Pressure propagation during hydraulic stimulation: case study of the 2000 stimulation at Soultz-sous-Forêts

Hydraulic stimulation of pre-existing fractures and faults plays a significant role in improving hydraulic conductivity of the fracture network around injection and production wells in deep geothermal reservoirs. In present work, a three-dimensional distinct element method (3DEC, Itasca) is used to simulate the year 2000 hydraulic stimulation of GPK2 well of Soultz-sous-Forêts geothermal reservoir, where several major hydraulic stimulations have been performed and are well documented. The field scale numerical model of the reservoir (about 6000 × 4500 × 4500 m³) includes an explicit description of the main fault (FZ4770), was developed to constrain the large-scale hydromechanical properties of the fault, in particular, its behavior in terms of non-linear elastic response related to fault aperture changes. The first phase of the stimulation is modelled as a constant flow rate of 30 ls⁻¹ of water injection into the center of a deformable fault at the depth of approximately 4.7 km. We observed that the fluid pressure front migration from the injection point along the fracture follows, under the in-situ stress condition and the moderate injection pressure, a pseudo-diffusion behavior as power-law function of time with a 0.5 exponent (√t) when the injection flow rate is constant. It is demonstrated that the dynamic evolution of aperture opening due to fluid injection into the fracture is responsible for the pressure propagation behavior, owing to a hydraulic aperture change rather than a fluid pressure diffusion process. This numerically observed propagation process is compatible with a high fault effective diffusivity of 13 m²/s as that observed in the field. In case of a linear increase of the injection flow rate, the pseudo-diffusion process disappears leading to a time dependent power-law migration of the pressure front with exponent of 0.75. The pressure propagation is shown to be strongly influenced by the injection scheme.