Identification of unstable zones in wellbores using Cation Exchange Capacity in Gonbadli Gas field, eastern Kope Dagh, North- East Iran

To continue drilling operation, identification of unstable zones in some parts of oil and gas wells is important. Wellbore wall collapse or swelling, in addition to stop drilling operations can lead to variety of mechanical problems. The most important factors in this process are presence of clay and clay facieses in sedimentary sequences of oil and gas fields. There are several methods to identify clay type and volume such as determination of Cation Exchange Capacity (CEC). In some minerals like smectite and vermiculite, while expanding property is noticeable, cation exchange capacity is high. This study has been done in Shurijeh Formation (Early Cretaceous) in Gonbadli gas field. First, for 20 samples, CEC has been measured applying Bower method and then by neural network estimator, based on the petrophysical logs and laboratory results, an appropriate model was fitted to estimate this parameter in well interval. The cation exchange capacity values obtained and Caliper log were used to examine the relationship between wellbore collapse and CEC. This study implied that more than 75 percent of the points in which CEC are higher than 70 in well No.1 and more than 74 percent in well No.2, have been collapsed. As a result this parameter can be used to detect and prevent collapses in clayey zones of well.