**2. Microseismic applications**

There are many case studies in the literature that illustrate how microseismicity can be used to aid in the exploitation of unconventional reservoirs. One very evident one was provided by Mayerhofer et al. [22] for a two-well, multi-stage, multi-perforation-clusters completion in the Marcellus. Figure 1 shows a plan view and side view of the microseismic data color coded by the well being stimulated. In these views, there is enough information to decide if the well trajectory is correct (assuming transverse fractures are desired), if the number of stages is sufficient to access all of the reservoir, if the number and spacing of perf clusters is giving the desired behaviour, if the treatment fluids, rates, and volumes are generating appropriate lengths without causing excessive height growth, and many other more subtle aspects of completion. This example shows the type of information that one should expect to obtain in such a monitoring project.

**Figure 1.** Example Marcellus microseismic maps for two adjacent wells.

While most interest about microseismicity tends to be focused within final dimensions of the fracture, the growth patterns often provide valuable information for designing fracture treatments. Many treatments show extremely rapid initial growth in either height or length,

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**Figure 1.** Example Marcellus microseismic maps for two adjacent wells.

Nevertheless, there have been several validation experiments where other measurement technologies have been used to verify the accuracy and interpretation of microseismicity, and these have been very helpful in promoting an understanding of the process of microseismic activation during a fracturing treatment. The most comprehensive of these tests was the M-Site test funded by GRI and DOE; it was developed as a fracture diagnostics laboratory in the Piceance basin of Colorado [5-8]. Intersection wells, downhole tiltmeters, tracers, pressure interference, and other technologies were used to show the accuracy of determining the fracture azimuth, length, and height by these methods in typical sandstone reservoir rocks. In these tests, it became clear that microseismicity does not necessarily occur on the hydraulic fracture, but can develop along planes of weakness at an offset distance that depends on both

While there have been no published tests about fracturing in shale reservoirs that provide the full detail available from M-Site, the project described by Fisher et al. [12] in the Barnett shale has many of the same elements as M-Site. Both downhole and surface tiltmeters were used to supplement the microseismic data, and numerous offset producing wells were used to monitor the movement of fracturing fluid during the treatment. Wells that were "bashed" (i.e., loaded up with fracturing fluids) provided direct evidence of actual fluid presence at that location that could be compared to the microseismicity. This comprehensive test verified the actual

With a reasonable level of accuracy and interpretability established by validation tests, such as those described, microseismicity can be used for field development, completion design, stimulation optimization, and addressing environmental concerns. The last aspect, with respect to aquifers and seismicity, is very important for current unconventional reservoir

There are many case studies in the literature that illustrate how microseismicity can be used to aid in the exploitation of unconventional reservoirs. One very evident one was provided by Mayerhofer et al. [22] for a two-well, multi-stage, multi-perforation-clusters completion in the Marcellus. Figure 1 shows a plan view and side view of the microseismic data color coded by the well being stimulated. In these views, there is enough information to decide if the well trajectory is correct (assuming transverse fractures are desired), if the number of stages is sufficient to access all of the reservoir, if the number and spacing of perf clusters is giving the desired behaviour, if the treatment fluids, rates, and volumes are generating appropriate lengths without causing excessive height growth, and many other more subtle aspects of completion. This example shows the type of information that one should expect to obtain in

the formation and the treatment.

124 Effective and Sustainable Hydraulic Fracturing

formation of a "network" in this reservoir.

development throughout the world.

**2. Microseismic applications**

such a monitoring project.

While most interest about microseismicity tends to be focused within final dimensions of the fracture, the growth patterns often provide valuable information for designing fracture treatments. Many treatments show extremely rapid initial growth in either height or length, followed by a highly reduced late-time development. Figure 2 shows an example of length development as a function of time, with each side of the y axis representing one wing of a planar fracture. The bounding dashed line is square-root-of-time behaviour, which is very common and would suggest high leakoff conditions, such as into natural fractures (e.g., [23]). The color coding represents tip-related events (green) and interior events (red). Generally, half or more of the microseismic events occur after the tip has passed the event location, again suggesting natural fracture interactions [15,21] as the source of much of the microseismicity.

actions that jeopardize the treatment. The remote likelihood that source mechanisms can be used to evaluate the hydraulic fracture behaviour (other than dimensions from the event locations) can be easily understood in terms of both energy and volumetric considerations. The total microseismic energy released (or at least what can be detected with current instru‐ mentation) is typically on the order of one millionth or less of both the energy input into the treatment and the strain energy that would be calculated for the fracture based on microseismic dimensions and measured pressures [21]. Similarly, the volumes associated with the sum total of the microseismic displacements are generally on the order of a few liters or less compared to hundreds or thousands of cubic meters of fluid injected. This small volume cannot be

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Figure 3 shows a plot of the total seismic energy associated with microseismicity in a fracture as a function of the largest event and the "b" value. The b value is the negative slope of the Gutenberg-Richter frequency distribution for earthquakes in a region over some time period. For earthquakes, it is usually near 1.0. For microseisms associated with hydraulic fracturing, it is quite variable and often between 1.0 and 2.0. Given a b value, maximum magnitude event, and low end cutoff (in this case magnitude -4), the energy released can be found by integrating the energy as a function of magnitude over the distribution. For the overwhelming majority of treatments, the maximum magnitude is less than 0 (and often much less than 0), so the typical energy released is on the order of hundreds of kilojoules or less. Fracture injections in shale

stimulations usually imparts hundreds of millions kilojoules of energy.

**Figure 3.** Seismic energy released as a function of magnitude and b value.

representative of SRV or other fracture parameters.

**Figure 2.** Fracture length development versus time and conditions.

#### **3. Beyond dots, or beyond verification**

It is well-understood that microseismicity is a scaled-down version of conventional seismicity and tools from earthquake seismology should be applicable in some sense for evaluating microseismic behaviour [16]. Certainly, the fault plane solutions that can be derived from a moment tensor inversion provide some information about the planes that are activated during fracturing. Unfortunately, there is no validation that such information can be taken much beyond a resolution of the fault planes, nor is it necessarily clear how the fault planes are being activated (stress effects, leakoff, actual tip extension processes, etc.).

To suggest that any change in behaviour of the source mechanism, such as a difference between pure shear and a large volumetric component, is somehow diagnostic of fracture behaviour is pure hypothesization without any supporting field, lab, or theoretical results. This type of theorizing is useless, and possibly deleterious, without validation because it could lead to actions that jeopardize the treatment. The remote likelihood that source mechanisms can be used to evaluate the hydraulic fracture behaviour (other than dimensions from the event locations) can be easily understood in terms of both energy and volumetric considerations. The total microseismic energy released (or at least what can be detected with current instru‐ mentation) is typically on the order of one millionth or less of both the energy input into the treatment and the strain energy that would be calculated for the fracture based on microseismic dimensions and measured pressures [21]. Similarly, the volumes associated with the sum total of the microseismic displacements are generally on the order of a few liters or less compared to hundreds or thousands of cubic meters of fluid injected. This small volume cannot be representative of SRV or other fracture parameters.

followed by a highly reduced late-time development. Figure 2 shows an example of length development as a function of time, with each side of the y axis representing one wing of a planar fracture. The bounding dashed line is square-root-of-time behaviour, which is very common and would suggest high leakoff conditions, such as into natural fractures (e.g., [23]). The color coding represents tip-related events (green) and interior events (red). Generally, half or more of the microseismic events occur after the tip has passed the event location, again suggesting natural fracture interactions [15,21] as the source of much of the microseismicity.

It is well-understood that microseismicity is a scaled-down version of conventional seismicity and tools from earthquake seismology should be applicable in some sense for evaluating microseismic behaviour [16]. Certainly, the fault plane solutions that can be derived from a moment tensor inversion provide some information about the planes that are activated during fracturing. Unfortunately, there is no validation that such information can be taken much beyond a resolution of the fault planes, nor is it necessarily clear how the fault planes are being

To suggest that any change in behaviour of the source mechanism, such as a difference between pure shear and a large volumetric component, is somehow diagnostic of fracture behaviour is pure hypothesization without any supporting field, lab, or theoretical results. This type of theorizing is useless, and possibly deleterious, without validation because it could lead to

**Figure 2.** Fracture length development versus time and conditions.

activated (stress effects, leakoff, actual tip extension processes, etc.).

**3. Beyond dots, or beyond verification**

126 Effective and Sustainable Hydraulic Fracturing

Figure 3 shows a plot of the total seismic energy associated with microseismicity in a fracture as a function of the largest event and the "b" value. The b value is the negative slope of the Gutenberg-Richter frequency distribution for earthquakes in a region over some time period. For earthquakes, it is usually near 1.0. For microseisms associated with hydraulic fracturing, it is quite variable and often between 1.0 and 2.0. Given a b value, maximum magnitude event, and low end cutoff (in this case magnitude -4), the energy released can be found by integrating the energy as a function of magnitude over the distribution. For the overwhelming majority of treatments, the maximum magnitude is less than 0 (and often much less than 0), so the typical energy released is on the order of hundreds of kilojoules or less. Fracture injections in shale stimulations usually imparts hundreds of millions kilojoules of energy.

**Figure 3.** Seismic energy released as a function of magnitude and b value.

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

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