SUMMARY
Along with the development of drilling to offshore or deep and complex formation, the drilling risk is becoming more and more serious, which leads to continuous increase of drilling cost. Controlling drilling cost and ensuring drilling safety become more and more attractive. So a method was proposed to solve this problem. Firstly, the drilling risk was evaluated by the method of risk degree judgment, which was divided into the dangerous zone, transition zone and safe zone. Measure must be taken to avoid risk in the dangerous zone. The drilling process should be cautious in the safe zone without the need to avoid risk. Risk in transition zone was the focus in this paper. The risk treatment and risk aversion cost model were established to calculate the risk treatment and aversion cost. Then the coordination mechanism of risk and cost was proposed to take both safety and cost into consideration. Finally, the optimized casing program based on the coordination mechanism was presented. The case study shows that the drilling risk can efficiently be reduced based on the optimized casing program. At the same time, the cost can be reduced to the greatest extent.