## Exploration Activity: Numerical Modeling At Coso Geothermal Area (2010)

To determine conditions when fractures nucleate

A numerical model was developed using Poly3D to simulate the distribution and magnitude of stress concentration in the vicinity of the borehole floor, and determine the conditions under which petal-centerline fractures nucleate. As a whole, the simulations have demonstrated that a borehole under the stress boundary conditions present at the Coso 58A-10 borehole is able to amplify the stress concentration to produce tension below the borehole floor consistent with the occurrence of petal-centerline fractures in image logs. In vertical boreholes, the maximum magnitude of _1 below the borehole floor reaches tension within only a very small fraction of the frictionally permissible stress states, corresponding to low mean stress. However, deviation towards SHmax in the stress state relevant to Coso increases the potential for tension below the borehole floor, whereas deviation towards SHmin leads to enhanced compression. These results prove that the occurrence of petal-centerline fractures can be used to constrain principal stress magnitudes and determine faulting regime.

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