Insights into a Greener Stimuli-Responsive Fracturing Fluid for Geothermal Applications

Enhanced geothermal system (EGS) techniques allow the development of geothermal resource where no active hydrothermal fields are present. These potential reservoirs of heat at depths ranging from 900-4000 m and temperature in the range of 150 - 400°C are almost impermeable and one of the primary challenges faced by EGS is to create safe and cost-effective high-permeability reservoirs in such conditions. Recent advancements in the hydraulic fracturing techniques and horizontal drilling in the oil and gas industry provided a critical driver for EGS however the fracturing fluids used in the oil and gas industry are problematic for EGS. The macro-polymers developed to modify fluid rheology in typical hydraulic fracturing are not applicable in high-temperature conditions due to their thermal decay. An additional difficulty is the removal of such fluids from the formation after fracture creation which in turn decreases flow rates and heat transfer. We have recently developed a new fracturing fluid, StimuFrac^TM, which is a non-toxic polyallylamine aqueous solution that can undergo chemically-induced reactions with CO₂ to form a single-phase fluid/hydrogel with concomitant large and rapid volume expansion and viscosity increase. StimuFrac^TM has shown at the lab-scale to consistently fracture rock cores at significantly lower net pressures in a range of representative geothermal pressure/temperature conditions. We will present the two main mechanisms that we have identified as responsible for fracturing rock at lower net pressures. The first mechanism is the additional overpressure generated by aqueous polymer solution during its CO₂-triggered volume expansion. The second mechanism is associated to a reduction in pore invasion pressure as a result of the lower interfacial tension of the fluid. This reduction in pore invasion pressure results in a reduction in fracturing net pressure as previously reported. The results presented in this work illustrate the potential of StimuFrac^TM as a greener and less-energy intensive fracturing fluid for geothermal and fossil energy production.