Oil-based drilling fluid suitable for shale oil-gas well and preparation method of oil-based drilling fluid

Abstract

The invention relates to an oil-based drilling fluid suitable for a shale oil-gas well and a preparation method thereof, and belongs to the technical field of drilling fluids for petroleum drilling engineering. The drilling fluid is mainly prepared from a water-in-oil base solution, a main emulsifier, an auxiliary emulsifier, organic soil, a filtrate reducer, a wetting agent, quick lime and a blocking agent. (2-4) g of organic soil, (3-5) g of filtrate reducer and (2-4) g of blocking agent are correspondingly added into every 100 mL of water-in-oil base fluid; the mass ratio of base oil to saline water in the water-in-oil base fluid is (7-8): (2-3); the blocking agent is emulsified asphalt; the filtrate reducer is a modified asphalt resin filtrate reducer. The oil-based drilling fluid provided by the invention is high in inhibitive ability and good in plugging capacity, and can effectively cope with hydration expansion of shale and maintain the stability of a well wall.

Description

Technical Field

[0001] The invention relates to an oil-based drilling fluid suitable for shale oil and gas wells and a preparation method thereof, belonging to the technical field of drilling fluids for petroleum drilling engineering. Background Art

[0002] Shale oil and gas development primarily relies on horizontal well placement. Due to the abundance of fractures and microfractures in shale formations, drilling in horizontal sections can easily lead to downhole complications such as wellbore collapse, massive block shedding, and shale hydration and expansion. Furthermore, drilling highly deviated wells often encounters issues such as severe downhole mocha and poor sand carryover due to extended openhole sections. Therefore, the performance of drilling fluids directly impacts drilling efficiency and the occurrence of downhole complications, placing higher demands on drilling fluid performance.

[0003] Oil-based drilling fluids, which use oil as the continuous phase, are becoming increasingly common in high-temperature, high-pressure deep wells, horizontal wells, highly deviated directional wells, slim-hole drilling, and shale gas drilling. Compared to water-based drilling fluids, oil-based drilling fluids offer advantages such as high-temperature resistance, corrosion resistance, excellent lubricity, and minimal damage to oil and gas formations. They have become a crucial tool for drilling challenging high-temperature deep wells, highly deviated directional wells, horizontal wells, and various complex formations.

[0004] Commonly used oil-based drilling fluids primarily include oil, water, emulsifiers, oxidized asphalt, organic acids, alkalis, and other reagents. However, in the development of hard and brittle shale, due to the development of microcracks in the hard and brittle shale and the high collapse stress, the fluid entering the microcracks generates pressure transmission under the action of the hydrostatic column. Commonly used oil-based drilling fluids have insufficient sealing capacity and cannot effectively control the pressure transmission of the shale, resulting in wellbore instability, which seriously restricts the development of shale oil and gas.

[0005] A Chinese invention patent, published on November 18, 2015, with publication number CN105062438A, discloses an organic emulsion drilling fluid with an ultra-low water-to-oil ratio of 40:60. The patent discloses that the drilling fluid comprises 40 parts 3# white oil, 60 parts 30% emulsion stabilizer, 2-5 parts high-efficiency emulsifier, 1-3 parts emulsion stabilizer, 1-3 parts fluid loss additive, 1-5 parts lime, and 20-50 parts barite. This organic emulsion drilling fluid exhibits excellent emulsion stability, rock-carrying capacity, and wellbore stability, significantly reducing the cost of organic drilling fluid preparation and oil and gas drilling. However, the patent is limited to a 40:60 oil-to-water ratio and has a temperature resistance of only 120°C. The drilling fluid exhibits significant viscosity and shearing, making implementation difficult and the demulsification voltage below the required value.

[0006] A Chinese invention patent with a publication date of July 30, 2021 and publication number CN113185955A discloses an oil-based drilling fluid, a preparation method and application thereof. The oil-based drilling fluid includes the following components: white oil, organic soil, calcium chloride solution, calcium oxide, primary emulsifier, auxiliary emulsifier, wetting agent, plugging agent, barite, and filtration reducer; the plugging agent is one or more of 1250 mesh ultrafine calcium carbonate, 2250 mesh ultrafine calcium carbonate, nano plugging agent, and asphalt plugging agent ME-CAL; the nano plugging agent is nano-graphene modified with sodium dodecylbenzene sulfonate or heavy alkylbenzene sulfonate, and the average particle size of the nano plugging agent is 80-210 nm. This oil-based drilling fluid achieves multi-faceted synergistic plugging through bridging and filling under the action of high temperature in the well. It is used in the drilling process in deep shale layers to enhance the plugging ability of the drilling fluid. However, according to the test results, the penetration depth of the drilling fluid is still large, and the stability performance of the drilling fluid is also unknown. Summary of the Invention

[0007] The first object of the present invention is to provide an oil-based drilling fluid suitable for shale oil and gas wells, and to provide an oil-based drilling fluid with good sealing properties and strong stability in high temperature environments.

[0008] The second object of the present invention is to provide a method for preparing an oil-based drilling fluid suitable for shale oil and gas wells, and to provide an oil-based drilling fluid with good sealing properties and strong stability in high temperature environments.

[0009] In order to achieve the above-mentioned object, a technical solution of an oil-based drilling fluid suitable for shale oil and gas wells in the present invention is:

[0010] Disclosed is an oil-based drilling fluid suitable for shale oil and gas wells, mainly comprising a water-in-oil base fluid, a primary emulsifier, a secondary emulsifier, organic soil, a fluid loss reducer, a wetting agent, quicklime, and a plugging agent. For every 100 mL of the water-in-oil base fluid, 2-4 g of organic soil, 3-5 g of the fluid loss reducer, and 2-4 g of the plugging agent are added. The mass ratio of base oil to brine in the water-in-oil base fluid is (7-8):(2-3). The plugging agent is emulsified asphalt, and the fluid loss reducer is a modified asphalt resin-based fluid loss reducer.

[0011] The beneficial effect of the above technical solution is that the oil-based drilling fluid suitable for shale oil and gas wells of the present invention is an improved invention. By optimizing the composition and dosage of the oil-based drilling fluid, the present invention forms an oil-based drilling fluid composed of a water-in-oil base fluid, a primary emulsifier, a secondary emulsifier, organic soil, a fluid loss additive, a wetting agent, quicklime, a plugging agent, and barite. The synergistic effect of these components gives the oil-based drilling fluid of the present invention strong inhibitory and plugging properties, and it can effectively maintain system stability even at higher temperatures, effectively addressing the hydration expansion of shale and maintaining wellbore stability.

[0012] Specifically, the emulsifier is a stabilizer for the emulsion, which can prevent the small droplets of the dispersed phase from condensing with each other. The membrane formed by using two emulsifiers in the present invention is stronger and stronger than the membrane of a single emulsifier, and its surface activity is greatly enhanced. The liquid phases are less likely to aggregate, and the formed emulsion is more stable.

[0013] The organic clay is a lipophilic clay made by the interaction between hydrophilic bentonite and quaternary ammonium salt cationic surfactants. It is easily dispersed in the oil phase and has good viscosity-enhancing and suspension effects. Preferably, the organic clay is BP-324.

[0014] Fluid loss control agents can reduce fluid loss by slowing the flow of oil through the filter cake near the interface between emulsion droplets and solid particles. They can also reduce fluid loss by adsorbing and depositing on the wellbore wall with other lipophilic colloids in the system to form a dense filter cake. Preferably, the resin fluid loss control agent is MOTEX.

[0015] Adding certain wetting agents to oil-based drilling fluid can change the contact angle of the solid / liquid interface. These substances will make the solid phase preferentially wetted by the oil and maintain the stability of the emulsion.

[0016] Plugging agents can effectively improve the plugging performance of oil-based drilling fluids. The present invention uses emulsified asphalt as a plugging agent, which works closely with the components of the oil-based drilling fluid. This not only improves the stability of the drilling fluid at high temperatures, but also effectively enhances the plugging performance of the drilling fluid, controlling the pressure transmission of shale, thereby preventing wellbore instability and improving wellbore stability. Preferably, the emulsified asphalt is FF-III.

[0017] As a further improvement, the primary emulsifier is a long-chain alkyl acid salt; the auxiliary emulsifier is a fatty alcohol polyether; and for every 100 mL of water-in-oil base liquid, (1-2) g of the primary emulsifier and (1-2) g of the auxiliary emulsifier are added.

[0018] The beneficial effect of the above technical solution is that the above-mentioned type of primary emulsifier and auxiliary emulsifier are more compatible with the oil-based drilling fluid system of the present invention, and the formed oil-in-water emulsion is relatively dense and stable, which effectively improves the stability of the oil-based drilling fluid.

[0019] Preferably, the primary emulsifier is long-chain alkyl salt OME; and the secondary emulsifier is alkyl alcohol polyether OME-2.

[0020] As a further improvement, the base oil is one of white oil or diesel.

[0021] As a further improvement, the wetting agent is a phosphate anionic surfactant; (1-2) g of the wetting agent is added to every 100 mL of the water-in-oil base liquid.

[0022] Preferably, the phosphate anionic surfactant is anionic surfactant OW.

[0023] As a further improvement, quicklime (1-3) g is added to every 100 mL of water-in-oil base liquid.

[0024] As a further improvement, the brine is an aqueous solution formed by adding (20-25) g of calcium chloride to 100 mL of water.

[0025] As a further improvement, the density of the oil-based drilling fluid is (1.20-1.40) g / cm 3 .

[0026] In order to achieve the above-mentioned object, the technical solution of a method for preparing an oil-based drilling fluid suitable for shale oil and gas wells in the present invention is:

[0027] A method for preparing an oil-based drilling fluid suitable for shale oil and gas wells comprises the following steps:

[0028] (1) adding a primary emulsifier and a secondary emulsifier to the base oil, mixing them evenly, and adding salt water to obtain a water-in-oil emulsion;

[0029] (2) adding organic soil, fluid loss reducer, wetting agent, quicklime, plugging agent and barite to the water-in-oil emulsion in step (1) and mixing them evenly to obtain the product.

[0030] The beneficial effects of the above technical solution are: the present invention is applicable to the preparation method of oil-based drilling fluid for shale oil and gas wells, the preparation process is simple, it can be carried out on existing production equipment, and it has good adaptability to on-site applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 This is a bar graph showing the detection of the organic soil colloid rate in Experimental Example 2 of the present invention;

[0032] Figure 2 This is an experimental diagram of the detection of the colloid rate of organic soil in Experimental Example 2 of the present invention (in the figure, from left to right are BP-324, HFGel-120, ZSHG, and LT);

[0033] Figure 3 This is a comparison chart of ES (emulsion breaking voltage) of different types of fluid loss reducers in Experimental Example 5 of the present invention;

[0034] Figure 4 This is a bar chart comparing the invasion depths of different plugging agent addition amounts in Experimental Example 5 of the present invention. DETAILED DESCRIPTION

[0035] Shale contains vast quantities of oil and gas resources, making deep shale gas development fundamental for future increases in China's oil production. However, due to the abundance of fractures and microfractures in shale formations, horizontal drilling can easily lead to complex downhole conditions, such as wellbore collapse, significant block shedding, and shale hydration and expansion. Furthermore, the drilling of highly deviated wells often encounters issues such as severe downhole mocha and poor sand carryover due to extended openhole sections. While the technology for drilling horizontal shale wells using oil-based drilling fluids is now largely mature, wellbore stability remains a critical concern. Therefore, improving the stability and sealing capabilities of existing oil-based drilling fluids is a pressing technical challenge that needs to be addressed. The present invention provides an oil-based drilling fluid suitable for shale oil and gas wells, which is mainly composed of a water-in-oil base fluid and a primary emulsifier, an auxiliary emulsifier, organic soil, a fluid loss reducer, a wetting agent, quicklime, and a plugging agent; for every 100 mL of the water-in-oil base fluid, (2-4) g of organic soil, (3-5) g of the fluid loss reducer, and (2-4) g of the plugging agent are added; the mass ratio of base oil to brine in the water-in-oil base fluid is (7-8):(2-3); the plugging agent is emulsified asphalt; and the fluid loss reducer is a modified asphalt resin-based fluid loss reducer. The present invention optimizes the composition and dosage of the oil-based drilling fluid to form an oil-based drilling fluid composed of a water-in-oil base fluid, a primary emulsifier, a secondary emulsifier, organic soil, a fluid loss reducer, a wetting agent, quicklime, a plugging agent and barite. The components cooperate with each other and act synergistically, so that the oil-based drilling fluid of the present invention has strong inhibitory and plugging properties, and can still effectively maintain the stability of the system at higher temperatures, can effectively cope with the hydration expansion of shale, and maintain the stability of the well wall.

[0036] The present invention will be further described below in conjunction with specific embodiments. It should be noted that, unless otherwise specified, the various embodiments or technical features described below can be arbitrarily combined to form new embodiments. The equipment and raw materials used are all commercially available or commonly used in the art. The methods in the following embodiments are all conventional methods in the art unless otherwise specified.

[0037] In the following examples, unless otherwise specified, the operations described are routine operations in the art.

[0038] The white oil used in the examples of the present invention was purchased from Henan Jinma Petroleum Technology Co., Ltd.; the primary emulsifier OME, the secondary emulsifier OME-2, the wetting agent OW, and the plugging agent FF-III were purchased from Deshunyuan Petroleum Technology Co., Ltd.; the organic soil BP-324 was purchased from Xiongguan Technology Development Co., Ltd.; the fluid loss reducer MOTEX was purchased from Hanke New Technology Co., Ltd.; quicklime, calcium chloride, and barite were purchased from Yanan Industrial Co., Ltd.

[0039] 1. Specific embodiments of an oil-based drilling fluid suitable for shale oil and gas wells and its preparation method of the present invention

[0040] Example 1

[0041] The oil-based drilling fluid for shale oil and gas wells of this embodiment is composed of 320g of white oil and 80g of brine solution in parts by weight; the brine solution is a calcium chloride solution formed by dissolving 25g of calcium chloride in 100mL of water; based on 400mL of oil-based water-based fluid, the amounts of the remaining components are: 8g of primary emulsifier (OME), 4g of auxiliary emulsifier (OME-2), 16g of organic soil (BP-324), 20g of fluid loss reducer (MOTEX), 8g of wetting agent (OW), 12g of quicklime, and 16g of plugging agent (FF-III). The drilling fluid density is 1.40g / cm 3 Add barite.

[0042] The oil-based drilling fluid suitable for shale oil and gas wells of this embodiment is prepared by the following steps:

[0043] (1) Add the primary emulsifier, the secondary emulsifier and the calcium chloride aqueous solution to the white oil in order according to the formula amount to obtain a water-in-oil emulsion.

[0044] (2) Add organic soil, filtration reducer, wetting agent, quicklime, plugging agent and barite powder to the oil-in-water emulsion in the prescribed amount and stir evenly.

[0045] Example 2

[0046] The oil-based drilling fluid suitable for shale oil and gas wells of this embodiment is composed of 300g of white oil and 100g of brine solution in parts by weight; the brine solution is a calcium chloride aqueous solution formed by dissolving 25g of calcium chloride in 100mL of water; based on 400mL of oil-based water-based fluid, the amounts of the remaining components are: 8g of primary emulsifier (OME), 8g of auxiliary emulsifier (OME-2), 16g of organic soil (BP-324), 12g of fluid loss reducer (MOTEX), 8g of wetting agent (OW), 4g of quicklime, and 16g of plugging agent (FF-III). According to the drilling fluid density of 1.40g / cm 3 Add barite.

[0047] The preparation method of the oil-based drilling fluid suitable for shale oil and gas wells in this embodiment is as described in Example 1.

[0048] Example 3

[0049] The oil-based drilling fluid for shale oil and gas wells of this embodiment is composed of 280g of white oil and 120g of brine solution in parts by weight; the brine solution is a calcium chloride solution formed by dissolving 25g of calcium chloride in 100mL of water; based on 400mL of oil-based water-based fluid, the amounts of the remaining components are: 4g of primary emulsifier (OME), 4g of auxiliary emulsifier (OME-2), 8g of organic soil (BP-324), 12g of fluid loss reducer (MOTEX), 4g of wetting agent (OW), 8g of quicklime, and 14g of plugging agent (FF-III). According to the drilling fluid density of 1.40g / cm 3 Add barite.

[0050] The preparation method of the oil-based drilling fluid suitable for shale oil and gas wells in this embodiment is as described in Example 1.

[0051] Example 4

[0052] The oil-based drilling fluid of this embodiment suitable for shale oil and gas wells is composed of 320g of white oil and 80g of brine solution in parts by weight; the brine solution is a calcium chloride solution formed by dissolving 25g of calcium chloride in 100mL of water; based on 400mL of oil-based water-based fluid, the amounts of the remaining components are: 8g of main emulsifier (OME), 4g of auxiliary emulsifier (OME-2), 12g of organic soil (BP-324), 16g of fluid loss reducer (MOTEX), 4g of wetting agent (OW), 8g of quicklime, and 8g of plugging agent (FF-III). According to the drilling fluid density of 1.20g / cm 3 Add barite.

[0053] The preparation method of the oil-based drilling fluid suitable for shale oil and gas wells in this embodiment is as described in Example 1.

[0054] 2. Comparative Example

[0055] Comparative Example 1

[0056] The oil-based drilling fluid of this comparative example is different from that of Example 1 in that the amount of plugging agent added is 4 g. The specific formula is as follows: in parts by weight, the water-in-oil base slurry is composed of 320 g of white oil and 80 g of brine solution; the brine solution is a calcium chloride aqueous solution formed by dissolving 25 g of calcium chloride in 100 mL of water; based on 400 mL of water-in-oil base fluid, the amount of the remaining components is: main emulsifier (OME) 8 g, auxiliary emulsifier (OME-2) 4 g, organic soil (BP-324) 16 g, filtration reducer (MOTEX) 20 g, wetting agent (OW) 8 g, quicklime 12 g, plugging agent (FF-III) 4 g. According to the drilling fluid density of 1.40 g / cm 3 Add barite.

[0057] The preparation method of the oil-based drilling fluid of this comparative example is as described in Example 1.

[0058] Comparative Example 2

[0059] The oil-based drilling fluid of this comparative example is different from that of Example 2 in that the amount of organic soil added is 6 g. The specific formula is as follows: in parts by weight, the oil-in-water base slurry is composed of 300 g of white oil and 100 g of brine solution; the brine solution is a calcium chloride aqueous solution formed by dissolving 25 g of calcium chloride in 100 mL of water; based on 400 mL of oil-in-water base fluid, the amounts of the remaining components are: main emulsifier (OME) 8 g, auxiliary emulsifier (OME-2) 8 g, organic soil (BP-324) 6 g, filtration reducer (MOTEX) 12 g, wetting agent (OW) 8 g, quicklime 4 g, plugging agent (FF-III) 16 g. According to the drilling fluid density of 1.40 g / cm 3 Add barite.

[0060] The preparation method of the oil-based drilling fluid of this comparative example is as described in Example 1.

[0061] Comparative Example 3

[0062] The oil-based drilling fluid of this comparative example is different from that of Example 3 in that the amount of fluid loss reducer added is 6 g. The specific formula is as follows: in parts by weight, the water-in-oil base slurry is composed of 280 g of white oil and 120 g of brine solution; the brine solution is a calcium chloride aqueous solution formed by dissolving 25 g of calcium chloride in 100 mL of water; based on 400 ml of water-in-oil base fluid, the amount of the remaining components is: main emulsifier (OME) 4 g, auxiliary emulsifier (OME-2) 4 g, organic soil (BP-324) 8 g, fluid loss reducer (MOTEX) 6 g, wetting agent (OW) 4 g, quicklime 8 g, plugging agent (FF-III) 14 g. According to the drilling fluid density of 1.40 g / cm 3 Add barite.

[0063] The preparation method of the oil-based drilling fluid of this comparative example is as described in Example 1.

[0064] Comparative Example 4

[0065] The oil-based drilling fluid of this comparative example is different from that of Example 4 in that the additive does not contain a plugging agent. The specific formula is as follows: in parts by weight, the oil-in-water base slurry is composed of 320g of white oil and 80g of brine solution; the brine solution is a calcium chloride aqueous solution formed by dissolving 25g of calcium chloride in 100mL of water; based on 400mL of oil-in-water base fluid, the amounts of the remaining components are: 8g of main emulsifier (OME), 4g of auxiliary emulsifier (OME-2), 12g of organic soil (BP-324), 16g of filtration reducer (MOTEX), 4g of wetting agent (OW), and 8g of quicklime. According to the drilling fluid density of 1.20g / cm 3 Add barite.

[0066] The preparation method of the oil-based drilling fluid of this comparative example is as described in Example 1.

[0067] Comparative Example 5

[0068] The oil-based drilling fluid of this comparative example is different from that of Example 1 in that no fluid loss reducer is added, and the amount of emulsified asphalt added is 36g. The specific formula is as follows: in parts by weight, the oil-in-water base slurry is composed of 320g of white oil and 80g of brine solution; the brine solution is a calcium chloride aqueous solution formed by dissolving 25g of calcium chloride in 100mL of water; based on 400mL of oil-in-water base fluid, the amount of the remaining components is: main emulsifier (OME) 8g, auxiliary emulsifier (OME-2) 4g, organic soil (BP-324) 16g, wetting agent (OW) 8g, quicklime 12g, plugging agent (FF-III) 36g. According to the drilling fluid density of 1.40g / cm 3 Add barite.

[0069] Comparative Example 6

[0070] The oil-based drilling fluid of this comparative example is different from that of Example 1 in that no emulsified asphalt is added, and the amount of fluid loss reducer added is 36g. The specific formula is as follows: in parts by weight, the oil-in-water base slurry is composed of 320g of white oil and 80g of brine solution; the brine solution is a calcium chloride aqueous solution formed by dissolving 25g of calcium chloride in 100mL of water; based on 400mL of oil-in-water base fluid, the amount of the remaining components is: main emulsifier (OME) 8g, auxiliary emulsifier (OME-2) 4g, organic soil (BP-324) 16g, fluid loss reducer (MOTEX) 36g, wetting agent (OW) 8g, quicklime 12g. According to the drilling fluid density of 1.40g / cm 3 Add barite.

[0071] 3. Experimental Examples

[0072] Experimental Example 1 Optimization of emulsifier type and dosage

[0073] 1. Optimal emulsifier type

[0074] An emulsion is a dispersion system formed by dispersing one liquid in the form of droplets in another immiscible liquid. The droplets are called the dispersed phase or internal phase; the other, continuous phase is called the dispersion medium or continuous phase. Emulsifiers act as stabilizers for emulsions. When dispersed on the surface of the dispersion medium, they form a thin film or double layer, imparting a certain charge to the dispersed phase. This prevents the small droplets from agglomerating, resulting in a more stable emulsion.

[0075] To strengthen the interfacial film, using a mixed emulsifier is more effective than using a single emulsifier. A film composed of two emulsifiers is stronger and more robust than a single emulsifier, with significantly enhanced surface activity. This makes the liquid phases less likely to coalesce, resulting in a more stable emulsion. The first emulsifier, called the primary emulsifier, primarily forms the film's backbone. The second emulsifier, called the secondary emulsifier, further stabilizes the primary emulsifier and increases the viscosity of the external phase.

[0076] In this experimental example, four emulsifiers, OME, DLJ, HK and XG, were selected as alternative main emulsifiers, and OME-2, DLJ-2, HK-2 and XG-2 were selected as alternative auxiliary emulsifiers. The experimental method is as follows, and the specific results are shown in Table 1.

[0077] Experimental methods:

[0078] ① Use a measuring cylinder to measure 320 mL of white oil, add 8 g of primary emulsifier and 6 g of secondary emulsifier, and stir for 20 minutes;

[0079] ②Add 12g of organic soil and stir for 20 minutes;

[0080] ③ Add 80 mL of CaCl2 solution (25 g of calcium chloride in 100 mL of water) to the solution to prepare an emulsion and stir for 20 minutes;

[0081] ④Measure the emulsion breaking voltage;

[0082] ⑤ Heat the solution at 120℃ for 16 hours and measure the emulsion breaking voltage again.

[0083] Table 1 Demulsification voltage data of drilling fluid with different emulsifier combinations

[0084]

[0085]

[0086] Note: The main emulsifier of OME is a long-chain alkyl acid salt emulsifier, the main emulsifier of DLJ is an amide emulsifier, the main emulsifier of HK is a phosphate emulsifier, and the main emulsifier of XG is calcium cyclohexanecarboxylate; the auxiliary emulsifier of OME-2 is a fatty alcohol polyether auxiliary emulsifier, the auxiliary emulsifier of DLJ-2 is an oleic acid emulsifier, the auxiliary emulsifier of HK-2 is a petroleum sulfonic acid emulsifier, and the auxiliary emulsifier of XG-2 is a polyhydrocarbon carboxylate emulsifier.

[0087] It can be seen from the above table that the combination of OME primary emulsifier and OME-2 auxiliary emulsifier, the combination of OME primary emulsifier and XG-2 auxiliary emulsifier, and the combination of XG primary emulsifier and OME-2 auxiliary emulsifier have better electrical stability. After the emulsion ages, the demulsification voltage is higher than that of other combinations.

[0088] The rheological properties of the above three groups were further tested in the liquid of the following formula before and after hot rolling at 120°C for 16 hours. The main and auxiliary emulsifiers with better overall performance were selected. The formula was: 320mL white oil + 12g organic soil + 8g primary emulsifier + 6g auxiliary emulsifier + 80mL CaCl2 solution (25g calcium chloride added to 100mL water) + 8g quicklime. The experimental method is as follows, and the specific results are shown in Table 2.

[0089] Experimental methods:

[0090] ① Use a measuring cylinder to measure 320 mL of white oil, add 8 g of primary emulsifier and 6 g of secondary emulsifier, and stir for 20 minutes;

[0091] ②Add 12g of organic soil and stir for 20 minutes;

[0092] ③ Add 80 mL of CaCl2 solution (25 g of calcium chloride in 100 mL of water) to the solution to prepare an emulsion and stir for 20 minutes;

[0093] ④ Add 8g of quicklime and stir for 20 minutes;

[0094] ⑤Measure the emulsion breaking voltage and viscosity;

[0095] ⑥ Heat the solution at 120℃ for 16 hours and measure the emulsion breaking voltage and viscosity again.

[0096] Table 2 Rheological properties of drilling fluids with different emulsifier combinations

[0097]

[0098]

[0099] It can be seen from the above table that, when using the same formula, the rheological properties of the combination of OME primary emulsifier and OME-2 auxiliary emulsifier before and after aging are better than those of the other two combinations. Therefore, OME and OME-2 are preferred as the primary emulsifier and auxiliary emulsifier of the oil-based drilling fluid system.

[0100] 2. Optimal emulsifier dosage

[0101] The effects of varying amounts of primary and secondary emulsifiers on the performance of oil-based drilling fluids were studied. The basic drilling fluid formula consisted of: 320 mL of white oil + 12 g of organic soil + primary emulsifier + secondary emulsifier + 80 mL of CaCl₂ solution (25 g of calcium chloride in 100 mL of water) + 8 g of quicklime. The primary emulsifier dosage ranged from 8 g to 16 g, and the secondary emulsifier dosage ranged from 4 g to 12 g. The experimental method is as follows, and the specific results are shown in Table 3.

[0102] Experimental methods:

[0103] ① Use a graduated cylinder to measure 320 mL of white oil, add different amounts of primary emulsifier and auxiliary emulsifier, and stir for 20 minutes;

[0104] ②Add 12g of organic soil and stir for 20 minutes;

[0105] ③ Add 80 mL of CaCl2 solution (25 g of calcium chloride in 100 mL of water) to the solution to prepare an emulsion and stir for 20 minutes;

[0106] ④ Add 8g of quicklime and stir for 20 minutes;

[0107] ⑤Measure the emulsion breaking voltage and viscosity;

[0108] ⑥ Heat the solution at 120℃ for 16 hours and measure the emulsion breaking voltage and viscosity again.

[0109] Table 3 Comprehensive performance data of drilling fluid with different emulsifier addition amounts

[0110]

[0111]

[0112] It can be seen from the experimental results that when the addition amount of the main emulsifier is 8g and the addition amount of the auxiliary emulsifier is in the range of 4g to 8g, the system formula can maintain good electrical stability, and the change range of the system formula before and after aging is not large. The optimal addition amount is 8g of the main emulsifier and 4g of the auxiliary emulsifier.

[0113] Experimental Example 2 Optimization of organic soil type and dosage

[0114] 1. Optimal selection of organic soil types

[0115] Organic soil is a lipophilic clay made by the interaction between hydrophilic bentonite and quaternary ammonium salt cationic surfactants. It is easily dispersed in the oil phase and has good viscosity-enhancing and suspension effects, allowing oil-based emulsions to be accurately adjusted like water-based drilling fluids, and playing a stabilizing role in the entire oil-based drilling fluid system. The presence of organic soil can construct a filter cake for the oil-based drilling fluid, and it is the presence of the filter cake that prevents foreign solid and liquid phases from further invading the oil and gas layer.

[0116] Through preliminary research, this experimental example selected 8 kinds of advanced organic soils for oil-based drilling fluids. According to the "QSY 17817-2021 Organic Soil Standard for Oil-Based Drilling Fluids", the organic soil colloid rate experiment was carried out. The results are as follows: Figure 1 、 Figure 2 shown.

[0117] As can be seen from the figure, the colloid rates of the three products BP-324, BP-500A, and LT are above 90%. Therefore, the rheological properties of the oil-based drilling fluid formulas with the above three organic soils added were tested before and after hot rolling at 120°C for 16 hours, and the organic soil with better comprehensive performance was selected. The basic formula of the drilling fluid is: 320mL white oil + 12g organic soil + 8g primary emulsifier + 4g auxiliary emulsifier + 80mL CaCl2 solution (25g calcium chloride added to 100mL water) + 8g quicklime. The experimental method is as follows, and the specific results are shown in Table 4.

[0118] Experimental methods:

[0119] ① Use a graduated cylinder to measure 320 mL of white oil, add 8 g of primary milk and 4 g of secondary milk, and stir for 20 minutes;

[0120] ② Add 12g of organic soil and stir at high speed for 20 minutes;

[0121] ③ Add 80mL of CaCl2 solution (25g of calcium chloride in 100mL of water) to the solution to prepare the emulsion and stir at high speed for 20min;

[0122] ④ Add 8g of quicklime and stir for 20 minutes;

[0123] ⑤Measure the emulsion breaking voltage and viscosity;

[0124] ⑥ Heat the solution at 120℃ for 16 hours and measure the emulsion breaking voltage and viscosity again.

[0125] Table 4 Rheological properties of drilling fluids with different organic soils added

[0126]

[0127] During the experiment, tiny bubbles appeared in BP-500A and LT. As can be seen from the table above, BP-324 has better rheological properties than other three products. Therefore, BP324 is the preferred choice for organic soil in oil-based drilling fluids.

[0128] 2. Optimal amount of organic soil

[0129] In-house, we studied the effects of varying amounts of organic soil on the electrical stability and rheological properties of oil-based drilling fluids. The basic formula: 320 mL of white oil + organic soil + 8 g of primary emulsifier + 4 g of secondary emulsifier + 80 mL of CaCl₂ solution (25 g of calcium chloride in 100 mL of water) + 8 g of quicklime. The experimental method is as follows, and the results are shown in Table 5.

[0130] Experimental methods:

[0131] ① Use a graduated cylinder to measure 320 mL of white oil, add 8 g of primary emulsifier and 4 g of secondary emulsifier, and stir for 20 minutes;

[0132] ②Add different amounts of organic soil and stir for 20 minutes;

[0133] ③ Add 80mL of CaCl2 solution (25g of calcium chloride in 100mL of water) to the solution to prepare the emulsion and stir at high speed for 20min;

[0134] ④ Add 8g of quicklime and stir for 20 minutes;

[0135] ⑤Measure the emulsion breaking voltage and viscosity;

[0136] ⑥ Heat the solution at 120℃ for 16 hours and measure the emulsion breaking voltage and viscosity again.

[0137] Table 5 Electrical stability and rheological properties of drilling fluids with different organic soils added

[0138]

[0139] The data in the table show that organic soil, as an oleophilic colloid, has significant effects on increasing viscosity, improving shear strength, and reducing fluid loss. With increasing organic soil dosage, the system's viscosity and shear strength are significantly improved, as is the demulsification voltage. Overall, when the organic soil dosage is 8g to 16g, the system's rheological properties and electrical stability are good, but the overall economic benefits are better when the organic soil dosage is 12g, so the optimal organic soil dosage is 12g.

[0140] Experimental Example 3 Optimal Type and Dosage of Wetting Agent

[0141] The shear force of oil-based drilling fluid is generally low. If the weighting material and drill cuttings maintain their hydrophilicity and tend to aggregate with the water phase, causing high viscosity and sedimentation, their suspension ability will be poor, resulting in system instability.

[0142] Adding certain wetting agents to oil-based drilling fluids can change the contact angle (wettability) of the solid / liquid interface. These substances will make the solid phase preferentially wetted by oil. Therefore, adding wetting agents to maintain the proper oil wetting state of the solid phase is the guarantee for maintaining a stable emulsion.

[0143] 1. Preferred type of wetting agent

[0144] Through preliminary experimental research, we selected three advanced oil-based drilling fluid wetting agents for use in wetting agent settling experiments. The experimental methods are as follows, and the specific results are shown in Table 6.

[0145] Experimental methods:

[0146] ① Use a graduated cylinder to measure 25 mL of white oil and add 4 g of different wetting agents;

[0147] ②Add 5g of barite powder;

[0148] ③Measure the sedimentation volume of barite powder.

[0149] Table 6 Wetting agent suspension volume data table

[0150] time OW wetting agent (mL) XG wetting agent (mL) LX wetting agent (mL) 0min 25 25 25 10min 24.5 10 24 30min 24 4 23.5 1h 22.5 3.5 12.5 2h 17.5 1.5 2

[0151] Note: OW wetting agent is a phosphate ester anionic surfactant, XG wetting agent is a betaine zwitterionic surfactant, and LX wetting agent is a biosurfactant.

[0152] From the data in the above table, it can be concluded that with the increase of time, the suspended volume of barite powder in OW is the largest and the sedimentation volume is the smallest, so OW wetting agent is preferred.

[0153] 2. Optimal amount of wetting agent

[0154] After determining the dosage ratio of emulsifier and organic soil, wetting agent was added, and the rheological properties, electrical stability and filtration reduction performance of the oil-based drilling fluid formula were tested before and after hot rolling at 120°C for 16 hours to determine its dosage. The basic drilling fluid formula is as follows: 320mL white oil + 12g organic soil + 8g main emulsion + 4g auxiliary emulsion + 80mL CaCl2 solution (25g calcium chloride added to 100mL water) + wetting agent + 8g quicklime + barite, density 1.4g / cm 3 ,The experimental method is as follows, and the specific results are shown in Table 7.

[0155] Experimental methods:

[0156] ① Use a graduated cylinder to measure 320 mL of white oil, add 8 g of primary emulsifier and 4 g of secondary emulsifier, and stir for 20 minutes;

[0157] ②Add different amounts of organic soil and stir at high speed for 20 minutes;

[0158] ③ Add 80 mL of CaCl2 solution (25 g of calcium chloride in 100 mL of water) to the solution to prepare an emulsion and stir for 20 minutes;

[0159] ④ Add 8g of quicklime and stir for 20 minutes;

[0160] ⑤Add different amounts of wetting agent and stir for 20 minutes;

[0161] ⑥Add barite and stir for 20 minutes;

[0162] ⑦Measure the emulsion breaking voltage and viscosity;

[0163] ⑧The solution was hot-rolled at 120℃ for 16 hours, and the high-temperature and high-pressure filtration loss, emulsion breaking voltage and viscosity of the emulsion were measured.

[0164] The results, shown in Table 7, show that a wetting agent dosage of 4g has little effect on the system's performance after aging. However, excessive wetting agent dosages can affect the system's electrical stability and fluid loss performance. During field use, the appropriate wetting agent dosage should be adjusted based on actual conditions such as the weighting material and drill cuttings concentration. The optimal wetting agent dosage in this experiment was 4g.

[0165] Table 7 Effects of different wetting agent additions on drilling fluid properties

[0166]

[0167]

[0168] Experimental Example 4 Optimal filtration loss agent

[0169] The purpose of a fluid loss additive in a system is to reduce fluid loss and increase the stability of the water-in-oil emulsion. Therefore, it is necessary to optimize the fluid loss additive and select the appropriate fluid loss additive and dosage.

[0170] 1. Optimal type of fluid loss reducer

[0171] Different types of fluid loss additives were added to study their effects on the electrical stability, rheological properties, and fluid loss reduction performance of the oil-based drilling fluid. The basic formula was: 320 mL white oil + 12 g organic soil + 8 g primary emulsifier + 4 g auxiliary emulsifier + 80 mL CaCl2 solution (25 g calcium chloride added to 100 mL water) + 4 g wetting agent + 8 g lime + barite. The experimental method is as follows, and the specific results are shown in Table 8.

[0172] Experimental methods:

[0173] ① Use a graduated cylinder to measure 320 mL of white oil, add 8 g of primary emulsifier and 4 g of secondary emulsifier, and stir for 20 minutes;

[0174] ②Add 12g of organic soil and stir for 20 minutes;

[0175] ③ Add 80 mL of CaCl2 solution (25 g of calcium chloride in 100 mL of water) to the solution to prepare an emulsion and stir for 20 minutes;

[0176] ④ Add 12g of fluid loss reducer and stir for 20 minutes;

[0177] ⑤ Add 8g of lime and stir for 20 minutes;

[0178] ⑥Add different amounts of wetting agent and stir for 20 minutes;

[0179] ⑦Add barite and stir for 20 minutes;

[0180] ⑧Measure the emulsion breaking voltage and viscosity;

[0181] ⑨ Heat the solution at 120℃ for 16 hours and measure the high temperature and high pressure filtration loss, emulsion breaking voltage and viscosity of the emulsion.

[0182] Table 8 Drilling fluid performance data with different fluid loss reducers

[0183]

[0184]

[0185] Note: DLJ fluid loss control agent is an organic lignite-based fluid loss control agent, DSY fluid loss control agent is an acrylic acid-humic acid amide-based fluid loss control agent, MOTEX fluid loss control agent is a modified asphalt resin-based fluid loss control agent, and LX is a fluid loss control agent and a humic acid amide-based fluid loss control agent.

[0186] As shown in Table 8, MOTEX has the best fluid loss reduction effect, with a high temperature and high pressure fluid loss of only 2.2 mL. In addition, the demulsification voltage before and after aging is also high. Therefore, MOTEX fluid loss reducer is preferred.

[0187] 2. Optimal dosage of fluid loss reducer

[0188] By varying the amount of MOTEX in the basic formula, the general properties of the water-in-oil emulsion were measured, and the dosage of the treatment agent that provided the best fluid loss reduction was selected. The basic formula: 320 mL of white oil + 12 g of organic clay + 8 g of primary emulsifier + 4 g of secondary emulsifier + 80 mL of CaCl2 solution (25 g of calcium chloride in 100 mL of water) + 4 g of wetting agent + 8 g of lime + barite. The experimental method is as follows, and the specific results are shown in Table 9.

[0189] Experimental methods:

[0190] ① Use a graduated cylinder to measure 320 mL of white oil, add 8 g of primary emulsifier and 4 g of secondary emulsifier, and stir for 20 minutes;

[0191] ②Add 12g of organic soil and stir for 20 minutes;

[0192] ③ Add 80 mL of CaCl2 solution (25 g of calcium chloride in 100 mL of water) to the solution to prepare an emulsion and stir for 20 minutes;

[0193] ④Add different amounts of fluid loss additives and stir for 20 minutes;

[0194] ⑤ Add 8g of lime and stir for 20 minutes;

[0195] ⑥Add different amounts of wetting agent and stir for 20 minutes;

[0196] ⑦Add barite and stir for 20 minutes;

[0197] ⑧Measure the emulsion breaking voltage and viscosity;

[0198] ⑨ Heat the solution at 120℃ for 16 hours and measure the high temperature and high pressure filtration loss, emulsion breaking voltage and viscosity of the emulsion.

[0199] Table 9 Effects of different amounts of fluid loss reducers on drilling fluid properties

[0200]

[0201]

[0202] It can be seen from Table 9 that when the amount of fluid loss reducer is 8g, the system's fluid loss is large and the demulsification voltage is slightly lower. When the amount is 12g to 20g, the demulsification voltage can be stabilized at above 900V, and the high-temperature and high-pressure fluid loss is also stable at about 2.2mL, and the performance is relatively excellent.

[0203] Experimental Example 5 Optimal Selection of Plugging Agents

[0204] Shale gas formations share the following common characteristics: lithology is primarily muddy shale; clay mineral content is high, with a predominance of illite or illite-montmorillonite mixed layers; microcracks and fractures are well-developed; and rock fragments exhibit weak to moderate expansion and lack dispersion. Shale gas formations have well-developed microcracks and fractures, allowing water or drilling fluid filtrate to easily enter, disrupting the original balance of the microcracks. Once completely filled with filtrate, the rock mass undergoes a "hydraulic cutting" process, manifesting macroscopically as shale fragmentation and fragmentation, which can lead to complex downhole conditions. Therefore, in-house research on oil-based drilling fluid plugging agents has been conducted.

[0205] 1. Optimal type of plugging agent

[0206] A visual sand bed filter loss meter was used. The oil-based drilling fluid system was evaluated for permeation loss in a 20-40 mesh sand bed using an oil-based drilling fluid formulation containing four plugging agents, namely HK (rubber powder), emulsified asphalt, DLJ (modified plant fiber plugging agent), and LX (oil-soluble phenolic resin). The fluid loss was evaluated by hot rolling aging at 120°C for 16 hours under conditions of 0.69 MPa for 30 minutes. The basic formulation consisted of 320 mL of white oil, 12 g of organic soil, 8 g of primary emulsifier, 4 g of auxiliary emulsifier, 80 mL of CaCl2 solution (25 g of calcium chloride added to 100 mL of water), 12 g of fluid loss reducer, 4 g of wetting agent, 8 g of lime, barite, and plugging agent. The experimental method was as follows.

[0207] Experimental methods:

[0208] ① Use a graduated cylinder to measure 320 mL of white oil, add 8 g of primary emulsifier and 4 g of secondary emulsifier, and stir for 20 minutes;

[0209] ②Add 12g of organic soil and stir for 20 minutes;

[0210] ③ Add 80 mL of CaCl2 solution (25 g of calcium chloride in 100 mL of water) to the solution to prepare an emulsion and stir for 20 minutes;

[0211] ④ Add 12g of fluid loss reducer and stir for 20 minutes;

[0212] ⑤ Add 8g of lime and stir for 20 minutes;

[0213] ⑥Add 4g of wetting agent and stir for 20 minutes;

[0214] ⑦Add different types of plugging agents and stir for 20 minutes;

[0215] ⑧Add barite and stir for 20 minutes;

[0216] ⑨ Heat the solution at 120°C for 16 hours and measure the depth of its penetration into the sand bed.

[0217] The results are as follows Figure 3 As shown in the figure, it can be seen that emulsified asphalt has the best plugging effect, and the maximum invasion depth is only 4.2 cm, which shows that the oil-based drilling fluid with emulsified asphalt added can form an effective sealing layer in the micro-cracks. Therefore, emulsified asphalt is preferred as the plugging agent.

[0218] 2. Optimal amount of plugging agent

[0219] The oil-based drilling fluid with 4g, 8g, 10g, 12g, 14g, and 16g of plugging agent added was used as the basic formula. The experimental method was as shown above, and the results were as follows: Figure 4 As shown in the figure, when the amount of plugging agent added is 8g, the rate of decrease of invasion depth slows down and can be reduced to 2.8cm at the minimum. The plugging effect is good when the amount of plugging agent added is 8g to 16g.

[0220] Experimental Example 6 Comprehensive Performance Test

[0221] This experimental example compares the comprehensive performance of the oil-based drilling fluids in Examples 1-4 and Comparative Examples 1-6. The specific operation is as follows:

[0222] The oil-based drilling fluids from Examples 1-4 and Comparative Examples 1-6 were placed in an aging tank, rolled at 150°C for 16 hours, removed from the tank, and cooled to room temperature. The drilling fluids were tested for comprehensive properties, including rheological properties and fluid loss control, before and after aging according to GB / T 16783.2-2012 Field Testing of Drilling Fluids in the Petroleum and Natural Gas Industry - Part 2: Oil-Based Drilling Fluids. The results are shown in Table 10.

[0223] Table 10 Comprehensive properties of the oil-based drilling fluids in Examples 1-4 and Comparative Examples 1-6

[0224]

[0225]

[0226] The demulsification voltage of an oil-based drilling fluid refers to the ability of the electric field strength applied to the drilling fluid by the demulsifier to separate water and oil per unit time. A higher value indicates better separation performance. Table 6 shows that the basic performance of the oil-based drilling fluids of Examples 1-4 meets the requirements before and after aging. In comparison, the oil-based drilling fluids of Comparative Examples 1-6 have lower demulsification voltages and higher fluid loss. Among them, in Comparative Example 1, the amount of plugging agent added is insufficient, the plugging effect is slightly poor, and the filtration loss is large; in Comparative Example 2, the amount of organic soil added is insufficient, which affects the stability of the oil-based drilling fluid and the shear force is insufficient; the main functions of the filtration reducer are to reduce the filtration loss of the oil-based drilling fluid system, increase the viscosity of the oil-based drilling fluid system, and increase the suspension performance of the oil-based drilling fluid system. In Comparative Example 3, the amount of filtration reducer added is insufficient, which affects the shear force, viscosity, and demulsification voltage; Comparative Example 4 does not contain a plugging agent, and the demulsification voltage and filtration loss are greatly affected; Comparative Example 5 does not add a filtration reducer, and the amount of emulsified asphalt added is 36g. The rheology and demulsification voltage of the system do not decrease much, but the filtration loss is relatively high; Comparative Example 6 does not add emulsified asphalt, but 36g of filtration reducer increases the viscosity of the system compared to Comparative Example 4, improves the plugging performance to a certain extent, but the filtration loss is still relatively high, and the overall performance is still not as good as the embodiment.

[0227] Experimental Example 7 Inhibition Performance Test

[0228] This experimental example compares the inhibition performance of the oil-based drilling fluids in Examples 1-4 and Comparative Examples 1-6. The specific operation is as follows:

[0229] The inhibition performance of the oil-based drilling fluids in Examples 1-4 and Comparative Examples 1-6 was compared, and this experimental example was evaluated by rolling recovery rate and linear expansion rate.

[0230] 1. Rolling recovery rate evaluation

[0231] The rolling recovery method uses a roller heating furnace to load the drilling fluid to be evaluated into a stainless steel heating tank (which contains rock cores or drill cuttings from a stratum prone to collapse). The tank is then heated at 150°C for 16 hours. The sample recovery rate is then measured. This recovery rate is used to evaluate the drilling fluid system's ability to inhibit the dispersion of the rock sample prone to collapse. Specific results are shown in Table 11.

[0232] 2. Linear expansion rate evaluation

[0233] The linear expansion test utilizes a linear expansion tester. Cores or drill cuttings from a wellbore in a readily expansive formation are pressed into a cylinder of a specified thickness and placed inside a stainless steel test tube. This cylinder is then placed on the instrument. The drilling fluid filtrate to be evaluated is then poured into the test tube. The expansion after 16 hours is measured. The linear expansion percentage is used to characterize the drilling fluid system's ability to suppress the expansive rock sample. Specific results are shown in Table 11.

[0234] Table 11 Inhibition properties of oil-based drilling fluids in Examples 1-4 and Comparative Examples 1-6

[0235] Serial number Rolling recovery rate (%) Linear expansion rate (%) clear water 49.8 88.3 Example 1 98.7 3.1 Example 2 98.1 3.5 Example 3 96.9 4.8 Example 4 97.2 4.1 Comparative Example 1 95.1 12.2 Comparative Example 2 93.0 12.6 Comparative Example 3 92.2 14.4 Comparative Example 4 91.4 15.1 Comparative Example 5 93.0 13.5 Comparative Example 6 93.2 12.8

[0236] As shown in Table 11, the oil-based drilling fluid system suitable for shale oil and gas wells of the present invention has good performance in inhibiting the hydration expansion of clay minerals, with a shale rolling recovery rate of 98.7% and a linear expansion rate as low as 3.1%.

[0237] Experimental Example 8: Plugging Performance Test

[0238] This experimental example evaluates the plugging performance of the oil-based drilling fluids in Examples 1-4 and Comparative Examples 1-6 through a sand bed test. The specific operation is as follows:

[0239] Take 500cm respectively 3 The drilling fluid prepared in the examples and comparative examples was used as standby. A visual sand bed filter loss meter was used to take 150 cm of 75 mesh cuttings from the easily collapsed layer. 3 、200 mesh rock chips 100cm 3 and 1000 mesh rock cuttings 100cm 3 The mixture was stirred evenly and poured into a cylindrical, see-through drilling fluid cup to simulate the heterogeneous porosity of shale formations. The prepared drilling fluid was then added. The cup lid was tightened, and the air source was connected to adjust the pressure to 0.69 MPa. The air valve was opened to allow air to enter the drilling fluid cup, simulating the state of drilling fluid being squeezed and seeping into the wellbore wall. The penetration depth of the drilling fluid was observed and recorded through the cup. A comparison of the drilling fluid's plugging performance is shown in Table 12.

[0240] Table 12 Plugging performance of different drilling fluid systems

[0241] Serial number Sand bed penetration depth (cm) Example 1 0.8 Example 2 1.0 Example 3 1.3 Example 4 1.2 Comparative Example 1 4.1 Comparative Example 2 4.2 Comparative Example 3 4.6 Comparative Example 4 4.8 Comparative Example 5 4.0 Comparative Example 6 4.2

[0242] It can be seen from Table 12 that the sand bed invasion depth of the oil-based drilling fluid system suitable for shale oil and gas wells of the present invention is as low as 0.8 cm, and has good sealing performance.

[0243] In summary, the oil-based drilling fluid of the present invention has good plugging and inhibition properties and is suitable for efficient drilling of shale oil and gas wells.

[0244] The above description is merely a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by a person skilled in the art within the technical scope disclosed in the embodiments of the present invention should be included in the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be based on the scope of protection of the claims.

Claims

1. An oil-based drilling fluid suitable for shale oil and gas wells, characterized by: The invention is mainly composed of a water-in-oil base liquid, a primary emulsifier, an auxiliary emulsifier, organic soil, a fluid loss reducer, a wetting agent, quicklime, and a plugging agent. For every 100 mL of the water-in-oil base liquid, 2-4 g of organic soil, 3-5 g of the fluid loss reducer, and 2-4 g of the plugging agent are added. The mass ratio of the base oil to the brine in the water-in-oil base liquid is (7-8):(2-3). The plugging agent is emulsified asphalt. The fluid loss reducer is a modified asphalt resin-based fluid loss reducer.

2. The oil-based drilling fluid suitable for shale oil and gas wells according to claim 1, characterized in that: The primary emulsifier is a long-chain alkyl acid salt; the secondary emulsifier is a fatty alcohol polyether; and (1-2) g of the primary emulsifier and (1-2) g of the secondary emulsifier are added to every 100 mL of the water-in-oil base liquid.

3. The oil-based drilling fluid suitable for shale oil and gas wells according to claim 1, characterized in that: The base oil is one of white oil or diesel.

4. The oil-based drilling fluid suitable for shale oil and gas wells according to claim 1, characterized in that: The wetting agent is a phosphate anionic surfactant; 1-2 g of the wetting agent is added to every 100 mL of the water-in-oil base liquid.

5. The oil-based drilling fluid suitable for shale oil and gas wells according to claim 1, characterized in that: Add (1-3) g of quicklime for every 100 mL of water-in-oil base liquid.

6. The oil-based drilling fluid suitable for shale oil and gas wells according to any one of claims 1 to 5, characterized in that: The brine is an aqueous solution formed by adding (20-25) g of calcium chloride to 100 mL of water.

7. The oil-based drilling fluid suitable for shale oil and gas wells according to claim 6, characterized in that: The density of the oil-based drilling fluid is (1.20-1.40) g / cm 3 .

8. A method for preparing an oil-based drilling fluid suitable for shale oil and gas wells according to any one of claims 1 to 7, characterized in that: The following steps are involved: (1) adding a primary emulsifier and a secondary emulsifier to the base oil, mixing them evenly, and adding salt water to obtain a water-in-oil emulsion; (2) adding organic soil, fluid loss reducer, wetting agent, quicklime, plugging agent and barite to the water-in-oil emulsion in step (1) and mixing them evenly to obtain the product.

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