Numerical Modeling of the Nucleation Conditions of Petal-Centerline Fractures below a Borehole Floor, A Sensitivity Study and Application to the Coso Geothermal Field

Abstract

A boundary element model using Poly3D© has been developed to investigate the conditions in which the stress concentration below the floor of a borehole can cause tensile stress necessary to nucleate petal-centerline fractures. The remote stress state, borehole geometry, and traction boundary conditions on the borehole surface are taken from direct measurements in the 58A-10 borehole of the Coso Geothermal Field, CA in which the occurrence of petal-centerline fractures are confirmed by image logs. A sensitivity analysis revealed that outward directed traction boundary conditions on the borehole surface including weight on bit and excess drilling mud pressure suppress tensile stress below the borehole. Simulations of the potential for tension below the borehole floor due to remote stresses limited by the frictional strength of rock in the Earth’s brittle crust reveal that petal-centerline fractures in vertical boreholes are favored in normal faulting environments. However, this behavior is highly sensitive to borehole deviation and deviation direction such that increased deviation towards SHmax increases tension by more than an order of magnitude over the vertical borehole in the stress state at the Coso Geothermal Field. These results demonstrate that the occurrence of petal-centerline fractures in cores or image logs of borehole walls constrain both the principal stress directions and to a lesser extent the magnitude of stresses in the shallow Crust.