Lessons from Fort Worth Basin Induced Earthquake Sequences

The Fort Worth basin (FWB) in north Texas has experienced over 30 magnitude (M) 3.0+ earthquakes and one M4.0 since 2008. Events have been linked to disposal of produced wastewater into the Ellenburger formation, which is in hydrogeologic fault contact with the crystalline basement. Here, seismicity rates have decreased as wastewater injection has decreased. This presentation provides an overview of the current research regarding induced earthquakes in the FWB, a discussion of outstanding research questions, and information to place the FWB results in context with the now higher seismicity rates in south and west Texas.

Understanding compartmentalization, anisotropy, and local or long-distance pore fluid pressure and/or poroelastic driven stress changes remains a target of 3D geomechanical modeling and shear-wave splitting studies in the FWB. Seismicity rates have significantly decreased since peak injection volumes in 2012, but earthquakes continue to occur into 2019. The spatio-temporal history of the now nine identified earthquake sequences relative to wastewater injection data varies. Earthquakes have occurred on northeast-southwest trending basement normal faults and to a more limited extent, within the overlying Ellenburger formation. The seismogenic faults do not appear unique relative to other similarly oriented faults in the system imaged using 2D and 3D seismic reflection data, and some seismogenic faults do not have very large fault slip potential, assuming a 1 MPa regional pressure increase. The cumulative history of wastewater injection and fluid production within the Ellenburger, and the evolution of pressure in the injection formation and units in hydrogeological contact, appear to be key drivers of the seismogenic evolution of the basin.

In 2019, the highest seismicity rates in Texas are spatially correlated to the Delaware sub-basin of the Permian basin and to the Eagle Ford shale play. Attempts to provide improved subsurface fault maps and 3D geological models are ongoing, earthquake catalog accuracy has vastly improved due to the Texas Seismic Network, and more detailed stress information is becoming available. These key datasets, combined with more timely and detailed information on production and disposal operations, can help push forward research in other regions of Texas.