Kinetics of the Gelation of Colloidal Silica at Geothermal Conditions, and Implications for Reservoir Modification and Management

Abstract

Gel deployments to divert, direct and control fluid flow have been extensively and successfully used in the oil and gas industries for enhanced oil recovery, and for the containment of radioactive and toxic wastes. However, to the best of our knowledge, gels have not been applied to hydrothermal systems to enhance heat extraction. The ability to divert and control fluid flow in the subsurface would be extremely useful to the geothermal industry. Gels may be able to block hydraulic short circuits, minimize losses of injected geothermal fluid to the surrounding formation, and divert working fluids to the hotter regions of a formation. These applications will all in turn maximize heat extraction from a geothermal reservoir and therefore maximize the longevity and the economic potential of the reservoir. Inorganic, non-toxic gels, such as colloidal silica gels, may be ideal blocking agents for geothermal systems if suitable gelation times and control of gelation behavior can be achieved. In the current study, we detail colloidal silica gelation times, behavior, and gel stability as a function of silica concentration, pH, salt concentration, and temperature up to 300 °C. Results indicate that while colloidal silica gels will have limited use in high-temperature geothermal systems (T > 200 °C) due to fast gelation times and lack of long-term thermal stability of silica gels at such high temperatures, colloidal silica gels are generally well-behaved at lower temperatures, with a large range of predictable gel times. Colloidal silica gels may therefore have broad application to lower-temperature (T ≤ 200 °C) geothermal systems and regions of lower temperature within hotter systems. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.