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1. (WO2013116381) DUAL GRADIENT MANAGED PRESSURE DRILLING

Pub. No.:    WO/2013/116381    International Application No.:    PCT/US2013/023916
Publication Date: Fri Aug 09 01:59:59 CEST 2013 International Filing Date: Thu Jan 31 00:59:59 CET 2013
IPC: E21B 21/08
Applicants: WEATHERFORD/LAMB, INC.
Inventors: FEASEY, Guy F.
PAVEL, David
MITCHELL, Mark A.
Title: DUAL GRADIENT MANAGED PRESSURE DRILLING
Abstract:
A method of drilling a subsea wellbore includes drilling the wellbore by injecting drilling fluid through a tubular string extending into the wellbore from an offshore drilling unit (ODU) and rotating a drill bit disposed on a bottom of the tubular string. The method further includes, while drilling the wellbore: mixing lifting fluid with drilling returns at a flow rate proportionate to a flow rate of the drilling fluid, thereby forming a return mixture. The lifting fluid has a density substantially less than a density of the drilling fluid. The return mixture has a density substantially less than the drilling fluid density. The method further includes, while drilling the wellbore: measuring a flow rate of the returns or the return mixture; and comparing the measured flow rate to the drilling fluid flow rate to ensure control of a formation being drilled.