| The geothermal gradient of Jinlin Oilfield is relativly high and the average is 3.5 to 4℃. The deepest target structure--the Shahezi structure is 5400 meters deep and the bottom hole static temperature is as high as 220℃. The Yingchengzu structure's gas bearing formation is with a very high formational pressure, and the formation pressure coefficient of Changshen 1 well which reached this formation was as high as 1.13. Besides, there are many formation fractures in this very formation and the drilling fluid density window is very narrow, so, it is easy to leak off. There is very much mud stone in the Qingshankou structure, it is of high risk of flopping. Brittle vulcanite of the yingchengzu structure falls in part easily because of its nature, and sand tuff in this structure swells and flops easily when it meets water. As a result, it is crucial to handle these problems. But, all the existing types of drilling fluid can not meet the demands of deep prospect hole, so it is needed to develop a new type of drilling fluid which can satisfy the requirements of deep exploration well of Jinlin oilfield. Mechanism was first studied. And guided by this mechanism, polyacrylonitrile which was treated by a series of chemical reactions, such as multi-hydrolysis, sulfomethylation, oxidization, and recombination, was served as host treating chemical for YDH series drilling fluid. High temperature filtration-prevent stablizer was developed with YDH series-3 as the main component: 2.5~3%YDH-3+0.5%Na2SO4. Also, the type of drilling fluid which is very suitable for Jinlin oilfield was decided: DKW+1%FK+2%HA+2~3%SF-260. in house test shows that this type of drilling fluid's rheological property is very stable and with high temperature resistance, good anti-sloughing, and it can bring up the cuttings carring capacity.
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