Through-Casing Formation Conductivity Measurement Based on Transient Electromagnetic Logging Data

Transient electromagnetic (TEM) logging is a promising noncontact method for through-casing formation conductivity measurements. We studied the through-casing TEM logging method based on the processing of TEM logging data measured in a production well. Similar to Doll's work in borehole induction logging, we presented the expressions of the ‘useful signal’ and the ‘useless signal’ in casedhole logging based on which, the methods of removing the ‘useless signal’ and obtaining the formation conductivity curve are introduced. We analyzed the influence of the casing on the TEM signals, described the characteristics of TEM response signals, and obtained the ‘useful signal’ carrying formation conductivity data. Casedhole formation conductivity curves, which are subsequently compared with the known openhole conductivity log, are obtained by dealing with the ‘useful signal’. We identified the characteristics of casedhole formation conductivity curves, and some problems that need to be considered in their practical application. Due to the influence of the casing, the radial detection depth of the TEM logging tool in a cased hole is small, so the detection result is mainly the equivalent conductivity of the cement ring and formation near the outer casing wall. Although the casedhole conductivity curves are in good agreement with openhole logging results in regular formations, due to the influence of the casing and the changes in the physical environment in the well, complete consistency is unrealistic for these two kinds of curves in all well intervals. Therefore, a thorough analysis is required before practical application. Moreover, the effects of well temperature and casing deformation must be corrected for accordingly.