Results and Lessons Learned from an Extended Single-well Tracer Campaign Performed in the Handil Field, Indonesia.

Handil is a mature oil and gas field in the Mahakam Delta, Indonesia. It was discovered in 1974 and developed since 1975. It was firstly produced by natural depletion and then by waterflooding. From 1995 gas injection was implemented as tertiary recovery mechanism. In 2007 a first Single-well tracer test (SWTT) campaign was carried out in the field showing a low Remaining Oil Saturation (ROS) that was confirmed by coring technique in the selected reservoir. Uncertainty existed on the origin of such low ROS (12-14%), thus in 2011 an ambitious SWTT campaign was launched to assess the ROS distribution in different reservoirs under waterflood and simultaneous water and gas flood strategies. The objective was to better understand EOR mechanisms prevailing in the field and help assess ROS under different flooding strategies. The challenging campaign consisted of a program of consecutive SWTT trials performed in a mature and offshore field environment. This paper focuses on the operations and results of the SWTT campaign obtained from three tests out of five that were performed in different reservoirs. The interpretation of these tests was challenging and numerical simulation was compulsory for a reliable ROS estimation. In reservoir A, we suspected that the tested zone have been invaded by the gas cap. Therefore, in order to get an uncertainty range of ROS in presence of gas, the partitioning coefficient of primary tracer (Ethyl Acetate) between gas and water was measured at reservoir conditions. Results indicate that even with high trapped gas saturation, the ROS is in the order of 22%. For the second test in Reservoir B, the mass balance was excellent but profiles showed non ideal behavior due either to drift or a wellbore effect. The two hypotheses were investigated and numerical simulations helped identify that wellbore effect was the main non-ideality. This was included and a reliable ROS value of ~30% was determined. The third SWTT was performed in Reservoir C and showed an excellent tracer recovery with low scattering data. However production phase was characterized by an unstable flow regime which required the use of instantaneous production rate during numerical simulation. Matching of tracer profiles indicates an average ROS in this reservoir of 20%. These ROS values that range between 20-30% allow us today to move forward in the identification of potential EOR candidates and they will help us advance in the location of future pilot zones for different EOR processes.