Incremental U-tube process to retrieve of bottom hole assembly during casing while drilling operations

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

• (a) lightening the density of the fluid in the casing string above the bottom hole assembly to a lesser density than the fluid in the casing string annulus;
• (b) moving the bottom hole assembly upward in the casing string in response to an upward force created by the fluid in the casing string annulus being heavier than the column of fluid above the bottom hole assembly;
• (c) flowing fluid into the casing string annulus as the bottom hole assembly moves upward;
• (d) flowing from the casing string less dense fluid being displaced by the upward movement of the bottom hole assembly;
• (e) when the flow rate of the less dense fluid being displaced in step (d) drops below a selected level, suspending the bottom hole assembly in the casing string;
• then
• (f) while the bottom hole assembly is suspended, lightening the density of the fluid in the casing string below the bottom hole assembly without significantly changing the density of the fluid in the annulus, again creating an upward force on the bottom hole assembly that causes the bottom hole assembly to move upward in the casing string.

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

• (a) dropping a retrieving tool down the casing string and following it with a less dense fluid that is lighter in density than fluid in the annulus;
• (b) latching the retrieving tool to the bottom hole assembly and unlocking the bottom hole assembly from the casing string with the retrieving tool, thereby creating a retrievable unit, the retrievable unit having slips that extend into engagement with the casing string to allow upward movement but not downward movement of the retrievable unit;
• then
• (c) moving the retrievable unit upward in the casing string in response to an upward force created by the fluid in the casing string annulus being heavier than the less dense fluid above the bottom hole assembly;
• (d) flowing fluid into the casing string annulus as the retrievable unit moves upward;
• (e) flowing from the casing string the less dense fluid being displaced by the upward movement of the retrievable unit;
• and
• (f) when the less dense fluid being displaced in step (e) substantially ceases flowing, pumping additional less dense fluid into the casing string below the retrievable unit while the retrievable unit remains suspended by the slips of the retrievable unit, the less dense fluid again creating an upward force on the bottom hole assembly that causes the retrievable unit to again move upward in the casing string.