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`Chapter 44
`
`Hydraulic Fracturing Mine Back Trials —
`Design Rationale and Project Status
`
`Peter K. Kaiser, Benoît Valley,
`Maurice B. Dusseault and Damien Duff
`
`Additional information is available at the end of the chapter
`
`http://dx.doi.org/10.5772/56260
`
`Abstract
`
`Last year, a joint Mining and Oil & Gas industry consortium was established in Canada to
`conduct hydraulic fracturing (HF) tests accompanied by a mine-back of fractured regions to
`assess HF models and microseismic monitoring data during controlled experiments. Details
`about the displacement field, fracture aperture and extent, and micro-seismic parameters
`could then be verified and used as calibration data for modeling of HF processes in igneous
`and dense sedimentary rocks.
`
`Various injection experiments are planned and they will include pre-fracturing rock mass
`characterisation using best available current techniques, dense arrays of multi-parameter
`wall and borehole-mounted instruments, and the treated volume will be mined through to
`assess fracturing effectiveness, existing fractures and new fracture interactions, and to deter‐
`mine if pathways can be identified for improving currently available numerical and fracture
`network modeling tools.
`
`In this paper we present the results of the experimental design and planning phase, outlin‐
`ing objectives and justifications for planned experimental layouts. Preliminary plans for a
`first mine-through trial at Newcrest Mining’s Cadia East mine in New South Wales, Austral‐
`ia are described. The hypotheses advanced in this experimental design, supported by evi‐
`dence from the literature, are that activation and development of a fracture network by
`hydraulic stimulation is possible if the injection procedure is designed such that injection
`pressures and rates are maintained within an optimal window, thereby producing condi‐
`tions under which effective stress management for risk mitigation in deep mining can best
`
`© 2013 Kaiser et al.; licensee InTech. This is an open access article distributed under the terms of the Creative
`Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,
`distribution, and reproduction in any medium, provided the original work is properly cited.
`
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`be achieved. The evaluation of these hypotheses is the focus of the current high level experi‐
`mental plan presented in the paper.
`
`Keywords stress management, stiffness modification, shale gas analogue, mine-back experi‐
`ments, model calibration, hydraulic fracture, naturally fractured rocks
`
`1. Introduction
`
`Hydraulic fracturing (HF) has been widely used in the oil & gas (O&G) and mining
`industry: in O&G to stimulate reservoirs [1] and in mining, primarily to initiate caving and
`to improve fragmentation (e.g. [2-4]). Attempts have also been made to initiate slip on faults
`or shears [5] and research including mine-backs of hydraulically fractured zones has been
`conducted [6,7] in order to better understand the characteristics of the propagated
`hydraulic fractures. However, to the authors’ knowledge, although there are many
`anecdotal indications of hydraulically induced changes to rock mass properties and stress,
`hydraulic fracturing has so far not been successful in inducing sufficient changes in the in
`situ or mining-induced stress field to be of practical value for risk mitigation related to
`violent seismic energy release in deep and high stress mining. It is speculated that the latter
`can only be achieved by the stimulation, mobilisation and enhancement of a natural
`fracture network rather than by solely generating a new system of induced hydraulic
`fractures. Hence, an innovative testing program, focussed on natural fracture network
`stimulation and the development of these techniques for stress management purposes is
`pursued. The mobilisation and development of a fracture network is also relevant for the
`optimal exploitation of tight gas or oil shale reservoirs, which closely resemble hard-rock
`situations (low permeability block, naturally fractured, stiff, low to moderate Poisson’s
`ratio, etc.). The success of the proposed hydraulic injection program will be investigated
`during a mine-back test, and the results applied to mining and O&G applications.
`
`In this paper, the results of the experimental design phase, outlining objectives and justifica‐
`tions for planned experimental layouts, are presented. Preliminary plans for the first mine-
`through trial at Newcrest Mining’s Cadia East mine in New South Wales, Australia are
`described.
`
`2. Project objectives
`
`The practical justification for the overall HF project is different for the mining and O&G sector
`consortium sponsors. However, both sectors are interested in advancing the state of knowl‐
`edge in three broad areas: (a) fracture network stimulation and development, (b) stress field
`modification, and (c) micro-seismic data interpretation during hydraulic fracturing and
`reservoir stimulation. Hence, the broad objectives of the program meets the primary needs of
`both sectors and will advance the understanding of hydraulic fracture network stimulation
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`
`based on experiments permitting near-field monitoring followed by investigation of the
`treated volume via mapping and monitoring during mine-through.
`
`2.1. Mining perspective
`
`Various hydraulic fracturing (HF) experiments have been undertaken in mines, some with
`mine-through experiments (e.g. [6]) for various purposes: to better understand fracture
`propagation, fracture interaction with natural joints, fragmentation changes, penetration of
`proppants, etc. Successes have been reported with respect to the use of HF for rock mass
`preconditioning, for rock fragmentation and cave initiation (e.g. [2]) but unanswered questions
`remain about its effectiveness in affecting stress redistribution and in controlling energy
`release from critically stressed rock mass structures. There are much anecdotal but little
`scientifically proven evidence that HF can help manage stresses, or not. The authors suggest
`that it may be the methodology of fracturing that may be the source of the apparent contra‐
`dictions reported in the literature. As mines progress to greater depth stress management for
`the control of seismically releasable energy becomes of strategic importance. Furthermore,
`with the introduction of mechanized excavation techniques for rapid mine development (e.g.,
`by Rio Tinto, AngloGold Ashanti, and others), new risks related to strain-bursting are
`introduced because of the less-damaging nature of these excavation techniques.
`
`For the mining sector the motivations are to broaden the application of hydraulic fracturing
`and rock mass stimulation beyond cave initiation, propagation and fragmentation manage‐
`ment by introducing methodologies for hydraulic stress and rock mass stiffness management
`that will eventually find introduction for risk mitigation in deep and high stress mining
`operations. In particular, the problem of fault-slip rockbursting is perplexing and, it is thought,
`can possibly be addressed through the creation of “damage zones” around potentially unstable
`structures, thereby reducing the energy emission levels and rates and improving constructa‐
`bility in highly stressed ground.
`
`It is hypothesised that current hydraulic injection techniques deployed in cave mine applica‐
`tions are predominantly propagating hydraulic fractures and that shear dilation is a secondary
`process. Indeed, opening Mode I fractures develop within a narrow (almost planar) zone
`normal to σ3, and their irregular nature promotes asperity locking resulting in little final net
`shear strength or stiffness reduction. It is recognised that as fluids are lost in the rock mass
`surrounding the hydraulic fracture some distributed shearing of critically oriented natural
`fractures will also occur (e.g. [3]), however in order to enable stress management, one must
`promote volumetrically distributed irreversible changes to the rock mass and the development
`of injection techniques that achieve this objective is at the core of the planned research. Section
`3 presents the output of a review of current injection practices for various applications and
`their effect on the rock mass. It served as background for the development of the experimental
`approach presented in Section 4.
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`2.2. O&G perspective
`
`The advent of numerous staged HF stimulations along the lengths of deep horizontal wells [8]
`has unlocked huge quantities of natural gas and oil in low permeability formations that had
`heretofore been considered non-commercial. Typically, a 1 to 2 km long horizontal well (Fig.
`1) is drilled parallel to σ3, and a series of hydraulic fractures are installed along the length of
`the well, injecting into one or several perforated or open sites each time, until from 10 to 40
`sites are fracture-stimulated. The optimum design of each stage is still the subject of consid‐
`erable debate, in part because existing mathematical models of fracturing, founded on single-
`plane Sneddon crack type assumptions in unjointed continua, are inadequate to predict
`fracture length, stimulated volume, or surface contact area in naturally fractured rock and
`more complex approaches using fracture network models are difficult to calibrate. Thus,
`design is largely empirical, based on remote field measurements that may be inadequate or
`difficult to interpret (tilt measurements, microseismic measurements and post-fracture well
`tests). For each new field, there is an extensive period of experimentation with different
`sequences of fluids and proppants, using different rates and materials, along with limited field
`measurements (generally microseismic monitoring) to try and optimize the stimulation
`process to achieve a maximum contacted volume without wasteful fracture propagation into
`non-productive overlying strata. Each stimulated well may cost 5-10 million dollars, and the
`eastern United States Marcellus Shale alone may require over 500,000 wells for complete
`development, as the deposit covers over 95,000 square miles, and at least 6 horizontal wells
`are needed for each square mile (100 acre spacing). Furthermore, the deeper lying Utica Shale,
`which also extends into Canada, will eventually be developed, requiring a similar number of
`wells [9, 10]. Sub-optimal fracture design because of incomplete understanding and inade‐
`quate predictive tools quickly becomes a costly luxury.
`
`Figure 1. Staged hydraulic fracturing along a horizontal well axis for shale gas stimulation.
`
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`These low permeability strata that contain natural gas or low-viscosity oil are often called
`“shales”, although many of them are better classified as siltstones or even argillaceous
`limestones (marls). The rock matrix is a stiff (30 to 110 GPa), low-porosity (0.04-0.10), low
`permeability (microDarcy to nanoDarcy) material. The rock mass is naturally fractured,
`generally with one dominant set orthogonal to bedding, and one or two minor sets, also
`orthogonal to the bedding planes. Interestingly, these properties are substantially more similar
`to those of igneous and metamorphic rocks encountered in “hard rock” mines than they are
`to typical sedimentary rocks such as heavy oil-rich sandstones, or conventional higher porosity
`(0.15-0.25) limestones and sandstones. Hence, it is attractive for improving O&G reservoir
`stimulation techniques to perform tests in a deep mining context.
`
`The O&G dimension of a HF mine-back experiment is to provide an experimental platform
`for testing predictive models and stimulation procedures suitable for the oil industry. Frac‐
`turing igneous rock at depth in a mining context is therefore of interest because the rocks are
`similar (naturally fractured, stiff, low Poisson’s ratio, anisotropic, almost impermeable matrix
`blocks…), because the deep mine provides access to a high stress environment (1.5 to 3 km
`deep) at one tenth the cost of a vertical oilfield borehole, and because a direct mine-back of a
`fracture-stimulated region can verify assumptions about stimulated volumes, fracture
`aperture, relationship to microseismic emissions, and the rock mass strains [11].
`
`The concept of a stimulated volume that is far larger than the sand-filled fracture propagation
`volume (Fig. 2) is fundamental to understanding shale oil or shale gas stimulation, but cannot
`be easily verified directly, nor can it be predicted by design models that are commonly
`available. The calibration and validation of advanced model permitting complex behaviour
`including branching needs data rarely available and the proposed experimental work will
`contribute to provide such validation data. Fig. 2 presents a 2-D simplification of a complex,
`3-D process involving many natural fractures near a wellbore that have been propped, and a
`large zone surrounding the sand zone where block rotation and shear have created open
`fractures and self-propped dilated fractures [8]. In mining, this process is called rock mass
`bulking due to geometric incompatibilities between, displaced and rotated, strong blocks of
`rock. These bulking induced fractures are favored through high-rate injection, and they are
`thought to be the primary source of microseismic emissions, whereas the zone into which sand
`is transported, the propped aperture, and the number of near-wellbore propped natural
`fractures are favored by injection of a highly viscous fluid. Remote displacement measure‐
`ments (i.e. tilt measurements) cannot distinguish amongst individual fractures, only suitable
`local instrumentation and a mine-back test can give confidence in the actual geometry and
`disposition of the dilated or propped regions.
`
`Thus, the motivation for the O&G industry is to optimize HF treatment in tight reservoirs by
`calibrating design software and hydraulic fracturing propagation monitoring techniques, that
`is to relate the geophysical observables from fracture initiation and propagation, particularly in
`the case of microseismic monitoring, and to better understand the development of hydraulic
`fractures in tight and low permeability naturally fractured lithologies. These objectives can be
`achieved by performing experiments in deep mines, in which the rock properties are similar to
`the O&G lithological context because of their stiff, fractured, low permeability characteristics.
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`Figure 2. The sand zone and the dilated zone (the stimulated volume).
`
`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.
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`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.
`
`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.
`
`100
`106
`
`101
`
`Injection rate [l/min]
`102
`103
`
`104
`
`105
`
`enhanced geothermal
`
`tight shale
`
`cave mining
`
`stress measurement
`
`enhanced geothermal
`
`tight shale
`
`cave mining
`
`104
`
`102
`
`100
`
`10−2
`
`10−4
`
`Injection volume [m3]
`
`20 days
`1 week
`
`1 day
`
`3 hours
`1 hour
`1/2 hour
`
`Injection duration [min]
`
`1 min.
`100
`
`stress measurement
`
`101
`
`103
`102
`Injection rate [l/min]
`
`104
`
`105
`
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`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:
`
`(1)
`
`(2)
`
`
`
`σn = 12 (σ1 + σ3) + 12 (σ1 - σ3)cos2θ
`
`
`
`τ = 1
`2 (σ1 - σ3)sin2θ
`
`Figure 4. Cross plot of minimum principal stress and maximum injection pressure.
`
`Bossier formation, Dowdy Ranch field, USA
`
`Soultz − GPK3
`
`Basel − BS1 − stimulation inj.
`
`Newcrest, trial
`
`Newcrest
`
`Fenton Hill, Phase II
`
`
`
`Soultz − GPK1Soultz − GPK1
`
`Urach 3, Phase 2
`
`Northparkes E26
`El Teniente
`Cooper Basin, Murteree Formation, Well B6
`Northparkes E48
`Buffelsfontein Mine
`Coso Geothermal Field, Well 38C−9
`Rosemanowes
`
`Northparkes
`E26
`
`Mining
`Geothermal
`O&G
`Opening dominated
`Shear dominated
`
`Cooper
`Basin
`Habanero
`
`Desert Peak
`Moonee Coal Mine
`
`Fenton Hill, Phase I
`
`Basel − BS1 −charact. Inj.
`
`80
`
`70
`
`60
`
`50
`
`40
`
`30
`
`20
`
`10
`
`Maximum injection pressure [MPa]
`
`0
`
`0
`
`20
`
`80
`60
`40
`Minimum principal stress [MPa]
`
`100
`
`120
`
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`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):
`
`with
`
`R ≥cos2θ
`
`R = P f
`
`- 1 / 2(σ1 + σ3)
`1 / 2(σ1 - σ3)
`
`
`
`(3)
`
`(4)
`
`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.
`
`P =b
`3 -s
`P =f
`s
`s
`for =1.5
`
`3
`
`1
`
`s
`
`1
`
`3
`
`t
`
`P =f s
`
`1
`
`s3
`
`s1
`q
`sn
`
`Pf
`
`P =b
`3 -s
`P =f
`s
`for =1.73
`
`3
`
`1
`
`1
`
`s
`s
`
`3
`
`P =b
`3 -s
`P =f
`s
`for =2s
`
`3
`
`3
`
`1
`
`Jacking (Pf> )s
`
`n
`
`s
`
`1
`
`P =f s
`
`3
`
`m
`=1.0
`
`m = 0.8
`m = 0 . 5
`
`t
`m
`shearing (|
`| > (
`
`s
`
`-
`
`n
`
`Pf
`) )
`
`30
`
`120
`90
`60
`s
`- angle between fracture normal and 1
`
`q
`
`150
`
`180
`
`1
`
`0.5
`
`0
`
`-0.5
`
`-1
`
`-1.5
`
`-2
`
`-2.5
`
`0
`
`3)
`
`3)
`1+s
`
`1-s
`
`1/2(s
`-1/2(s
`
`Pf
`
`R=
`
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`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 ≥cos2θ - 1
`μ sin2θ
`
`(5)
`
`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
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`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 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 σ1
`= 73 MPa (~horizontal E-W), σ2 = 49 MPa (~horizontal N-S) and σ3 = 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).
`
`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.
`
`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 gas sector pre-
`fracture treatment modeling routines in order to fine tune the injection procedure.
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`Stage I
`
`Stage II
`
`Stage III
`
`Stage IV
`
`Stage V
`
`Establishing base line
`
`Stimulation injection in virgin rock mass
`
`Connect fracture network using hydraulic fracturing to enhance stimulation potential
`
`Solids injection
`
`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 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
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`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.
`
`Acknowledgements
`
`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
`Australia are thanked for their incredible support and data sharing for this project.
`
`Author details
`
`Peter K. Kaiser1, Benoît Valley1,2, Maurice B. Dusseault3 and Damien Duff1
`
`1 CEMI - Centre for Excellence in Mining Innovation, Sudbury, Canada
`
`2 Geological Institute, ETH Zurich, Switze