Patent ID: 7604057

Claim:
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 column of fluid in the casing string above the bottom hole assembly to a lesser density than the column of 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 denser column of fluid in the casing string annulus than the column of fluid in the casing string above the bottom hole assembly; (c) preventing downward movement of the bottom hole assembly when the upward force becomes insufficient to continue satisfactory upward movement of the bottom hole assembly; then (d) lightening the density of the column of fluid in the casing string below the bottom hole assembly, again creating an upward force on the bottom hole assembly that causes the bottom hole assembly to move upward in the casing string.