Geomechanics of Fluid Injection

As with many issues surrounding the oil and gas industry, the dramatic uptick in both hydraulic fracturing and seismic activity through the first 15 years of the twenty-first century generated plenty of media swirl. Although many of the earthquakes have magnitudes of less than 3 and most are not felt by humans, some high-profile incidents geographically located near oil and gas operations catalyzed the media and public to question the relationship between these occurrences. Many concluded that hydraulic fracturing causes earthquakes, but as with all trends, correlation does not equal causation. The main question remains: does hydraulic fracturing cause earthquakes? And if so, precisely how does that happen?

As noted in the previous lesson, earthquakes happen mainly at plate boundaries and less frequently within plates. The earthquakes associated with oil and gas operations usually fall into the latter category, the most salient examples being in north Texas and Oklahoma.

Human activity, such as oil and gas operations, can activate or reactivate pre-existing intraplate faults. Contrary to popular belief, however, the activity most closely associated with such activation is not usually stimulation (hydraulic fracturing) or oil and gas extraction, but rather the consequent wastewater disposal. Production operations from most oil and gas wells, not just those that have been hydraulically fractured, produce wastewater, which then must be managed. The most cost-effective method to manage this wastewater is often reinjection, but recent experience has brought this practice into question because of increased earthquakes in the vicinity of a small fraction of the thousands of active disposal wells.  The question then becomes, why do only certain disposal wells seem to be associated with earthquakes, and what are the key factors that could be examined to predict this occurrence.

Faults within the Earth’s crust exist at various levels of likelihood of slipping, and those poised to slip are called “critically stressed”, owing to several factors: the ambient stress level in the earth’s crust, the fault’s orientation relative to the Earth’s principal stress directions, and the pore pressure of the fluids in the rocks around the fault. The stress level in the crust and the orientation of the fault relative to those stresses control how much shear stress is acting on the fault plane, driving it toward slip, as well as how much normal stress is squeezing the fault planes together, creating frictional resistance to slip.  The pore pressure in the rocks around the fault can counteract the normal stresses to reduce frictional resistance to sliding.  Even if a fault is initially stable, where the shear stress applied is effectively counter-balanced by frictional resistance, an increase in pore pressure along a fault, for example as a result of adding large volumes of wastewater to a formation, could decrease the fault’s natural resistance to slipping, causing an earthquake.

The more information petroleum geologists have about the locations of existing faults, their size and their likelihood of slipping, the better they will be able to avoid injecting wastewater in their proximity.

One of our experts expands on the explanation here on the geomechanics of liquids at subsurface faults and how human activity may affect seismic activity.

Transcript

Geomechanics of Fluid Injection – Jon Olson – The University of Texas at Austin

It’s very important that the oil and gas community, whether it be industry personnel, regulatory personnel or academics, is attuned to any environmental issues related to operations. New technologies, approaches and techniques may create new concerns which must be addressed through research, innovative solutions and possible changes in regulations. A recent example of this dynamic is a concern over earthquakes and their potential relationship to oil and gas operations. The debate has focused on whether the recent upswing in earthquake activity in places like Oklahoma and North Texas is a result of oil and gas activity. If that is true, what specific activities are linked to the earthquakes, and what changes can be made to reduce the likelihood of these earthquakes?

Recent research on the issue of induced seismicity suggests that under certain conditions waste water injection into deep subsurface rock formations can increase the likelihood of earthquakes. This is a complex geomechanical process, so let’s review some facts about oil and gas operations before we examine the controlling factors that are thought to impact earthquake risk.

Oil and gas production often has water associated with it. Depending on the geology, the amount of water could be very significant. Because this water typically comes from the deep subsurface, it has a very high salinity that makes it unusable for human consumption or other use without applying expensive desalination procedures. Therefore, the industry treats it as a waste and re-injects it deep underground. Underground disposal of produced water is not new. Many states have hundreds to thousands of these wells. What’s different now that could be responsible for the link between some waste water injection and earthquakes? It turns out that there have been suspected induced earthquakes related to oil and gas activity in the past, but they have not been very common and occurred mostly in sparsely populated areas.

The shale gas and tight oil boom that began in the early 2000’s greatly increased the rate of waste water injection in places like Oklahoma and North Texas because of the increase in water production associated with unconventional oil and gas. But injection does not cause an earthquake at every well. As a matter of fact, it’s an extremely low percentage of wells that are associated with seismicity. Most of these induced earthquakes are magnitude three or less on the Richter scale, which means most are barely strong enough for humans to detect. What is it about certain injection wells that results in an increase in seismicity in a particular area?

The first factor controlling earthquake risk around an injection well is its proximity to a preexisting fault. Researchers agree that induced seismicity is most certainly the reactivation of existing fault planes. If we plan our injection wells away from those existing faults, risk can be reduced. Given the presence of a nearby fault, we need to assess the mechanical state of that fault. In particular, how much natural shear stress is being exerted by the earth to make the fault want to slip, and how much natural frictional resistance is present to prevent slip from happening? If a fault is what we call critically stressed, only a small amount of perturbation to the mechanical state can cause slip to occur. Whether a fault is critically stressed will depend on the fault’s orientation relative to the earth’s principle stress directions, the magnitude of the different stresses and the pore pressure acting on the fault. In an area like the Fort Worth Basin in Texas, there was no measurable seismicity for decades. Then all of a sudden earthquakes began to occur, albeit small ones, less than magnitude three for the most part. What happened?

As I mentioned earlier, waste water injection increased tremendously as the shale gas boom erupted in the Barnett Shale. The produced water had to be injected and most of that was sent to a very deep carbonate sedimentary formation called the Ellenburger. There are preexisting faults in the Ellenburger and in the rocks below it. The increase in pore pressure caused by injection actually weakens the frictional resistance of the faults in the subsurface without reducing the natural shear stresses that are trying to make the fault slip. If the shear resistance or strength of the fault is reduced too much, the fault will slip. Let’s say an injection well causes pore pressure that induces slip on a preexisting fault. Does a nearby town experience any earthquake? Not necessarily. The shaking caused by an earthquake is roughly proportional to its magnitude, which in turn depends on the size of the fault that slips, the stiffness of the rock and the initial natural shear stress.

For an earthquake to be felt, we need a magnitude of about three, which corresponds to about three millimeters of slip on a fault that has an area of 300 meters by 300 meters. The biggest Fort Worth Basin earthquake suspected to be induced was a magnitude four, corresponding to a fault a kilometer long and a kilometer high slipping by 10 centimeters. How do these details translate to assessing risk? The larger the fault, the more rare it is, but the easier it is to locate using modern geophysical methods. Therefore, if we drill enough injection wells without paying attention to the geology underground, we are likely to put a few wells near big faults. If we inject a large volume at high enough pressure, we might get an earthquake. However, if we explore our surroundings more thoroughly before drilling, we increase our ability to locate the big faults and avoid injecting close to them.

One last factor to consider is depth below the ground surface. The earth stresses increase as we go deeper, including the shear stresses applied to the faults. If we inject in shallow formations, say less than a few thousand vertical feet, the stresses on the faults probably aren’t high enough to cause significant earthquakes. However, in order to guarantee the fluids we inject don’t come back to the surface, we don’t like to inject shallow. The benefit is that when we inject deep, we are more confident that our waste water is completely isolated from the near surface fresh water resources. The concern with injecting deep is that the shear stresses in this environment are higher, and the weakening effect of the injection pressure can reach big faults in the basin. This is what is suspected to have happened in Ohio and Oklahoma, where earthquakes on the order of magnitude five are thought to be related to recent injection.

Now, what about hydraulic fracturing and induced earthquakes? Hydraulic fracture treatments are high rate, high pressure injections, but they are short-lived lasting on average a few hours. On the other hand, waste water injection operations, which can also be high rate and high pressure, can operate intermittently or continuously for months or years or even decades. The volumes injected for waste water disposal are orders of magnitude larger than those in hydraulic fracturing. Consequently, only a very small fraction of suspected felt induced earthquakes in the United States have been attributed to hydraulic fracturing. The more common activity associated with hydraulic fracturing is what we call microseismicity, which is of magnitude one or less. A typical microseismic event is comparable to the energy released by knocking a gallon jug of milk off the kitchen table. These micro-earthquakes have such small energy that we have to put our seismometers deep underground in nearby wells to even be able to detect the evidence of slip events, usually fractures that are a few meters on a side slipping by a fraction of a millimeter.

When researchers are trying to determine whether seismicity is induced, they start by looking for correlations in time and space. Did the earthquake happen after or during significant injection of waste water into the subsurface, and is the earthquake close to an injection well? If the answer to these questions is yes, then further work can be done using physics-based modeling of fluid flow and fault slip mechanics. Oftentimes this sophisticated modeling approach confirms the plausibility of an earthquake being induced, but sometimes it suggests otherwise. The shortcomings in our ability to confidently interpret every earthquake may be related to a poor understanding of the occurrence of preexisting faults underground, a need for better physics in our models or a poor understanding of the frequency and location of natural seismicity. These challenges are being tackled by researchers in academia and industry. Increased deployment of seismic stations will help better locate and describe earthquakes. Further geologic mapping and theoretical investigations of earthquake mechanics will help establish a better baseline understanding of the world beneath us.

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