The application of the Carbon Dioxide Enhanced Oil Recovery (CO₂-EOR) to an offshore oilfield in Vietnam has been investigated through an international joint study between Japan and Vietnam since 2007. In order to reduce and mitigate uncertainties and risks for future scale application, a CO₂-EOR pilot test was conducted in the Rang Dong field, offshore Vietnam in 2011.

The pilot test was conducted as a single-well “Huff-n-Puff” operation which was designed to minimize the test duration, required CO₂ volume to observe the meaningful CO₂-EOR effects and to reduce the risk of corrosion to the existing facility. Over 100 tons of CO₂ were successfully injected into the Lower Miocene sandstone reservoir and the well was flowed back after two days of soaking time. Various parameters were monitored including production rate, bottom-hole pressure and production fluid properties. To investigate the CO₂-EOR effects and for the purpose of monitoring the reservoir fluid saturation change, cased hole pulsed neutron saturation logging was carried out during every stage of the test. These logging operations and their timing were carefully designed to avoid any possibility of fluid contamination and to minimize operation time.

Through the analysis of the acquired logs, vertical contrast of preferential CO₂ injection, zonal oil saturation change, oil saturation change with time within single run, etc., were clearly identified. These observations played important roles in evaluating the CO₂-EOR effect in detail by comparing them with the reservoir simulation prediction.

In this paper the detailed design for the reservoir fluid saturation monitoring, observed results and three-phase fluid saturation changes with time are discussed.