Controlling Asphaltene Deposition in Ageing Wells

Abstract

Abstract ADNOC Onshore produces asphaltene-unstable crudes subject to severe wellbore deposition, which is exacerbated by miscible WAG injection. In one of the oilfields, commissioned in 1973 and producing approximately 400,000 BPD from 4 key reservoirs a cluster of approximately 20 wells experienced severe asphaltene deposition. Over time field conditions changed, reservoir pressure declined and water cut increased from 0% to 30%. With these changes the number of shut-ins due to asphaltene deposition started to change. Regular well intervention by gauge cutters or PLT tools confirmed that little to no asphaltene build up occurred in wells with high water cut. In this paper it was evaluated whether changes to asphaltene deposition severity can be linked to water production, or that other production factors might contribute. Field data were reviewed for the period 2003 to 2022, for a total of 12 wells with a history of moderate to severe asphaltene deposition. The field data included PVT data and stock tank oil properties, well testing data, production allocation data, wireline inspection data, field lab analysis of wellbore deposits and chemical soaks. The review indicated that changes in asphaltene deposition were not influenced by a gradual decline in reservoir pressure or gas break through causing changes to the AOP location. A scoring matrix was developed to determine the ‘deposition severity’ for each well for every year and plotted against the average water cut of that year. From this scoring matrix it was concluded that there is a weak correlation between the severity of asphaltene deposition and water cut. Even at water cuts around 40% asphaltene deposition has not stopped completely, but the number of required interventions has dropped. Or long-term goal is to develop correlations that can be used to predict future behavior of asphaltenes, for optimization of field development plans in reservoirs subject to WAG and surfactant/polymer flooding. A correlation can help to understand whether asphaltene deposition will be permanent or reduce over time for different development scenarios and whether mitigation by inhibitor injection or periodic cleaning with solvents is most effective and economical.