**4. Environmental aspects**

Proving the safety of hydraulic fracturing is a welcome side benefit of microseismic surveys. One of the issues fostered by fracturing opponents is that large shale treatments could contaminate aquifers by fracturing into near-surface water supplies. There is, first of all, a very large base of literature that demonstrates that fracture height growth is severely limited by geologic conditions; [26] provides a brief review of some of the pertinent literature. Factors such as stress changes and material property variations across layers, interface characteristics associated with the horizontal bedding, and higher-permeability layers that allow high levels of fluid leakoff are all commonly present in sedimentary basins where oil and gas are being exploited. These features cause fractures to propagate much farther laterally than vertically. There are also theoretical considerations that clearly demonstrate that fracturing volumes are smaller by an order of magnitude or more than the volumes that would be required to propagate fractures the multi-kilometer distances to approach the surface.

others result in upward growth, while others can veer in a different horizontal direction. These faults are easily detectable in the microseismic data because the magnitudes of the events are much larger than the magnitudes of normal reservoir events. Figure 4 shows that faults have only a limited effect on height growth. The behaviour of a hydraulic fracture when it intersects a fault is not an unanswered question. There could be some additional height growth, though

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Microseismic data, which is essentially the monitoring of micro-earthquakes, also provides information to show the extremely small likelihood of damaging seismic activity [27]. Figure 5 shows the maximum microseismic magnitude for all fracture treatments in major North American basins up to mid-year 2011, where each point is the maximum magnitude microse‐ ism obtained in a fracture stage. The largest monitored microseism found in several thousand stages has not exceeded +1.0, which is about 1,000 times less energy than the threshold that

The question of fault interaction comes into play with induced seismicity as well as height growth. Again, the data show what happens when faults are intersected. Most seismicity induced by shale stimulations, although somewhat variable across formations, is in the magnitude range of -4 to -1. Larger events are the direct result of interaction with faults, some of which are seismic and were known to exist, and others were subseismic prior to being marked by microseismicity. The general increase in magnitude that occurs as a result of fault interaction is usually one to two. The Bowland shale and Horn River basin experience suggest

all measurements show that it is limited.

**Figure 4.** Hydraulic fracture height growth in shale basins of North America.

can just be felt at the surface.

With the microseismic monitoring database that has been obtained throughout the last decade, there is now a large amount of information available that shows clearly that fracturing does not propagate the enormous distances required to cause contamination. Figure 4 provides an example of fracture height data from all monitored fractures in six major North American shale basins sorted by depth [26]. This plot shows thousands of fracture stages ordered by depth of the perforations. The fracture tops, as indicated by the shallowest microseism detected on each tests, are shown in red, while the fracture bottoms are green. While fractures occasionally exhibit significant vertical growth (both upward and downward), the distances are small compared to the distance required to approach typical water wells. The formations included in this data set are the Barnett, Marcellus, Woodford, EagleFord, Haynesville, and Muskwa/ Evie.

It is also important to note that many of the "spikes" in the data of Figure 4 are attributed to fault interactions. There are many faults, some well below seismic detection, that can influence the behaviour of the fracture. As can be seen, many of these faults result in downward growth,

**Figure 4.** Hydraulic fracture height growth in shale basins of North America.

The actual source mechanism is a result of the geomechanical processes that occur during fracturing. There is a large perturbation in the stresses around a fracture and a bigger pertur‐ bation in pore pressure as the high pressure fracturing fluid leaks off into the reservoir through the pore space or into natural fractures. These changes alter the existing in situ conditions and impact the behaviour of any slippage or opening that might occur around the fracture. Geomechanical calculations can be useful to understanding these perturbations, and they can also provide improved understanding of the microseismic distribution by assessing the stress

The linkage of geomechanics and source mechanisms should be helpful to understanding the reservoir and how it is impacted by the stimulation. The slippage planes that are activated should have higher permeability and could provide clues about the reservoir itself (e.g., natural

Proving the safety of hydraulic fracturing is a welcome side benefit of microseismic surveys. One of the issues fostered by fracturing opponents is that large shale treatments could contaminate aquifers by fracturing into near-surface water supplies. There is, first of all, a very large base of literature that demonstrates that fracture height growth is severely limited by geologic conditions; [26] provides a brief review of some of the pertinent literature. Factors such as stress changes and material property variations across layers, interface characteristics associated with the horizontal bedding, and higher-permeability layers that allow high levels of fluid leakoff are all commonly present in sedimentary basins where oil and gas are being exploited. These features cause fractures to propagate much farther laterally than vertically. There are also theoretical considerations that clearly demonstrate that fracturing volumes are smaller by an order of magnitude or more than the volumes that would be required to

With the microseismic monitoring database that has been obtained throughout the last decade, there is now a large amount of information available that shows clearly that fracturing does not propagate the enormous distances required to cause contamination. Figure 4 provides an example of fracture height data from all monitored fractures in six major North American shale basins sorted by depth [26]. This plot shows thousands of fracture stages ordered by depth of the perforations. The fracture tops, as indicated by the shallowest microseism detected on each tests, are shown in red, while the fracture bottoms are green. While fractures occasionally exhibit significant vertical growth (both upward and downward), the distances are small compared to the distance required to approach typical water wells. The formations included in this data set are the Barnett, Marcellus, Woodford, EagleFord, Haynesville, and Muskwa/

It is also important to note that many of the "spikes" in the data of Figure 4 are attributed to fault interactions. There are many faults, some well below seismic detection, that can influence the behaviour of the fracture. As can be seen, many of these faults result in downward growth,

fractures) and optimum methods to enhance permeability in the reservoir.

propagate fractures the multi-kilometer distances to approach the surface.

and failure conditions around the fracture [21, 25].

**4. Environmental aspects**

128 Effective and Sustainable Hydraulic Fracturing

Evie.

others result in upward growth, while others can veer in a different horizontal direction. These faults are easily detectable in the microseismic data because the magnitudes of the events are much larger than the magnitudes of normal reservoir events. Figure 4 shows that faults have only a limited effect on height growth. The behaviour of a hydraulic fracture when it intersects a fault is not an unanswered question. There could be some additional height growth, though all measurements show that it is limited.

Microseismic data, which is essentially the monitoring of micro-earthquakes, also provides information to show the extremely small likelihood of damaging seismic activity [27]. Figure 5 shows the maximum microseismic magnitude for all fracture treatments in major North American basins up to mid-year 2011, where each point is the maximum magnitude microse‐ ism obtained in a fracture stage. The largest monitored microseism found in several thousand stages has not exceeded +1.0, which is about 1,000 times less energy than the threshold that can just be felt at the surface.

The question of fault interaction comes into play with induced seismicity as well as height growth. Again, the data show what happens when faults are intersected. Most seismicity induced by shale stimulations, although somewhat variable across formations, is in the magnitude range of -4 to -1. Larger events are the direct result of interaction with faults, some of which are seismic and were known to exist, and others were subseismic prior to being marked by microseismicity. The general increase in magnitude that occurs as a result of fault interaction is usually one to two. The Bowland shale and Horn River basin experience suggest that greater increases are possible, but they must be exceedingly rare given the common experience, as shown in Figure 5.

most treatments are conducted within a fairly limited range, it stands to reason that the largest

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events will generally be found when treating under those conditions.

**Figure 6.** Effect of rate and volume on induced seismicity in three shale basins.

**Figure 5.** Microseismic magnitudes associated with fracturing in North American shale basins.

Although these results show that induced seismicity is not likely to be a problem in hydraulic fracturing, there likely will be rare cases (e.g., Bowland shale, Horn River basin) where the fracturing interacts with a major fault system that is critically stressed. In such a case, it could be possible to experience seismicity that is significant enough to be felt at the surface.

It is always useful to investigate how the seismicity might be mitigated so that development work can continue. Figure 6 addresses the issue of whether rate and volume adjustments might reduce the strength of the seismicity. These results show that the magnitudes generated by hydraulic fracturing, whether fault induced or not, is not generally a function of either rate or volume, for the range considered by this data. Data from three US basins are plotted and it can be seen that there is no definitive trend, except at very low rates and volumes. When consid‐ ering the volumes necessary for effective fracturing of shale resources, the ability to influence the seismicity is very limited.

It should be noted that the maximum that occurs in each plot is most likely because most of the treatments are conducted at those particular rates and volumes. The likelihood of experi‐ encing larger-than-normal events is predicated on the likelihood of intersecting a fault. Because most treatments are conducted within a fairly limited range, it stands to reason that the largest events will generally be found when treating under those conditions.

that greater increases are possible, but they must be exceedingly rare given the common

**Figure 5.** Microseismic magnitudes associated with fracturing in North American shale basins.

Although these results show that induced seismicity is not likely to be a problem in hydraulic fracturing, there likely will be rare cases (e.g., Bowland shale, Horn River basin) where the fracturing interacts with a major fault system that is critically stressed. In such a case, it could

It is always useful to investigate how the seismicity might be mitigated so that development work can continue. Figure 6 addresses the issue of whether rate and volume adjustments might reduce the strength of the seismicity. These results show that the magnitudes generated by hydraulic fracturing, whether fault induced or not, is not generally a function of either rate or volume, for the range considered by this data. Data from three US basins are plotted and it can be seen that there is no definitive trend, except at very low rates and volumes. When consid‐ ering the volumes necessary for effective fracturing of shale resources, the ability to influence

It should be noted that the maximum that occurs in each plot is most likely because most of the treatments are conducted at those particular rates and volumes. The likelihood of experi‐ encing larger-than-normal events is predicated on the likelihood of intersecting a fault. Because

be possible to experience seismicity that is significant enough to be felt at the surface.

experience, as shown in Figure 5.

130 Effective and Sustainable Hydraulic Fracturing

the seismicity is very limited.

**Figure 6.** Effect of rate and volume on induced seismicity in three shale basins.
