**5. Better understanding of physical processes associated with microseismicity**

The microseismic case studies by De Meerman et al. (2009) andCastellanos and Van der Baan (2012) do not include fluid injection; yet they already demonstrate that analysis of the micro‐ seismic cloud of event locations can reveal important insights into the local geology and subsurface deformations. Pore pressure and stress changes during hydraulic fracturing lead to a propagating cloud of microseismic events, which can be recorded and analyzed to constrain the volume of the stimulated zone. Because pressures and stresses diffuse/propagate beyond the fluid-filled fractures and affect the (generally jointed) rock mass in all directions, the microseismic cloud represents a volumetric map of the extent of shear and opening of naturally fractured rock.

try that satisfies geometrical requirements for azimuthal coverage of the source region

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Other fundamental descriptions of microseismic sources include the seismic moment and associated energy release, in addition to spectral source characteristics that reveal the timeand spatial-scales of rupture. Recent developments in earthquake seismology suggest that rock-deformation processes commonly occur across a broad spectrum of time scales (and frequency), wherein earthquakes merely represent a high-frequency end member (e.g., Beroza and Ide, 2011). We postulate that rock deformation processes associated with hydraulic fracturing obey scaling laws that are similar to earthquakes. If so, microseismic activity recorded conventionally using geophones, which are relatively insensitive to ground motion below their natural frequency (typically ~ 10 Hz), could represent a high-frequency end

Seismic moment-tensors provide a general mathematical representation of seismic point sources (Ben-Menahem and Singh, 2000). Inversion techniques to estimate moment tensors from seismic recordings are well developed in earthquake seismology, but are only starting to be used in microseismic monitoring applications (Baig and Urbancic, 2010). The determination of moment tensors can potentially provide useful insights into rupture processes, but care is required to ensure that survey design is adequate (Eaton and

The spatial dimensions of microseismic events are encoded in the spectra of the radiated seismic waves. Microseismic events can therefore be analyzed using spectral methods (e.g. Eaton, 2011), providing an alternative approach for characterizing sources. For example, models for shear slip on a circular crack (Brune 1970, 1971; Madariaga, 1977) predict the shape of source spectra and provide scaling relationships between spectral parameters and source parameters (slip area and seismic moment). These source attributes complement those derived

Tensile microseismic events are believed to play an important role during hydraulic fracture treatment of unconventional reservoirs (Baig and Urbancic, 2010). Tensile microseismic events may be associated with self-propping (remnant aperture), or wedging open of natural fractures because of the induced strain field. Walter and Brune (1993) developed a model for far-field source spectra for tensile rupture, and compared these with modeled far-field spectra for shearslip events and showed that anomalously low S/P spectral amplitude ratios are a diagnostic characteristic of tensile rupture. Building on this approach, Eaton et al. ("Scaling relations and spectral characteristics of tensile microseisms", manuscript in preparation for Geophysics) investigate source characteristics of microseismic events induced by hydraulic-fracturing, with application to microseismic data from the previously described multistage treatment in northeastern British Columbia. They show that although spectral estimates of magnitude are relatively unaffected by uncertainty in seismic attenuation, for typical microseismic magni‐ tudes accurate knowledge of seismic attenuation is necessary to estimate some spectral parameters. They also document microseismic events with spectral characteristics that reflect

a complex rupture pattern, such as rapid opening and closing of tensile cracks.

(Eaton and Forouhideh, 2011).

Forouhideh, 2010; 2011).

from moment-tensor inversion.

member of the complete deformation spectrum.

A key element in current research is to develop interpretation methods that bridge the gap between geophysical data analysis and engineering applications of microseismic data. Ultimately, operators would like to know how to optimize the fracturing treatment given the in situ stress regime, dominant natural fracture orientations, pre-existing faults and other zones of weaknesses, and the prevailing lithologies. Phsyically, there exists an intimate link between the above geologic features, employed stimulation strategies and resulting micro‐ seismicity. Existing unknowns can be summarized using the following two fundamental questions: (1) Given a known stress field, geology, rock mass fabric and injection strategy, what are the most likely resulting microseismic characteristics (e.g., hypocentres, source mecha‐ nisms and magnitudes)? (2) What does measured microseismicity reveal about the existing stress field and local geomechanical properties of the rockmass? The first question involves solving the forward model (given the physical parameters, what are the resulting observa‐ tions?) The second question involves solving the inversion problem (given our observations, what can we determine about the current physical state?).

From an engineering point of view, answering these questions will have an immediate impact on first creating optimal drainage and fracturing strategies and then confirming their success or failure prior to starting production. From a geophysical perspective, recorded microseis‐ micity and integration of the results with surface seismic data should significantly enhance our understanding of the existing subsurface geologic conditions and the geomechanical behavior of the reservoir, thus providing pertinent information to the completion engineers.

Pertinent considerations include: (1) Obtaining accurate locations for microseismic events to support meaningful volumetric analysis of the associated microseismic cloud. (2) Inferring the failure mechanism (i.e., are fractures opening, closing or shearing?). (3) Determination of why failure is occurring in specific locations but not in others (why are fractures not always symmetric with respect to the injection well and what is the geomechanical behavior of the reservoir)? The last question, in particular, is difficult to answer from the recorded seismicity alone since the geomechanical behavior depends on the in-situ stress field, the local rock properties (lithologies), and any existing areas of weakness including faults, fractures and joints (Grob and Van der Baan, 2011, Chorney et al., 2012).

#### **5.1. Advanced microseismic source analysis**

Robust characterization of microseismic sources has the potential to provide important information about deformation mechanisms. Borrowing from earthquake seismology, seismic moment tensors can be used to describe microseismic point sources in general terms of a set of force couples. Moment tensors can be represented in terms of source type (Hudson et al., 1989), a classification scheme that includes shear slip (double couple), dipole, compensated linear vector dipole and volumetric sources. The reliability of these classifica‐ tion schemes depends critically upon the use of a recording array with a suitable geome‐ try that satisfies geometrical requirements for azimuthal coverage of the source region (Eaton and Forouhideh, 2011).

constrain the volume of the stimulated zone. Because pressures and stresses diffuse/propagate beyond the fluid-filled fractures and affect the (generally jointed) rock mass in all directions, the microseismic cloud represents a volumetric map of the extent of shear and opening of

A key element in current research is to develop interpretation methods that bridge the gap between geophysical data analysis and engineering applications of microseismic data. Ultimately, operators would like to know how to optimize the fracturing treatment given the in situ stress regime, dominant natural fracture orientations, pre-existing faults and other zones of weaknesses, and the prevailing lithologies. Phsyically, there exists an intimate link between the above geologic features, employed stimulation strategies and resulting micro‐ seismicity. Existing unknowns can be summarized using the following two fundamental questions: (1) Given a known stress field, geology, rock mass fabric and injection strategy, what are the most likely resulting microseismic characteristics (e.g., hypocentres, source mecha‐ nisms and magnitudes)? (2) What does measured microseismicity reveal about the existing stress field and local geomechanical properties of the rockmass? The first question involves solving the forward model (given the physical parameters, what are the resulting observa‐ tions?) The second question involves solving the inversion problem (given our observations,

From an engineering point of view, answering these questions will have an immediate impact on first creating optimal drainage and fracturing strategies and then confirming their success or failure prior to starting production. From a geophysical perspective, recorded microseis‐ micity and integration of the results with surface seismic data should significantly enhance our understanding of the existing subsurface geologic conditions and the geomechanical behavior of the reservoir, thus providing pertinent information to the completion engineers. Pertinent considerations include: (1) Obtaining accurate locations for microseismic events to support meaningful volumetric analysis of the associated microseismic cloud. (2) Inferring the failure mechanism (i.e., are fractures opening, closing or shearing?). (3) Determination of why failure is occurring in specific locations but not in others (why are fractures not always symmetric with respect to the injection well and what is the geomechanical behavior of the reservoir)? The last question, in particular, is difficult to answer from the recorded seismicity alone since the geomechanical behavior depends on the in-situ stress field, the local rock properties (lithologies), and any existing areas of weakness including faults, fractures and

Robust characterization of microseismic sources has the potential to provide important information about deformation mechanisms. Borrowing from earthquake seismology, seismic moment tensors can be used to describe microseismic point sources in general terms of a set of force couples. Moment tensors can be represented in terms of source type (Hudson et al., 1989), a classification scheme that includes shear slip (double couple), dipole, compensated linear vector dipole and volumetric sources. The reliability of these classifica‐ tion schemes depends critically upon the use of a recording array with a suitable geome‐

what can we determine about the current physical state?).

joints (Grob and Van der Baan, 2011, Chorney et al., 2012).

**5.1. Advanced microseismic source analysis**

naturally fractured rock.

450 Effective and Sustainable Hydraulic Fracturing

Other fundamental descriptions of microseismic sources include the seismic moment and associated energy release, in addition to spectral source characteristics that reveal the timeand spatial-scales of rupture. Recent developments in earthquake seismology suggest that rock-deformation processes commonly occur across a broad spectrum of time scales (and frequency), wherein earthquakes merely represent a high-frequency end member (e.g., Beroza and Ide, 2011). We postulate that rock deformation processes associated with hydraulic fracturing obey scaling laws that are similar to earthquakes. If so, microseismic activity recorded conventionally using geophones, which are relatively insensitive to ground motion below their natural frequency (typically ~ 10 Hz), could represent a high-frequency end member of the complete deformation spectrum.

Seismic moment-tensors provide a general mathematical representation of seismic point sources (Ben-Menahem and Singh, 2000). Inversion techniques to estimate moment tensors from seismic recordings are well developed in earthquake seismology, but are only starting to be used in microseismic monitoring applications (Baig and Urbancic, 2010). The determination of moment tensors can potentially provide useful insights into rupture processes, but care is required to ensure that survey design is adequate (Eaton and Forouhideh, 2010; 2011).

The spatial dimensions of microseismic events are encoded in the spectra of the radiated seismic waves. Microseismic events can therefore be analyzed using spectral methods (e.g. Eaton, 2011), providing an alternative approach for characterizing sources. For example, models for shear slip on a circular crack (Brune 1970, 1971; Madariaga, 1977) predict the shape of source spectra and provide scaling relationships between spectral parameters and source parameters (slip area and seismic moment). These source attributes complement those derived from moment-tensor inversion.

Tensile microseismic events are believed to play an important role during hydraulic fracture treatment of unconventional reservoirs (Baig and Urbancic, 2010). Tensile microseismic events may be associated with self-propping (remnant aperture), or wedging open of natural fractures because of the induced strain field. Walter and Brune (1993) developed a model for far-field source spectra for tensile rupture, and compared these with modeled far-field spectra for shearslip events and showed that anomalously low S/P spectral amplitude ratios are a diagnostic characteristic of tensile rupture. Building on this approach, Eaton et al. ("Scaling relations and spectral characteristics of tensile microseisms", manuscript in preparation for Geophysics) investigate source characteristics of microseismic events induced by hydraulic-fracturing, with application to microseismic data from the previously described multistage treatment in northeastern British Columbia. They show that although spectral estimates of magnitude are relatively unaffected by uncertainty in seismic attenuation, for typical microseismic magni‐ tudes accurate knowledge of seismic attenuation is necessary to estimate some spectral parameters. They also document microseismic events with spectral characteristics that reflect a complex rupture pattern, such as rapid opening and closing of tensile cracks.

#### **5.2. Geomechanical response and reservoir analysis**

As indicated above, the reliability with which moment tensors can be determined depends strongly on the acquisition geometry (Eaton and Forouhideh, 2010; 2011). There is thus a need for alternative and complementary analysis methods to reveal more about the in situ stress field. Fortunately, independent information on the in situ stress field can also be obtained by analyzing the frequency-magnitude distribution of microseismic events. This is achieved by plotting the distribution of event magnitudes on a semi-log plot (Figure 4). This distribution, also called the Gutenberg-Richter relation, usually shows a power law behavior. Its linear slope gives the so-called b-value. Schorlemmer et al. (2005) have shown that this b-value changes depending on the stress regime by plotting b-values versus rake angles (indicating slip direction of the hanging wall) for a large variety of earthquakes. For a b-value less than 1, the vertical stress is the least principal compressive stress and we are in a thrust-fault regime. If the vertical stress is intermediate, the b-value will likely be around 1, indicating a strike-slip faulting regime. And if it exceeds 1, then the stress regime is extensional, with the maximum principal stress vertical, creating a normal fault regime.

The case study of Grob and Van der Baan (2011) using a microseismic dataset recorded over a heavy oil field drained using cyclic steam stimulation revealed that the in situ stress state changed from extensional to compressive with an intermediate strike-slip regime, indicating initial opening and then closing of fractures. This occurred over an 8-month period where pure injection in the first four months was followed by combined injection and production in different parts of the field (Figure 4). We postulate that analysis of the statistical b-values will provide complementary information to temporal and spatial variations in the in situ stress field as determined by moment-tensors inversions, and therefore contains a wealth of infor‐ mation to facilitate reservoir management.

#### **5.3. Relating geomechanical properties to microseismic observables**

Various observations suggest that microseismic events tend to occur preferentially in specific lithologies only (e.g., a sand) but not in some others (e.g., a shale), even if fluids are known to traverse both lithologies in a hydraulic fracturing experiment, shown in Figure 5 (Rutledge et al., 2004, Pettitt et al., 2009). This suggests that deformation in some rock types may occur aseismically, especially in higher-permeability, ductile shales, or simply that the radiated elastic energy for microseismic events in some rock types may occur at frequencies that are too low to be detected using conventional recording systems. More‐ over, anecdotal information suggests that the abundance and intensity of microseismic events may not necessarily correlate to the effectiveness of the fracture treatments (Maxwell et al., 2008; Boroumand and Eaton, 2012).

energy to pry apart the walls of a single very large fracture, and the radiated energy observed

**Figure 4.** Analysis of frequency-magnitude variations in microseismic events recorded over a heavy-oil field drained using cyclic-steam stimulation (after Grob and Van der Baan, 2011). Top: Distribution of event sizes for the whole da‐ taset. Shown is the cumulative number of events smaller than a given magnitude. A fit on the linear part of the curve gives a b-value of 1.35 indicating overall extensional faulting. Bottom: Temporal evolution of b-values for this dataset. Three stages are visible: at the beginning high b-values larger than 1.0 (implying extensional faulting or opening of fractures) until November 2009, followed by b-values around 1.0 and finally a last stage with values around 0.65 (indi‐ cating closing of fractures or compressive faulting), starting end of January 2010. Pure steam injection took place prior to November 2009, followed by a combined injection and production in different parts of the field. The statistical analysis of frequency-magnitude variations in microseismic data provide us with invaluable information on changes in

radiated seismic energy, and the fracture energy is inferred to be 15–40% of the input energy

The three most likely factors to dominate the geomechanical behavior of a reservoir are the local in situ stress regime, pre-existing fractures (and other zones of weaknesses), and the actual rock properties (e.g., whether they are more ductile or brittle as expressed by their Young's modulus or Poisson's ratio and thus the Lamé parameters). In order to better understand why

–107

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times larger than the estimated

from recorded seismicity. The injected energy is 104

(Maxwell et al., 2008; Boroumand and Eaton, 2012).

the underlying stress fields.

The concept of microseismic efficiency represents the ratio of radiated seismic energy (Vassiliou and Kanamori, 1982) to the total deformation energy. Analysis of deformation energy is being done by using pressure, rate, fluid/proppant volume and other relevant data curves produced from the surface equipment in order to calculate the total energy/work produced to generate fractures in the ground. Often substantial differences are estimated between the total input energy inferred from fluid injection rates and pressures, the fracture

**5.2. Geomechanical response and reservoir analysis**

452 Effective and Sustainable Hydraulic Fracturing

principal stress vertical, creating a normal fault regime.

**5.3. Relating geomechanical properties to microseismic observables**

mation to facilitate reservoir management.

et al., 2008; Boroumand and Eaton, 2012).

As indicated above, the reliability with which moment tensors can be determined depends strongly on the acquisition geometry (Eaton and Forouhideh, 2010; 2011). There is thus a need for alternative and complementary analysis methods to reveal more about the in situ stress field. Fortunately, independent information on the in situ stress field can also be obtained by analyzing the frequency-magnitude distribution of microseismic events. This is achieved by plotting the distribution of event magnitudes on a semi-log plot (Figure 4). This distribution, also called the Gutenberg-Richter relation, usually shows a power law behavior. Its linear slope gives the so-called b-value. Schorlemmer et al. (2005) have shown that this b-value changes depending on the stress regime by plotting b-values versus rake angles (indicating slip direction of the hanging wall) for a large variety of earthquakes. For a b-value less than 1, the vertical stress is the least principal compressive stress and we are in a thrust-fault regime. If the vertical stress is intermediate, the b-value will likely be around 1, indicating a strike-slip faulting regime. And if it exceeds 1, then the stress regime is extensional, with the maximum

The case study of Grob and Van der Baan (2011) using a microseismic dataset recorded over a heavy oil field drained using cyclic steam stimulation revealed that the in situ stress state changed from extensional to compressive with an intermediate strike-slip regime, indicating initial opening and then closing of fractures. This occurred over an 8-month period where pure injection in the first four months was followed by combined injection and production in different parts of the field (Figure 4). We postulate that analysis of the statistical b-values will provide complementary information to temporal and spatial variations in the in situ stress field as determined by moment-tensors inversions, and therefore contains a wealth of infor‐

Various observations suggest that microseismic events tend to occur preferentially in specific lithologies only (e.g., a sand) but not in some others (e.g., a shale), even if fluids are known to traverse both lithologies in a hydraulic fracturing experiment, shown in Figure 5 (Rutledge et al., 2004, Pettitt et al., 2009). This suggests that deformation in some rock types may occur aseismically, especially in higher-permeability, ductile shales, or simply that the radiated elastic energy for microseismic events in some rock types may occur at frequencies that are too low to be detected using conventional recording systems. More‐ over, anecdotal information suggests that the abundance and intensity of microseismic events may not necessarily correlate to the effectiveness of the fracture treatments (Maxwell

The concept of microseismic efficiency represents the ratio of radiated seismic energy (Vassiliou and Kanamori, 1982) to the total deformation energy. Analysis of deformation energy is being done by using pressure, rate, fluid/proppant volume and other relevant data curves produced from the surface equipment in order to calculate the total energy/work produced to generate fractures in the ground. Often substantial differences are estimated between the total input energy inferred from fluid injection rates and pressures, the fracture

**Figure 4.** Analysis of frequency-magnitude variations in microseismic events recorded over a heavy-oil field drained using cyclic-steam stimulation (after Grob and Van der Baan, 2011). Top: Distribution of event sizes for the whole da‐ taset. Shown is the cumulative number of events smaller than a given magnitude. A fit on the linear part of the curve gives a b-value of 1.35 indicating overall extensional faulting. Bottom: Temporal evolution of b-values for this dataset. Three stages are visible: at the beginning high b-values larger than 1.0 (implying extensional faulting or opening of fractures) until November 2009, followed by b-values around 1.0 and finally a last stage with values around 0.65 (indi‐ cating closing of fractures or compressive faulting), starting end of January 2010. Pure steam injection took place prior to November 2009, followed by a combined injection and production in different parts of the field. The statistical analysis of frequency-magnitude variations in microseismic data provide us with invaluable information on changes in the underlying stress fields.

energy to pry apart the walls of a single very large fracture, and the radiated energy observed from recorded seismicity. The injected energy is 104 –107 times larger than the estimated radiated seismic energy, and the fracture energy is inferred to be 15–40% of the input energy (Maxwell et al., 2008; Boroumand and Eaton, 2012).

The three most likely factors to dominate the geomechanical behavior of a reservoir are the local in situ stress regime, pre-existing fractures (and other zones of weaknesses), and the actual rock properties (e.g., whether they are more ductile or brittle as expressed by their Young's modulus or Poisson's ratio and thus the Lamé parameters). In order to better understand why

shearing) in the surrounding areas due to block rotations. Wedging creates fracture openings well beyond the proppant tips (or infiltration extents) due to normal extensional forces on the surfaces of the joint leading to tensile (mode I) failure and facilitating slurry/proppant penetration. It also leads to a large increase in the effective permeability in a zone beyond the

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**Figure 6.** Analogues can help us understand how fluid and/or proppant injection into a reservoir zone affects the re‐ sulting rock deformation. In this case a solid material is injected into a material comprised of rigid blocks, showing clearly where propping, wedging, rotation and shearing will occur. Such observations provide important clues on the anticipated moment tensors throughout the resulting microseismic event cloud. From: Dusseault et al. (2011).

Block rotation continues beyond the area of proppant infiltration and tensile opening at the proppant tips. It involves large changes in both the normal and shear forces excited on the joint surfaces, yielding predominantly mode II fracturing (i.e., shearing). This may cause slip on existing joints in naturally fractured rocks, and even facilitate fault reactivation if the effective stresses are sufficiently to close to criticality. Shear displacement along natural fractures is associated with self-propping where irregular joint surfaces after slip create remnant aper‐ tures, facilitating subsequent fluid flow (Dusseault et al., 2011). Such observations provide important clues on the anticipated moment tensors throughout the resulting microseismic event cloud, demonstrating that tensile source mechanism are likely to dominate close to the

Obviously fluid and/or proppant infiltration into naturally fractured rock is significantly more complex since the exact behaviour will depend on the situ stress field, pre-existing in natural fractures and lithologies. The interaction of brittle failure in intact rock and the slip/shearing in naturally fractured areas can be complex (Figure 7); yet the principles deduced from the

proppant tips, but double-couple events in all other areas.

study of analogues should help unravel the various competing processes.

proppant infiltration.

**Figure 5.** Hydraulic fracturing of a tight-gas sand. 1408 events are recorded over 5 hours. Events are colour shaded by time: green (earliest) to red (latest). Events occur in two formations with very few detected events in between. Yet the event history reveals that brittle failure occurs first in the right-most part of the bottom formation, and then suddenly jumps to the top formation indicating the presence of a possible aseismic fault. After Pettitt et al. (2009).

the seismic efficiency is so low, and what precisely happens when we are injecting fluids at high pressures into rocks we need to improve our understanding of what the various geo‐ physical observations (moment tensors, hypocentres, resonance frequencies, etc.) truly reveal of the newly induced fracture networks specifically, and the geomechanical reservoir response in general. Three general options to achieve this objective are analogues, computational modelling, and physical modelling in the laboratory.

#### **5.4. Analogues**

Dusseault et al. (2011) use analogues to explain many of the fracturing processes that may occur when fluids and/or proppants are injected at high pressure into intact and naturally fractured rock. They consider a medium composed of rigid blocks and injection of a solid. This leads to many insights despite the fact that this is clearly a great simplification of reality.

In Figure 6 a solid material is injected into a material composed of rigid blocks, producing tensile mode I fracturing (i.e., wedging) at the tips of the proppant inclusions, and mode II (i.e., shearing) in the surrounding areas due to block rotations. Wedging creates fracture openings well beyond the proppant tips (or infiltration extents) due to normal extensional forces on the surfaces of the joint leading to tensile (mode I) failure and facilitating slurry/proppant penetration. It also leads to a large increase in the effective permeability in a zone beyond the proppant infiltration.

**Figure 6.** Analogues can help us understand how fluid and/or proppant injection into a reservoir zone affects the re‐ sulting rock deformation. In this case a solid material is injected into a material comprised of rigid blocks, showing clearly where propping, wedging, rotation and shearing will occur. Such observations provide important clues on the anticipated moment tensors throughout the resulting microseismic event cloud. From: Dusseault et al. (2011).

Block rotation continues beyond the area of proppant infiltration and tensile opening at the proppant tips. It involves large changes in both the normal and shear forces excited on the joint surfaces, yielding predominantly mode II fracturing (i.e., shearing). This may cause slip on existing joints in naturally fractured rocks, and even facilitate fault reactivation if the effective stresses are sufficiently to close to criticality. Shear displacement along natural fractures is associated with self-propping where irregular joint surfaces after slip create remnant aper‐ tures, facilitating subsequent fluid flow (Dusseault et al., 2011). Such observations provide important clues on the anticipated moment tensors throughout the resulting microseismic event cloud, demonstrating that tensile source mechanism are likely to dominate close to the proppant tips, but double-couple events in all other areas.

the seismic efficiency is so low, and what precisely happens when we are injecting fluids at high pressures into rocks we need to improve our understanding of what the various geo‐ physical observations (moment tensors, hypocentres, resonance frequencies, etc.) truly reveal of the newly induced fracture networks specifically, and the geomechanical reservoir response in general. Three general options to achieve this objective are analogues, computational

jumps to the top formation indicating the presence of a possible aseismic fault. After Pettitt et al. (2009).

**Figure 5.** Hydraulic fracturing of a tight-gas sand. 1408 events are recorded over 5 hours. Events are colour shaded by time: green (earliest) to red (latest). Events occur in two formations with very few detected events in between. Yet the event history reveals that brittle failure occurs first in the right-most part of the bottom formation, and then suddenly

Dusseault et al. (2011) use analogues to explain many of the fracturing processes that may occur when fluids and/or proppants are injected at high pressure into intact and naturally fractured rock. They consider a medium composed of rigid blocks and injection of a solid. This leads to many insights despite the fact that this is clearly a great simplification of reality.

In Figure 6 a solid material is injected into a material composed of rigid blocks, producing tensile mode I fracturing (i.e., wedging) at the tips of the proppant inclusions, and mode II (i.e.,

modelling, and physical modelling in the laboratory.

**5.4. Analogues**

454 Effective and Sustainable Hydraulic Fracturing

Obviously fluid and/or proppant infiltration into naturally fractured rock is significantly more complex since the exact behaviour will depend on the situ stress field, pre-existing in natural fractures and lithologies. The interaction of brittle failure in intact rock and the slip/shearing in naturally fractured areas can be complex (Figure 7); yet the principles deduced from the study of analogues should help unravel the various competing processes.

Baig and Urbancic (2010) from field observations of hydraulic fracturing (Figure 8). Baig and Urbancic (2010) find dominant failure mechanisms of double couple (shearing) and fracture opening and closing (tensile failure and closing). This confirms insights gained from the analogues (Figures 6 and 7) where shearing and tensile failure seem to dominate, respectively,

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**Figure 8.** Hudson plots of the failure mechanisms for microseismic events in the bonded-particle simulations for triax‐ ial compression tests with confining pressures of 0 MPa (left) and 40 MPa (right). The colors represent the time: prepeak stress events are in black; events happening around peak stress are in red and post-peak events are displayed in blue (modified from Chorney et al., 2012). Both fracture opening and closing (tensile failure and closing) occurs. CLVD: Compensated-linear vector dipole. The simulated seismicity shows a surprising correspondence with real field meas‐

Chorney et al. (2012) also monitor the total input energy of the system, the total kinetic energy emitted from bond breakages, and the energy deduced from the moment magnitudes of the microseismic events. The kinetic energy represents approximately 5% of the input energy; the radiated seismic energy is 50-100 times smaller than the kinetic energy. The radiated energy calculated using the Gutenberg-Richter relationship between moment magnitude and energy may thus underestimate the energy incurred from brittle failure. Both the radiated and kinetic energy from brittle failure are substantially lower than the input energy. This confirms observationsbyMaxwelletal.(2009)andBoroumandandEaton(2012).Ductileorslow,aseismic deformationmustthusconstituteasignificanttermintheenergybudgetforboththesenumerical simulations of triaxial compression and for hydraulic fracturing experiments in general.

Approaches such as bonded-particle models are thus useful to study the anticipated geome‐ chanical behavior of a reservoir; in particular anticipated brittle failure (as expressed by a microseismic event) as well as any aseismic deformation (due to semi-brittle or plastic flow). Ultimately, they may help to investigate how resulting deformation and microseismic emissions depend on (1) in the in situ stress regime, which relates to the magnitude and ratio of the vertical stress Sv and the maximum and minimum horizontal stresses SH and Sh; (2) pre-existing fractures and other zones of weakness most likely to break; and finally (3) the local rock properties defined by the Young's modulus and Poisson's ratio (both related to the Lamé parameters). Constraints on many of these factors can be obtained using the processing and

Unfortunately, discontinuum-based methods such as bonded-particle approaches may be less suitable to simulate fluid injection as fluids can only be described as small particles.

in the surrounding area and at the tips of the proppant infiltrations.

urements from hydraulic fracturing experiments (e.g., Baig and Urbancic, 2010).

interpretation techniques described previously.

**Figure 7.** Fluid and/or proppant injection into a reservoir zone will create new fractures, as well as close, shear or pop open existing fractures. The various failure mechanisms may lead to a larger microseismic cloud surrounding the area of injected fluids, thereby improving reservoir drainage. The microseismic events are therefore also characterized by a variety of earthquake mechanisms. Their analysis can yield a wealth of knowledge on the underlying failure mecha‐ nisms beyond mere locations. From: Dusseault et al. (2011).

#### **5.5. Geomechanical modelling**

Analogues provide a first understanding on how fluid and/or proppant injection is likely to deform the surrounding rock mass (Figures 6 and 7). They also provide pertinent clues on where to expect brittle failure (and thus microseismic events) and their most probable failure mechanism (source mechanism). Geomechanical modeling is subsequently a great aid for improving our understanding on links between fluid-induced rock failure, the occurrence of microseismicity and underlying geomechanical behaviour, beyond the assumption of rigid blocks and no fluid diffusion (i.e., no leak off).

Bonded-particle modeling is becoming an important computational tool for modeling the complex dynamical behavior of rocks rupturing given a set of boundary conditions (Potyondy and Cundall, 2004). This approach simulates rock deformation using an assemblage of rigid, round particles that are bonded together. This grid of particles can deform freely and bonds can be broken to represent local failure. Bonds are character‐ ized by normal and shear strengths as well as friction coefficients to model respectively tensile and shear failure. Such a discontinuum-based approach seems more appropriate to model rock deformation through failure since it eliminates the need for complex consti‐ tute relations required for continuum approaches (Hazzard and Young, 2000). Also microseismic moment tensors can be inferred by integrating local bond failure in both space and time (Hazzard and Young, 2004).

Chorney et al. (2012) use bonded-particle modelling to examine resulting seismicity for triaxial compression tests using different confining pressures. The resulting Hudson plots (i.e., moment-tensor distribution) show a surprising similarity with those obtained for real data by Baig and Urbancic (2010) from field observations of hydraulic fracturing (Figure 8). Baig and Urbancic (2010) find dominant failure mechanisms of double couple (shearing) and fracture opening and closing (tensile failure and closing). This confirms insights gained from the analogues (Figures 6 and 7) where shearing and tensile failure seem to dominate, respectively, in the surrounding area and at the tips of the proppant infiltrations.

**Figure 8.** Hudson plots of the failure mechanisms for microseismic events in the bonded-particle simulations for triax‐ ial compression tests with confining pressures of 0 MPa (left) and 40 MPa (right). The colors represent the time: prepeak stress events are in black; events happening around peak stress are in red and post-peak events are displayed in blue (modified from Chorney et al., 2012). Both fracture opening and closing (tensile failure and closing) occurs. CLVD: Compensated-linear vector dipole. The simulated seismicity shows a surprising correspondence with real field meas‐ urements from hydraulic fracturing experiments (e.g., Baig and Urbancic, 2010).

**5.5. Geomechanical modelling**

456 Effective and Sustainable Hydraulic Fracturing

blocks and no fluid diffusion (i.e., no leak off).

nisms beyond mere locations. From: Dusseault et al. (2011).

and time (Hazzard and Young, 2004).

Analogues provide a first understanding on how fluid and/or proppant injection is likely to deform the surrounding rock mass (Figures 6 and 7). They also provide pertinent clues on where to expect brittle failure (and thus microseismic events) and their most probable failure mechanism (source mechanism). Geomechanical modeling is subsequently a great aid for improving our understanding on links between fluid-induced rock failure, the occurrence of microseismicity and underlying geomechanical behaviour, beyond the assumption of rigid

**Figure 7.** Fluid and/or proppant injection into a reservoir zone will create new fractures, as well as close, shear or pop open existing fractures. The various failure mechanisms may lead to a larger microseismic cloud surrounding the area of injected fluids, thereby improving reservoir drainage. The microseismic events are therefore also characterized by a variety of earthquake mechanisms. Their analysis can yield a wealth of knowledge on the underlying failure mecha‐

Bonded-particle modeling is becoming an important computational tool for modeling the complex dynamical behavior of rocks rupturing given a set of boundary conditions (Potyondy and Cundall, 2004). This approach simulates rock deformation using an assemblage of rigid, round particles that are bonded together. This grid of particles can deform freely and bonds can be broken to represent local failure. Bonds are character‐ ized by normal and shear strengths as well as friction coefficients to model respectively tensile and shear failure. Such a discontinuum-based approach seems more appropriate to model rock deformation through failure since it eliminates the need for complex consti‐ tute relations required for continuum approaches (Hazzard and Young, 2000). Also microseismic moment tensors can be inferred by integrating local bond failure in both space

Chorney et al. (2012) use bonded-particle modelling to examine resulting seismicity for triaxial compression tests using different confining pressures. The resulting Hudson plots (i.e., moment-tensor distribution) show a surprising similarity with those obtained for real data by

Chorney et al. (2012) also monitor the total input energy of the system, the total kinetic energy emitted from bond breakages, and the energy deduced from the moment magnitudes of the microseismic events. The kinetic energy represents approximately 5% of the input energy; the radiated seismic energy is 50-100 times smaller than the kinetic energy. The radiated energy calculated using the Gutenberg-Richter relationship between moment magnitude and energy may thus underestimate the energy incurred from brittle failure. Both the radiated and kinetic energy from brittle failure are substantially lower than the input energy. This confirms observationsbyMaxwelletal.(2009)andBoroumandandEaton(2012).Ductileorslow,aseismic deformationmustthusconstituteasignificanttermintheenergybudgetforboththesenumerical simulations of triaxial compression and for hydraulic fracturing experiments in general.

Approaches such as bonded-particle models are thus useful to study the anticipated geome‐ chanical behavior of a reservoir; in particular anticipated brittle failure (as expressed by a microseismic event) as well as any aseismic deformation (due to semi-brittle or plastic flow). Ultimately, they may help to investigate how resulting deformation and microseismic emissions depend on (1) in the in situ stress regime, which relates to the magnitude and ratio of the vertical stress Sv and the maximum and minimum horizontal stresses SH and Sh; (2) pre-existing fractures and other zones of weakness most likely to break; and finally (3) the local rock properties defined by the Young's modulus and Poisson's ratio (both related to the Lamé parameters). Constraints on many of these factors can be obtained using the processing and interpretation techniques described previously.

Unfortunately, discontinuum-based methods such as bonded-particle approaches may be less suitable to simulate fluid injection as fluids can only be described as small particles. Continuum-based approaches such as finite-element methods may be required for coupled fluid-flow and geomechanical simulation (Dean et al., 2003; Minkoff et al., 2003; Angus et al., 2010). On the other hand, particle-based methods are highly appropriate to modelling crack propagation and brittle failure. Although this is feasible with continuum-based approaches it leads to highly expensive computations. Angus et al. (2010), for instance, circumvent the requirement for modelling fracture propagation by assuming that the differential effective stress tensor at the local point of failure is a first-order approxima‐ tion to the local failure mechanism (Zoback and Zoback, 1980). For failure in intact rock this is likely a reasonable assumption, but not for failure along pre-existing weaknesses (Gephart and Forsyth, 1984).

Due to a strong desire for near-real time information by completion engineers, acquisition and service companies have focused predominantly on providing hypocentre locations and moment magnitudes. Microseismic recordings contain, however, a wealth of information beyond event locations, including moment tensors and resonance frequencies. Thus, many pertinent research questions on microseismic acquisition, processing and interpretation

Microseismic Monitoring Developments in Hydraulic Fracture Stimulation

http://dx.doi.org/10.5772/56444

459

Nonetheless, microseismic monitoring has a bright future with long-standing applications such as monitoring of shaft stability in mines and the creation of engineered geothermal systems; more recent applications involve monitoring of hydraulic stimulation of "tight" hydrocarbon reservoirs and steam-injection in heavy-oil fields. Future applications may incorporate surveillance of CO2 storage as well as slurried waste solids disposal through

The first two authors would like to thank the sponsors of the Microseismic Industry Consor‐ tium for financial support. Arc Resources, Nanometrics and ESG Solutions are particularly thanked for their support of the field project. All authors would like to thank their collabora‐

and Maurice Dusseault3

[1] Aki, K., Fehler, M., and Das, S., 1977, Source mechanism of volcanic tremors: fluiddriven crack model and their application to the 1963 Kilauea eruption: Journal of

tors, students and postdocs whose work has contributed tremendously to this paper.

remain to be answered before full use of microseismic recordings can be achieved.

continuous injection.

**Acknowledgements**

**Author details**

Mirko van der Baan1

**References**

, David Eaton2

1 University of Alberta, Edmonton, Alberta, Canada

2 University of Calgary, Calgary, Alberta, Canada

3 University of Waterloo, Waterloo, Ontario, Canada

\*Address all correspondence to: Mirko.vanderBaan@ualberta.ca

Volcanology and Geothermal Research, 2, 259–287.

#### **5.6. Physical modelling**

Ultimately physical modelling in the laboratory is required to confirm our inferences from the study of analogues and numerical simulations, thereby completing the circle between fluidinduced rock failure, the occurrence of microseismicity and underlying geomechanical deformation. Many authors have studied the links between microseismic event locations and fracture growth in both triaxial compression and hydraulic fracturing tests (Solberg et al., 1980; Sondergeld and Estey, 1981; Kranz et al., 1990; Lockner et al., 1991; Lockner, 1993; Chitrala et al., 2010). Most of these studies were successful in determining the event hypocenters; yet few provided reliable full moment tensor solutions. The latter are essential for better under‐ standing the actual rock failure mechanisms.

The analogues are very useful for building a first understanding on what to expect when injecting fluids and/or proppants into the rock matrix (Figures 6 and 7) but the combina‐ tion of numerical simulations and their verification using physical experiments in the laboratory will help to bridge the gap between geophysical data analysis and engineer‐ ing applications of microseismic data by providing a framework for advanced interpreta‐ tion strategies, thereby facilitating completion of the the circle between acquisition, processing and interpretation.
