Well Measurements: Wire-line Logs A well log is a record of one or more physical measurements as a function of depth in a borehole Gamma Ray Resistivity Density Depth SLIDE 11 There are many ‘tools’ that can be lowered into a well bore to take measurements Usually a section is drilled, a string of tools is lowered to the (current) bottom, and measurements are recorded as the tools are slowly pulled up the well bore Some tools work only before the well bore is lined with steel casing – called open-hole logs Other tools can work in cased holes In the old days, the measurements were recorded on a strip of paper that was registered by the depth of the toll as the measurements were made – a log (right image) Now everything is captured digitally, but the data are still displayed as ‘logs’ Photo on left some the wire-line on a drum The tools are attacked to the end of the wire-line and lowered into the well bore The wire-line is pulled back and measurements are recorded. The rotation of the drum is calibrated to translate into the depth of each tool as measurements come in The white/blue building houses the recording equipment and the operator Courtesy of ExxonMobil L 3 - Types of Data