Numerical analysis of horizontal wellbore state during drilling at the first offshore hydrate production test site in Shenhu area of the South China Sea

Horizontal well drilling in the unconsolidated marine hydrate sediments is a challenging issue due to high risk of wellbore instability. This paper analyzed this issue under different conditions through numerical simulations based on the field available data in Shenhu area of the South China Sea. Besides, the potential secondary hydrate formation in the annulus and the influence of mud cake on the wellbore stability were discussed. The results show that the assumed horizontal wellbore may not collapse at the site SHSC-4 even though some local areas may enter the yielded state, however, the drilling pipe sticking may occur after drilling because of slight displacement of well wall caused by stress release. The sensitivity analyses exhibit that increasing the drilling fluid density and salinity may slightly accelerate hydrate dissociation, but these effects on wellbore stability is negligible. A significant increase in the drilling fluid temperature may be a disaster for maintaining the wellbore stability, and even cause secondary hydrate formation and well blowout. The model also shows that the wellbore instability can exacerbate with the decrease in the initial hydrate saturation and irreducible water saturation while the low permeability mud cake is conductive to reducing filtrate loss and facilitate the wellbore stability.