Hydraulic stimulation is a technique often used in order to improve the formation permeability near the wellbore and to establish flow connections with the productive zones. In many geothermal reservoirs, due to the role of natural fractures, hydraulic stimulation can enhance the formation permeability either through shear or tensile fracturing. Even under the conditions when tensile propagation dominates, shear failure can play an important role away from the wellbore and in determining fluid leak off in formations with low matrix permeability. Results of stimulation treatments are controlled by in situ stresses, flow and mechanical properties of the formation, properties of the injected material and treatment parameters. Conventional hydraulic fracturing treatments tend to be of short duration, lasting hours to days, and poro-mechanical effects tend to dominate the system behavior. Shear stimulation treatments can be of much longer duration, lasting weeks to months; thus thermo-mechanical effects also become important on these time scales. The locations of the microseismic events can serve as indicators of the zones of enhanced permeability, thus providing vital information for verification of the coupled THM models. The ability to model the coupled thermal-hydrologic-mechanical (THM) processes in fractured geological formations is important in analyzing stimulation treatments. We are developing a general purpose computational code, FEHM, that models coupled THM processes during multi-phase fluid flow and deformation in fractured porous media. The code is control volume – finite element based. The equations representing all three of the physical processes being considered – thermal, hydrologic, and mechanical- are formulated simultaneously in a consistent fashion. Nonlinearities in the equations and the material properties are handled using a full Jacobian Newton-Raphson technique. The code incorporates several models of fracture aperture and stress behavior combined with permeability relationships, including the directional dependence of permeabilities on normal as well as shear stresses. We present examples including permeability enhancement due to shear stimulation using a 3-dimensional model with data published from the Desert Peak reservoir in Nevada, and show sensitivity studies for parameter values in the ranges published for the Brady’s field in Nevada, USA.

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