## 3 Results
YP is the initial resistance to flow caused by the electrochemical forces between the particles [20; 21]. Technically, it indicates how much pump pressure must be applied during a drilling operation in order to bring rock cuttings up to the surface level from the static state. Meanwhile, in terms of surface interaction, high YP means an attractive surface force is exhibited and low YP means steric surface force is exhibited. When YP was determined as a function of a nanoparticle, a reduction of YP in the form of a 3 rd order polynomial relationship was revealed, as shown in Figure 5a,b. The statistical correlations for the yield point measurement in the presence of nanoparticle (NP) are (i) YP = 0.325 NP\({}^{3}\)\(-\) 2.55 NP\({}^{2}\) + 4.875 NP + 21.35 (for graphene nanoplatelet) and YP = 0.1667 NP  
Figure 4: Plastic viscosity (PV) measurement of water-based muds with 5% error bar. (**a**) PV measurements, and (**b**) PV percent increment when graphene nanoplatelet and nanosilver are present. 2.5 NP\({}^{2}\) + 4.3333 NP + 22 (for nanosilver) with both of the correlations having R\({}^{2}\) = 1. Mud samples with graphene nanoplatelet added were not significantly affected. The percentage reduction is less than 14%. Meanwhile, the presence of 0.1 g nanosilver gave a YP reduction of 8.3%. At 0.5 g, the reduction is more than 3 rd-magnitude (i.e., 29.1%). At 1.0 g, the percentage of reduction is in the magnitude of 5 th (i.e., 58.3%). The effect of reduced YP revealed the existence of steric force formed under fixed ionic strength condition, between the nanoparticles in a colloidal water-based mud system. In general, water-based mud with a presence of nanoparticle has a lower YP as compared to a conventional type. This means  
Figure 5: Yield point (YP) measurement of water-based muds with 5% error bar. (**a**) YP measurements, and (**b**) YP percent reduction when graphene nanoplatelet and nanosilver are present. that only low force or stress is needed for the drilling fluid to flow. It is indeed highly desirable to have a low YP to allow sand and shale cuttings to settle out and entrained gas to escape, to minimize swabbing effect during pulling the string out of hole, and permit the circulation to be started at a low pump pressure (i.e., economically feasible) [22]. In this study, we found that nanosilver is a potential candidate as it exhibits lower YP than that of graphene nanoplatelet. Previous studies have found that graphene compound acts as a surfactant, due to its capability in creating highly stable Pickering emulsions of organic solvents like solid particles [23]. It can also act as a molecular dispersing agent to process insoluble materials such as graphite and carbon nanotubes in water [24]. Conversely, the insignificant reduction of YP revealed in this study is likely due to particle bridging effect caused by the interaction of polar functional groups, located in an ideal position to interact with interacting adsorbed layers form nearby particles [17]. The presence of additional attractive force, such as a bridging interaction caused by adsorbed nanoparticles, was found to accentuate the bridging interactions which extent the flocculation state of suspension [25] and significantly increases the strength of interparticle interactions by up to 10-fold [26]. Figure 6a shows the performance of water-based mud in the presence of graphene nanoplatelet and nanosilver on the volume of filtrate (fluid loss); whilst, the percentage reduction is shown in Figure 6b. The filtration test was conducted at 500 psi and it is within an acceptable range of hydrostatic pressure (200-500 psi). In designing drilling mud, the hydrostatic pressure to be imposed by the drilling mud must be higher than the formation pressure. If the drilling mud was incorrectly formulated, the expected hydrostatic pressure may not be attained. Hence it will initiate the influx of formation fluid to occur and a phenomenon that is known as 'kick' will be triggered and may result in blowout.