Editor’s note: Professionals in the oil and gas industry often receive questions about how industry operations affect public health, the environment, and the communities in which they operate. Of particular concern today is the impact of hydraulic fracturing on the environment. In this new column, JPT is inviting energy experts to put those questions and concerns about industry operations into perspective. Additional information about the oil and gas industry, how it affects society, and how to explain industry operations and practices to the general public is available on SPE’s Energy4me website at www.energy4me.org.
Can activities related to shale oil and gas production cause earthquakes? Yes. Can these be avoided? Yes, again.
In discussing this, we first need to break the issue down into two buckets. One is the hydraulic fracturing of rock as a method of stimulation. This is necessary because the reservoir being accessed has very low permeability, which is the rock property that allows flow of fluids. Fracturing causes artificial permeability that enables the oil or gas to flow. The second bucket is the injection of wastewater from production operations into rock as a means of disposal. These two operations are quite different as potential contributors to earthquake activity.
The Basics
Earthquakes are generated when there is Earth movement in a fault. A fault is a fracture in the Earth with significant displacement of the rock on either side of the fracture. To cause movement, the human activity would need to pour energy into what is known as an active fault. This is a fault which, when energized, will cause Earth movement. According to the US Geological Survey, the magnitude of an earthquake will be directly proportional to the length of the active fault.
Faults can be detected. Modern 3D seismic imaging can identify the location and lengths of all but the smallest faults. Small faults are not a concern as a source of damaging earthquakes. So the remedy in large part lies in performing these surveys before a fracturing or wastewater disposal operation.
Hydraulic Fracturing
Seismic activity directly associated with stimulation operations has been studied extensively. Thousands of fracturing events have been observed using a technique known as microseismic monitoring. As the name implies, the seismic events are expected to be small compared with earthquakes. The monitoring was originally done to make operations more efficient. It is in the interest of the operator to direct the fracturing energy into the oil or gas bearing layer. Properly designed operations achieve this. The monitoring has shown that the vast majority of fracturing operations generate low intensities, well below 1.0 on the Richter scale. A handful of instances of higher levels of seismic activity have been documented (out of hundreds of thousands of wells fractured over the past 50 years). For example, an event of 2.5 was observed in the United Kingdom. Although this has not been fully explained, it is believed to have been associated with a proximal active fault. This appears to be the case for most of the other known instances as well. Virtually all oil and gas operations conduct 3D seismic monitoring prior to planning the wells. Faults can be, and ought to be, identified and avoided.
Water Disposal Wells
Fracturing fluids returning to the surface usually contain high concentrations of salt and must be disposed of safely. One method is to inject them into what the US Environmental Protection Agency classifies as UIC Class II wells. The water is sequestered in deep formations, preferably porous bodies such as saline aquifers. There is little doubt that some of these wells are implicated in felt earthquakes in Arkansas, Ohio, Texas, and Oklahoma. A few have been of a magnitude up to 5.2. In Arkansas, as well as in Ohio, when the injections were stopped, seismic activity was reduced or stopped. Even smaller earthquakes not causing damage are worrisome for people who have never experienced one. Remedies must be sought and, fortunately, these are available.
The simplest remedy is to just not do it! While somewhat tongue in cheek, this is largely possible by reusing flowback water. Technology now allows for highly saline fluids to be used in fracturing fluid. Consequently, the treatment for reuse can be very economical. In states such as Pennsylvania, where the geology is not suited for disposal wells, this is happening routinely. But even if this were done on a wide scale, disposal would be required at the cessation of all operations. But now the volumes will be substantially less.
Before the design of the disposal well, a seismic survey ought to be conducted and the well not be placed proximal to an active fault. The cost of such a survey would be a fraction of the total well cost. Another approach, with a somewhat higher associated cost, would be to place microseismic monitoring stations at least during the early stages of injection. Injecting liquid into most formations will produce stress. The microseismic monitoring will help to identify a threshold injection rate that will prevent seismic activity. The tools for accomplishing this are readily available.
Earthquakes are very unlikely to be caused directly by fracturing operations, where injection operations typically last a few hours to days. Wastewater disposal wells pose a greater concern because of higher volumes and ongoing injection. However, if wastewater disposal wells are properly designed and placed, the potential for earthquakes is quite small. Keep in mind that there currently are more than 150,000 disposal wells in the US alone. Very few of them are causing incidents. With care, even these can be avoided.
Takeaways
- Earthquakes are generated when there is Earth movement in a fault
- Seismic imaging can identify all but the smallest faults
- Earthquakes are very unlikely to be caused directly by fracturing operations
- Improperly designed and/or placed wastewater disposal wells can produce earthquake activity.
