Abstract

The objective of this work was to develop a supercritical CO2 foam with a liquid phase composed of a polyelectrolyte complex nanoparticle system interacting with a viscoelastic surfactant solution. This is intended to be used as a fracturing fluid to reduce fluid loss and increase post-fracture clean-up efficiency for improved productivity in unconventional reservoirs. In a previous study, we optimized the polycation/polyanion ratio and polyelectrolyte pH using zeta potential and phase-angle light scattering. Rheological and foam stability tests were used to further optimize surfactant/polyelectrolyte ratio. In this work, dynamic fluid loss tests were performed on supercritical CO2 foam generated using the optimized ratio of surfactant/PECNP to investigate the effect of PECNP addition to the surfactant solution on the fluid loss. The same foam systems were used to investigate post fracture clean up using sand pack tests. Interfacial tension was measured on both air-(surfactant/PECNP) and supercritical CO2-(surfactant/PECNP) systems to understand the effect of PECNP on IFT. Fluid loss to the formation during hydraulic fracturing causes water blockage, formation damage and capillary pressure shift, lowering the conductivity of the reservoir, and impeding the flow of oil and gas. Supercritical CO2 foam generated by surfactant solution exhibited low fluid loss, thus lower values of fluid loss coefficient compared to unfoamed surfactant systems. However, addition of polyelectrolyte complex nanoparticles further reduced the total fluid loss and fluid loss coefficient. Foam fracturing fluids break in the presence of crude oil because the oil penetrates the foam lamellae causing drainage and lamellar rupture; this results in effective post fracture clean up using supercritical CO2 foam as fracturing fluid. Foam systems generated by surfactant showed promising clean up results, and the addition of PECNP further increased clean up efficiency. From IFT test results we concluded that the addition of PECNP decreased interfacial tension of both air-(surfactant/PECNP) and supercritical CO2-(surfactant/PECNP) systems. The addition of optimized polyelectrolyte complex nanoparticles to surfactant in a supercritical CO2 foam fracturing fluid can reduce interfacial tension and fluid loss, which will reduce formation damage, resulting in better hydrocarbon flow. PECNP stabilized surfactant-supercritical CO2 foam drains rapidly in the presence of crude oil, leading to good clean-up, which will result in better flow back and higher productivity.

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