PNNL

Abstract

Working fluid’s flow rates of over 5 m3/min are required for economically viable geothermal energy production in EGS. Hydraulic stimulation is considered an efficient way for enhancing reservoir permeability and increasing heat exchange efficiency in EGS. Although different fracturing fluids have been studied, water fracturing is the main stimulation method currently used in EGS due to its recognized permeability enhancement efficiency in tight oil and gas stimulation operations. Alternative stimulation methods under development include supercritical CO2, CO2 foams, propellants, and liquid nitrogen. CO2 is a water-free fracturing technique that can form fracture networks with multiple bedding planes opened. CO2 fracturing can generate complex fractures with lower breakdown pressure than water or thickened water fracturing, which is attributed to the viscosity of the fluid. However, due to the low viscosity and the resultant high leak-off of CO2 compared with water, it has been reported that it takes significantly longer times for CO2 to break down the rock. CO2 foam fracturing has been investigated to address the high leak-off problem of CO2. Foam can also increase the proppant carrying ability, which can enhance proppant transport into the deeper portion of the reservoir. However, due to the thermodynamically unstable nature of foams, foam stability under high-temperature EGS conditions remains a big challenge for fracturing applications. Cyclic soft stimulation and liquid nitrogen fracturing have been demonstrated to produce more complex and branched fractures as compared to cyclic water fracturing. However, the high cost of this stimulation approach could prohibit its large-scale applications. The use of gaseous propellants in the formation has been reported, where a reaction is subsequently initiated, leading to high pressures that can increase permeability. Nevertheless, questions remain about safe deployment, fracture propagation further away from wellbore, and the method’s inability to transport proppants. StimuFracTM, a CO2-responsive aqueous polymer solution combined with CO2, has been reported as an alternative fracturing fluid for EGS, showing permeability enhancement of up to two orders of magnitude higher than water in foot-scale granite samples. Nonetheless, no work has been performed beyond the laboratory. In addition to fracturing fluids, proppants are critical to maintaining long-term reservoir permeability. The introduction of proppants with the hydraulic-fracturing fluid, which is a common practice in tight oil and gas recovery, has been restricted in the geothermal industry due to various technical limitations, including the uncertainties in proppant’s long-term durability caused by EGS-associated thermo-mechanical and chemical stresses. This chapter discusses different stimulation approaches and proppants with a critical view of the technical barriers that need to be addressed before they can be effectively applied in EGS.