ケーシング設計方法の改良 その過程と将来像

Abstract

The traditional casing design method for Japan Petroleum Exploration Co., Ltd. (JAPEX) was based on specific margins (either ratios or differences) between the anticipated load and the strength of the casing. The specific margins, however, had never been explained as to the downhole conditions on which the margins are based. The size of the margins had been accepted by the company engineers without reasonable explanations.In the mid-1980's, when JAPEX drilled a 6, 000-m Japanese record depth well, the triaxial stress design method was studied. However, due to its relative complexity, the triaxial stress design method was utilized only for some special cases to confirm the soundness of the traditional design.In 1989, JAPEX decided to disuse the traditional design method and adopted a new design method. The new design method assumes specific downhole conditions for each part of the design, namely collapse, burst and tension. Design factors were adopted from a combination of the JAPEX traditional design method and the Maximum Load Casing Design method presented by Mr. C. Prentice.Through the repeated field use since then, the new JAPEX method has been modified to better reflect the downhole conditions and to reduce casing cost. Application of different sets of design scenarios to exploratory wells and development wells is an example of such modifications.In the future, the concept of probabilistic distributions of the strength and the load of the casing shall be introduced in casing design. This approach will quantify the risk of casing failures and will provide a new methodology to preserve an acceptable level of safety without sacrificing economic considerations.