Applicability Assessment of Viscoelastic Surfactants and Synthetic Polymers as a Base of Hydraulic Fracturing Fluids

Abstract

Hydraulic fracturing (HF) is currently the most widespread and effective method of oil production stimulation. The most commonly used fracturing fluid is crosslinked guar gels. However, when using these systems, problems such as clogging of the pore space, cracking, and proppant packing with the remains of the undestroyed polymer arise. Therefore, the efficiency of the hydraulic fracturing process decreases. In this work, compositions based on viscoelastic surfactants (VES) and synthetic polymers (SP) were considered as alternatives capable of minimizing these disadvantages. Most often, the possibility of using a composition as a fracturing fluid is evaluated using rotational viscometry. However, rotational viscometry is not capable of fully assessing the structural and mechanical properties of fracturing fluid. This leads to a reduced spread of systems based on VES and SP. This paper proposes an integrated approach to assessing the effectiveness of a water-based fracturing fluid. The proposed comprehensive approach includes an assessment of the main characteristics of water-based fracturing fluids, including an analysis of their structural and mechanical properties, which is based on a combination of rotational and oscillatory rheology and a comparative analysis of methods for studying the influence of fluids on the reservoir rock. The use of the developed approach to assess the technological properties of fracturing fluids makes it possible to demonstrate the potential applicability of new, unconventional fracturing fluids such as systems based on VES and SP.

1. Introduction

Petroleum remains the most important energy source, and in 2019, the share of oil in the global energy balance was 33.1% [1]. Many old and developed hydrocarbon fields are hard-to-recover (HTR) [2,3,4,5].

HF is an effective and the most widely used method of developing such HTR reserves [6,7,8,9,10,11]. In the 1960s, the first hydraulic fracturing fluids were used on a hydrocarbon basis. From the 1950s to the 1980s, the proportion of oil-based fluids in the fracturing process decreased from 100 to 20%, which reduced the costs and risks of treatment [12,13,14]. Aqueous solutions of polymers, such as starches [15], guar gum [16,17], xanthan gum [18,19,20], carboxymethyl cellulose [21,22,23,24], and polyacrylamide [25,26], have come to replace hydrocarbon-based fluids [27,28]. Despite the low content of chemical reagents in water-based hydraulic fracturing fluids (less than 2–3 vol.% [27,29]), many environmental organizations oppose hydraulic fracturing operations. The main argument against hydraulic fracturing is the pollution of underground sources of fresh water, which occurs due to the ingress of hydrocarbons and process fluids into the aquifers through the resulting cracks [30,31,32]. When modeling the hydraulic fracturing process, many researchers note that the probability of groundwater contamination with hydrocarbons and process fluids is extremely small. During the simulation, 0.02% of the injected fluid reaches the aquifer, and more than 60% of the process fluid is recovered from the reservoir when production is resumed [33,34]. The problem of groundwater contamination during hydraulic fracturing can arise in the absence of detailed information about the geological features. Possible problems can also be caused by human factors [35].

The most widespread are liquids thickened with guar gum [27,36]. When performing HF operations using traditional guar-based fracturing fluids, problems consisting of significant clogging of the pore space and the resulting cracks with residues of undestroyed fluid arise [37]. The high viscosity of crosslinked guar gels makes it difficult to transport the proppant deep into the low-permeability formation. As a result, cracks are formed predominantly growing in height along the reservoir, while the main goal during treatment is to create a long conductive fracture that propagates as deep as possible into the productive interval [38].

In the case of operations using guar-based fracturing fluids, the fluid composition is selected individually for each field and, depending on the characteristics of the candidate well, may include, in addition to the main components (gelling agent, crosslinker, and destructor), a number of additional modifier additives. Such fracturing fluids are sensitive to the quality of water—which forms the base of the gel and leads to increased preparation costs associated with bringing fresh water to the well and heating it up. In addition, gels based on guar gum, even after destruction, form sediments that are insoluble in water and oil, thus clogging the proppant pack by up to 10–50%, and sometimes even more.

In modern HF practice, the tasks of developing and introducing fracturing fluids that reduce the above negative consequences to a minimum are becoming urgent. Such fluids can include systems based on viscoelastic surfactants (VES, which are non-polymer fracturing fluids) [39,40,41,42,43,44,45,46,47,48] and synthetic polymers such as polyacrylamide (PAM) and products based on it [22,25,26]. Each of these systems has its own advantages.

According to the practice of Russian oil and service companies, the recommended viscosity of a fracturing fluid based on guar gum varies in the range of 350–1000 mPa·s at 100 s⁻¹. Fluid viscosity values are published in documents titled “Technical requirements for the development of a hydraulic fracturing fluid formulation”, which are published along with the tender documentation for the purchase of reagents. For example, for the conditions of the Rosneft Nyagan branch in 2021, an initial viscosity parameter of 600 cP was specified for a cross-linked guar system. It is believed that it is the effective viscosity parameter that describes the ability of the fracturing fluid to retain the proppant in volume and transport it to the formed fracture. Systems with such a viscosity can hold the proppant volume in the well and transport it to the resulting fracture, thereby ensuring high efficiency of the whole operation. Rotational viscometry is a widely used method for determining the effective viscosity of fracturing fluids [49,50,51,52,53]. However, this type of measurement does not fully describe the structural and mechanical properties of liquids, which leads to a reduced spread of systems based on VES and synthetic polymers. This problem can be solved by using an additional method for assessing the structural and mechanical properties of fracturing fluids based on oscillatory rheology [43,54,55,56].

Some hydrocarbon reservoirs are characterized by the presence of clay minerals. The penetration of fresh water as part of HF fluid into such reservoirs during various technological operations can lead to swelling of clay minerals, a decrease in permeability and, as a consequence, a decrease in the efficiency of the work performed [54,57,58,59]. As a result, one of the important parameters of the quality of the HF fluid is its effect on the water-sensitive areas of the reservoir. There is a long list of methods used to study the swelling of clays, but there is an acute issue related to the interpretation of the obtained experimental data and their extrapolation to the real conditions of the reservoir.

Various methods are used to assess the suitability of new fracturing fluids. In this article, the authors propose an approach using well-known techniques: a combination of rotational and oscillatory rheology and comprehensive studies of the effect of fluid on the rock. This paper compares traditionally used water-based fracturing fluids (cross-linked guar gels) with surfactant and synthetic polymer-based fluids that are widely used in fracturing processes.

2. Materials and Methods

Four types of water-based HF fluids were selected as objects of study:

• “Linear” guar gel: guar polymer solution at a concentration of 3.5 kg/m³;
• “Crosslinked” guar gel: a solution of guar polymer at a concentration of 3.3 kg/m³ crosslinked by the addition of 2.4 L/m³ borate crosslinker with the inclusion of 2.0 L/m³ clay stabilizer, 2.5 L/m³ demulsifier, 2.0 L/m³ destruction activator, and 0.3 kg/m³ destructor (ammonium persulphuric acid);
• Non-polymer composition: 70.0 L/m³ solution of ampholytic surfactant (NEFTENOL-VES Pat. RU 2 746 499 Basic chemical compound oleylamidopropyl betaine (OAPB)) with the addition of 18.0 L/m³ of the RS-1 structuring reagent;
• Liquid based on synthetic polymer: solution of 8.0 L/m³ synthetic polymer (gelling agent PolyGel, based on partially hydrolyzed polyacrylamide (PHPA) suspension in hydrocarbon) with the addition of 2.0 L/m³ clay stabilizer, 2.5 L/m³ demulsifier, and 0.5 kg/m³ destructor (ammonium persulfate).

For the above-mentioned systems, a comparative analysis was carried out of their viscous, elastic, structural, and mechanical properties, their sand-carrying and sand-holding capacity, and the effect of “destroyed” systems on the swelling of the clay component of the core material.

In this work, all experiments using the above techniques were carried out at least three parallel experiments to ensure reproducibility, reliability and repeatability of the results.

2.1. Rheological Investigation

Most of the fracturing fluids are Newtonian pseudoplastic systems. Such systems are described using the formula:

$$\tau =k·{\gamma }^n,$$

(1)

where $\tau$ is the shear stress in Pa; $k$ is the consistency coefficient in mPa·s; $\gamma$ is the shear rate in s⁻¹; n is the flow behavior index. In turn, the effective viscosity (μ) of such systems is determined by the following formula [60,61]:

$$\mu =\frac{\tau }{\gamma }.$$

(2)

Rotational viscometry is commonly used method to determine the viscosity of HF fluids. The essence of the method is that the test fluid is placed in a gap between two coaxial cylinders, one of which (the rotor) rotates at a constant speed. The studies were carried out in accordance with the ISO 13503-1 [62]. The standard specifies the parameters of measuring cells traditionally used to determine the effective viscosity of fracturing fluids. The torque transmitted by the fluid from the rotor to the stationary cylinder is a measure of the viscosity of the investigated fluid. In this work, the effective viscosity was measured on a GRACE M5600 rotary rheometer at a constant shear rate corresponding to 100 s^–1.

In the project called “Rational development of liquid hydrocarbons of the planet” at Gubkin University a comprehensive approach to assessing the structural and mechanical properties of HF fluid, based on the use of not only rotational, but also oscillatory rheology, was proposed.

Oscillatory rheology quantifies the assessment of both the viscous and elastic properties of a material. Oscillation studies were carried out on a rotary rheometer GRACE M5600. In this type of research, the magnitude of the moment of the forces applied to the rotor changes according to the harmonic law. In this experiment, the frequency was varied in the range from 0.01 to 3.0 Hz at a fixed voltage amplitude. During measurements, the test sample is minimally deformed, which allows for a more detailed and accurate assessment of the properties of the test sample. In oscillatory studies, the frequency dependencies of such parameters as the modulus of accumulation (elasticity, G′) and the modulus of losses (viscosity, G″) are determined [46,55,56,63,64,65,66].

The rheological properties of structured fluids in the presence of worm-like micelles or long polymer chains in the system can be described by the Maxwell model for viscoelastic bodies. According to this model, in the mode of vibrational shear deformations, the modules G′ and G″ obey the following relations [43,56,63,66,67]:

$$G^{\prime }=\frac{{\omega }^2{\tau }_R^2}{1+{\omega }^2{\tau }_R^2}{G}_0;G^{″}=\frac{\omega {\tau }_R}{1+{\omega }^2{\tau }_R^2}{G}_0,$$

(3)

where G′ is the module of accumulation corresponding to the elastic response of the system, Pa; G″ is the module of losses characterizing the viscosity properties of the sample, Pa; ω is the vibration frequency, Hz; τ_R is stress relaxation time, s; and G₀ is the module of elasticity at its high-frequency plateau, Pa.

The dependencies curves of the modulus of accumulation and the modulus of losses on the oscillation frequency have an intersection at a frequency equal to the reciprocal of the relaxation time [63]:

$${\tau }_R=1/\omega ,\mathrm{for}{G}^{\prime }=G^{″},$$

(4)

The stress relaxation time is a parameter that can be used to evaluate the structural properties of viscoelastic systems. This parameter is determined by the time required to relieve stress in the material due to structural changes [64].

In addition, knowing G₀ and using the known values of τ_R to assess the structuredness of the system, it is possible to determine the cell size of intertwined micellar or polymer networks (correlation length, ξ) [68,69]:

$$\xi ={\left(\frac{k_{B}T}{G_0}\right)}^{1/3},$$

(5)

where k_B is the Boltzmann constant (J/K) and Т is the temperature (K).

2.2. Sand-Carrying and Sand-Holding Capacity

The fracturing fluid must be held in suspension in order to transport the proppant or sand. The free fall of proppant particles in the fracture fluid sample is considered to be the fluid retention capacity. To carry out the test, a sample of the fluid being investigated is poured into glass cylinders. A proppant or sand particle is placed in the sample and the time for the particle to pass through 10 cm of the liquid column and the rate of its settling in cm/min are measured.

To determine the sand-carrying capacity, the proppant is introduced into the fracturing fluid at the stage of its preparation. The suspension is then poured into a graduated cylinder and the settling of the proppant in the volume of the liquid over time is measured. The HF fluid must maintain a given concentration of proppant without visible settling during the entire time of its transportation to the hydraulic fracture [70].

2.3. Rock Swelling Studies

The sources provide the following methods for assessing the effect of fluid on the rock [71,72,73,74]:

• determining the swelling coefficients on the Zhigach–Yarov device;
• determining the linear deformation of a rock sample using a Zhigach–Yarov device or similar;
• determining the rock swelling in free volume by sedimentation stability of the suspension;
• determining the rock swelling using a capillary suction timer;
• determining the volumetric deformation of the compressed tablet;
• determining the stability of the core material in the roller furnace;
• studying the changes in the filtration-capacity properties of artificial (bulk) models or samples of natural cores.

Such a variety of techniques leads the researcher to a difficult choice—which technique to use to assess the stability of clays in clayey rock samples and to assess the effectiveness of the reagent/stabilizer.

2.4. Zhigach–Yarov Method

The method for measuring the swelling coefficient of rocks and clay powders is based on determining the volume of the rock (clay powder) as it swells in the studied liquid medium. The values of the sample height before and after swelling obtained from the readings of the dial gauge, when multiplied by the inner area of the cell, give the volumes. Further, the results can be processed in graphical (Figure 1) and mathematical ways.

The swelling coefficient is determined by the formula:

$$K=\frac{\gamma · \alpha }{m}+tg\beta -1,$$

(6)

where K is the swelling coefficient equal to the ratio of the volume of liquid V_liq bound by the sample to the volume of dry particles V₀, γ is the density of dry clay (g/сm³), m is the mass of the sample (g), tgβ is the coefficient showing what proportion of the pore volume in a dry sample is retained in a swollen sample (in the form of an immobilized liquid), and α is the coefficient depending on the properties of the clay and the value of tgβ.

The volume of the swollen sample V_sw at any value of the initial volume of the dry sample V_beg is equal to the sum of the volumes, Equation (7) (see Figure 2):

$$V_{sw}=V_0+V_{liq}+V_{liq}^{\prime }$$

(7)

where $V_0$ is the volume of dry particles of clay rock (ratio of mass to density), V_liq is the volume of the swelling liquid, and $V_{liq}^{\prime }$ is the volume of the immobilized liquid.

$$V_{liq}^{\prime }=\left(V_{beg}-V_0\right)·tg\beta .$$

(8)

Substituting the value $V_{liq}^{\prime }$ in Equation (5), we obtain:

$$V_{liq}^{\prime }=\left(V_{beg}-V_0\right)·tg.$$

(9)

To determine the value of $tg\beta$, it is necessary to solve at least two Equation (9) with different values:

$$\begin{array}{c}{V}_{liq}=V_{sw1}-V_0-\left(V_{beg 1}-V_0\right)·tg\beta ,\\ V_{liq}=V_{swi}-V_0-\left(V_{begi}-V_0\right)·tg\beta .\end{array}$$

It should be noted that an important aspect for the correct design of the experiment and for obtaining good results (with a small deviation) is to take into account the ratio of the masses of the test sample and the liquid in which the measuring cell is placed. The description of the method does not indicate the exact ratio of the inhibitor solution and the mass of the sample, it is only important that the liquid level be above the piston level. For distilled water and saturated brines, the effect caused by different ratios of the weighed portions of the sample and liquid do not give any practical errors in measurements. When examining solutions of inhibitors with working concentrations (1–10 L/m³), it is possible to obtain rather large discrepancies in the results. This effect is associated with the number of molecules of the active substance of the solution. A change in the volume of the solution entails a change in the concentration of the active substance relative to the weighed portion of the test sample.

2.5. Linear Deformation Method

This method for determining the coefficient of linear deformation simplifies the calculations of the method given above, and the coefficient of swelling here determines the value of the ratio of the final volume of the sample (swollen during the experiment) to the initial volume. Similar measurement methods are implemented in the Model 2100 Linear Swell Meter manufactured by Fann [73].

2.6. Capillary Suction Timer (CST)

This method uses the capillary suction pressure of porous paper to influence filtration. When the suspension is filtered by this suction pressure, the rate at which the filtrate spreads from the suspension is predominantly determined by the filterability of the suspension. CST automatically measures the filtrate advance time between radially located electrodes when a specified area of special filter paper is impregnated with the liquid phase of the suspension [71].

The method for studying the swelling of clays, based on the action of the CST, is easy to use but does not take into account the rate of sorption of the clay stabilizer on the surface of the suspended rock in an aqueous medium. In addition, this technique does not consider the possible adsorption of surfactants on the surface of the paper filter and the possible change in the rate of soaking.

2.7. Visual Assessment of the Sedimentation Stability of the Suspension

This technique allows one to visually assess the effect of the clay swelling inhibitor. With good inhibition, the clay suspension quickly stratifies. In the case of using an uninhibited solution, the suspension remains stable for a long time (provided that well-swelling clay powder samples are used) [72].

Despite the simplicity of it, preparing and conducting the experiment, the method suffers from the low accuracy inherent in all methods of visual assessment of the effectiveness of reagents. Therefore, it should be used mainly for the initial determination of the presence or absence of the inhibitory ability of substances and for the determination of the working concentration.

2.8. Testing the Dynamic Stability of Core Samples

Testing was carried out in accordance with API RP 13I [74]. This assessment method allows one to determine the effect of a fluid on large samples of core material.

3. Results and Discussion

3.1. Investigation of the Structural and Mechanical Properties of Fracturing Fluids

In contrast to rotational viscometry, oscillatory rheology allows the evaluation of both the viscosity and elastic properties of a material. The sample is minimally deformed during measurements, which allows a more detailed and accurate assessment of its structural and mechanical properties [63,67,75].

In order to evaluate and compare the structural and mechanical properties of various water-based HF fluids, a comparative analysis of the frequency dependencies of the modules of elasticity and viscosity of these compositions was carried out. The results are shown in Figure 3.

As can be seen from the results in the “linear” guar gel the viscosity modulus prevails over the elastic modulus over the entire frequency range. Compositions based on “cross-linked” guar gel, VES, and synthetic polymers have different frequency dependencies for the elasticity and viscosity modulus. In the low-frequency region, the value of G″ is higher than the value of G′ and the compositions exhibit viscous fluid properties. However, in the high-frequency range the behavior of the curve’s changes and the value of G′ increases and reaches a plateau while the values of G′ for compositions based on VES and PHPA are significantly higher than for the “cross-linked” guar gel (at 2.0 Hz, the G′ values are about 6.0 and 8.0 Pa for the compositions PolyGel and VES, respectively, while for the “cross-linked” gel the G′ value is 2.7 Pa). At the same time, with increasing frequency the value of viscosity parameter G″ is significantly reduced.

For the composition based on VES, relatively low viscosity values and high elasticity values in comparison with the “crosslinked” gel can be explained by the formation of a complex viscoelastic system of interwoven VES surfactant micelles characterized by high elastic properties. The micellar surfactant system is formed due to weak non-covalent interactions; therefore, the deformation of the structure under mechanical influences is reversible. For comparison, polymer systems can be irretrievably destroyed under the influence of forces applied to them [76]. For systems based on a synthetic polymer, a possible explanation for similar dependencies is the formation of such structures due to hydrogen complex interactions between the amide groups of the polymer.

It should be noted that the frequency dependencies of G′ and G″ of all the systems under study, except for the “linear” guar gel, can be considered from the point of view of the Maxwellian behavior of viscoelastic bodies.

At a certain frequency, the graph shows the intersection of the curves of the elastic G′ modulus and the viscosity G″ modulus. The intersection point corresponds to the reciprocal of the relaxation time and can be used to judge the structural properties of viscoelastic systems. The relaxation time increasing indicates a complex spatial structure. In the “cross-linked” guar gel—composed of PolyGel and VES—there is a shift of the point of intersection to the low-frequency zone, which indicates the high structuredness of the systems.

A comparative analysis of the effective viscosity, sand-holding capacity, and structural and mechanical properties of the studied fluids was carried out (

Table 1. Comparative analysis of the structural and mechanical properties of various types of fracturing fluids.

Fracturing Fluid Type	η 1 at 25 °С for 100 s–1, mPa·с	G′ 2, Pa	G″ 2, Pa	τR, s	ξ, nm	ʋ 3, cm/ min
“Linear” guar gel	55	1.02	1.23	-	-	18.65
“Cross-linked” guar gel	760	3.11	1.28	8.23	114.09	0.24
Non-polymer composition based on VES	360	8.97	1.50	33.22	77.14	0.01
Fluid based on synthetic polymer	90	7.57	1.80	40.71	84.07	0.10

¹ effective viscosity; ² moduli of elasticity G′ and viscosity G″ are measured at a frequency of 3 Hz; ³ sedimentation rate of proppant particles in the volume of liquid, 16/20 fraction proppant was used.

).

It was found that at lower values of the effective viscosity the system based on VES and synthetic polymer have higher elastic properties in comparison with crosslinked guar gel (G′ is equal 8.97, 7.57, and 3.11 Pa for VES, synthetic polymer, and crosslinked gel, respectively).

The VES composition has the highest value for the elastic modulus. This is explained by the presence in the system of a three-dimensional mesh of topological links of long cylindrical micelles. Due to the high elastic component, these compositions have a better sand-holding capacity in comparison with the cross-linked gel (the deposition rate of proppant particles in the liquid is 0.01, 0.1, and 0.24 cm/min for VES, PolyGel, and a cross-linked gel, respectively).

In addition, the smallest values of the correlation length ξ were observed in the VES and PolyGel systems (77.14 nm and 84.07 nm, respectively). This indicates a lot of entanglements between molecules.

VES systems are characterized by a large relaxation time (33.22 s), which can be explained by the relatively large time spent on the restoration of the complex three-dimensional structure of entanglements of long cylindrical micelles. The great value for the relaxation time in compositions based on a synthetic polymer (40.71 s) is explained by the time spent on the formation of a complex structure of intertwined polymer chains in which interactions occur due to hydrogen bonds between the active amide centers of molecules.

Further studies of the sand-carrying capacity of the investigated fracturing fluids (

Table 2. Evaluation of the sand-carrying capacity of water-based fracturing fluids after holding under static conditions for 30 min. Proppant fraction 16/20 was used.

“Linear” Guar Gel	“Crosslinked” Guar Gel	Non-Polymer Composition Based on VES	Liquid Based on Synthetic Polymer

) showed that the systems based on VES and synthetic polymers are characterized by the ability to retain the proppant in its volume comparable to the highly viscous “cross-linked” guar gel. These results can be explained by the high indices of the elastic modulus of these systems as shown by oscillatory rheology.

Based on these studies, it can be concluded that to assess the effectiveness and applicability of new systems for hydraulic fracturing one should not rely only on established methods. It is instead advisable to use an integrated approach in the analysis of the structural and mechanical properties of fluids for hydraulic fracturing, which consists of using a combination of rotational and oscillatory rheology.

To illustrate the use of the method of describing hydraulic fracturing fluids, the figure shows a study of a fluid based on a synthetic polymer.

This Figure 4 shows the frequency dependences of the elastic moduli and viscosity moduli of a fluid based on a synthetic polymer with different dosages. With an increase the polymer concentration, we observe an increase G′ and G″. One can also observe a shift in the intersection point to the low-frequency region. The data indicates an increase in the relaxation time.

Figure 5 shows the dependence of the proppant settling rate on the relaxation time. There is a dependence that with an increase in the relaxation time, the proppant settling rate decreases. For the studied synthetic polymer solution, it was noticed that at a dosage of 6 L/m³ or more, the system achieves the best values of the sand-holding capacity index, which corresponds to the highest values of the relaxation time. As a result, the relaxation time can act as a criterion for selecting the concentration of the gelling agent.

3.2. Investigation of the Influence of Filtrates of Destroyed Liquids on the Rock

When conducting experiments on the influence of fluids on the reservoir rock, the following reagents were used: potassium chloride solution 10%, potassium chloride solution 2%, distilled water, and filtrates of the destroyed liquids, including “cross-linked” guar gel, non-polymer composition based on VES, and liquid-based synthetic polymers.

The viscosity of the destroyed systems weas measured on a Brookfield rotary viscometer using a UL cell adapter. The measurement results presented in Figure 6 show the degree of destruction of liquids, which we use in further studies.

3.2.1. CST

The experiment was carried out using the CST manufactured by Fann Instruments. A total of 5 g of core material was placed in 50 mL of the test sample. The results are shown in Figure 7.

As it is possible to see from the results of the experiment, the maximum impregnation time was found for the sample placed in the filtrate of the degraded “cross-linked” guar gel. This effect was caused by the presence of guar that was not completely destroyed, the molecules of which created a filter cake that prevented impregnation of the filter paper.

3.2.2. Visual Assessment of the Sedimentation Stability of the Suspension

During the experiment, a rock sample from an oil field in Russia was used. A weighed portion of 1.7 g was placed in 100 mL of the test liquid sample. Then, the sample was vigorously shaken for 1 min and left in the dark for 24 h. After a predetermined period of time, the samples were shaken again, and the time of the suspension settling was recorded using a stopwatch. For convenience, a camera with a time-lapse function was used.

The results of the experiment are shown in Figure 8 where the dependence of the percentage of suspension sedimentation on time is presented. As can be seen, destroyed “crosslinked” guar gel and destroyed synthetic-based gel showed the worst results. We assume that this result is associated with the residual viscosity of the solutions, and the presence of large incompletely destroyed polymer molecules in the filtrate does not allow the solid phase to settle.

3.2.3. Testing the Dynamic Stability of Core Material

A clay-covered core with a fraction of 1–2 mm was taken as a test sample, and 20 g of the sample was placed in a cell together with 350 mL of the test solution. The cell was heated in a roller oven at 50 °C for 24 h with rotation. The contents of the cell were then poured through a 1 mm sieve. The core remaining on the sieve was washed with a potassium chloride solution, dried at 105 °C for 2 h, and weighed to determine the percentage of weight loss.

The dependence of the percentage of weight loss of the core material on the composition of the liquid phase is shown in Figure 9. The destroyed non-polymer fracturing fluid based on VES has the least effect on the rock, and distilled water has the greatest effect.

3.2.4. Measuring Swelling and Linear Expansion Coefficients on the Zhigach–Yarov Device

The experiment was carried out on 3D printed cells with an inner diameter of 25 mm connected by a rod to an ICh-10 0.01 dial indicator (Figure 10). The assembled device is shown in Figure 11. The study was carried out with 1 g and 3 g of core material. The samples were kept in the device for a week. The measurements were taken every hour for the first 6 h and then once daily after 1, 2, 3, 6, and 7 days.

When processing the data, the highest sensor indicators for the entire period were selected, and both coefficients were calculated. The results of the calculation are shown in Figure 12, and coefficients describe the effect of the liquid on the swelling of the core material in different ways.

When comparing the calculated coefficients of linear expansion and swelling, it was found that the values of the coefficient of swelling differ depending on the liquid phase under study. To confirm the reliability of the results received, it is necessary to search for alternative research methods.

3.2.5. Comprehensive Assessment of the Influence of Fluids on the Rock

Conducting studies of the clay’s stabilizing ability of liquids is an extremely important task. The obtained results also testify that the choice of research methodology has the same important role. During the research, different results were obtained—the same test fluid showed high efficiency in one method and low efficiency in another. This indicates the imperfection of the methodological approaches to the study of rock swelling and the need to develop a comprehensive method for assessing the inhibitory capacity of clay stabilizers.

One of the options for this approach can be the study and analysis of the results using several methods to assign values and to calculate the average parameter value.

Table 3. Table summarizing the results of assessing the swelling of clays using various methods.

	Potassium Chloride Solution 10%	Potassium Chloride Solution 2%	Filtrate of Destroyed “Cross-Linked” Guar Gel	Filtrate of Destroyed Non-Polymer Composition Based on VES	Filtrate of Destroyed Liquid Based on Synthetic Polymer	Distilled Water
Capillary suction time (CST)	1	4	6	2	3	5
Sedimentation stability of the suspension	1	3	6	2	6	4
Disintegration test by hot rolling	4	5	3	1	2	6
Swelling coefficients by the Zhigach–Yarov method	3	4	1	2	5	6
Linear swelling coefficients	4	5	1	3	2	6
Average score	2.6	4.2	3.4	2	3.6	5.4

shows the results of using this approach.

The study involved six different fluid samples. The first place was given to the fluids that showed the least effect on the sample during the experiment, the sixth place—the greatest effect compared to the rest. In the case of Sedimentation stability of the suspension, two sixth places can be observed—this is due to the fact that the suspension did not separate during the experiment in the study of Filtrate of destroyed “cross-linked” guar gel and Filtrate of destroyed liquid based on synthetic polymer. The closer the Average score is to one, the less impact the fluid has on the rock according to the results of the five evaluation methods.

Averaging the results obtained by different methods can help to assess the overall inhibitory ability of the test fluid. Based on a comprehensive approach to assessing the effect of fluid on the reservoir rocks, it is possible to conclude that the best sample which has the least effect on the rock is the sample of destroyed gel based on VES. This result is due to the fact that a sufficient amount of surfactant remains in the filtrate of the destroyed liquid to prevent swelling.

Unfortunately, when testing a fluid for fracturing applicability, the broad approach described above is not used. The best fluid should have optimal structural-mechanical properties combined with minimal impact on the reservoir rock. The choice of the determining criterion depends on the type of hydraulic fracturing fluid: in the case of compositions based on surfactants or synthetic polymers, the most significant are the structural parameters of the system and not the viscosity, but in all cases, it is necessary to assess the effect of fluids on the rock.

4. Conclusions

Based on previous approaches to assessing the technological properties of hydraulic fracturing fluids, one of the main criteria was the viscosity parameter obtained on a rotational viscometer. Such an approach to assessing the quality of hydraulic fracturing fluid does not allow for a comprehensive and complete assessment of the structural and mechanical properties of fluids. In addition, the standard fluid applicability analysis includes only one of a long list of methods for evaluating the impact of a fluid on a rock, which does not unambiguously judge the degree of fluid impact.

This paper presents a broader approach to evaluating the performance of a water-based fracturing fluid. The proposed set of methods includes the assessment of the main characteristics of water-based sand carrier fluids. This method involves the analysis of structural and mechanical properties using a combination of rotational and oscillatory rheology together with comparative analysis of methods for studying the influence of fluids on the reservoir rock. Rotational viscometry in combination with oscillatory viscometry allows you to evaluate the efficiency of a fluid based not only on viscosity, but also on elastic properties. The comprehensive assessment of the fluid’s influence on the rock using techniques based on various physical processes will allow us to more accurately draw conclusions about the applicability of systems and avoid problems caused by the swelling of water-sensitive formations. The use of this approach to assess the technological properties of water-based fracturing fluids demonstrates the high potential applicability of new, unconventional fracturing fluids such as fracturing fluid based on VES and PHPA.