Abstarct: The problem of wellbore stability in shales has frustrated oil-field engineers from the start of oil and gas well drilling. When water sensitive shale is exposed to conventional water baxsed muds, shale has an immediate tendency to take up water from the drilling fluid. This can result in rapid swelling or dispersion of the shale, pipe stocking and wellbore instability. Invert oil emulsion muds are typically the first choice in drilling water sensitive shale formations. Along with high performance, these fluids have disadvantages such as high cost, environmental impact and detrimental effects on the drilling and completion of the pay zone. It is shown that polymeric water baxsed muds can be an environmentally friendly alternative to oil baxsed muds. Shale encapsulator polymers coat cuttings and exposed shales, limiting interaction with water. Polyglycols have inverse solubility in water with temperature. When polyglycol heated, becomes partially insoluble and emulsion block thus created will avoid further fluid invasion and mud pressure penetration, stabilizing the shale. In 2011 National Iranian Drilling Company designed a special formulation of polymeric mud to provide near oil muds performance. In this study the formulation designed in NIDC was reviewed for optimizing by change in the sequence of adding special additives into the mud. Twenty two drilling fluid samples with same formulation and different sequence in adding additives were prepared. Also an invert oil emulsion mud prepared as an ideal fluid to control shale swelling. After preparing fluids the rheological properties were measured. The ability of controlling the shale swelling of each fluid was studied by linear swelling test. Their ability were different due to different sequence of adding the additives. Finally, the best sequence of adding the additives to the water-baxsed drilling fluid that shows the best performance in swelling reduction was suggested for better treatment.