Theory, Design, Realization, and Field Results of an Inductive Casing Collar Locator

During well-logging operations, the depth of a logging tool is usually obtained by measuring the length of the wireline cable that suspends the tool in the well. In some cases, a secondary depth reference is obtained with a casing collar locator (CCL) that detects the collars along the steel casing lining the well. The thicker steel at these collars is detected by measuring the perturbations it causes to the field of a permanent magnet in the CCL logging tool. Due to its simplicity and efficiency, the CCL tool has not seen many changes throughout the years. However, for small, untethered sensor platforms, neither of the above methods is practical. This paper describes the theory, design, construction, and testing of an inductive sensor that detects the gap on the inside of the casing where two pipe joints mate together within a collar. Two identical cylindrical coils along a common axis are connected in a bridge configuration and driven by an ac signal. When one coil is near the gap, its inductance is slightly larger than that of the other, misbalancing the bridge. The sensor was implemented in a wireline tool and tested in a well. The field data show a clear double pulse of ac signal across the bridge each time the tool passes a collar. This sensor is smaller and lighter than CCLs that use permanent magnets, making it suitable for small, untethered sensor platforms.