Editing Wireline (cabling) (section)

==Wireline logs==

First developed by Conrad and Marcel Schlumberger in 1927, wireline logs measure formation properties in a well through electrical lines of wire. Different from [[measurement while drilling]] (MWD) and mud logs, wireline logs are constant downhole measurements sent through the electrical wireline used to help geologists, drillers and engineers make real-time decisions about the reservoir and drilling operations. Wireline instruments can measure a host of petrophysical properties that form the basis of geological and petrophysical analysis of the subsurface. Measurements include self-potential, natural gamma ray, acoustic travel time, formation density, neutron porosity, resistivity and conductivity, nuclear magnetic resonance, borehole imaging, well bore geometry, formation dip and orientation, fluid characteristics such as density and viscosity and formation sampling.

The logging tool, also called a [[wikt:sonde|sonde]], is located at the end of the wireline. The measurements are made by initially lowering sonde using the wireline to the prescribed depth and then recorded while raising it out of the well. The sonde responses are recorded continuously on the way up creating a so-called "log" of the instrument responses. The tension on the line assures that the depth measurement can be corrected for elastic stretch of the wireline. This elastic stretch correction will change as a function of cable length, tension at surface (called surface tension, Surf.Ten) and at the tool end of the wireline (called cablehead tension, CHT) and the elastic stretch coefficient of the cable. None of these are constants, so the correction has to be adjusted continuously between when starting the logging operation to recovery to the reference point (usually surface, or zero depth point, ZDP).