DRILLING SYSTEM AND METHOD FOR CONTROLLING EQUIVALENT CIRCULATING DENSITY DURING DRILLING OF WELLBORES

摘要

A drilling system for drilling subsea wellbores includes a tubing-conveyed drill bit (130) that passes through a subsea wellhead. Surface supplied drillng fluid flows through the tubing (121), discharges at the drill bit, returns to the wellhead through a wellbore annulus (122), and flows to the surface via a riser extending from the wellhead. A flow restriction device (164) positioned in the riser restricts the flow of the returning fluid while an active fluid device controllably discharges fluid from a location below to just above the flow restriction device in the riser, thereby controlling bottomhole pressure and equivalent circulating density ("ECD"). Alternatively, the fluid is discharged into a separate return line (206) thereby providing dual gradient drilling while controlling bottomhole pressure and ECD. A controller (180) controls the energy and thus the speed of the pump in response to downhole measurement(s) to maintain the ECD at a predetermined value or within a predetermined range.