METHOD AND APPARATUS FOR CONTROLLING BOTTOM-HOLE PRESSURE DURING DUAL-GRADIENT DRILLING

摘要

A method for controlling wellbore pressure in a wellbore during drilling. The method includes operating a drilling system to have a first fluid pressure gradient inside a drillstring (2) extending from the earth's surface to a drill bit (4) at the bottom of the wellbore. The drilling system has a second fluid pressure gradient lower than the first fluid pressure gradient in an annular space between the drillstring (2) and the wellbore from a selected depth in the wellbore to the earth's surface. Introduction of drilling fluid to the inside of the drillstring is stopped, and fluid flow in the annular space from a point below the selected depth to a point above the selected depth is selectively controlled (44) to cause a substantially constant fluid pressure at a predertermined depth in the wellbore.