Dynamic Asphaltene Behavior for Gas Injection Risk Analysis
Authors: Yonebayashi Hideharu , Al Mutairi Ali Mohamed , Al-Habshi Ali Mohamed , Urasaki Daisuke 
Published: March 2015

Abstract
Asphaltene study is now becoming a regular menu as a part of gas injection studies 1–11. The asphaltene onset pressure (AOP) is one of the most important factors to understand asphaltene precipitating behavior. The SDS (solid detection system) based on light scattering technique has been quite popular and widely used in all over the world 1,7–9,12–15. The simple experiments to measure AOP are usually conducted using mixture of reservoir fluid and injection gas, and various gas mixing volume are assumed to be investigated. These various experimental specification of gas mixing volume are useful to understand asphaltene risks during gas injection projects. However, what this investigation can show is just 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 NIR (neear infrared) 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 combination of mixtures (sampled reservoir fluid mixed with 0, 25, 37.5, 43.5 and 50 mol% injection gas) were examined to measure AOP. Furthermore, the numerical models were generated and calibrated with the experimental findings. In order 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 bottomhole of some wells in the GIP area. Namely, we were obliged to judge that our current experimental results of static asphaltene behavior overlooked at the actual asphaltene risks. What is insufficient for a realistic modeling ? Our hypothesis is the dynamic asphaltene behavior.
During gas injection process, the injected gas composition is changed due to vaporizing gas drive (VGD) mechanism, in which gas was enriched with the intermediate molecular weight hydrocarbons from reservoir oil. Our latest experimental investigation of static asphaltene behavior did not include this process. Therefore, the sensitivity analyses of the VGD effects were carried out with the calibrated model to realistically evaluate the actual APE. Various enriched gas composition were assumed, and the affects of these enriched gas on APE were investigated. Consequently, it was found that the enrichment of intermediate components expanded APE, and the operating condition of asphaltene problematic wells could be explained to be inside APE. Therefore, we concluded that the dynamic asphaltene behavior must be understood for a realistic risk evaluation in the gas injection project.

Introduction

Background and Histories
The target field was discovered in 1963 and started production in 1967. It is currently operated by ADMA-OPCO. It produces from two carbonate reservoirs (A and B) and its oil is transported and processed at the plant near an island. To maintain reservoir pressure, the dump flood water injection started in 1972, followed by powered water injection in 1978 and the crestal gas injection in 2003. In addition to this project, gas injection pilot project at the flank area has been carried out at western flank area of the field.