In deepwater drilling operations, natural gas may enter the wellbore from oil reservoirs. As the wellbore and pipelines in deepwater wells are under low-temperature and high-pressure conditions, natural gas hydrates are easy to form, which will bring new problems and challenges to drilling, well control and riser design. When drilling a horizontal wellhole, the heat transfer mode in openhole section will change, for the existence of openhole section. In this paper, a model to calculate the wellbore temperature profile when drilling horizontal wells to deepwater reservoirs is established, considering the effects of openhole. A case study is carried out, based on which, the wellbore temperature profile and the hydrate formed region are depicted. The sensitivity analysis about parameters including horizontal section length, water depth, geothermal gradient, mud parameters and additives are also presented. According to the equilibrium curve and annulus temperature curve, hydrate formed region is affected significantly by water depth, mud specific heat capacity and additives. Horizontal section length and geothermal gradient only have little influence on hydrate formed region. This study will provide certain guidance for engineering practice of deepwater drilling.
Temperature profile is one of the most important parameters when drilling a well, for it will affect the downhole pressure and lead to the wellbore stability problems. In deepwater drilling, the temperature of sea water decreases with the water depth and will cool the drilling fluid, and then formation temperature increases with depth and heat the fluid. As known, gas hydrate will form under conditions of high pressure and low temperature. The temperature/pressure condition in annulus makes it easy to form gas hydrate which will lead to serious accidents when drilling in deepwater environment (Barker and Gomez 1989). Therefore, the wellbore temperature profile should be calculated accurately and the occurrence of hydrate accumulation should be predicted in advance. However, the temperature of drilling fluid will be difficult to determine, especially when drilling horizontal wells into deepwater sediments.
