| This paper was supported by Science and Technology Study Project from Ministry of Education (No:205135):"Mechanism Study of In-situ Stress and Casing Failure on Complex Formation Based on Coupling Problem", this paper was also supported by project of Outstanding Youth Foundation of Sichuan (No: 03ZQ026-044):"Mechanism Study of Casing Failure of Special Wells".Many of references about mechanism study of casing failure were investigated, gathered and read through, the former studies were synthetically analyzed, the mechanism of casing failure on complex formation was analyzed and studied in detail, and then the study contents and methods were presented. Based on the general casing strings design, program of casing strings design on complex formation was developed, aimed at the section of special formation, such as abnormal high pressure, salt rock formation that the in-situ stress was varied, based on study of this paper, special casing strings design was performed。The study contents and contributions as follows:1. Analyzed and studied the geological and engineering factors of casing failure in detail.2. Analyzed the status and limitation of casing strings design at home and abroad, presented that determining the external pressure of production casing by the actual most in-situ stress or non-uniform in-situ stress on complex formation, performing special casing strings design at special section taking into account the non-uniform in-situ stress.3. Through studying the data from certain oil field of west, such as the service condition of casing strings, the formation and faults' strike, the distribution of the water flooding, production history and casing failure on the region and section, and the relation of the location of casing failure and faults, the reason and manner of casing failure were obtained, and the corresponding preventive measures were presented, provided references for the reasonable adjustment of parameters of water flooding and production and preventing casing failure.4. In this paper, the relation of the horizontal in-situ stress and the coefficient of the tectonic stress were derived, in accordance with the curve of fracture reversely derived the horizontal in-situ stress, and provided actual in-situ stress for the casing strings design of complex formation.5. Analyzed and studied the main factor that affecting the casing strength in theory, namely, the affect of cement mantle on casing strength. According to the relation of displacement displacement associate with deformation, derived the expression of the pass coefficient of external pressure that the load transmitted from formation to the external wall of casing throughout the cement mantle, in accordance with these mechanics and mathematical expressions, we can conveniently analyze the mechanics affect of cement mantle on casing.6. Establish the mechanics model of casing deformation on non-uniform in-situ stress, inaccordance with the mathematics mechanics relation and boundary condition, based on the former study, derived the mathematical model of casing strength, can be directly used to the special section casing strings design of complex formation.7. In accordance with former study, and combining with the actual parameters, studied the affect of perforation on casing strength, and derived the theoretical formula of the perforation resistance strength coefficient, provided simple and convenient computing method forperforation casing strength design.8. In accordance with the circumstance of casing strings on complex formation, derived the mathematical expression for casing strings design under the triaxial stress, also derived the mathematical expression of equivalent resistance strength of casing strings under non-uniform in-situ stress, provided correct and reasonable mathematical models for developing special program of casing strings on complex formation. The model and program have been applied to casing strings design on complex formation of the oil field, and have achieved satisfying results. |
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