**8. Limitations of biopolymers**

Generally, the biopolymers have lower thermal stability and degrade at higher temperatures. The salts, calcium, and bacterial resistance also negatively impact the performance of the mud containing biopolymers. Thus other additives such as bactericides are added to prevent the bacterial attack. Owing to the reduced thermal stability, either the native materials are modified by adding more functionalized groups into their main chain or the addition of two or more biopolymers is used. The former is economical compared to the second one. In a study, the synergic effect of xanthan and diutan gum enhanced the performance of water-based drilling fluids and showed shear-thinning behavior [28].

Different researchers have attempted to modify the biopolymers for drilling fluid applications. Starch is one of the most widely used additives, which improves both rheology and the filtration behavior of the muds. Starches have been modified using chemical, physical and enzymatic approaches. The usage of starch in the petroleum industry is in the drilling fluids, enhanced oil recovery, and completion fluids. A variety of starches are available, which are generally used for food applications. Pregelatinized starch (PGS) has been used in the oil industry due to its lower costs. Carboxymethyl starches (CMSs) are also very common, showing better thermal stability and salt resistance in WBMs. The combination of polyanionic cellulose (PAC) with the modified starch shows significant improvements in terms of mud rheology and filtration. Acetylated and grafted starches also showed promising results in WBMs.
