The asphaltene risk map recommendation: Asphaltene Risk Estimation on an Offshore Field Utilizing Experimental Results

Abstract

ABSTRACT It is widely known that pressure drop and compositional change of reservoir fluid causes asphaltene precipitation. Regarding compositional change, gas injection is believed to be a typical trigger of asphaltene deposition. The injected gases are mixed with reservoir fluid and gradually change the composition in the reservoir as gas front is heading. For such fluids, compositional changes are believed to take place not only near gas injectors but also in a wider area. Thus, asphaltene precipitation risks should be considered on field scale. This paper demonstrates how to generate "Asphaltene Risk Map on field scale" based on the correlation between Asphaltene Onset Pressure (AOP) and fluid composition. A Middle Eastern offshore oil field which encounters asphaltene problems was selected as an example because the above two key parameters are mainly working in the field for destabilizing asphaltene. For creating risk map, compositions of reservoir fluid near the asphaltene problem wells were calculated from reservoir simulation. The calculated compositions were applied for the asphaltene model with calibrating measured AOP. Various combinations were evaluated on the plots of AOP versus composition to find well-converged correlation. Eventually, the C1-C19 component was selected to achieve the most reasonable correlation to AOP for this case. According to pressure variation from the correlated AOP, risk ratings were proportionally distributed on the plot. Based on these risk ratings, "Asphaltene Risk Map" was translated from the relationship between C1-C19 component and pressure. The created risk map shows a reasonable tendency to replicate the timing of actual asphaltene precipitation. In addition, two types of asphaltene precipitation mechanism were confirmed on the Risk Map. Namely, higher risk was estimated in two producers; one is located in the gas breakthrough area due to gas cap expansion, and the other is isolated from the gas breakthrough area but is in the vicinity of free gas observed area where compositional change occurs. This Asphaltene Risk Map is considered useful to optimize gas injection plan, in particular. Time-lapse prediction of asphaltene risk can suggest timing and/or priority to prepare mitigation plan.