A method for preparing carbonaceous microparticle materials from natural starch and its use in water-based drilling fluids

Abstract

The application discloses a method for preparing carbonaceous microparticle material from natural starch and application of the carbonaceous microparticle material in water-based drilling fluid. The method uses natural starch as raw material, and performs heat treatment at 250-450 DEG C under a controlled atmosphere to make the starch undergo dehydration, cracking and partial carbonization reaction to obtain carbonaceous residues, and then the carbonaceous residues are mechanically pulverized and subjected to particle size grading treatment to obtain carbonaceous microparticle material with a particle size of 5-200 microns. The obtained material has a stable carbonaceous skeleton structure and good temperature resistance, and when used as a fluid loss additive in water-based drilling fluid, can reduce API fluid loss by more than 46%, and when used as a plugging agent, can form an effective bridging structure in microcracks with a size of 100-300 microns. After aging at 150 DEG C for 16 hours, the fluid loss performance increases by only about 20%, which is much better than that of original starch. The raw material is cheap and easy to obtain, the preparation process is simple, and the carbonaceous microparticle material has both fluid loss reduction and plugging functions.

Description

Technical Field

[0001] This invention relates to the field of oil and gas drilling engineering technology, specifically to a method for preparing carbonaceous particulate materials from natural starch under controlled conditions, and the application of these carbonaceous particulate materials as filtration reducers and plugging agents in water-based drilling fluid systems. This invention belongs to the interdisciplinary field of oilfield chemicals, drilling fluid engineering, and biomass material conversion technology. Background Technology

[0002] In oil and gas drilling, drilling fluid plays several crucial roles, including carrying cuttings, cooling and lubricating the drill bit, maintaining wellbore stability, balancing formation pressure, and protecting the reservoir. Among these, the filtration control performance and wellbore sealing capacity of the drilling fluid directly affect the safety and efficiency of drilling operations. When drilling fluid filtration is excessive, the liquid phase invades formation pores and microfractures, causing anything from mud cake thickening and wellbore enlargement to wellbore instability, differential sticking, and even lost circulation, resulting in severe economic losses and safety risks for drilling operations.

[0003] Currently, filtration loss reducers widely used in water-based drilling fluid systems are mainly divided into two categories: natural polymers and synthetic polymers. Natural polymer filtration loss reducers mainly include modified starch, sodium carboxymethyl cellulose (CMC), polyanionic cellulose (PAC), and xanthan gum; synthetic polymer filtration loss reducers include polyacrylamide, acrylic acid-acrylamide copolymers (such as AMPS copolymers), and sulfonated phenolic resins (SMP / SMPL). The mechanism of action of these filtration loss reducers is mainly through the adsorption, entanglement, and bridging of macromolecular chains, forming a dense mud cake on the wellbore surface, thereby reducing the liquid phase permeation rate.

[0004] However, the aforementioned conventional filtration loss reducers all have limitations in practical applications. Taking natural starch and its derivatives as an example, although starch is widely available and inexpensive, its thermal stability is poor. When the bottom-hole temperature exceeds 120°C, the glycosidic bonds in starch molecules are easily hydrolyzed and broken, leading to a decrease in molecular weight and a sharp decline in filtration loss reduction. Even after chemical modification treatments such as cross-linking, esterification, or grafting, modified starch still struggles to maintain effective filtration loss control in high-temperature well sections above 150°C. Similarly, CMC and PAC also undergo significant degradation at temperatures exceeding 140°C. While synthetic polymer-based filtration loss reducers have relatively better temperature resistance, they are more expensive, and some products have poor biodegradability, potentially causing environmental problems.

[0005] In wellbore sealing, addressing leakage caused by formation microfractures or micropores typically requires adding bridging materials to the drilling fluid system. Commonly used bridging materials include calcium carbonate particles, graphite powder, asphaltene, walnut shell particles, fruit shell powder, cellulose fibers, and mica flakes. These materials form a sealing layer at fracture inlets or pore throats through the particle bridging principle. The effectiveness of the sealing depends on whether the particle size distribution matches the target fracture or pore size, the so-called particle size matching principle.

[0006] However, traditional bridging materials also face many problems in practical use. While calcium carbonate particles have controllable particle size, their high specific gravity (approximately 2.7 g / cm³) makes them prone to settling in low-shear regions, affecting the rheological stability of drilling fluids. Graphite powder has good lubricity and plugging properties, but high-quality natural graphite resources are limited and their prices fluctuate significantly. Asphaltene materials soften or even melt at high temperatures, potentially causing the plugging structure to fail. Although organic granular materials such as walnut shells have a moderate specific gravity, their raw material sources are unstable, and the control precision for particle hardness and size distribution is limited. Furthermore, many traditional plugging materials have relatively singular functions, primarily used for either reducing filtration loss or primarily for plugging, making it difficult to effectively combine both functions within a single material.

[0007] In recent years, the preparation of carbonaceous functional materials from biomass feedstocks through thermochemical conversion has become a research hotspot in materials science and energy chemical engineering. Starch, as one of the most widely distributed and abundant renewable carbohydrates in nature, has a molecular structure composed of numerous α-D-glucose units polymerized through glycosidic bonds, mainly consisting of amylose (approximately 20%–30%) and amylopectin (approximately 70%–80%). Under high-temperature conditions, starch undergoes a series of reactions, including dehydration, glycosidic bond breaking, cyclization, aromatization, and condensation, ultimately forming carbon-rich residues. Existing research indicates that starch begins to decompose significantly above 250℃, and within the temperature range of 300℃ to 400℃, it can form carbonaceous residues with a certain carbon content and microporous structure.

[0008] Based on the above background, the inventors, through in-depth research, discovered that by controlling natural starch under appropriate temperature and atmosphere conditions through reasonable process parameters, a solid product with a stable carbonaceous skeleton structure can be obtained. Further, through mechanical crushing and particle size classification, carbonaceous microparticle materials with controllable particle size distribution can be prepared. This material exhibits good thermal stability of its carbonaceous skeleton, overcoming the defect of natural starch's easy degradation at high temperatures. Simultaneously, its particle morphology and particle size distribution can be artificially controlled, meeting the dual technical requirements of reducing filtration loss and plugging in drilling fluids. This material uses natural starch as a raw material, which is widely available, inexpensive, and has a relatively simple preparation process, showing promising prospects for industrial application.

[0009] Therefore, developing a comprehensive technical solution for preparing carbonaceous microparticle functional materials from natural starch as the starting material and applying them to water-based drilling fluid systems to achieve both filtration loss reduction and plugging functions is of great theoretical significance and engineering application value for solving the problems of insufficient temperature resistance of filtration loss reduction agents and high cost of plugging materials in existing technologies. Summary of the Invention

[0010] To address the shortcomings of the existing technology, the present invention aims to provide a method for preparing carbonaceous microparticles as a filtration reducer and a blocking agent from natural starch. This method utilizes a controlled atmosphere heat treatment process to cause starch to undergo dehydration, pyrolysis, and partial carbonization reactions under specific temperature and atmosphere conditions, obtaining a solid residue with a stable carbonaceous structure. This residue is then mechanically pulverized and particle size classified to obtain carbonaceous microparticle materials.

[0011] Another object of the present invention is to provide a water-based drilling fluid system comprising the above-mentioned carbonaceous particulate material, and the application of the carbonaceous particulate material in the water-based drilling fluid as a filtration reducer and a plugging agent.

[0012] To achieve the above-mentioned objectives, the present invention adopts the following technical solution:

[0013] A method for preparing carbonaceous particulate materials from natural starch includes the following steps:

[0014] Step 1, Raw Material Preparation: Select natural starch raw materials, wherein the natural starch is selected from at least one of corn starch, potato starch, cassava starch, wheat starch, sweet potato starch, or rice starch. Industrial-grade starch raw materials are preferred, with a moisture content not exceeding 15 wt%. The selected starch raw materials are dried and pretreated to reduce the moisture content to below 10 wt%, and then placed in a high-temperature resistant reaction vessel for later use.

[0015] Step 2, Controlled Atmosphere Heat Treatment: The reaction vessel containing the starch raw material is placed in a heating furnace and heat-treated under a controlled atmosphere. The controlled atmosphere refers to an environment with an oxygen content lower than normal atmospheric levels, which can be a nitrogen atmosphere, an inert gas atmosphere, or a semi-enclosed condition restricting airflow. During the heat treatment, the temperature is first increased from room temperature to the target temperature at a heating rate of 5–20°C / min. The target temperature is 250–450°C, preferably 300–400°C, and more preferably 320–380°C. After reaching the target temperature, it is maintained at a constant temperature for 30–120 minutes, preferably 45–90 minutes. During the heat treatment process, the starch raw material undergoes the following stages in sequence: a dehydration stage (approximately 100–200°C), where it mainly loses bound water and some free water; a pyrolysis stage (approximately 200–350°C), where glycosidic bonds break, molecular chains fragment, and dehydration condensation occurs; and a partial carbonization stage (approximately 350–450°C), where organic fragments undergo further dehydrogenation, cyclization, and aromatization, forming an amorphous carbonaceous structure. The gaseous products generated during the heat treatment process include water vapor, carbon dioxide, carbon monoxide, and small amounts of volatile organic compounds. After the isothermal treatment is completed, the heating device is turned off, and the reaction vessel is allowed to cool naturally to room temperature under a controlled atmosphere.

[0016] Step 3, Crushing and Particle Size Classification: The cooled carbonaceous residue is removed from the reaction vessel and mechanically crushed. The crushing method can be one or a combination of ball milling, air jet milling, hammer milling, or planetary ball milling. The crushed carbonaceous powder is then classified by sieving or air jet classifier to obtain carbonaceous microparticles within the target particle size range. The particle size range of the carbonaceous microparticles is 5–200 micrometers, where D10 is 5–20 micrometers, D50 is 20–120 micrometers, and D90 is 80–200 micrometers.

[0017] In the above method, the mass loss rate of the raw material during heat treatment is 40% to 75%, preferably 50% to 70%, and more preferably 55% to 68%. The mass loss rate is defined as the percentage of the difference in mass of the solid material before and after heat treatment to the mass of the solid material before heat treatment.

[0018] The carbonaceous microparticle material has the following physicochemical characteristics: the carbon content is 40wt% to 75wt% (on a dry basis), and the remaining components are non-carbon elements such as oxygen and hydrogen, as well as a small amount of ash; the material is dark brown to black and has an irregular particle morphology; in terms of microstructure, the particle surface and interior contain micropores and mesopores, and the BET specific surface area is 5 to 80 m² / g; the particle bulk density is 0.4 to 0.9 g / cm³, and the true density is 1.3 to 1.8 g / cm³.

[0019] This invention also provides a water-based drilling fluid system comprising the following components: a base slurry (containing water, bentonite, and alkali), a viscosifier, the aforementioned carbonaceous microparticle material, and other conventional drilling fluid treatment agents added as needed. The amount of the carbonaceous microparticle material added to the drilling fluid system is 1% to 8% of the total mass of the drilling fluid, preferably 2% to 5%.

[0020] When carbonaceous microparticles are used as filtration loss reducers, their addition amount is preferably 1% to 4%. The carbonaceous microparticles, with their irregular morphology and microporous surface structure, are embedded in the sludge cake layer, working together with bentonite particles and other treatment agent molecules to form a dense sludge cake, reducing the porosity of the sludge cake and thus lowering the liquid phase permeation rate.

[0021] When carbonaceous microparticles are used as plugging agents, their addition amount is preferably 2% to 6%. Carbonaceous microparticles form bridging structures at the entrances of formation microfractures or micropores through a particle bridging mechanism. Subsequently, fine particles and colloidal substances are deposited and filled on the surface of the bridging structure, constructing a plugging layer with a certain pressure-bearing capacity. Carbonaceous microparticles can be compounded with calcium carbonate particles, ultrafine graphite, or cellulose fibers to form a multimodal particle gradation system.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] (1) It uses natural starch as raw material, which is widely available and inexpensive. The raw material is not limited by region or season, making it suitable for large-scale industrial production.

[0024] (2) Starch is converted into carbonaceous microparticles through a controlled atmosphere heat treatment process, which gives the material excellent thermal stability and overcomes the defect of natural starch being easily degraded in high-temperature drilling environments. After aging at 150°C for 16 hours, the filtration loss increased by only about 20%, while the filtration loss of the original starch increased by more than 100% under the same conditions.

[0025] (3) The particle size distribution of carbonaceous microparticles can be precisely controlled by crushing and grading processes, which can meet the requirements of particle size distribution for different well sections and different formation conditions.

[0026] (4) Carbonaceous microparticle materials have both the functions of reducing filtration loss and plugging, achieving functional integration on the same material, simplifying drilling fluid formulation design, and reducing the total amount of treatment agent added and overall cost.

[0027] (5) The carbonaceous microparticle material has a moderate density (bulk density 0.4 to 0.9 g / cm³), good suspension stability in drilling fluid system, and is not easy to settle.

[0028] (6) Carbonaceous particulate materials are derived from natural biomass and have better environmental friendliness and biodegradability compared to synthetic polymer materials. Attached Figure Description

[0029] Figure 1 This is a schematic diagram of the process flow for preparing carbonaceous particulate materials from natural starch according to the present invention.

[0030] Figure 2 The graph shows the particle size distribution of the carbonaceous microparticles obtained in Examples 1 and 2.

[0031] Figure 3 This is a schematic diagram illustrating the mechanism by which carbonaceous microparticles form bridging and plugging structures at microcracks in the wellbore.

[0032] Figure 4 A comparative graph showing the effect of different amounts of carbonaceous microparticles on the API filtration loss of drilling fluid.

[0033] Figure 5 A comparison of the pressure-bearing capacity of carbonaceous microparticle plugging systems under different joint widths.

[0034] Figure 6 This is a comparison chart of API filtration loss before and after high-temperature aging of carbonaceous microparticle materials and raw starch.

[0035] Figure 7 A comparison chart showing the effect of carbonaceous microparticles on reducing filtration loss in different drilling fluid systems.

[0036] Figure 8 TGA / DSC simultaneous thermal analysis curves for different starch raw materials.

[0037] Figure 9 The images show SEM microstructures of different carbonaceous microparticle materials, where (a) is the product of Example 1 and (b) is the product of Example 2.

[0038] Figure 10 Comparison of XRD patterns of raw starch and carbonaceous microparticles treated at different temperatures. Detailed Implementation

[0039] The technical solution of the present invention will be further described in detail below with reference to specific embodiments and accompanying drawings. The following embodiments are for illustrative purposes only and do not constitute a limitation on the scope of protection of the present invention. All equivalent substitutions or transformations made based on the technical solution of the present invention are within the scope of protection of the present invention. Unless otherwise stated, the reagents used in the following embodiments are all commercially available analytical grade or industrial grade products, and the experimental methods all employ conventional techniques in the art.

[0040] Example 1: Preparation of carbonaceous microparticle materials using corn starch as raw material

[0041] Industrial-grade corn starch with a moisture content of approximately 12% was selected as the raw material. First, the raw material was dried in an oven at 105℃ for 4 hours to reduce the moisture content to below 8%. 500g of the dried corn starch was placed in a stainless steel reactor, sealed with a lid, but the exhaust valve was left open to allow gaseous products to escape. The reactor was placed in a muffle furnace, and the temperature was increased from room temperature to 320℃ at a rate of 10℃ / min. During the heating process, water vapor was observed escaping from the exhaust valve when the temperature reached approximately 120℃; when the temperature reached approximately 220℃, a slight caramel odor began to appear in the exhaust gas, indicating that the starch had begun a thermal decomposition reaction. After reaching 320℃, the temperature was maintained at a constant level for 60 minutes. During the constant temperature phase, the venting gradually decreased. After the constant temperature period, the power to the muffle furnace was turned off, and the reactor was allowed to cool naturally to room temperature while remaining sealed. The reactor was opened, the product was removed, and the solid residue was weighed and determined to be 220g, with a calculated mass loss rate of 56%.

[0042] The resulting carbonaceous residue was dark brown to black and relatively brittle. It was first coarsely crushed using a hammer crusher, then finely crushed using a planetary ball mill (milling conditions: ball-to-material mass ratio 4:1, rotation speed 350 rpm, milling time 2 hours). The crushed material was preliminarily classified by passing it through a 200-mesh standard sieve, and the undersize material was collected. The particle size distribution was further determined using a laser particle size analyzer. The physicochemical properties of the obtained carbonaceous microparticle material are summarized in Table 1.

[0043] Table 1 Comparison of physicochemical properties of carbonaceous particulate materials in Examples 1 and 2 Heat treatment temperature (°C) 320 380 Isothermal time (min) 60 45 Quality loss rate (%) 56 67 Exterior color Dark brown to black black D10 (μm) 12 8 D50 (μm) 48 50 D90 (μm) 135 140 BET specific surface area (m² / g) 15 28 Carbon content (wt%, dry basis) 52.3 61.5 Hydrogen content (wt%, dry basis) 4.1 2.8 Ash content (wt%) 2.8 3.2

[0044] Example 2: Preparation of carbonaceous microparticle materials using potato starch as raw material

[0045] 500g of food-grade potato starch was selected as raw material and, after undergoing the same drying pretreatment as in Example 1, was placed in a stainless steel reactor. The temperature was increased to 380°C in a muffle furnace at a rate of 8°C / min, and maintained at this temperature for 45 minutes. After cooling, the product was removed and weighed; the solid residue weighed 165g, and the calculated mass loss rate was approximately 67%. This higher mass loss rate compared to Example 1 is related to the higher heat treatment temperature, indicating that the starch underwent a deeper carbonization reaction at 380°C. The resulting residue was nearly pure black in color.

[0046] The pulverization and particle size classification methods were the same as in Example 1. The physicochemical properties of the obtained carbonaceous particles are detailed in Table 1. As can be seen from Table 1, the carbon content of the product in Example 2 (61.5%) was significantly higher than that in Example 1 (52.3%), and the BET specific surface area (28 m² / g) was also higher than that in Example 1 (15 m² / g), indicating that the higher heat treatment temperature promoted more complete carbonization and volatile matter release. The particle size distribution curve is shown in [Table 1]. Figure 2 .

[0047] Example 3: Particle Size Control

[0048] The carbonaceous residue prepared in Example 1 was subjected to crushing and grading under different conditions to obtain products with different particle size grades. The results are summarized in Table 2.

[0049] Table 2. Particle size distribution of carbonaceous particles under different grinding and classification conditions 3a (coarse-grained) Hammer crusher The material on the sieve is passed through a 100-mesh sieve. 55 110 190 Large seam sealing 3b (Medium-grained) Planetary ball mill 200~400 mesh screen room 15 45 72 Reduce filtration loss + seal the middle seam 3C (Fine-grained) Air jet mill (0.6MPa) Airflow classification 3 12 28 Filtration reduction + micropore sealing

[0050] As shown in Table 2, by selecting different crushing equipment and grading process conditions, products with large differences in particle size distribution range can be obtained, which provides a basis for subsequent particle size distribution design for different well section conditions.

[0051] Performance Evaluation

[0052] (1) Evaluation of the filtration loss reduction performance of carbonaceous particulate materials

[0053] Preparation of the base slurry: Add 15g of sodium bentonite to 350mL of deionized water, stir at high speed for 20 minutes, and then let it stand for pre-hydration for 24 hours. Then add 0.5g of sodium carbonate to adjust the pH to 9-10, and then add 1g of low-viscosity PAC as a basic filtration loss reducer. Stir at high speed for 30 minutes to obtain the base slurry. The formulations of each system and the API filtration loss test results are shown in Table 3.

[0054] Table 3. Effect of different amounts of carbonaceous microparticles on API filtration loss in drilling fluid. A (Control Group) — 0 18.2 — B Example 1 Product 1 14.6 19.8 C Example 1 Product 3 9.8 46.2 D Example 1 Product 5 7.5 58.8 E Example 2 Product 3 9.2 49.5

[0055] The experimental method followed GB / T 16783.1-2014 standard for API filtration loss testing at room temperature and pressure (test conditions: 0.69 MPa pressure difference, Whatman No. 50 filter paper, collection of filtrate volume over 30 minutes). After each system was prepared, it was stirred at high speed for 30 minutes to ensure uniform dispersion of carbonaceous particles. The bar chart comparing the experimental results is shown below. Figure 4 .

[0056] From Table 3 and Figure 4 It can be seen that carbonaceous microparticles can effectively reduce the API filtration loss of drilling fluid, and the filtration loss decreases with increasing addition amount within the range of 1% to 5%. Carbonaceous microparticles prepared from potato starch (system E, with a higher degree of carbonization and a larger specific surface area) showed slightly better filtration loss reduction effect than corn starch-based products (system C) at the same addition amount. Observation of the formed filter cake revealed that the filter cake was denser and smoother after the addition of carbonaceous microparticles, and its thickness was reduced, indicating that the carbonaceous microparticles effectively participated in the construction of the filter cake structure.

[0057] (2) Evaluation of the plugging performance of carbonaceous particulate materials

[0058] The sealing performance was evaluated using a high-temperature, high-pressure sealing test instrument. Artificial stainless steel fracture plates were used to simulate wellbore microfractures, with fracture widths set at 100 micrometers, 200 micrometers, and 300 micrometers. The drilling fluid formulation for the sealing test consisted of a base slurry + carbonaceous microparticles + fine-grained calcium carbonate. The experimental results are shown in Table 4.

[0059] Table 4. Plugging performance of carbonaceous microparticle plugging system under different crack widths 100 3% Sample 3b (medium particle size) 2% (D50=15μm) 3.5 >7.0 (not broken through) 200 3% Sample 3b (medium particle size) 2% (D50=15μm) 3.5 6.2 300 3% Sample 3b (medium particle size) 2% (D50=15μm) Ineffective sealing — 300 2% sample 3a + 2% sample 3b 1% (Ultrafine grade) 3.5 4.5

[0060] From Table 4 and Figure 5 It can be seen that carbonaceous microparticles possess excellent microcrack sealing capabilities. Under a 100-micrometer crack width, effective bridging occurs in approximately 2 minutes after applying a pressure differential of 3.5 MPa, and the sealing layer remains stable even after gradually increasing the pressure to 7.0 MPa. For larger crack widths (300 micrometers), effective sealing can also be achieved through a multi-stage blend of coarse and medium-sized carbonaceous microparticles combined with ultrafine calcium carbonate filling. This indicates that the sealing requirements for different crack widths can be matched by adjusting the particle size distribution.

[0061] (3) Evaluation of performance stability after high temperature aging

[0062] To investigate the performance stability of carbonaceous microparticle materials under high-temperature conditions, system C (base slurry + 3% carbonaceous microparticles) from Example 4 was subjected to a high-temperature and high-pressure aging test. A system with 3% added raw corn starch (without heat treatment) was used as a comparative example (Comparative Example 1). Each system was placed in an aging tank and subjected to rolling aging at 120°C and 150°C for 16 hours, respectively. After aging, the systems were cooled to room temperature and stirred at high speed for 20 minutes, and then the API filtration loss was measured. The results are shown in Table 5.

[0063] Table 5 Comparison of API filtration loss before and after high-temperature aging System C 3% carbonaceous particles 9.8 10.5 7.1 11.8 20.4 Comparative Example 1 3% raw corn starch 12.5 19.3 54.4 26.7 113.6

[0064] From Table 5 and Figure 6 It is evident that the high-temperature stability of carbonaceous microparticles is far superior to that of untreated raw starch. After aging at 120℃ for 16 hours, the filtration loss of the carbonaceous microparticle system increased by only 7.1%, while that of the raw starch system reached a staggering 54.4%. Under the even more demanding condition of 150℃, the increase in filtration loss for the carbonaceous microparticle system was 20.4%, while that for the raw starch system reached 113.6%. This is because the heat treatment of carbonaceous microparticles transforms their thermally unstable glycosidic bond structure into a more stable carbonaceous framework structure, allowing them to maintain good structural integrity and functional effectiveness even in high-temperature drilling fluid environments.

[0065] (4) Comparison with conventional filtration loss reducers

[0066] To further illustrate the technical advantages of the carbonaceous microparticle material of this invention, its performance was compared with that of several conventional filtration loss reducing agents under the same base slurry conditions. Different treatment agents (3 wt%) were added to the base slurry, and the API filtration loss at room temperature and after aging at 150℃ for 16 hours were measured. The results are shown in Table 6.

[0067] Table 6. Performance Comparison of Carbonaceous Microparticles and Conventional Filtration Loss Reduction Agents Carbonaceous microparticles (this invention) 9.8 11.8 20.4 3000～5000 raw corn starch 12.5 26.7 113.6 2000～3000 Modified starch (CMS) 10.2 18.5 81.4 6000～8000 Low viscosity CMC 9.5 16.8 76.8 8000～12000 AMPS copolymer 8.8 10.2 15.9 15000～25000

[0068] As shown in Table 6, the room-temperature filtration loss reduction effect of the carbonaceous microparticle material of this invention is on par with modified starch and CMC, and slightly inferior to AMPS copolymer. However, in terms of high-temperature aging performance, the carbonaceous microparticle material is significantly better than modified starch and CMC, second only to AMPS copolymer. Considering the material cost, the cost of the carbonaceous microparticle material is only 1 / 5 to 1 / 3 of that of AMPS copolymer, demonstrating a significant advantage in cost-effectiveness. In addition, the carbonaceous microparticle material also has a sealing function, which is not possessed by the aforementioned conventional filtration loss reducing agents.

[0069] (5) Compatibility evaluation of carbonaceous microparticles with various drilling fluid systems

[0070] To verify the compatibility of carbonaceous microparticles with different types of water-based drilling fluid systems, 3 wt% of carbonaceous microparticles (product from Example 1) were added to three drilling fluid systems, and their rheological properties and filtration loss were tested. The results are shown in Table 7.

[0071] Table 7 Performance evaluation of carbonaceous particulate materials in different drilling fluid systems Freshwater bentonite system No additions 15 9 16.0 Freshwater bentonite system Add 3% 18 11 9.5 Inhibitory KCl polymer system No additions 22 14 8.5 Inhibitory KCl polymer system Add 3% 23 15 5.8 Low solid phase polymer system No additions 12 7 11.0 Low solid phase polymer system Add 3% 14 8 7.2

[0072] From Table 7 and Figure 7 It can be seen that carbonaceous microparticles exhibit good compatibility with the three common water-based drilling fluid systems. Adding 3% carbonaceous microparticles only slightly increases the apparent viscosity and plastic viscosity of each system (the increase is less than 20%), without significantly adversely affecting the rheological properties of the drilling fluid. Simultaneously, the API filtration loss of each system shows a significant decrease (reduction of 32%–41%), indicating that carbonaceous microparticles can effectively reduce filtration loss in water-based drilling fluids of different compositions.

[0073] (6) Structural and morphological characterization of carbonaceous particulate materials

[0074] To gain a deeper understanding of the structural characteristics of carbonaceous microparticle materials, thermogravimetric analysis (TGA / DSC), scanning electron microscopy (SEM), and X-ray diffraction (XRD) were performed on the original starch and carbonaceous microparticle products, respectively.

[0075] ① Thermogravimetric analysis (TGA / DSC)

[0076] Thermogravimetric-differential scanning calorimetry (TGC-DSC) was used to analyze corn starch and potato starch raw materials using a simultaneous thermal analyzer (model: STA 449 F3 Jupiter, NETZSCH GmbH, Germany). Test conditions: nitrogen atmosphere (flow rate 50 mL / min), heating rate 10 °C / min, temperature range 30–500 °C. Results are as follows: Figure 8 As shown, the TGA curves of both starch raw materials exhibit typical three-stage weight loss characteristics: the first stage (30–150℃) is the dehydration stage, with a mass loss of approximately 8%–10%, corresponding to the endothermic peak at approximately 100℃ on the DSC curve; the second stage (200–350℃) is the main thermal decomposition stage, where weight loss is most severe, with a mass loss of approximately 45%–55%, corresponding to the strong endothermic peak near approximately 300℃ on the DSC curve, where glycosidic bond breaking and molecular chain fragmentation are the main reactions; the third stage (350–500℃) is the slow carbonization stage, with a small amount of volatiles continuing to escape. At the temperature node corresponding to 320℃, the residual mass of corn starch is approximately 44%, consistent with the 56% mass loss rate (i.e., 44% residual rate) in Example 1; at the temperature node of 380℃, the residual mass of potato starch is approximately 33%, also basically consistent with the 67% mass loss rate (i.e., 33% residual rate) in Example 2. Thermogravimetric data verified the accuracy of the mass loss data reported in Examples 1 and 2.

[0077] ② Morphological analysis by scanning electron microscopy (SEM)

[0078] The microstructure of the carbonaceous microparticles was observed using a field emission scanning electron microscope (ZEISS Sigma 300) with an accelerating voltage of 5 kV. The samples were sputter-coated with gold before testing. SEM morphology illustration. Figure 9 The observation results show that the particles of the product in Example 1 (treated at 320℃) are mainly irregular blocks and flakes, with relatively rough surfaces and a small number of surface micropores. The particles of the product in Example 2 (treated at 380℃) are similar in morphology to those in Example 1, but the number of surface micropores is significantly increased, with pore sizes ranging from approximately 0.5 to 2 micrometers, consistent with its higher BET specific surface area (28 m² / g vs. 15 m² / g). The increased number of surface micropores facilitates the filling and adsorption of carbonaceous particles in the cake, thereby improving the cake's density. The particle size range for both products is between 10 and 150 micrometers, consistent with the laser particle size analysis results.

[0079] ③ X-ray diffraction (XRD) analysis

[0080] The crystal structure of raw corn starch and two carbonaceous particulate products was analyzed using an X-ray diffractometer (Rigaku SmartLab, Cu Kα radiation, λ=0.15406nm). The scanning range was 2θ=5°~60°, and the scanning rate was 5° / min. XRD patterns are shown below. Figure 10 The original corn starch exhibited typical A-type crystalline peaks at approximately 15°, 17°, 18°, and 23° at 2θ, indicating a distinct semi-crystalline structure. The XRD pattern of the product from Example 1 (treated at 320°C) showed that the A-type crystalline peaks had largely disappeared, with only a weak diffraction peak remaining near 15°. Simultaneously, a significantly broadened diffuse peak appeared at approximately 22° at 2θ, a typical characteristic of amorphous carbonaceous structures, indicating that the crystalline structure of the starch had been destroyed and transformed into amorphous carbonaceous material. In the XRD pattern of the product from Example 2 (treated at 380°C), the A-type crystalline peaks completely disappeared, leaving only two broadened diffuse peaks at approximately 23° and 43° at 2θ, corresponding to the (002) and (100) planes of amorphous carbon, respectively, indicating that higher temperature treatment promoted a more thorough carbonization process.

[0081] Table 8 Comparison of structural parameters between carbonaceous microparticle materials and raw starch XRD crystallization characteristics The type A crystallization peak is obvious. The crystallization peaks have largely disappeared, and the amorphous carbon peaks have largely disappeared. The crystallization peak completely disappeared, and the amorphous carbon peak was absent. TGA residue (corresponding temperature) — 44%（320℃） 33%（380℃） DSC main decomposition peak temperature (°C) 305 — — SEM Surface Features Smooth spherical particles Irregular block shape with a few micropores Irregular block shape with numerous micropores BET specific surface area (m² / g) <1 15 28 Carbon content (wt%) 39.8 52.3 61.5

[0082] The characterization results from TGA, SEM, and XRD (summarized in Table 8) clearly show that after controlled atmosphere heat treatment, the original semi-crystalline structure of natural starch is completely destroyed, transforming it into a carbonaceous material dominated by amorphous carbon. Higher heat treatment temperatures result in deeper carbonization, higher carbon content, and larger specific surface area. These structural changes are the structural basis for the excellent high-temperature stability and good filtration loss reduction and plugging function of carbonaceous particulate materials.

[0083] Distinguishing features from the closest existing technology

[0084] The present invention is compared and analyzed with the closest existing technology as follows:

[0085] The closest existing technology is modified starch-based filtration loss reducers. In existing technologies, modified starches (such as cross-linked starch, carboxymethyl starch, and starch graft copolymers) are the most commonly used natural polymeric filtration loss reducers in water-based drilling fluids. The technical approach for these materials is to chemically modify and retain the polymeric chain structure of starch, utilizing the hydration, adsorption, and bridging effects of the molecular chains to construct a dense mud cake. However, modified starch is still essentially a polysaccharide polymer, and its glycosidic bonds inevitably undergo hydrolysis and breakage at high temperatures. In contrast, this invention adopts a completely different technical approach: instead of attempting to retain and reinforce the polymeric chain structure of starch, it actively breaks it down and transforms it into a carbonaceous skeleton structure. This fundamental structural transformation enables the material to achieve high-temperature stability that modified starch cannot achieve. As can be seen from the comparative data in Table 6, the filtration loss increase of the carbonaceous particles of this invention after aging at 150℃ is only 20.4%, while that of modified starch is 81.4%. Furthermore, the carbonaceous microparticles of the present invention are particulate materials rather than polymer solutions, and therefore also have the function of bridging and blocking particles, which is not possessed by modified starch-based filtration loss reducers.

[0086] The closest existing technology is biochar or biochar materials. Existing technologies have reported the preparation of biochar from biomass raw materials such as sawdust, straw, and coconut shells through pyrolysis or carbonization, and its use for environmental adsorption or soil improvement. A few studies have also explored the potential applications of biochar in drilling fluids. However, this invention differs from existing biochar technologies in the following key ways: First, the raw materials are different—this invention specifically uses natural starch as the starting material, rather than lignocellulosic raw materials. The chemical composition of starch (mainly glucose units, without lignin and cellulose) and its thermal decomposition behavior are fundamentally different from lignocellulosic raw materials, resulting in different structural characteristics of the carbonaceous products. Second, the purpose is different—the purpose of preparing carbonaceous microparticles in this invention is specifically for reducing filtration loss and plugging in drilling fluids. Therefore, there are clear design requirements for the particle size distribution of the product (D10=5~20μm, D50=20~120μm, D90=80~200μm), which is precisely controlled through pulverization and grading processes. Existing biochar technologies typically do not involve this kind of particle size distribution design for drilling fluid applications. Third, the application evaluation of the system—this invention not only provides a preparation method, but also demonstrates the filtration loss reduction effect and plugging performance of carbonaceous microparticles in various drilling fluid systems through a complete set of examples, and conducts a systematic comparison with conventional treatment agents, providing sufficient data support for engineering applications.

[0087] The closest existing technology (3): conventional plugging materials such as calcium carbonate and graphite. Traditional plugging materials mainly achieve their plugging function through particle bridging, but usually do not have the function of reducing filtration loss. Calcium carbonate has a high density (about 2.7 g / cm³) and is prone to settling; graphite has good lubricity but its source is limited and its price is unstable. The carbonaceous microparticle material of this invention has a bulk density of only 0.4-0.9 g / cm³, and its suspension stability is significantly better than that of calcium carbonate. At the same time, it has the dual functions of reducing filtration loss and plugging, which is a functional integration that is difficult to achieve with traditional plugging materials.

Claims

1. A method for preparing carbonaceous particulate materials from natural starch, characterized in that, The steps include the following: (1) Raw material preparation: Select natural starch raw materials and dry them to reduce the moisture content to below 10wt%; (2) Controlled atmosphere heat treatment: The starch raw material is heat treated under a controlled atmosphere, wherein the controlled atmosphere is an atmosphere with an oxygen content lower than the normal atmospheric level, the heat treatment temperature is 250-450℃, and the holding time is 30-120 minutes, so that the starch undergoes dehydration, cracking and partial carbonization reaction. (3) Crushing and particle size classification: The carbonaceous residue obtained after cooling is mechanically crushed and particle size classified to obtain carbonaceous microparticles with a particle size range of 5 to 200 micrometers.

2. The method according to claim 1, characterized in that, The natural starch raw material is selected from at least one of corn starch, potato starch, cassava starch, wheat starch, sweet potato starch, or rice starch.

3. The method according to claim 1, characterized in that, The heat treatment temperature is 300–400℃, preferably 320–380℃.

4. The method according to claim 1, characterized in that, The controlled atmosphere is a nitrogen atmosphere, an inert gas atmosphere, or a semi-enclosed condition that restricts airflow.

5. The method according to claim 1, characterized in that, The heating rate of the heat treatment process is 5–20 °C / min.

6. The method according to claim 1, characterized in that, The mass loss rate of raw materials during heat treatment is 40% to 75%, preferably 50% to 70%.

7. The method according to claim 1, characterized in that, The carbonaceous microparticle material has a D10 of 5–20 micrometers, a D50 of 20–120 micrometers, and a D90 of 80–200 micrometers.

8. The method according to claim 1, characterized in that, The carbonaceous particulate material has a carbon content of 40wt% to 75wt% and a BET specific surface area of ​​5 to 80 m² / g.

9. The application of the carbonaceous particulate material prepared by the method according to any one of claims 1 to 8 as a filtration loss reducer in water-based drilling fluids.

10. The application of carbonaceous particulate materials prepared by the method according to any one of claims 1 to 8 as plugging agents in water-based drilling fluids.

11. The application according to claim 9, characterized in that, The amount of carbonaceous microparticle material added to the water-based drilling fluid is 1% to 4% of the total mass of the drilling fluid.

12. The application according to claim 10, characterized in that, The amount of carbonaceous microparticle material added to the water-based drilling fluid is 2% to 6% of the total mass of the drilling fluid.

13. The application according to claim 10, characterized in that, The carbonaceous microparticle material is used in combination with at least one of calcium carbonate particles, graphite, or cellulose fibers.

14. A water-based drilling fluid system, characterized in that, The drilling fluid system comprises carbonaceous particulate material prepared by the method according to any one of claims 1 to 8, wherein the amount of carbonaceous particulate material added to the drilling fluid system is 1% to 8% of the total mass of the drilling fluid.

15. The water-based drilling fluid system according to claim 14, characterized in that, The amount of carbonaceous microparticle material added is 2% to 5% of the total mass of the drilling fluid.

Images