Stimulation in enhanced geothermal systems

Working fluid's flow rates of over 5m3/min are required for an economically viable geothermal energy production in EGS. Hydraulic stimulation is considered an efficient way to enhance the permeability of reservoirs and increase efficiency of heat exchange 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 that are under development include supercritical CO2, CO2 foams, propellants, and liquid nitrogen. CO2 fracturing 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 was reported to take significantly longer times to break down the rock. CO2 foam fracturing has been investigated to address the high leak-off problem of CO2. Foams can also increase the proppant carrying ability, enhancing 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 with cyclic water fracturing. However, the high cost of this type of stimulation 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 the safe deployment, fracture propagation further away from the wellbore, and the method's inability to transport proppants. StimuFrac, 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 in maintaining long-term permeability of reservoirs. 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 uncertainties in proppant's long-term durability caused by EGS-associated thermomechanical 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.