Objectives/Scope: StimuFrac, a CO2-reactive polymer aqueous solution [polyallylamine (PAA) 1wt% in water], can be used as a less water-intensive fracturing fluid for enhanced geothermal systems (EGS) based on laboratory-scale investigations. Fracturing tests from inch scale samples indicate that StimuFrac in the presence of CO2 could generate fractures with high conductivity at lower breakdown pressures compared with water, CO2, and a combination of water/CO2 fracturing processes. The objective of this work is to study the performance of patented StimuFrac fluid in ½ foot side rock samples under representative EGS pressure/temperature conditions and reveal the mechanisms governing the fracturing process assisted with multiphase flow numerical simulations using Subsurface Transport Over Multiple Phases (STOMP). Methods/Procedures/Process: StimuFrac was evaluated using a high-temperature true-triaxial fracturing apparatus and ½ foot side granite cubic samples. Three representative fracturing fluids including water, CO2, CO2 with water were used as control. The fluid transport was simulated with STOMP based on a home-built model which is designed with feedback from the experimental setup and conditions. Results/Observations/Conclusions: For all the water “only” fracturing tests, the conductivity of the rock fractured is quite low (less than 2 μm3 based on radial flow assumption). All three CO2-based fracturing fluids, i.e., CO2 injected in hot dry rock (HDR), CO2 injected in rock partially saturated with water, and CO2 injected in rock partially saturated with aqueous PAA (1wt%), fractured granite at higher breakdown pressures, high transient flow rates, and generated higher-conductivity fractures as compared to water. In addition, faster pressurization rates with CO2-based fracturing fluids are found to be associated with higher fracture conductivities. When partially saturating the rock sample with 1wt% PAA aqueous solution followed by fracturing with CO2, the volume expansion and viscosity increase triggered by CO2-induced cross-linking of PAA leads to a faster pressure increase than CO2/water and dry CO2. This faster pressurization rate is possibly caused by (1) decrease in relative permeability of CO2 compared to that for the uncrosslinked CO2/water system, and (2) a decreased leakoff due to the increase in viscosity of PAA. It was also found that CO2 as a fracturing fluid injected in HDR can generate high fracture conductivity only when injected at very high flow rates (10 mL/min). However, the conductivity of CO2 fracturing in HDR is highly variable while CO2 injected in rock partially saturated PAA consistently generates large fractures with significantly lower variability in conductivity values. In addition, CO2/PAA fracturing fluid system generates fractures with the highest conductivity independently of injection flow rates and using 1/6 of the mass of CO2 as compared to CO2 injected in HDR. The fractured rock samples show that all three CO2-based fracturing fluids, CO2 injected in HDR, CO2 injected in rock partially saturated with water, and CO2 injected in rock partially saturated with aqueous PAA, can generate larger fracture planes than water. From the simulation pressure distribution map of water fracturing, we observed a plateau (water vapor pressure) of the pressure distribution of water in the radial direction of rock which hinders the fracture propagation. STOMP modeling results show a higher pressurization length for PAA/CO2 than other fluids, which may contribute to the consistently higher conductive fractures generated. Applications/Significance/Novelty: The results of this study suggest CO2/PAA as the best performing stimulation fluid under the studied geothermal P/T conditions. CO2/PAA offers the following three additional advantages over waterless CO2, and CO2/water fracturing fluids: 1) it requires significantly lower volumes of CO2 due to the reduced leak off, 2) large fractures can be generated reproducibly and independently of CO2 injection flow rate, and 3) the reversible viscosity (Shao et al. 2015) increase is beneficial to transport proppants when they become available for enhanced geothermal systems.

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