A world first "Controlled Annular Mud Level" type Dual Gradient Drilling (DGD) system was successfully applied on an ultra-deepwater well drilled in May - July 2012.

Water depth was 2260m and the formation was generally carbonates with potential for severe or total losses. DGD was applied to prevent losses from occurring. Dynamic circulation pressure effects were eliminated. The DGD topside system was rigged up offline and when running the riser, the DGD pump was launched and attached to the riser for the last 400m. During drilling of the 17 ½" hole section, a Dual Gradient mud weight was selected and the riser level was maintained between 150 and 200m below flow line most of the time. On connections the riser level was kept unchanged. The level was decided according to PWD Readings. On several occasions it was demonstrated that lowering the riser level increased ROP whilst raising it again decreased ROP in hard rock.

In the 12 ¼" hole section, drilled to well TD, the same approach was taken, but here the riser level was raised 50 m (approximately 75 psi) on connections to account for some of the reduced bottom hole pressure when shutting down the rig mud pumps. In this section some gas bearing formations were penetrated with a reduced riser level. No gas at all was seen in the riser top.

During drilling of the reservoir and surrounding formations, no losses were seen and the hole was in good condition with no indication of fill or drag.

The paper will present the technology applied and analyze the results achieved on this well.

You can access this article if you purchase or spend a download.