## Materials and Methods  
Graphene nanoplatelet was synthesized at Inha University, Korea via the modified Hummers method [16; 17]. Figure 1a shows the distinctive layered structure of the graphene nanoplatelet obtained from exfoliated graphite identified by transmission electron microscope (TEM) (model: CM200, Philips, Amsterdam, The Netherlands). The crystalline structure of the graphene nanoplatelet was identified by X-ray diffraction (XRD) (model: DMAX-2500, Rigaku, The Woodlands, TX, USA) in Figure 1b displaying a peak at approximately 10.88\({}^{\circ}\). Graphene nanoplatelet is colloidal suspended particles in aqueous solution produced by the chemical modification of graphite. Graphene nanoplatelet has multi-functional properties and it consists of 3-8 layers (average thickness), a diameter of particle ranging from sub-micron to 50 \(\upmu\)m and the surface area is 750 m\({}^{2}\)/g. It signifies the new class of nanocarbon. Graphene nanoplatelets can improve the matrix material such as the surface hardness, stiffness, and strength. Such attributes of graphene nanoplatelets are suitable for inhibiting clay swelling and wellbore strengthening. Nanosilver was supplied by Terra Techno Engineering, Shah Alam, Selangor, Malaysia. Its physical appearance is grey in color with an average size particle of ~50 nm and an average surface area of 3.7 m\({}^{2}\)/g. Figure 2 shows the UV-visible absorption (UV-Vis) spectrum of the nanosilver particle showing a band at a range of 400-425 nm. Figure 1: _Cont._  
Table 2 shows a commercial formulation of water-based mud obtained from the Scomi Group Berhad company. The selected materials follow the ANSI/API 13A/ISO 13500 Specification for drilling mud materials standard. Eight samples of water-based mud were prepared using an electronic mixer (model: Hamilton Beach, Hamilton, OH, USA). Every time an addition of a component was made, a continuous stirring for about five minutes was performed. Step wisely, fresh water and pre-hydrated gel were added, followed by soda ash, potassium chloride, Hydrozan(r), Hydro star NF(r)[18], and barite. Graphene nanoplatelet and nanosilver were added at different amounts as an additive to the prepared drilling mud samples. The mixture was stirred for about one hour to confirm homogeneity. All tests were conducted according to the ANSI/API 13B-1/ISO 10414-1 standard. Figure 1: (**a**) Transmission electron microscope (TEM) image; and (**b**) X-ray diffraction (XRD) spectrum of graphene nanoplatelet. Figure 2: UV-Vis spectrum of nanosilver. Density of water-based mud was measured using a mud balance equipment of accuracy to within 0.01 gcm\({}^{-3}\). The lid of the balance was removed from the balance cup and drilling fluid sample was placed inside the cup. The cup was covered with the lid and some drilling fluid was ensured to overflow from a hole of the lid to make sure the cup was fully filled with drilling fluid. Then, the arm balance was placed on the base, with the knife-edge resting in the fulcrum. The rider was moved along the graduated arm until the vial was centered. Density measurement was recorded at the left hand edge of the rider. Rheological properties of water-based mud were determined by using a viscometer (model: Fann, Houston, TX, USA). The equipment was switched on and allowed to stabilize, after which the viscosity of distilled water was tested to check the integrity of the equipment. The mud sample was poured into the cup of the viscometer and was placed on the viscometer stand. The stand was adjusted and held in position as the rotor sleeve was immersed in the mud exactly to the fill line. The speed selector knob was selected to rotate at 600 rpm (revolutions per minute), and the power was switched on. When a steady dial reading was reached, this was recorded as the 600 rpm dial reading. The above process was repeated for 300 rpm.