Stress Drop Changes with Distance from Disposal Wells in the Fort Worth Basin, Texas

Abstract:

Since 2008, earthquakes in the Fort Worth Basin (FWB) have been induced by the disposal of recovered waste-fluid associated with extraction of unconventional gas. Five larger sequences were well recorded on local seismic networks with the data used to estimate kinematic source properties. Source spectra and associated source parameters including corner frequency, seismic moment and stress drop are estimated using a modified generalized inversion technique (GIT), which is developed for situations where there are no hard-rock sites. As an assessment of the validity of the modified GIT approach, corner frequencies and stress drops from the GIT are compared to estimates using the traditional empirical Green’s function (EGF) method for 11 magnitude 3.0+ events. For these events, corner frequency residuals, calculated as GIT minus EGF, have a mean of 0.18 HZ with a standard deviation of 1.59 Hz. We find consistent mean stress drops from the GIT and EGF methods, 7.96 MPa and 8.01 MPa, respectively. For the total dataset of 89 earthquakes, stress drop estimates using GIT are similar to tectonic intraplate earthquakes with one exception. The Dallas-Fort Worth (DFW) Airport sequence, the first within the basin, exhibit relatively low stress drops that increase with radial distance from the injection point. This unique feature extends to a distance of 1.5 km, which is a region of significantly increased fluid pressure based on a simple pore pressure model. The model suggests that the stress drop variation is inversely proportional to the pore pressure diffusion within the region of high pore pressure perturbation. The DFW sequence exhibits the shortest time between injection and earthquakes with the injection well very near the fault (< 1.5 km). Other sequences show no time or space correlation with modeled pore pressure perturbations but wells were located at 2 km or greater distances in those cases. We conclude that the normal faulting earthquakes within the crystalline basement of the FWB, are largely driven by a perturbation to pre-existing tectonic stresses and only at very near distances within the high fluid pore pressure window does stress drop decrease.