The single-component performance of the fracturing fluid directly influences its behavior under high temperatures and is crucial for successful fracturing operations and enhanced production. When conducting fracturing in deep, high-temperature oil and gas reservoirs to boost production, it is indispensable to employ stable thickeners for suspending and transporting the proppant-laden fracturing fluid. Considering the depth of the well, the crosslinker must demonstrate a longer delayed crosslinking time to control filtration loss in the fracturing fluid. Furthermore, minimizing the viscosity of the fracturing fluid is of paramount importance for reducing the pumping pressure during operations. Additionally, the gel formed at elevated temperatures should have a strong carrying capacity after prolonged shear and excellent delayed breaking characteristics.By the meticulous selection of key single agents of the fracturing fluid, such as the modification technology of the ultra-high temperature guar gum system and the ultra-high temperature crosslinking agent, the interrelationships between the fundamental formula of the ultra-high temperature fracturing fluid and its performance under various high-temperature levels and different additive dosages are investigated, the optimal formula system that complies with the low damage performance and low friction resistance performance is determined, and the fracturing fluid system that accommodates the stimulation and reformation requirements of ultra-deep and ultra-high temperature is established.Experimental methodsMethodsDeep, high-temperature fracturing operations necessitate specific requirements for the thickening agents in the fracturing fluid. The stability of these agents at elevated temperatures is crucial to ensure the carrying capacity and gel-breaking performance of the fracturing fluid. Additionally, due to the well’s depth, the crosslinker must not only possess an extended delayed crosslinking time but also exhibit exceptional carrying capacity and effective delayed breaking gel performance following prolonged shear. To address these demands, it was conducted experimental studies on super-high-temperature thickeners, delayed crosslinkers, temperature stabilizers, anti-swelling agents, and breakers. Through comprehensive assessment and optimization efforts, a suite of ultra-high-temperature fracturing fluid systems suitable for enhancing deep-well fracturing was successfully developed (Fig. 1). This ultra-high-temperature fracturing fluid system demonstrates outstanding stability at elevated temperatures and effectively mitigates various challenges that may arise during deep, high-temperature reservoir fracturing operations. Throughout the experimental process, diverse additives were fine-tuned for varying depths, temperatures, and shear conditions—providing robust support for optimizing the performance of the fracturing fluid.Fig. 1Research technology route.Thickening agentThe laboratory conducted experiments to investigate the key factors influencing the performance of modified high-temperature guanidine gum products, including alkali concentration, alcohol content, water quantity, ethylene oxide dosage, reaction temperature and time. The modification experiment of guanidine gum was carried out using a 500 mL four-hole round-bottom flask equipped with a stirrer, condenser, dropper and thermometer. Initially, an appropriate amount of industrial ethanol was added at room temperature and stirred thoroughly. Then, a suitable quantity of guanidine collagen powder was introduced into the mixture. Subsequently, NaOH solution was slowly added at room temperature to initiate the alkalization reaction. As soon as the alkalization reaction completed, the temperature started to rise gradually while ethylene oxide and cationic etherifying agent were slowly dripped into the flask simultaneously through a dropper. The reactant-containing flask was placed in a water bath to maintain consistent reaction conditions at a predetermined temperature for a specific duration. Upon completion of the reactions, the flask was cooled to room temperature and hydrochloric acid was gradually added during the cooling process until reaching a pH value of 7.0. The resulting product underwent centrifugation, drying at 100 °C, milling, and filtration through a screen mesh with an aperture size of 0.125 mm to yield ultra-high-temperature modified guanidine gum (GHPG) products. The evaluation experiment focused on investigating the impact of different alkali amounts, water amounts, reaction temperatures, reaction times, and ethylene oxide amounts on the water insoluble content of ultra-high temperature modified guar gum (GHPG).Crosslinking agentTo meet the performance requirements of the crosslinking agent under high temperature and shear conditions, boron and zirconium were selected as the central ions, with the total mass of boric acid and zirconium oxychloride set at 16% of the mass of the crosslinking agent. Initially, zirconium oxychloride was combined with its corresponding ligand in a suitable solvent, followed by heating and stirring in a water bath until it dissolved into a colorless and transparent liquid. Subsequently, 6 parts of boric acid were dissolved in 2 parts of solvent, with an addition of a specific amount of NaOH for complete hydrolysis. After adding a certain amount of ligand, this mixture was also heated and stirred to form another colorless and transparent liquid. Finally, these two mixtures were combined and continuously heated while being stirred to yield a faint yellowish slightly viscous transparent liquid—the desired crosslinking agent.The organic boron zirconium crosslinking agent exhibits delayed crosslinking functionality, and its mechanism of slow crosslinking is as follows: under alkaline conditions, there is a strong affinity between the organic coordination entity and the boron ion. Leveraging this affinity, the excess coordination entity encapsulates the colloidal particles of the organic boron zirconium crosslinking agent, with a stronger affinity observed at higher levels of alkalinity. The coordination entity functions as a protective film formed on the surface of the organic boron zirconium crosslinking agent, and the tightness of this film depends on adjusting alkalinity levels. By manipulating alkalinity, one can effectively control exposure levels of the organic boron zirconium crosslinking agent and regulate its potential reaction with hydroxyl groups in vegetable gums to achieve precise control over crosslinking time. The delayed crosslinking experiment results indicate that the viscosity of the organic boron-zirconia crosslinking agent reaches its first inflection point at approximately 6 min. At this stage, the system demonstrates a localized linear structure, showing strong flowability and low friction characteristics. Around 14 min, the second inflection point emerges, leading to the formation of a reticular structure in the system. This structure displays high viscoelasticity and is accompanied by an observed “picking tongue” characteristic (Table 1 and Fig. 2).Table 1 Dynamic viscosity changes of gel at different crosslinking times.Fig. 2Dynamic viscosity curve of organic boron-zirconium crosslinker under delayed crosslinking.Clay stabilizing agentThe binder utilized is a low-concentration organic cationic polymer with multiple adsorption sites, exhibiting resistance to acids and saltwater. Through dissociation, the organic cationic polymer generates positively charged cations that effectively neutralize the negative charge on the clay surface (Fig. 3), thereby demonstrating excellent prevention of clay swelling.Fig. 3Synthesis method of small cationic clay stabilizer.The starch-cadmium iodide approach was utilized to investigate the saturated adsorption capacity and adsorption attributes of the clay stabilizer. At first, the working curve of the cationic polymer was ascertained by means of a spectrophotometer to establish a quantitative correlation between optical density and concentration. Then, the optical density of the supernatant from the developed color in the adsorption system was gauged, and the adsorption capacity of the system was computed using the established working curve. The experiment contrasted polymers with different degrees of cationization and evaluated how cationization influenced their saturated adsorption capacity (Table 2, Fig. 4). As the degree of cationization rises, the saturated adsorption quantity of the polymer on the clay surface and the positive charge adsorption unit within its molecule also escalate, thereby strengthening the stability of the clay.Table 2 Influence of cationization degree on its saturated adsorption capacity.Fig. 4Adsorption characteristic curves of different degrees of cationization.Temperature stabilizersIn this experiment, TS-1 was chosen as the temperature stabilizer. It is a colorless or light-yellow liquid with a density range of 1.05–1.18 g/cm3 and pH value ranging from 9.0 to 11.0. Gel samples were loaded into sample cups of a high-temperature and high-pressure coaxial cylinder rotary viscometer for controlled-rate heating experiments at 3 ± 0.2 °C per minute while rotating at a shear rate of 170 s−1. The fracturing fluid sample underwent continuous shearing during heating until its apparent viscosity decreased to reach a value of 50 mPa·s in order to assess its temperature resistance. The enhancement in the sample’s temperature resistance was evaluated by comparing differences before and after adding cross-linked gel.$$ \Delta T = T_{1} – T_{2} $$where ΔT represents the amount of temperature increase, °C. T1 represents the measurement temperature without adding the temperature stabilizer, °C. T2 represents the measurement temperature after adding the temperature stabilizer, °C.Experimental instruments and materialsThe main instruments and reagents used in the experiments are shown in Tables 3 and 4 below. These experimental reagent products are all industrial-grade items of qualified quality, provided by chemical enterprises located in Sichuan Province in western China, and some of the products are synthesized within the laboratory.Table 3 Comparison of performances of commonly used fracturing fluid thickeners.Table 4 Comparison of performances of commonly used fracturing fluid thickeners.Principle of single agent performanceThickener
(1)
Guan gum thickeners
Plant gum serves as the primary thickener for water-based fracturing fluid, constituting over 90% of usage. Among various thickeners such as guar powder or modified guar powder, soybean gum, hydroxy methyl cellulose (CMC), guar gum or its modified derivatives, modified guar gum is recognized as the most effective thickener due to its strong affinity for water and ability to swell upon contact with water, leading to water coagulation34. This is attributed to its abundance of hydroxyl groups that form hydrogen bonds with water, allowing the polymer chains formed by these groups to disperse in water. The resulting dispersed solution resembles swollen long fiber bundles suspended in water, which intertwine and obstruct solution flow, thereby increasing viscosity35. While guan gum and its derivatives can meet the basic requirements of conventional fracturing operations, their thermal performance is limited to temperatures below 180 °C. To address this limitation and meet operational needs, corresponding crosslinking agents, temperature stabilizers and other additives must be developed to enhance the thermal performance of the fracturing system36.The resistance of guar gum-based thickeners to high temperatures (180 °C) in fracturing fluid systems is still relatively uncommon, primarily due to the hydrolysis of sugar glycosidic bonds in the molecular main chain under high temperature and alkaline conditions, resulting in a rapid decrease in system viscosity and failure to meet sand-carrying requirements.
(2)
Complex Thickener
A composite thickener involves the combination of two thickeners, such as guar gum and polyacrylamide. These high-molecular-weight compounds can form a hydrogen bond structure resembling a ladder, which allows the chains to protect each other and enhance their thermal stability37. The presence of this ladder structure ensures that the breakage of one chain does not disrupt the overall network structure, provided it occurs in different sections of the ladder grid. Therefore, incorporating an additional compatible thickener alongside guar gum can significantly enhance the thermal and shear resistance properties of the fracturing fluid system38.
(3)
Synthetic Polymer Thickeners
Conventional synthetic polymer fracturing fluids suffer from poor temperature and shear stability. The primary approach to improving the shear resistance of polymer fracturing fluid is currently by increasing the rigidity of the main chain molecules of the polymer thickener. In comparison with plant-based natural high molecular weight materials, synthetic polymer thickeners offer advantages such as strong thickening ability, easy flow restoration after breakage, and low residue content. Particularly for polymers, they exhibit excellent thermal stability. Due to the large bond energy in C–C single bonds that constitute the high molecular chains of synthetic polymers, they are less prone to breaking at elevated temperatures. Therefore, when designing the molecular chains of synthetic thickeners, functional requirements should be considered and side chains with functional groups such as hydroxyl groups and cross-linking groups should be selected for incorporation into the high molecular chains. Additionally, heat resistance of these side chains must also be taken into account. The crushing, drying, and dispersing processes during solution synthesis of synthetic polymer thickeners significantly impact their heat resistance performance in process configuration. Thus, fracturing fluid prepared using a powder preparation method exhibits markedly lower heat resistance performance compared to that prepared using a gel preparation method.Comparing the three types of thickeners, guar gum-based thickeners offer the advantages of consistent performance, easy product accessibility, cost-effectiveness, and minimal reservoir damage. Furthermore, their heat resistance can be enhanced through modification.Crosslinking agentsTo meet the performance requirements of the crosslinking agent under high temperature and shear conditions, boron and zirconium were selected as the central ions. The mass ratio of boric acid to zirconium oxychloride was set at 16% of the total mass of the crosslinking agent. Initially, zirconium oxychloride was combined with its corresponding ligand, followed by addition of an appropriate amount of solvent. The mixture was heated and stirred in a water bath until it dissolved into a colorless and transparent liquid. Subsequently, 6 parts of solvent were added to 2 parts of boric acid and stirred until dissolution occurred. A certain quantity of NaOH was then introduced to facilitate complete hydrolysis of boric acid, along with a specific amount of ligand. The resulting mixture was heated and stirred in a water bath to yield a colorless and transparent liquid. Finally, the two liquids were blended together and continuously heated and stirred to produce a faint yellow, slightly viscous transparent liquid—the crosslinking agent.The conventional crosslinking agents used at high temperatures pose two main challenges. Firstly, the rapid crosslinking rate leads to increased flow resistance of fracturing fluid in the pipeline. Secondly, the continuous shearing of fracturing fluid in deep wells makes it difficult to maintain its performance. Therefore, there is a need for a delayed crosslinking agent that can prevent or partially delay the crosslinking of fracturing fluid in the surface pipe string and gradually achieve the desired viscosity before entering the wellbore and reaching the target layer. This not only facilitates construction pumping but also ensures optimal fracturing fluid performance, thus introducing delayed crosslinking technology. Currently, delayed crosslinking technology primarily focuses on four aspects: incorporating delaying agents to extend the crosslinking time; utilizing formation heat; controlling dissolution rate; adjusting pH change and polymer hydration reaction. Organic zirconium-based crosslinking agents are suitable for high-temperature (200 °C) formations as they do not hydrolyze and can maintain viscosity for an extended period at elevated temperatures. Additionally, their residue after breaking down can serve as a clay stabilizer. Commonly used organic zirconium compounds include triethanolamine zirconium, acetylacetone zirconium, sodium zirconium citrate, and sodium zirconium tartrate chelates. Combining boron with zirconium not only combines boron’s shear stability advantages with zirconium’s heat stability but also reduces expensive usage of zirconium while remaining effective at high/ultra-high temperatures.An exceptional organic boron-zirconium crosslinker has been developed, which exhibits superior temperature resistance and delayed crosslinking compared to inorganic boron crosslinkers. Furthermore, it mitigates fracturing fluid-induced damage to formation permeability when compared to organic titanium or chromium crosslinkers. The organic boron-zirconium crosslinker is synthesized under strictly controlled reaction conditions for the incorporation of organic zirconium complexes into organic boron. The resulting larger colloidal ions are attributed to stronger complex bonds formed between the zirconium ions and the organic boron colloidal particles, thereby further enhancing the gel’s crosslinking density and strength while improving its temperature and shear resistance properties. This principle is illustrated in Fig. 5.Fig. 5Schematic diagram of crosslinking of organic boron-zirconium crosslinking agent.Anti swelling agentThe presence of clay in the formation presents two challenges: firstly, expansible clay minerals (mainly montmorillonite) undergo expansion upon exposure to water, leading to a reduction in reservoir permeability; secondly, non-expansible clay minerals (such as kaolinite, illite, chlorite) migrate when fluid flow rate exceeds a certain threshold, resulting in pore structure blockage and further reduction of reservoir permeability. Therefore, addressing clay stability requires measures to prevent the expansion of expansible clay minerals and inhibit the migration of non-expansible clay minerals.Clay stabilizers are chemical agents that inhibit the swelling of clay or migration of clay particles. Various chemical methods and techniques have been employed to mitigate or prevent reservoir damage caused by clays and particles. Essentially, montmorillonite, kaolinite, illite, and their mixed forms are the most prevalent clays found in natural accumulations within reservoirs, characterized by a significant presence of negatively charged clay surfaces. These negative charges exhibit high sensitivity towards liquids, forming the basis for the development of most clay stabilizing additives. Similar to pore lining or filling minerals, clay minerals exist naturally in accumulations or occasionally occur as flakes within pores. They are surrounded by saline primary formation water primarily composed of Na+ and Ca2+, which form salt ions that adsorb onto the clay surface through electrostatic interactions acting as neutralizing agents for negative ions, thereby maintaining stability under normal conditions. However, infiltration of external fluids such as low-salt well treatment fluids or non-productive water dilutes the primary water and reduces its salt concentration. Consequently, diffusion of cations covering the clay surface occurs leading to expansion in montmorillonite and certain mixed clays while causing fractures in kaolinite, illite shale, and mixed rocks.The main chemicals that possess both swelling and migration inhibitory properties are organic cationic polymers. Organic cationic polymer stabilizers offer several advantages, including a wide temperature range (24–260 °C), resistance to acid treatment, oil and water resistance, low dosage requirement with quick effect, long shelf life, and ease of use. Moreover, they exhibit good compatibility with working fluids and reservoirs while preventing issues such as oil-wet inversion on the hydrophilic surface of treated formations.Demulsifying agentUpon entering the oil and gas layer, fracturing fluid creates capillary pressure. The oil–water interface tension typically ranges from 25–35 mN/m but is affected by temperature, pressure, and oil composition. Gas–water interface surface tension can range from 74 to 40 mN/m, depending on temperature and pressure conditions. Most of the outlets for oil within rock layers have diameters greater than 1 μm. When contact angle equals 0° with σ = 30 mN/m, capillary pressure on the oil–water contact surface is approximately 0.12 MPa. Conversely, it is higher on gas–water contact surfaces. Water existing in bead form leads to increased capillary pressures due to Jamin effect at low permeability (0.1 × 10–3 μm2) and a saturation level of 60%, where even reaching up to 1.4 MPa may pose challenges during flowback processes when reservoir inflow rates are low or discharge times are prolonged.The displacement aid is a chemical substance utilized to decrease the surface tension of fracturing fluid or oil–water interface tension, enhance the contact angle with rock, augment energy, diminish capillary resistance during fracturing fluid return, facilitate complete return, and mitigate formation pollution. The critical micelle concentration (CMC) value of Gemini surfactants is typically 10–100 times lower than that of traditional surfactants. In comparison to conventional surfactants, Gemini surfactants exhibit superior efficacy in reducing water surface tension. For instance, Gemini-type surfactants possessing exceedingly high surface activity, excellent water solubility, and hydrophilic bonding groups demonstrate remarkably low kraff points. Ionic Gemini surfactants possess two polar groups and twice the charge which enables them to exert stronger attraction towards other electroneutral or oppositely charged surfactants. The synthetic pathway for the selected Gemini surfactant (Fig. 6) involves refluxing secondary amine and 3-chloroepoxypropane to generate bridging quaternary ammonium salt followed by reacting the quaternary ammonium salt with hydrogen peroxide to obtain oxidized quaternary ammonium salt. Finally purifying the product through recrystallization.Fig. 6Synthesis route of Gmeini surfactant.Gel breakerIn high-temperature layers, layer degradation typically occurs automatically with increased residence time. However, to expedite this process, a breaker is incorporated into the fracturing fluid. When natural polymers are employed as gelling agents, biological enzymes are often utilized as breakers, which do not affect the layers themselves. Suitable biological enzymes include α-amylase, β-amylase, maltose, cellulase, and hemicellulosic acid. Nevertheless, due to their limited thermal stability, biological enzymes can only be used for treatment when the temperature of the layers remains below 60 °C. Additionally, for fracturing fluids prepared from synthetic polymers, biological enzymes generally have negligible impact on layer degradation. Another category of oxidants can serve as film-breaking agents including potassium dichromate (K2Cr2O7), potassium permanganate (KMnO4), hydrogen peroxide (H2O2), persulfate (S2O82−), perethylenedibenzo-p-dioxin (PEDD), and peroxyt-butyl (–C(CH3)3). The former several types of inorganic oxidants are suitable for higher temperature layers while the latter two organic types are applicable to lower temperature layers. Oxidizing materials achieve film breaking effects by generating free radicals. When there exists a substance capable of providing reducing metallic ions (FeSO4), the formation of free radicals can be further accelerated to enhance film breaking rate. Oxidizing materials exhibit film breaking effects on both natural and synthetic polymers. However, it is generally believed that crosslinked natural polymers can also undergo self-decomposition at temperatures exceeding 70 °C.The third type of gel breaker consists of inorganic or organic acids, which disrupt the gel structure by reducing the pH of the fracturing fluid. Commonly used acids include sulfuric acid, hydrochloric acid, acetic acid, and p-toluenesulfonic acid. Organic esters undergo hydrolysis at elevated temperatures, enabling their dispersion or dissolution in the fracturing fluid to automatically break down the gel within the formation. Examples of esters include ethyl formate, ethyl chloroacetate, titanium dibutyl ether, maleic dibutyl ether, and adipic dibutyl ether. It is widely acknowledged that dicarboxycyclopentadienyl dialkyl ethers exhibit enhanced effectiveness due to their slow hydrolysis upon injection into the formation; subsequently accelerating and disrupting the gel when temperatures reach 65 °C or higher.The requirements for breaking the gel and flowback of fracturing fluid are opposite to those for sand fracturing. During the addition of sand, the fracturing fluid must have high viscosity and stability to effectively transport the sand, minimize filtration losses, and enhance liquid utilization efficiency. Conversely, during flowback, it is crucial for the viscosity of the fracturing fluid to rapidly decrease in order to reduce flow resistance, increase discharge speed, shorten discharge time, and ultimately maximize discharge efficiency. The stability of fracturing fluids is primarily influenced by two factors: high-speed shear degradation and high-temperature oxidation degradation. High-speed shear degradation mainly occurs during pumping between the wellbore and burst point; whereas high-temperature oxidation mainly occurs when the fracturing fluid comes into contact with formation rock resulting in heat exchange. Even with sufficient heat stabilizers at high temperatures, there may still be small amounts of dissolved oxygen due to chemical reaction equilibrium. These residual traces can cause partial degradation of fracturing fluids and require appropriate dosing of degumming agents for rehydration.The ammonium sulfate functions as a breaker by decomposing free radicals (Fig. 7) within the formation via persulfate, which subsequently attacks the polymer chain of guanidine gum. This results in a complete reduction in viscosity of the fracturing fluid through the cascade reaction of free radicals.Fig. 7Decomposition reaction of ammonium persulfate.The concentration of polymer in the fracturing fluid will increase by 5–7 times during the pumping and fracture closure process, due to filtration phenomenon. This presents challenges for breaking the fracturing fluid. When ordinary breaker is dissolved in the liquid, it flows into the reservoir with filtration phenomenon. However, due to the concentration of polymer on the filter cake, the dissolved breaker cannot effectively penetrate it and may cause permanent damage to the formation. At the same time, a high enough concentration of breaker must be added during construction to fully achieve polymer breaking effects. However, increasing breaker concentration prematurely reduces viscosity of fracturing fluid and may lead to increased sand removal and blockage problems. Ensuring viscosity of fracturing fluid during construction while achieving complete breaking after construction creates a mutually restrictive relationship. The new type of coated capsule breaker effectively solves this problem by delaying release time through a thin film coating instead of using ordinary particles form. This maintains required viscosity during construction while gradually releasing effective components under closure stress for section cleaning.