixing the polymer thickener with an optional cross-linking agent and water to obtain a high-viscosity jelly fracturing fluid, a low-viscosity slippery water fracturing fluid and a medium-viscosity linear gel fracturing fluid, and then obtaining a fracturing fluid system. The above-mentioned preparation method is simple and easy to operate. The prepared fracturing fluid can achieve continuous mixing and seamless viscosity change to achieve the construction effect of alternating fracturing, which is more suitable for the actual construction process.

[0036] In a preferred embodiment, step S1 includes the following steps: step S11, mixing the solvent and the emulsifier, stirring and heating to obtain a first solution; step S12, adding a polymer monomer to the first solution under an inert gas atmosphere, stirring and emulsifying to obtain a second solution; step S13, adding an initiator to the second solution, stirring and initiating to obtain a third solution; step S14, adding a phase transfer agent to the third solution under an air atmosphere, and performing a phase transfer to obtain a polymer thickener.

[0037] The present invention first mixes the solvent and the emulsifier, and stirs and heats to dissolve the emulsifier to obtain a first solution; then, a polymer monomer is added in an inert gas atmosphere, stirred and emulsified, and dissolved oxygen in the mixed solution is removed. After deoxygenation for about 30 minutes, a second solution is obtained, an initiator is added, and stirring and initiation are performed to obtain a third solution; finally, a phase inversion agent is added in an air atmosphere, and phase inversion is performed to obtain a polymer thickener. The above-mentioned inverse emulsion polymerization method can better control the molecular weight of the polymer, realize rapid dissolution of the polymer, and improve the viscosity-increasing performance.

[0038] In a preferred embodiment, in step S11, the solvent includes white oil and water, preferably in a weight ratio of 1:(1.2-2.7), which has better solubility for each material; more preferably, the weight ratio of the solvent to the polymer monomer is (58-62):(23-48), which can ensure that each polymer monomer is fully dissolved while ensuring a certain concentration, which is more conducive to the full progress of the subsequent reaction; and / or the emulsifier includes Span-80 and Tween-40, preferably in a weight ratio of (4-6):1; more preferably, the weight ratio of the emulsifier to the polymer monomer is (7-7.5):(23-48), under the above conditions, the emulsification degree can be further improved, so that each polymer monomer can be more fully emulsified; and / or the stirring and heating temperature is 20-30°C, the stirring rate is 1000-1500r / min, and the stirring time is 30-40min, which has better mixing uniformity of each material.

[0039] For the purpose of further improving the mixing and emulsifying effect of the stirring emulsification, in a preferred embodiment, in step S12, the inert gas includes one or more of nitrogen, carbon dioxide and helium; and / or the stirring emulsification temperature is 20-30°C, the stirring rate is 1000-1500 r / min, and the stirring time is 30-40 min.

[0040] For the purpose of further improving the mixing and initiation reaction effect of stirring initiation, in a preferred embodiment, in step S13, the initiator is a redox initiator; preferably, the redox initiator includes one or more of potassium persulfate-sodium bisulfite, ammonium persulfate-sodium bisulfite, potassium persulfate-ascorbic acid and ammonium persulfate-ascorbic acid; more preferably, the weight ratio of the initiator to the polymer monomer is (0.02-0.03):(23-48), so as to further ensure that the polymerization reaction of each polymer monomer is fully initiated, which is more conducive to the formation of the above-mentioned polymer thickener of the present invention; based on similar reasons, it is further preferred that after adding the initiator, the pH value of the feed solution is adjusted to 6.5-7.5, and then stirring initiation is carried out; preferably, the stirring initiation temperature is 38-45°C, the stirring rate is 100-150r / min, and the stirring time is 3.5-4.5h.

[0041] In a preferred embodiment, in step S14, the phase inversion agent includes one or more of OP-10, NP-10 and polyoxyethylene ether; preferably, the weight ratio of the phase inversion agent to the polymer monomer is (0.3-0.4): (23-48), and the above conditions are more conducive to the synthesis of a polymer thickener for tight limestone fracturing. The diversion agent can also be adjusted according to the optimal dissolution rate and dissolution state of the polymer thickener (no floating oil phase or floccules in the solution).

[0042] In another typical embodiment of the present invention, a fracturing production method for a dense limestone reservoir is provided, which is carried out using the above-mentioned fracturing fluid system for dense limestone of the present invention. First, a high-viscosity gel fracturing fluid is used as a fracture-making fluid to press open at least one main fracture in the dense limestone reservoir; then a low-viscosity slickwater fracturing fluid with excellent drag-reducing performance is used to complicate the main fracture network; finally, a medium-viscosity linear gel fracturing fluid is used as a sand-carrying fluid to carry out the fracturing of the dense limestone reservoir at a well depth of 3000 meters and 3 1 / 2 Oil pipe, construction displacement 4.0~6.0m 3 / min condition, construction sand ratio (sand pile volume / sand carrying liquid volume, m 3 / m 3 ) can be up to 30%, with excellent variable viscosity sand carrying performance, can carry proppant to support the fracture network, various viscosities of fracturing fluid can achieve online continuous mixing and seamless viscosity conversion, to achieve the construction effect of alternating fracturing.

[0043] Typically, but not limiting, the high viscosity jelly fracturing fluid comprises 0.5wt%, 0.6wt%, 0.7wt%, 0.8wt%, 0.9wt%, 1wt% or a range consisting of any two of a polymer thickener, 0.1wt%, 0.15wt%, 0.2wt%, 0.25wt%, 0.3wt% or a range consisting of any two of a cross-linking agent, and the balance is water, with an apparent viscosity of 500mPa·s, 600mPa·s, 700mPa·s, 800mPa·s, 900mPa·s, 1000mPa·s or a range consisting of any two of a polymer thickener.

[0044] Typically, but not limiting, low viscosity slick water fracturing fluid comprises 0.01 wt%, 0.02 wt%, 0.04 wt%, 0.05 wt%, 0.06 wt%, 0.08 wt%, 0.1 wt% or a range consisting of any two of them, a polymer thickener, the balance being water, and an apparent viscosity of 9 mPa·s, 10 mPa·s, 11 mPa·s, 12 mPa·s, 13 mPa·s, 14 mPa·s, 15 mPa·s or a range consisting of any two of them.

[0045] Typically, but not limiting, the medium viscosity linear gel fracturing fluid comprises 0.5wt%, 0.6wt%, 0.7wt%, 0.8wt%, 0.9wt%, 1wt% or a range consisting of any two of them, and the balance is water, and the apparent viscosity is 90mPa·s, 100mPa·s, 110mPa·s, 120mPa·s, 130mPa·s, 140mPa·s, 150mPa·s or a range consisting of any two of them.

[0046] Typically, but not limiting, the polymer monomers include 20, 21, 22, 23, 24, 25 parts of acrylamide or a range consisting of any two of them, 1, 2, 3, 4, 5, 6, 7, 8 parts of dimethyldiallylammonium chloride or a range consisting of any two of them, 1, 2, 3, 4, 5 parts of sodium p-styrene sulfonate or a range consisting of any two of them, and 1, 2, 3, 4, 5 parts of N-acryloylmorpholine or a range consisting of any two of them.

[0047] The present application is further described in detail below in conjunction with specific embodiments. These embodiments should not be construed as limiting the scope of protection claimed in the present application.

[0048] Unless otherwise specified, "parts" in the following examples and comparative examples refer to parts by weight, and "%" refers to percentage by weight.

[0049] Example 1

[0050] S1: In a four-necked flask equipped with a stirrer, a thermometer, and a reflux condenser, 58 parts of a mixed solvent (white oil and deionized water in a mass ratio of 1:1.5) and 7 parts of an emulsifier (Span-80 and Tween-40 in a mass ratio of 6:1) were added, and the four-necked flask containing the mixed solution was placed in a water bath for water bath heating, and mixed and stirred for 35 minutes, wherein the water bath heating temperature was controlled at 30°C and the stirring rate was controlled at 1000 r / min;

[0051] S2: Connect the four-necked flask to the ventilation pipe, and then introduce N2 for 20 minutes to remove the dissolved oxygen in the mixed solution; slowly add 23.68 parts of acrylamide monomer, 8 parts of dimethyldiallylammonium chloride monomer, 2 parts of sodium p-styrenesulfonate monomer, and 1 part of N-acryloylmorpholine monomer through the additive addition port of the four-necked flask, and then continue to maintain the temperature at 30°C, continue stirring for 35 minutes, and control the stirring rate at 1200r / min to fully emulsify;

[0052] S3: After emulsification, continue to slowly add 0.02 parts of initiator (potassium persulfate-sodium bisulfite) through the additive addition port of the four-necked flask, and adjust the pH value of the reaction solution to 7.0; adjust the water bath heating temperature to 40°C, and then continue stirring the reaction for 4 hours, wherein the stirring rate is controlled at 120r / min;

[0053] S4: After the above reaction is completed, stop passing N2 and continue to keep warm for 1 hour to allow all the residual monomers to react to obtain an oil-in-water emulsion. Add 0.3 parts of a phase inversion agent (OP-10) to obtain a polymer thickener.

[0054] S5: Mix the polymer thickener with the cross-linking agent (zirconium citrate cross-linking agent) and water, and stir them thoroughly to obtain a fracturing fluid, which includes: low-viscosity slippery water system: 0.05wt% polymer thickener, the balance is water; medium-viscosity linear gel system: 0.8wt% polymer thickener, the balance is water; high-viscosity jelly gel system: 1.0wt% polymer thickener, 0.3wt% cross-linking agent, the balance is water.

[0055] Example 2

[0056] S1: In a four-necked flask equipped with a stirrer, a thermometer, and a reflux condenser, add 62 parts of a mixed solvent (white oil and deionized water in a mass ratio of 1:1.5) and 7.5 parts of an emulsifier (Span-80 and Tween-40 in a mass ratio of 6:1), and place the four-necked flask containing the mixed solution in a water bath for water bath heating, while mixing and stirring for 35 minutes, wherein the water bath heating temperature is controlled at 30°C and the stirring rate is controlled at 1000 r / min;

[0057] S2: Connect the four-necked flask to the ventilation pipe, and then introduce N2 for 20 minutes to remove the dissolved oxygen in the mixed solution; slowly add 20 parts of acrylamide monomer, 7.08 parts of dimethyldiallylammonium chloride monomer, 2 parts of sodium p-styrene sulfonate monomer, and 1 part of N-acryloylmorpholine monomer through the additive addition port of the four-necked flask, and then continue to maintain the temperature at 30°C, continue stirring for 35 minutes, and control the stirring rate at 1200r / min to fully emulsify;

[0058] S3: After emulsification, continue to slowly add 0.02 parts of initiator (ammonium persulfate-sodium bisulfite) through the additive addition port of the four-necked flask, and adjust the pH value of the reaction solution to 7.0; raise the water bath heating temperature to 40°C, and then continue stirring the reaction for 4 hours, wherein the stirring rate is controlled at 120r / min;

[0059] S4: After the above reaction is completed, stop passing N2 and continue to keep the temperature for 1 hour to allow all the residual monomers to participate in the reaction to obtain an oil-in-water emulsion. Add 0.4 parts of a phase inversion agent (OP-10) to obtain a polymer thickener;

[0060] S5: Mix the polymer thickener with the cross-linking agent (zirconium citrate cross-linking agent) and water, and stir them thoroughly to obtain a fracturing fluid, which includes: low-viscosity slippery water system: 0.05wt% polymer thickener, the balance is water; medium-viscosity linear gel system: 0.8wt% polymer thickener, the balance is water; high-viscosity jelly gel system: 1.0wt% polymer thickener, 0.3wt% cross-linking agent, the balance is water.

[0061] Example 3

[0062] S1: In a four-necked flask equipped with a stirrer, a thermometer, and a reflux condenser, 58 parts of a mixed solvent (white oil and deionized water in a mass ratio of 1:1.5) and 7.2 parts of an emulsifier (Span-80 and Tween-40 in a mass ratio of 6:1) were added, and the four-necked flask containing the mixed solution was placed in a water bath for water bath heating, and mixed and stirred for 35 minutes, wherein the water bath heating temperature was controlled at 30°C and the stirring rate was controlled at 1000 r / min;

[0063] S2: Connect the four-necked flask to the ventilation pipe, and then introduce N2 for 20 minutes to remove the dissolved oxygen in the mixed solution; slowly add 23.42 parts of acrylamide monomer, 4 parts of dimethyldiallylammonium chloride monomer, 5 parts of sodium p-styrenesulfonate monomer, and 5 parts of N-acryloylmorpholine monomer through the additive addition port of the four-necked flask, and then continue to maintain the temperature at 30°C, continue stirring for 35 minutes, and control the stirring rate at 1200r / min to fully emulsify;

[0064] S3: After emulsification, 0.03 parts of initiator (potassium persulfate-ascorbic acid) was slowly added through the additive adding port of the four-necked flask, and the pH value of the reaction solution was adjusted to 7.0; the water bath heating temperature was raised to 45°C, and then the reaction was stirred for 4 hours, wherein the stirring rate was controlled at 120 r / min;

[0065] S4: After the above reaction is completed, stop passing N2 and continue to keep the temperature for 1 hour to allow all the residual monomers to participate in the reaction to obtain an oil-in-water emulsion. Add 0.35 parts of a phase inversion agent (OP-10) to obtain a polymer thickener;

[0066] S5: Mix a polymer thickener with a cross-linking agent (zirconium citrate cross-linking agent) and water, and stir them thoroughly to obtain a fracturing fluid, which includes: a low-viscosity slippery water system: 0.01wt%, 0.05wt%, and 0.1wt% of a polymer thickener are mixed with water respectively, and stir them thoroughly to obtain three groups of low-viscosity slippery water fracturing fluids, low viscosity 3-1, low viscosity 3-2, and low viscosity 3-3; a medium-viscosity linear gel system: 0.8wt% of a polymer thickener, and the balance is water; a high-viscosity gel system: 1.0wt% of a polymer thickener, 0.3wt% of a cross-linking agent, and the balance is water.

[0067] Example 4

[0068] S1: In a four-necked flask equipped with a stirrer, a thermometer, and a reflux condenser, add 60 parts of a mixed solvent (white oil and deionized water in a mass ratio of 1:1.5) and 7.3 parts of an emulsifier (Span-80 and Tween-40 in a mass ratio of 6:1), and place the four-necked flask containing the mixed solution in a water bath for water bath heating, and mix and stir for 35 minutes, wherein the water bath heating temperature is controlled at 30°C and the stirring rate is controlled at 1000 r / min;

[0069] S2: Connect the four-necked flask to the ventilation pipe, and then introduce N2 for 20 minutes to remove the dissolved oxygen in the mixed solution; slowly add 20.135 parts of acrylamide monomer, 1 part of dimethyldiallylammonium chloride monomer, 1 part of sodium p-styrenesulfonate monomer, and 10 parts of N-acryloylmorpholine monomer through the additive addition port of the four-necked flask, and then continue to maintain the temperature at 30°C, continue stirring for 35 minutes, and control the stirring rate at 1200r / min to fully emulsify;

[0070] S3: After emulsification, continue to slowly add 0.025 parts of initiator (potassium persulfate-sodium bisulfite) through the additive addition port of the four-necked flask, and adjust the pH value of the reaction solution to 7.0; raise the water bath heating temperature to 40°C, and then continue stirring the reaction for 4 hours, wherein the stirring rate is controlled at 120r / min;

[0071] S4: After the above reaction is completed, stop passing N2 and continue to keep the temperature for 1 hour to allow all the residual monomers to participate in the reaction to obtain an oil-in-water emulsion. Add 0.36 parts of a phase inversion agent (OP-10) to obtain a polymer thickener;

[0072] S5: Mix the polymer thickener with the cross-linking agent (zirconium citrate cross-linking agent) and water, and stir them thoroughly to obtain a fracturing fluid, which includes: low-viscosity slippery water system: 0.05wt% polymer thickener, the balance is water; medium-viscosity linear gel system: 0.8wt% polymer thickener, the balance is water; high-viscosity jelly gel system: 1.0wt% polymer thickener, 0.3wt% cross-linking agent, the balance is water.

[0073] Example 5

[0074] S1: In a four-necked flask equipped with a stirrer, a thermometer, and a reflux condenser, 58 parts of a mixed solvent (white oil and deionized water in a mass ratio of 1:1.5) and 7 parts of an emulsifier (Span-80 and Tween-40 in a mass ratio of 6:1) were added, and the four-necked flask containing the mixed solution was placed in a water bath for water bath heating, and mixed and stirred for 35 minutes, wherein the water bath heating temperature was controlled at 30°C and the stirring rate was controlled at 1000 r / min;

[0075] S2: Connect the four-necked flask to the ventilation pipe, and then introduce N2 for 20 minutes to remove the dissolved oxygen in the mixed solution; slowly add 25 parts of acrylamide monomer, 5 parts of dimethyldiallylammonium chloride monomer, 1.68 parts of sodium p-styrene sulfonate monomer, and 3 parts of N-acryloylmorpholine monomer through the additive addition port of the four-necked flask, and then continue to maintain the temperature at 30°C, continue stirring for 35 minutes, and control the stirring rate at 1200r / min to fully emulsify;

[0076] S3: After emulsification, continue to slowly add 0.02 parts of initiator (potassium persulfate-sodium bisulfite) through the additive addition port of the four-necked flask, and adjust the pH value of the reaction solution to 7.0; adjust the water bath heating temperature to 40°C, and then continue stirring the reaction for 4 hours, wherein the stirring rate is controlled at 120r / min;

[0077] S4: After the above reaction is completed, stop passing N2 and continue to keep the temperature for 1 hour to allow all the residual monomers to participate in the reaction to obtain an oil-in-water emulsion. Add 0.3 parts of a phase inversion agent (OP-10) to obtain a polymer thickener;

[0078] S5: Mix the polymer thickener with the cross-linking agent (zirconium citrate cross-linking agent) and water, and stir them thoroughly to obtain a fracturing fluid, which includes: low-viscosity slippery water system: 0.05wt% polymer thickener, the balance is water; medium-viscosity linear gel system: 0.8wt% polymer thickener, the balance is water; high-viscosity jelly gel system: 1.0wt% polymer thickener, 0.3wt% cross-linking agent, the balance is water.

[0079] Example 6

[0080] The difference from Example 1 is that in S5, the polymer thickener is mixed with a cross-linking agent (zirconium citrate cross-linking agent) and water, and the fracturing fluid is obtained after sufficient stirring, which includes: low-viscosity slippery water system: 0.01wt% polymer thickener, the balance is water; medium-viscosity linear gel system: the fracturing fluid includes 0.5wt% polymer thickener, the balance is water; high-viscosity jelly gel system: the fracturing fluid includes 0.5wt% polymer thickener, 0.3wt% cross-linking agent, and the balance is water.

[0081] Example 7

[0082] The difference from Example 1 is that in S5, the polymer thickener is mixed with a cross-linking agent (zirconium citrate cross-linking agent) and water, and the fracturing fluid is obtained after sufficient stirring, which includes: low viscosity slippery water system: 0.1wt% polymer thickener, the balance is water; medium viscosity linear gel system: the fracturing fluid includes 1wt% polymer thickener, the balance is water; high viscosity jelly gel system: the fracturing fluid includes 1wt% polymer thickener, 0.1wt% cross-linking agent, and the balance is water.

[0083] Example 8

[0084] The difference from Example 1 is that in S1, 58 parts of a mixed solvent (white oil and deionized water in a mass ratio of 1:1.2) and 7 parts of an emulsifier (Span-80 and Tween-40 in a mass ratio of 4:1) are added to a four-necked flask equipped with a stirrer, a thermometer, and a reflux condenser, and the four-necked flask containing the mixed solution is placed in a water bath for water bath heating, and mixed and stirred for 30 minutes, wherein the water bath heating temperature is controlled at 20°C and the stirring rate is controlled at 1000r / min.

[0085] Example 9

[0086] The difference from Example 1 is that in S1, 58 parts of a mixed solvent (white oil and deionized water in a mass ratio of 1:1.7) and 7 parts of an emulsifier (Span-80 and Tween-40 in a mass ratio of 6:1) were added to a four-necked flask equipped with a stirrer, a thermometer, and a reflux condenser, and the four-necked flask containing the mixed solution was placed in a water bath for water bath heating, and mixed and stirred for 40 minutes, wherein the water bath heating temperature was controlled at 30°C and the stirring rate was controlled at 1500 r / min.

[0087] Example 10

[0088] The difference from Example 1 is that in S2, after the monomer is added, the temperature is maintained at 20° C., stirring is continued for 40 minutes, and the stirring rate is controlled at 1000 r / min to achieve sufficient emulsification.

[0089] Embodiment 11

[0090] The difference from Example 1 is that in S2, after the monomer is added, the temperature is maintained at 30° C., stirring is continued for 30 min, and the stirring rate is controlled at 1500 r / min for sufficient emulsification.

[0091] Example 12

[0092] The difference from Example 1 is that in S3, the water bath heating temperature is raised to 45°C, and then the reaction is continuously stirred for 4.5 hours, wherein the stirring rate is controlled at 100 r / min.

[0093] Example 13

[0094] The difference from Example 1 is that in S3, the water bath heating temperature is raised to 38°C, and then the reaction is stirred continuously for 3.5 hours, wherein the stirring rate is controlled at 150 r / min.

[0095] Comparative Example 1

[0096] The difference from Example 1 is that no acrylamide monomer is used.

[0097] Comparative Example 2

[0098] The difference from Example 1 is that no dimethyldiallylammonium chloride monomer is used.

[0099] Comparative Example 3

[0100] The difference from Example 1 is that no sodium p-styrene sulfonate monomer is used.

[0101] Comparative Example 4

[0102] The difference from Example 1 is that no N-acryloylmorpholine monomer is used.

[0103] The apparent viscosity of the fracturing fluids of the above examples and comparative examples is shown in Table 1. The reaction mechanism diagram of the polymer thickener preparation process of Example 1 is shown in Figure 1 The appearance photo of the polymer thickener of Example 2 is shown in Figure 2 .

[0104] The drag reduction performance of the low-viscosity slickwater fracturing fluids prepared in the above examples and comparative examples was tested, and the drag reduction rate results are shown in Table 1: The test was carried out using a pipeline friction tester, and the pipeline flow rate was 6m / s. Example 3 The drag reduction performance results of the low-viscosity slickwater fracturing fluids with different polymer thickener concentrations are shown in Table 1. Figure 3 As shown, it can be seen that the relative drag reduction rate of the low-viscosity slick water fracturing fluid of the present invention meets the requirement of greater than 70% in the general technical standard for fracturing fluids. When the polymer thickener content is 0.1wt%, the drag reduction rate can reach up to 80.21%, which has excellent drag reduction performance.

[0105] The medium viscosity linear gel fracturing fluid prepared in the above examples and comparative examples was subjected to temperature and shear resistance tests. The endpoint viscosity results are shown in Table 1. The test was conducted using a high temperature rheometer. 50 mL of the fracturing fluid was poured into a rotating cylinder test system. -1 The shear rate was 120°C and the heat and shear resistance test was carried out. The heat and shear resistance results of the viscose fracturing fluid in Example 4 are as follows: Figure 4 As shown, it can be seen that at the end of the test, the endpoint viscosity of the medium-viscosity linear gel fracturing fluid of the present invention can still be maintained at 80.44 mPa·s, which is much greater than the requirement of greater than 50 mPa·s in the general technical standards for fracturing fluids, showing excellent temperature and shear resistance.

[0106] The cross-linking degree of the high-viscosity jelly fracturing fluid prepared in the above examples and comparative examples was tested, and the cross-linking effect is shown in Table 1: The high-viscosity jelly fracturing fluid was hung at 30°C, 90°C and 120°C to observe its cross-linking degree. Figure 5 As shown, it can be seen that with the increase of temperature, the cross-linking effect of the high-viscosity jelly fracturing fluid of the present invention becomes better and better, and temperature-controlled cross-linking can be achieved.

[0107] The state photos of the low viscosity 3-3 low viscosity slippery water fracturing fluid of Example 3, the medium viscosity linear gel fracturing fluid (carrying sand) of Example 4 and the high viscosity gel fracturing fluid of Example 5 are as follows: Figure 6 As shown, a is the low viscosity 3-3 of Example 3, b is Example 4, and c is Example 5.

[0108] Table 1

[0109]

[0110]

[0111] It can be seen from the above that the inverse emulsion thickener of the present invention has good solubility and dissolution viscosity, and each monomer has a good synergistic effect. Different monomers play their respective functions in the system. Example 4 has the best heat resistance, shear resistance and cross-linking performance.

[0112] It can be seen from the above that, compared with the comparative example, the embodiments of the present invention can form fracturing fluids of different viscosities by adjusting the content of the components of the fracturing fluid system, and realize online free conversion between fracturing fluids of different viscosities, thereby breaking the system barriers, realizing online continuous mixing and seamless viscosity conversion, and achieving the construction effect of alternating fracturing, without the need for advance fluid preparation, easy sand addition, and moderate construction pressure.

[0113] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A fracturing fluid system for dense limestone, characterized in that: It includes high-viscosity jelly fracturing fluid, low-viscosity slippery water fracturing fluid and medium-viscosity linear gel fracturing fluid; in terms of weight percentage, the high-viscosity jelly fracturing fluid includes 0.5-1wt% of a polymer thickener, 0.1-0.3wt% of a cross-linking agent, and the balance is water, and the apparent viscosity is 500-1000mPa·s; the low-viscosity slippery water fracturing fluid includes 0.01-0.1wt% of the polymer thickener, and the balance is water, and the apparent viscosity is 9-15mPa·s; the medium-viscosity linear gel fracturing fluid includes 0.5-1wt% of the polymer thickener, and the balance is water, and the apparent viscosity is 90-150mPa·s; The polymer thickener is formed by polymer monomers through inverse emulsion polymerization, and the polymer monomers include acrylamide, dimethyldiallylammonium chloride, sodium p-styrene sulfonate and N-acryloylmorpholine.

2. The fracturing fluid system according to claim 1, characterized in that: In parts by weight, the polymer monomer includes 20 to 25 parts of acrylamide, 1 to 8 parts of dimethyldiallylammonium chloride, 1 to 5 parts of sodium p-styrenesulfonate, and 1 to 10 parts of N-acryloylmorpholine; and / or the cross-linking agent includes one or more of a zirconium citrate cross-linking agent, a zirconium lactate cross-linking agent, and a zirconium acetate cross-linking agent.

3. The fracturing fluid system according to claim 1 or 2, characterized in that: The polymer thickener has a structure shown in general formula (I): Among them, * is the chemical bond connection position, x is 11000-28000, y is 200-500, z is 2000-4000, and w is 100-400.

4. The method for preparing a fracturing fluid system for dense limestone according to any one of claims 1 to 3, characterized in that: The following steps are involved: Step S1, performing inverse emulsion polymerization on polymer monomers to obtain a polymer thickener; the polymer monomers include acrylamide, dimethyldiallylammonium chloride, sodium p-styrenesulfonate and N-acryloylmorpholine; Step S2, respectively mixing the polymer thickener with an optional cross-linking agent and water to obtain a high-viscosity jelly fracturing fluid, a low-viscosity slickwater fracturing fluid and a medium-viscosity linear gel fracturing fluid, and further obtaining the fracturing fluid system.

5. The preparation method according to claim 4, characterized in that: The step S1 comprises the following steps: Step S11, mixing the solvent and the emulsifier, stirring and heating, to obtain a first solution; Step S12, adding the polymer monomer to the first solution under an inert gas atmosphere, and stirring and emulsifying to obtain a second solution; Step S13, adding an initiator to the second solution, and stirring and initiating the mixture to obtain a third solution; Step S14, adding a phase transition agent to the third solution under air atmosphere, and performing phase transition to obtain the polymer thickener.

6. The preparation method according to claim 5, characterized in that: In the step S11, The solvent comprises white oil and water, preferably the weight ratio of the two is 1:(1.2-1.7); more preferably, the weight ratio of the solvent to the polymer monomer is (58-62):(23-48); and / or The emulsifier includes Span-80 and Tween-40, preferably the weight ratio of the two is (4-6):1; more preferably, the weight ratio of the emulsifier to the polymer monomer is (7-7.5):(23-48); and / or The stirring temperature is 20-30° C., the stirring rate is 1000-1500 r / min, and the stirring time is 30-40 min.

7. The preparation method according to claim 5 or 6, characterized in that: In the step S12, The inert gas comprises one or more of nitrogen, carbon dioxide and helium; and / or The stirring emulsification temperature is 20-30° C., the stirring rate is 1000-1500 r / min, and the stirring time is 30-40 min.

8. The preparation method according to any one of claims 5 to 7, characterized in that: In step S13, The initiator is a redox initiator; preferably, the redox initiator includes one or more of potassium persulfate-sodium bisulfite, ammonium persulfate-sodium bisulfite, potassium persulfate-ascorbic acid and ammonium persulfate-ascorbic acid; more preferably, the weight ratio of the initiator to the polymer monomer is (0.02-0.03):(23-48); Preferably, after adding the initiator, the pH value of the liquid is adjusted to 6.5-7.5, and then the stirring initiation is performed; Preferably, the stirring initiation temperature is 38-45° C., the stirring rate is 100-150 r / min, and the stirring time is 3.5-4.5 h.

9. The preparation method according to any one of claims 5 to 8, characterized in that: In step S14, The phase inversion agent includes one or more of OP-10, NP-10 and polyoxyethylene ether; Preferably, the weight ratio of the phase inversion agent to the polymer monomer is (0.3-0.4):(23-48).

10. A fracturing production method for a dense limestone reservoir, characterized in that: The fracturing fluid system for dense limestone described in any one of claims 1 to 3 is used, firstly, a high-viscosity gel fracturing fluid is used to press open at least one main fracture in the dense limestone reservoir, then a low-viscosity slickwater fracturing fluid is used to complicate the main fracture network, and finally a medium-viscosity linear gel fracturing fluid is used to carry a proppant to support the fracture network for production.

Images