Study on Stability of Horizontal Wellbore Drilled in Marine Natural Gas Hydrate Reservoir

Natural gas hydrate reservoirs at sea areas are shallow, loose and porous, and poor in mechanical properties in general, so wellbore instability is likely to occur after the drilling fluid infiltrates into the reservoir during drilling. In this study, effects of different factors on the stability of horizontal wellbore have been analyzed by establishing a coupling model of multi-physical fields of the drilled wellbore and reservoir of marine natural gas hydrate. The results show that when horizontal well is drilled into a gas hydrate reservoir, the plastic yield zone of the wellbore is not uniform in distribution and no longer a regular fan ring shape due to the different stress distribution around the wellbore from vertical well. Meanwhile, the closer the formation to the wellbore is, the longer the exposure of the formation to drilling fluid is, so the more thorough the hydrate decomposition, the greater the effective plastic strain, and the greater the risk of wellbore instability is. With the increase of pressure and temperature of drilling fluid and reservoir permeability, the range of mechanical strength drop around the wellbore would expand, making the yield zone of the wellbore and thus the risk of wellbore instability rise. But with the increase of reservoir porosity, the yield zone of wellbore decreases gradually, which is good for the wellbore stability during drilling, but once hydrate around the wellbore decomposes completely during long term production, the risk of wellbore instability may rise sharply. The results of this study can provide a theoretical reference for the stability analysis of horizontal well drilled, the safety pressure control during wellbore drilling and the evaluation of limit extension length of horizontal section in hydrate reservoir at sea areas.