Experimental and Mechanistic Study of Stabilized Dry CO2 Foam Using Polyelectrolyte Complex Nanoparticles Compatible with Produced Water To Improve Hydraulic Fracturing Performance

Abstract

The amount of fresh water used in hydraulic fracturing can be significantly reduced by employing produced water-compatible supercritical CO2 (scCO2) foams. Foams generated using surfactants only have suffered from long-term stability issues resulting in low viscosity and proppant-carrying problems. In this work, foam lamella stabilization with polyelectrolyte complex nanoparticles (PECNPs) and wormlike micelles (WLMs) is investigated. Electrostatic interactions are studied as the defining factors improving the hydraulic fracturing performance using the PECNP system prepared in produced water. Two oppositely charged polyelectrolytes are investigated to generate a more stable lamellae between the aqueous phase and the scCO2 while degrading in the presence of crude oil. The generated dry foam system is used as a hydraulic fracturing fluid in a tight shale formation. The strong compatibility of the synthesized PECNPs with zwitterionic surfactants prepared in highly concentrated brine in the form of wormlike micelles above critical micelle concentration (CMC) helps develop a highly viscous, dry foam capable of using produced water as its external phase. This foam system improves fracture propagation and proppant transport fracture cleanup compared to the base case foam system with no PECNPs. The formation of PEC–surfactant nanoparticles was verified via zeta potential, particle size analysis, and transmission electron microscopy; the underlying mechanism was identified as electrostatic rearrangement of WLMs along the PECNP’s perimeter or formation of electrostatically bonded micelles with the nanoparticle to create a new enhanced nanoparticle. A Raman spectroscopic model was developed to understand the PECNP–surfactant spectra and subsequent spectroscopic and hence structural changes associated with complexation. Enhanced bulk viscosity and improved foam quality as a result of complexation at the interface was identified with rheometry in addition to sand pack experiments with PECNP–surfactant ratios of 1:9 and 4:6 in 33.3 kppm and 66.7 kppm salinity brine systems, respectively. Enhancement in the shear thinning and cleanup efficiency of the fracturing fluid was observed. Formation damage was controlled by the newly introduced mixtures as fluid loss volume decreased across the tight Kentucky sandstone cores by up to 78% and 35% for scCO2 foams made with PECNP–WLMs in 33.3 and 66.7 kppm salinity brine, respectively. The produced water compatibility and reduction of water disposal presented the prospect of environmentally friendly scCO2 foams for hydraulic fracturing of unconventional reservoirs.