Influence of thermal conductivity of annular protective fluids on wellbore hydrate formation and hydrate decomposition in shallow formation during deep-water drilling and well testing

Key words:  deep-water gas wells  annular protective fluid  wellbore hydrate formation  shallow formation hydrate decomposition

Abstract:

During deep-water drilling of hydrate bearing formations, natural gas hydrates in near wellbore formation can decompose, and in well testing or production of deep-water oil and gas reservoirs, hydrates can be formed in wellbore, causing blockage. Special annular protective fluids can be used to mitigate those problems.In this study, taking a deep-water gas well in the South China Sea as an example, a numerical simulation method was used to study the influence of the thermal conductivity of annular protective fluids on wellbore hydrate formation and shallow hydrate decomposition under different drilling and well test conditions. The simulation results indicate that the lower the thermal conductivity of the annular protective fluid, the lower the critical gas production rate for hydrate generation in wellbore. When the thermal conductivity of the annular protective fluid is as low as 0.10 W/(m·℃),the critical gas rate for no hydrate generation in wellbore can be reduced to 5×10⁴ m³/d. Low thermal conductivity of the annular protective fluid is also beneficial to reduce heat transfer between wellbore and nearby rock formation, which can inhibit hydrate decomposition during drilling when drilling fluid with higher temperature is used. The accuracy of the simulation model has been verified by comparing the wellhead temperature measured during field gas production testing with the simulation results, with errors being less than 5%.