Lower environmental impact formulations for European shale gas plants

Nowadays, shale gas is the fastest growing natural unconventional gas resource worldwide, as a result of the combination between major technological developments and geopolitical events. Despite the great potential in terms of exploitable reservoirs, several environmental issues have been associated to shale gas extraction and hydraulic fracturing, like induced seismicity, greenhouse gas emissions, and water contamination. In this perspective, the formulation of environmentally-safe fracturing fluids plays a crucial role in the minimization of the environmental footprint of shale gas exploitation. In this work we proposed two completely green and eco-compatible fracturing fluid formulations specific for the shale gas formations in Europe. These formulations must possess well-defined properties in terms of composition and operative performances, including the absence of controlled or toxic substances, an appropriate viscosity for proppant transport, the effectiveness in high pressure, temperature and salinity conditions and the presence of specific additives to limit the extraction of Naturally Occuring Radioactive Materials (NORM). The first formulation we developed is a linear gel based on four different green polysaccharides, namely guar gum, sodium hyaluronate, sodium alginate and hydroxpropylcellulose. The second is a viscoelastic surfactant-based (VES) fluid, composed by an eco-compatible surfactant, sodium oleate, and a inorganic salt, KCl. Rheological measurements, Optical Microscopy, Differential Scanning Calorimetry (DSC) and Small Angle X-Ray Scattering experiments were performed in order to assess the main physico-chemical properties of the formulations and their fluid behaviour. All the formulations show a very high viscosity (optimal to avoid proppant sedimentation), a high shear resistance and a good thermal stability. Moreover, we tested their effectiveness at high temperature, high pressure and salinity content, demonstrating that the variation in these geochemical parameters can be effectively used to modulate the fluid rheological behaviour. The polysaccharide-based and the VES formulations were then implemented by the inclusion of two specific additives, Carbon Black and Azorubine, which induces a fluid responsiveness to specific external stimuli, namely an applied electrical voltage and UV light. Rheological, DSC and SAXS experiments demonstrated that in both cases the fluid viscosity can be modulated using an applied input. The addition of Carbon black brings about also an enhanced thermal resistance and the possibility to recover and re-use the fluid. In the last part of the work we developed two completely safe and environmental friendly approaches to limit the amount of NORM in the flowback water and for the treatment of wastewaters after the extraction of the shale gas. The first method is a physical treatment consisting in the exposure of the fracfluid to a weak static magnetic field before being pumped in the pipelines (Magnetic Water Treatment, MWT). The second method is based on the use of green innovative scale inhibitors, polyglutammate and polyaspartate, to replace the non-ecocompatible polyacrilates in fracfluid formulations. Optical microscopy, Fluorescence Spectroscopy and X-Ray Diffraction experiments showed that the combined effect of an external magnetic field and high temperature reduces the amount of insoluble alkali earth metals scales (CaCO3, BaCO3, SrCO3and CaSO4) and limits NORM extraction, since the formation of fewer scales of larger dimensions makes more difficult the flowback of NORM ions. Qualitative precipitation and solubility experiments demonstrated the effectiveness of the green antiscalant agents in reducing the precipitate and controlling the crystals size of CaSO4 and SrSO4. The high temperature does not affect the anti-scaling performances, which are even enhanced when added to a polysaccharide-based formulation. For this reason, the use of very small amounts of these green additives could be a simple way to limit the formation al of mineral scales, also in the presence of radioactive materials, such as radium salts.