An Investigation of The Stability of Colloidal Asphaltene in Petroleum Reservoirs

Abstract

Abstract Asphaltene precipitation from petroleum fluids has proved to be a difficult problem to define and study. Asphaltene problems may occur deep in the reservoir formation and cause permeability reduction as well as contribute to serious plugging problems in oil well tubing and surface facilities. The stability of a dispersion in colloid science terms refers to the resistance of the particles to flocculation. The degree of this resistance is a measure of stability. Asphaltene colloidal dispersions in petroleum reservoirs are considered physically stable if free from any changes in physical properties. This paper first examines theoretically particles (asphaltene) in crude oil to determine, if they are electrically charged, how important it is to their inter-particle repulsion (stability). The theoretical result is then compared with the surface potential and steric effect of asphaltene and/or solid in petroleum fluids. Both theoretical and experimental results indicated that the electrical charge effects of asphaltene were found to play an utterly negligible role in the interaction between asphaltene particles and either surface or other particles. The stability of colloidal asphaltene in oil was investigated and determined to be caused by the steric effect of high molecular weight material (resins) adsorbed on asphaltene particles. The aggregation kinetic rates of asphaltene were also investigated using a laser back-scattering technique and determined to be influenced by the resin chemical potential concentration and the dispersion medium solvency. The latter needs to be a "good solvent" for the resin so that the aliphatic chains can be extended and move freely, thus enhancing the steric repulsion force. Introduction The particles in a colloidal dispersion are thermodynamically unstable owing to their excess surface free energy. However, this energy will be reduced by the aggregation which reduces the exposed surface area. Aggregation can lead to reversible loosely bound structures (flocs), or can produce irreversible structures (agglomerates). The latter may produce a thick deposit material giving irreversible separation of the two phases. The stability of a colloid dispersion can be defined as its resistance to flocculation or coagulation. The degree of the "resistance" is used as a measure of the dispersion stability. Colloidal asphaltene can be considered physically stable in petroleum reservoirs as a result of the resins present when no deposition problems occur1. A principal cause of aggregation is the van der Waals attractive forces between the particles. To promote stability, a repulsive force must be present of sufficient range to overcome the van der Waals attraction. Such a force could in principle arise from electrical double layer overlap or from steric repulsion. So-called lyophobic sols are stabilised entirely by electric double layer interactions. The DLVO theory [Deryagin-Landau2 & Verwey-Overbeek3] was developed to predict the behaviour of such dispersions from an understanding of the attractive and repulsive forces between adjacent particles. However, the applicability of the DLVO theory in understanding the behaviour of colloidal asphaltene in petroleum reservoirs must be treated with caution, since the theory was not developed for the deposition of crude oil heavy ends, like asphaltene. Using the theory involves the measurement of the zeta potential of colloids, whereas in the case of crude oil it is difficult to measure the zeta potential due to the low permittivity and high viscosity of crude oil. Moreover, as will be shown in this paper, electric charge is ineffective at producing a repulsive barrier in such crude oils or solvents.