Analytical Model and Experimental Testing of the Limits of Hydraulic Fracture Caging

Enhanced Geothermal Systems (EGS) are a promising concept for unlocking the great potential of Hot Dry Rock (HDR) resources for clean and sustainable energy production. It can be argued that three of the foremost unsolved challenges for EGS are: induced seismicity, uneconomically low flow rates, and premature cooling of the produced fluid. We propose that fracture caging could be a solution to these three challenges. Fracture caging is the placement of a ‘cage’ of boundary wells around injection wells before fluid stimulation or circulation begins. This fracture cage is intended to contain injected fluids and to thereby limit fracture growth. In the long term, this fracture cage permits sustained high-pressure fluid injection to hold fractures open using hydraulic pressure (i.e., ‘hydropropping’) instead of by using proppant particles or by shear asperity propping. In this study, we present an analytical model and laboratory experiments that quantitatively explore the limits of tensile hydraulic fracture caging. Discoveries from this work include: (1) the maximum flow rates that can be caged are limited by flow constrictions in the boundary wells but are not limited by the injection pressure, (2) a hydraulic fracture can be caged with as few as two boundary wells, and (3) tensile fractures can be hydropropped without growing larger during sustained high-pressure fluid injection into a cage.