Geomechanical aspects of induced microseismicity during CO2 injection in Illinois Basin

Carbon sequestration activities are increasing in a global effort to mitigate the effects of greenhouse gas emissions on the climate. Injection of wastewater and oil-field fluids is known to induce seismic activity. This makes it important to understand how that risk relates to CO2 injection. Injection of supercritical CO2 into the Cambrian Mt. Simon sandstone in Illinois Basin induced microseismicity that was observed below the reservoir, primarily in the Precambrian crystalline basement. Geomechanical and flow properties of rock samples from the involved formations were measured in the laboratory and compared with geophysical log data and petrographic analysis. The controlling factors for induced microseismicity in the basement seem to be the hydraulic connection between the reservoir and basement rock and reactivation of pre-existing faults or fractures in the basement. Additionally, the presence of a laterally continuous low-permeability layer between reservoir and basement may have prevented downward migration of pore pressure and reactivation of critically stressed planes of weakness in the basement. Results of the geomechanical characterization of this intermediate layer indicate that it may act as an effective barrier for fluid penetration into the basement and that induced microseismicity is likely to be controlled by the pre-existing system of faults. This is because the intact material is not expected to fail under the reservoir stress conditions.

Velocity and azimuthal anisotropy structure underneath the Reelfoot Rift region from Rayleigh wave phase velocity dispersion curves

SUMMARY Phase velocity and azimuthal anisotropy maps for fundamental mode Rayleigh waves are determined for a portion of the central United States including the seismically active Reelfoot Rift (RFR) and the enigmatic Illinois Basin. Dense seismic array installations of the Northern Embayment Lithosphere Experiment, the EarthScope transportable array and the Ozarks Illinois Indiana Kentucky array allow a detailed investigation of phase velocity and anisotropy in a broad period range (20–100s).We obtain more than 12 000 well-constrained, unique two-station paths from teleseismic events. The two-station method is used to determine dispersion curves and these are inverted for isotropic phase velocity maps and azimuthal anisotropy maps for each period. The presence of fast phase velocities at lower crustal and uppermost mantle depths is found below the RFR, and Ste. Genevieve and Wabash Valley fault zones. At periods of 30s and higher, the RFR is underlain by slow phase velocities and is flanked to the NW and SE by regions of fast velocity. Fast phase velocities are present below the centre of the Illinois Basin in the period range 75–100s. Anisotropy fast axis orientations display complex patterns for each period and do not trend parallel to the direction of absolute plate motion. Anisotropy fast directions are consistently parallel to the trend of the RFR from 50s to higher periods, suggesting the presence of either frozen-in anisotropic fabric or fabric related to material transport from a recently discovered, pronounced low velocity zone below the Mississippi Embayment.

Stress Analysis Using Direct-S Wavelets Produced by a Vertical Vibrator

We demonstrate how to extract the azimuth of maximum horizontal stress (Shmax) in deep rocks by doing a simple, 360-degree, mathematical rotation of a down-going, direct-S, wavelet generated at the base plate of a surface-based vertical vibrator. We worked with direct-S wavelets that travel through stressed rocks to a deep, horizontal, VSP geophone. We show that the azimuth where a polarity reversal occurs in mathematical rotations of this down-going direct-S wavelet defines the azimuth of Shmax in the rocks between the receiver and the surface source. We tested this direct-S wavelet rotation method for determining Sh-max azimuth at a site in the Illinois Basin using legacy VSP data acquired in 2013. SHmax azimuths indicated by this simple wavelet-rotation method were determined when vertical vibrators were stationed at zero-offset, at far-offset, and at different azimuths around a VSP receiver well. These VSP-based SHmax azimuths agreed with the azimuth of SHmax found by traditional mini-frack tests in the VSP receiver well. This simple VSP procedure seems to not be discussed in geophysical literature. The publication of this technical finding should be of interest to the worldwide geophysical community, especially to those who need to monitor how stress fields shift when fluids are injected into, or extracted from, deep porous reservoirs.