Reevaluation of Asphaltene Precipitation Risk Depending on Field Operational Condition Change/Variation: Case Study Comparing Risks in Past 2008 and Present 2016 for Future Prediction

Abstract

The objective of this paper is to overcome the asphaltene risk evaluation usually conducted snapshot basis. We evaluate the temporal change in the asphaltene risks as gas injection proceeds. In reservoirs under gas injection, in-situ fluid component gradually changes by multiple contacting with the injection gas. Those compositional changes affect asphaltene stability and bring difficulty into the risk predictions by asphaltene models. This study aims to reduce the risk uncertainty depending on operational condition changes. Periodical upgrading of asphaltene model is essential for understanding the time-depending changes of asphaltene risks. In the previous study (Yonebayashi et al. 2011), the asphaltene risk was evaluated for an offshore oil field in the Arabian Gulf in 2008 by use of cubic-plus-association equation of state (CPA-EoS) models on the basis of all available date at the time. After the previous study, additional experimental data was accumulated forthe future gas injection plan. Then, the update study was performed by incorporating those newly collected data. Subsequently, both findings in the past and the present were compared with each other. According to the previous study recommendation, additional asphaltene laboratory studies were conducted on the basis of newly collected samples. All Asphaltene On-set Pressure (AOP) detected from the new samples were higher than those of the previous study. Especially, a large difference was observed from the past/present results of the lower reservoir's AOPs even though samples collected from the identical well. Asphaltene precipitation risk was observed to increase largely because the new AOP was detected at the reservoir temperature while no AOPs detected in the previous study. The difference might be occurred by saturation pressure increase. Then, the numerical asphaltene models were revised, and accordingly, the asphaltene risk estimation were updated higher in the lower reservoir. For the upper reservoir, the past/present AOPs were slightly changed to become higher. The reference sample fluids were collected from two different wells showing minor difference of asphaltene contents. Those variations might be caused by geological heterogeneity that could affect on fluid maturity. Then, the risk rating was updated to be slight higher, too. In this paper, through the comparison between the previous and current studies, it was pointed out the importance of regular monitoring asphaltene risks. This study provides the valuable findings of time-lapse evaluation of asphaltene precipitation risks for reservoir under gas injection. The evaluations currently conducted in the industry are snapshots of instantaneous risks. Through entire field life, the risks have varied depending on operating conditions. This study argued the risk-change in the unique field by the identical workflow but using each representative data collected at different times. Finally, this study demonstrated the importance of time-depending fluid dynamics.