A low-viscosity, low-water-loss, safe and environmentally friendly drilling mud for diamond wireline coring of pulverized coal seams and its preparation method.

Abstract

This invention discloses a low-viscosity, low-water-loss, safe and environmentally friendly drilling mud for diamond wireline coring in pulverized coal seams and its preparation method, relating to the field of geological and mineral drilling technology. By weight percentage, it comprises the following components: 3.5%-4.5% composite bentonite, 0.8%-1.2% acrylamide-intercalated modified nano-bentonite, 1.8%-2.8% GLA filtration loss reducer, 0.15%-0.25% FS-16S polymer, with the balance being water. This invention utilizes composite bentonite to construct a low-viscosity base, acrylamide-intercalated modified nano-bentonite to seal microfractures in carbonaceous mudstone, and GLA and FS-16S to synergistically reduce filtration loss and improve salt and temperature resistance. This makes the prepared mud suitable for diamond wireline coring with annulus diameters of 0.5-3mm, applicable to pulverized coal seam coring at depths of 200-800m. It achieves the effects of low pump pressure, low API filtration loss, stable borehole walls, high core recovery rate, and low operating costs.

Description

Technical Field

[0001] This invention relates to the field of geological and mineral drilling technology, specifically to a low-viscosity, low-water-loss, safe and environmentally friendly drilling mud for diamond wireline coring of pulverized coal seams and its preparation method. Background Technology

[0002] Diamond wireline coring has become a core technology for geological and mineral exploration in pulverized coal seams due to its "high core recovery rate, fast drilling efficiency, and minimal in-hole interference." However, the mud wall protection under small annulus (0.5-3mm) is a technical challenge in diamond wireline coring operations in pulverized coal seams. Because carbonaceous mudstone (cement strength <0.5MPa) and peat deposits (water content >30%) in pulverized coal seams are easily affected by mud properties, leading to spalling / collapse, wireline coring places even stricter requirements on the mud's "low viscosity (ensuring smooth circulation), low water loss (ensuring borehole stability), and low solids (avoiding drill bit clogging)."

[0003] Existing mud solutions for diamond wireline coring of pulverized coal seams are mainly divided into three categories:

[0004] (1) Water + simple bentonite slurry (common foundation solution in the industry)

[0005] Composition: Water + 3%-5% sodium bentonite + 0.1%-0.2% CMC (carboxymethyl cellulose);

[0006] Application scenarios: shallow holes (<200m), simple pulverized coal seams (carbonaceous mudstone content <5%);

[0007] Core weakness:

[0008] a. Uncontrolled water loss: API filtration loss is 15-20 mL / 30 min. A large amount of filtrate invades carbonaceous mudstone, causing clay mineral hydration and swelling (swelling rate > 15%), borehole wall spalling rate > 40%, and in severe cases, the drill bit is buried (wired coring drill rod recovery rate < 80%).

[0009] b. Large viscosity fluctuations: Bentonite is prone to agglomeration, with an apparent viscosity of 12-15 mPa·s. The flow resistance increases dramatically in the 0.5-3 mm annulus, and the pump pressure rises to 1.8-2.2 MPa, which can cause the annulus wall to collapse due to compression.

[0010] c. Core contamination: CMC is adsorbed on the core surface, resulting in a coal core recovery rate of less than 85%, which does not meet the "core fidelity" requirement of wireline coring.

[0011] (2) Iron-chromium salt-CMC composite slurry (traditional improved scheme)

[0012] Composition: 5%-6% sodium bentonite + 1.5%-2% iron-chromium lignin sulfonate (to reduce filtration loss) + 0.8%-1% CMC + 0.3% NaOH;

[0013] Application scenarios: Medium-deep holes (200-500m), pulverized coal seams containing a small amount of carbonaceous mudstone;

[0014] Core weakness:

[0015] a. Environmental protection standards not met: Iron chromium salts contain Cr 3+ (Concentration in waste liquid is 0.05-0.1 mg / L), exceeding the "Cr" limit in the "Emission Standard of Pollutants for Coal Industry" (GB 20426-2006). 3+ The requirement of "≤0.03mg / L" necessitates additional treatment, increasing processing costs by 30%.

[0016] b. High viscosity leads to low efficiency: With an apparent viscosity of 15-20 mPa·s, the wireline coring advance is shortened to 2-3m per cycle (normally it should be 4-5m), the hourly efficiency is <3m, and the inner wall of the drill pipe is prone to scaling (scale thickness 0.2-0.3mm), which affects core lifting;

[0017] c. Poor sealing performance: The mud cake formed by iron-chromium salts alone (thickness > 1.5 mm) cannot seal the micro-fractures (0.1-1 μm) in carbonaceous mudstone, and the spalling rate is still 30%-35%.

[0018] (3) Ordinary polymer-modified pulp (such as PAM-KHm pulp, which has been promoted in recent years)

[0019] Composition: 4% sodium bentonite + 0.3%-0.5% PAM (polyacrylamide) + 0.5%-0.8% KHm (guar gum) + 0.2%-0.3% AMPS polymer;

[0020] Application scenarios: medium to deep boreholes (300-800m), complex pulverized coal seams;

[0021] Core weakness:

[0022] a. Difficulty in balancing low viscosity and low water loss: In order to control the filtration loss (9-12 mL / 30 min), the amount of PAM needs to be increased, which causes the apparent viscosity to rise to 11-14 mPa·s and the pump pressure to be 1.6-1.9 MPa, and there is still a risk of pore wall compression.

[0023] b. Microfracture sealing failure: PAM molecular chains are long (molecular weight > 500,000), making it impossible to enter the microfractures of carbonaceous mudstone. The colloids formed by KHm alone cannot prevent the intrusion of filtrate, and the peat sediment swelling rate is > 8%.

[0024] c. Poor thermal stability: At 80℃ (earth temperature at depths of 500m and above), PAM is easily hydrolyzed, with a performance degradation rate of >20% / 16h, requiring frequent replenishment of the agent (replenishment frequency 2-3 times / day), increasing operating costs.

[0025] It is evident that existing drilling mud has several drawbacks in small annulus (0.5-3mm), including difficulty in balancing low viscosity and low water loss (either high viscosity leading to pump pressure exceeding 1.6MPa or high filtration loss leading to borehole wall spalling), insufficient microfracture sealing (spalling rate >30%), poor suppression of peat deposition swelling (swelling rate >8%), low core recovery rate (<88%), and insufficient environmental friendliness and thermal stability. These shortcomings prevent it from meeting the core requirements of diamond wireline coring drilling for pulverized coal seams. Summary of the Invention

[0026] This invention addresses the technical problems of existing drilling mud, including difficulty in balancing low viscosity and low water loss in small annulus conditions (either high viscosity leading to pump pressure exceeding 1.6 MPa or high filtration loss causing borehole wall spalling), insufficient microfracture sealing (spalling rate > 30%), poor suppression of peat deposition swelling (swelling rate > 8%), low core recovery rate (< 88%), and insufficient environmental friendliness and thermal stability. The aim is to provide a low-viscosity, low-water-loss, safe, and environmentally friendly drilling mud for diamond wireline coring in pulverized coal seams, along with its preparation method. This involves constructing a low-viscosity base with composite bentonite, sealing microfractures in carbonaceous mudstone with acrylamide intercalation-modified nano-bentonite, and synergistically reducing filtration loss with GLA and FS-16S, while improving salt and temperature resistance. This results in a drilling mud suitable for 0.5-3 mm small annulus conditions in diamond wireline coring, applicable to 200-800 m pulverized coal seam wireline coring, achieving pump pressures of 1.2-1.5 MPa and API... The filtration loss is ≤7.0mL / 30min, the carbonaceous mudstone stripping rate is <5%, the swelling rate is <3%, the core recovery rate is ≥92%, and the operating cost is low.

[0027] The present invention is achieved through the following technical solution.

[0028] The first objective of this invention is to provide a low-viscosity, low-water-loss, safe, and environmentally friendly drilling mud for diamond wireline coring in pulverized coal seams, comprising the following components by weight percentage:

[0029] The composition consists of 3.5%-4.5% composite bentonite, 0.8%-1.2% acrylamide intercalated modified nano-bentonite, 1.8%-2.8% GLA filtration loss reducer, 0.15%-0.25% FS-16S polymer, and the balance being water.

[0030] The FS-16S polymer is generated by free radical copolymerization of AMPS and acrylamide.

[0031] GLA is an abbreviation for "Grafted Lignin-Acrylamide," a typical naming convention in the industry for polymer-based filtration loss reducers (core monomer + abbreviation of grafting modification process). The specific components of GLA filtration loss reducers include hydroxyl groups in the molecular chain that form hydrogen bonds with rocks / peat. The core component is a copolymer with lignin as the backbone, incorporating acrylamide derivatives (containing hydroxyl and amide functional groups) through graft polymerization. The auxiliary modification unit may contain a small amount of acrylic acid or 2-acrylamide-2-methylpropanesulfonic acid (AMPS) monomer copolymers (to improve salt and temperature resistance), but the core functional unit is the lignin-acrylamide graft chain. The core function of this component design is to utilize the hydrogen bonds formed between hydroxyl groups and pore walls, and the entanglement of amide groups with the FS-16S polymer, to synergistically form a thin and tough cake, achieving low filtration loss and inhibiting swelling.

[0032] The technical principle of this invention is as follows:

[0033] The mud formulation of this invention uses composite bentonite to provide a low-viscosity colloidal base, controlling the plastic viscosity at 9.0-9.8 mPa·s, suitable for flow in small annular spaces of 0.5-3 mm. Acrylamide-intercalated modified nano-bentonite is added; after acrylamide intercalation, the interlayer spacing of the bentonite increases, allowing it to penetrate into 0.1-1 μm microcracks to form a physical sealing layer (sealing rate > 90%), preventing filtrate intrusion, reducing pore wall spalling, and improving pore wall stability. The acrylamide derivative in the GLA filtrate reducer contains hydroxyl groups (-OH), which can form hydrogen bonds with the pore wall rock and simultaneously form a thin mud cake (thickness < 0.8 mm) on the pore wall, reducing filtrate penetration. It also combines with the hydroxyl groups of peat organic matter to inhibit swelling, further improving pore wall stability. FS-16S The sulfonic acid groups of AMPS in the polymer can enhance the salt resistance and heat resistance of the mud, and the amide groups can also entangle with the GLA chain. The two work together to reduce filtration loss, form a thin and tough mud cake, and extend the mud cake life.

[0034] Therefore, based on the above scheme, the apparent viscosity of the mud in this invention is controlled at 9.0-9.8 mPa·s, and the API filtration loss is ≤7.0 mL / 30 min. This achieves a balance between low viscosity and low water loss in small annulus conditions, with a carbonaceous mudstone exfoliation rate of <5%, a swelling rate of <3%, good microcrack sealing effect, and effectively improved pore wall stability. Furthermore, after aging at 80℃ for 16 h, the performance degradation rate is <8%, indicating high thermal stability. Frequent replenishment of chemicals is not required, thus reducing operating costs.

[0035] Furthermore, the composite bentonite, by weight percentage, comprises 70%-85% sodium bentonite and 15%-30% low molecular weight (molecular weight 5000-10000) sodium / potassium polyacrylate. The mud formulation of this invention uses a blend of sodium bentonite and low molecular weight sodium / potassium polyacrylate to construct a low-viscosity foundation for diamond wireline coring mud in pulverized coal seams. The sodium / potassium polyacrylate electrostatic repulsion disperses the bentonite particles, preventing agglomeration and achieving a balance between "colloidal stability" and "low viscosity," controlling the plastic viscosity at 8.0-8.8 mPa·s, suitable for 0.5-3 mm small annular flow, laying the foundation for subsequent low-viscosity mud.

[0036] Furthermore, the acrylamide intercalated modified nano-bentonite has an interlayer spacing of 2.2-2.8 nm and a particle size ≤100 nm.

[0037] Furthermore, the preparation method of the acrylamide intercalated modified nano-bentonite is as follows:

[0038] First, bentonite is modified by sodium carbonate, then mixed with an acrylamide monomer solution, and potassium persulfate-sodium bisulfite is added. Intercalation polymerization is carried out at 40~60℃ to obtain acrylamide intercalated modified nano-bentonite.

[0039] Furthermore, the amount of acrylamide monomer added is 10%-25% of the weight of bentonite, and the amount of potassium persulfate-sodium bisulfite added is 0.5%-2.0% of the weight of bentonite.

[0040] Furthermore, the preparation method of the FS-16S polymer is as follows:

[0041] AMPS and acrylamide were dissolved in water, and ammonium persulfate was added. Free radical polymerization was initiated at 50-80°C to obtain FS-16S polymer.

[0042] Furthermore, the mass ratio of AMPS, acrylamide, and ammonium persulfate is (30-50):(50-70):(0.8-2.0). This ratio can balance the salt resistance and temperature resistance of FS-16S polymer with the synergistic effect of reducing filtration loss. The amount of ammonium persulfate is 1%-2% of the total monomer, which is in line with the conventional addition range of free radical polymerization initiators.

[0043] Optionally, if special requirements are needed, 0.3%-0.5% of lubricant and 0.3%-0.5% of calcium carbonate sealant may be added, wherein the lubricant is polyether modified silicone oil or fatty acid amide lubricant.

[0044] The second objective of this invention is to provide a method for preparing low-viscosity, low-water-loss, safe, and environmentally friendly drilling mud for diamond wireline coring in pulverized coal seams, comprising the following steps:

[0045] Weigh out sodium bentonite and low molecular weight sodium polyacrylate according to the formula, mix them to obtain composite bentonite, add it to water, mix evenly, and then hydrate and stand at room temperature for 24-28 hours to obtain the base slurry.

[0046] Acrylamide-intercalated modified nano-bentonite was added to the base slurry, followed by the addition of GLA filtration loss reducer and FS-16S polymer, and the mixture was thoroughly mixed.

[0047] Finally, adjust the pH of the system to 8-9 to obtain a low-viscosity, low-loss cement slurry.

[0048] In the preparation process of this invention, the bentonite is allowed to fully hydrate and expand by being left to stand for a relatively long time (24-28 hours) to form a stable colloidal skeleton, thus avoiding particle agglomeration and blockage of the annulus.

[0049] Furthermore, during the preparation process, after each addition of a substance, stirring and mixing are performed, with the stirring speed gradually decreasing from high to low. Specifically, the stirring speed is reduced from 11000-12000 r / min to 8000-10000 r / min in each step to ensure uniform dispersion of all components. The mud preparation method of this invention, through the synergistic effect of hydration and settling at room temperature for 24-28 hours, step-by-step stirring (with gradually decreasing speed), and precise pH adjustment (8-9), ensures stable mud performance.

[0050] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0051] The apparent viscosity of the mud in this invention is controlled at 9.0-9.8 mPa·s, and the API filtration loss is ≤7.0 mL / 30 min. It achieves a balance between low viscosity and low water loss in small annulus, with a carbonaceous mudstone exfoliation rate of <5%, a swelling rate of <3%, good microcrack sealing effect, and effectively improves pore wall stability. Moreover, after aging at 80℃ for 16 h, the performance degradation rate is <8%, and the thermal stability is high. It does not require frequent replenishment of chemicals, thus reducing operating costs. Detailed Implementation

[0052] To make the objectives, technical solutions, and advantages of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments. Obviously, the illustrative embodiments and descriptions of the present invention are only used to explain the present invention and are not intended to limit the present invention.

[0053] The embodiments of the present invention will be described in detail below. However, unnecessary details may be omitted. For example, detailed descriptions of well-known matters and repeated descriptions may be omitted. This is to avoid making the following description unnecessarily lengthy and to facilitate understanding by those skilled in the art.

[0054] The "scope" disclosed in this invention is defined in the form of a lower limit and an upper limit. A given scope is defined by selecting a lower limit and an upper limit, which define the boundaries of the specific scope. The scope defined in this way can include or exclude end values, and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a scope.

[0055] Unless otherwise specified, all embodiments and optional embodiments of the present invention can be combined with each other to form new technical solutions.

[0056] Unless otherwise specified, all technical features and optional technical features of this invention can be combined to form new technical solutions.

[0057] Unless otherwise specified, the terms "comprising" and "including" as used in this invention can be open-ended or closed-ended. For example, "comprising" and "including" can mean that other substances not listed may also be included, or that only the listed substances may be included.

[0058] Unless otherwise specified, all steps of the present invention may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, the mention that the method may also include step (c) indicates that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0059] The technical solution of the present invention will be further described in detail below with reference to the embodiments.

[0060] It should be noted that, unless otherwise specified, the experimental methods used in the embodiments are conventional methods. Unless otherwise specified, the materials, reagents, methods, and instruments used are all conventional materials, reagents, methods, and instruments in the art, and can be obtained commercially by those skilled in the art.

[0061] Example 1

[0062] The preparation method of acrylamide intercalation modified nano-bentonite includes the following steps:

[0063] Bentonite pretreatment: Bentonite is modified by sodiumation, and sodium carbonate and other sodium-containing substances are added. + Reagents replace interlayer cations, enhance interlayer cation exchange capacity and hydrophilicity, and create conditions for intercalation;

[0064] Intercalation reaction: The amount of acrylamide monomer added is 15% of the weight of bentonite, and the amount of potassium persulfate-sodium bisulfite (redox initiator) added is 1.2% of the weight of bentonite. The intercalation polymerization reaction is carried out at 50°C with stirring, so that acrylamide molecules are inserted into the interlayer domain of bentonite.

[0065] Post-processing: After the reaction is complete, the product is washed, dried and ground to obtain modified bentonite with interlayer spacing increased from 1.2 nm to 2.2-2.8 nm, and the particle size after dispersion is ≤100 nm.

[0066] Example 2

[0067] The preparation method of FS-16S polymer includes the following steps:

[0068] Raw material preparation: Weigh 40g of AMPS (2-acrylamide-2-methylpropanesulfonic acid), 60g of acrylamide, and 1.5g of ammonium persulfate in a mass ratio of 40:60:1.5. Also prepare 250mL of deionized water.

[0069] Polymerization reaction: Dissolve AMPS and acrylamide in 250 mL of deionized water and stir at 25 °C for 10 min until completely dissolved; purge the solution with high-purity nitrogen for 15 min to remove oxygen, transfer to a three-necked flask and heat to 60 °C; add 1.5 g of ammonium persulfate and stir continuously at 500 r / min for 3.5 h until the solution viscosity rises to 500-800 mPa·s, then stop the reaction;

[0070] Post-processing: After the reaction is complete, place the polymer gel in a 60℃ hot air drying oven for 8 hours (or vacuum dry at 50℃ for 4 hours). When the moisture content is ≤1.5%, pulverize it with a universal pulverizer and pass it through a 100-mesh sieve to obtain the FS-16S polymer product (molecular weight test results are 250,000-300,000, suitable for low viscosity requirements).

[0071] Example 3

[0072] (1) A low-viscosity, low-water-loss, safe and environmentally friendly mud formula for diamond wireline coring drilling in pulverized coal seams, comprising the following components by weight percentage: 4.0% composite bentonite (3.2% sodium bentonite + 0.8% low molecular weight sodium polyacrylate), 1.0% acrylamide intercalated modified nano-bentonite, 2.0% GLA filtration loss reducer, 0.2% FS-16S polymer, 0.4% fatty acid amide lubricant, and 92.4% water.

[0073] (2) A method for preparing a low-viscosity, low-water-loss, safe and environmentally friendly drilling mud for diamond wireline coring of pulverized coal seams, comprising the following steps:

[0074] Step 1: Prefabrication of composite bentonite

[0075] Weigh out sodium bentonite and low molecular weight sodium polyacrylate (molecular weight 5000-10000) that meet API standards according to the formula ratio; put the two materials into a high-speed mixer and stir continuously at 11000r / min for 12min, stop once in the middle (3min) to scrape the wall and continue stirring. The resulting composite bentonite has a particle size distribution of 5-20μm and no visible agglomerates.

[0076] Step 2: Preparation of basic colloidal slurry

[0077] Add deionized water to the mixing tank, then slowly add the composite bentonite prepared in step 1 while stirring. Stir at 11000r / min for 20min using a high-speed mixer, then hydrate and stand at room temperature (25℃±2℃) for 26h.

[0078] The base slurry prepared in this step has an apparent viscosity of 8.5-9.0 mPa·s and a colloidal stability of >95% (no sedimentation after 24 hours). This step allows the bentonite to fully hydrate and expand through hydration and settling, forming a stable colloidal skeleton and preventing subsequent annular blockage.

[0079] Step 3: Construction of the microfracture sealing layer

[0080] Acrylamide-intercalated modified nano-bentonite was added to the base slurry obtained in step 2, and the mixture was stirred at 8000 r / min for 10 min. Samples were taken every 4 min to observe the dispersion state of the nanoparticles. After stirring, the particle size distribution was detected using a laser particle size analyzer. The nanoparticles were found to have a suspension stability >90% / 24h, a particle size ≤100nm, and a proportion ≥95%.

[0081] This step ensures the uniform dispersion of nanoparticles, preparing for subsequent sealing of 0.1-1μm microfractures in carbonaceous mudstone.

[0082] Step 4: Composite system for reducing filtration loss

[0083] Add GLA filtration loss reducer to the slurry obtained in step 3, stir at 8000 r / min for 10 min to ensure that the polymer chains are fully expanded, then add FS-16S polymer, stir at 8000 r / min for 10 min, and finally add lubricant at 8000 r / min for 8 min to obtain composite slurry.

[0084] Step 5: Performance Adjustment and Quality Inspection

[0085] The slurry pH was adjusted to 8.5 by slowly adding 10% sodium hydroxide solution. The viscosity was measured to be 9.2 mPa·s and the filtration loss was 6.8 mL / 30 min, which met the standards.

[0086] (3) Application of diamond wireline coring in ordinary coal seams

[0087] Drilling was conducted at a 300m deep pulverized coal seam in a coal mine in Qinghai Province, using a diamond wireline coring drill (outer drill rod φ73mm, inner core tube φ50mm, annular space 1.5mm). The carbonaceous mudstone content was 10%-15%, the formation water salinity was 3000mg / L, and the ground temperature was 45℃.

[0088] The application results are as follows:

[0089] Thermal stability: The peat swelling rate is 2.2% (after aging at 80℃ for 16 hours, the swelling rate is still ≤2.8%, which meets the core indicator of "swelling rate <3%" in the invention and is better than the defect of "swelling rate >8%" in existing PAM-KHm slurry).

[0090] Circulation performance: Pump pressure 1.3MPa, flow rate 1.0m / s, no annular blockage;

[0091] The borehole walls are stable: the carbonaceous mudstone has a spalling rate of 4% and no collapse has occurred;

[0092] Core quality: 93% coal core recovery rate, 85% core integrity;

[0093] Efficiency: 4.5m advance per cycle, 4.2m per hour, which is 40% higher than the existing PAM-KHm slurry.

[0094] Example 4

[0095] (1) A low-viscosity, low-water-loss, safe and environmentally friendly mud formula for diamond wireline coring drilling in pulverized coal seams, comprising the following components by weight percentage: 4.0% composite bentonite (3.2% sodium bentonite + 0.8% low molecular weight sodium polyacrylate), 1.0% acrylamide intercalated modified nano-bentonite, 2.8% GLA filtration loss reducer, 0.25% FS-16S polymer, 0.4% fatty acid amide lubricant, and 91.55% water.

[0096] (2) The preparation method of the low viscosity and low loss cement slurry in this embodiment is the same as that in embodiment 3, except that the hydration time is extended to 28h.

[0097] (3) Diamond wireline coring of ordinary pulverized coal seams

[0098] A coalbed methane well in Shaanxi Province has a 450m deep pulverized coal seam, a wireline coring ring space gap of 2.0mm, a peat content of 20%, and a ground temperature of 60℃.

[0099] The performance and application effects are as follows:

[0100] Performance: Viscosity 9.5 mPa·s, filtration loss 6.2 mL / 30 min, peat swelling rate 2.5%;

[0101] The pore walls were composed of peat deposits with no spalling, and the integrity of the pore walls was 98%.

[0102] Core samples: 92% recovery rate, no organic contamination.

[0103] Example 5

[0104] (1) A low-viscosity, low-water-loss, safe and environmentally friendly mud formula for diamond wireline coring drilling in pulverized coal seams, comprising the following components by weight percentage: 4.0% composite bentonite (3.0% sodium bentonite + 1.0% sodium polyacrylate), 1.0% acrylamide intercalated modified nano-bentonite, 2.0% GLA filtration loss reducer, 0.2% FS-16S polymer, 0.4% ultrafine calcium carbonate (particle size 3μm), and 92.4% water.

[0105] Mechanism of action: Increased sodium polyacrylate content in the composite bentonite results in stronger electrostatic repulsion, with viscosity controlled at 9.6 mPa·s and filtration loss at 6.2 mL / 30 min; the combination of nanoparticles and ultrafine calcium carbonate forms a dual "nano + micro" sealing effect, achieving a 92% crack sealing rate.

[0106] (2) The preparation method of the low viscosity and low loss cement slurry in this embodiment is the same as that in embodiment 3.

[0107] (3) Application of pulverized coal seams with well-developed microfractures in carbonaceous mudstone

[0108] Application scenario: A 600m deep pulverized coal seam in a coal mine in Guizhou Province, with a wireline coring ring space gap of 1.0mm, well-developed microfractures (0.5-1μm) in carbonaceous mudstone, and a ground temperature of 75℃.

[0109] Performance and application effects:

[0110] Thermal stability: After aging at 80℃ for 16h, the viscosity was 9.8mPa·s (decay of 2.1%), and the filtration loss was 6.5mL / 30min (decay of 4.8%).

[0111] Borehole wall: carbonaceous mudstone with a spalling rate of 1%, pump pressure 1.5 MPa;

[0112] Core recovery rate: 92%, 4.0m per cycle.

[0113] Comparative Example 1

[0114] The difference between this comparative example and Example 3 is that sodium bentonite is used instead of composite bentonite, i.e., low molecular weight sodium polyacrylate is not added. The mud preparation method is the same as in Example 3, and the mud properties obtained are as follows:

[0115] Viscosity: Apparent viscosity 12.5-14.0 mPa·s. Reason: The core function of low molecular weight sodium polyacrylate (0.8% in the original formula) is to disperse sodium bentonite particles through electrostatic repulsion and avoid agglomeration; without this component, sodium bentonite easily forms agglomerates (particle size > 50 μm), and the flow resistance in the 0.5-3 mm small annular spaces increases dramatically, resulting in a significant increase in viscosity compared to Example 3 (9.2 mPa·s). Without sodium polyacrylate, the slurry viscosity fluctuates greatly.

[0116] Filtration loss: API filtration loss 18.5-22.0 mL / 30 min. Reason: Agglomerated bentonite particles cannot form a dense colloidal skeleton, the mud cake on the pore wall is loose (thickness > 1.5 mm), and the filtrate easily penetrates the carbonaceous mudstone; it is significantly higher than that of Example 3 (6.8 mL / 30 min), and the filtration loss of "clear water + simple bentonite slurry" is out of control.

[0117] Thermal stability: After aging at 80℃ for 16 hours, the viscosity decreased to 10.2-10.8 mPa·s (attenuation rate 12.5%-15.0%); the filtration loss increased to 23.0-26.5 mL / 30 min (attenuation rate 20.0%-25.0%). Reason: Without the dispersing and stabilizing effect of sodium polyacrylate, the agglomerated particles further settled at high temperature, the colloidal skeleton collapsed, and the thermal stability was much worse than that of Example 3 (attenuation 2.1% / 4.8%). "Without sodium polyacrylate, the mud water separation rate increased and the stability decreased."

[0118] Comparative Example 2

[0119] The difference between this comparative example and Example 3 is that: the acrylamide intercalated modified nano-bentonite is replaced with ordinary bentonite, that is, no modification treatment is performed. The mud preparation method is the same as in Example 3, and the mud properties obtained are as follows:

[0120] Viscosity: Apparent viscosity 10.5-11.5 mPa·s. Reason: Ordinary bentonite has a particle size >200nm (much larger than the ≤100nm of modified nano-bentonite), which cannot penetrate into the micro-fractures of carbonaceous mudstone, and has poor dispersibility (no interlayer expansion effect of intercalation modification). The frictional resistance between particles is slightly higher than in Example 3, resulting in a slight increase in viscosity.

[0121] Filtration loss: API filtration loss 15.0-18.0 mL / 30 min. Reason: Ordinary bentonite cannot seal the 0.1-1 μm microcracks in carbonaceous mudstone. The filtrate invades in large quantities through the microcracks, and the filtration loss is double that of Example 3 (6.8 mL / 30 min), which is consistent with the pattern of "poor sealing performance and high filtration loss of unmodified bentonite".

[0122] Thermal stability: After aging at 80℃ for 16 hours, the viscosity decreased to 9.2-9.8 mPa·s (attenuation rate 8.0%-10.0%), and the filtration loss increased to 19.5-22.5 mL / 30 min (attenuation rate 12.0%-15.0%). Reason: Ordinary bentonite lacks the stable interlayer structure formed by acrylamide intercalation, making it prone to dehydration and shrinkage at high temperatures, leading to damage to the cake's integrity and a decrease in filtration loss control. Although superior to Comparative Example 1, it is still far inferior to Example 3.

[0123] Comparative Example 3

[0124] The difference between this comparative example and Example 3 is that FS-16S polymer is not added, while everything else remains the same. The resulting mud properties are as follows:

[0125] Viscosity: Apparent viscosity 8.8-9.5 mPa·s. Reason: The amount of FS-16S polymer added is only 0.2% (Formulation of Example 3), and its molecular chain entanglement with GLA contributes little to the viscosity. Without this component, the viscosity fluctuates slightly (slightly lower than in Example 3), still maintaining a low viscosity range, but losing the synergistic effect with GLA.

[0126] Filtration loss: API filtration loss 10.5-13.0 mL / 30 min. Reason: The sulfonic acid groups of AMPS in FS-16S can entangle with the amide groups of GLA, enhancing the compactness of the cake; without this component, the cake formed by GLA alone is thin but lacks toughness (thickness <0.5 mm), and the filtration loss is more than 50% higher than in Example 3, proving the conclusion that "GLA needs to work synergistically with the polymer to reduce filtration loss".

[0127] Thermal stability: After aging at 80℃ for 16 hours, the viscosity decreased to 7.0-7.5 mPa·s (attenuation rate 18.0%-22.0%); the filtration loss increased to 18.0-22.0 mL / 30 min (attenuation rate 25.0%-30.0%). Reason: FS-16S is the temperature-resistant core (AMPS sulfonic acid group is resistant to 80℃ high temperature). Without this component, the GLA chain is easily hydrolyzed at high temperature, the cake structure is destroyed, and the performance degradation is "the degradation rate of ordinary polymer slurry (PAM-KHm) is >20%".

[0128] Comparative Example 4

[0129] The difference between this comparative example and Example 3 is that step 2 does not involve hydration and settling; that is, after mixing evenly, step 3 is carried out directly, while the rest remains unchanged. The properties of the resulting mud are as follows:

[0130] Viscosity: Apparent viscosity 10.0-13.0 mPa·s (large fluctuation range). Reason: Sodium bentonite was not hydrated and allowed to stand for 24-28 hours (Step 2 of Example 3), so it could not fully expand to form a stable colloidal skeleton. The particles were in a "semi-hydrated" state, with partial agglomeration (particle size 20-50 μm), resulting in a higher and fluctuating viscosity compared to Example 3 (9.2 mPa·s), consistent with the "unstable viscosity of unhydrated bentonite slurry".

[0131] Filtration loss: API filtration loss 16.0-20.0 mL / 30 min. Reason: The colloidal stability of insufficiently hydrated bentonite is <80% (the colloidal stability after hydration in Example 3 is >95%), there are voids in the mud cake on the pore wall, the filtrate can easily penetrate, and the filtration loss is double that of Example 3, which is consistent with the technical logic of "incomplete colloidal skeleton, uncontrolled filtration loss".

[0132] Thermal stability: After aging at 80℃ for 16 hours, the viscosity decreased to 8.0-9.5 mPa·s (attenuation rate 12.0%-15.0%); the filtration loss increased to 21.0-25.0 mL / 30 min (attenuation rate 18.0%-22.0%). Reason: The semi-hydrated particles further dehydrated and agglomerated at high temperature, and the colloidal structure collapsed faster than the fully hydrated system. The performance degradation was significantly higher than that in Example 3, which confirms that "hydration and settling is a key step to ensure thermal stability".

[0133] Finally, it should be noted that the above specific embodiments are only used to describe the purpose, technical solution, and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific implementation of the present invention and is not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to the foregoing specific embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions or improvements can be made to some or all of the technical features. These modifications, equivalent substitutions, and improvements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention, and they should all be covered within the scope of the claims and specification of the present invention.

Claims

1. A low-viscosity, low-fluid-loss, safe and environmentally friendly mud for use in the powder coal bed diamond wire-line coring drilling, characterized in that, By weight percentage, comprising the following ingredients: Composite bentonite 3.5%-4.5%, acrylamide intercalation modified nano bentonite 0.8%-1.2%, GLA fluid loss additive 1.8%-2.8%, FS-16S polymer 0.15%-0.25%, the balance is water; The FS-16S polymer is generated by free radical copolymerization of AMPS and acrylamide.

2. The low-viscosity, low-fluid-loss, safe and environmentally friendly slurry for use in the powder coal bed diamond wire line coring according to claim 1, characterized in that, The composite bentonite contains sodium bentonite 70%-85% and low molecular weight sodium / potassium polyacrylate 15%-30% by weight percentage.

3. The low-viscosity, low-fluid-loss, safe and environmentally friendly mud for use in the powder coal bed diamond wire line coring according to claim 1, characterized in that, The interlayer spacing of the acrylamide intercalation modified nano bentonite is 2.2-2.8 nm, and the particle size is ≤100 nm.

4. The low-viscosity, low-fluid-loss, safe and environmentally friendly mud for use in the powder coal bed diamond wire line coring according to claim 1 or 3, characterized in that, The preparation method of the acrylamide intercalation modified nano bentonite is: First, the bentonite is modified by sodium modification with sodium carbonate, then mixed with acrylamide monomer solution, and potassium persulfate-sodium bisulfite is added, and the intercalation polymerization reaction is carried out at 40-60℃ to obtain acrylamide intercalation modified nano bentonite.

5. The low viscosity and low fluid loss safe and environment-friendly slurry for the powder coal bed diamond wire line core drilling according to claim 4, characterized in that, The addition amount of acrylamide monomer is 10%-25% of the weight of bentonite, and the addition amount of potassium persulfate-sodium bisulfite is 0.5%-2.0% of the weight of bentonite.

6. The low viscosity and low fluid loss safe and environment-friendly slurry for the powder coal bed diamond wire line core drilling according to claim 1, characterized in that, The preparation method of the FS-16S polymer is: AMPS, acrylamide is dissolved in water, ammonium persulfate is added, and free radical polymerization reaction is initiated at 50-80℃ to obtain FS-16S polymer.

7. The low viscosity and low fluid loss safe and environment-friendly slurry for the powder coal bed diamond wire line core drilling according to claim 6, characterized in that, The mass ratio of AMPS, acrylamide, ammonium persulfate is (30-50):(50-70):(0.8-2.0).

8. The low viscosity and low fluid loss safe and environment-friendly slurry for the powder coal bed diamond wire line core drilling according to claim 1, characterized in that, The low viscosity and low fluid loss mud also includes lubricant 0.3%-0.5%, calcium carbonate 0.3%-0.5%.

9. The method for preparing the low-viscosity and low-fluid-loss safe and environmentally friendly mud for the powder coal bed diamond wire line coring according to any one of claims 1-8, characterized in that, Including the following steps: Sodium bentonite and low molecular weight sodium polyacrylate are weighed according to the formula, composite bentonite is obtained, and water is added, mixed uniformly, then hydrated and placed at room temperature for 24-28h to obtain the base slurry; Add acrylamide intercalation modified nano bentonite to the base slurry, then add GLA fluid loss additive and FS-16S polymer in turn, and mix uniformly; Finally, adjust the pH of the system to 8-9 to obtain low viscosity and low fluid loss mud.

10. The method according to claim 9, characterized in that, During preparation, after adding each substance, stirring and mixing are carried out, and the stirring speed gradually decreases from high to low.