A plugging agent, synthetic-based drilling fluid, and preparation method and application thereof

Abstract

The present invention relates to a plugging agent, a synthetic-based drilling fluid for drilling in high-pressure reservoirs with a narrow safety density window, and a preparation method and application thereof. The synthetic-based drilling fluid comprises, by mass, 25 to 30 parts of base oil; 1 to 2 parts of a primary emulsifier; 1 to 2 parts of an auxiliary emulsifier; 1 to 2 parts of a wetting agent; 2 to 4 parts of calcium chloride brine; 1 to 3 parts of quicklime; 1 to 2 parts of organic bentonite; 2 to 4 parts of a fluid loss reducer; 1 to 2 parts of a plugging agent; and a density regulator added according to density requirements. The formula of the synthetic-based drilling fluid provided by the present invention can reduce the bottom hole circulating pressure loss equivalent drilling fluid density to 0.05 g / cm after using a vibrating screen (such as 325 mesh) to enhance solid control. 3 the following.

Description

Technical Field

[0001] The present invention relates to the technical field of drilling fluids, and in particular to a plugging agent, a synthetic-based drilling fluid for drilling in a narrow safety density window high-pressure reservoir, and a preparation method and application thereof. Background Art

[0002] The offshore oil fields of Nigeria and Cameroon are located in the Niger Delta of West Africa, with water depths of 30 to 200 meters. They are block oil and gas fields with developed faults, broken formations, and easy collapse and leakage. The formation pressure coefficient in the reservoir section is as high as 2.0 MPa, and the safe drilling equivalent drilling fluid density window can be as narrow as 0.1 g / cm 3 When Western companies drilled in this oil field, the loss of synthetic drilling fluid in a single well was more than 500m 3 When the density of synthetic drilling fluid exceeds 2.00g / cm 3 The fluidity is poor, and the bottom hole pressure loss during circulation is 0.15-0.2g / cm higher than that during static state. 3 When overflow pressure occurs, the pump leaks when it is turned on and surges when it is stopped. Furthermore, the drilling fluid has poor sedimentation stability after being contaminated by oil and gas, and the weighting agent has severe sedimentation. However, the current technologies for expanding the safe density window of water-based and diesel-based drilling fluids, such as pressure-bearing plugging agents and viscosity reduction, are not suitable for synthetic-based drilling fluids. The currently used high-density synthetic-based drilling fluids cannot solve the problems of well leakage in high-pressure reservoirs with narrow safe density windows, nor the sedimentation stability of synthetic-based drilling fluids after being contaminated by oil and gas.

[0003] The patent "An ultra-high density environmentally friendly gas-to-liquid synthetic-based drilling fluid and its preparation method" (CN201910777190.0) introduces a synthetic-based drilling fluid composed of gas-to-liquid, chloride brine solution, organic soil, emulsifier, calcium oxide, fluid loss additive and barite. The density can reach 2.6g / cm 3 , high temperature and high pressure filtration loss ≤ 1.5mL, filter cake thickness 1.5mm. This synthetic-based drilling fluid has good rheological properties under high temperature and high pressure conditions, meeting the environmental protection requirements of various regions.

[0004] The patent, "A High-Temperature-Resistant, High-Density Synthetic-Based Drilling Fluid" (CN201210020644.8), describes a synthetic-based drilling fluid composed of a base fluid, a CaCl2 aqueous solution, a primary emulsifier, a secondary emulsifier, a wetting agent, a viscosity enhancer, a temperature-resistant agent, a fluid loss reducer, organic soil, an alkalinity regulator, and a weighting agent. This fluid addresses the challenges of cuttings carryover and poor lubricity during drilling, and can reduce the risk of differential pressure sticking in extended-reach and ultra-deep horizontal wells.

[0005] In general, the existing synthetic-based drilling fluid technology cannot meet the requirements for safe drilling in high-pressure reservoirs with a narrow safety density window. Summary of the Invention

[0006] In order to overcome the problems existing in the prior art, the present invention provides a plugging agent, a synthetic-based drilling fluid for use in a narrow safety density window, and a preparation method and application thereof. For example, one of the purposes of the present invention is to provide a plugging agent that is added to a synthetic-based drilling fluid for plugging while drilling, which can improve the pressure-bearing capacity of a reservoir with a narrow safety density window and reduce the loss of the synthetic-based drilling fluid. For another example, another purpose of the present invention is to provide a synthetic-based drilling fluid for use in a narrow safety density window, which can solve the problems of loss of synthetic-based drilling fluid when drilling in a high-pressure reservoir with a narrow safety density window, sedimentation of weighting agents after the drilling fluid is contaminated, and other problems. For another example, another purpose of the present invention is to provide a preparation method for a synthetic-based drilling fluid corresponding to the above-mentioned synthetic-based drilling fluid.

[0007] To achieve the above object, the first aspect of the present invention provides a plugging agent, the raw materials of which include synthetic fiber, graphite, calcium carbonate and SPAN80 in a mass ratio of 70-80:10-15:10-15:1-3.

[0008] In some embodiments of the present invention, the length of the synthetic fiber is 0.1 to 3 mm and the diameter is 20 to 25 μm; preferably, the synthetic fiber is selected from one or more of spandex fiber, acrylic fiber and polyester fiber.

[0009] According to the present invention, there are no other strict restrictions on spandex fiber, acrylic fiber and polyester fiber. Conventional commercially available products that meet the above-mentioned limiting conditions are applicable to the present invention, and those skilled in the art can also select them according to actual conditions.

[0010] In some embodiments of the present invention, the graphite has a particle size of 0.1 to 2 mm and a particle size D50 of 1 mm.

[0011] In some embodiments of the present invention, the calcium carbonate has a particle size of 0.02 to 0.2 mm and a particle size D50 of 0.1 mm.

[0012] According to the present invention, SPAN80 (sorbitan monooleate) is a liquid nonionic, oil-soluble surfactant. There are no other strict restrictions on SPAN80. Any conventional commercially available product that meets the aforementioned requirements is suitable for use in the present invention, and those skilled in the art may also select a suitable surfactant based on actual circumstances. The SPAN80 in the plugging agent of the present invention can characterize the plugging agent surface, preventing the plugging agent from affecting the electrical stability of the synthetic-based drilling fluid.

[0013] The plugging agent provided by the present invention is added to the synthetic-based drilling fluid for plugging while drilling, which can improve the pressure bearing capacity of the narrow safety density window reservoir and reduce the loss of the synthetic-based drilling fluid.

[0014] The second aspect of the present invention provides a synthetic-based drilling fluid for a narrow safe density window, comprising the plugging agent as described in the first aspect above.

[0015] In some embodiments of the present invention, the raw materials of the synthetic-based drilling fluid include, by weight:

[0016]

[0017] and optionally a density modifier.

[0018] In the present invention, the density regulator is added according to the density requirement of the synthetic-based drilling fluid in actual application.

[0019] In some embodiments of the present invention, the density regulator is modified ultrafine iron ore powder; preferably, the modified ultrafine iron ore powder is formed by modification with octadecyl ammonium bromide and / or polyoxyethylene ether diammonium.

[0020] In some embodiments of the present invention, the method for preparing the modified ultrafine iron ore powder comprises: grinding ultrafine iron ore powder, octadecyl ammonium bromide and polyoxyethylene ether diammonium in a gas-to-liquids process to obtain the modified ultrafine iron ore powder.

[0021] In some embodiments of the present invention, the volume ratio of the gas-to-liquids, ultrafine iron ore powder, octadecyl ammonium bromide, and polyoxyethylene ether diammonium is 80-120:50-80:5-7:3-5.

[0022] In the present invention, when grinding ultrafine iron ore powder, octadecyl ammonium bromide and polyoxyethylene ether diammonium in gas-to-liquids, the grinding is stopped when the powder passes through a 3500-mesh sieve cloth to obtain modified ultrafine iron ore powder.

[0023] In the present invention, there is no strict restriction on the grinding time of the ultrafine iron ore powder, octadecyl ammonium bromide and polyoxyethylene ether diammonium in the gas-to-liquids process, as long as the powder can pass through a 3500-mesh sieve.

[0024] In some embodiments of the present invention, the average particle size of the ultrafine iron ore powder is 1 to 3 μm, and the particle size D50 is 1.5 μm.

[0025] In some embodiments of the present invention, the density of the density regulator is 1.6 to 3.0 g / cm 3 .

[0026] The present invention uses modified ultrafine iron ore powder instead of traditional barite as a density regulator to reduce the solid content of high-density drilling fluid. The synthetic-based drilling fluid prepared by combining other raw materials (such as emulsifiers, etc.) can reduce the bottom hole circulation pressure loss to 0.05g / cm after using a vibrating screen (such as 325 mesh) to strengthen solid control. 3The following; using ultrafine iron ore powder with an average particle size of 1 to 3 μm can solve the problem of weighting agent sedimentation after the drilling fluid in the narrow safe density window reservoir is contaminated by oil and gas.

[0027] In the present invention, the base oil is a specialized oil used to prepare synthetic-based drilling fluids. It is a commonly available commercial product and is not strictly limited in this application. Those skilled in the art will be able to flexibly select the base oil. For example, the base oil can be gas-to-liquid (GTL), a liquid oil produced from natural gas. It is non-toxic, biodegradable, and has a high flash point, making it suitable for offshore drilling. It is available from Total, France. This base oil can be the same GTL used in the preparation of modified ultrafine iron ore powder. It is understood that the above-mentioned raw materials are not limited to these, and those skilled in the art will be able to flexibly select the base oil.

[0028] In some embodiments of the present invention, the primary emulsifier is selected from one or more of polyamide carboxylic acid, calcium naphthenate and polyamide.

[0029] In the present invention, polyamide carboxylic acid is a condensation product of oleic acid and polyethylene polyamine, an oil-soluble nonionic emulsifier, sourced from the Sinopec Petroleum Engineering Technology Research Institute; calcium naphthenate is an ionic surfactant obtained by saponifying cyclohexane acid with sodium hydroxide and then performing a metathesis reaction using a calcium salt, and is commercially available; polyamide is a polyamide product obtained by reacting pentaerythritol with acrylonitrile, resulting in a star-shaped polycarboxylic acid, with hexadecylamine, and is an oil-soluble emulsifier, sourced from the Sinopec Petroleum Engineering Technology Research Institute. It is understood that the above raw materials are not limited thereto, and those skilled in the art can flexibly select from them.

[0030] In some embodiments of the present invention, the auxiliary emulsifier is selected from one or more of sodium dodecylbenzenesulfonate and sorbitan monooleate.

[0031] In the present invention, sodium dodecylbenzenesulfonate is prepared by sulfonating dodecylbenzene with oleum and then neutralizing it with caustic soda. It is an anionic surfactant available through conventional commercial channels. Sorbitan monooleate is an oil-soluble nonionic surfactant available under the trade name SPAN80 and is available through conventional commercial channels. It is understood that the above raw materials are not limited to these, and those skilled in the art may flexibly select from them.

[0032] It should be noted that auxiliary emulsifiers such as SPAN80 help synthetic-based drilling fluids form oil-in-water emulsions and improve the emulsification effect.

[0033] In some embodiments of the present invention, the wetting agent is selected from one or more of octylphenol polyoxyethylene ether and sorbitan monolaurate.

[0034] In the present invention, octylphenol polyoxyethylene ether is an oil-soluble nonionic surfactant, trade name OP-4, available through conventional commercial channels; sorbitan monolaurate is an oil-soluble nonionic surfactant, trade name SP-20, available through conventional commercial channels. It is understood that the above raw materials are not limited to these, and those skilled in the art can flexibly select them.

[0035] In some embodiments of the present invention, the mass concentration of the calcium chloride brine is 25% to 30%.

[0036] According to the present invention, quicklime and organobentonite are substances that can be understood by those skilled in the art. The present invention has no strict restrictions on quicklime and organobentonite, and those skilled in the art can select them according to specific circumstances.

[0037] In some embodiments of the present invention, the fluid loss reducer is selected from one or more of oxidized asphalt, modified humic acid, natural asphalt and cationic emulsified asphalt.

[0038] In this invention, oxidized asphalt is an asphalt oxidation product with a softening point of 110-150°C. It is highly dispersible in oil and is used as a fluid loss reducer in synthetic drilling fluids. It is sourced from the Sinopec Petroleum Engineering Technology Research Institute. Modified humic acid is a humic acid amide fluid loss reducer, a product of the reaction between humic acid chloride and octadecylamine. It is used as a fluid loss reducer in synthetic drilling fluids with minimal impact on drilling fluid viscosity and is available from Kaiping Zhongmao Import and Export Co., Ltd. Natural asphalt is the product of the oxidation of the heavy components of crude oil after the light components evaporate. It is commercially available. Cationic emulsified asphalt is produced through a specialized process using a combination of surfactants, cationic shale inhibitors, and asphalt with a specific softening point range. It is available from Shandong Kekai Petroleum Technology Co., Ltd. It is understood that these raw materials are not limited to these, and those skilled in the art can flexibly select from these raw materials.

[0039] The third aspect of the present invention provides a method for preparing the synthetic-based drilling fluid as described in the second aspect above, comprising the steps of:

[0040] The synthetic drilling fluid is obtained by mixing base oil, primary emulsifier, secondary emulsifier and wetting agent, and then sequentially adding calcium chloride brine, quicklime, organic bentonite, fluid loss reducer, plugging agent and optional density regulator and mixing.

[0041] In the present invention, there is no strict limitation on the temperature, rotation speed and mixing time during mixing, and those skilled in the art can determine them according to actual conditions.

[0042] In some embodiments of the present invention, the method for preparing the synthetic-based drilling fluid comprises the steps of:

[0043] Step 1: Take 25-30 parts by mass of gas-based oil and put it in a high-stirring cup, add 1-2 parts by mass of primary emulsifier, 1-2 parts by mass of auxiliary emulsifier, and 1-2 parts by mass of wetting agent, and stir at high speed for 10-20 minutes at a speed of 10000-12000 r / min;

[0044] Step 2: Add 1-2 parts by mass of calcium chloride brine and stir at high speed for 10-20 minutes at a speed of 10,000-12,000 r / min;

[0045] Step 3: Add 1 to 2 parts by mass of quicklime and stir at high speed for 10 to 20 minutes at a speed of 10,000 to 12,000 r / min;

[0046] Step 4: Add 1 to 2 parts by mass of organic bentonite and stir at high speed for 15 to 20 minutes at a speed of 10,000 to 12,000 r / min;

[0047] Step 5: Add 2 to 4 parts by mass of high-temperature resistant fluid loss reducer and 1 to 2 parts by mass of plugging agent, and stir at high speed for 10 to 20 minutes at a speed of 10,000 to 12,000 r / min;

[0048] Step 6: Add density regulator in corresponding parts by mass according to density requirements, and stir at high speed for 20 to 30 minutes at a speed of 10,000 to 12,000 r / min to prepare a synthetic drilling fluid.

[0049] Any numerical value mentioned in the present invention includes all values ​​from the lowest value to the highest value in increments of one unit if there is only a two-unit interval between any minimum value and any maximum value. For example, if the amount of a component, or the value of a process variable such as temperature, pressure, or time is stated to be 25-30, it is meant in this specification to specifically list values ​​such as 26-29, ... and 27-28. For non-integer values, it is appropriate to consider units of 0.1, 0.01, 0.001, or 0.0001. These are just some specifically indicated examples. In this application, in a similar manner, all possible combinations of numerical values ​​between the lowest and highest values ​​listed are considered to have been disclosed.

[0050] A fourth aspect of the present invention provides a use of the synthetic-based drilling fluid described in the second aspect in drilling in a high-pressure reservoir with a narrow safety density window.

[0051] Compared with the prior art, the present invention has at least one of the following beneficial effects:

[0052] 1) The plugging agent provided by the present invention is prepared to obtain a synthetic-based drilling fluid for plugging while drilling, which can improve the pressure bearing capacity of the narrow safety density window reservoir and reduce the loss of the synthetic-based drilling fluid;

[0053] 2) The synthetic-based drilling fluid formula provided by the present invention can reduce the bottom hole circulation pressure loss equivalent drilling fluid density to 0.05 g / cm after using a vibrating screen (such as 325 mesh) to enhance solid control. 3 the following;

[0054] 3) The synthetic-based drilling fluid provided by the present invention can replace traditional barite by surface-modified ultrafine iron ore powder, thereby reducing the solid content of high-density drilling fluid;

[0055] 4) The synthetic drilling fluid provided by the present invention can solve the problems of high formation pressure coefficient (such as 2.0) and narrow safety density window (such as 0.1g / cm 3 ) oilfield reservoirs faced with the problems of synthetic-based drilling fluid loss and weighting agent sedimentation after the drilling fluid is contaminated by oil and gas. DETAILED DESCRIPTION

[0056] The present invention is described in detail below through examples, but the protection scope of the present invention is not limited to the following description.

[0057] The synthetic drilling fluids prepared in the following examples are used in the offshore oil fields of Nigeria and Cameroon, which are located in the Niger Delta in West Africa, covering an area of ​​4200 km. 2 The offshore oilfield has a water depth of 30 to 200 meters. There are 5 major faults and more than 10 minor faults in the oilfield, which is a block oil and gas reservoir with well-developed faults and broken formations. The safe drilling window density of the reservoir is as low as 0.1g / cm 3 However, it should be noted that the synthetic-based drilling fluid of the present invention is not limited thereto and can be used in oil and gas fields with the same or similar narrow safety density window and high-pressure reservoirs.

[0058] In the following examples, if the specific conditions are not specified, the experiments were carried out under conventional conditions or those recommended by the manufacturer. All reagents or instruments used, if the manufacturer is not specified, are commercially available conventional products.

[0059] In the following examples, the base oil used is natural gas oil, trade name EDC95-11, from Total, France; the primary emulsifier is polyamide carboxylic acid, from Sinopec Petroleum Engineering Technology Research Institute; the secondary emulsifier is sorbitan monooleate (i.e., SPAN80), from Sinopec Petroleum Engineering Technology Research Institute; the wetting agent is sorbitan monolaurate, from Sinopec Petroleum Engineering Technology Research Institute; the mass concentration of calcium chloride brine is 25% to 30%; the quicklime is a conventional commercial product; the organobentonite is cationic organobentonite, from Sinopec Nanjing Chemical Research Institute; the fluid loss reducer is oxidized asphalt, from Sinopec Petroleum Engineering Technology Research Institute; synthetic fibers are spandex, acrylic, and polyester fibers in a mass ratio of 1:1:1, with a diameter of 0.1 to 3 mm and a diameter of 20 to 25 μm, sourced from Shandong Kekai Petroleum Technology Company; graphite has a particle size of 0.1 to 2 mm and a D50 of 1 mm, sourced from Shandong Kekai Petroleum Technology Company; calcium carbonate has a particle size of 0.02 to 0.2 mm and a D50 of 0.1 mm, sourced from Shandong Kekai Petroleum Technology Company; SPAN80 comes from the Sinopec Petroleum Engineering Technology Research Institute; ultrafine iron ore powder has an average particle size of 1 to 3 μm, with a D50 particle size of 1.5 μm, sourced from the Sinopec Petroleum Engineering Technology Research Institute.

[0060] Example 1

[0061] The plugging agent of this embodiment can be prepared by mixing synthetic fiber, graphite powder, calcium carbonate and SPAN80 in a mass ratio of 70:15:15:2.

[0062] The density regulator of this embodiment is modified ultrafine iron ore powder. The volume ratio of gas-to-liquid, ultrafine iron ore powder, octadecyl ammonium bromide, and polyoxyethylene ether diammonium is 90:80:5:3. The ultrafine iron ore powder, octadecyl ammonium bromide, and polyoxyethylene ether diammonium are ground in gas-to-liquid until they can pass through a 3500 mesh sieve. The density is 2.7 g / cm 3 Modified ultrafine iron ore powder.

[0063] The preparation of the synthetic-based drilling fluid of this embodiment includes:

[0064] Step 1: Take 30 parts by mass of gas-based oil and put it in a high-stirring cup, add 2 parts by mass of primary emulsifier, 2 parts by mass of auxiliary emulsifier, and 2 parts by mass of wetting agent, and stir at high speed for 10 minutes at a speed of 12000 r / min;

[0065] Step 2: Add 1 part by mass of calcium chloride saline and stir at high speed for 10 minutes at a speed of 12000 r / min;

[0066] Step 3: Add 2 parts by mass of quicklime and stir at high speed for 10 minutes at a speed of 12000 r / min;

[0067] Step 4: Add 1 part by mass of organic bentonite and stir at high speed for 20 minutes at a speed of 12000 r / min;

[0068] Step 5: Add 3 parts by mass of high-temperature resistant fluid loss additive and 1 part by mass of plugging agent, and stir at high speed for 10 minutes at a speed of 12000 r / min;

[0069] Step 6: Add 270 parts by mass of a density regulator and stir at high speed for 30 minutes at a rotation speed of 12,000 r / min to prepare a synthetic drilling fluid A1.

[0070] The properties of the prepared synthetic-based drilling fluid A1 were tested according to GB / T 16783.2-2012 Field Testing of Drilling Fluids in the Petroleum and Natural Gas Industry Part 2: Testing of Oil-Based Drilling Fluids. The bottomhole circulation pressure loss was calculated using the API RP 13D Rheology and Hydraulics of Oil-well Drilling Fluids method. The results are shown in Table 1 below.

[0071]

[0072] Example 2

[0073] Example 2 is substantially the same as Example 1, except that:

[0074] The density regulator in step 6 is added in an amount of 150 parts by mass to prepare a synthetic-based drilling fluid A2.

[0075] The prepared synthetic-based drilling fluid A2 was tested according to the method in Example 1 above, and the test results are shown in Table 2 below.

[0076]

[0077] Example 3

[0078] Example 3 is basically the same as Example 1, except that:

[0079] The amount of plugging agent added in step 5 is 2 parts by mass to prepare synthetic-based drilling fluid A3.

[0080] The prepared synthetic-based drilling fluid A3 was tested according to the method in Example 1 above, and the test results are shown in Table 3 below.

[0081]

[0082] Comparative Example 1

[0083] The raw materials used in Comparative Example 1 include gas-to-liquids, a primary emulsifier, a secondary emulsifier, a wetting agent, calcium chloride brine, quicklime, organic bentonite, a fluid loss reducer and conventional barite. The barite is barium sulfate with a particle size of 35 to 75 μm, of which the D50 particle size is 48 μm, and is from Shandong Kekai Petroleum Technology Co., Ltd.

[0084] The preparation of the synthetic-based drilling fluid of Comparative Example 1 includes:

[0085] Step 1: Take 30 parts by mass of gas-based oil and put it in a high-stirring cup, add 1 part by mass of primary emulsifier, 1 part by mass of auxiliary emulsifier, and 1 part by mass of wetting agent, and stir at high speed for 10 minutes at a speed of 12000 r / min;

[0086] Step 2: Add 6 parts by mass of calcium chloride brine and stir at high speed for 10 minutes at a speed of 12000 r / min;

[0087] Step 3: Add 1 part by mass of quicklime and stir at high speed for 10 minutes at a speed of 12000 r / min;

[0088] Step 4: Add 3 parts by mass of organic bentonite and stir at high speed for 20 minutes at a speed of 12000 r / min;

[0089] Step 5: Add 2 parts by mass of high temperature resistant fluid loss additive and stir at high speed for 10 minutes at a speed of 12000 r / min;

[0090] Step 6: Add 92 parts by mass of barite and stir at high speed for 30 minutes at a speed of 12000 r / min.

[0091] Made into synthetic-based drilling fluid B1.

[0092] The properties of the prepared synthetic-based drilling fluid B1 were tested according to GB / T 16783.2-2012 Field Testing of Drilling Fluids in the Petroleum and Natural Gas Industry Part 2: Testing of Oil-Based Drilling Fluids. The bottomhole circulating pressure loss was calculated using the API RP 13D Rheology and Hydraulics of Oil-well Drilling Fluids method. The results are shown in Table 4 below.

[0093]

[0094]

[0095] Test Case

[0096] The anti-pollution and sand bed plugging capabilities of synthetic-based drilling fluid A1 obtained in Example 1 and synthetic-based drilling fluid B1 obtained in Comparative Example 1 were tested. These were determined in accordance with GB / T 16783.2-2012, Field Testing of Drilling Fluids in the Petroleum and Natural Gas Industry - Part 2: Oil-Based Drilling Fluids, and SY / T 5840-2007, Indoor Test Methods for Bridging and Plugging Materials for Drilling Fluids, respectively.

[0097] The measured results are shown in Tables 5 and 6 below:

[0098]

[0099]

[0100] In Table 6, multi-graded calcium carbonate is a compound product of calcium carbonate with different particle sizes, with a particle size range of 0.01 to 0.20 mm and a particle size D90 of 0.18 mm. The dosage is 3% of the drilling fluid (weight to volume ratio), and it is from Shandong Kekai Petroleum Technology Co., Ltd. under the trade name of multi-graded calcium carbonate; conventional while-drilling plugging agent is plant fiber with a size of 0.05 to 0.20 mm and a particle size D90 of 0.18 mm. The dosage is 3% of the drilling fluid (weight to volume ratio), and it is from Shandong Kekai Petroleum Technology Co., Ltd. under the trade name of pressure plugging agent.

[0101] Comparing Tables 5 and 6 above, we can see that synthetic-based drilling fluid B1 exhibits poor sedimentation stability after being contaminated by oil and gas, resulting in severe weighting agent sedimentation. This results in poor plugging effectiveness against 20-40 mesh sand beds, making it unsuitable for drilling in reservoirs with a narrow safe density window. The synthetic-based drilling fluid A1 developed in this application exhibits improved resistance to oil and gas contamination and weighting agent sedimentation. Its sand bed has a pressure bearing capacity of 7 MPa, making it suitable for drilling in reservoirs with a narrow safe density window.

[0102] The synthetic drilling fluid B1 prepared in Comparative Example 1 was used in the FOX-Z well in the offshore oil fields of Nigeria and Cameroon. A 200-mesh vibrating screen was used to drill the narrow safety density window (0.1 g / cm 3 ) reservoir, leakage and gushing problems were prominent, and a total loss of synthetic drilling fluid reached 646m during drilling. 3 During the well kick and well killing period, oil and gas invaded, the weighting agent settled, 700m of drilling tools were buried, the oil well was scrapped, and the economic losses were huge.

[0103] The synthetic drilling fluid A1 prepared in Example 1 was used in the Hotel 13 well (on the same platform as the FOX-Z well) in the Nigerian and Cameroon offshore oil fields. A 325-mesh vibrating screen was used to drill the narrow safety density window (0.1 g / cm 3 ) reservoir, there is no leakage or gushing problem, the drilling fluid performance is stable, and there is no weighting agent settling phenomenon. During the drilling period, a total of 143m3 of synthetic drilling fluid was consumed. 3The leakage rate was reduced by 77.4% compared with the FOX-Z well. The drilling was smooth and no complicated situations occurred throughout the well. The non-production time was 23%, which was reduced by 49% compared with the FOX-Z well.

[0104] In summary, when comparing Example 1 with Comparative Example 1, the synthetic-based drilling fluid A1 of Example 1 is used, the bottom hole circulation pressure loss equivalent drilling fluid density is increased from 0.2 g / cm 3 Reduced to 0.05g / cm 3 The synthetic-based drilling fluid can plug a 20-40 mesh sand bed, and the resulting plugging layer has a pressure bearing capacity of up to 7 MPa. In addition, the synthetic-based drilling fluid A1 can resist 20% oil contamination, has no weighting agent sedimentation, and its drilling fluid performance can meet the requirements of safe drilling.

[0105] It should be noted that the embodiments described above are only used to explain the present invention and do not constitute any limitation of the present invention. The present invention has been described with reference to typical embodiments, but it should be understood that the words used therein are descriptive and explanatory words, rather than restrictive words. The present invention may be modified as specified within the scope of the claims of the present invention, and the present invention may be revised without departing from the scope and spirit of the present invention. Although the present invention described therein relates to specific methods, materials and embodiments, it does not mean that the present invention is limited to the specific examples disclosed therein. On the contrary, the present invention can be extended to all other methods and applications with the same function.

Claims

1. A synthetic-based drilling fluid for a narrow safety density window, characterized in that: The raw materials of the synthetic drilling fluid include, by weight: 25~30 parts of base oil; 1~2 parts of main emulsifier; 1~2 parts of auxiliary emulsifier; 1~2 parts of wetting agent; 2-4 parts calcium chloride brine; 1-3 parts quicklime; 1~2 parts of organic bentonite; 2~4 parts of fluid loss additive; 1~2 parts of leak-proof agent; and density regulators, The raw materials of the plugging agent include synthetic fiber, graphite, calcium carbonate and SPAN80 in a mass ratio of 70-80:10-15:10-15:1-3; The density regulator is modified ultrafine iron ore powder surface-modified with octadecyl ammonium bromide and polyoxyethylene diamine. The preparation method of the modified ultrafine iron ore powder comprises: The ultrafine iron ore powder, octadecyl ammonium bromide and polyoxyethylene diamine are ground in a gas-to-liquids process to obtain the modified ultrafine iron ore powder. The volume ratio of the gas-to-liquids, ultrafine iron ore powder, octadecyl ammonium bromide, and polyoxyethylene diamine is 80-120:50-80:5-7:3-5.

2. The synthetic-based drilling fluid according to claim 1, characterized in that The average particle size of the ultrafine iron ore powder is 1-3 μm.

3. The synthetic-based drilling fluid according to any one of claims 1-2, characterized in that The density of the density regulator is 1.6-3.0 g / cm 3 .

4. The synthetic-based drilling fluid according to any one of claims 1-2, characterized in that The primary emulsifier is selected from one or more of polyamide carboxylic acid, calcium naphthenate and polyamide; And / or, the auxiliary emulsifier is selected from one or more of sodium dodecylbenzenesulfonate and sorbitan monooleate; And / or, the wetting agent is selected from one or more of octylphenol polyoxyethylene ether and sorbitan monolaurate; And / or, the mass concentration of the calcium chloride brine is 25% to 30%; And / or, the fluid loss reducer is selected from one or more of oxidized asphalt, modified humic acid, natural asphalt and cationic emulsified asphalt.

5. The synthetic-based drilling fluid according to claim 1, characterized in that: The length of the synthetic fiber is 0.1-3 mm and the diameter is 20-25 μm; And / or, the particle size of the graphite is 0.1-2 mm; And / or, the particle size of the calcium carbonate is 0.02-0.2 mm.

6. The synthetic-based drilling fluid according to claim 5, characterized in that The synthetic fiber is selected from one or more of spandex fiber, acrylic fiber and polyester fiber.

7. A method for preparing a synthetic-based drilling fluid according to any one of claims 1 to 6, comprising the steps of: The synthetic drilling fluid is obtained by mixing base oil, primary emulsifier, auxiliary emulsifier and wetting agent, and then sequentially adding calcium chloride brine, quicklime, organic bentonite, fluid loss reducer, plugging agent and density regulator and mixing.

8. Use of the synthetic-based drilling fluid according to any one of claims 1 to 6 in drilling high-pressure reservoirs with a narrow safety density window.

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