Oil and gas industry continues to increasing demand of more cost-effective well design and operations. Thus, PTTEP Thai onshore drilling team response to the mission by enhancing well design of deep wells in Sirikit Field from three strings to two strings well. This optimization not only reduce cost per well but also unlock reserve in deeper section which used to be uneconomic. To implement two strings design for deep wells with long open hole section (more than 2,000 mMD), there are key challenges which has to be overcome as per listed below; Directional control issue Higher torque, drag and side force Difficulty for wireline logging and casing running operation Formation stability and lost circulation Revised Casing Design To overcome these challenges, not only suite of tools and technologies have to be studied and field proven but drilling practice also has to be reviewed. Risk assessment and feasibility study has been conducted to ensure that this design optimization would results in positive outcome Since the first implementation in 2015, more than 50 wells have been successfully drilled and completed. The longest open hole section of 2,899.3 mMD with TD of 4,202 mMD has been achieved and average cost saving of more than 25% per well has been realized. Below list the technology applied to overcome challenges; All targets have been penetrates without directional control issue using Rotary Steerable System (RSS). High drilling torque have been mitigated using high torque drill pipe and torque reduction tools. Reamer and back-reaming out of hole have been used to smoothen wellbore which facilitate wireline logging and casing running operation. Eccentric casing shoe and low-friction centralizers have been used to further aid running casing in high angle wells. High drilling fluid weight has been used in advance to combat formations stability issue. Hence, lost circulation materials (LCMs) have been prepared to mitigate loss circulation due to high ECD. Polymer based spacer system have been used for the wells which have potential of lost circulations while cementing.