| Natural gas hydrate is a white crystalline cage shaped compound formed by methane gas and water under low temperature and high pressure.1m3 hydrate can be decomposed into 0.8m3 water and 164m3 methane gas under the condition of standard,commonly known as "combustible ice".The resources are mainly distributed in the deep sea area of 300～3000m of the land permafrost zone and the coastal continental shelf,of which about 80.2%are stored in the shallow area within the buried depth of 300m.The total resources of China are 80 trillion and 100 billion parties(including 64 trillion and 200 billion of the South China Sea).In the face of such a huge amount of resources,it is of great strategic significance to speed up the safe and efficient development of gas hydrate reservoirs to alleviate the contradiction between the supply and demand of energy and to ensure the national energy security.In the process of drilling horizontal wells in the marine natural gas hydrate layer,the gas hydrate debris particles flow up with the drilling fluid in the bottom level of the well,and the change of temperature and pressure in the horizontal section is small,so the decomposition of the gas hydrate particles in the horizontal section is very small.The increase of friction and torque caused by poor wellbore purification seriously restricts the extension ability of horizontal well,but due to the poor stability of the hole wall and poor cementation of the reservoir,the phenomenon of borehole enlargement may occur.The bed will eventually cause difficulties in extending horizontal wells,poor wellbore stability and increased friction and torque of drilling tools.To this end,the following work has been carried out in this paper,based on the horizontal wells of marine gas hydrate drilling:firstly,the critical back rate model of drilling fluid in horizontal segment rolling and jumping is established.Two,starting from horizontal and vertical sections,three kinds of expansion methods are set up for horizontal and vertical section,45 degree angle expansion and arc expansion.Solid model;three.Through coupling simulation,the critical flow velocity needed to carry rock was studied under three ways of expanding the horizontal section.Four,the flow field changes under the critical velocity of the vertical section were studied by simulation;five,the horizontal and vertical section of the horizontal well was simulated by the marine gas hydrate drilling.The results show that the diameter of circular arc enlargement is the most easy to carry in the three horizontal section,45 degree angle enlargement is the second.the right angle enlargement is the most difficult.With the increase of hydrate abundance in the horizontal and vertical segments,the smaller the critical velocity is,the solid content increases and the liquid content increases with the increase of the mechanical drilling speed reduce.The experimental results are finally coupled with the numerical simulation,and the results are within the allowable range of error.Finally,the optimization of the process parameters is carried out by an example of a horizontal well of marine gas hydrate.From the optimization results,it provides theoretical guidance for the actual drilling horizontal wells.The understanding of the flow pattern of horizontal well drilling is of positive significance for optimizing drilling parameters and reducing drilling safety risks.Through this study,it lays a theoretical foundation for the efficient development of horizontal wells in marine gas hydrate formation,and provides a basis for developing the risk assessment of marine natural gas hydrate by drilling horizontal well technology,and has improved the high efficiency and safety of drilling horizontal wells under the condition of developing marine natural gas hydrate. |
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