IMPROVED THERMAL MODEL FOR HYDRATE FORMATION DRILLING CONSIDERING MULTIPLE HYDRATE DECOMPOSITION EFFECTS

Hydrate is ice-like solid non-stoichiometric crystalline compound, which is stable at favorable low temperature and high-pressure conditions. The predominant gas component stored in naturally-occurring hydrate bearing sediment is CH4 and is estimated about 3000-20000 trillion cubic meter worldwide. Thus, it has attracted significant research interests as an energy source from both academic and industry for the past two decades. Ensuring drilling safety is much important to realize efficient exploitation of hydrate source. Additionally, accurate prediction of wellbore temperature field is of great significance to the design of drilling fluid and cement slurry and the analysis of wellbore stability. However, the heat transfer process in wellbore and hydrate layer during drilling through hydrate formation is a complex phenomenon. The calculation method used in the conventional formation cannot be fully applied to hydrate reservoir drilling, largely due to the complex interactions between the hydrate decomposition, multiphase flow and heat transfer behaviors. In this study, an improved thermal model of wellbore for hydrate layer drilling process is presented by coupling the dynamic decomposition of hydrate, the transportation of hydrate particles in cuttings and heat transfer behaviors in multiphase flow. The distribution of temperature field and rules of hydrate decomposition both in wellbore and hydrate layers are thoroughly analyzed with case study, which is very helpful for the designing drilling parameters, avoiding the gas kick accidents. As well as making a detailed guidance of wellbore stability analysis. This proposed mathematical model is a more in-depth extension of the conventional temperature field prediction model of wellbore, it can present some important implications for drilling through gas-hydrate formation for practical projects.