Poro-Thermoelastic Mechanisms in Wellbore Stability and Reservoir Stimulation

Abstract

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 us-ing
analytical and boundary element models.
Results indicate that cooling induces a pore
pressure drop inside the formation that tends
to increase the e .ective stresses near the well-bore
but, it reduces the total stresses in the
rocknearthe well more significantly (decreas-ing
the stress di .erence, i.e., Mohrís circle ra-dius).
Thus, cooling causes wellbore stabil-ity
with respect to shear failure and insta-bility
with respect to tensile failure. Con-sequently,
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 frac-ture
tip leading to crack growth. As a re-sult,
fracture slip may occur leading to per-meability
enhancement. On the other hand,
increased pore pressure in the rock matrix re-duces
fracture width.