Casing String Design New Methods And Software Development

Unreasonable casing design is one of the most important reason why casing damage. It mainly means: First, the understanding about forms of stresses of casing is not adequate. In the older method of casing design, external pressure of casing is considered only dependent on hydrostatic pressure and mud pressure, the forces of formation in production are not considered. Second, the understanding of modes of external pressure is not adequate, external pressure is considered uniform distribution along radius of casing, non-uniform loads, symmetrical concentrated load and shear-type load that casing may meet in production are not considered. Thirst, the understanding about strength is not adequate, casing strength is considered API strength when the loads of casing are uniform distribution. In fact, the loads of casing are not uniform distribution and casing strength is not the API strength.In forms of stresses of casing, the paper analyzed the non-uniform loads caused by shale creep and formation slide, and put forward the casing ovality at yield can considered as a mechanics standard of casing failure under non-uniform loads. In strength of casing, based on analyzing the influences made by cementing, perforating and non-uniform, the concept of decrease factor of casing strength.A new method has been put forward, in the method, the triaxial stress design of casing string method is used in the design, but in the formations that shale creep and slide are easily found, special design methods are adopted. Based on the proposed method and in combination with the practice of oilfields, the software for strength design of casing string is developed. Designed the casing of xing13-ding2-23 well by the software, compared the result with that of the usual method of casing design, it displayed that the software can be used in oilfields.