**3. Review of injection practices and their effect on the rock mass**

The generic term "hydraulic injection" covers a spectrum of practices with distinct objectives. With the contribution of Itasca, we conducted a literature survey to capture current injection practices in three sectors: mining, deep geothermal and O&G. A case study database, including 14 mining cases, 46 deep geothermal cases, and 4 O&G cases (to be expanded), includes information on the geomechanics context (stress state, rock strength,...), the injection metrics (flow rate, pressure record, injection volume and duration,...), the monitoring program and the measured or observed effect on the rock masses (main activated mechanisms, stimulated volume, fracture extent...).

Fig. 3 illustrates the breadth of injection practices. At the low end of the spectrum, we included some metrics from the ISRM suggested method for hydraulic fracturing stress measurements [12] where a short interval is injected at a very low rate (2 – 3 l/min) for a short time (1 – 3 min). The mechanism in this case is borehole wall failure in tension, captured by the breakdown pressure in the pressure record followed by a limited extension of the hydraulic fracture and its closure after well shut-in (instantaneous shut-in pressure, ISIP) which is used as an indicator of the σhmin magnitude, assuming that the borehole is vertical and that the fracture has propagated beyond the near-wellbore region.

An up-scaled version of the stress measurement method is used in cave mining operations to pre-condition the rock for improved caveability or fragmentation. A short packed interval is injected to initiate and propagate fractures, and rates, duration and volumes are about two to three orders of magnitude larger than for stress measurements. This propagates fractures typically several tens of meters from the borehole and injections are repeated to generate a zone of fractured rock. Observed fractures typically grow perpendicular to the minimum principal stress and their trajectory is relatively little influenced by natural features (e.g., joints) unless the later makes an sharp angle with the growing hydraulic fracture path.

**3. Review of injection practices and their effect on the rock mass** 

The generic term "hydraulic injection" covers a spectrum of practices with distinct objectives. With the contribution of Itasca, we conducted a literature survey to capture current injection practices in three sectors: mining, deep geothermal and O&G. A case study database, including 14 mining cases, 46 deep geothermal cases, and 4 O&G cases (to be expanded), includes information on the geomechanics context (stress state, rock strength,...), the injection metrics (flow rate, pressure record, injection volume and duration,...), the monitoring program and the measured or observed effect on the rock masses (main activated mechanisms,

Fig. 3 illustrates the breadth of injection practices. At the low end of the spectrum, we included some metrics from the ISRM suggested method for hydraulic fracturing stress measurements [12] where a short interval is injected at a very low rate (2 – 3 l/min) for a short time (1 – 3 min). The mechanism in this case is borehole wall failure in tension, captured by the breakdown pressure in the pressure record followed by a limited extension of the hydraulic fracture and its closure after well shut-in

stimulated volume, fracture extent...).

**3. Review of injection practices and their effect on the rock mass**

**Figure 2.** The sand zone and the dilated zone (the stimulated volume).

volume, fracture extent...).

882 Effective and Sustainable Hydraulic Fracturing

propagated beyond the near-wellbore region.

The generic term "hydraulic injection" covers a spectrum of practices with distinct objectives. With the contribution of Itasca, we conducted a literature survey to capture current injection practices in three sectors: mining, deep geothermal and O&G. A case study database, including 14 mining cases, 46 deep geothermal cases, and 4 O&G cases (to be expanded), includes information on the geomechanics context (stress state, rock strength,...), the injection metrics (flow rate, pressure record, injection volume and duration,...), the monitoring program and the measured or observed effect on the rock masses (main activated mechanisms, stimulated

Fig. 3 illustrates the breadth of injection practices. At the low end of the spectrum, we included some metrics from the ISRM suggested method for hydraulic fracturing stress measurements [12] where a short interval is injected at a very low rate (2 – 3 l/min) for a short time (1 – 3 min). The mechanism in this case is borehole wall failure in tension, captured by the breakdown pressure in the pressure record followed by a limited extension of the hydraulic fracture and its closure after well shut-in (instantaneous shut-in pressure, ISIP) which is used as an indicator of the σhmin magnitude, assuming that the borehole is vertical and that the fracture has

An up-scaled version of the stress measurement method is used in cave mining operations to pre-condition the rock for improved caveability or fragmentation. A short packed interval is injected to initiate and propagate fractures, and rates, duration and volumes are about two to three orders of magnitude larger than for stress measurements. This propagates fractures typically several tens of meters from the borehole and injections are repeated to generate a zone of fractured rock. Observed fractures typically grow perpendicular to the minimum principal stress and their trajectory is relatively little influenced by natural features (e.g., joints)

unless the later makes an sharp angle with the growing hydraulic fracture path.

Figure 3. A broad spectrum of injection practices with specific injection metrics for each industry; related objectives are demonstrated by this cross **Figure 3.** A broad spectrum of injection practices with specific injection metrics for each industry; related objectives are demonstrated by this cross plot of injection volume and injection durations vs. maximum injection rate.

plot of injection volume and injection durations vs. maximum injection rate. An up-scaled version of the stress measurement method is used in cave mining operations to pre-condition the rock for improved caveability or fragmentation. A short packed interval is injected to initiate and propagate fractures, and rates, duration and volumes are about two to three orders of magnitude larger than for stress measurements. This propagates fractures typically several tens of meters from the borehole and injections are repeated to generate a zone of fractured rock. Observed fractures typically grow perpendicular to the minimum principal stress and their trajectory is relatively little influenced by natural features A different situation is encountered in deep geothermal projects with high rate, long duration injections performed in long open-hole sections for reservoir stimulation. The injection metrics are one to two orders of magnitude higher than for cave pre-conditioning cases and extensive monitoring is used to understand fracture activation and propagation, permeability enhance‐ ment and fluid penetration [13, 14]. The predominant mechanisms stem from natural fracture system activation [15] leading to fracture self-propping by shear displacement, causing permanent permeability increases. Critically stressed fractures, oriented optimally to the deviatoric stress field for shear failure are the most prone to activation (see Fig. 2), and slip is accompanied by microseismicity.

(e.g., joints)unless the later makes an sharp angle with the growing hydraulic fracture path.

A different situation is encountered in deep geothermal projects with high rate, long duration injections performed in long openhole sections for reservoir stimulation. The injection metrics are one to two orders of magnitude higher than for cave preconditioning cases and extensive monitoring is used to understand fracture activation and propagation, permeability enhancement and fluid penetration [13] [14]. The predominant mechanisms stem from natural fracture system activation [15] leading to fracture self-propping by shear displacement, causing permanent permeability increases. Critically stressed fractures, oriented optimally to the deviatoric stress field for shear failure are the most prone to activation (see Fig. 2), and slip is accompanied by microseismicity.

At the upper end, shale gas well practices involve high rate injection at a number of sites along the well; injections that are carefully sequenced at each stage with massive injection (up to 3000 m3 per site) of fluids of different viscosity at elevated rates (typical rates

 Insights into the role of variable injection metrics on rock mass response is gained in Fig. 4 where the maximum pressure reached during an injection is plotted against the local estimate of the minimum principal stress magnitude as well as the predominantly activated mechanism (Mode I opening fracture propagation vs. shear re-activation). The dominant activated mechanisms on this

of 12 m3/min are reported) to optimize proppant penetration and the generation of shear dilated zone volume.

At the upper end, shale gas well practices involve high rate injection at a number of sites along the well; injections that are carefully sequenced at each stage with massive injection (up to 3000 m3 per site) of fluids of different viscosity at elevated rates (typical rates of 12 m3 /min are reported) to optimize proppant penetration and the generation of shear dilated zone volume.

Insights into the role of variable injection metrics on rock mass response is gained in Fig. 4 where the maximum pressure reached during an injection is plotted against the local estimate of the minimum principal stress magnitude as well as the predominantly activated mechanism (Mode I opening fracture propagation vs. shear re-activation). The dominant activated mechanisms on this plot are clearly partitioned by the unit slope line: Mode I propagation cases plot above the unit slope while shear activation cases plot on or below the line.

This partition can in part be explained by considering the simple stability model of a cohe‐ sionless pressurized fracture in extension (opening) and shear (Fig. 5). The normal (*σn*) and shear stress (*τ*) resolved on a fracture can be expressed by the following expressions:

$$
\sigma\_n = \frac{1}{2}(\sigma\_1 + \sigma\_3) + \frac{1}{2}(\sigma\_1 - \sigma\_3)\cos 2\theta \tag{1}
$$

and the maximum principal stress direction. The criterion for opening is *Pf* ≥

$$
\tau = \frac{1}{2}(\sigma\_1 \cdot \sigma\_3)\sin 2\theta \tag{2}
$$

This partition can in part be explained by considering the simple stability model of a cohesionless pressurized fracture in extension

) resolved on a fracture can be expressed by the following

, the angle between the fracture normal

volume.

interest, is on average

with

ܴ ʹߠ െ <sup>ଵ</sup>




*R*

=

1/ 2(

1 - 3 )

*Pf* - 1/ 2 (

1 +

3 )



0

0.5

1

curve on Fig. 5):

with

unit slope on Fig. 4).

=90°). The

**4. Planned experimental approach** 

partially connected natural fracture network.

**4.1. Site conditions summary** 

[16]), lead to (red area on Fig. 5):

(5) ߠʹ

**Figure 5.** Stability in opening and shear of a cohesionless pressurised fracture.

shearing (| | > ( - ) ) <sup>n</sup> *Pf*

=0.8

*P =f P =b* 3 - <sup>3</sup> <sup>1</sup> for =1.73 1 3

> =1.0

*P =f P =b* 3 - <sup>3</sup> <sup>1</sup> for =2 1 3

=0.5

ఓ

with *σ1* and *σ3,* the maximum and minimum principal stress magnitude, respectively and *θ*, the angle between the fracture normal and the maximum principal stress direction. The criterion for opening is *Pf* ≥ *σn* which, if substituted in Eq. 1 and re-arranged, leads to (blue

*<sup>R</sup>* <sup>=</sup> *<sup>P</sup> <sup>f</sup>* - <sup>1</sup> / 2(*σ*<sup>1</sup> <sup>+</sup> *<sup>σ</sup>*3)

the fracture opening mode dominates requires a pressure larger than *Pf*

*P =f P =b* 3 - <sup>3</sup> <sup>1</sup> for =1.5 1 3

(perpendicular to σ3, i.e. *θ*=90°). The initiation of the hydraulic fracture at the borehole wall will require a larger pressure (the breakdown pressure, *Pb* on Fig. 5) that depends on the principal stress ratio. Thus, to initiate and propagate a fracture from the borehole wall where

slope on Fig. 4). Also a fracture that propagates exactly perpendicular to σ3, as a Mode I hydraulic fracture does, will not shear since the resolved shear stress on the fracture plane for

Figure 5. Stability in opening and shear of a cohesionless pressurised fracture.

0 30 60 90 120 150 180


The criterion for shearing of a cohesionless fracture is |

The minimum pressure to generate jacking is *Pf*

such an orientation is 0 (Eq. 2 for *θ*=90°).

*R* ≥cos2*θ* (3)

Hydraulic Fracturing Mine Back Trials — Design Rationale and Project Status

<sup>1</sup> / 2(*σ*<sup>1</sup> - *<sup>σ</sup>*3) (4)

*Pf*

*3*

*P =f* <sup>1</sup>

*P =f* <sup>3</sup>

*1*

*n* 

Jacking (Pf> ) <sup>n</sup>

= σ3, if the fracture is favorably oriented

= σ3 (above the unit

http://dx.doi.org/10.5772/56260

885


the coefficient of friction of the fracture. It can be seen from Fig. 5 that fractures optimally oriented (

Mode II shearing occurs (Fig 2), and shear displacement also occurs within the Mode I volume.

experimental design to fit local site conditions is presented in the next section.

*1* = 73 MPa (~horizontal E-W),

130°) will shear at a pressure *Pf* lower than the minimum jacking pressure (unless locking asperities give a high apparent cohesion). Thus, for injection with connectivity to the natural fracture network where the pressure is raised progressively so that the Mode I breakdown pressure at the borehole wall is not reached, shear mechanisms on critically oriented fractures will be the dominant mechanism and the maximum injection pressure will remain close to or below the minimum jacking pressure *Pf* =3 (below the

 There is thus the opportunity to generate stress and rock mass properties change through shearing mechanisms if injection is carried out such that pressure is kept in the gray area of Fig 5, i.e. below the breakdown pressure but above the minimum pressure required for shearing of critically oriented fractures. This situation is called *hydraulic stimulation* in the remainder of this article in contrast with the *hydraulic fracturing* that results in the initiation and propagation of a Mode I fracture. Of course, since Mode I fracture requires a larger pressure than Mode II shearing in rock masses with cohesionless joints, aggressive injection leads to Mode I-dominated fracturing closer to the wellbore, and this zone is surrounded by a pressurized volume within which stimulative

Based on these theoretical considerations and supported by the compiled literature, an experiment to be conducted at Cadia East mine (Newcrest Mining Ltd) in New South Wales, Australia, is being designed to focus on activating shear mechanisms to generate volumetrically distributed fractures and permanent rock mass change. The high level experimental design that will guide detailed

The HF experiment will be integrated with a cave conditioning operation using hydraulic injection in the Cadia East mine, PC2-S1 block. The borehole layout for the cave conditioning operation (Fig. 6) will comprise a borehole array with centres at 60 m to 80 m. Two holes will be extended to the undercut level for this experiment, allowing a subsequent mine-through of the stimulated

The local geology consists of a faulted monzonite body intruded into a volcaniclastic series. Typical uniaxial rock strength ranges from 130-170 MPa, and the rock mass quality is fair to good with two plus random, non-persistent discontinuity sets resulting in a

The boreholes will extend from 850 m depth to 1425 m depth, with the experiment taking place at the greater depth. The in-situ stress condition, estimated from an extensive stress measurement program above 1250 m, and then extrapolated to the depth of

stresses in the thrust fault condition (future experiments at other mines may be situated in strike-slip and normal fault conditions).

*2* = 49 MPa (~horizontal N-S) and

*<sup>3</sup>* = 42 MPa (~vertical). This places the

*<sup>n</sup>*-*Pf*) which, if combined with Eq. 1 and 2 and rearranged (see also

≅ 40° ‒ 80° and 100° ‒

*<sup>n</sup>* which, if substituted in Eq. 1 and re-arranged, leads

*n*) and shear stress (

The minimum pressure to generate jacking is *Pf* =3, if the fracture is favorably oriented (perpendicular to 3, i.e.

initiation of the hydraulic fracture at the borehole wall will require a larger pressure (the breakdown pressure, *Pb* on Fig. 5) that depends on the principal stress ratio. Thus, to initiate and propagate a fracture from the borehole wall where the fracture opening mode dominates requires a pressure larger than *Pf* =3 (above the unit slope on Fig. 4). Also a fracture that propagates exactly perpendicular to 3, as a Mode I hydraulic fracture does, will not shear since the resolved shear stress on the fracture plane for such

*3,* the maximum and minimum principal stress magnitude, respectively and

Figure 4. Cross plot of minimum principal stress and maximum injection pressure. **Figure 4.** Cross plot of minimum principal stress and maximum injection pressure.

(opening) and shear (Fig. 5). The normal (

� ��� � �������� (1)

=90°).

expressions:

� ��� � ��� � �

� ��� � �������� (2)

to (blue curve on Fig. 5):

���������� (4)

an orientation is 0 (Eq. 2 for

� � ����� (3)

� � �������������

�� � �

� � �

with *1* and 

with

with *σ1* and *σ3,* the maximum and minimum principal stress magnitude, respectively and *θ*, the angle between the fracture normal and the maximum principal stress direction. The criterion for opening is *Pf* ≥ *σn* which, if substituted in Eq. 1 and re-arranged, leads to (blue curve on Fig. 5):

$$R \geq \cos 2\theta \tag{3}$$

with

) resolved on a fracture can be expressed by the following

, the angle between the fracture normal

volume.

interest, is on average

with

ܴ ʹߠ െ <sup>ଵ</sup>

unit slope on Fig. 4).

=90°). The

**4. Planned experimental approach** 

partially connected natural fracture network.

**4.1. Site conditions summary** 

*<sup>n</sup>* which, if substituted in Eq. 1 and re-arranged, leads

/min are

At the upper end, shale gas well practices involve high rate injection at a number of sites along the well; injections that are carefully sequenced at each stage with massive injection (up to 3000

reported) to optimize proppant penetration and the generation of shear dilated zone volume.

Insights into the role of variable injection metrics on rock mass response is gained in Fig. 4 where the maximum pressure reached during an injection is plotted against the local estimate of the minimum principal stress magnitude as well as the predominantly activated mechanism (Mode I opening fracture propagation vs. shear re-activation). The dominant activated mechanisms on this plot are clearly partitioned by the unit slope line: Mode I propagation cases

This partition can in part be explained by considering the simple stability model of a cohe‐ sionless pressurized fracture in extension (opening) and shear (Fig. 5). The normal (*σn*) and

Figure 4. Cross plot of minimum principal stress and maximum injection pressure.

0 20 40 60 80 100 120

Minimum principal stress [MPa]

and the maximum principal stress direction. The criterion for opening is *Pf* ≥

(opening) and shear (Fig. 5). The normal (

**Figure 4.** Cross plot of minimum principal stress and maximum injection pressure.

Desert Peak Fenton Hill, Phase I

Moonee Coal Mine

Northparkes E48

El Teniente

Fenton Hill, Phase II

Northparkes E26

Newcrest Newcrest, trial

Bossier formation, Dowdy Ranch field, USA

� ��� � �������� (1)

Buffelsfontein Mine

Coso Geothermal Field, Well 38C−9

Rosemanowes

Cooper Basin, Murteree Formation, Well B6

Soultz − GPK1

=90°).

expressions:

0

10

20

Northparkes E26

30

40

Maximum injection pressure [MPa]

50

60

70

80

� ��� � ��� � �

� ��� � �������� (2)

to (blue curve on Fig. 5):

���������� (4)

an orientation is 0 (Eq. 2 for

� � ����� (3)

� � �������������

�� � �

� � �

with *1* and 

with

<sup>2</sup> (*σ*<sup>1</sup> - *σ*3)cos2*θ* (1)

This partition can in part be explained by considering the simple stability model of a cohesionless pressurized fracture in extension

*n*) and shear stress (

The minimum pressure to generate jacking is *Pf* =3, if the fracture is favorably oriented (perpendicular to 3, i.e.

initiation of the hydraulic fracture at the borehole wall will require a larger pressure (the breakdown pressure, *Pb* on Fig. 5) that depends on the principal stress ratio. Thus, to initiate and propagate a fracture from the borehole wall where the fracture opening mode dominates requires a pressure larger than *Pf* =3 (above the unit slope on Fig. 4). Also a fracture that propagates exactly perpendicular to 3, as a Mode I hydraulic fracture does, will not shear since the resolved shear stress on the fracture plane for such

*3,* the maximum and minimum principal stress magnitude, respectively and

<sup>2</sup> (*σ*<sup>1</sup> - *σ*3)sin2*θ* (2)

Basel − BS1 − stimulation inj.

Mining Geothermal O&G

Opening dominated Shear dominated

Soultz − GPK3

Urach 3, Phase 2

Basel − BS1 −charact. Inj.

Cooper Basin Habanero

per site) of fluids of different viscosity at elevated rates (typical rates of 12 m3

plot above the unit slope while shear activation cases plot on or below the line.

<sup>2</sup> (*σ*<sup>1</sup> + *σ*3) +

*<sup>τ</sup>* <sup>=</sup> <sup>1</sup>

*<sup>σ</sup><sup>n</sup>* <sup>=</sup> <sup>1</sup>

plot on or below the line.

shear stress (*τ*) resolved on a fracture can be expressed by the following expressions:

1

m3

884 Effective and Sustainable Hydraulic Fracturing

$$R = \frac{P\_f \cdot 1/2(\sigma\_1 + \sigma\_3)}{1/2(\sigma\_1 \cdot \sigma\_3)}\tag{4}$$

plot are clearly partitioned by the unit slope line: Mode I propagation cases plot above the unit slope while shear activation cases The minimum pressure to generate jacking is *Pf* = σ3, if the fracture is favorably oriented (perpendicular to σ3, i.e. *θ*=90°). The initiation of the hydraulic fracture at the borehole wall will require a larger pressure (the breakdown pressure, *Pb* on Fig. 5) that depends on the principal stress ratio. Thus, to initiate and propagate a fracture from the borehole wall where the fracture opening mode dominates requires a pressure larger than *Pf* = σ3 (above the unit slope on Fig. 4). Also a fracture that propagates exactly perpendicular to σ3, as a Mode I hydraulic fracture does, will not shear since the resolved shear stress on the fracture plane for such an orientation is 0 (Eq. 2 for *θ*=90°).

Figure 5. Stability in opening and shear of a cohesionless pressurised fracture.


the coefficient of friction of the fracture. It can be seen from Fig. 5 that fractures optimally oriented (

Mode II shearing occurs (Fig 2), and shear displacement also occurs within the Mode I volume.

experimental design to fit local site conditions is presented in the next section.

*1* = 73 MPa (~horizontal E-W),

130°) will shear at a pressure *Pf* lower than the minimum jacking pressure (unless locking asperities give a high apparent cohesion). Thus, for injection with connectivity to the natural fracture network where the pressure is raised progressively so that the Mode I breakdown pressure at the borehole wall is not reached, shear mechanisms on critically oriented fractures will be the dominant mechanism and the maximum injection pressure will remain close to or below the minimum jacking pressure *Pf* =3 (below the

 There is thus the opportunity to generate stress and rock mass properties change through shearing mechanisms if injection is carried out such that pressure is kept in the gray area of Fig 5, i.e. below the breakdown pressure but above the minimum pressure required for shearing of critically oriented fractures. This situation is called *hydraulic stimulation* in the remainder of this article in contrast with the *hydraulic fracturing* that results in the initiation and propagation of a Mode I fracture. Of course, since Mode I fracture requires a larger pressure than Mode II shearing in rock masses with cohesionless joints, aggressive injection leads to Mode I-dominated fracturing closer to the wellbore, and this zone is surrounded by a pressurized volume within which stimulative

Based on these theoretical considerations and supported by the compiled literature, an experiment to be conducted at Cadia East mine (Newcrest Mining Ltd) in New South Wales, Australia, is being designed to focus on activating shear mechanisms to generate volumetrically distributed fractures and permanent rock mass change. The high level experimental design that will guide detailed

The HF experiment will be integrated with a cave conditioning operation using hydraulic injection in the Cadia East mine, PC2-S1 block. The borehole layout for the cave conditioning operation (Fig. 6) will comprise a borehole array with centres at 60 m to 80 m. Two holes will be extended to the undercut level for this experiment, allowing a subsequent mine-through of the stimulated

The local geology consists of a faulted monzonite body intruded into a volcaniclastic series. Typical uniaxial rock strength ranges from 130-170 MPa, and the rock mass quality is fair to good with two plus random, non-persistent discontinuity sets resulting in a

The boreholes will extend from 850 m depth to 1425 m depth, with the experiment taking place at the greater depth. The in-situ stress condition, estimated from an extensive stress measurement program above 1250 m, and then extrapolated to the depth of

stresses in the thrust fault condition (future experiments at other mines may be situated in strike-slip and normal fault conditions).

*2* = 49 MPa (~horizontal N-S) and

*<sup>3</sup>* = 42 MPa (~vertical). This places the

*<sup>n</sup>*-*Pf*) which, if combined with Eq. 1 and 2 and rearranged (see also

≅ 40° ‒ 80° and 100° ‒

The criterion for shearing of a cohesionless fracture is |

**Figure 5.** Stability in opening and shear of a cohesionless pressurised fracture.

(5) ߠʹ

[16]), lead to (red area on Fig. 5):

ఓ

The criterion for shearing of a cohesionless fracture is |*τ*| ≥ *μ* (*σn*-*Pf* ) which, if combined with Eq. 1 and 2 and rearranged (see also [16]), lead to (red area on Fig. 5):

$$R \geq \cos 2\theta \cdot \frac{1}{\mu} \sin 2\theta \tag{5}$$

good with two plus random, non-persistent discontinuity sets resulting in a partially connected

Figure 6. Layout for the experiment to be conducted at Cadia East mine, Newcrest Ltd.

Hydraulic Fracturing Mine Back Trials — Design Rationale and Project Status

http://dx.doi.org/10.5772/56260

887

**Stage I** Establishing base line

**Stage IV** Solids injection **Stage V** Mine-through

gas sector pre-fracture treatment modeling routines in order to fine tune the injection procedure.

**Stage II** Stimulation injection in virgin rock mass

stimulation potential

flow rate) will be so low that it will be difficult not to exceed the optimal pressure for stimulation.

change of rock mass permeability induced by the applied hydraulic injection treatments.

The experimental design is constrained by logistical factors; particularly, the current pumping capacity available and water supply permits to pump 75,000 l of water per 12 hours shift at a maximum flow rate of 400 l/min and maximum pressure of about 70 MPa.

The suggested test sequence involves five stages (see Table 1). Stage I will focus on establishing a base line dataset and will involve geological and rock mass parameter characterisation, borehole televiewers and formation testing as well as using standard oil and

Stage II will comprise a stimulation of the lower section of the experimental holes. The length of the stimulated section will be determined based on televiewer data and formation testing in order to ensure connectivity with the natural fracture network. It is expected, since the natural fracture network is probably poorly connected (below the percolation threshold), that the borehole injectivity (the capacity of the formation to accept flow for a given pressure increase or reciprocally the pressure increase at a given

At Stage III, the low borehole injectivity will be remediated through increasing fracture network connectivity by creating an array of hydraulic fractures before performing a second stimulation of the borehole. A final injection stage (Stage IV) will focus on the placement of solids in the fractured rock mass in order to better understand proppant penetration, to modify its properties, and to

The final stage of the experiment (Stage V) will be a diagnostic exercise where the injected volume will be mined-through in small increments to evaluate the impact of the injection treatments on the fracturing, the rock mass behaviour and the stress state in

Characterisation will be repeated between stages in order to evaluate changes to the base line data collected in Stage I, including

Hydraulic fracturing (HF) currently has found current applications in mining environments in the promotion of rock caving and fragmentation control and has potential for stress and stiffness modification and rock mass pre-conditioning. In the O&G industry,

**Stage III** Connect fracture network using hydraulic fracturing to enhance

The boreholes will extend from 850 m depth to 1425 m depth, with the experiment taking place at the greater depth. The in-situ stress condition, estimated from an extensive stress measure‐ ment program above 1250 m, and then extrapolated to the depth of interest, is on average *σ<sup>1</sup>* = 73 MPa (~horizontal E-W), *σ2* = 49 MPa (~horizontal N-S) and *σ<sup>3</sup>* = 42 MPa (~vertical). This places the stresses in the thrust fault condition (future experiments at other mines may be

**4.2. High level experimental design** 

**Figure 6.** Layout for the experiment to be conducted at Cadia East mine, Newcrest Ltd.

The experimental design is constrained by logistical factors; particularly, the current pumping capacity available and water supply permits to pump 75,000 l of water per 12 hours shift at a

The suggested test sequence involves five stages (see Table 1). Stage I will focus on establishing a base line dataset and will involve geological and rock mass parameter characterisation, borehole televiewers and formation testing as well as using standard oil and gas sector pre-

fracture treatment modeling routines in order to fine tune the injection procedure.

Table 1. Proposed experimental stages.

maximum flow rate of 400 l/min and maximum pressure of about 70 MPa.

natural fracture network.

situated in strike-slip and normal fault conditions).

enhance shear slip.

stimulated volume.

**5. Conclusion** 

**4.2. High level experimental design**

with *μ* the coefficient of friction of the fracture. It can be seen from Fig. 5 that fractures optimally oriented (*θ* ≅ 40° ‒ 80° and 100° ‒ 130°) will shear at a pressure *Pf* lower than the minimum jacking pressure (unless locking asperities give a high apparent cohesion). Thus, for injection with connectivity to the natural fracture network where the pressure is raised progressively so that the Mode I breakdown pressure at the borehole wall is not reached, shear mechanisms on critically oriented fractures will be the dominant mechanism and the maximum injection pressure will remain close to or below the minimum jacking pressure *Pf* = σ3 (below the unit slope on Fig. 4).

There is thus the opportunity to generate stress and rock mass properties change through shearing mechanisms if injection is carried out such that pressure is kept in the gray area of Fig. 5, i.e. below the breakdown pressure but above the minimum pressure required for shearing of critically oriented fractures. This situation is called *hydraulic stimulation* in the remainder of this article in contrast with the *hydraulic fracturing* that results in the initiation and propagation of a Mode I fracture. Of course, since Mode I fracture requires a larger pressure than Mode II shearing in rock masses with cohesionless joints, aggressive injection leads to Mode I-dominated fracturing closer to the wellbore, and this zone is surrounded by a pressurized volume within which stimulative Mode II shearing occurs (Fig. 2), and shear displacement also occurs within the Mode I volume.

Based on these theoretical considerations and supported by the compiled literature, an experiment to be conducted at Cadia East mine (Newcrest Mining Ltd) in New South Wales, Australia, is being designed to focus on activating shear mechanisms to generate volumetri‐ cally distributed fractures and permanent rock mass change. The high level experimental design that will guide detailed experimental design to fit local site conditions is presented in the next section.

### **4. Planned experimental approach**

#### **4.1. Site conditions summary**

The HF experiment will be integrated with a cave conditioning operation using hydraulic injection in the Cadia East mine, PC2-S1 block. The borehole layout for the cave conditioning operation (Fig. 6) will comprise a borehole array with centres at 60 m to 80 m. Two holes will be extended to the undercut level for this experiment, allowing a subsequent mine-through of the stimulated volume.

The local geology consists of a faulted monzonite body intruded into a volcaniclastic series. Typical uniaxial rock strength ranges from 130-170 MPa, and the rock mass quality is fair to

Figure 6. Layout for the experiment to be conducted at Cadia East mine, Newcrest Ltd. **Figure 6.** Layout for the experiment to be conducted at Cadia East mine, Newcrest Ltd.

good with two plus random, non-persistent discontinuity sets resulting in a partially connected natural fracture network. The experimental design is constrained by logistical factors; particularly, the current pumping capacity available and water supply permits to pump 75,000 l of water per 12 hours shift at a maximum flow rate of 400 l/min and maximum pressure of about 70 MPa.

The boreholes will extend from 850 m depth to 1425 m depth, with the experiment taking place at the greater depth. The in-situ stress condition, estimated from an extensive stress measure‐ ment program above 1250 m, and then extrapolated to the depth of interest, is on average *σ<sup>1</sup>* = 73 MPa (~horizontal E-W), *σ2* = 49 MPa (~horizontal N-S) and *σ<sup>3</sup>* = 42 MPa (~vertical). This places the stresses in the thrust fault condition (future experiments at other mines may be situated in strike-slip and normal fault conditions). **4.2. High level experimental design**  The suggested test sequence involves five stages (see Table 1). Stage I will focus on establishing a base line dataset and will involve geological and rock mass parameter characterisation, borehole televiewers and formation testing as well as using standard oil and

The experimental design is constrained by logistical factors; particularly, the current pumping capacity available and water supply permits to pump 75,000 l of water per 12 hours shift at a maximum flow rate of 400 l/min and maximum pressure of about 70 MPa. gas sector pre-fracture treatment modeling routines in order to fine tune the injection procedure. **Stage I** Establishing base line **Stage II** Stimulation injection in virgin rock mass

#### **4.2. High level experimental design Stage III** Connect fracture network using hydraulic fracturing to enhance

enhance shear slip.

stimulated volume.

**5. Conclusion** 

The criterion for shearing of a cohesionless fracture is |*τ*| ≥ *μ* (*σn*-*Pf*

Eq. 1 and 2 and rearranged (see also [16]), lead to (red area on Fig. 5):

pressure will remain close to or below the minimum jacking pressure *Pf*

displacement also occurs within the Mode I volume.

**4. Planned experimental approach**

**4.1. Site conditions summary**

the stimulated volume.

slope on Fig. 4).

886 Effective and Sustainable Hydraulic Fracturing

the next section.

*<sup>R</sup>* <sup>≥</sup>cos2*<sup>θ</sup>* - <sup>1</sup>

with *μ* the coefficient of friction of the fracture. It can be seen from Fig. 5 that fractures optimally oriented (*θ* ≅ 40° ‒ 80° and 100° ‒ 130°) will shear at a pressure *Pf* lower than the minimum jacking pressure (unless locking asperities give a high apparent cohesion). Thus, for injection with connectivity to the natural fracture network where the pressure is raised progressively so that the Mode I breakdown pressure at the borehole wall is not reached, shear mechanisms on critically oriented fractures will be the dominant mechanism and the maximum injection

There is thus the opportunity to generate stress and rock mass properties change through shearing mechanisms if injection is carried out such that pressure is kept in the gray area of Fig. 5, i.e. below the breakdown pressure but above the minimum pressure required for shearing of critically oriented fractures. This situation is called *hydraulic stimulation* in the remainder of this article in contrast with the *hydraulic fracturing* that results in the initiation and propagation of a Mode I fracture. Of course, since Mode I fracture requires a larger pressure than Mode II shearing in rock masses with cohesionless joints, aggressive injection leads to Mode I-dominated fracturing closer to the wellbore, and this zone is surrounded by a pressurized volume within which stimulative Mode II shearing occurs (Fig. 2), and shear

Based on these theoretical considerations and supported by the compiled literature, an experiment to be conducted at Cadia East mine (Newcrest Mining Ltd) in New South Wales, Australia, is being designed to focus on activating shear mechanisms to generate volumetri‐ cally distributed fractures and permanent rock mass change. The high level experimental design that will guide detailed experimental design to fit local site conditions is presented in

The HF experiment will be integrated with a cave conditioning operation using hydraulic injection in the Cadia East mine, PC2-S1 block. The borehole layout for the cave conditioning operation (Fig. 6) will comprise a borehole array with centres at 60 m to 80 m. Two holes will be extended to the undercut level for this experiment, allowing a subsequent mine-through of

The local geology consists of a faulted monzonite body intruded into a volcaniclastic series. Typical uniaxial rock strength ranges from 130-170 MPa, and the rock mass quality is fair to

) which, if combined with

= σ3 (below the unit

*<sup>μ</sup>* sin2*θ* (5)

The suggested test sequence involves five stages (see Table 1). Stage I will focus on establishing a base line dataset and will involve geological and rock mass parameter characterisation, borehole televiewers and formation testing as well as using standard oil and gas sector prefracture treatment modeling routines in order to fine tune the injection procedure. stimulation potential **Stage IV** Solids injection **Stage V** Mine-through Table 1. Proposed experimental stages.

> Stage II will comprise a stimulation of the lower section of the experimental holes. The length of the stimulated section will be determined based on televiewer data and formation testing in order to ensure connectivity with the natural fracture network. It is expected, since the natural fracture network is probably poorly connected (below the percolation threshold), that the borehole injectivity (the capacity of the formation to accept flow for a given pressure increase or reciprocally the pressure increase at a given

> At Stage III, the low borehole injectivity will be remediated through increasing fracture network connectivity by creating an array of hydraulic fractures before performing a second stimulation of the borehole. A final injection stage (Stage IV) will focus on the placement of solids in the fractured rock mass in order to better understand proppant penetration, to modify its properties, and to

> The final stage of the experiment (Stage V) will be a diagnostic exercise where the injected volume will be mined-through in small increments to evaluate the impact of the injection treatments on the fracturing, the rock mass behaviour and the stress state in

> Characterisation will be repeated between stages in order to evaluate changes to the base line data collected in Stage I, including

Hydraulic fracturing (HF) currently has found current applications in mining environments in the promotion of rock caving and fragmentation control and has potential for stress and stiffness modification and rock mass pre-conditioning. In the O&G industry,

flow rate) will be so low that it will be difficult not to exceed the optimal pressure for stimulation.

change of rock mass permeability induced by the applied hydraulic injection treatments.


hydraulic fracturing and stimulation, and post-fracture characterization, including, where possible, mine-through of fractured zones. Type A predictions (before the event – [17]) based on the detailed ground characterization can be tested in practice, and implications for MS

Hydraulic Fracturing Mine Back Trials — Design Rationale and Project Status

http://dx.doi.org/10.5772/56260

889

Specifically, the hypothesis that stress management is best achieved by *hydraulic stimulation*, i.e. activation and development of a fracture network through Mode II shear dilation in contrast to *hydraulic fracturing*, i.e. initiation and propagation of Mode I hydraulic fractures, will be tested. Theoretically there are injection pressure windows favourable for rock mass stimulation and activation in shear of critically stressed fractures, a notion supported by a review of the current practices in the O&G, mining and geothermal industries. Of course, aggressive Mode I fracturing in a strongly deviatoric stress field in naturally fractured rock masses will always be accompanied by shear within and around the Mode I dominated zone. The proposed experimental setup aims at quantifying the changes in the rock mass permeability and stiffness

Tatyana Katsaga, David O. Degagné and Branko Damjanac from Toronto and Minneapolis Itasca offices, respectively, are warmly thanked for their contribution to this project in the form of a thorough literature compilation: Fig. 3 and 4 of this paper are directly built from their literature review database. Geoff Capes and Glenn Sharrock from Newcrest Mining Ltd in

3 Department of Earth and Environmental Sciences, University of Waterloo, Waterloo,

[1] Gale JFW, Reed RM, Holder J. Natural fractures in the Barnett Shale and their impor‐ tance for hydraulic fracture treatments. AAPG Bulletin 2007; 91(4): 603-622.

and Damien Duff1

Australia are thanked for their incredible support and data sharing for this project.

, Benoît Valley1,2, Maurice B. Dusseault3

1 CEMI - Centre for Excellence in Mining Innovation, Sudbury, Canada

2 Geological Institute, ETH Zurich, Switzerland

emission interpretation can be ground-truthed.

associated with hydraulic stimulation.

**Acknowledgements**

**Author details**

Peter K. Kaiser1

Canada

**References**

**Table 1.** Proposed experimental stages.

Stage II will comprise a stimulation of the lower section of the experimental holes. The length of the stimulated section will be determined based on televiewer data and formation testing in order to ensure connectivity with the natural fracture network. It is expected, since the natural fracture network is probably poorly connected (below the percolation threshold), that the borehole injectivity (the capacity of the formation to accept flow for a given pressure increase or reciprocally the pressure increase at a given flow rate) will be so low that it will be difficult not to exceed the optimal pressure for stimulation.

At Stage III, the low borehole injectivity will be remediated through increasing fracture network connectivity by creating an array of hydraulic fractures before performing a second stimulation of the borehole. A final injection stage (Stage IV) will focus on the placement of solids in the fractured rock mass in order to better understand proppant penetration, to modify its properties, and to enhance shear slip.

The final stage of the experiment (Stage V) will be a diagnostic exercise where the injected volume will be mined-through in small increments to evaluate the impact of the injection treatments on the fracturing, the rock mass behaviour and the stress state in stimulated volume.

Characterisation will be repeated between stages in order to evaluate changes to the base line data collected in Stage I, including change of rock mass permeability induced by the applied hydraulic injection treatments.

### **5. Conclusion**

Hydraulic fracturing (HF) currently has found current applications in mining environments in the promotion of rock caving and fragmentation control and has potential for stress and stiffness modification and rock mass pre-conditioning. In the O&G industry, HF in tight oil or gas shales, rocks of similar properties (low k, high E, naturally fractured…), is a vital technol‐ ogy used to develop unconventional oil and gas resources with long horizontal wells and numerous fracture stages at sites distributed along the axis of the horizontal well. We note that the properties of the rocks involved are quite similar in both industries, and the economical need for better HF predictive tools in the O&G industry is large, given the huge development costs predicted for the upcoming decades in North America.

Experiments in deep mines, one planned for 2013 in Australia, and two to follow later in Canada, will be based on extensive pre-characterization, intensive monitoring, staged hydraulic fracturing and stimulation, and post-fracture characterization, including, where possible, mine-through of fractured zones. Type A predictions (before the event – [17]) based on the detailed ground characterization can be tested in practice, and implications for MS emission interpretation can be ground-truthed.

Specifically, the hypothesis that stress management is best achieved by *hydraulic stimulation*, i.e. activation and development of a fracture network through Mode II shear dilation in contrast to *hydraulic fracturing*, i.e. initiation and propagation of Mode I hydraulic fractures, will be tested. Theoretically there are injection pressure windows favourable for rock mass stimulation and activation in shear of critically stressed fractures, a notion supported by a review of the current practices in the O&G, mining and geothermal industries. Of course, aggressive Mode I fracturing in a strongly deviatoric stress field in naturally fractured rock masses will always be accompanied by shear within and around the Mode I dominated zone. The proposed experimental setup aims at quantifying the changes in the rock mass permeability and stiffness associated with hydraulic stimulation.
