Modeling And Analysis Of Core Drilling In Complex Formation Of Submarine Gas Hydrate

As a clean energy in the future,natural gas hydrate has the characteristics of high energy,large scale and shallow burial.It is widely distributed in the seas of the world and has received great attention from countries around the world.However,due to its special accumulation mechanism and occurrence conditions,the submarine gas hydrate reservoirs are all distributed around the structural strata and complex strata with frequent layer changes,which makes the drilling and exploitation of natural gas hydrate extremely difficult.By researching drilling and coring of submarine gas hydrate under complex formations,this paper used the finite element software to simulate the process of drilling and coring,and analyzed the drilling speed and the disturbance of the core under different drilling parameters.Therefore,it provides guidance and theoretical basis for the optimization of drilling parameters for efficient drilling of low-disturbance gas hydrate.The specific research content and innovative points of this paper are as follows:Firstly,this paper reviewed the research background and significance of the subject,and summarized the related research status at home and abroad,including the composition and environmental factors of the submarine gas hydrate reservoir,the drilling and production dynamics model of the diamond bit,and the research on drilling and coring using numerical simulation.Then,the research content and specific research route of the thesis are described.The complex information drilling of submarine gas hydrate reservoirs is researched,and the influence of the drilling process on the core’s perturbation is explored.Secondly,the natural gas hydrate reservoir environment and gas hydrate-bearing sea rock model in the seabed are analyzed.The formation drill ability of the submarine gas hydrate reservoir is graded and a reasonable diamond bit is selected according to the principle of diamond bit selection.After simplifying the diamond bit,a finite element model of the drill bit drilling into the complex formation of gas hydrate is established.Then,the finite element simulation analysis of the drilling process of gas hydrate complex formation are carried out.The main content includes applying different drilling parameters to the diamond bit to obtain the footage of the diamond bit in complex formations,the motion state of diamond drilling in different seabed strata is divided into three stages and achieved stable drilling rates in all stages.At the same time,the simulation results obtained the distribution law of equivalent stress on core and well circumference in drilling.It lays the foundation for core disturbance analysis.Finally,Based on the finite element simulation analysis results of diamond drill bit drilling into complex submarine gas hydrate reservoirs,an evaluation method for core disturbance in drilling and coring is proposed.Python script is used to calculate the minimum radius of the undisturbed area under different drilling parameters and the change of the disturbance rate of the core with time.Thus,the influence of different drilling parameters on the core disturbance is analyzed.The results show that within a certain range,the weight on bit has a significant effect on the core disturbance during the drilling process,and the change of the rotational speed has no effect on the stable value of the core disturbance rate.Therefore,various factors should be comprehensively considered in actual drilling.When drilling sedimentary layers,the WOB and rotational speed can be appropriately reduced to improve the core quality.And when drilling gas hydrates at a certain drilling speed,the low-disturbance core is obtained by reducing the drilling pressure,and the rotation speed is appropriately increased to obtain higher drilling speed.