Dynamic Asphaltene Behavior for Gas-Injection Risk Analysis
Authors: Yonebayashi Hideharu , Al Mutairi Ali , Al Habshi Ali , Urasaki Daisuke 
Published: August 2011

Summary
Asphaltene study is now becoming a regular menu as a part of gas-injection studies (Kokal et al. 2003, 2004; Yin et al. 2000; Srivastava et al. 1999; Yin and Yen 2000; Parra-Ramirez et al. 2001; Sarma 2003; Jamaluddin et al. 2000; Negahban et al. 2005; Okwen 2006; Moghadasi et al. 2006). The asphaltene onset pressure (AOP) is one of the most important factors in understanding asphaltene precipitating behavior. The solid detection system (SDS) based on light-scattering technique has been quite popular and widely used in all over the world (Kokal et al. 2003, 2004; Jamaluddin et al. 2000; Negahban et al. 2005; Gholoum et al. 2003; Garcia et al. 2001; Oskui et al. 2006; Gonzalez et al. 2007) to measure AOP. The simple experiments to measure AOP are usually conducted using a mixture of reservoir fluid and injection gas, and various gas-mixing volumes are assumed to be investigated. These various experimental specifications of gas-mixing volume are useful in understanding asphaltene risks during gas-injection projects. However, this type of investigation can show only a static asphaltene behavior, and sometimes might overlook true asphaltene risks.
In the gas-injection pilot (GIP) project in an offshore carbonate oil field in the Arabian Gulf, the static asphaltene behavior was studied by the SDS using near-infrared (NIR) light-scattering technique. For this study, a single-phase bottomhole sample was collected from the same producing zone, but the sampling location was 90 ft shallower than the GIP area. Various combinations of mixtures were examined to measure AOP (i.e., reservoir fluid mixed with 0, 25, 37.5, 43.5, and 50 mol% injection gas). Furthermore, the numerical models were generated and calibrated with the experimental findings. To evaluate the asphaltene risks at the GIP area, the models were adjusted to the target oil composition by considering existing oil compositional gradient in the field. However, the modeling analyses showed that the operating conditions of producing wells are outside the estimated asphaltene-precipitation envelope (APE). This result was inconsistent with the field fact in which actual asphaltene deposits were observed and collected from the bottomhole of some wells in the GIP area. Thus we were obliged to recognize that our current experimental results of static asphaltene behavior overlooked the actual asphaltene risks. What is insufficient for realistic modeling? Our hypothesis is the dynamic asphaltene behavior.
During a gas-injection process, the injected-gas composition is changed because of a vaporizing-gas-drive (VGD) mechanism, in which gas was enriched with the intermediate-molecular-weight hydrocarbons from reservoir oil. Our latest experiments investigated a static asphaltene behavior only; that is, it did not include this process. Therefore, the sensitivity analyses were motivated to realistically evaluate the actual APE, counting the VGD effects with the calibrated model. Various enriched-gas compositions were investigated in terms of how these enriched gases would affect APE. Consequently, it was found that the enrichment of intermediate components expanded the APE, and the operating conditions of asphaltene-problematic wells could be placed inside the APE. Therefore, we concluded that the dynamic asphaltene behavior must be understood for a realistic risk evaluation in the gas-injection project.