Wellbore instability and formation collapse caused by lost circulation are vital issues during well excavation in the oil industry. This study reports the novel utilization of soy protein isolate (SPI) as fluid loss additive in bentonite-water based drilling fluids (BT-WDFs) and describes how its particle size and concentration influence on the filtration property of SPI/BT-WDFs. It was found that high pressure homogenization (HPH)-treated SPI had superior filtration property over that of native SPI due to the improved ability for the plugging pore throat. HPH treatment also caused a significant change in the surface characteristic of SPI, leading to a considerable surface interaction with BT in aqueous solution. The concentration of SPI had a significant impact on the dispersion state of SPI/BT mixtures in aquesous solution. At low SPI concentrations, strong aggregations were created, resulting in the formation of thick, loose, high-porosity and high-permeability filter cakes and high fluid loss. At high SPI concentrations, intercatlated/exfoliated structures were generated, resulting in the formation of thin, compact, low-porosity and low-permeability filter cakes and low fluid loss. The SPI/BT-WDFs exhibited superior filtration property than pure BT-WDFs at the same solid concentraion, demonstrating the potential utilization of SPI as an effective, renewable, and biodegradable fluid loss reducer in well excavation applications.

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A bentonite (BT) suspension is extensively used as a water-based drilling fluid (WDF) for well excavation; however, it suffers from insufficient rheology and poor filtration performance at low solid content. Cellulose nanocrystals (CNCs) with nanoscale dimensions, large surface area, high stiffness, and reactive functional groups are promising candidates to improve the rheology and filtration performance of BT-WDFs through surface interactions. Consequently, a fundamental understanding of the surface interaction between CNCs and BT platelets becomes critical. This work aims to reveal the crucial role of surface characteristics of CNCs on their interaction with BT platelets as well as the rheological and filtration performance of CNC/BT-WDFs. Two types of CNCs with distinctive surface characteristics [i.e., carboxylated CNCs (cCNCs) and cationic CNCs (caCNCs)] were rationally prepared and applied as modifiers in BT-WDFs. The cCNCs were shown to attach to the edge surface of BT platelets and acted as "bridges", connecting BT platelets via "edge-to-edge" association. On the contrary, the caCNCs were absorbed to the face surface of BT platelets and acted as "cross-linking agents", stacking numerous layers of BT platelets through "face-to-face" association. These differences led to the distinctive dispersion state of BT platelets and the overall performance of CNC/BT-WDFs. The cCNCs served as more effective rheological and filtration agents in the developed fluids compared with caCNCs, highlighting the importance of surface characteristics of CNCs in the development of low-solidcontent, high-performance CNC/BT-WDFs for well excavation.

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