An Experimental Study of Thermal and Hydraulic Geothermal Reservoir Stimulation of Brittle Impermeable Material

Abstract

Reservoir stimulation is one of the key technologies necessary for optimization of enhanced geothermal systems. The higher volumetric density of fractures created by stimulation allows for greater access to the rock, which potentially yields higher fluid flow rates and increased injectivity. To study this further a suite of ongoing experiments simulates a brittle, impermeable reservoir subjected to sequential hydraulic and thermal fracturing processes. It is anticipated that cooling and injection protocols could lead to an increase in injectivity that is greater than either of these stimulation processes alone. The combined hydraulic and thermal fracturing process is hypothesized to result in fracture geometries that are significantly different than those produced by either hydraulic or thermal fracturing alone, with potentially higher flow rates. The higher flow rates can directly result in an elevated heat transfer rate in a geothermal system. To assess this potential for increased injectivity and surface area, experiments were conducted in which acrylic specimens were stimulated using methods analogous to those that could be used on a reservoir scale. Fractures that have faces perpendicular to the maximum horizontal principal stress were theorized by Perkins and Gonzolaz (1985) to occur once a hydraulic fissure has been thermally fractured. These fractures were created in blocks of acrylic and validate their findings.