On shaky ground: understanding the correlation between induced seismicity and wastewater injection in the Fort Worth basin
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Starting in the mid-2000s, there has been an increase in seismic activity around areas where wastewater injection was occurring in association with produced water from oil and gas development. As the rate of wastewater injection increased, so did occurrences of earthquakes in the surrounding area. There are, however, many injectors that don’t spatially correlate with earthquakes, and the boundaries between safe and high risk practice have yet to be defined. The goal of the study is to understand both positive and negative correlations between seismic events and injection well locations by conducting fluid flow simulations and geomehcanical analysis. In order to identify areas of injection with and without seismic activity, a reservoir simulation model was utilized using the Computer Modeling Group (CMG) Implicit Explicit Black Oil (IMEX) simulator for the Fort Worth Basin (FWB), including 374 wells with relevant data located in the following counties: Denton, Ellis, Erath, Hill, Hood, Jack, Johnson, Palo Pinto, Parker, Somervell, Tarrant and Wise. The model integrated formation and injection data. The formation data incorporated includes the formation thickness and permeability; the injection data incorporated includes injection volume, time, and depth. Most of the high volume injection activities occurred in the Ellenburger formation, hence in depth analysis of the results for the Ellenburger formation was performed. Simulation results showed that there are spatial and temporal correlations between earthquake events and areas of pore pressure increase in the Ellenburger. However, not all areas with elevated pore pressure are correlated with seismic activity. Geomechanical analysis was performed by taking the pore pressure change results from the flow modeling and the in-situ stress data available in the literature as inputs. Earthquakes induced by wastewater injection would be predicted to occur in area where the simulated pore pressure increase met or exceeded the minimum frictional resistance of optimally oriented fault surface, given the absence of mapped faults in the area. The geomechanical strength limit for the location one of the earthquake events of interest, the DFW airport earthquakes, could be reached, however, by a 10x permeability reduction in the estimated permeability for the injection into the Ellenburger. This research highlight the need to consider pore pressure changes, more precise earthquake locations, better estimate of the permeability values in the injection horizon, and a more accurate stress state and geomechanical strength criteria to reduce modeling uncertainty in order to better understand the correlation between induced seismicity and wastewater injection.