Asphaltene is a complex macromolecule whose abundance strongly affects the physical and interfacial properties of crude oil. Asphaltene molecules may precipitate during crude oil production/transportation, which may lead to plugging/ clogging of wellbores, pipelines, and equipment. In this study, the solubility of asphaltene in toluene has been investigated by calculation of noncovalent interaction energies between asphaltenes in toluene medium. The results of this study revealed that the main interactions in the asphaltene−toluene system are Lifshitz−van der Waals and Lewis acid−base interactions, whereas the electrostatic double layer is of lower comparative order of significance specifically at lower separation distances and lower ζ potentials. However, the repulsive electrostatic double-layer interactions may assist in stabilizing the asphaltene−toluene system based on the comparative values of Lifshitz−van der Waals, Lewis acid−base, and electrostatic double-layer interactions. This is the case especially at higher separation distances and/or higher temperatures where asphaltene particles have greater values of ζ-potential. Furthermore, it is illustrated that when asphaltene has a lower electron-donor parameter, i.e., a lower basicity than toluene, then Lewis acid−base interactions between asphaltenes in toluene are repulsive. This repulsive Lewis acid−base interaction may compensate for the attractive van der Waals interactions between asphaltene particles at low asphaltene basicity. Finally, the electron donor/ acceptor component of the surface energy strongly determines the fate of asphaltene in crude oil colloidal system.

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An optimized design for hydraulic fracturing is of great importance especially with the growing demand for this method as a means of production enhancement from tight gas reservoirs. The first optimum fracture design (OFD) approach, which maximizes well productivity for a given fracture volume, was introduced by Prats in 1960 for single-phase Darcy flow systems. This method, which was later modified and presented in the form of Unified Fractured Design (UFD) charts by other investigators, is widely used in the petroleum industry, even for gas condensate systems. Recently some methodologies have been proposed to modify UFD considering the two-phase region around the fracture as a damage zone with reduced permeability. These methods are generally oversimplified as they neglect the phase change and variation of relative permeability with interfacial tension (IFT) and velocity for these low IFT systems. They also require data that are not readily available, in particular the pressure profile (the two-phase boundary) around the wellbore. We introduce an explicit formulation and a more general methodology for OFD that includes the important gas condensate flow parameters in both matrix and fracture. The optimum fracture dimensions are obtained by maximizing the effective wellbore radius, using the recently developed correlation by Mahdiyar et al. (2009). This formulation accounts for the mechanical and flow skins based on quite readily available information at wellbore conditions. In this paper, the integrity of the introduced formulation has been verified for many different prevailing conditions, whilst highlighting the errors of using conventional approaches with some important practical guidelines. In this exercise, the maximum productivity calculated using the proposed formulation is compared with results of the literature or our in-house simulator. This program, using a fine grid approach, simulates gas condensate flow around a hydraulically fractured well for various fracture length-width ratios and identifies the optimum fracture dimensions, for a given fracture volume, providing maximum mass flow rate. Introduction Optimization of a hydraulic fracture geometry provides the maximum productivity/injectivity of a hydraulically fractured well (HFW) with a fixed fracture volume. The importance of this subject has encouraged many investigators to direct their efforts to this topic. Prats (1961) was the first to introduce the issue of the optimum fracture geometry. According to his results, optimum fracture design for a HFW in a square drainage area under the Darcy flow regime is gained when dimensionless fracture conductivity, that is the ratio of flowability of the fracture (the permeability-width product) over that of the matrix (the permeability-fracture-length product), is 1.26. Valko et al. (1998) presented an optimization approach called Unified Fracture Design (UFD). They emphasize that "the key to formulating a meaningful technical optimization problem is to realize that penetration and dimensionless fracture conductivity are competing for the same source: the propped volume". In the UFD method, the propped number is introduced as two times of the ratio of the propped volume to the reservoir volume, weighted by their permeability contrast. Their charts present the dimensionless productivity index of Hydraulically Fractured Wells (HFWs) at Pseudo-Steady State (PSS) as a function of dimensionless propped number and fracture conductivity. In these graphs, it is clearly shown that for each propped number there is an optimum fracture conductivity at which the productivity index is a maximum. Economides et al. (2002) in discussing the optimal design claimed that "…In reality the existence of a transient flow period does not change the previous conclusions on optimal dimensions. Our calculations show that there is no reason to depart from the optimum compromise derived for the pseudo-steady state, even if the well will produce in the transient regime for a considerable time (say months or years). Simply stated, what is good for maximizing pseudo-steady state flow is also good for maximizing transient flow".