Preparation, Performance Evaluation and Mechanisms of a Diatomite-Modified Starch-Based Fluid Loss Agent

Abstract

Natural polymer materials are increasingly utilized in drilling fluid additives. Starch has come to be applied extensively due to its low cost and favorable fluid loss reduction properties. However, its poor temperature resistance and high viscosity limit its application in high-temperature wells. This study innovatively introduces for the first time diatomite as an inorganic material in the modification process of starch-based fluid loss additives. Through synergistic modification with acrylamide and acrylic acid, we successfully resolved the longstanding challenge of balancing temperature resistance with viscosity control in existing modification methods. The newly developed fluid loss additive demonstrates remarkable performance: It remains effective at 160 °C when used independently. When added to a 4% sodium bentonite base mud, it achieves an 80% fluid loss reduction rate—significantly higher than the 18.95% observed in conventional starch-based products. The resultant filter cake exhibits thin and compact characteristics. Moreover, this additive shows superior contamination resistance, tolerating 30% NaCl and 0.6% calcium contamination, outperforming other starch-based treatments. With starch content exceeding 75%, the product not only demonstrates enhanced performance but also achieves significant cost reduction compared to conventional starch products (typically containing < 50% starch content).

1. Introduction

With increasingly stringent environmental regulations, the development of natural polymer-based additives for drilling fluids has gained significant importance. Natural polymers find extensive applications in petroleum drilling operations, with commonly used types including cellulose derivatives (e.g., CMC), starches (e.g., CMS), plant gums, chitosan, lignins, and humic acids. These materials serve specific functions—fluid loss control (starch, cellulose, and lignin), viscosity (cellulose, plant gums, and xanthan gum), flocculation (chitosan), and viscosity reduction (humic acid and lignin). For instance, carboxymethyl cellulose (CMC) is widely adopted as a fluid loss additive, while sodium carboxymethyl starch (CMS) and polyanionic cellulose (PAC) are frequently employed for equivalent purposes.

Although environmentally benign drilling fluids formulated with natural polymers demonstrate satisfactory ecological compatibility, their poor thermal stability becomes problematic when formation temperatures exceed 130 °C, rendering them unsuitable for high-temperature well drilling. Recently, the focus of research has shifted toward starch-based fluid loss additives due to their cost effectiveness and dual mechanism of fluid loss reduction through adsorption and physical plugging effects. However, the inherent thermal limitations of native starch restrict its effective temperature threshold below 100 °C, making it incompatible with deep well operations.

Contemporary modification techniques, including etherification, oxidation, grafting, and crosslinking, have shown limited success in enhancing thermal stability. While these approaches moderately improve temperature resistance to the 120–140 °C range, they introduce significant drawbacks—substantial viscosity elevation in drilling fluid systems and the incorporation of nonbiodegradable monomers, thereby compromising both rheological control and environmental performance. This study proposes an innovative hybrid modification strategy employing inorganic materials coupled with thermally stable functional groups. Through organic–inorganic composite modification targeting high-temperature adsorption sites, the developed starch-based additive achieves effective fluid loss control under extreme conditions up to 160 °C, while maintaining optimal viscosity parameters and biodegradability.

Fanta et al. pretreated starch with radiation first, and then modified it with acrylamide. The obtained products exhibited a degree of grafting, high thermal stability, and an excellent filtration reduction effect [1]. Khalil et al. used potassium permanganate as initiator to initiate the polymerization of starch and olefins. It was found that different acid concentrations and types affected the grafting rate of the products, and also affected the anti-temperature and filtration loss properties of the products [2]. Biswas et al. made use of the dissolution effect of starch in ionic liquid to prepare acetate starch, but the thermal stability of this starch was poor [3].

Wang et al. took starch as the main material, selected one or several of acrylic acid (AA), acrylamide (AM), 2-acrylamide-2-methylpropanesulfonic acid (AMPS), cationic monomer, and anionic monomer to graft and developed a variety of modified starch-based fluid loss agents. They have good resistance to high temperatures, salt pollution, and filtration loss in drilling fluid, and the thermal stability of the optimized drilling fluids can reach 180 °C [4,5,6,7]. Guo et al. developed a kind of highly temperature resistant starch-based fluid loss agent by grafting sulfonic acid onto starch molecular chains [8].

To solve the problem of the poor biodegradability of grafted modified starch-based fluid loss agents, a modified starch filtration agent with better thermal stability than gelatinized starch can be obtained by the etherification and crosslinking of a monomer which is easier to degrade, but its thermal stability is generally below 130 °C. The main reason for the poor thermal stability of this kind of starch-based fluid loss agent is that the ether bond is easily degraded at high temperatures. Sifferman developed a cross-linked starch modified by epichlorohydrin etherification whose thermal stability was about 130 °C [9]. Katask et al. developed a carboxymethyl starch-based fluid loss agent via the carboxymethylation of starch. By increasing the degree of substitution, thermal stability was improved. When combined with another starch-based fluid loss agent at high temperatures, it could still maintain a certain viscosity and fluid loss reduction performance, but it had a great influence on the viscosity of the drilling fluid. The thermal stability of starch modified by etherification and crosslinking is better than that of etherified starch, because crosslinking can moderately increase the molecular chain strength of the modified product [10].

Yang et al. introduced a filamination reaction into the modification of starch, and combined with an etherification reaction, a starch-based fluid loss agent was synthesized in the laboratory, the thermal stability of the product reached 130 °C, and the filtration loss performance was good [11]. Through a large number of studies, the methods of starch modification to improve the thermal stability were summarized and it was concluded that the main ways to improve the thermal stability were to prevent the formation of hydroxyl salts and prevent the reaction between hydroxyl salts and nuclear loss agents. By preventing the formation of C-O bond in the synthesis reaction, a new starch-based fluid loss agent KFD can be obtained. The performance of this fluid loss agent is comparable to that of modified starch DFD-2, and the thermal stability can reach 130 °C in a saturated saline water-based drilling fluid. However, this method is complicated and needs optimization in pilot production [12].

This study pioneers the utilization of diatomite with intrinsic micro/nano porous structures, establishing a groundbreaking paradigm for developing thermal-resistant starch-based fluid loss reducers. Our innovative approach combines diatomite’s unique physical adsorption capacity with targeted chemical modification, effectively addressing both thermal degradation and rheological stability challenges. The proposed methodology not only overcomes conventional technical barriers but also drives technological innovation in eco-friendly water-based drilling fluids.

2. Materials and Methods

2.1. Materials

Corn starch, acrylamide (AM), acrylic acid (AA), N,N′-methylenebisacrylamide (MBA), diatomite, ammonium persulfate (APS), sodium hydroxide (NaOH), anhydrous ethanol, etc., were purchased from Aladdin Chemical Reagent Co., LTD. (Shanghai, China). The synthesis process employed corn starch, diatomite, AM, and AA as the principal raw materials, where N,N′-methylenebisacrylamide (MBA) functioned as a crosslinking agent, ammonium persulfate (APS) acted as an initiator, sodium hydroxide regulated the pH of the reaction mixture, and absolute ethanol facilitated product purification through solvent precipitation.

Material characterization was performed using a Nicolet Nexus 470 Fourier transform infrared (FTIR) spectrometer (Thermo Fisher Scientific, Madison, WI, USA) for functional group identification. The Regulus8100 field-emission scanning electron microscope (SEM, Hitachi High-Tech Corporation, Tokyo, Japan) was used for morphological examination. A TG209-F3 thermogravimetric analyzer (Netzsch, Selb, Germany) was used operating under an N₂ atmosphere (20–800 °C, 10 °C/min). The rheological properties were measured using a ZNN-D6 six-speed rotational viscometer (Haitongda, Qingdao, China) following API RP 13B-1 standards [13]. Filtration characteristics were determined with an API filter press under 100 psi (30 min, 25 °C). Thermal stability assessments employed a high-temperature roller furnace.

2.2. Methods

2.2.1. Preparation of Fluid Loss Agent

Add 200 mL of deionized water and 10 g of corn starch into the three-mouth flask, put them into a water bath at 75 °C, and blow with high-purity nitrogen for 30 min. Then, add 1 g of diatomite and continue stirring for 30 min before cooling to room temperature. Dissolve a 2.5% initiator, a 1 g monomer, and a 0.5% crosslinker in 10 mL deionized water, add them into the flask, and stir for 10 min. The temperature of the water bath was adjusted to 55 °C, after blowing nitrogen and stirring the reaction for 30 min, a 2.5% initiator and 0.5% crosslinker were added, and the reaction continued for 5 h. The pH of the product was adjusted to neutral with sodium hydroxide, and ethanol was added to precipitate the product and clean it. The precipitate was vacuum-dried at 45 °C, and then ground and crushed to obtain the diatomite-modified fluid loss agent, named GFLA (Figure 1).

2.2.2. Fourier-Transform Infrared Spectroscopy

Infrared spectrum analysis (FTIR) can analyze whether the product is successfully synthesized according to the intended design. By shining a beam of infrared rays of different wavelengths onto the molecules of a substance, different groups can absorb specific wavelengths of radiation and analyze whether certain groups are present in the product. The fluid loss agent was dissolved in water, and excess alcohol was added to precipitate the fluid loss agent, and this was repeated three times to obtain the purified product. Taking a small amount of product and adding a certain amount of potassium bromide to prepare the samples, the infrared spectral curve of wave number in 400–1000 cm⁻¹ was obtained [14,15,16].

2.2.3. Rheological Properties of Fluid Loss Agent Suspensions

A Brookfield rheometer was used to test the rheological properties of 0.25%, 0.5%, 1.0%, 1.5%, and 2.0% fluid loss agent suspensions, and to test the change in viscosity of the fluid loss agent suspensions in terms of dosage and shear rate [17,18,19]. The suspension was formulated by homogenizing deionized water with GFLA under high-shear mixing at 10,000 RPM for 15 min to achieve colloidal stability.

2.2.4. Performance Evaluation of Fluid Loss Agent

According to the conventional testing method of drilling fluid properties, an API filtration press was selected to test the filtration [20]. A six-speed viscometer was selected to test the readings of 600 RPM, 300 RPM, 200 RPM, 100 RPM, 6 RPM, and 3 RPM of the drilling fluid. The rheological properties of the drilling fluid were calculated according to the formula below [21,22].

AV = Φ600/2,

(1)

PV = Φ600 − Φ300,

(2)

YP = (2Φ300 − Φ600)/2,

(3)

where AV represents the apparent viscosity, the unit of which is mPa·s; PV represents the plastic viscosity, the unit of which is mPa·s; and YP stands for the yield point in Pa.

Evaluation of Thermal Stability of Fluid Loss Agent

A 4% prehydrated bentonite mud was amended with a 1% self-developed fluid loss agent, which was rolled for 16 h at 160 °C, 170 °C, and 180 °C, respectively [23]. The rheological properties and filtration properties of the mud were tested before and after hot rolling, and the thermal stability of the newly developed fluid loss agent was evaluated.

Evaluation of Salt Pollution Resistance

In the 4% prehydrated bentonite mud, 2% GFLA was added as the experimental mud, and 0%, 0.5%, 4.0%, 10.0%, and 30.0% sodium chloride was added to the experimental mud for hot rolling at 160 °C. The rheological properties and filtration properties of the experimental mud before and after hot rolling were, respectively, tested, and the salt pollution resistance of the newly developed GFLA was evaluated.

Evaluation of Calcium Pollution Resistance

In the 4% prehydrated bentonite mud, 2% GFLA was added as the experimental mud, and 0%, 0.1%, 0.3%, 0.6%, and 1.0% calcium chloride was added to the experimental mud for hot rolling at 160 °C. The rheological properties and filtration properties of the experimental mud before and after hot rolling were, respectively, tested, and the calcium pollution resistance of the newly developed GFLA was evaluated [24].

3. Results and Discussion

3.1. Infrared Spectral Analysis

The characteristic absorption peaks of Si-O bonds in diatomite (760 cm⁻¹ and 472 cm⁻¹) were found in the infrared spectrogram of the modified starch-based fluid loss agent, indicating that diatomite also participated in starch modification (Figure 2). At the same time, the C=O stretching vibration peak of -COO- (1562 cm⁻¹), the bending vibration peak of -CH₂ bond (1010 cm⁻¹), and the characteristic peak of amide group (1650 cm⁻¹) were also found, indicating that the anticipated molecular structure was synthesized [25,26,27].

3.2. Thermogravimetric Analysis

Diatomite exhibited negligible mass loss below 600 °C with a residual mass loss rate of 0.91%, attributed to the removal of free water and the partial decomposition of the organic constituents. The thermal degradation of starch occurred in two distinct stages—the primary stage demonstrated free water evaporation with a maximum mass loss rate at 71.5 °C, while the secondary stage involved the scission of starch molecular chains, initiating decomposition at 298.7 °C and reaching its maximum mass loss rate at 317.70 °C, ultimately achieving an 88% total mass loss (Figure 3).

GFAL displayed two-stage degradation characteristics—the initial stage corresponded to free water elimination (with a maximum mass loss rate at 66.0 °C), followed by the combined decomposition of starch molecules and grafted monomeric chains in the second stage, commencing at 312.93 °C with a peak mass loss rate at 339.78 °C, culminating in a 59.92% cumulative mass loss. Given the high product purity obtained from the synthesis reaction, a single smooth curve is evident in the thermogravimetric profile. When cross-referenced with the previously discussed FTIR spectral data through comprehensive analysis, this suggests the complete participation of all starting materials in the reaction system.

Notably, GFLA demonstrated enhanced thermal stability with the initial decomposition temperature elevated by 14.23 °C compared to native starch. Experimental data under controlled heating (10 °C/min) revealed that the starch reached its mass loss inflection point (59.84% loss) after a mere 28.65 °C temperature increment (2.865 min), whereas GFLA required a 78.95 °C increase (7.895 min) to attain its inflection point at a 36.97% mass loss. The thermal decomposition window expanded from 80 °C for starch to 150 °C for GFAL, indicating successful modification significantly broadened temperature applicability.

Thermogravimetric analysis conclusively demonstrates that GFLA is an effective starch derivative with substantially improved performance characteristics, validating the success of the chemical modification.

3.3. Scanning Electron Microscope Analysis

A ZEISS sigma300 (Oberkochen, Baden-Württemberg, Germany) scanning electron microscope was used to observe the microstructure of diatomite, sodium polyacrylate, corn starch, and the modified starch-based fluid loss agent [28]. The experimental results are shown in Figure 4.

It can be seen from the scanning electron microscope images that the corners of corn starch particles are not clear, and that they have a regular polygon shape. The sodium polyacrylate particles have obvious corners, the surface of the particles is relatively flat, and there are no obvious grooves and textures. The diatomite particles contain a large number of micro and nano pores, which provide space for the physical adsorption of diatomite. The surface of the modified starch-based fluid loss agent particles is rough, and it is difficult to observe the starch particles and some monomer auto polymerization products, which indicates that the morphology of the starch particles and the diatomite has been changed by the grafting reaction. However, the surface of the fluid loss agent is uneven, with a flaky, long-strip, and spherical distribution, indicating that the morphology of diatomite has changed after modification; its specific surface area has increased, and its adsorption area has increased.

3.4. Polarizing Microscope Analysis

It can be seen from the polarizing microscope photos that the corn starch particles are round and polygonal, and that the average particle size is about 6 μm (Figure 5). However, the particle size of the fluid loss agent is irregular, ranging from 1 μm to 20 μm, the distribution is not uniform, and the particle size distribution range is wide. At the same time, no diatomite particles were found under the polarizing microscope, indicating that diatomite particles were covered by the starch’s long-chain grafting during the modification process.

3.5. Performance Evaluation of Fluid Loss Agent Water Suspension

As can be seen from Figure 6 and Figure 7, the fluid loss agent suspension exhibits obvious shear thinning behavior, which is conducive to hole cleaning and drilling fluid flow in the wellbore. At a low shear rate (from 0.01 to 1 s⁻¹), the viscosity decreases rapidly with the increasing shear rate. When the shear rate is large, the viscosity almost does not change, showing the characteristics of a pseudoplastic fluid.

It can be seen from Figure 8 that the viscosity of the suspension gradually increases with the increase in the concentration of the fluid loss agent, indicating that the fluid loss agent has a certain effect in terms of increasing the viscosity of the system. And, in a certain range, the viscosity of the fluid loss agent suspension has a linear positive correlation with the dosage. After hot rolling at 160 °C, the fluid loss agent suspension still exhibits shear dilution characteristics, but its viscosity at low shear rate is significantly reduced compared with that before hot rolling, indicating that hot rolling at a high temperature improves the dispersion of the fluid loss agent particles and decreases the viscosity of the suspension. Also, it can be seen that after hot rolling the viscosity of the fluid loss agent suspension decreases significantly, indicating that the heat degradation of the fluid loss agent molecules occurs after hot rolling. With the increase in fluid loss agent concentration, the viscosity tends to increase, indicating that a partial degradation of the fluid loss agent molecules occurs.

3.6. Properties of GFLA and Other Similar Products

Carboxymethyl starch (CMS), a temperature-resistant modified starch (GCMS), and the modified starch-based fluid loss agent (GFLA) were added into water at 1% concentrations and stirred at 5000 r·min⁻¹ for 30 min. Their viscosities at different shear rates were measured by a Brookfield viscometer. The experimental results are shown in Figure 9.

Figure 9 shows that the suspension (or solution) of the three treatment agents exhibited shear dilution characteristics after they were added to water. At the same concentration, the viscosity of the CMS, GCMS, and GFLA suspension (or solution) was successively CMS > GCMS > GFLA. At the same concentration, the viscosity of the GFLA suspension was the lowest. The results indicated that the diatomite-modified starch-based fluid loss agent had the lowest viscosity.

3.7. Evaluation of Thermal Stability of GFLA

The rheological properties and filtration properties of 4% bentonite-based mud were tested by adding the 1% fluid loss agent at 160 °C, 170 °C, and 180 °C, respectively, to evaluate the thermal stability of the fluid loss agent. The experimental results are shown in

Table 1. Test results of thermal stability evaluation of GFLA.

Temperature/°C	Test Sample	AV /(mPa·s)	PV /(mPa·s)	YP /Pa	YP/PV	Gel10s/Gel10min /(Pa/Pa)	FLAPI /mL	Hk /mm	pH
Room temperature	Mud	5	3	2	0.67	1.5/2.5	28	1	11
	Mud with GFLA	6.25	5.5	0.75	0.14	0/0	6.4	0.5	10
160	Mud	2.5	2	0.5	0.25	0/0	38	1.5	10
	Mud with GFLA	2.5	2	0.5	0.25	0/0	7.6	0.5	9
170	Mud	1.5	1	0.5	0.5	0/0	40	1.5	9
	Mud with GFLA	2.75	2.5	0.25	0.10	0/0	7.2	0.5	9
180	Mud	1.75	1.5	0.25	0.17	0/0	41	1.5	9
	Mud with GFLA	2.25	1.5	0.75	0.5	0/0	10.0	0.5	9

.

It can be seen that the API filter cake, after hot rolling at different temperatures of bentonite mud, broke into small pieces after air-drying (Figure 10a). But the filter cake can still maintain the overall structure after hot rolling at 170 °C and 180 °C with the addition of GFLA, and no obvious breakage occurs, indicating that the fluid loss agent can make the structure of the filter cake more stable and enhance the damage resistance of the filter cake. Due to its large specific surface area and numerous micro and nano pores, the modified starch polymer can be adsorbed onto the surface and within the pores of the diatomite. Diatomite, with a chemical composition mainly consisting of SiO₂, exhibits stable chemical properties. When subjected to high temperatures, diatomite acts as a shield to some extent, thereby retarding the thermal degradation of the modified starch polymer and enhancing its thermal stability. This indicates that the API filtration mud cake still maintains good film-forming properties after hot rolling at 180 °C.

3.8. Evaluation of Salt Pollution Resistance of GFLA

A 2% GFLA was added to 4% bentonite-based mud, and then different concentrations of NaCl were added to evaluate the rheological properties and filtration properties before and after hot rolling at 160 °C. The experimental results are shown in

Table 2. Experimental results of salt resistance evaluation of GFLA.

Concentrations of NaCl /%	Term	AV /(mPa·s)	PV /(mPa·s)	YP /Pa	YP/PV	FLAPI /mL	pH	HK /mm	FLHTHP /mL
0	Before rolling	17.5	12	5.5	0.46	6.4	9	0.5	36
	After rolling	9	8	1	0.13	7.0	7	0.5
0.5	Before rolling	16.5	11	5.5	0.50	6.8	10	0.5	32
	After rolling	10	8	2	0.25	6.0	9	0.5
4.0	Before rolling	20	10	10	1.00	8.8	10	0.5	33
	After rolling	15	9	6	0.67	5.0	8	0.5
10.0	Before rolling	20	7	13	1.86	14.4	10	1	34
	After rolling	11	6	5	0.83	6	7	0.5
30.0	Before rolling	22	7	15	2.14	20.2	10	1	43
	After rolling	8.5	5	3.5	0.70	16	7	0.5

. Different concentrations of NaCl were added to the 4% bentonite-based mud, the API filtration loss of the experimental mud is mesured after hot rolling, and the result is shown in

Table 3. API filtration loss of 4% bentonite-based mud under different NaCl dosage.

Concentrations of NaCl/%	0	0.5	4.0	10.0	30.0
Filtration loss before hot rolling/mL	28	40	80	106	144
Filtration loss after hot rolling/mL	37	52	120	160	220

.

According to the data in Table 2, with the increase in NaCl concentration, the apparent viscosity of the experimental mud decreased and then tended to be stable, the plastic viscosity decreased, and the dynamic shear force increased before hot rolling at 160 °C. The filtrate loss of the experimental mud increased gradually, but the increase was small, and the thickness of the mud cake increased gradually. After hot rolling at 160 °C, with the increase in NaCl concentration, the apparent viscosity of the mud increased and then decreased, the plastic viscosity changed little, the dynamic plastic ratio gradually increased, and the filtration loss at a high temperature and high pressure decreased. Before and after hot rolling, with the increase in NaCl concentration, the filtration loss of the experimental mud changed little, indicating that the salt resistance of the filtration loss reducer was good, and the salt resistance could reach 30% percent. According to the comparison with Table 3, when the concentration of NaCl reaches 30% percent, the filtration loss reduction rate of the 2% fluid loss agent can reach 92.73%.

3.9. Evaluation of Calcium Pollution Resistance of Fluid Loss Agent

The rheological properties and filtration properties before and after hot rolling at 160 °C were tested by adding a 2% filtration reduction agent to a 4% bentonite-based slurry and adding calcium chloride at different concentrations. The experimental results are shown in

Table 4. Experimental results of calcium resistance evaluation of GFLA.

Concentrations of CaCl2/%	Term	AV /(mPa·s)	PV /(mPa·s)	YP /Pa	YP/PV	FLAPI /mL	pH	HK /mm	FLHTHP /mL
0	Before rolling	17.5	12	5.5	0.46	6.4	9	0.5	36
	After rolling	9	8	1	0.13	7.0	7	0.5
0.1	Before rolling	21	17	4	0.24	6.8	9	0.5	38
	After rolling	7.5	6	1.5	0.25	6	9	0.5
0.3	Before rolling	14	11	3	0.27	4.2	7	0.5	41
	After rolling	6.5	5	1.5	0.30	4.0	7	0.5
0.6	Before rolling	15	12	3	0.25	10.0	7	0.5	46
	After rolling	6	4.5	1.5	0.33	9.0	7	0.5
1.0	Before rolling	16	9	7	0.78	42	7	3	120
	After rolling	11	7	4	0.57	39.2	7	3

.

As shown in Table 4, before hot rolling at 160 °C, with the increase of calcium chloride concentration, the dynamic-to-plastic viscosity ratio of the experimental mud decreased, and the filtration loss decreased first and then increased. At a 1.0% concentration, the filtration loss increased to 42 mL, with a large increase and the thickness of the mud cake gradually increasing. After hot rolling at 160 °C, with the increase in the calcium chloride concentration, the apparent viscosity and plastic viscosity of the experimental mud first decreased and then increased. Before and after hot rolling, when the concentration of the calcium chloride is less than 0.6%, the API filtration loss of the experimental mud decreases first and then increases with the increase in calcium chloride concentration, and the change in high temperature and high pressure filtration loss is small, indicating that GFLA has good calcium resistance. However, when the concentration was 1.0%, the API filtration capacity of the experimental mud rapidly increased to 39.2 mL, and the high-temperature and high-pressure filtration capacity increased to 120 mL, indicating that the anti-calcium pollution concentration of GFLA was 0.6%. As the concentration continued to increase to 1.0%, the drilling fluid was over-flocculated, the dynamic shear force of the drilling fluid increased significantly, the colloidal state of the system was destroyed, and the fluid loss increased.

3.10. Mechanism Analysis of GFLA

3.10.1. Water Swelling

The fluid loss agent was added to deionized water at a 1% concentration and stirred at a high speed of 5000 r·min⁻¹ for different durations to test the particle size distribution of the suspension. The experimental results are shown in Figure 11.

It can be seen from Figure 11 that the particle size of the suspension increases with the increase in stirring time, indicating that the particle size of the filtration reducer is increased through the effect of water absorption after the addition of the agent to the water, which is conducive to the physical sealing effect of the filtration reducer particles. There are a large number of hydroxyl, amide, and other polar groups on the surface of the fluid loss agent, which can adsorb water molecules through hydrogen bonding. Therefore, in a certain temperature range, the fluid loss agent shows a good water absorption performance. This is conducive to reducing the free water content in drilling fluid, and the volume of the fluid loss agent after the adsorption of a large number of water molecules increases significantly as they become expandable elastic deformation particles, improving the compressibility of the mud cake and reducing the solid-phase content of the mud cake. The better the compressibility of the mud cake and the lower the solid-phase content, the more difficult it is for drilling fluid filtrate to invade.

3.10.2. The Formation of the Grid Structure

The API mud cake with a 4% bentonite experimental mud with GFLA added was freeze-dried, and its microstructure was observed by scanning electron microscopy. The experimental results are shown in Figure 12.

It can be seen from the scanning electron microscope photos that the API mud cake of bentonite mud is relatively loose and has no regular structure, but it can be seen that the clay particles are connected end–end and end–plane, and that the pores and cracks are large. The experimental mud with GFLA added can form a tight connection, with small and dense pores, and each part is connected to another to form a bridge in which the pores are formed after the sublimation of water molecules. Under the same magnification, the degree of porosity of the two samples is obviously different.

3.11. Biological Toxicity and Degradability

In the laboratory modification process of the diatomite-modified starch-based fluid loss agent, no heavy metal additives were introduced. Consequently, analysis of heavy metal elements was unnecessary, with evaluations focused solely on its biodegradability and biotoxicity.

The biodegradability of the fluid loss agent GFLA was assessed following the biotoxicity classification standards for oilfield chemicals and drilling fluids outlined in the China National Petroleum Corporation Enterprise Standard SY/T 6787-2010 [29]. The results indicated that GFLA exhibits readily degradable characteristics (BOD₅/COD_Cr ≥ 5%), demonstrating values slightly inferior to native starch (

Table 5. Experimental results of environmental performance evaluation.

Material	BOD5	CODCr	BOD5/CODCr	EC50 (×104 mg·L−1)
GFLA	232.7	823.7	28.3%	8.4
Starch	117.4	380.2	30.9%	11.2

). The biotoxicity evaluation confirmed that GFLA is nontoxic (>2.0 × 10⁴ mg·L⁻¹).

4. Conclusions

This article uses starch as the raw material and prepares a starch-based fluid loss agent named GFLA that can be used in oil drilling through grafting modification.

(1)

The graft-modified GFLA exhibits an irregular morphology with an increased specific surface area. This unique structural configuration facilitates enhanced adsorption potential, effectively reducing free water content in the drilling fluid and consequently achieving a significant filtrate reduction.

(2)

Compared to conventional starch-based fluid loss agents, the developed GFLA demonstrates a minimized viscosity effect after 160 °C hot rolling while maintaining temperature resistance up to 180 °C. Notably, even prior to rolling, it exhibits a negligible impact on the rheological properties of drilling fluids (Table 1).

(3)

At a 2% dosage concentration, GFLA shows excellent contamination resistance with a 30% NaCl and a 0.6% CaCl₂ tolerance. This indicates that GFLA is suitable for complex salt systems containing ≥ 20% monovalent salt ions, which enables GFLA to be effectively applied in high-salinity drilling environments.

(4)

Water absorption analysis reveals that GFLA’s 30 min uptake capacity significantly surpasses its 2 min absorption (Figure 10). Filter cake characterization (Figure 11) indicates improved compactness in additive-containing systems. These findings confirm dual working mechanisms: (i) hydration-induced expansion and (ii) three-dimensional network formation, synergistically contributing to effective fluid loss control.