**4. Biopolymers application in water-based muds**

Water-based drilling fluids are an economical and eco-friendly alternative for successfully drilling a wellbore. Both conventional and high-performance muds generally use biopolymers to enhance rheological characteristics, provide acceptable viscosity, suspend solids, and control filtrate loss in the wellbore. A few examples are natural biomaterials derived from plants or microorganisms such as starch, guar, xanthan, and their chemically modified substitutes.

Biopolymers used for drilling fluids can be classified as plant-based or microbialbased, depending on their source. For instance, guar gum and locust bean gum are the most widely used plant-based materials. Guar gum is a nonionic linear polymer obtained from the seeds of the guar plant composed of the sugars galactose and mannose. On the other hand, locust bean gum is isolated from the fruit of the legume *Ceratonia siliqua*, and its structure is similar to guar except for the ratio of galactose to mannose, which influences its solubility in water. It has been observed that the galactose content directly relates to the water solubility. Thus the guar shows better solubility than locust bean gum.

Another kind of biopolymer that is mostly employed in water-based systems is bacterially produced polymers. Xanthan gum is a typical and mostly used additive of this type. It has a more branched structure than other gums and is very efficient, offering shear-thinning rheological behavior that is almost optimal for drilling fluid applications. Similarly, welan gum is a negatively charged (anionic) gum formed by bacteria belonging to the Alcaligenes genus fermenting sugar. It is observed that it exhibits better viscosity and salt resistance.

Various studies have highlighted the importance of biopolymers for the improvement of mud properties. For example, nitrocellulose-based muds were developed in a study, which showed enhanced rheological and filtration properties. It was used in high-performance, water-based fluids as a renewable, non-hazardous, and cost-effective alternative to synthetic polymers, with the added feature of maintaining and optimizing fluid characteristics [6]. Likewise, diutan gum was used as a drilling mud viscosifier, resulting in retaining their viscosities up to 232°C, when sodium erythorbate, potassium formate, and polyethylene glycol were added to the formulations [7].
