Environmentally friendly, rheoreversible, hydraulic-fracturing fluids for enhanced geothermal systems

•CO2-expanded rheoreversible hydraulic fracturing fluid.•Significantly enhances permeability compared to existing fracturing fluids.•Creates fractures at effective stress considerably lower than current technology.•Potentially applicable in a wide range of pressure, temperatures and reservoir pH.•Environmentally friendly and potentially recyclable.

Cost-effective creation of high-permeability reservoirs inside deep crystalline bedrock is the primary challenge for the feasibility of enhanced geothermal systems (EGS). Current reservoir stimulation entails adverse environmental impacts and substantial economic costs due to the utilization of large volumes of water “doped” with chemicals including rheology modifiers, scale and corrosion inhibitors, biocides, friction reducers among others where, typically, little or no information of composition and toxicity is disclosed. An environmentally benign, CO2-activated, rheoreversible fracturing fluid has recently been developed that significantly enhances rock permeability at effective stress considerably lower than current technology based on laboratory-scale tests. In the present work we evaluate the potential of this novel fracturing fluid for application at geothermal sites under different chemical and geomechanical conditions, by performing laboratory-scale fracturing experiments with different rock sources under different confining pressures, temperatures, and pH environments. The results demonstrate that CO2-reactive aqueous solutions of environmentally amenable polyallylamine (PAA) represent a highly versatile fracturing fluid technology. This fracturing fluid consistently and reproducibly creates/propagates fracture networks through highly impermeable crystalline rock from Coso EGS and Newberry EGS sites at significantly lower effective stress as compared to conventional fracturing fluids. In addition, permeability was significantly enhanced (several orders of magnitude). This was evident in all laboratory-scale experiments, including variable rock source/type, operation pressure and temperature (over the entire range for EGS applications), as well as over a wide range of formation-water pH values. This effective, versatile, and environmentally-friendly fracturing fluid technology represents a significant advancement compared to industrially available fracturing fluids for cost-effective and competitive geothermal energy production.

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