Polymeric Dispersants Delay Sedimentation in Colloidal Asphaltene Suspensions

Abstract

Asphaltenes, among the heaviest components of crude oil, can become unstable under a variety of conditions and precipitate and sediment out of solution. In this report, we present sedimentation measurements for a system of colloidal scale asphaltene particles suspended in heptane. Adding dispersants to the suspension can improve the stability of the system and can mediate the transition from a power-law collapse in the sedimentation front to a rising front. Additional dispersant beyond a crossover concentration can cause a significant delay in the dynamics. Dynamic light scattering measurements suggest that the stabilization provided by the dispersants may occur through a reduction of both the size and polydispersity of the asphaltene particles in suspension.

Colloidal asphaltene deposition in laminar pipe flow: Flow rate and parametric effects

“…The precipitation or phase separation process involves molecular asphaltene association and growth of nanoparticles, followed by rapid colloidal aggregation to macro-scopic scales, and complete separation by sedimentation or deposition. 1,2 Colloidal asphaltene deposition on metal surfaces causes problems in industrial settings: as petroleum fluids are produced from reservoirs, depressurization causes asphaltene precipitation, ultimately resulting in deposition in wellbores and pipelines. Such deposition can impede production.…”

Effect of Dispersant on Asphaltene Suspension Dynamics: Aggregation and Sedimentation

“…While dispersants do not prevent the initial association of asphaltene molecules to the nano-or colloidal scale, they do induce a delay in the eventual sedimentation process of the asphaltenes. Dynamic light scattering results on dilute asphaltene suspensions showed that this lengthening of the sedimentation time is due in part to a decrease in the colloidal asphaltene particle size with the addition of dispersant (Hashmi, 2010). In addition, our initial results indicated that the dynamics of the asphaltene sedimentation process exhibits features similar to those seen in colloidal systems which aggregate and form gels (Poon, 1999).…”

“…In addition, our initial results indicated that the dynamics of the asphaltene sedimentation process exhibits features similar to those seen in colloidal systems which aggregate and form gels (Poon, 1999). However, the dynamic light scattering did not reveal aggregation in the diluted suspensions (Hashmi, 2010). In this study we investigate two petroleum fluids and present dynamic light scattering results which do show the dynamics of colloidal aggregation as a function of composition, both in terms of the amount of dispersant as well as the amount of heptane.…”

“…We define the heptane ratio χ, with units mL/g, indicating the number of mL of heptane (Fischer) combined with 1 g of oil. The asphaltene content of the oils is measured as reported previously, by mixing the oils with heptane at χ = 40 mL/g (Hashmi, 2010). The mixtures are soniccated for 1 minute, and filtered through 0.2 µm pore-size cellulose nitrate membrane filters (Whatman).…”

“…Given the addition of heptane, without any solvent addition, the asphaltenes are completely unstable and precipitate out of solution through a sedimentation process. In our previous work (Hashmi, 2010), we demonstrated that the use of dispersants can delay sedimentation and the completion of asphaltene precipitation in suspensions prepared with oil and heptane at a ratio of 40 mL heptane per g oil. While dispersants do not prevent the initial association of asphaltene molecules to the nano-or colloidal scale, they do induce a delay in the eventual sedimentation process of the asphaltenes.…”