Author: Mark Zoback
While the devastation accompanying the magnitude-9.0 Tohoku earthquake that occurred off the coast of Japan on March 11, 2011 still captures attention worldwide, the relatively stable interior of the U.S. was struck by a somewhat surprising number of small-to-moderate earthquakes last year. Most were natural events, the types of earthquakes that occur from time to time in all intraplate regions. Nevertheless, a number appear to be associated with the disposal of wastewater, at least in part related to natural gas production.
The occurrence of injection-related earthquakes is understandably of concern to the public, government regulators, policymakers, and industry alike. Yet it is important to recognize that with proper planning, monitoring, and response, they can be managed and their risk reduced.
No earthquake triggered by fluid injection has ever caused serious injury or significant damage. Approximately 140,000 wastewater disposal wells have been operating safely in the U.S. for many decades.
That said, we have known for over 40 years that earthquakes can be triggered by fluid injection. The first well-studied cases were earthquakes triggered by waste disposal at the Rocky Mountain arsenal in the early 1960s.
Increasing pore pressure in rock by injecting fluids reduces the stress clamping faults shut which prevents fault slip, thus allowing elastic energy to be released in earthquakes. These earthquakes would someday have occurred anyway as a result of slowly accumulating forces in the earth resulting from natural geologic processes — injection just speeds up the process.
The concern about triggered seismicity associated with shale gas development arises after hydraulic fracturing, when wastewater that flows back out of the wells is disposed of at dedicated injection wells. Five steps can be taken to reduce the risks of triggering seismicity whenever we inject fluid into the subsurface
Step 1: Avoid Injection into Active Faults
Earthquakes occur nearly everywhere as a result of natural geologic processes, even in continental interiors Modern 3-D seismic imaging methods can identify faults capable of producing earthquakes. Faults large enough to produce earthquakes above magnitude 6.0 should be easily detectable as part of geologic characterization studies of potential injection sites because they are many tens of kilometers in size. We also know a lot about the relationship between the orientation of potentially active faults and the ambient stress field in a given region from basic principles of structural geology and rock mechanics. This enables us to identify potentially problematic faults prior to injection.
Step 2: Minimize Pore Pressure Changes at Depth
Rocks in the upper part of Earth’s crust contain pre-existing pore space, fractures, and flaws. These void spaces are normally filled with freshwater near Earth’s surface (in approximately the first 1 kilometer) and filled with saline brines at greater depths. Injecting fluids into the subsurface will increase the pressure in these voids, depending on the rate at which it is injected and the volume of pore space available to accommodate the injected fluids. It should be noted that injection always occurs at depths where the injected fluids are isolated from near-surface water supplies.
To minimize the potential for seismicity, it is advantageous to minimize the pore pressure perturbations associated with injection. The best way to accomplish this is to minimize the injected volume of fluid. Another way is to utilize highly permeable regional saline aquifers to dispose of wastewater. Alternatively, weak, poorly cemented, and highly permeable sandstone formations would also be ideal for injection. Inevitably, cases will arise where well-cemented, less permeable, and more brittle formations must be used for injection. In those cases, care must be taken to avoid large pore pressure changes.
Step 3: Install Local Seismic Monitoring Arrays
Because smaller faults can escape detection, seismic monitoring arrays should be deployed in the vicinity of injection wells when there is concern that injection might trigger seismicity. By supplementing regional networks with local seismic arrays near injection wells, accurate locations of earthquakes that might be triggered by injection can be used to determine the locations and orientations of the causative faults. Although small faults cannot cause large earthquakes, even small earthquakes felt by the public will be a cause for concern and should be monitored.
Step 4: Establish Modification Protocols in Advance
Following precedents established for development of enhanced geothermal systems, operators and regulators should jointly establish protocols for injection sites in areas where there is concern about triggering seismicity. These protocols are sometimes referred to as “traffic light” systems.
Green means go: Once operational protocols and local seismic networks are in place and injection begins at agreed-upon rates, operators would have a green light to continue unless earthquakes begin to occur. The occurrence of seismicity would be a yellow light. Operators would slow injection rates and study the relationship between the seismicity and injection. Should seismicity cease, operations could potentially continue at reduced injection rates. In fact, it was demonstrated 40 years ago at Rangely that earthquakes could be turned on and off by modulating the injection rate. With such protocols in place, the response to triggered seismicity is pre-defined.
Step 5: Be Prepared to Alter Plans or Abandon Wells
In the same way that it’s important to plan for the possibility of triggered seismicity in advance, we have to be prepared to reduce injection rates, or even abandon wells if triggered seismicity cannot be stopped. That would be the red traffic light: Seismicity has been detected that appears to be associated with a fault potentially capable of producing a moderate-sized earthquake. In the Barnett Shale in Texas near the Dallas-Fort Worth metro area in 2008, the seismicity abated once injection in the problematic wells was terminated.
Overall, it is important for the public to recognize that the risks posed by injection of wastewater are extremely low. In addition, the risks can be minimized further through proper planning, careful monitoring, and taking a proactive response if triggered seismicity does occur.
This article is redacted from a previously published article in EARTH magazine. Republished by permission.