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