ASSESSMENT OF THE TECHNICAL BENEFITS OF DUAL GRADIENT DRILLING
DGD is a technology that makes use of separate fluids with different densities in the wellbore. The lighter fluid floats on top of the heavy fluid in the riser. The lighter fluid is only used for inducing pressure and is otherwise inactive. However the heavy fluid is used for the same purpose as used in the conventional drilling procedures. This helps to adjust the bottom hole pressure (BHP) in a shorter time, and make it able to adjust the well bore pressure curves with the formation pressure curves. The attractions that DGD highlights are the reduction in the cost of drilling and an increase in the production rate after well completion ( Gaup, 2014).
The development work on the DGD was accelerated during the 1990s when a joint industry project was undertaken with the aim to utilize such technology to be used in the high pressure, low fracture gradient in ultra-deep waters. Even though sufficient investments have been made on drilling rigs which can operate in depths greater than 8000ft, the resources present at these reservoirs cannot be extracted unless new procedures are developed to lower hydrostatic mud pressures to avoid fractures in the shallow zones. The problems faced in ultra-deep drilling include shallow water flowing, lost circulation and loss of well control. If any of these problems occur, they will prevent the completion of the well to be achieved. Multiple casing strings are used to avoid such problems. This means that the production string is quite small for a high production well and also for horizontal and multilateral completions in order to make the project economically viable. Pumps are used to reduce the hydrostatic head from the mud-line to the surface in DGD techniques. This is the reason why there is no balanced u-tube present in DGD as compared to the conventional drilling ( Kennedy 2001). The primary component that enables the DGD operations is the Mud Lift Pump (MLP). With the help of diaphragm pumps powered by the seawater, it pumps the drilling fluid and cutting back to the rig floor. The Subsea Rotating Device (SRD) maintains the boundary between the sea water density fluid in the drilling riser and the drilling fluid and redirects the mud through the MLP through the Solids Processing Unit (SPU). SPU is used to decrease the size of the drill cuttings which can be managed by the MLP (Ganpatye et al. 2013).