Because of the complex geological structure and unpredictable geologic environment, drilling engineering is characterized by high risk and high cost, especially the drilling processes of complex formation or deep water condition. Controlling drilling cost and ensuring the drilling safe has been the focus of research at home and abroad. So this article's objective is to establish the casing program optimization method based on risk and cost coordination. In this article, based on the drilling quantitative risk assessment and the drilling accidents report, the risk probability profile can be obtained. Then according to the theory of fuzzy mathematics, the model of risk degree judgment was established. The drilling risk probability profile is divided into three parts: the safe region, the transitional region and the dangerous region. No special treatment is necessary for the safe region. Risk in the dangerous region must be avoided before drilling. The risk in the transitional region must take account of both risks and costs. The risk treatment and aversion cost calculation model were established. Then the coordination of drilling risk and cost was proposed. Finally, the optimized casing program based on this method was obtained. After the summarization of this principle, taking the drilling data of one block of a basin in west China as the example, the risk level identification results were mainly identical with the realities, and indicated that the method could satisfy the needs of drilling engineering. Case study showed that the decision making for casing design of one pre-drill well was optimized based on the coordination mechanism. The well was successfully completed without risks and the drilling cost decreased by 15%.

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