Casing Failure Mechanism

This dissertation was supported by National Natural Science Foundation of China(NSFC) "Study of Mechanism of Casing Failure on the Formation of Complex Mechanics System Based on numerical simulation" (Project No.:50074025).This dissertation mainly dealt with the Mechanism of Casing Failure. Many of related references were studied in the dissertation, some methods and ideas are to help our study, including a lots of the cases of casing failure, it was to provide the decision-making for the study of which casing was failed. At the same time, the results of the new statistic were that the wells of casing failure were up to 17735 by the end of 2002, accounting for the 20 percentage of the whole China, it caused a loss of severe economy. There are many types of the casing failure and very complex casing failure. On the base of many researchers' literatures, the study of the dissertation mainly focused on the mechanism of casing failure of plastic-flow formation, the mechanism of worn casing failure, and the mechanism of the leakage and jumped-out failure of casing connection. The mechanics parameters of the rock core of the plastic-flow formation were tested in the paper, and the stress-strain curves of casing steels were tested, all of the parameters provided for the finite element models. The mechanism of casing failure was studied in details with elastic-plastic finite element method(FEM), the results of the study provided the theoretical supports for preventing casing failure.The main innovations of the paper as following:1. Based on the geological structure , geological data, seism data and testing well data, and so on, the FE mechanic models of the complex rock mechanics while injecting and producing were established, including the parameters, formation temperature grads, rock density, initial porous pressure, injection pressure, production pressure drop as well as the injection period. In light of elasto-plastic creep theory, nonlinear mathematics theory, mechanics theory and the coupling model of the in-situ stress and injection, and the ABAQUS software was extended with the nonlinear mathematics method to solve the problem, it is the breakout achievement in this areas.2. The modified Drucker-Prager/Cap model was introduced to the FEA of the casing failure of plastic-flow formation, the FE model was more reasonable and accurate than that before.3. The mechanism of the worn casing failure about its collapse, tension strength and burst was first studied in details, and the related FE models were established. The burst coefficient of the worn casing was first presented in the paper, the coefficient could be used to scale the burst strength of the worn casing. In the design of the casing string, if the wearing of the casing was considered, we presented that the burst safety coefficient divided by the burst strength coefficient was regarded as the design safety coefficient.4. The mechanism of the leakage and jumped-out failure of casing connection was studied in details with FEM, and the deferent flank angles and the deferent taped cone casing threads were analyzed.