Wellbore stability in high reactive shales is one of the operator challenges leading them to use OBM and WBM with different inhibitors such as KCL, glycols, amines ant etc. Environmental concerns restrict the OBM and Amines usages, but knowledge shortage in relation with inhibition mechanisms, still preserve KCL\polymer mud as an efficient solution even in sensitive areas. Cost effective silica-alumina nano drilling fluids as KCL substitution, provides inhibition both mechanically and chemically which is not achievable by other inhibitors. The silica-alumina nano-drilling fluid system is designed in different mud weights without consuming KCL. This fluid system is subjected to hot rolling for 4, 8 and 16 hrs at 250 and 300 °F in the presence of highly reactive montmorillonite shale. All mud properties including shale recovery, rheological properties, fluid loss, foaming, lubricity and mud cake quality were evaluated before and after hot rolling. Also the stability of the nano based drilling fluid system in different salinities and probable contaminations were examined and finally all the results and cost evaluation were compared with ordinary high performance drilling fluid systems such as KCL\polymer, KCL\PHPA\Glycol. The collected data from experimental results are compared to high performance water based drilling fluids such as KCL\polymer or KCL\PHPA\Glycol drilling fluid systems, somewhat cationic polymers or amine chemistry. This nano-drilling fluid possesses the most shale inhibition characteristic between 90-99% which is higher than the other aforementioned systems. (This value is really comparable with OBM having the highest shale inhibition among the others). Rheological properties like PV (plastic viscosity) and YP (Yield Point) remain relatively constant in an acceptable level; in that PV and YP decrease level is lower than 20% which is the half of the efficient KCL containing muds. The fluid loss is sufficiently controlled, less than 5 cc even at high temperatures, and lubricity effect provides us with a really acceptable Kf (friction of coefficient factor), under 0.2, which enables applying the system in high deviated zone or directional drilling. Stability tests show that this system has no incompatibility with high salinities and is adequately resistant in presence of contaminations. Also this system is applicable in the presence of weighting agents. Finally cost comparisons and environmental evaluations introduce this system as a cost benefit and environmental friendly system. The most striking feature of this research is to apply cost effective silica-alumina nano drilling fluid to replace the conventional inhibitors which are not economical and environmentally friendly such as KCL. Significantly by introducing this system, we can diminish the high costs of synthesizing cationic polymers or amine compounds for shale drilling in sensitive areas and highly reactive shale.