Abstract

Measuring rock properties using logging tools is extremely difficult when the formation is on the other side of the casing. Old wells are already cased pre-empting the use of in vogue resistivity measurements in their appraisal for bypassed oil or when monitoring saturation and depletion to optimally manage oil and gas fields. To help achieve the objective presently two state of art tools are available namely carbon-oxygen logging, which has been in use for several years and recently introduced cased hole resistivity logging. The former uses nuclear activation of the formation behind casing and its response but has shallower depth of investigation than the latter one, which is the resistivity measurement behind casing with far deeper depth of investigation. Cased hole resistivity measurements were envisaged long ago but could become a reality recently thanks to improvements in technology. Typically, the cost of acquiring the data from cased holes is far less than that of drilling a new well solely to gather data. The risk of cased hole logging operations is also substantially less than that of drilling operations. This paper presents some case histories to manifest the succour that the technology has rendered in the appraisal of old wells. Case histories are from Western region of ONGC covering the carbon-oxygen logging and from Mumbai offshore area covering cased hole resistivity logging.