Abstract: X gas field is an important area for China's offshore oil development, with the main reservoir HG group being a low porosity, medium-low permeability to ultra-low porosity, and ultra-low permeability reservoir. The reservoir has strong heterogeneity and production capacity needs to be improved. In order to ensure the development of the gas field, a number of high inclination directional wells, horizontal wells, and extended-reach wells, whose average open hole length exceed 1200 m, have been implemented. In response to the challenges of reservoir damage caused by poor physical properties, susceptibility to contamination, difficulty in wellbore cleaning due to high inclination rock debris beds and oil mud accumulation, long backflow time of oil-based drilling fluid in the long open hole section of horizontal wells, and high risk of natural gas cold venting, we conducted analysis of reservoir damage factors and mechanisms, optimization of oil-based drilling fluid performance and evaluation of reservoir protection effect, optimization of wellbore structure and well trajectory, optimization of lower completion string structure, innovation of high-efficiency cleaning technology and high-efficiency flowback technology for wellbore. The long open hole oil-base drilling fluid completion operations have been implemented for the first time on a large scale. And it ensured the safe and efficient production of X gas field and led to excellent capacity release effect. The application shows that this technology can effectively assist in the economic and effective development of low-permeability oil and gas fields, and can be promoted in similar oil and gas fields.