Accurate depth determination in well logging

In well logging, where a measuring instrument or "tool" is suspended from a cable in a well bore, accurate information on the instantaneous depth of file tool is difficult to obtain because the tool tends to bounce up and down as the cable is drawn up the borehole. A particularly difficult situation is "stick and slip," i.e., where the sonde stops momentarily, and then jumps forward as cable tension overcomes static friction. Past efforts at depth determination have generally relied only on measuring cable travel uphole at the winch. However, increasing applications require greater accuracy in tool depth determination. This paper presents a method for estimating tool depth which performs well even in the presence of "stick and slip." It uses a downhole accelerometer, and a Kalman filter to estimate depth in conjunction with a method to detect sonde sticking and to modify the filter accordingly. Its performance on real data is demonstrated.