World's energy demand is on upsurge and Oil & Gas industry has been directed towards the exploitation of deepwater oil and gas resources. To make this operation successful petroleum industry is continuously demanding new innovative drilling technology that can be easily implemented. One of the constraints that hinders the deepwater exploration is its continuously shrinking narrow window between pore pressure and fracture pressure that further limits the reliability of old conventional riser drilling technique.

Dual gradient drilling (DGD) is a managed pressure drilling that involves use of two different annular fluids for drilling a prospect providing a favorable annular pressure profile and simpler, safer & economical well design.

In any of the rotary drilling operation, drilling hydraulics is the most complicated and least understood drilling variable as it involves complex relationship between drilling fluid, drill bit and formation. Riserless drilling, one of the type of DGD, eliminates the use of marine riser (u-tube is imbalance) and involves use of an additional subsea mud pump at seabed which, supplements to the complication for hydraulics computation as compared to conventional drilling.

In drilling HPHT deepwater wells, drilling fluid is constantly exposed to its own column pressure and formations geothermal gradient which subsequently affects the mud rheology and density. These altered rheological parameters leads to inadequate hole cleaning and lowers the drilling efficiency. High temperature in the wellbore cause's fluid expansion and high pressure causes fluid compression and these changes get extremely severe as water depth and drilling depth increases. Hence it becomes necessary to consider the HPHT effects on Equivalent Circulating Density (ECD) and to correct & optimize the hydraulic program to ensure the best utilization of the hydraulic energy for effective hole cleaning.

This paper presents a detailed study of Well hydraulics for riserless drilling considering the HPHT effect on Equivalent Circulating Density and its optimization so as to ensure riserless drilling operation is conducted in safe and effective manner. Also the paper is concluded with some recommendations on operational and design strategies.

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