|Alternative Title||Study of deformation and failure caused by hydrate decomposition on wellhead soil layer|
|Place of Conferral||北京|
|Keyword||水合物分解 开采 井口安全性 气体喷发|
Natural gas hydrate is an ideal clean alternative energy in the twenty-first Century. It mainly exists in deep-sea sediments and permafrost zones. The trial production of gas hydrate has been carried out inseveral countries. In 2013, Japan successfully carried out the first trial production of offshore hydrate in the world. In 2017, China realized trial exploitation lasting for 60 days.
During the exploitation, the decomposition of hydrates will cause the increase of pore pressure and the decrease of stratum’s effective stress, resulting in softening of soil, damage to offshore structure such as wellbore. At the same time, if the permeability of overlying soil is low, the drainage rate of methane gas produced by the decomposition of phase equilibrium is much less than that the gas accumulation rate, which leads to the accumulation of excess pore pressure. Accordingly the surrounding soil is extruded, resulting in large deformation of the soil around the wellbore and even the phenomenon of gas eruption. Therefore, it is required to study the stability of the sediment surrounding the wellbore and the gas-induced eruption. The main studies are as follows.
Firstly, the decomposition range of the hydrate in a certain time is obtained by the theory according to the geological data obtained from the site. In view of the possible location of the exploitation zone, the local deformation of the sediment caused by hydrate decomposition is numerically simulated, and the wellbore safety is also evaluated. Under the condition of decomposition zone of 3m, the maximum settlement is about 2.0-3.0cm. It is safety of the wellbore in this case. However, with the increase of the decomposition zone and the thickness of gas hydrate layer, the settlement of the sediment increases, and zone with plastic damage becomes larger, which can result in sand production, wellbore collapse and other disasters.
Secondly, we analyzed the influences of hydrate decomposition and the occurrence of horizontal well on the formation stability during the process of hydrate exploitation. It can be seen that the settlement also becomes larger with lager hydrate decomposition radius during exploitation with the horizontal well. The maximum settlement occurs mainly over the horizontal well, and the horizontal displacement increases gradually. The vertical deformation relative to the hydrate is relatively small. The influence of horizontal displacement can be neglected.
Thirdly, the phenomenon of gas eruption is analyzed referencing the large holes caused by gas explosion after gas hydrate decoposition in Bermuda area. The main factors are found based on dimensional analysis. On this basis, the numerical simulation is carried out to analyze the specific conditions. The results show that the value of the radius of the hydrate decomposition zone and the thickness of the overlying soil layer is larger, the excess pore pressure ratio is smaller, the damage occurs more easily.
Key Words: Hydrate decomposition, exploitation, wellhead safety, gas eruption
|王晶. 水合物分解引起井口周围土层变形与破坏的研究[D]. 北京. 中国科学院大学,2018.|
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