US Patent 7779920 Controlling backflow pressure during retrieval of bottom hole assembly

A bottom hole assembly in a casing-while-drilling operation is retrieved by displacing the fluid in the casing string with a less dense fluid than the fluid in the annulus. The bottom hole assembly moves upward in the casing string in response to an upward force due to the different densities of fluid in the casing string and in the annulus. Displaced fluid flows out of the casing string and through a restrictive orifice of a choke. The flow area of the orifice is varied as the bottom hole assembly moves upward to control the rate at which the bottom hole assembly moves upward.