controlled time survey

1. n. [Production Logging]
 A method of determining injection-flow profiles by monitoring the reduction in tracer material as it moves down the well. A slug of radioactive tracer is added to the injection fluid. As the slug moves down the well, several gamma ray logs are recorded at well-defined time intervals. The position of the slug is seen as a large gamma ray peak whose size is proportional to the flow rate. A reduction in the size of the peak indicates a loss of fluid into the formation. Fluid velocity can be calculated from the time interval and the distance the peak has moved using timed-slug analysis. Tracer-loss measurements produce a type of radioactive-tracer log, used mainly to give a general idea of fluid flow in low flow-rate wells. In very low flow-rate wells, an alternative technique has been used in which the gamma ray detector is held stationary at some depth until the slug has passed. The detector is then moved down to another depth to observe the slug again. With these data, it is possible to make quantitative estimates of fluid flow.