Fracturing Fluid Cleanup by Controlled Release of Enzymes from Polyelectrolyte Complex Nanoparticles

Guar-based polymer gels are used in the oil and gas industry to viscosify fluids used in hydraulic fracturing of production wells, in order to reduce leak-off of fluids and pressure, and improve the transport of proppants. After fracturing, the gel and associated filter cake must be degraded to very low viscosities using breakers to recover the hydraulic conductivity of the well. Enzymes are widely used to achieve this but injecting high concentrations of enzyme may result in premature degradation, or failure to gel; denaturation of enzymes at alkaline pH and high temperature conditions can also limit their applicability. In this study, application of polyelectrolyte nanoparticles for entrapping, carrying, releasing and protecting enzymes for fracturing fluids was examined. The objective of this research is to develop nano-sized carriers capable of carrying the enzymes to the filter cake, delaying the release of enzyme and protecting the enzyme against pH and temperature conditions inhospitable to native enzyme. Polyethylenimine-dextran sulfate (PEI-DS) polyelectrolyte complexes (PECs) were used to entrap two enzymes commonly used in the oil industry in order to obtain delayed release and to protect the enzyme from conditions inhospitable to native enzyme. Stability and reproducibility of PEC nanoparticles was assured over time. An activity measurement method was used to measure the entrapment efficiency of enzyme using PEC nanoparticles. This method was confirmed using a concentration measurement method (SDS-PAGE). Entrapment efficiencies of pectinase and a commercial high-temperature enzyme mixture in polyelectrolyte complex nanoparticles were maximized. Degradation, as revealed by reduction in viscoelastic moduli of borate-crosslinked hydroxypropyl guar (HPG) gel by commercial enzyme loaded in polyelectrolyte nanoparticles, was delayed, compared to equivalent systems where the enzyme mixture was not entrapped. This indicates that PEC nanoparticles delay the activity of enzymes by entrapping them. It was also observed that control PEC nanoparticles decreased both viscoelastic moduli, but with a slower rate compared to the PEC nanoparticles loaded with enzyme. Preparation shear and applied shear showed no significant effect on activity of enzyme-loaded PEC nanoparticles mixed with HPG solutions. However, fast addition of chemicals during the preparations showed smaller particle size compared to the drop-wise method. PEC nanoparticles (PECNPs) also protected both enzymes from denaturation at elevated temperature and pH. Following preparation, enzyme-loaded PEC nanoparticles were mixed with borate crosslinked HPG and the mixture was injected through a shear loop. Pectinase-loaded nanoparticles mixed with gelled HPG showed no sensitivity to shear applied along the shear loop at 25 °C. However, EL2X-loaded PEC nanoparticles showed sensitivity to shear applied along the shear loop at 40 °C. Filter cake was formed and degraded in a fluid loss cell for borate crosslinked HPG solutions mixed with either enzymes or enzyme-loaded PEC nanoparticles. Cleanup slopes of filter cake degraded using enzyme-loaded PEC nanoparticles and systems with enzymes mixed with HPG gel were significantly higher than for the filter cake formed with HPG gel mixed with no enzyme. In a different application, enzyme-loaded PEC nanoparticles showed significantly slower reduction in viscosity of HPG solution over time compared to the HPG systems mixed with enzyme. Increasing the viscosity of low concentration HPG, used as slick-water, decreases the proppant settling velocity. This is of specific interest in fracturing fluids used for unconventional reservoirs

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

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

Polyelectrolyte Complex Nanoparticles for Protection and Delayed Release of Enzymes in Alkaline pH and at Elevated Temperature during Hydraulic Fracturing of Oil Wells

Please note that this is an author-produced PDF of an article accepted for publication following peer review. The definitive version is available at www3.interscience.wiley.comPolyethylenimine-dextran sulfate polyelectrolyte complexes (PEC) were used to entrap two enzymes used to degrade polymer gels following hydraulic fracturing of oil wells in order to obtain delayed release and to protect the enzyme from harsh conditions. Degradation, as revealed by reduction in viscoelastic moduli, of borate-crosslinked hydroxypropyl guar gel by commercial enzyme loaded in polyelectrolyte nanoparticles was delayed up to 11 hours, compared to about three hours for equivalent systems where the enzyme mixture was not entrapped. PEC nanoparticles also protected both enzymes from denaturation at elevated temperature and pH