In recent years, with the advancement of exploration and development technology, more and more unconventional reservoirs have been explored and developed (Jia et al. 2012; Sun et al. 2019; Li et al. 2020). They are characterized by high temperature, tight and deep burial, so reservoir stimulation has become an indispensable and efficient development method (Lau et al. 2016; Yang et al. 2018; You et al. 2018; Dai et al. 2019). Currently, for the development of tight oil and gas, and other resource reservoirs, it is particularly important to improve the performance of fracturing fluids (Scanlon et al. 2015; Zhao et al. 2018). Conventional water-based fracturing fluids include guar gum fracturing fluids, cellulose fracturing fluids, and polymer fracturing fluids (Adewole and Muritala 2019). Among them, guar gum and polymers have some disadvantages such as relatively narrow supply, high residue content, and high cost (Barati and Liang 2014; Thombare et al. 2016; Sharma et al. 2018). Therefore, cellulose, which has the advantages of low cost, wide sources, and no pollution, has become the best choice for fracturing fluids thickeners for low-cost and efficient development of unconventional reservoirs (Yang et al. 2018; Gao et al. 2019).
Despite the excellent properties of cellulose, its industrial application is limited owing to its poor water solubility. Given the poor water solubility of traditional cellulose, previous researches focused on improving its water solubility by etherification (Sang and Xiao 2009; Chieng and Chen 2010; Hebeish et al. 2010; Sehaqui et al. 2016), esterification (Wang et al. 2018), graft copolymerization (Raus et al. 2011; Thakur et al. 2013). To ensure its good water solubility, a branched chain was introduced into the main chain, which breaks the length of the chain, so that the molecular chains are not entangled enough (Chang and Zhang 2011). Therefore, the thickening effect is not good, and the shear resistance and temperature resistance of the network structure are limited. As a result, cellulose fracturing fluids can not meet the temperature resistance requirements of the unconventional reservoir at 120 oC using the modified methods mentioned above.
Some scholars innovatively added nanomaterials to viscoelastic fluids and found that adding nanomaterials can significantly enhance the strength of the network structure (Nettesheim et al. 2008; Helgeson et al. 2010). Nanoparticles with a high specific surface area can interact with more crosslinking agents to enhance the crosslinking effect between crosslinking agents and polymers. This effect not only improves the network density but also significantly reduces the amount of crosslinking agent, which ensures the good stability of the nanocomposites at high temperatures (Pal et al. 2015; Budnyak et al. 2018). In recent years, nanomaterials have been widely used in the field of oil and gas field stimulation (Lafitte et al. 2012; Lau et al. 2016; Villada et al. 2021). At present, nano Al2O3 (Savvashe et al. 2016), TiO2 (Hurnaus and Plank 2015), ZrO2 (Tana et al. 2017), and other materials have been added into the polymer to form a hydrogel with excellent mechanical properties. Fakoya and Shah (2018) added nano-SiO2 to surfactant fracturing fluids and polymer fracturing fluids, which improved the rheological properties and temperature resistance of the fracturing fluids and changed the wettability of the rock. Crews and Huang (2008) added nanoparticles with a mass ratio of 0.12 wt.% to 0.18 wt.% to clean fracturing fluids with a surfactant molecule volume ratio of 2 wt.% and maintained an apparent viscosity of 300 mPa·s at 65°C and 100 s− 1. Regarding fluid loss prevention and sand carrying stability, the performance of nano-composite fracturing fluids is significantly improved. Luo Mingliang (2012). also used nano-modification of a fracturing fluids based on anionic surfactant fatty acid methyl ester sulfonate. Under the conditions of 70 ℃ and 170 s− 1, its apparent viscosity can reach 50 mPa·s.
Among many nanomaterials, we focus on cellulose nanocrystals(CNCs), which are highly crystalline rod-like nanomaterials isolated from cellulose. Compared with traditional nanomaterials, CNCs has the advantages of environmental friendly, wide source, renewable, mass production, and low cost, and also can be well matched with cellulose and its derivatives(Lu and Hsieh 2010; Moon et al. 2011; Liu et al. 2021). In addition, cellulose nanocrystals have excellent mechanical properties, such as high crystallinity (60%~90%), high elastic modulus (150 GPa), and high tensile strength (10 GPa) (Habibi et al. 2007; Iwamoto et al. 2009; Yang et al. 2021). Yang and co-workers (2013) covalently cross-linked cellulose nanocrystals and polyacrylamide to prepare two component composite hydrogels with excellent mechanical properties and recovery ability. By means of the indentation depth sensor, the contribution of CNCs skeleton to hardness and elasticity increased by 500% compared with the original PAAm hydrogel. Ahn and Song (2016) analyzed the viscoelastic characteristics of all cellulose nanocrystals and carboxymethyl cellulose (CMC-Na) suspensions and verified that cellulose nanocrystals can be well dissolved together, which provided a favorable theoretical foundation for cellulose nano fracturing fluids. However, there have been few studies on hydrogels formed by Cellulose fracturing fluids and CNCs. Inspired by the above, this paper innovatively mixed cellulose nanocrystals and cellulose to form hydrogel with high strength.
Herein, we hybridized CNCs with sodium CMC-Na to form cellulose nano fracturing fluids, analyzed the mechanism of cellulose nanocrystals and cellulose, studied the rheological properties, temperature resistance, and gel-breaking performance of nano-hybrid fracturing fluids. The present work provides a new approach to improve the properties of cellulose fracturing fluids via the use of CNCs as both cross-linker and nanofiller.