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This paper describes an integrated discipline approach to investigate and mitigate asphaltene challenges in an onshore Abu Dhabi oil field. This large carbonate field produces from three reservoirs with substantial lateral variation of fluid properties. Several wells in the northeastern part of the field producing from one of the three oil reservoirs suffer from asphaltene deposition in the wellbore. A multidisciplinary team of experts from Shell and ADCO worked closely together to evaluate the natural controls on asphaltene stability in this oilfield and to predict whether deposition in other areas of the field may be expected in the future. In addition, a robust inhibition and mitigation strategy has been developed. A thorough review of production and well intervention data, combined with detailed analysis of field deposits confirmed that wellbore deposition was due to asphaltenes in approximately one third of all the affected wells. In order to understand the variability in fluid properties, a wellhead sampling and fluid analysis campaign was carried out (39 wells distributed across the field). Analyses of basic fluid properties such as API, S%, Ni, V, SARA supported by more sophisticated geochemical characterization data of the reservoir fluids clearly demonstrated that the lateral variability in fluid properties from NE to SW is attributed to two processes: (1) geological mixing of fluids from different sources into the reservoir, and (2) maturity variation of fluids form the same source in the reservoir. Furthermore, stock tank oil asphaltene stability tests clearly indicated that fluids from the NE have a high tendency to be unstable with respect to asphaltene precipitation, and fluids from the SW of the field are stable. These results are in agreement with field observations. The confirmation of the field data with simple laboratory tests allowed for the development of empirical correlations between certain fluids properties and the risk of asphaltene precipitation and deposition. Based on this it was concluded that deposition is not expected to expand to the SW part of the field during the production lifespan. Moreover, a cluster of wells has been identified in the NE were deposition has not occurred to date but with a high propensity for deposition if production conditions will change. For these wells and the ones with confirmed asphaltene deposition problems additional live-oil testing, thermodynamic modeling as well as a surveillance plan have been suggested, which will further improve the correlations and help to identify challenges ahead of actual deposition. |
