Abstract: The high temperature stability and reservoir protection performance are key factors for the development of Dongfang Gas Field in the South China Sea which has high formation temperature and low-permeability reservoirs.In view of these problems,rolling aging test and compatibility test were used to sort out high temperature treating agents,and the high density (2.2 kg/L) high temperature resistance (170 ℃) water based drilling fluid and solid free cesium-formate completion fluid were developed.Core flow experiment,plugging experiment,rheology and filtration test were adopted to evaluate its comprehensive performance.According to the laboratory evaluation results,the high temperature and high density water based drilling fluid showed good rheological behavior under high temperature with filtration of less than 15 mL and the permeability recovery of more than 85%.Its tolerances to salt,calcium,poor soil contamination were 5.0%,0.5%,8.0% respectively and the extreme pressure lubrication factor was 0.131.The solid free cesium-formate completion fluid with the permeability recovery of more than 88%,and the rolling recovery of up to 87.28%.It could lower the corrosion of drilling tools to 0.0669 mm/a.So it is concluded that the drilling fluid and completion fluid can meet the demand of HTHP gas well drilling and provide technical support for safe and efficient development of Dongfang Gas Field.