PROPOSED MINNESOTA WELLBORE RE-ENTRY TO MEASURE STRESS ORIENTATIONS AND GEOPHYSICAL PROPERTIES FOR EARTHQUAKE HAZARD CHARACTERIZATION

The state of stress in the Earth’s crust determines which subset of faults is most likely to produce damaging earthquakes, as well as the style of ground shaking expected to occur in an earthquake. Despite Minnesota’s large population centers, two nuclear power stations, and other critical infrastructure, almost no information is available about the stress field, including the orientation of the maximum horizontal principal stress (S_Hmax) and relative stress magnitudes (faulting regime) throughout the state and most of the surrounding region.

Although Minnesota is within a relatively stable intraplate area, damaging earthquakes can occur even where seismicity rates are low, including due to anthropogenic activities such as deep well injection. Recently, the Minnesota Geological Survey released a new map of major faults and geologic units within the state's crystalline basement, where felt earthquakes are most likely to occur. If information regarding S_Hmax orientation and the style of faulting become available, this information will help determine which mapped faults are most hazardous.

In this presentation, we outline a proposed effort to re-enter inactive boreholes in and surrounding Minnesota for the purpose of identifying stress indicators (borehole breakouts and drilling-induced tensile fractures) and collecting a suite of additional geophysical data. The State of Minnesota maintains a borehole database, which contains many viable targets for re-entry, including 184 boreholes with total depths >1 km. We will use an acoustic televiewer as well as standard wireline logging tools including 3-arm caliper, full-waveform sonic, formation resistivity, natural gamma, and fluid temperature/resistivity. These data will be employed for constraining near-surface seismic velocities for characterizing earthquake ground motions, fracture orientations for identifying subsurface fault populations, fluid content for groundwater studies, and lithologies and other rock properties for numerous applications. In addition to obtaining a suite of S_Hmax orientations for Minnesota, this proposed project will provide benefits for evaluating mineral resources potential, supporting groundwater modeling, and better characterizing seismic hazard and risk.