Strength Design Of Casing String In Deep Water Wells

In recent years, with increasing demand for oil and natural gas and the development of oil and natural gas exploration and exploitation. Deepwater oil and gas exploration has become a hot spot of oil and gas exploration and exploitation in the world today and it is also the new bright spot of increasing oil and gas reserves and production. Besides, deepwater oil and gas exploration is an inevitable development trend of offshore oil and gas exploration and it is an important strategic area to replace conventional oil and gas resources. However, the design of casing string strength in deepwater wells has been perplexing the domestic and foreign drilling researchers. Casing stress in deep-water wells is very complicated, therefore, in the casing design, it is needed to analyze the casing stress in the engineering applications comprehensively. Casing design not only needs to consider the performance degradation of casing caused by corrosion, abrasion, but also takes the changes of wellbore pressure caused by annulus pressure into consideration. And the changes of temperature, pressure and fluid flow rate caused by different oil and gas production of casing design should be considered. Moreover, the uneven load distribution caused by the decomposition of natural gas hydrate should also be taken into account for casing design. In short, all these factors are needed to be considered in the design of casing strength in deep-water wells.In this paper, through analyzing and summarizing the domestic and international design methods of casing string strength, a set of strength design models of casing string for deepwater wells is established. The influence of corrosion, abrasion and annulus pressure on the strength design of casing string is studied in this model. And the strength calculation method, the calculation method of external load and the values of safety factors are studied. Besides, the influence of factors such as corrosion and annulus pressure on the safety coefficient of casing string is analyzed. In this paper, a finite element model of casing string-cement sheath-formation (Thermodynamic properties of natural gas hydrate) is established, the unbalanced load caused by the decomposition of natural gas hydrate is simulated, the influence of thermodynamic and mechanical properties of cement sheath on the load distribution of casing string is studied, the effects of the geometric dimension of the cement sheath loss on the maximum von Mises stress distribution of casing string is discussed.In this paper, Aimed at the geological and engineering characteristics of a deepwater well, the safety factor distribution of each layer of the casings is studied by reference to the conventional strength design and check methods of casing string. Then according to real casing program, annulus fluid and annulus pressure conditions, the changes of casing strength with their service time are calculated and the effects of corrosion of casing and annulus pressure on wellbore casing string safety is analyzed in this paper.