Methods for sub-surface fluid shut-off

A method for reducing the production of undesirable fluid from a well having a production zone, the production zone including an undesirable fluid-producing interval and a desirable fluid-producing interval, said method comprising:

• placing an obstruction in the production zone near a base of the desirable fluid-producing interval;
• and
• releasing a buoyant plugging material below said obstruction, whereby said buoyant plugging material forms a barrier to the flow of undesirable fluid from the undesirable fluid-producing interval into said desirable fluid-producing interval while producing from said desirable fluid-producing interval.

A method for reducing the production of undesirable fluid from a well having a gravel-containing zone traversing a production zone having a perforated tubing surrounded by a first screen separated from a second screen by a blank area, and a third screen separated from the second screen by a blank area, the production zone having gravel around the first screen, the second screen, and the third screen, the production zone including an undesirable fluid-producing interval and at least one desirable fluid-producing interval, said method comprising:

• placing a first obstruction in the perforated tubing at a location corresponding to the blank area between the first screen and the second screen;
• and
• placing a second obstruction in the perforated tubing at a location corresponding to the blank area between the second screen and the third screen, said second obstruction being in fluid communication with said first obstruction.