Oil & Gas
Induced seismicity in oil and gas production has been observed ever since the 1930s, i.e., ever since large-scale extraction of fluids occurred. The most famous early instance was in Wilmington, California, where oil production triggered a series of damaging earthquakes. In this instance, the cause of the seismicity was traced to subsidence due to rapid extraction of oil without replacement of fluids. Once this was realized, the oil extraction was balanced with water injection to mitigate the seismicity. Ever since then, the oil and gas industry has adopted these practices not only to mitigate seismicity, but also to mitigate damage to the oil wells in the producing field (wells would be sheared off in the subsurface as subsidence occurred). This is a good example of community and industry interests overlapping to produce a solution.
In the last decade, a number of examples of earthquake activity related to oil and gas production as well as injection of liquids under high pressure have been observed, although not with the serious consequences seen in Wilmington. Almost all induced seismicity associated with petroleum extraction can be traced to either fluid injection or extraction. In some recent cases, injection of produced water (excess water extracted during oil and gas extraction) has produced significant seismic activity. Examples are in Colorado and Texas, where gas and oil production yield large volumes of water that must be put back underground. In some cases, the water cannot be put back exactly where it was produced, and over-pressurization of the water causes induced seismicity. Mitigation can be achieved through abatement and/or redistribution of the fluids to different areas or depths.
This type of seismicity should not be confused with hydrofracturing . As discussed in the previous section on general induced seismicity, there are two main types of induced seismicity, shear and tensile (repeated here for convenience). Almost all of the significant (recorded activity and in some cases felt activity) is associated with the type of failure called shear failure. These types of earthquakes can be very small or large, depending on the geologic environment and available forces to cause an earthquake (the excess water injection type). Another type of induced seismicity is that associated with hydrofracturing. Hydrofracturing is done by injecting fluid into the subsurface to create distinct fractures, in order to to link to existing fractures to create permeability in the subsurface. This is done to to extract in situ fluids (such as oil and gas). Hydrofracturing is distinct from many types of induced seismicity, because hydrofracturing is by definition only created when the forces applied create a tensile fracture, creating a “driven” fracture. Shear failure has been observed associated with hydrofracturing operations, as the fluid leaks off into existing fractures, but due to the very high frequency of the tensile failure (seismic source at the crack tip only), only the associated shear failure is observed by microseismic monitoring . However, hydofracturing is such a small perturbation that it is rarely, if ever, a hazard when it is used to enhance permeability in oil and gas or other types of fluid extraction activities. To our knowledge, hydrofracturing to intentionally create permeability rarely creates unwanted induced seismicity large enough to be detected on the surface with very sensitive sensors, let alone be a hazard or annoyance.
Therefore, oil- and gas-induced seismicity has been dealt with successfully and is well understood. In the last several years, induced seismicity is receiving more attention, not because it is a hazard (although some recent cases in Texas have drawn attention to it), but (as in the case of geothermal) because it can be used to trace the success of inducing permeability.