Abstract
Natural gas hydrate (NGH) is a kind of new type green energy source with giant reserves which has been thought of highly by energy explorers in the world. However, NGH breaks down to produce some natural gas that enters the annulus and flows together with the drilling fluid. The gas-liquid two-phase flow can have an impact on the work of the drill string. Therefore, it is important to study gas-liquid two-phase flow in the annulus on the dynamic characteristics of the drill string. In this article, taking a single drill string as the research object, a fluid-structure coupled finite element mathematical model of two-phase flow in the annulus and drill string is established based on computational fluid dynamics and computational structural dynamics theory. The finite element numerical simulation method is used to analyze the influence of drilling fluid and natural gas in the annulus on the dynamic characteristics of the drill string. The simulation analysis shows the following: (1) The motion of drilling fluid or natural gas in the annulus will reduce the natural frequency of the drill string, and the drilling fluid has a greater impact on the natural frequency of the drill string. (2) When single-phase drilling fluid flows in the annulus, the displacement peak in different directions, maximum equivalent stress, and strain of the drill string increase with the increase of the drilling fluid flow velocity or pressure, and the drilling fluid pressure has a more significant effect. (3) When the gas-liquid two-phase fluid flows in the annulus, the displacement peak, maximum equivalent stress, velocity amplitude, and acceleration amplitude of the drill string all increase with the natural gas flow velocity and natural gas content increase, and the natural gas flow velocity has a more significant effect.
Highlights
Energy plays an important role for social stability and national construction
When the drill string encounters natural gas hydrate (NGH), NGH breaks down to produce some natural gas that enters the annulus and flows together with the drilling fluid, which exacerbates the damage to the drill string. erefore, it is of great significance to study the influence of fluid-structure interaction on dynamic characteristics of drill string, which provides safety guidance for NGH exploitation
A fluid-structure coupled finite element mathematical model of two-phase flow and drill string in the annulus was established by the finite element method, and the influence of the two-phase flow of drilling fluid and natural gas in the annulus on the dynamic characteristics of the drilling column was investigated by numerical simulation methods. e conclusions were shown as follows: (1) Both movement of the drilling fluid and natural gas in the annulus will reduce the drill string’s natural frequency and increase the mode shape vibration amplitude, but the drilling fluid has a greater effect on the drill string’s natural frequency than the natural gas
Summary
Energy plays an important role for social stability and national construction. In recent years, there is an increasing demand for energy in various fields, which led to decreasing or even near depletion of oil and gas resources on land. Erefore, it is of great significance to study the influence of fluid-structure interaction on dynamic characteristics of drill string, which provides safety guidance for NGH exploitation. Yang et al [14] analyzed the effect of annulus drilling fluid on the drill string lateral vibration by numerical simulation technology. The studies on the influence of gas-liquid two-phase flow in the annulus on the dynamic characteristics of the drill string are rare. E calculation results are transferred on the interface between the fluid domain and the structural domain to ensure that the boundary conditions of the coupling surface are met and numerical simulation methods are used to build a finite element calculation model and analyze the effects of two fluids in the annulus, drilling fluid and natural gas, on the dynamic characteristics of the drill string The studies on the influence of gas-liquid two-phase flow in the annulus on the dynamic characteristics of the drill string are rare. erefore, taking a single drill string as the research object, fluid governing equations and structural governing equations are established based on fluid dynamics and structural dynamics theory, and the fluid and structure motion equations are derived by using Galerkin finite element method. e calculation results are transferred on the interface between the fluid domain and the structural domain to ensure that the boundary conditions of the coupling surface are met and numerical simulation methods are used to build a finite element calculation model and analyze the effects of two fluids in the annulus, drilling fluid and natural gas, on the dynamic characteristics of the drill string
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