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High Temperature, High Pressure Stability of Aqueous Foams for Potential Application in Enhanced Geothermal System (EGS) 1

Enhanced Geothermal Systems (EGS) are typically constructed by injecting high-pressure water into deep hot dry rocks (HDR) under carefully controlled conditions to create new or re-open existing fractures, which usually uses an immense quantity of water. In lieu, a more sustainable technique is to utilize aqueous foams as fracturing fluid to reduce water consumption and waste-water treatments of conventional hydraulic fracturing. Although foam-based fracturing has shown promising results in oil and gas industries, its feasibility has not been demonstrated in EGS conditions that usually involve high temperature and high pressures. One potential barrier for utilizing foams in EGS applications is that foams are thermodynamically unstable and will become more unstable with increasing temperature due to liquid drainage, bubble coarsening, and coalescence. This work focuses on evaluation of the stability of selected aqueous foams under high-temperature and high-pressure conditions. Specifically, foams generated with surfactant alfa olefin sulfonate (AOS) were studied at temperature up to 200 o C, while the pressure ranged between 100 psi and 1000 psi. The effect of additional stabilizing agents was also examined, including guar gum, bentonite clay, borate salt crosslinker, silicon dioxide nanoparticles (SiO 2 ), and graphene oxide (GO) dispersion. Results showed that the stabilizing agents can enhance the foam thermal stability. Foams made with AOS and the borate salt crosslinker exhibited the longest half-life of 20 min at 200°C when the pressure was 1000 psi. An analysis of the data showed that foam stability decreased exponentially as temperature increased. On the other hand, pressure increased the foam half-life according to a power model. This study indicates that it may be possible to obtain highly stable foams at high-temperature and high-pressure conditions with appropriate stabilizing agents. generated with AOS as the surfactant had the best stability as indicated by the foam half-life (Figure 1). In this paper, we report the foam stability at high temperatures (up to 200 o C) and high pressures (up to 1000 psi) for various AOS based foams with additional stabilizing agents. The new results indicated that the foam stability can be extended by adding stabilizing agent such as crosslinker, which led to the longest half-life of 20 min at 200°C and 1000 psi. Analytical models were developed to comprehend the effect of temperature (exponential dependence) and pressure (power-law dependence) on the foam half-life.