Time-lapse monitoring of stress-field variations within the Lower Permian shales in Kansas

Abstract

Azimuthal stress-field variations in Permian shale overlying voids in salt appear related to localized void roof failure based on interpretations of time-lapse recordings of passive seismic surface-wave data. Salt-dissolution voids in south-central Kansas exist within the Hutchinson Salt Member as a result of solution mining operations early in the 20th century. Since shear-wave velocity can be estimated from surface-wave analysis and is directly related to the shear modulus, or material rigidity, it can be used to indicate shale cap-rock competency. Temporal bulk-velocity variations in bedrock acquired over a known void from 2013 to 2018 are consistent with cyclic stress-field behavior expected with repetitive localized failure within the shale cap rock. It was determined that roof failure and associated subsidence began prior to the 2013 surveys along a north–south failure plane with consistent progression from southwest to northeast until 2017. A low-velocity anomaly observed in 2014–2015 on both profiles indicated a load redistribution in the shale cap rock. This low-velocity region was gradually replaced by an increasing velocity field in subsequent years. These bulk-velocity variations are interpreted as representations of stoping, which occurs when overlying roof layers or sidewall segments progressively fail. As of 2018, the velocity trend in the affected area remained lower than the recorded baseline conditions, with no low-velocity anomalies observed since 2015. Shear-wave velocities are not likely to return to 2013 conditions due to loss of roof material and redistribution of load. Annual surveys will monitor for conditions consistent with the beginning of a stress buildup and roof collapse unless monitoring indicates a change in the subsurface that warrants invasive analysis or other actions.