  Wellbore cooldown is often employed before well stimulation and/or hydraulic
fracture stress testing in EGS and high temperature petroleum reservoirs. The
thermo-elastic stress resulting from heat conduction during the cooling
activity can have important influence on the behavior of the hydraulic
fractures. A coupled numerical model has been developed to study the
thermo-mechanical effect associated with pre-injection wellbore cooldown on the
wellbore pressure and geometry of the hydraulic fracture (either longitudinal
or transverse to the wellbore axis). The main novelty of this numerical study
is the consideration of significant near-wellbore thermal stresses in the
coupled non-linear problem of hydraulic fracturing initiation and propagation,
which enables investigation of the thermo-mechanical effect under different
fracture propagation regimes. Simulation results show earlier fracture
initiation and lower breakdown pressure caused by cooling circulation.
Extensive wellbore cooling also significantly alters the evolution of wellbore
pressure, as evidenced by the differences observed under various cooling
conditions. Most importantly, cooling promotes the transverse initiation of
hydraulic fractures in situations where the initiation would have been
longitudinal in the absence of cooling. The cases most susceptible to the
complete change of fracture initiation geometry are those horizontal wells
drilled parallel to the minimum horizontal stress but also applicable to
vertical wells in cases where the vertical stress is the lower in magnitude
than either horizontal principal stress. These results combine to indicate a
profound potential for cooling to impact hydraulic fracture initiation and
early growth, and therefore needs to be considered in the planning and
interpretation of stress testing and reservoir stimulation when cooling
operations are necessary.
