An LWD Open-Hole Sigma for Time-Lapse Reservoir Monitoring - Case Study from South East Asia

Abstract

Abstract Sigma log has long been crucial in cased-hole evaluation as it is one of a few measurements that can be made through complicated well completions. In fresh water environments, sigma log was used to discriminate gas from liquid; in some cases can differentiate water from oil, hence used subsequently for time-lapse reservoir monitoring to indicate changes in fluid type over production. A problem arises when no baseline sigma is available for time-lapse comparison. Traditionally a synthetic baseline sigma log may be artificially derived but suffers from uncertainty due to compounding of any errors in original volumetric interpretation as well as errors in the endpoint estimations. The measurement of formation sigma under open hole condition has been enabled by a downhole pulsed neutron generator (PNG), installed in a multi-function logging while drilling (LWD) tool. The relatively large LWD collar reduces significantly the influence of borehole salinity which results in accurate sigma measurement while drilling. Thus a direct sigma baseline can be obtained in open-hole without errors brought from volumetric interpretation and sigma endpoint estimation. Verification of mud filtrate invasion can be done through Multi Depth of Investigation (MDOI) sigma measurements. In order for an LWD baseline sigma measurement to be suitable for time-lapse analysis, the instrument response should be accurate, precise and repeatable by the subsequent cased-hole logging instrument. Sigma logs from both open-hole LWD and cased-hole wireline conveyed tools, over intervals of similar depth, were compared in offset wells within the same license block. Some 25 datasets from 5 producing fields in the region were examined and statistical analysis affirmed the response characteristics of both logs were emanating compatible. It confirms the data compatibility measured by both tools and supports the potential application of open-hole sigma log to serve as a baseline log for future time-lapse application.