Effect of lubricating additives on the tribological properties of polymer-clay drilling fluid in a metal-to-metal sliding contact

Original article

Abusal Yusef A.Y., Yakhin A.R., Silnov D.V., Araslanova D.I., Gorshkov V.A. Effect of lubricating additives on the tribological properties of polymer-clay drilling fluid in a metal-to-metal sliding contact. Nanotechnologies in Construction. 2026;18(4):521–528. https://doi. org/10.15828/2075-8545-2026-18-4-521-528. – EDN: OCBSKY.

Абусал Юсеф А.Ю., Яхин А. Р., Сильнов Д.В., Арасланова Д.И., Горшков В.А. Исследование влияния смазывающих добавок на трибологические характеристики полимер-глинистого бурового раствора в паре трения «металл–металл». Нанотехнологии в строительстве. 2026;18(4):521–528. – EDN: OCBSKY.

Modern technologies for constructing oil and gas wells involve a significant increase in the length of horizontal sections and the complexity of the wellbore profile [1, 2]. According to estimates by service companies, the share of wells with horizontal completions in the total drilling volume in 2023–2025 exceeded 60% in a number of oil and gas regions [25]. Under such conditions, the drill string experiences increased loads due to friction against the wellbore walls and casing pipes. High torques and resistance forces during tripping operations not only increase energy consumption but also create the risk of differential sticking, which leads to accidents and increased well construction costs [3, 4].

Traditional water-based lubricating additives can reduce friction, but their effectiveness is limited under high contact loads, and some formulations may negatively affect the technological parameters of the fluid [5–9]. Furthermore, at high inclination angles, conventional

Nanotechnologies in construction

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2026; 18 (4): 521–528

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lubricants can be washed away from the friction surface, failing to provide a stable protective layer [8].

In recent years, the direction of using surfactants and hydrocarbon components as modifiers of lubricating properties has been actively developing [10, 11]. Due to their ability to adsorb on the metal surface and form boundary layers, such additives can significantly alter the friction mechanism, reducing the coefficient of friction even at relatively low concentrations [12, 13]. Studies show that compositions based on waste oil and surfactants can improve the tribological characteristics of drilling fluids [14, 15, 21–24].

Despite the existing progress, a practical gap remains in the available literature: many works are focused on individual types of additives, do not always consider synergistic effects when they are used together, and also do not provide comparisons in a single base formulation and under identical test conditions on a «metal-to-metal» pair. In this regard, there is a need for a systematic study that allows evaluating both the individual efficiency of various additives and their combined effect in a polymerclay fluid.

To fill this gap, the present work investigates the effect of paraffins, the EZО additive, fatty acids (soaps), waste oil, as well as compositions with surfactants on the coefficient of friction of a polymer-clay fluid in a «metal-to-metal» pair. The evaluation was carried out for additive concentrations of 0.2%; 0.5%; 1.0%; and 2.0% by volume with determination of friction reduction efficiency.

Stages of the study:

• -    Preparation of the base polymer-clay fluid and its modification with lubricating additives at specified concentrations.

• -    Conducting tribological tests on an OFITE E.P. lubricity tester and determination of optimal types and concentrations of additives.

• -    Evaluation of synergistic effects when using additives together against a background of waste oil.

• -    Analysis of the effect of oil concentration on the efficiency of lubricating components.

METHODS AND MATERIALS

Preparation of the base polymer-clay fluid

For the experiments, a polymer-clay fluid (PCF) of the following composition was used as a base, wt.%:

• -    technical water – balance;

• -    bentonite (PBMB) – 4%;

• -    polyanionic cellulose low viscosity (PAC-LV) – 0.4%; - marble chips (CaCO₃, fraction 5–50 µm) – 5%.

The fluid was prepared using a high-speed mixer with a rotation speed of 1500 rpm. Sequence of component addition:

• -    Bentonite – mixing for 20 min for hydration;

• -    PAC-LV – mixing for 15 min;

• -    CaCO₃ – mixing for 10 min.

After preparation, the fluid was aged for 24 hours to stabilize its properties.

Tested additives and their introduction

The following types of additives were studied in this work:

• -    liquid paraffins (concentrations 0.2%; 0.5%; 1.0%);

• -    EZО additive (0.2%; 0.5%);

• -    fatty acids (soaps) (0.2%; 0.5%);

• -    waste motor oil (0.25%; 0.5%; 1.0%; 2.0%);

• -    composition «surfactant + liquid paraffins» (2% surfactant).

The introduction of additives was carried out in two stages: first, the additive was mixed with a small volume (10% of the total volume) of the base fluid in a laboratory beaker, then the resulting mixture was mixed on a highspeed mixer (IKA T25) at 10,000 rpm for 15 min. After that, the composition was added to the remaining volume of the fluid and mixed for 20 min at 1500 rpm. The fluid was aged for 24 hours before testing.

Tribological tests

The evaluation of lubricating ability was carried out on an OFITE E.P. Lubricity Tester (model 112-00), compliant with API 13B-1 requirements.

Test conditions:

• -    friction pair: standard block made of AISI 3135 steel pressed against a rotating ring made of AISI 4620 steel;

• -    load: 150 inch-pounds (16.95 N·m);

• -    ring rotation speed: 60 rpm;

• -    test duration: 10 min until steady-state friction regime was established;

• -    temperature: 24±2 °C.

The coefficient of friction (µ) was calculated using the formula:

µ = M·K·F,                                 (1)

where M – measured torque (N·m);

K – instrument calibration coefficient (1.47);

F – normal force (N).

Friction reduction efficiency (∆µ, %) was determined relative to the base fluid:

• .      ц. base-ц. test

Д|1 = ———— X 100%,        (2)

p_base where µ_base – coefficient of friction of the base fluid (without additives);

µ_test – coefficient of friction of the fluid with the tested additive.

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For each composition, no fewer than five parallel measurements were performed, and gross errors were excluded (Grubbs' criterion). Statistical processing was carried out using one-way analysis of variance (ANOVA) followed by post-hoc testing (Tukey HSD). The significance level was taken as p < 0.05.

RESULTS AND DISCUSSION

Effect of additives on the coefficient of friction and efficiency in the base fluid

Table 1 presents the test results of additives introduced directly into the base formulation without the addition of oil.

As can be seen from the data, the highest efficiency is demonstrated by the EZO additive, providing a reduction in the coefficient of friction by 87.4–87.7% even at a minimum concentration of 0.2%. However, given the high cost of this reagent, its use in pure form may be economically unjustified. An alternative is waste oil, which, despite its modest efficiency (26.5–26.8%), has a low cost.

Effects of combined use of lubricating additives against a background of oil

To evaluate synergistic effects, the additives were introduced into a fluid containing waste oil at concentrations of 0.25–2.0%. The results are presented in Table 2.

Analysis of the obtained data allows identifying several patterns

When small amounts of oil (0.25%) are added, a synergistic effect is observed. For the composition of BF

Table 1. Effect of additives on the coefficient of friction and friction reduction efficiency in the base fluid

Fluid composition	Additive concentration, %	Coefficient of friction (CoF)	Efficiency, %
BF (Base Formulation)	–	0,373	–
BF + paraffins	0.2	0.333	10.72%
	0.5	0.319	14.48%
BF + EZO	0.2	0.047	87.40%
	0.5	0.046	87.67%
BF + fatty acids (soaps)	0.2	0.304	18.50%
	0.5	0.289	22.52%
BF + waste oil	0.2	0.274	26.54%
	0.5	0.273	26.81%

Table 2. Effect of additives against a background of oil in a freshwater fluid

Fluid composition	Additive concentration	Parameter	Oil concentration
			0.25%	0.5%	1.0%	2%
BF + liquid paraffins	0.20%	CoF	0.245	–	0.311	–
		Efficiency, %	34.32%	–	16.62%	–
	0.50%	CoF	0.295	–	–	–
		Efficiency, %	20.91%	–	–	–
	1%	CoF	0.345	–	–	–
		Efficiency, %	7.51%	–	–	–
BF + EZO	0.20%	CoF	0.244	–	0.312	–
		Efficiency, %	34.58%	–	16.35%	–
BF + fatty acids	0.20%	CoF	0.265	–	–	–
		Efficiency, %	28.95%	–	–	–
	0.50%	CoF	0.241	–	–	–
		Efficiency, %	35.39%	–	–	–
BF + surfactant + liquid paraffins	2%	CoF	0.229	0.167	0.151	0.108
		Efficiency, %	38.61%	55.23%	59.52%	71.05%

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containing oil and liquid paraffins (0.2%), the efficiency percentage is 34.32%, which is significantly higher than that of the same additive but without oil (10.72% in Table 1). A similar pattern is observed for the EZO additive and fatty acids.

An increase in oil concentration above 0.5% leads to a decrease in additive efficiency. For BF containing oil and liquid paraffins, increasing the oil fraction from 0.25% to 1.0% reduces efficiency from 34.32% to 7.51%. This dynamics indicates either a dilution effect or incompatibility of components: an excess of oil prevents the active elements of the additives from creating a stable protective film.

When using surfactants in the composition of BF with oil and liquid paraffins, it leads to a reduction in the coefficient of friction. At a low content of oil (0.25%) and liquid paraffins (0.2%) in BF, the coefficient of friction is 0.229, and the friction reduction efficiency is 38.61%. However, at high oil concentrations (2%), the presence of surfactants demonstrates anomalously high efficiency of up to 71.05% (coefficient of friction is 0.108). It is possible that the surfactant is a critical component that works only in highly concentrated oil media, acting as an emulsifier or stabilizer of the lubricating layer structure.

Effect of drilling fluid mineralization on additive efficiency

To assess the effect of drilling fluid mineralization on additive efficiency, experiments were conducted with BF mineralized with potassium chloride (KCl). The results are presented in Table 3. The coefficient of friction for BF + KCl is 0.123.

Thus, in the KCl environment, the overall level of the coefficient of friction is lower than in the base fluid. At high oil concentrations (2%), the fluid with surfactant provides a friction reduction of 53.66%.

Conclusion

Based on the experimental data, three key mechanisms can be identified that operate in the studied compositions.

Formation of an adsorption film. Polar molecules (e.g., fatty acids and surfactants) settle on the metal surface, creating a thin boundary layer that is easily sheared. This leads to a reduction in the coefficient of friction under boundary lubrication conditions [5, 8]. If hydrocarbons are additionally introduced, the stability of this layer increases, which enhances the effect.

Table 3. Effect of additives against a background of oil in a mineralized fluid

Fluid composition	Additive concentration	Parameter	Oil concentration
			0.25%	0.5%	1.0%	2%
BF + KCl + oil		CoF	0.106	–	–	–
		Efficiency, %	13.82%	–	–	–
BF + KCl + liquid paraffins	0.20%	CoF	–	0.106	0.074	–
		Efficiency, %	–	13.82%	39.84%	–
	0.50%	CoF	0.121	–	–	–
		Efficiency, %	1.63%	–	–	–
	1%	CoF	0.116	–	–	–
		Efficiency, %	5.69%	–	–	–
BF + KCl + EZO	0.20%	CoF	–	0.114	0.115	–
		Efficiency, %	–	7.32%	6.50%	–
	0.5	CoF	0.106	0.101	–	–
		Efficiency, %	13.82%	17.89%	–	–
BF + KCl + fatty acids	0.20%	CoF	–	0.117	–	–
		Efficiency, %	–	4.88%	–	–
	0.50%	CoF	0.095	–	–	–
		Efficiency, %	22.76%	–	–	–
BF + KCl + surfactant + liquid paraffins	2%	CoF	–	0.087	0.061	0.057
		Efficiency, %	–	29.27%	50.41%	53.66%

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Appearance of a «third body». Microscopic solid particles (including wear products or components of waste oil) can act as an interlayer separating the rubbing pairs. However, when there is too much oil (more than 0.5%), the active components literally «drown» in it — hence the drop in efficiency when it is in excess.

Colloidal structure with the participation of surfactants. At high oil dosages (2.0%), the surfactant begins to act as an emulsifier, stabilizing a fine emulsion. Such an emulsion forms a stable film with anomalously low frictional resistance. This conclusion is confirmed by a sharp increase in efficiency when transitioning from 1.0% to 2.0% oil when a surfactant is present in the composition.

Our findings are in good agreement with literature data [14, 15, 17], which show that the combination of hydrocarbons and surfactants provides synergy in friction reduction.

CONCLUSION

It has been experimentally proven that mixed lubricants based on waste oil, surfactants and standard additives (paraffins, EZO, fatty acids) effectively improve the tribology of polymer-clay drilling fluids.

The greatest effect (friction reduction of 87.7%) is provided by the EZO additive, but its use is constrained by its high cost. Waste oil itself without additives reduces friction by 26.5–26.8%, which is advantageous in terms of cost.

A synergistic effect was found in the presence of 0.25% oil: the efficiency of liquid paraffins increases from 10.72% to 34.32%, and of fatty acids — from 22.52% to 35.39%.

Exceeding the threshold of 0.5% oil worsens the performance of most additives due to competition for the surface and dilution. The only exception is the composition with surfactant: at 2.0% oil, its efficiency reaches 71.05% due to the formation of a stable emulsion structure.

In a saline environment (KCl), the patterns are not disrupted: the mixture of surfactant with 2.0% oil reduces friction by 53.66%, which indicates the reliability of the proposed mechanism even under conditions of high mineralization.

Thus, the developed compositions based on waste oil and surfactants can be recommended as a cheap and effective alternative for lubricating «metal-to-metal» sliding contact during drilling of horizontal and directional wells with high contact loads.