ABSTRACT
Conventional methods for monitoring the movement of formation fluid contacts in low salinity environments, using pulsed-neutron-capture (PNC) logs, employ the "time lapse" technique. This technique is based on the comparison between base logs recorded before production commenced and logs recorded later in the life of a producing well. The recording of a base log requires the well to be re-entered some weeks or months after completion and, in the case of fields which are developed using subsea completions, this is not practical.
This paper describes a technique which was developed to interpret PNC logs in low salinity environments without the need for base logs. The technique involves Computing synthetic base logs from the open hole log Interpretation of a well and the sigma response to each mineral and fluid in the formation. This method has proved successful in the monitoring of changes in oil-water contacts in two fields in the Timor Sea, Jabiru and Challis, both of which have formation water salinities of about 40,000 ppm, NaCl equivalent.
Verification of the accuracy of the technique is provided by means of monitoring the change in natural radioactivity in the formation due to water movement This has been observed by changes in relative gamma ray responses between open hole and cased hole logs beneath the new oil-water contacts.
