Abstract: The well Shi-49H1, located in the block Shi-49 in Yingxi area, Qaidam Basin, Qinghai Oilfield, was drilled to enhance the productivity of the anticline structure of the block, especially the productivity of the E₃²-V reservoir group, and to further understand the oil and gas enrichment patterns of the E₃² reservoir of the Shizigou. Well test data will be used to calculate reserves of the oilfield. A BH-WEI drilling fluid was used to drill the third interval, a slim hole section in which the use of high density drilling fluid resulted in downhole problems such as high annular pressure loss, rheology control, narrow drilling window, contamination to the drilling fluid, and high risk of well control etc. These problems have been successfully addressed partially because of the use of the BH-WEI drilling fluid. Average hole size was 159.88 mm, with percent hole enlargement of only 4.9%, indicating that the borehole wall was stabilized by the drilling fluid. The density of the drilling fluid was 2.10 g/cm³, the highest density that has ever been used in horizontal drilling in Yingxi area. No hindrance to exerting pressure on bit has ever been encountered during horizontal drilling. Tripping of the drilling string was conducted smoothly. The well was successfully completed. A high industrial oil production rate was acquired, as determined by well test, indicating that the BH-WEI mud has good reservoir protecting ability.