Coulomb stress change during and after tensile fracture opening in a geothermal reservoir

Stress shadowing and the ratio of shear to normal stress in the rock surrounding a newly created tensile fracture are investigated. Shearing on plane of weakness near the stimulated volume can be inhibited or promoted by change in poro- and thermo-elastic stress, while pore pressure increase tends to promote failure, via reduction of effective stress when acting on a failure plane. A numerical model has been used to calculate the coupled poro-elastic response and it has been verified against analytical solution available in literature. The triggering of seismicity, by pore pressure change and stress changes, can be delayed by stress shadowing with respect to the expected occurrence triggered by pore pressure diffusion only. Different shut-in schedule may be employed to mitigate unwanted seismicity response to stimulation treatment, according to our result an important role can be played by proppants in keeping the fracture open and therefore keeping the stress shadow in place while the overpressure diffuses.