taking pressure off deep water
Drilling in deep water poses far greater challenges than drilling on land. That’s because deep water is a high-pressure environment created by the combination of the weight of the seawater plus the weight of the formations at the bottom of the ocean. The extreme variations in pressure between the two create tremendous challenges. But what if we had a technology that enabled us to execute our deepwater wells more like wells on land? Enter Subsea Mudlift Drilling (SMD) – formerly known as dual-gradient drilling.
Until now, deepwater drilling was done within a steel riser extending from a drillship to the blowout preventer (BOP) situated near the seabed. Mud is pumped down a rotating drill pipe to lubricate and cool the drill bit and carries rock cuttings back up within the riser to surface tanks. But the mud weight makes pressure inside the riser nearly twice that of the sea water outside.
With SMD, we can fill the riser with a seawater-like fluid, so riser and ocean pressure match. The technology mimics the natural pressure of the well bore and essentially takes water out of the way by effectively removing the impact of the water depth on the well design. It’s like operating a drillship on the seafloor.
In addition, SMD can be used to control the mudline pressures of conventional, or single-gradient, wells. Seawater is pumped in a separate pipe from the drillship into a subsea mudlift pump on top of the BOP at the seafloor. The seawater forces mud back up its own pipe to the drillship, relieving bottom-hole pressure. This is a form of single-gradient managed pressure drilling.
In anticipation of the successful development and deployment of the new technology, we had the world’s first SMD drillship, Pacific Santa Ana, designed to our specifications to accommodate the system. After completing testing in the U.S. Gulf of Mexico, the vessel is expected to drill our first SMD well in late 2015.
Updated: July 2015