Carbon Black Suspension as Electrically Conductive Fracturing Fluid for Fracture Diagnostics in Enhanced Geothermal Systems

Summary

Fracture diagnostics in subsurface energy systems using a low-frequency electromagnetic (EM) induction method requires a high electrical conductivity contrast between propped fractures and the surrounding formations. Conventional electrolyte-based fluids suffer from salt precipitation and corrosion at enhanced geothermal system (EGS) temperatures exceeding 150°C, which limits their applicability. While carbon black (CB) suspensions are widely used in composite materials and industrial fluids, their performance at EGS reservoir conditions and suitability as a fracturing fluid additive have not been previously characterized. In this study, we investigate CB suspensions as dual-function fracturing fluid additives that simultaneously enhance electrical conductivity for EM-based fracture diagnostics and improve proppant transport through viscosity enhancement at high temperature and pressure. CB suspensions at 0.1 wt%, 0.5 wt%, and 1.0 wt% were tested at pressures up to 2,000 psi and temperatures up to 200°C, with viscosity measured via the Hagen-Poiseuille pressure-drop analysis and cross-validated using a rotational rheometer. CB suspensions achieved electrical conductivities up to 0.5 S m⁻¹ at 200°C and 2,000 psi, exceeding granitic formation conductivity by more than four orders of magnitude. Conductivity follows Arrhenius behavior with temperature and a percolation power law with concentration, and an empirical correlation was developed for EGS stimulation design. Adding 1 wt% CB increases fluid viscosity by 30–40% at high temperature and pressure, which reduces proppant settling velocity by 19–21% at 200°C and improves proppant transport without the corrosion and precipitation risks associated with conventional electrolyte-based solutions. A 3D frequency-domain EM forward model confirms that a conductive proppant pack can generate induction logging signals above the formation background, which can be used to detect propped fractures in EGS.