| Title | Validation of a Coupled Thermal-Hydrological-Mechanical Model Through a Comparative Study of Shear Stimulation in Two Geothermal Environments: USA and New Zealand |
|---|---|
| Authors | David Dempsey, Jonathon Clearwater, Sharad Kelkar and Irene Wallis |
| Year | 2013 |
| Conference | New Zealand Geothermal Workshop |
| Keywords | injectivity, stimulation, numerical model, coupled flow-stress, permeability |
| Abstract | Injection of cold water has been demonstrated to improve injectivity at Desert Peak EGS well 27-15, Nevada, as well as numerous wells at Ngatamariki and other New Zealand geothermal fields. The injectivity index (....) of a well is generally observed to evolve according to a power law. i.e., .... å ...., where the exponent .. ranges between 0.3 and 0.7. Conceptually, injectivity gain is attributed to the creation of a stimulated volume of either shear-enhanced fracture permeability or thermal contraction of matrix rock in a confined aquifer. In earlier work, a coupled thermo-hydro-mechanical model was constructed for the 2010 Desert Peak EGS shear stimulation. Here, we further validate that model through its application to a 30-day cold-water injection stimulation of a well in the Ngatamariki geothermal field. The model calculates anisotropic permeability enhancement as a function of thermal and pressure induced Mohr-Coulomb shear failure; only the reactivation of existing fractures is considered. not the creation of new ones. Applied to Desert Peak, the model matched an observed 15-fold increase in .... with .. = 0.33 and suggested that a spherical stimulation zone had been created. In this study we extend the model to conditions appropriate for the Ngatamariki field. Both fields are located within an extensional stress regime; however, Ngatamariki exhibits much higher initial permeability, greater fracture density and more anisotropy in fracture orientations than observed at Desert Peak. In contrast to Desert Peak, during the stimulation of a Ngatamariki well, an 8-fold increase in injectivity was observed with .. .. 0.62. Comparisons between such disparate datasets improve our understanding of injectivity evolution and its connection to changes in the reservoir. For example, a model for the Ngatamariki stimulation indicates that the damaged zone is more planar than at Desert Peak; this may represent stimulation of a single fracture or narrow fracture zone. More generally, sensitivity analysis indicates that Desert Peak and Ngatamariki represent two endmembers of a more general relationship between .. and stimulation geometry. |