Controlling Backflow Pressure During Retrieval of Bottom Hole Assembly

A method of retrieving a bottom hole assembly in a casing-while-drilling operation wherein the casing string and a casing string annulus each contain a column of fluid, comprising:

• (a) displacing the fluid in the casing string with a less dense fluid than the fluid in the annulus;
• (b) releasing the bottom hole assembly and allowing it to move 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;
• (c) in response to the upward movement of the bottom hole assembly, flowing displaced fluid out of the casing string and through a restrictive orifice of a choke, and varying the flow area of the orifice as the bottom hole assembly moves upward to control the rate at which the bottom hole assembly moves upward;
• and
• (d) as step (c) is occurring, filling the casing string annulus with fluid more dense than the less dense fluid in the casing string.