Modeling the Impact of Thermal Stresses Induced by Wellbore Cooldown on the Breakdown Pressure and Geometry of a Hydraulic Fracture

Wellbore cooldown is often employed before well stimulation and/or hydraulic fracture stress testing in Enhanced Geothermal Systems and high-temperature petroleum reservoirs to prevent equipment from being overheated due to high temperatures. 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 (i.e., in the same plane as the well) in the absence of cooling. The cases most susceptible to the complete change of fracture initiation geometry are those where the well is drilled parallel to the least compressive stress, typically 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.