Abstract: For horizontal wells drilled for development of Tertiary reservoirs, conventional weak gel drilling fluid may not fully satisfy all demands for drilling safety and reservoir protection. With consideration of specific properties of Shahejie Formation, proportions of potassium formate and KCl used in the formula have been modified to reduce the activity of the drilling fluid with the intent to match the drilling fluid used in reservoir formation with the activity of the Shahejie Formation. By utilizing capacities of potassium formate ions in compression of double electric layers in clay gel particles, inhibition performances to the mud shale can be enhanced effectively; To protect reservoir formations, step-by-step matching and filling have been adopted by selecting CaCO3 grains with optimal sizes to achieve perfect match with pore diameters in reservoir formation. Duirng completion, recessive acid can be used as gel breaker to obtain desirable unplugging performances and effectively remove problems related to sidewall instability and proper protection of deep formation in the Shahejie Formation. The innovative technique has been applied successfully in Well BZ34-7-A2H to achieve goals in safe drilling and reservoir protection.