State of stress in the Illinois Basin and constraints on inducing failure

The stress regime in the Illinois Basin was investigated to assess how the rock column might respond to the injection of fluids, including coproduced formation brines and supercritical CO2.This response is a concern because injection practices could increase pore fluid pressure and potentially induce seismicity. Data were collected to determine the magnitude and orientation of a three-component stress field: vertical stress (Sv) and minimum (Sh) and maximum (SH) horizontal stresses. The Sv was evaluated with a six-layer lithostratigraphic column. A two-layer pressure–depth Sv model was generated for the central part of the basin, and a single pressure gradient model was constructed for the surrounding region. In the central part of the basin, the Sv gradient is 24.9 MPa/km (1.11 psi/ft) to a depth of 2134 m (7000 ft), followed by a gradient of 27.1 MPa/km (1.20 psi/ft) below 2134 m (7000 ft). For the area surrounding the deep basin, the Sv gradient was 25.5 MPa/km (1.13 psi/ft). The Sh was evaluated from multiple data sources, primarily hydraulic fracture records or extended leak-off tests. The Sh gradient calculations ranged from 24.1 to 27.3 MPa/km (1.07 to 1.21 psi/ft). The Sh values for the basal Paleozoic clastic units are lower than those for units in the overlying horizons. The SH was based on a critically stressed model yielding values between 40.0 and 82.6 MPa/km (1.77 to 3.65 psi/ft). Stress orientation data for the Illinois Basin were collected from multiple sources. The orientation of SH across the study area is relatively uniform in strike at approximately N60°E. Marked deviations in SH result from localized structural discontinuities.