Asphaltene Adsorption Isotherm in the Pores of Reservoir Rock Cores

Abstract

Abstract New data relating to the adsorption of asphaltenes on rock pore surfaces are reported. The data were obtained from the measurements of the electrokinetic potential of several different sandstone core samples with asphaltenic oils. The adsorption of asphaltene is accompanied by a numerical reduction in the (negative) surface potential of the pore walls, eventually stabilizing at a small positive potential, attributed to the asphaltene itself. After increasing to around 30% of the pore radius, the asphaltene adsorbed layer thickness stopped growing either with time or with concentration of asphaltene in the flowing liquid. This behaviour is attributed to the effect on the disjoining pressure across the adsorbed layer described by Derjaguin and Churaev1. Alternative hypotheses involving asphaltene adsorption isotherm have been investigated. A theoretical treatment advanced describing particle adsorption in the same terms as molecular adsorption and the Langmuir isotherm, with the free energy of asphaltene adsorption on the rock surface (modeled on silica), calculated on the basis of van der Waals attraction. Acceptable agreement was obtained with the electrokinetic measurements. Introduction Asphaltenic oils flowing through reservoir formations often results in severe permeability damages near the well bore causing a decrease in well productivity2–6. Beside permeability damage, the adsorbed asphaltene layers may also induce wettability alteration of the oil-bearing formation7. The asphaltenic flow depends significantly on the dispersive properties of the microstructure formation, its stability, and the characteristic of flow8–9. The adsorption of asphaltene is controlled by a number of factors such as, stability of colloidal asphaltene in the oil phase, the thickness and stability of water film adsorbed on rock pores surfaces, and the nature of the rock mineral surfaces10–11. The magnitude of the interaction forces between the three previous factors control the degree to which such adsorption is irreversible with respect to the remediation treatment. Asphaltenes defined operationally as the polar fraction of crude oil that can be precipitated by the addition of low molecular weight alkanes but is soluble in aromatic solvents (i.e. toluene, diesel)12–14. Asphaltene structure were found to be associated of comprising several condensed aromatic discs, each of them representing asphaltene molecule core, with approximately parallel planes stacked one over other. The chains of aliphatic and/or naphthene-cyclic systems are attached with asphaltene aromatic discs with known structure of asphaltene molecules15–17. The molecular weight of asphaltene ranges from one thousand to several hundred thousand with a microparticles density of approximately 1.2 g/cc and a spheroidal shape 30 to 65 Å in diameter18. In order to investigate the extent of formation damage by asphaltenes in crude oil this work has used electrokinetic technique to study the adsorption of asphaltenes in rock pores. Alternative hypotheses involving uniform adsorption have been investigated with a theoretical treatment describing asphaltene adsorption isotherm calculated on the basis of van der Waal attraction. Acceptable agreement was obtained. Kinetics of adsorption in porous system Before describing the effect of asphaltenes on reservoir rock properties, it is important to understand the kinetics of adsorption in porous system. The kinetics of adsorption of molecules can be contributed by: transport towards the surface by diffusion, attachment and reconformation to the interface. The shape and flexibility of the molecules may play a role on the kinetics process at a timescale8–9. In order to assess the contributions of each of the three processes it is essential to measure the kinetics of adsorption under hydrodynamic conditions. Most researchers investigate the kinetics of adsorption by monitoring changes in the concentration of asphaltene or polymer in a dispersion of adsorbent particles or capillaries11,19. This method is less suitable to describe the kinetics process since the hydrodynamic conditions are non-uniform and difficult to specify. Others investigated the measurements of the adsorbed layer thickness on the solid surfaces. Several techniques have been applied to determine the thickness of the adsorbed layer, such as macroscopic flow methods, disjoining pressure measurements, surface force methods, and photon correlation spectroscopy20–24.