The petroleum industry is demanding new drilling concepts that make exploitation of deepwater resources economically feasible. Dual gradient drilling concepts have been introduced to overcome the challenges associated with deep water drilling, by providing simpler, safer, more economic well designs and therefore increasing the ultimate development and utilization of deepwater resources14 .

In dual gradient drilling, the wellbore is drilled with two different annulus fluid gradients in place. By having two distinct pressure gradients, a favorable pressure profile in deepwater wells is expected specifically a profile closer to what naturally exists in the subsurface formations. This causes the utilization of dual gradient drilling will reduce the number of casing strings required, reduce the non-productive time and also enhances well control7 .

Even though dual gradient drilling provides a system that makes the pressure profile closer to what naturally exists in the formation, the pressure across the annulus is also affected by changes of pressure and temperature over depth, as both of changes will affect mud density and its rheology. The study of the effects of pressure and temperature variation is required to understand the distribution of mud density and rheology over the depth and to ensure that the dual gradient drilling operation is able to be conducted in a safe manner.

As dual gradient drilling introduces additional circulation and injection lines, the calculation of injection fluid flowrate-based on the minimum flowrate required to lift the cutting to the surface-, bit hydraulic optimization and the injection fluid density-based on required pressure profile across the wellbore-is very important, since failure in determination of those parameters will cause hole cleaning problems, inefficient drilling operation, or even kick and lost circulation problems.

This paper presents a detailed hydraulic analysis of a Dual Gradient Drilling operation, which uses a liquid and gas-lift system and provides the determination of cutting transport parameters, hydraulic optimization, required injection fluid density, required injection fluid flowrate, and the pressure profile due to different annulus fluid gradients in place. To achieve an accurate determination of hydraulic parameters, a software application has been created and developed. A previous deepwater model that has been proposed by Lopes is used as the input for this software.

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