Abstract: Methods for reducing or eliminating undesirable fluid-production in a producing well by releasing a plugging material below an obstruction placed in the producing zone. The obstruction is placed near the base of a desirable fluid-producing interval. The plugging material flows outward to form a barrier to the flow of undesirable fluids into the desirable fluid-producing interval. For most applications, a buoyant plugging material is used so that undesirable fluid crossflow carries the plugging material to the location where it is needed to form a barrier to the undesirable fluid production. The present invention can be used in gravel-packed wells, open hole wells, slotted-liner wells, monobore completion wells, or cased-hole wells. The present invention can also be used with a pair of obstructions with a plugging material released between them to shut off multiple intermediate intervals producing undesirable fluids.