Study on the buckling behavior of continuous tubing column in deepwater natural gas hydrate reservoirs

Although the gas hydrate in the South China Sea has been successfully exploited, it is still in the exploration stage. In order to conduct large-scale commercial development, a large number of technical difficulties need to be overcome. The burial depth of the combustible ice in the South China Sea is more than 200 meters below the seabed mudline, and the vertical depth of the overlying strata that can be used for deflecting is small. The implementation of short radius horizontal wells has become the best technical means to ensure the drilling rate of the reservoir, increase the drainage area of the reservoir, and improve the development efficiency under the current technical conditions. Coiled tubing has more advantages than conventional drill string in the development of short radius horizontal wells because of its flexibility, which provides a new focus for cost saving. The buckling of coiled tubing is different from that of conventional rods. Its lateral displacement will be constrained by the size of the wellbore, resulting in sinusoidal and helical buckling. Therefore, it is very important to understand the downhole buckling behavior of coiled tubing for hydrate exploitation.In this paper, based on the previous research, the calculation model of coiled tubing buckling load is derived by using the energy method, and the load calculation method of straight well section is improved according to the rationality of finite element simulation, and then the critical buckling load of different well sections is analyzed; Considering the influence of hydrate decomposition in hydrate reservoir, assuming that the density of drilling fluid increases linearly with the well depth after the natural gas from hydrate decomposition enters the annulus, the average density formula of drilling fluid under different gas ratios at the top of the annulus is established, so as to obtain the calculation formula of coiled tubing floating weight and equivalent axial load under different gas ratios at the top of the annulus; The geometric nonlinear finite element buckling analysis model of coiled tubing is established based on the nonlinear large deformation theory, and the buckling process of coiled tubing under different axial loads and different top annulus gas ratios is simulated under different well sections.