Numerical Simulation on the Drilling Fluid Flowing in Gas Hydrate-Bearing Sediment at the Laboratory Scale

The invasion of drilling fluid into sediments around borehole will influence their physical properties,for example,strength and pore pressure.In the gas hydrate-bearing sediments(GHBS),the invasion process may couple with the dissociation and reformation of gas hydrates around the borehole,and then affect wellbore stability and log interpretation in GHBS.Thus,the investigation on the invasion dynamics of drilling fluid into GHBS is important for drilling and logging.Here,we used a numerical simulation method to analyze the dynamic behaviors of drilling mud invasion and flowing in GHBS at the laboratory scale according to field data and an experimental simulation system.The results show that the hydrate layer is obviously influenced by the mud invasion,and the density,temperature and salt content of the drilling mud all have great effects on the process of the invasion,especially the density which controls the heat and salt transfer efficiency to some content.Also,it is found that hydrate might decompose in the process of the fluid migration because of reducing the dissolved methane in the pore water and destroying the dynamic balance between natural gas hydrate,water and dissolved gas although the phase equilibrium state of gas hydrate was not broken.