In hydraulic fracturing high pressures are applied to induce fractures within shale formations. Sand and other solid particles (known as proppants) are then injected to prevent the collapse of the fractures once the pressure is released.These propped fractures ease the extraction of natural gas/oil. Sand has the disadvantages of not distributing evenly within the fractures, and of not reaching fractures far from the wellbore. This limits the amount of natural gas/oil that can be extracted from a given shale formation. Research is being conducted in materials sciences to produce proppantsthat are uniform in size (so that they will not clog the fractures), small (so that they canpenetrate narrow fractures), mechanically strong (to maintain the fractures open even when exposed to high subsurface pressures), and light (to diffuse farther from the wellbore, allowing the extraction of larger amounts of natural gas/oil from one fracture). This proposal intends to explore whether Pickering emulsions could be used to prevent the collapse of the hydraulic fractures once the pressure is released. Pickering emulsions e.g., water in oil) are stabilized by solid particles. They find numerous applications in areas as diverse as food preparation, cosmetics,drug delivery, and catalytic processes. The results obtained so far in the PIs laboratory,summarized in the narrative, have been obtained for bulk Pickering emulsions.Such studies have quantified the effect of solid particles on the water-decane interfacial tension, and the mechanism of dropletcoalescence. Current research is being conducted to clarify the interfacial structure anddynamical properties for systems composed by mixtures of different solid particles.This proposal stems from the hypothesis that emulsions might allow for a bettercontrolled distribution of the proppants within a fracture, possibly far from the wellbore. In the systems of interest the proppants will have the dual role of fracture stabilizers and emulsion formers.It is anticipated that once the emulsions are destabilized because of particle-rock interactions,the proppants will adsorb onto the rock surface.By controlling the emulsion stability within different sub-surface formations it will be possible to design shale-specific proppants.
The intellectual merit of the proposed research consists in the evaluation of a new method to inject proppants within hydraulic fractures (Pickering emulsions instead of aqueous dispersions). If successful, this exploratory research could lead to the exploitation of larger amounts of the natural gas/oil stored in shale formations.
Broader impacts. One post-doctoral researcher will conduct this exploratory research over a period of one year. The results obtained will be used to test our hypothesis. If successful, the proposed research could bring enormous economical advantages to the nation because the hydraulic fracturing stage constitutes one of the most expensive stages in the well drilling, and because more natural gas will be produced from a single well. Potential impacts are also expected on other technologies (e.g., cosmetics and drugdelivery), as our research will lead to better fundamental understanding of Pickeringemulsions. The research results will be integrated in teaching and outreach activities, following our track record.
This exploratory project was awarded to investigate whether Pickering emulsions could be used in the practice of hydraulic fracturing to reduce the environmental impact of this technology. To produce oil and natural gas from shale formations it is necessary to increase the porosity of the rock formation to allow the fluids to be transported. This is achieved by injecting water at high pressure that fractures the rock (hydraulic fracturing). Because the shale formations are deep underground, once the water pressure is released the fractures tend to heal, thus preventing the gas from flowing to the surface. To maintain the fractures 'open', sand (or other 'proppants') are injected with water. Unfortunately the sand tends to deposit not too far from the well, with the consequence that most of the fractures generated by hydraulic fracturing do not produce the desired oil and gas. The goal of this project was to understand whether it is possible to transport the sand further into the rock, perhaps using Pickering emulsions. Pickering emulsions are obtained by stabilizing oil (or water) droplets dispersed in water (or in oil) using solid particles (e.g., sand with small grains for hydraulic fracturing). The solid particles are needed because oil and water do not mix, and within the solid particles two phases would form (one ricj in water, the other rich in oil). Typical emulsions are salad dressings, which requires mechanical agitation to be prepared and quicky destabilize because solid particles (or other chemicals) are not present. The project was conducted using computer simulations at the coarse grained level. At this level the methodology doe not describe water, oil, and particles with atomistic precision, but allows for the investigation of systems rather large, yet small in comparison to experiments. During the project one post-doctoral student and one Ph.D. student were trained to implement coarse-grained simulations, five peer-reviewed journal articles were published, one is in preparation, and a number of presentations were given at international conferences. All results have been widely disseminated and are available to the public. We studied: 1. how spherical particles behave and move at water-oil interfaces as a function of their surface properties, of their loading at the interface, and of the presence of particles of different types. We found that using different particles can lead to the destabilization of emulsions because the particles tend to have higher mobility when surrounded by different particles. 2. how ellipsoidal particles protrude from a water-oil interface. We found that if the particles are too long (aspect ratio 4 or longer) they tend to orient parallel to the interface. This suggests that using particles that are too long will not lead to the stabilization of emulsions. 3. how the behavior of the particles depend on the size of the droplet on which they are adsorbed. We found that when the droplet diameter is less than 40 times the main axis of the particle the particle behavior strongly depend on the curvature of the interface, because of an interplay of different driving forces, one of the most important being the elastic behavior of the interface. This could be important for the production of new materials and the use of Pickering emulsions in advanced applications such as catalysis. 4. how the shape of the particles affect the stability of droplets when they impact a solid surface. We found that cuboidal particles are more effective at stabilizing the droplets than spherical particles, most likely because they pack better at the liquid-liquid interface, thus preventing the liquid inside the droplet from adsorbing on the wall. This has a direct application for the use of Pickering emulsions in hydraulic fracturing, as controlling the particles features seems to be directly related with how far they can be transported into the fractured rock. These results set the basis of productive interactions with industry, which are being explored now that the grant is completed. The goal fo these collaborations is to secure that technological developments will lead to energy production with limited environmental impact, which will secure the wellbeing of the society at large.
