An Answer to Ever-Challenging Question of Selecting the Right Test Method (Dispersion or Deposition) for Asphaltene Stability Determination

Abstract

Changing thermodynamic and compositional conditions of producing fields can cause decreased asphaltene stability and initiate aggregation, subsequent precipitation, and eventual deposition within flowlines. Usage of asphaltene inhibitors that prevent aggregation and tackle the problem right at the inception is widely preferred. However, such chemistries were observed to be counter-productive and led to higher asphaltene deposition in many cases. Thus, raising the question of what approach works best for assessing asphaltene stability: Dispersion or Deposition? The focus of this study is to explore the relationship between the underlying working mechanism of dispersion and deposition-based test methods. Multiple crude oil samples produced from different regions of the world were evaluated using asphaltene inhibitor chemistries with optical transmittance, thermoelectric, and flow loop methods. Optical transmittance method evaluates sedimentation rate and cluster size distribution of asphaltene cluster within the test fluid medium. Thermoelectric method describes the dispersion state of asphaltenes within native crude oil. Flow loop setup assesses total mass deposited when the oil (blank or dosed) and precipitant mixture is flown through capillary tubes. The results from these tests indicated that a fine balance between the dispersion and deposition mechanisms must be maintained as these may not respond linearly or in direct relationship at all conditions. It was seen that dispersing the asphaltene clusters too small may lead to high diffusional rate within the low flow shear regime and build up more deposit in depositional dominant test methods. Variation in treatment concentration (especially overtreatment) of an effective asphaltene inhibitor can result in lowering of cluster size to a range which in effect can cause more deposition. The overall assessment suggests that not having a holistic overlook at these test methods and following the standard process of giving specific focus on a singular approach, can mislead the asphaltene stability and inhibitor performance evaluation. The key role of asphaltene cluster size as a bridge relating the dispersion and deposition-based test method is revealed in this paper. It is seen that there exists an effective range of cluster size within which the results from different test methods correlate well. Therefore, it is imperative that the asphaltene inhibitor development philosophy must include test screening methods focusing on each instability stage (precipitation, aggregation, and deposition) individually and combine the learnings to come up with the best recommendation.