High-density oilfield wastewater sinks
In some regions of the central United States, oilfield wastewater is characterized by very high dissolved solids concentration. This causes the wastewater to have a much higher density than natural fluids in the seismogenic zone. Our research shows that this may cause the wastewater to sink and locally maintain elevated fluid pressure below injection wells for a decade or more after wastewater injections.
Observations: Mean annual earthquake depth (black circles) systematically increases in three counties within Oklahoma, even after injection rates decline in some areas. We analyzed the USGS National Produced Water Geochemical Database and found that fluids in the deep basement rocks (where earthquakes occur) are characterized by dissolved solids concentrations of 70,000 - 100,000 ppm, while wastewater from oil and gas producing formations in three Oklahoma counties are:
Alfalfa County: ~207,000 ppm
Oklahoma County: ~198,000 ppm
Lincoln County: ~189,000 ppm
Earthquakes do not appear to deepen in the Raton Basin, and we find that wastewater from oil and gas production is ~2,000 ppm in dissolved solids.
We modeled a typical high-rate wastewater injection well to understand how different dissolved solids concentrations between wastewater and host rocks fluids affect fluid pressure in the seismogenic zone.
Our model simulates 10 years of oilfield wastewater disposal and 40 years of post-injection fluid pressure recovery using geologic and fluid characteristics of the Anadarko Shelf in northern Oklahoma.
High-density wastewater displaces lower density host-rock fluids as it sinks. This causes fluid pressure to increase, even after injections cease.
Comparing our model with the earthquake data from Oklahoma, we find that wastewater sinks at the same rate as earthquakes deepen: ~0.5 km per year during injection and ~0.2 km after injection.