Phase-Field Simulation of Hydraulic Fracturing by CO₂ and Water with Consideration of Thermoporoelasticity

We have advanced phase-field simulation of hydraulic fracturing with consideration of thermoporoelasticity and discretization based on the mixed finite element in temperature, pressure, and the phase field. The key application is intended for hydraulic fracturing by water and by CO₂ in hot dry rock. In geothermal fracturing, the injection fluid may have much lower temperature than the hot-volcanic rock and consideration of thermoporoelasticity may have a significant effect. We provide numerical simulations and comparison with laboratory data and examine the effect of thermoporoelasticity on breakdown pressure and fracture intensity. The thermal effect is more pronounced under unconfined conditions, especially for CO₂ fracturing. The change of granite rock strength in the Brazilian tests at different temperatures without specific fluid confinement may not apply to high stress boundary conditions. Based on simulation of hydraulic fracturing experiments using water in heated and unheated granite, we conclude that the critical energy release rate G_c which is a key parameter of the phase field is not affected by temperature in the range of 20–300 °C. In that respect, there is similarity on the independency of Young’s modulus from temperature. The critical stress is, however, known to be a function of temperature. An important observation relates to simulation of fracturing by water and CO₂ in a domain larger than laboratory scale. CO₂ fills the created fractures quickly. Filling of created fractures by water takes time, and as a result fractures propagate in many stages. We observe from simulations that fracture intensity from CO₂ is higher than by water in line with laboratory measurements. Higher fracture intensity and fracture surface area is an important consideration in renewable energy production from geothermal formations due to low thermal conductivity in volcanic rocks.