(270e) Multi Scale Flow Observation and Stability Measurement for Environmentally Friendly Waterless Fracturing Using Supercritical CO2 Foam

The energy sector is experiencing a historic boost in oil and natural gas production thanks to development of hydraulic fracturing for unconventional recovery from tight shale formations. However, the process suffers from large dependency on water resources with reported negative environmental impacts. Reduction of freshwater use in hydraulic fracturing via high internal phase emulsions (foams) is a promising approach to protect drinking water resources and enhance ground and surface water quality to avoid large water disposal which may lead to seismic activities. A sustainable approach for reduction of freshwater consumption and produced water disposal is to inject water-less fracturing fluid with minimum aqueous phase volumes obtained from the production cycle itself. The non-aqueous phase, however, could be obtained from an external gas source such as N₂ or CO₂. Application of compressed CO₂ as a non-aqueous phase improves the fluid viscosity, fracture propagation and oil production and it’s been introduced as a promising candidate for CO₂ storage and sequestration in shale formations. The mixture of produced water and compressed CO₂ in supercritical state must be stabilized to form homogeneous supercritical CO₂ (scCO₂) foam. In this work the multi scale observation and stability measurement of scCO₂ foam in different stages of generation, proppant transport, and fracture propagation during a hydraulic fracturing are presented.

Our approach addresses microstructural observation of high internal phase emulsion containing a gas-water lamella stabilized by polyelectrolyte complex nanoparticles (PECNP) and wormlike micelles (WLMs) with electrostatic interactions as defining factors to improve the disjoining pressure over capillary drainage for a better foam stability. The length scale involved in shale production covers multiple orders of magnitude ranging from macro- to micro- scale. Therefore, millimetric view cell observation was coupled with micrometric fluidic visualization to shed light on multi scale observation of physical structure, geometry, dynamic and stability of electrostatically enhanced scCO₂ foam. Lamella stability as a result of complexation of two oppositely charged polyelectrolytes with zwitterionic surfactant were investigated in view cell and glass microchips. The electrostatically stabilized interface between the highly concentrated electrolyte and the scCO₂ is attributed to carrying the proppants and fast degradation of lamella in the presence of crude oil to maintain fracture conductivity and fast flow back to the surface.

Formation of highly viscous, dry foam, capable of improving fracture propagation, proppant transport, and fracture cleanup were observed in miniature field of view of vertical view cell column where the foam is generated, and stability is influenced by gravity drainage. Foam microstructural graphs were examined to obtain bubble size and size distribution. Subsequently, morphology and structural parameters of foam bubbles including size, size distribution and lamella thickness as well as film drainage and rate of deformation were studied via visualization techniques, post processing software and image analysis. Finally, the hydrodynamic and flow regime parameters were derived to correlate chemical properties of lamella and multiphase flow dynamic and foam physical/geometrical properties. Superior stability of scCO₂ foam enhanced with complexes of PECNP and surfactants over a lamella containing sole surfactant was observed in millimetric scale whereas polyhedral structure of dry foam degrades faster in lamellae enhanced with supercharged particles when the foam meets oil. This phenomena occurs due to interaction of oil with the chemical components residing in the lamella such as PECNPs. Synthesized ionic complexes are susceptible to degradation and secession of coiled PEI-DS chains and electrostatic desorption of amine functional groups.

Multiphase flow in fractured medium was emulated using a lab-on-a-chip device. Different etching strategies (wet/dry) for fabrication of pressure resistive microfluidic chips were introduced to study microscale scCO₂ bubble geometry, transport and stability in simulated pathways in harsh geological conditions that may range from simplified patterns to complex microcracks based on tomography data. Wet etching technique on glass was performed via UV photolithography, surface layer removal and thermal bonding, whereas, dry etching was conducted with selective laser etching (SLE) inside the glass bulk followed by acid/base wetting for glass debris removal. Micrographs obtained with two etching techniques were analyzed to obtain structural and geometrical parameters for confined scCO₂ bubbles in fractured networks. The foam stability and fluid loss in a simulated fracture were investigated and the rate of fluid leak-off was compared for scCO₂ foam stabilized with surfactant and PECNP/Surfactant complexes. The microscale observation sheds light on the strong electrostatic compatibility of the synthesized PECNP with zwitterionic surfactants prepared in highly concentrated brine which helps to develop a long term stable scCO₂ bubble in fractured medium and effectively addresses the common stability issues with surfactant generated foams suffering low viscosity and proppant carrying capability problems in hydraulic fracturing application.