Molecular-dynamics simulation is used to investigate the nature of two-phase (oil/water) flow in organic capillaries. The capillary wall is modeled with graphite to represent kerogen pores in liquid-rich resource shale. We consider that the water carries a nonionic surfactant and a solubilized terpene solvent in the form of a microemulsion, and that it was previously introduced to the capillary during hydraulic-fracturing operation. The water has already displaced a portion of the oil in place mechanically and now occupies the central part of the capillary. The residual oil, on the other hand, stays by the capillary walls as a stagnant film.Equilibrium simulations show that, under the influence of organic walls, the solvent inside the microemulsion droplets enables not only the surfactant but also the complete droplet to adsorb to the interfaces. Hence, delivering the surfactant molecules to the oil/water interface is achieved faster and more effectively in the organic capillaries. After the droplet arrives at the interface, the droplet breaks down and the solvent dissolves into the oil film and diffuses. This process is similar to drug delivery at nanoscale.Using nonequilibrium simulations based on the external forcefield approach, we numerically performed steady-state flow measurements to establish that the solvent and the surfactant molecules play separate roles that are both essential in mobilizing the oil film. The surfactant deposited at the oil/water interface reduces the surface tension and acts as a linker that diminishes the slip at the interface. Hence, it effectively enables momentum transfer from the mobile water phase to the stagnant oil film. The solvent penetrating the oil film, on the other hand, modifies flow properties of the oil. In addition, as a result of selective adsorption, the solvent displaces the adsorbed oil molecules and transforms that portion of the oil into the free oil phase. Consequently, the fractional flow of oil is additionally increased in the presence of solvent. The results of this work are important for understanding the effect of microemulsion on flow in organic capillaries and its effect on shale-oil recovery.
In recent years the use of microemulsions for remediating damaged wells has been a success story. Microemulsions are unique, thermodynamically stable, optically clear single phase blends of water, biodegradable water-immiscible solvent, co-solvent, and specially designed surfactant. Engineered mixtures of these ingredients can form stable microemulsions over a rather broad range of water to solvent to surfactant ratios. Are all microemulsions equally effective in their performance? The main focus of our study was to prepare various microemulsions and to examine a link between microemulsion composition and their performance. Microemulsion treatments have proved to be effective for improving gas production rates by increasing formation permeability and for enhancing fluid recovery from sand-packed and shale-packed columns. For achieving maximum benefits, microemulsion formulations have to be properly designed. Both microemulsion composition and dose are important for their end performance. The effectiveness of a particular microemulsion may vary depending on the application, and may be sensitive to the type of formation. Introduction In recent years a significant number of publications describing the use of microemulsions and surfactant solutions in stimulating oil and gas wells have appeared. These publications covered both studies performed in the lab, as well as the results from field case studies [1–5]. The benefits of surfactants and microemulsions are related to their surface activity, which is revealed as their ability to lower surface tension and contact angle on air/water interfaces. Surface activity is associated with both surfactants and environmentally friendly terpene solvents. Since the latter are insoluble in water, the synergistic effects between surfactants and solvents can be achieved by formulating these additives into microemulsions, which are thermodynamically stable optically transparent colloidal solutions. Many functions of microemulsions, essential for an effective remediation of damaged wells, are very well summarized in [2]. Microemulsions have been shown to be effective in improving core permeability, reducing emulsion tendencies between reservoir oil and treatment fluids, enhancing fluid recovery from proppant packs, and minimizing leak-off into the formation [2–5]. Superior performance of microemulsions is believed to be related to microemulsion structure, described by the so-called Voronoi model [6], which provides a maximized surface area of contact between surfactants and formation. In a number of studies it has been emphasized that microemulsions play an important role in altering capillary pressure, and especially capillary end effects [4,8]. Capillary pressure is given by a well-known equation where ? is the surface tension, ? is contact angle, and r is the radius of pores. For a liquid of density ??, capillary rise, h, determining the depth of liquid penetration, can be calculated as
In recent years a number of laboratory studies on the use of surfactants and microemulsions in hydraulic fracturing of shale formations have been reported. These studies mainly focused on such metrics as improvement in permeability regain and enhancement of fluid recovery from packed columns upon the use of surfactant-containing chemicals. Laboratory studies have also been backed by the documented observations from the field illustrating benefits of using microemulsions for the increase in gas production from shale formations. It is a commonly accepted view that these additives benefit gas production by lowering capillary pressure and altering wetability of shale formation. Although it is recognized that the interaction of surfactants and microemulsions with shale is governed by the energetics of solid/liquid, liquid/gas and solid/gas interfaces, there are practically no studies in which surface energies have been determined for different shales. The surface energy, as well as dispersive, non-dispersive, Lifshitz-van der Waals, and Lewis Acid-Lewis Base components of surface energy of several North American shales have been determined from contact angle measurements. It has been discovered that the surface energy of all shale rocks is rather low, typically in the range of 40-50 mJ/m 2 a contribution from non-dispersion ("polar") component of about 8-11 dyn/cm. Consequently, shales are capable of interacting with liquids predominantly via dispersion and Lifshitz-van der Waals molecular interactions, which should substantially influence the orientation of surfactant molecules and microemulsion moieties at the shale surface. Furthermore, there was no significant variation in surface energy of shales from different basins, which suggests that individuality of shale surface chemistry should play only a secondary role in the development of shale-specific chemical treatments, and factors other than surface chemistry should be considered first.
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