Contribution from Laboratory to Field: Case Studies of Asphaltene Precipitation Risk Evaluation

Three case studies pertairling to the precipitation risk of asphaltene, which comprise comprehensive evaluation from laboratory measurement to practical feedback into the field, are described. Two cases included aspects of gas injection, and the third corresponds to formation damage at a naturally depleted field. Asphaltene onset pressure (AOP) is a fundamental characteristic that represents in situ asphaltene behavior from a single-phase fluid sample; therefore, all evaluations were performed on the basis of laboratory-measured AOP data. The evaluations reported reveal how practical interpretation of laboratory-measured AOPs links appropriately to actual on-site phenomena. In the first two case studies, numerical asphaltene fluid models, calibrated using measured AOPs, were generated to evaluate asphaltene precipitation envelopes (APEs) by applying the cubic plus association equation of state. In the first case, a sensitivity analysis based on a numerical model of the original reservoir fluid was performed for several injection gases to investigate the impact of gas injection on APE behavior from the subsurface and surface points of view. In the second study, the variation of asphaltene deposits, actually observed at the gas injection pilot area, was explained by considering the vaporizing gas drive (VGD) process in terms of APE behavior. A sensitivity study was conducted to estimate how enriching the injection gas by this effect could expand the APE. An expanded APE could cause the variation of asphaltene deposits observed in the field: deposits were observed at certain times when enriched injection gas was accumulated but not when lean injection gas accumulated. Control of pressure depletion is considered. an effective countermeasure to mitigate asphaltene precipitation in a naturally depleted field. For a field with a potential asphaltene deposition problem in the reservoir, it is recommended to maintain the reservoir pressure above the AOP. Such mitigation needs reliable AOP data. Once destabilized, solid particles of asphaltene grow continuously from the nanoscale (precipitation) to the microscale by aggregation. Our AOP measurements adopted the latest laboratory techniques. Three data sets were acquired using filtration, high-pressure microscopy analysis, and laser light scattering techniques. Despite using the same single-fluid sample, the AOP results were not identical because each measuring technique had different detection limits for the minimum asphaltene particle size. This paper demonstrates how we assessed the AOP data to determine the cause of asphaltene-induced formation damage by comparison to actual flowing bottomhole pressure behavior. This practical AOP measurement could suggest a more optimum pressure control target that would allow for maximum oil production while mitigating production potential loss as a result of shutdown for cleanup or removal jobs.