A method for preparing an anti-calcium thickening agent, a brine-based drilling fluid and a method for preparing the same

Abstract

The present application relates to the technical field of oil drilling fluid, and particularly relates to a preparation method of calcium-resistant thickening agent, a brine-based drilling fluid and a preparation method; the preparation method of the calcium-resistant thickening agent comprises the following steps: A, uniformly mixing white oil and an emulsifier to form an oil phase, and reserving; B, dissolving 2-acrylamido-2-methyl-1-propanesulfonic acid, acrylonitrile, a cationic monomer and a carboxyl-containing vinyl monomer in pure water, adjusting the pH value to 7.0, and uniformly stirring to form an aqueous phase; C, dropwise adding the aqueous phase in step B into the oil phase in step A, and uniformly mixing to obtain a stable reverse-phase emulsion; D, in a nitrogen atmosphere, adding an initiator into the stable reverse-phase emulsion in step C, and obtaining the calcium-resistant thickening agent after reaction; the brine-based drilling fluid can maintain good rheological properties and stability in a high-temperature environment of 180 DEG C.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of oil drilling fluid, and particularly relates to a preparation method of calcium-resistant viscosity increasing agent, a brine-based drilling fluid and a preparation method. BACKGROUND

[0002] Drilling fluids play a key role in oil and gas exploration and development, and are widely used for cooling drill bits, removing cuttings, stabilizing well walls, controlling well pressure, and the like. According to the type of base liquid, drilling fluids are mainly divided into water-based drilling fluids and oil-based drilling fluids. Among them, water-based drilling fluids, with their lower cost, good environmental friendliness and reliable rheological performance, have become the most widely used type. However, as oil and gas exploration gradually extends to deep wells, ultra-deep wells and complex geological conditions, the performance of traditional water-based drilling fluids in extreme environments such as high temperature, high pressure and high salt has gradually shown limitations, mainly in the dependence on bentonite and the application problem of barite weighting agent.

[0003] Traditional water-based drilling fluids rely on bentonite as a thickening agent and barite (BaSO4) as a weighting agent. The main role of bentonite is to increase the viscosity of the drilling fluid and provide good rheological performance. However, bentonite has poor salt and temperature resistance, especially in high-concentration salt water and high-temperature environments, bentonite easily loses its thickening effect, causing the viscosity and rheological performance of the drilling fluid to drop sharply, thus causing the performance of the drilling fluid to degrade or even fail. On the other hand, barite is widely used as a weighting agent to increase the density of drilling fluids, especially in deep wells and high-pressure formations. However, barite particles tend to settle in high-temperature or high-salt environments, leading to a decrease in the stability of the drilling fluid. Particle settling not only affects the rheological performance of the drilling fluid, but also can cause well wall instability, reduce drilling efficiency and increase operational risks. In addition, barite particles are not dissolved in the drilling fluid and, if not fully dispersed, can cause solid material accumulation, increasing the risk of reservoir contamination and affecting oil and gas production and formation stability. The larger particles of barite can also cause additional frictional wear on the drill bit, drilling tools and well walls, thereby affecting drilling efficiency and equipment life. The above defects make the application of traditional water-based drilling fluids in high-temperature, high-salt and complex environments challenging, and a new solution is urgently needed to overcome these problems.

[0004] Brine-based drilling fluid, as a new type of water-based drilling fluid system, has been increasingly applied in deep wells, ultra-deep wells and complex oil and gas reservoir drilling operations in recent years. Compared with traditional fresh water-based drilling fluids, brine-based drilling fluids use salt solutions such as calcium chloride, sodium chloride and sodium formate as base fluids, directly increasing the density of the drilling fluid by increasing the salt concentration, thereby avoiding the problem of barite particle settling, resulting in a more stable rheological performance of the drilling fluid.

[0005] In addition to the inhibition mechanism of traditional calcium chloride drilling fluid, high-concentration calcium chloride soil-free water-based drilling fluid mainly enhances the inhibition by reducing the activity of the drilling fluid. The non-ideal semi-permeable membrane is formed between the shale well wall surface and the water-based drilling fluid, and some inorganic salts (such as calcium chloride) can significantly reduce the water activity of the drilling fluid and improve the efficiency of the semi-permeable membrane, thereby better stabilizing the well wall. Studies have shown that, compared with monovalent metal salts such as NaCl and KCl, the calcium chloride solution can generate greater osmotic pressure and higher membrane efficiency, and is more conducive to maintaining the stability of the shale well wall.

[0006] However, in the preparation process of the brine-based drilling fluid, how to effectively select and optimize the additives, especially the tackifiers, fluid loss reducers and plugging agents, to improve the performance of the brine-based drilling fluid in high-temperature, high-pressure and high-salinity environments, is still a hot research topic. The existing brine-based drilling fluid system still has some problems, such as limited temperature resistance and calcium resistance of the tackifier and the fluid loss reducer, and poor stability of some additives in high-salt concentration or high-calcium environment. These problems can lead to unstable performance of the drilling fluid, and even affect the safety and efficiency of downhole operations. SUMMARY

[0007] To solve the above technical problems, embodiments of the present application provide a preparation method of a calcium-resistant tackifier, a brine-based drilling fluid and a preparation method.

[0008] To achieve the above-mentioned purposes, embodiments of the present application adopt the following technical solutions:

[0009] In one aspect, the present application provides a preparation method of a calcium-resistant tackifier, comprising the following steps:

[0010] A. Mix white oil and emulsifier uniformly to form an oil phase, for standby use;

[0011] B. Dissolve 2-acrylamido-2-methyl-1-propanesulfonic acid, acrylonitrile, cationic monomer and carboxyl-containing vinyl monomer in pure water, and adjust the pH value to 7.0, and stir uniformly to form an aqueous phase;

[0012] C. Add the aqueous phase in step B dropwise into the oil phase in step A, and mix uniformly to obtain a stable inverse emulsion;

[0013] D. In a nitrogen atmosphere, add an initiator to the stable inverse emulsion in step C, and obtain a calcium-resistant tackifier after reaction.

[0014] In some embodiments, in step A, the volume ratio of the white oil to the mass of the emulsifier is 75 mL:2 g.

[0015] In some embodiments, the emulsifier is composed of sorbitan oleate and polysorbate-80 at a mass ratio of 1:1.

[0016] In some embodiments, in step B, the mass ratio of 2-acrylamido-2-methyl-1-propanesulfonic acid, acrylonitrile, cationic monomer to carboxyl-containing vinyl monomer is (40~50):(10-20):(20~30):(15~25).

[0017] In some embodiments, the cationic monomer includes at least one of dimethyl diallyl ammonium chloride, methacryloyloxyethyl trimethyl ammonium chloride, and (3-acrylamidopropyl)trimethyl ammonium chloride.

[0018] In some embodiments, the carboxyl-containing vinyl monomer includes acrylic acid and / or methacrylic acid.

[0019] In some embodiments, in step D, the initiator includes potassium persulfate and / or sodium bisulfite.

[0020] In some embodiments, in step D, the mass of the initiator is 0.1%-0.3% of the total mass of 2-acrylamido-2-methyl-1-propanesulfonic acid, acrylonitrile, cationic monomer and carboxyl-containing vinyl monomer, and the reaction temperature is 40-55°C.

[0021] On the other hand, the present invention also provides a brine-based drilling fluid, which, by weight, comprises the following raw materials: 400 parts of brine, 8-12 parts of anti-calcium filtration reduction agent, 4-10 parts of anti-calcium thickener, 12-16 parts of plugging agent and 16-20 parts of bridging agent, wherein the anti-calcium thickener is an anti-calcium thickener prepared by the above preparation method;

[0022] The brine comprises calcium chloride and / or calcium bromide; the sealing agent is asphalt powder; and the bridging agent is ultrafine calcium carbonate.

[0023] On the other hand, the present invention also provides a method for preparing brine-based drilling fluid, comprising the following steps:

[0024] Add calcium-resistant filtration reducer, calcium-resistant thickener, plugging agent and bridging agent to the brine and mix well to obtain brine-based drilling fluid.

[0025] Compared with the prior art, the present invention has at least the following beneficial effects:

[0026] 1. The thickener of this invention combines the synergistic effects of multiple monomers, exhibiting excellent resistance to calcium and salt deposits, thickening effect, and high-temperature stability. The quaternary ammonium salt groups in the cationic monomer form zwitterionic polymers with the anionic groups in AMPS and MAA, significantly enhancing calcium and salt resistance through the anti-polyelectrolyte effect, while reducing calcium ion interference through electrostatic shielding and complexation. The rigid structure of acrylonitrile enhances the cohesive force and high-temperature stability of the molecular chain, while the carboxyl groups provided by methacrylic acid further strengthen the complexation with calcium ions. Furthermore, the hydrophobic groups in the quaternary ammonium salt monomer optimize the thickening performance through hydrophobic association, enabling the thickener to exhibit significant viscosity improvement, rheological stability, and durability in deep wells, ultra-deep wells, and complex salt formations. Therefore, the thickener of this invention possesses excellent temperature and calcium resistance properties; when added to drilling fluid systems, it effectively improves the viscosity of drilling fluids under high calcium ion concentration conditions compared to other thickeners.

[0027] 2. The brine-based drilling fluid of this invention maintains good rheological properties and stability at a high temperature of 180℃. By directly increasing the drilling fluid density through the dissolution of salts such as calcium chloride and calcium bromide, the particle settling problem associated with traditional barite weighting methods is avoided. This weighting method not only makes the drilling fluid more fluid and stable but also reduces the risk of reservoir contamination caused by solid particles, further enhancing the system's environmental characteristics. By optimizing the brine concentration and introducing anti-calcium filtration loss reducers and anti-calcium thickeners, the system exhibits excellent salt resistance, remaining stable in environments with high calcium ion concentrations and high salt content.

[0028] In summary, this invention provides a brine-based drilling fluid system and its preparation method, which has significant technological advancements and broad application prospects, providing an efficient, environmentally friendly, and high-performance drilling fluid solution for deep wells and complex downhole operations. Detailed Implementation

[0029] The technical solutions in some embodiments of this disclosure will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of this disclosure, and not all embodiments. Based on the embodiments provided in this disclosure, all other embodiments obtained by those skilled in the art are within the scope of protection of this disclosure.

[0030] 1. Preparation Examples of Anti-Calcium Thickening Agents

[0031] Example 1

[0032] A method for preparing an anti-calcium thickener, comprising the following steps:

[0033] (1) Add 150 mL of No. 3 white oil to a beaker, add 4 g of emulsifier (Span80:Tween80=1:1, that is, the mass ratio of sorbitan oleate to polysorbate-80 is 1:1), stir and disperse at 4000 r / min for 10 minutes, and stir evenly to form an oil phase;

[0034] (2) Dissolve 45g of 2-acrylamido-2-methyl-1-propanesulfonic acid, 12g of acrylonitrile, 25g of methacryloyloxyethyltrimethylammonium chloride and 18g of methacrylic acid in 100mL of pure water, adjust the pH value to 7.0 with 40% sodium hydroxide aqueous solution, and stir evenly to form an aqueous phase;

[0035] (3) Slowly add the aqueous phase from step (2) to the oil phase from step (1) while stirring, and stir at 6000 r / min for 40 minutes to form a stable reverse emulsion;

[0036] (4) Add the emulsion obtained in step (3) into a three-necked flask and continuously purge with nitrogen gas;

[0037] (5) Heat the reaction system obtained in step (4) to 40°C, add 0.1g ammonium persulfate and 0.1g sodium bisulfite initiator, and carry out the reaction;

[0038] (6) After the reaction in step (5) has been complete for 4 hours, the mixture is allowed to cool naturally to room temperature to obtain a white, viscous emulsion-like liquid.

[0039] Example 2

[0040] A method for preparing an anti-calcium thickener, comprising the following steps:

[0041] (1) Add 150mL of No. 3 white oil to a beaker, add 4g of emulsifier (Span80:Tween80=1:1), stir and disperse at 4000r / min for 10 minutes, and stir evenly to form an oil phase;

[0042] (2) Dissolve 45g of 2-acrylamido-2-methyl-1-propanesulfonic acid, 15g of acrylonitrile, 22g of methacryloyloxyethyltrimethylammonium chloride and 18g of acrylic acid in 100mL of pure water, adjust the pH value to 7.0 with 40% sodium hydroxide aqueous solution, and stir evenly to form an aqueous phase;

[0043] (3) Slowly add the aqueous phase from step (2) to the oil phase from step (1) while stirring, and stir at 6000 r / min for 40 minutes to form a stable reverse emulsion;

[0044] (4) Add the emulsion obtained in step (3) into a three-necked flask and continuously purge with nitrogen gas;

[0045] (5) Heat the reaction system obtained in step (4) to 40°C, add 0.05g ammonium persulfate and 0.05g sodium bisulfite initiator, and carry out the reaction;

[0046] (6) After the reaction in step (5) has been complete for 4 hours, the mixture is allowed to cool naturally to room temperature to obtain a white, viscous emulsion-like liquid.

[0047] Example 3

[0048] A method for preparing an anti-calcium thickener, comprising the following steps:

[0049] (1) Add 150mL of No. 3 white oil to a beaker, add 4g of emulsifier (Span80:Tween80=1:1), stir and disperse at 4000r / min for 10 minutes, and stir evenly to form an oil phase;

[0050] (2) Dissolve 50g of 2-acrylamido-2-methyl-1-propanesulfonic acid, 15g of acrylonitrile, 25g of (3-acrylamidopropyl)trimethylammonium chloride and 20g of acrylic acid in 110mL of pure water, adjust the pH value to 7.0 with 40% sodium hydroxide aqueous solution, and stir evenly to form an aqueous phase;

[0051] (3) Slowly add the aqueous phase from step (2) to the oil phase from step (1) while stirring, and stir at 6000 r / min for 40 minutes to form a stable reverse emulsion;

[0052] (4) Add the emulsion obtained in step (3) into a three-necked flask and continuously purge with nitrogen gas;

[0053] (5) Heat the reaction system obtained in step (4) to 40°C, add 0.15g ammonium persulfate and 0.15g sodium bisulfite initiator, and carry out the reaction;

[0054] (6) After the reaction in step (5) has been complete for 4 hours, the mixture is allowed to cool naturally to room temperature to obtain a white, viscous emulsion-like liquid.

[0055] Example 4

[0056] A method for preparing an anti-calcium thickener, comprising the following steps:

[0057] (1) Add 150mL of No. 3 white oil to a beaker, add 4g of emulsifier (Span80:Tween80=1:1), stir and disperse at 4000r / min for 10 minutes, and stir evenly to form an oil phase;

[0058] (2) Dissolve 40g of 2-acrylamido-2-methyl-1-propanesulfonic acid, 12g of acrylonitrile, 30g of (3-acrylamidopropyl)trimethylammonium chloride and 18g of methacrylic acid in 100mL of pure water, adjust the pH value to 7.0 with 40% sodium hydroxide aqueous solution, and stir evenly to form an aqueous phase;

[0059] (3) Slowly add the aqueous phase from step (2) to the oil phase from step (1) while stirring, and stir at 6000 r / min for 40 minutes to form a stable reverse emulsion;

[0060] (4) Add the emulsion obtained in step (3) into a three-necked flask and continuously purge with nitrogen gas;

[0061] (5) Heat the reaction system obtained in step (4) to 40°C, add 0.1g ammonium persulfate and 0.1g sodium bisulfite initiator, and carry out the reaction;

[0062] (6) After the reaction in step (5) has been complete for 4 hours, the mixture is allowed to cool naturally to room temperature to obtain a white, viscous emulsion-like liquid.

[0063] Comparative Example 1

[0064] A method for preparing an anti-calcium thickener is as described in Example 1, except that methacryloyloxyethyltrimethylammonium chloride is not added.

[0065] Comparative Example 2

[0066] A method for preparing an anti-calcium thickener is described in Example 1, except that: the methacryloyloxyethyltrimethylammonium chloride in the reaction is replaced with dimethyl diallyl ammonium chloride.

[0067] Comparative Example 3

[0068] A method for preparing an anti-calcium thickener is as described in Example 1, except that acrylonitrile is not added.

[0069] Comparative Example 4

[0070] A method for preparing an anti-calcium thickener is as described in Example 1, except that methacrylic acid is replaced with acrylic acid in the reaction.

[0071] 2. Performance Testing:

[0072] The anti-calcium thickeners prepared in Examples 1-4 and Comparative Examples 1-4 were subjected to performance testing, and the specific methods are as follows:

[0073] Preparation of brine-based slurry: Add 160g CaCl2 to 400mL of distilled water and stir at 5000r / min for 20min. After complete dissolution, a 40% CaCl2 brine-based slurry is obtained.

[0074] 2% of the anti-calcium thickener prepared in Examples 1-4 and Comparative Examples 1-4 was added to 400 mL of brine-based slurry (i.e., 8 g of anti-calcium thickener was added to 400 mL of brine-based slurry). The mixture was stirred at 10000 r / min for 20 min. The apparent viscosity, plastic viscosity, dynamic shear force, and other rheological properties of the prepared drilling fluid were evaluated according to GB / T16783.1-2014 Petroleum and Natural Gas Industry Drilling Fluid Field Testing Part 1 Water-based Drilling Fluids. The prepared drilling fluid was then added to an aging tank and aged at 180℃ for 16 h in a roller furnace. The apparent viscosity, plastic viscosity, dynamic shear force, and other rheological properties of the drilling fluid were measured again. The experimental results are shown in Table 1.

[0075] Table 1. Performance Evaluation Results of Tackifiers

[0076]

[0077] Table 1 shows that both the example and comparative samples significantly increased the viscosity of the brine-based slurry before aging. Furthermore, the example sample exhibited a better viscosity-enhancing effect. After aging at 180℃, the viscosity of both the example and comparative drilling fluid samples decreased due to the high-temperature aging effect. However, the viscosity reduction of the example sample was less than that before aging, and it still maintained a good viscosity-enhancing effect.

[0078] Among the comparative samples, the performance varied significantly due to different monomer designs. Comparative Example 1, lacking cationic monomers, did not possess the anti-polyelectrolyte effect in its molecular chains, resulting in poor viscosity-enhancing properties of the drilling fluid and a significant decrease in viscosity after high-temperature aging.

[0079] Comparative Example 2 replaced methacryloyloxyethyltrimethylammonium chloride with dimethyl diallyl ammonium chloride. It can be seen that the thickener with anti-polyelectrolyte effect can still maintain a high viscosity in brine-based slurry, indicating that the anti-polyelectrolyte effect of the molecular chain plays a dominant role in improving the temperature and salt resistance of the thickener, while the hydrophobic association structure is secondary.

[0080] Comparative Example 3 does not contain acrylonitrile. Acrylonitrile has a good rigid structure, which can effectively improve the temperature resistance of the polymer. Without the participation of acrylonitrile, the molecular chain of the tackifier lacks a rigid structure, which makes it easy to degrade at high temperatures, resulting in poor temperature resistance and significant performance degradation after aging.

[0081] In Comparative Example 4, methacrylic acid was replaced with acrylic acid. Acrylic acid, compared to methacrylic acid, increased the flexibility of the polymer to some extent, but reduced its stability in high-temperature environments.

[0082] In summary, it can be seen that the anti-calcium thickener prepared in the embodiments of the present invention has excellent anti-calcium thickening properties. When applied to prepare a brine-based drilling fluid system, the performance of the resulting drilling fluid system is significantly better than that of the thickener prepared in the comparative example. The present invention selects the anti-calcium thickener prepared in Example 1 to prepare a drilling fluid system for illustration.

[0083] The anti-calcium thickener used in the following examples and comparative examples is the anti-calcium thickener prepared in Example 1.

[0084] The following anti-calcium filtration loss reducer is a hyperbranched polymer filtration loss reducer, which was prepared according to Example 1 in Chinese Patent Document CN113527575B. The specific preparation process is as follows:

[0085] 1) Preparation of pentaerythritol polyene monomer: Weigh 10g of pentaerythritol, 66.8g of allyl bromide, 50g of sodium hydroxide and 6.68g of tetrabutylammonium bromide and add them to a 500mL three-necked flask. Stir at room temperature for 5h and at 60℃ for 24h. After the reaction is complete, filter to remove the white solid. Wash the solid with 80mL of diethyl ether and repeat the washing 3 times. Dry the filtrate with anhydrous sodium sulfate and remove the diethyl ether by rotary evaporation at room temperature to obtain a pale yellow oily liquid, which is the pentaerythritol polyene monomer.

[0086] 2) Preparation of hyperbranched polymer filtration loss reducer: Weigh component A, component B, component C, pentaerythritol polyene monomer and water according to the weight percentage of each component mentioned above.

[0087] Component A: 4g of N,N,N-trimethyl-3(2-methylallylamino)-1-ammonium chloride, 4g of dimethyldiallylammonium chloride, 4g of N,N,N-trimethylphenylmethylammonium chloride, and 4g of methacryloyloxyethyltrimethylammonium chloride. Component A accounts for 3.2% of the total weight of all reaction mixtures.

[0088] Component B: 8g of sodium 2-acrylamide-2-methylpropanesulfonate, 8g of sodium methacryloyloxyethylsulfonate, 8g of N-vinylpyrrolidone, and 8g of sodium styrenesulfonate. Component B accounts for 6.4% of the total weight of all reaction mixtures.

[0089] Component C: 16g acrylamide, 16g sodium acrylate, 16g vinyl acetate. Component C accounts for 9.6% of the total weight of all reaction mixtures.

[0090] Pentaerythritol polyene monomer: 20g, accounting for 4% of the total weight of all reaction mixtures.

[0091] Water: 384g.

[0092] 116g of the above-mentioned components A, B, C, and pentaerythritol polyene monomer were placed in 384g of water and stirred. Nitrogen gas was continuously introduced and the temperature was raised to 50°C. 0.05g each of ammonium persulfate and sodium bisulfite were added. After reacting for 5 hours, the reactants were cooled to room temperature to obtain a transparent and viscous product. The product was purified with anhydrous ethanol to obtain a white solid. After drying at 100°C for 24 hours, it was pulverized to obtain a white powdery hyperbranched polymer filtration loss reducer.

[0093] 3. Drilling fluid preparation examples

[0094] Example 1

[0095] The preparation method of brine-based drilling fluid includes the following steps: First, add 160g of calcium chloride to 400mL of distilled water and stir at 5000r / min for 20min. After the solution is fully dissolved, brine-based slurry is obtained.

[0096] Take 400 mL of brine-based slurry and add 10 g of anti-calcium filtration loss reducer, 8 g of anti-calcium thickener, 12 g of sealing agent, and 16 g of bridging agent in sequence. After adding each raw material, stir at high speed for 20 minutes; the high-speed stirring speed is 10000 r / min, resulting in a density of 1.25 g / cm³. 3 Brine-based drilling fluid.

[0097] Example 2

[0098] The preparation method of brine-based drilling fluid includes the following steps: First, add 320g of calcium chloride to 400mL of distilled water and stir at 5000r / min for 20min. After the solution is fully dissolved, brine-based slurry is obtained.

[0099] Take 400 mL of brine-based slurry and add 10 g of anti-calcium filtration loss reducer, 8 g of anti-calcium thickener, 12 g of sealing agent, and 16 g of bridging agent in sequence. After adding each raw material, stir at high speed for 20 minutes; the high-speed stirring speed is 10000 r / min, resulting in a density of 1.4 g / cm³. 3 Brine-based drilling fluid.

[0100] Example 3

[0101] The preparation method of brine-based drilling fluid includes the following steps: First, add 300g of calcium chloride and 230g of calcium bromide to 400mL of distilled water, stir at 5000r / min for 20min, and obtain brine-based slurry after complete dissolution.

[0102] Take 400 mL of brine-based slurry and add 10 g of anti-calcium filtration loss reducer, 8 g of anti-calcium thickener, 12 g of sealing agent, and 16 g of bridging agent in sequence. After adding each raw material, stir at high speed for 20 minutes; the high-speed stirring speed is 10000 r / min, resulting in a density of 1.6 g / cm³. 3 Brine-based drilling fluid.

[0103] Comparative Example 1

[0104] The brine-based drilling fluid differs from that in Example 1 in that it does not contain an anti-calcium filtration loss agent.

[0105] Comparative Example 2

[0106] The brine-based drilling fluid differs from that in Example 1 in that it does not contain an anti-calcium thickener.

[0107] Comparative Example 3

[0108] The brine-based drilling fluid differs from Example 1 in that, based on Example 1, barite barium sulfate is used to increase the weight of the solution to 1.4 g / cm³. 3 .

[0109] 4. Performance Testing:

[0110] The drilling fluids prepared in Examples 1-3 and Comparative Examples 1-3 were subjected to performance testing, and the specific methods are as follows:

[0111] (1) Rheological filtration performance evaluation

[0112] The apparent viscosity, plastic viscosity, dynamic shear force, and other rheological properties of the prepared drilling fluid system were evaluated according to GB / T16783.1-2014 Petroleum and Natural Gas Industry Drilling Fluid Field Testing Part 1 Water-based Drilling Fluids. The API filtration loss of the drilling fluid was also measured. The drilling fluid system was transferred to a stainless steel high-temperature aging tank and aged at 180°C for 16 hours. The aforementioned rheological parameters, API filtration loss, and high-temperature, high-pressure filtration loss at 180°C were then measured again. The experimental results are shown in Table 2.

[0113] (2) Evaluation of suppression performance

[0114] The mudstone and shale from the reservoir section were washed, dried, and pulverized, then passed through a 6-10 mesh sieve. 50g of each sample was added to different aging tanks, along with shale inhibitor aqueous solutions of varying concentrations. The aging tanks were then subjected to rolling aging at 180℃ for 16 hours. The aged drill cuttings were passed through a 40 mesh sieve, cleaned, dried, and weighed (recorded as m). The rolling recovery rate was calculated using the following formula. The experimental results are shown in Table 3.

[0115] S = m / 50 * 100%

[0116] In the formula:

[0117] S – Shale rolling recovery rate, %

[0118] m — weight of residue on sieve, in grams.

[0119] Measurement results

[0120] Table 2 Evaluation of Rheological Filtration Properties of Brine-Based Drilling Fluids

[0121]

[0122] As can be seen from Table 2, the drilling fluids of both the examples and the comparative examples before aging exhibited good filtration loss reduction effects. After aging, the viscosity of Examples 1, 2, and 3 decreased slightly, the filtration loss under medium pressure was very small, and the filtration loss under high temperature and high pressure was less than 15 mL, indicating that the drilling fluid systems of the examples have good temperature and calcium resistance as well as filtration loss reduction performance.

[0123] In Examples 3 and 2, the calcium ion content was higher than in Example 1, resulting in slightly higher viscosity and shear stress, and slightly lower filtration loss. This indicates that the introduction of calcium ions enhanced the ionic strength and stability of the solution, further improving the ion shielding effect and making the molecular chains more stable. Simultaneously, the synergistic effect of the anti-calcium filtration loss reducer, anti-calcium thickener, and plugging agent enhanced the temperature and salt resistance of the drilling fluid system, forming a denser filter cake in high-concentration salt environments. Comparative Example 1, lacking the anti-calcium filtration loss reducer, showed significantly higher filtration losses under medium pressure and high temperature / high pressure than Example 1. This indicates that the anti-calcium filtration loss reducer played a crucial role in controlling filtration loss and enhancing filter cake stability. The lack of the filtration loss reducer led to a sharp decline in the system's filtration performance. Comparative Example 2 lacked the anti-calcium thickener, so its apparent viscosity and plastic viscosity were significantly lower than in Example 1, initially 16.5 mPa·s and 13.0 mPa·s respectively, and further decreased to 12.0 mPa·s and 10.0 mPa·s after aging. This indicates that anti-calcium thickeners are indispensable for maintaining the viscosity and stability of the system; the lack of thickeners significantly reduces the rheological properties of the system. In Comparative Example 3, the use of barite for weighting resulted in a significant increase in filtration loss to 28.4 mL after aging under high temperature and pressure. This is because barite particles tend to settle easily, leading to poor filter cake stability, and the insufficient dispersibility of barite makes the high-temperature performance of the system inferior to that of the brine-weighted system.

[0124] Table 3 Evaluation of Drilling Fluid Inhibition Performance and Rolling Recovery Rate

[0125]

[0126] The hydration inhibition performance of drilling fluids is crucial for suppressing the hydration swelling of clay minerals and stabilizing the wellbore formation. Plain water has a poor inhibition effect on shale, leading to easy hydration, swelling, and mud production, resulting in low recovery rates. Plain water cannot effectively prevent shale fracturing, easily causing the expansion of pores and fractures. The addition of calcium chloride enhances the chemical stability of shale through Na+ / Ca²⁺ exchange, reduces its hydration capacity, and improves the stability of the rock interface. Calcium salts effectively reduce the swelling and fracturing of shale through chemical inhibition, thus improving recovery rates. The example contains key additives such as anti-calcium filtration loss reducers, anti-calcium thickeners, and plugging agents. The synergistic effect of these additives significantly enhances the inhibition performance of the drilling fluid. Anti-calcium filtration loss reducers control the filtration loss of the drilling fluid, anti-calcium thickeners increase viscosity, and plugging agents enhance the sealing effect on the rock formation, effectively improving the stability of shale and thus significantly increasing the rolling recovery rate. In Comparative Example 1, the lack of an anti-calcium filtration reducer resulted in insufficient effectiveness in inhibiting shale hydration and swelling, leading to a low recovery rate. Comparative Example 2, lacking an anti-calcium thickener, had a lower drilling fluid viscosity, affecting its stability under high temperature and pressure conditions, reducing its adhesion and plugging properties, and consequently decreasing its recovery rate. Comparative Example 3 had the lowest recovery rate among all comparative samples. This sample used barite for weighting; the large barite particles easily settle and have uneven particle size, resulting in poor rheological properties and poor filtration. Although barite provides higher density, it cannot effectively improve inhibition like brine-based drilling fluids, thus its recovery rate is significantly lower than the examples.

[0127] In the description of this specification, specific features, structures, materials, or characteristics may be combined in any suitable manner in one or more embodiments or examples.

[0128] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A brine-based drilling fluid, characterized in that, By weight, it includes the following raw materials: 400 parts brine, 8-12 parts anti-calcium filtration loss reducer, 4-10 parts anti-calcium thickener, 12-16 parts sealing agent and 16-20 parts bridging agent; The brine includes calcium chloride and / or calcium bromide; The sealing agent is asphalt powder; The bridging agent is ultrafine calcium carbonate; The preparation method of the anti-calcium thickener includes the following steps: A. Mix the white oil with the emulsifier to form an oil phase, and set aside. B. Dissolve 2-acrylamido-2-methyl-1-propanesulfonic acid, acrylonitrile, cationic monomer and carboxyl-containing vinyl monomer in pure water, adjust the pH to 7.0, and stir until homogeneous to form an aqueous phase; C. Add the aqueous phase from step B dropwise to the oil phase from step A, and mix well to obtain a stable reverse emulsion; D. In a nitrogen atmosphere, an initiator is added to the stable reverse emulsion in step C, and an anti-calcium thickener is obtained after the reaction. In step B, the mass ratio of 2-acrylamido-2-methyl-1-propanesulfonic acid, acrylonitrile, cationic monomer to carboxyl-containing vinyl monomer is (40~50):(10-20):(20~30):(15~25). The cationic monomer includes at least one of dimethyl diallyl ammonium chloride, methacryloyloxyethyltrimethylammonium chloride, and (3-acrylamidopropyl)trimethylammonium chloride; The carboxyl-containing vinyl monomers include acrylic acid and / or methacrylic acid.

2. The brine-based drilling fluid according to claim 1, characterized in that, In step A, the volume ratio of the white oil to the mass ratio of the emulsifier is 75 mL: 2 g.

3. The brine-based drilling fluid according to claim 2, characterized in that, The emulsifier is composed of sorbitan oleate and polysorbate-80 in a mass ratio of 1:

1.

4. The brine-based drilling fluid according to claim 1, characterized in that, In step D, the initiator includes potassium persulfate and / or sodium bisulfite.

5. The brine-based drilling fluid according to claim 1, characterized in that, In step D, the initiator is 0.1%-0.3% of the total mass of 2-acrylamido-2-methyl-1-propanesulfonic acid, acrylonitrile, cationic monomer and carboxyl-containing vinyl monomer, and the reaction temperature is 40-55°C.

6. A method for preparing a brine-based drilling fluid as described in any one of claims 1 to 5, characterized in that, Includes the following steps: Add calcium-resistant filtration reducer, calcium-resistant thickener, plugging agent and bridging agent to the brine and mix well to obtain brine-based drilling fluid.

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