An untethered sensor for well logging

A golf ball-sized untethered sensor platform for logging oil wells is described. This “sensor ball” is deployed through the wellhead, falls to a programmed depth, releases a small weight to change its buoyancy, returns to the surface and is recovered through the wellhead. Data are logged from onboard sensors as the sensor ball falls and rises in the well. These data are downloaded after the sensor ball is recovered. Most of the cost of wireline logging comes from the crews and heavy equipment required to deploy the wireline cable into the well. Small, untethered sensors can be deployed by one person with minimal equipment and could become a less costly alternative for acquiring downhole data in vertical wells.