A method for determining the extent of recovery of materials injected or otherwise introduced into oil wells or subsurface formations is practiced using a portable device. The portable device can also be used to determiner the occurrence of a predetermined condition in an oil well such as water break through in a production zone, or the opening or closing of a sliding sleeve. When, for example, water breakthrough is detected, the zone producing too much water can be plugged, using, for example, a flow-through bridge plug, if there are other producing zones further downhole.