Poro-thermoelastic Mechanisms in Wellbore Stability and Reservoir Stimulation

Some fundamental mechanisms associated with cooling/heating of the rock in the context of drilling and reservoir stimulation in enhanced geothermal systems are described. The role of temperature and pore pressure in wellbore failure and fracture width is considered using analytical and boundary element models. Results indicate that cooling induces a pore pressure drop inside the formation that tends to increase the effective stresses near the wellbore but, it reduces the total stresses in the rock near the well more significantly (decreasing the stress difference, i.e., Mohr’s circle radius). Thus, cooling causes wellbore stability with respect to shear failure and instability with respect to tensile failure. Consequently, one often observes tension cracks whereas compressional wellbore breakouts may be absent. The cooling mechanism is useful in stimulation by cold fluid injection to enhance fracture permeability. Cooling increases joint aperture and the stress intensity at the fracture tip leading to crack growth. As a result, fracture slip may occur leading to permeability enhancement. On the other hand, increased pore pressure in the rock matrix reduces fracture width.