Managed pressure drilling episode 6
5- Dual-Gradient Drilling:
Dual-Gradient drilling refers to drilling with two different fluid-density gradients.
Fig. 1 shows the dual-gradient pressure profile. In this case, using a single density fluid for this wellbore will cause the wellbore pressure to exceed the formation pressure and result in lost circulation. With dual-gradient drilling, a lighter fluid is used in the upper portion of a wellbore and a heavier fluid at the lower portion. This enables the pressure to remain in the pressure window between the pore pressure and fracture pressure.
To achieve a dual gradient, a less-dense fluid such as air, inert gas, or light liquid is injected at a certain point in the wellbore. Introducing this less dense fluid at this point would decrease the density of the fluid from that point up to the surface.
Another technique is used for offshore environments. A small diameter return line is run from the seafloor to circulate the drilling fluid and cuttings. The marine riser is kept full of seawater. A subsea pump is used to lift the drill cuttings and the drill fluid from the wellbore annulus up to the rig floor. By using seawater in the marine riser, a more dense mud is used in the wellbore to achieve the bottomhole pressure required.
Riserless Dual-gradient drilling:
Dual-gradient drilling can also be achieved in deep water without a riser when first starting a subsea drilling location. A subsea RCD (RotatingControl Device) and remote operating vehicle are used. The ROV is able to adjust backpressure at the mudline by adjusting the choke. When the ROV closes the subsea choke, the BHP increases.
This results in drilling with a slight overbalance as if a marine riser filled with drilling fluid were present. The advantage of being able to drill with a slight overbalance is that it helps to prevent shallow gas or water flow. The seawater is used as the drilling fluid so the drilling fluid and cuttings can be left on the sea floor.
Fig. 2 Pressure profile for drilling dual gradient without a riser (From Hannegan).
Kick detection in dual-Gradient drilling:
With dual-gradient drilling, pressure gauges installed on the rig floor are more sensitive to changes than the gauges used in conventional drilling. A decrease in circulating pressure caused by an increase in flow will be more easily seen. If a kick occurs, the annular flow rate of the drilling fluid will increase by an amount equal to the influx rate. If the subsea pump were set to operate at a constant inlet pressure, the subsea pump rate would increase. This increase would be seen on the computers at the rig floor and would give a good indication of a kick. The procedures used to circulate the kick out are very similar to the ones used in conventional drilling.