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Hydraulic Fracture Growth Through FrictionalInterfaces in Adelaide Black Granite: Phase IExperimentsAndrew Bunger, James Kear, Andrew Rohde, and Rob JeffreyReport Number: EP104290

Enquiries should be addressed to:

Andrew BungerCSIRO Earth Science and Resource EngineeringIan Wark Laboratory, Bayview Avenue, Clayton 3169, Victoria, AustraliaPrivate Bag 10, Clayton South, 3169, Victoria, AustraliaTelephone : +61 3 9545 8334Fax : +61 3 9545 8331Email : Andrew.Bunger@csiro.au

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Contents

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2 Theoretical Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

3 Scaling and Experimental Design . . . . . . . . . . . . . . . . . . . . . . . . . . 5

4 Description of Experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

4.1 Geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

4.2 Laboratory Apparatus. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74.2.1 Fracture Fluid Injection System. . . . . . . . . . . . . . . . . . . . . 74.2.2 External Pressure Application System. . . . . . . . . . . . . . . . . . 104.2.3 Polyaxial Load Frame. . . . . . . . . . . . . . . . . . . . . . . . . . 104.2.4 Injection Tool . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104.2.5 Flow Control. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114.2.6 Pressure Monitoring. . . . . . . . . . . . . . . . . . . . . . . . . . . 11

4.3 Ultrasound Monitoring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114.3.1 Ultrasound system. . . . . . . . . . . . . . . . . . . . . . . . . . . . 124.3.2 Modelling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

4.4 Rock and Fluid Properties. . . . . . . . . . . . . . . . . . . . . . . . . . . . 154.4.1 Elastic Properties. . . . . . . . . . . . . . . . . . . . . . . . . . . . 154.4.2 Tensile Strength. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164.4.3 Fracture Toughness. . . . . . . . . . . . . . . . . . . . . . . . . . . 164.4.4 Interface Friction Coefficient. . . . . . . . . . . . . . . . . . . . . . 164.4.5 Fluid Viscosity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

5 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

5.1 Summary of Experimental Results. . . . . . . . . . . . . . . . . . . . . . . 19

5.2 Ultrasound Monitoring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

1

2

SummaryHydraulic fracturing is a vital technology used to increasethe permeability of petroleum andgeothermal reservoirs to fluid flow. Unconventional gas and geothermal reservoirs, which com-prise an important part of the Australian and Worldwide emerging energy portfolio, are typicallynaturally fractured and therefore the little-understood problem of hydraulic fracture growththrough naturally fractured rocks is an important area of research. In order to better understandthis issue, ten experiments have been completed in which hydraulic fractures have been createdin Adelaide Black Granite specimens such that they impinge orthogonally on a frictional inter-face. Depending on the conditions of each experiment the fractures either cross or are blunted atthe interface. The experiments are designed in light of scaling principles in order that the rela-tive importance of different physical processes can be understood, particularly when comparingthese small scale laboratory experiments with large scale field treatments. Ultrasound moni-toring, which was developed in conjunction with these experiments, was successful providedthat the transducers were close enough to the fracture plane. In one example, non-symmetricfracture growth about the injection point was determined based on ultrasound data and corrob-orated with observations. The crossing behaviour observedin all ten experiments, combinedwith past experiments, contributes to an emerging picture of hydraulic fracture growth throughan orthogonal natural fracture that is not dependent on the friction coefficient of the interface.This result challenges the current modelling paradigm in which the details of the interface plas-ticity are considered to be vital for understanding whetherand how a hydraulic fracture willcross a natural fracture. If additional experiments in other rock types also give indication thatthe friction coefficient of the interface is not a parameter governing the crossing behaviour, thena new paradigm ought to be proposed in which interface plasticity plays a reduced role.

KeywordsHydraulic Fracturing; Naturally Fractured Rocks; Fracture Mechanics; Experimental Mechan-ics; Ultrasound Monitoring

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1 IntroductionHydraulic fracturing (HF) is the process of driving cracks with high pressure fluids. Typicallythe interest is to make petroleum or geothermal reservoirs more conducive to fluid flow. Hy-draulic fractures provide highly conductive pathways in the reservoir either by creating openconductive cracks or by shearing naturally-occuring joints or fractures. This is usually ac-complished through pumping water or other more complex fluids into an isolated portion of awellbore that has been drilled into the reservoir.

Numerical models provide the primary tools for both design of hydraulic fracture treatmentsand for assessment of a treatment through history matching of model predictions to measure-ments such as the downhole pressure. Because these models are so important, much has beeninvested into hydraulic fracture simulation over the last 50 years. However, some fundamentaland important issues remain poorly understood. One of theseis the issue of how hydraulic frac-tures grow through pre-existing fractures in the rock mass,henceforth called “natural fractures”(NFs). Active research is being conducted in this area because unconventional petroleum andhot rock geothermal reservoirs, both key growth areas in theworldwide and Australian energysectors, are typically naturally fractured.

Intuitively one can imagine a case where the strength of the NF is identical to the rock itself, inwhich case the hydraulic fracture would be expected to propagate directly through the interfaceunaffected. On the other hand, if the interface is very weak,the hydraulic fracture might bedeviated such that subsequent growth takes place only on theweak interface. These two endmembers are typically referred to as “crossing” and “blunting” cases, respectively. With theaddition of crossing with an offset, i.e. “offsetting”, we have the three basic types of behaviourdiscussed byThiercelin et al.(1987).

While conceptually relatively simple, hydraulic fractureinteraction with NFs is difficult to im-plement in numerical models. In the present state of affairs, one must either model the plasticity,that is the opening and slipping behaviour, on the NF explicitly (e.g.Zhang et al., 2007), or re-sort to a simplified model that predicts crossing or non-crossing behaviour, such as those ofRenshaw and Pollard(1995) andBlanton(1986). The first approach can be computationallyexpensive, while the second approach relies on multiple ad hoc simplifications of the actualunderlying problem, including a neglecting of the effects of fluid flow. Current research isaimed at gaining new insight into the mechanics of HF interaction with NFs in order to bench-mark existing numerical predictions and to seek concise andyet rigorous crossing criteria forimplementation in numerical simulators.

This report describes a series of laboratory experiments directed at providing an experimentalbasis for advancement of our ability to model hydraulic fracture growth in naturally fracturedreservoirs. To this end, experiments are performed in whichhydraulic fractures are grown soas to interact with frictional interfaces between multipleplates of Adelaide Black Granite. Asan initial paradigm, the results are compared to the predictions obtained from the criterion ofRenshaw and Pollard(1995). Additionally, ultrasound is used to monitor HF growth, and thefirst developments of a new monitoring system are discussed.

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2 Theoretical BackgroundThe problem of a hydraulic fracture approaching, interacting with, and growing along and/orcrossing an interface requires modelling the coupled problem of rock deformation, fluid flow,and interface plasticity. While this problem is difficult, requiring sophisticated solution methodsand, often, significant computation time, some progress hasbeen made recently on the problemof a hydraulic fracture interacting with a single interfaceusing a model that couples linearelastic model of rock deformation, laminar flow of a Newtonian fluid, and a Mohr-Coulombyield criterion on the interface (Zhang et al., 2007).

For the present work, however, we will begin by comparing results with the simplified solutionof Renshaw and Pollard(1995). The premise of this solution is:

1. Consideration is limited to a semi-infinite crack nearly impinging on the interface, thatis, consider that the tip is a distanceδ from the interface wereδ is much smaller than thelength of the finite crack, hence simplification to the semi-infinite problem.

2. The distanceδ is also assumed to be much smaller thanrc, the radius of the process zone.Furthermore, the stresses inside the process zone are assumed to be smaller than or equalto those atrc and the leading order term of near-tip expansion of the elasticity solutionis taken to give a good estimate of the stress state at radiusr = rc. Henceσij(max), themaximum for each component of the stress, is taken to correspond toσij [rc(±π/2)], thatis, at a distancerc above/below the fracture as it impinges on the interface.

3. The heart of the criterion is then to simultaneously require zero slippage on the interfaceand that the maximum tensile stress acting parallel to the interface matches the tensilestrength of the rock. The second requirement is used to solvefor rc as a function of thetensile strengthTo for substitution into the first, no-slip requirement.

Upon substitution and evaluating constants,Renshaw and Pollard(1995) arrive to a criterionthat predicts crossing if

σh

To + σv>

0.35 + 0.35f

1.06, (2.1)

wheref is the friction coefficient of the presumed-cohesionless interface, and we have modifiednotation slightly to useσh andσv, the horizontal and vertical stress in our experiments, respec-tively, with a compression positive sign convention. Hencewe see that crossing is predicted,in a broad sense, provided that the horizontal compressive stressσh acting across the interfaceis sufficiently large relative to the sum of the tensile strength To and the vertical compressivestressσv acting along the interface, where “sufficiently large” is determined as a function thatdecreases as the value of the interface friction coefficientf increases. It will be useful to definethe “Crossing Stress Ratio”

T =σh

To + σv

(2.2)

3 Scaling and Experimental DesignIt is apparent that the solution ofRenshaw and Pollard(1995) does not attempt to capture allaspects of this complex, coupled problem. In particular, wewish in future stages of this experi-mental campaign to assess the influence of the viscous fluid flow and of the angle at which thehydraulic fracture impinges on the interface. From an experimental design perspective, the sec-

5

ond issue is relatively straightforward to address, in principle, by varying the angle of the platesto the applied principal stresses. The first issue, however,requires some attention to modellinghydraulic fracture growth when fluid flow is taken into consideration. Because the hydraulicfractures created in the laboratory are expected to be initially circular in their plan-view shape,the scaling relationships will be based on the governing equations for a circular hydraulic frac-ture (SeeSavitski and Detournay(2002), with modification to account for fluid lag byBunger(2005)).

The basic premise for design of circular hydraulic fractures in the laboratory that conform todifferent propagation regimes is outlined byBunger et al.(2005) andBunger and Detournay(2008). Details that can be ascertained from these previous workswill not be recapped here.To summarize, in the absence of fluid leak-off into the rock formation, one may describethe behaviour of circular hydraulic fractures as a simultaneous evolution: 1) From an earlytime regime in which the fluid occupies only a small portion ofthe fracture near its centre(Bunger and Detournay, 2007) to a large time regime in which the so-called lag between thefluid and fracture front vanishes, and 2) From and early time “viscosity dominated” regimein which viscous fluid flow is the predominant energy dissipation mechanism to a large time“toughness dominated” regime in which breaking of rock ahead of the crack tip is the predom-inant energy dissipation mechanism. Note that the toughness dominated regime is, therefore,characterised by vanishing viscous dissipation so that thefluid pressure in the fracture is spa-tially uniform.

These transitions can be shown to each take place relative toa particular characteristic timescale(e.g.Detournay, 2004). However, following the approach ofBunger and Detournay(2008), aconvenient approach for laboratory design is to express these transitions as taking place as thehydraulic fracture length increases relative to two characteristic length scales

Lom =E ′µ′1/3Q

1/3o

σ4/3v

, Lmk =E ′3µ′Qo

K ′4, (3.1)

whereLom andLmk are associated with the large lag to zero lag transition and the large viscousdissipation to zero viscous dissipation transition, respectively. Hereσv is the applied experi-mental stress acting perpendicular to the opening direction of the hydraulic fracture (i.e. theleast compressive principal stress),Qo is the constant volumetric injection rate, and

E ′ =E

1 − ν2, µ′ = 12µ, K ′ = 4

(

2

π

)1/2

KIc,

whereE is the Young’s modulus,ν is Poisson’s ratio,µ is the dynamic fluid viscosity, andKIc

the the rock fracture toughness. We note thatE ′ is the plane strain elastic modulus, while thealternate viscosity and toughness are used to eliminate numerical factors that would otherwiseclutter the governing equations.

The scaling analysis then leads to experimental design whereby we seekLom andLmk to take onvalues that are large or small relative to the distance to thefirst interfaceLs in order to achievefracture propagation under desired conditions. Based on the theoretical arguments outlined byBunger and Detournay(2008), we takeLs/Lmk . 0.06 to imply propagation in the viscositydominated regime andLs/Lmk & 2.4 to imply propagation in the toughness dominated regime,although these restrictions can be relaxed somewhat in a practical setting. Furthermore, fluidlag is not relevant to toughness dominated hydraulic fractures (e.g.Garagash and Detournay,2000), however, for viscosity dominated hydraulic fractures suppression of fluid lag is pre-

6

dicted theoretically forLs/Lom & 5.7, although again this restriction can likely be relaxedsomewhat in its practical application. For the present experiments (see summary in Table5.1),we haveLs/Lmk ∼ 100 andLs/Lom ∼ 0.5, and so we expect these experiments to be toughnessdominated but with the possibility of observable fluid lag.

Once a particular rock is selected for experimental use, therock propertiesE ′ andKIc are takenas fixed values. The friction coefficientf is also essentially a fixed value; it is a function ofthe chosen rock type and the surface grinding technique, neither of which are taken to varyfrom one test to the next, although these may vary within the context of a broader experimentalprogram that considers multiple rock types and/or multiplesurface preparation methods. Insummary, then, experimental design for this project consists of adjusting the stressesσh andσv,the fluid viscosityµ, and the injection rateQo in order to obtain either toughness or viscositydominated hydraulic fractures with negligible fluid lag that are expected to either cross or notcross according to the prediction of Eq. (2.1).

4 Description of Experiments

4.1 Geometry

The experiments were performed in nominally 400x360x350 mmspecimens assembled froma combination of one 400x360x150 mm and four or six 400x360x50 mm or 400x360x30 mmslabs of an Australian gabbro marketed as Adelaide Black Granite. Hydraulic fractures wereinitiated from one of three, 2 mm deep slots cut into the wall of a 16 mm borehole that wasdrilled into the central, 150 mm thick slab. These slots werecut at 100, 200, and 300 mm depthfrom the top of the specimen, with the purpose of allowing 3 experiments to be performedon each specimen. Figure4.1 shows a sketch of the specimen geometry, including a cartoonof a growing hydraulic fracture an one possible transducer array configuration for ultrasoundmonitoring of the fracture growth.

The specimens were prepared by grinding the 400x360 mm surfaces flat on a surface grinderusing a 60 grit Kinik silicon carbide (GC 60 K V) grinding wheel. The 5 or 7 slabs that wouldbecome a single specimen were then assembled and held together in a jig so that the outersurfaces of the specimen could be ground flat and square relative to one another. This ensuresthat the slabs used to make a single specimen are the same sizeto machine tolerance ( 10microns). Figure4.2shows the specimen as it is being surface ground.

4.2 Laboratory Apparatus

4.2.1 Fracture Fluid Injection System

In order to grow a hydraulic fracture through a rock specimensubject to horizontal and verticalexternal load, two separate pump systems are utilised.

The first of these systems uses a computer-controlled syringe pump with a stepping motor toinject fracture fluid into the rock at the location of the notch (Figure4.3). In this system distilledwater is pumped from the syringe pump into a fluid interface vessel. This interface vesselpressurises fracture fluid via a silicone balloon.

7

Figure 4.1: Specimen geometry.

8

Figure 4.2: Surface grinding of a rock specimen.

Figure 4.3: Fracture Fluid Injection System

9

Figure 4.4: External Pressure Application System

4.2.2 External Pressure Application System

The second pump system supplies high pressure water to a set of flat jacks positioned on eachof the block faces. These flat jacks expand under the applied pressure to exert a confining stresson the block. The confining stresses can be different for eachof the X,Y and Z axis, althoughfor these experiments both the horizontal stresses were identical while the vertical stress washeld at a value that was less than the horizontal stress and determined according to the designmethod discussed in Section3.

As shown in Figure4.4, the pressure in the horizontal flat jacks was applied firstlyvia a HaskelPump then for more fine control with an ISCO 260D syringe pump.The vertical pressure wasapplied via a single Haskel Pump with a pressure accumulatorincluded in the line to providesome elasticity to the external loading system. This softening of the system aids in maintaininga constant pressure throughout crack growth.

4.2.3 Polyaxial Load Frame

The force applied by the flat jacks on the sample block is contained within a polyaxial loadframe. This frame surrounds the sample in the horizontal plane and reacts against the flat jacksin order for the flat jacks to provide loading to the specimen.The reaction frame for the verticalstress consists of a steel lid that is connected to a steel floor plate by way of four threadedcolumns.

4.2.4 Injection Tool

In order to initiate a hydraulic fracture at a controlled depth, pressurised fracture fluid mustbe supplied specifically at the location of the borehole notch. This pressurisation at a specific

10

Figure 4.5: Fracture Fluid Injection Tool

location is achieved via an injection tool that isolates theoutlets of the fracture fluid injectionsystem via two o-rings. A channel allows air to bypass the o-rings thus equalising pressureabove and below the tool during insertion and removal.

Figure4.5shows a close-up view of the injection tool along with a steelrule for reference. Theo-rings are clearly seen above and below the outlet holes on the lower section of the tool. Notethat the bypass channel is not visible in the figure.

4.2.5 Flow Control

The flow of the fracture fluid is controlled via a needle valve.This valve allows fine adjust-ment to the cross-sectional area of the pipe that the fracture fluid flows through. For mostexperiments the valve is sufficiently open so that the pressures stabilise between the upstreamand downstream sides of the valve during injection, while sufficiently closed to prevent a highvelocity flow into the sample upon breakdown.

4.2.6 Pressure Monitoring

The pressures of both the flat jacks and the fracture fluid are monitored and recorded everysecond over the duration of the experiment. AB type in-line pressure transducers are utilisedfor this function. The fracture fluid pressure is monitored both upstream and downstream of theflow-control valve to enable precise throttling of the fluid injection system.

4.3 Ultrasound Monitoring

During the experiments, the rock sample is continuously monitored using ultrasonic P-waves.The approach and system that is being developed is similar tothat introduced byGroenenboom and Fokkema(1998) and should eventually allow real-time determination of thecrack tip location and measurement of the full-field fracture width. The present work, however,

11

Figure 4.6: Wave fields generated by the interaction of a plane wave with a hydraulic fracture

only considers the problem of fracture detection, with estimation of the full-field fracture widthbeing reserved for later study.

The physical basis for ultrasound monitoring is depicted inFigure4.6. A plane ultrasonic P-wave propagates toward a hydraulic fracture from below and,owing to the sudden change inacoustic properties, excites three new wave fields:

1. A diffracted wave field emanating from the fracture tip,

2. A reflected wave field, and,

3. Interface-waves along the hydraulic fracture.

A portion of the incident wave field also propagates through the hydraulic fracture into the upperhalf-space as a transmitted wave. For low viscosity fluids, interface waves are not significant.

Ultrasound monitoring of hydraulic fractures therefore amounts to detecting wave reflections,transmissions and diffractions, and relating certain characteristics of those waves to the fracturegeometry. Wave diffractions are, however, only associatedwith the fracture tip and quantifica-tion of the fracture geometry can, for the most part, be achieved using measurements of onlythe reflected and/or transmitted wave field.

The primary goal of the ultrasound monitoring experiments conducted in this study is to detectthe presence of a hydraulic fracture using measurements of the reflected wave field.

4.3.1 Ultrasound system

The core component of the ultrasound system is a DSPUT5000 pulser-receiver unit manufac-tured by US-Ultratek which is connected internally to a Dellworkstation computer via conven-tional 32bit PCI bus. This unit has separate output and inputchannels which can be operatedin either pulse-echo or pitch-catch mode. The output signalis a 50-480ns square wave pulsewhich is appropriate for generating ultrasonic waves in thefrequency range of 1-10MHz. Theonboard digital signal processor digitises the received signals at rates of up to 100MHz andis also able to perform basic numerical processing such as band-pass filtering and ensembleaveraging. The output/input channel of the DSPUT5000 is dynamically switched between 32independent actuator/receiver channels by 2 DT16b PCI multiplexor boards, also manufacturedby US-Ultratek.

Each experiment utilises portions of a 5x5 array of transducers arranged as a rectangular arrayusing a custom built loading platen (Fig.4.7). The platen has recesses for VT102 Videoscan

12

Figure 4.7: Ultrasound transducer platten that was used in all experiments.

transducers from OlympusNDT, which generate 1MHz P-waves,and machined conduits forthe signal wires. The transducers are each fluid coupled to the surface of the rock sampleusing honey, which is known to be one of the best ultrasound couplants for P-wave transducers(Pongracz et al., 1996). The transducers are typically placed on the side of the specimen that isparallel and nearest to the expected fracture plane. The entire array is interrogated at a rate inthe range 1-5Hz. Figure4.8shows the transducer locations used for each of the experiments.

4.3.2 Modelling

The main premise for modelling wave propagation through hydraulic fractures is that the frac-ture width varies only slowly across the fracture. Also, in the context of wave propagation, thefracturing fluid is treated as being inviscid and the rock material is assumed to be isotropic. Thefracture can thence be treated locally as a thin, uniform width layer of fluid that is sandwichedbetween two semi-infinite half space which represent the rock material. The model is a 3 layersystem, as depicted in Figure4.9.

In generating an analytical description of the problem, there are 4 possible wave fields in eachlayer that need to be considered, namely upward and downwardtravelling P- and S-waves. It issomewhat customary in the ultrasound literature to describe these waves in terms of two elasticpotentials,

φ = P+eiω[ xα

cos γ+ y

αsin γ−t] + P−eiω[ x

αcos γ− y

αsinγ−t] (4.1)

ψ = S+eiω[ xβ

cos ν+ y

βsin ν−t] + S−eiω[ x

βcos ν− y

βsin ν−t] (4.2)

whereω is the angular frequency of the incident waves,α andβ are the wave velocities ofP- and S-waves respectively with the overbar being used to differentiate the velocities of thefluid {α, β} from those of the rock material,{P+, P−, S+, S−} are the wave amplitudes and,

13

Figure 4.8: Location of transducers elements used in each ofthe experiments. Active element,# inactive element, − machined rock surfaces.

14

Figure 4.9: Three-layer model of wave propagation through ahydraulic fracture.

φ is related to dilatational wave motions andψ to shear motions. The model is developed bysuperimposing these fields and applying appropriate boundary conditions at each of the tworock-fluid interfaces. The wave amplitudes can then be determined and constitute a completeanalytical description of the problem.

For example,Rokhlin and Wang(1991) have formulated this approach into a matrix problemfrom which they derive expressions for the amplitude of the reflected and transmitted wavesfor when the incident wave field is propagating normal to the fracture plane, i.e.γ = ν =γ = ν = π/2. They simplify the analysis by assuming that the fluid layer is very thin so thatkh ≪ 1, wherek is the wavenumber for P-waves in the fluid andh is the layer thickness. Inthe case of an inviscid fluid, the appropriate boundary conditions are continuity of normal stressσyy and normal displacementuy, and vanishing shear stressσxy = 0, which leads to a reflectioncoefficient given by,

Rll =

Z2

f

Z2r− 1

Z2

f

Z2r

+ 1 − 2iZf

Zr

αhω

(4.3)

whereZR = αRρR is the acoustic impedance of the rock material andZF = αFρF that of thefluid. Noting the imaginary component in the denominator, the presence of a thin fluid layertherefore alters both the amplitude and phase of the reflected waves.

The variation of the reflection coefficient with respect toZr/Zf andhω/α is shown in Figure4.10. Here it is seen that the dynamic range ofRll is greatly affected by these parameters.For smallZr/Zf , one can expect to resolve a large range of fracture width with a fairly coarseresolution, whereas at largeZr/Zf one can expect very fine resolution for barely open fractures,with complete reflection of the incident wave occuring at a relatively lower fracture width. SinceAustralian Black Granite hasZr/Zf ≈ 8.5, the present experiments should be characterised bythe latter behaviour and the ultrasound monitoring system is thus expected to provide excellentdetection sensitivity for growing hydraulic fractures in this rock material.

4.4 Rock and Fluid Properties

4.4.1 Elastic Properties

Axial compression tests were carried out on 25 mm diameter cores of the Adelaide Black Gran-ite and the stress-strain response was measured. From a plotof axial stress vs axial strain

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0 2 4 6 8 100

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Acoustic Impedance Ratio, Zr / Z

f

Fra

ctur

e W

idth

, kfh

R

ll = 0.1

Rll = 0.3

Rll = 0.5

Rll = 0.7

Figure 4.10: Variation of reflection coefficient for a thin fluid layer.

(Figure4.11) the Young’s modulus was determined to be 102 GPa.

Poisson’s ratio was obtained by comparing the axial strain against the circumferential strain. Ascan be seen in Figure4.12the data points are tightly grouped and the results from the two testsare very similar. The average value of Poisson’s ratio from the two axial compression tests forAdelaide Black Granite was calculated as being 0.27.

4.4.2 Tensile Strength

The tensile strength of the Adelaide Black Granite has been determined through a series ofBrazilian (indirect) tension tests. The Brazillian test series consisted of ten specimens, eachmachined to a diameter of approximately 62 mm and a thicknessof 32 mm. The mean tensilestrength from 10 experiments was found to be 10.9 MPa with a standard deviation of 1.50 MPa.

4.4.3 Fracture Toughness

The nominal value for the fracture toughness (KIc) was found using semi-circular beam tests(Lim et al., 1993) to be in the range of 1.27-2.98 MPa m1/2, increasing with specimen size, withthe notch length ranging from 1.5-14 mm (Makhnenko et al., 2010). For these experiments weadopt and intermediate value from this range,KIc = 2.5 MPa m1/2.

4.4.4 Interface Friction Coefficient

The friction coefficient (f ) of the surface-ground rock surfaces (Section4.1) has been measuredthrough a series of Direct Shear experiments. The reaction and normal forces from the DirectShear experiments have been compared in Figure4.13. The average friction coefficient (f ) hasbeen calculated as 0.17.

4.4.5 Fluid Viscosity

Two different fracture fluids were prepared. The fluids were designed to have different vis-cosities to enable fracturing under both viscosity dominated and toughness dominated regimes,

16

Test 1- Young's Modulus (E) = 100.1GPa

0

5

10

15

20

25

30

35

0.0000% 0.0050% 0.0100% 0.0150% 0.0200% 0.0250% 0.0300%

Axial Strain (�)

Ax

ial

Str

es

s (

MP

a)

Test 2- Young's Modulus (E) = 104.6 GPa

Figure 4.11: Axial Stress vs Strain from Unconfined Compressive Tests

Test 1- Poisson's Ratio (�) = 0.28

0.0000%

0.0010%

0.0020%

0.0030%

0.0040%

0.0050%

0.0060%

0.0070%

0.0080%

0.0090%

0.0000% 0.0050% 0.0100% 0.0150% 0.0200% 0.0250% 0.0300%

Axial Strain (�)

Cir

cu

mfe

ren

tia

l S

tra

in (�)

Test 2- Poisson's Ratio (�) = 0.27

Figure 4.12: Strain Values from Unconfined Compression Test

17

y = 0.1734x

0

200

400

600

800

1000

1200

0 1000 2000 3000 4000 5000 6000

Normal Force (N)

Re

ac

tio

n F

orc

e (

N)

Figure 4.13: Direct Shear Test Results

although this first series of experiments makes use of only the lower viscosity fluid (TF50). Themajor ingredient in the less viscous fluid (TF50) was glycerol, while the major ingredient in themore viscous fluid (TF49) was glucose. The recipes for both fluids can be seen in Table4.1.

TF49 TF50Ingredient % by mass % by massGlycerol 0% 79.2%Glucose 94% 0%

Blue Food Dye 1.25% 2.6%H20 3.75% 17.4%

TiO2 Powder 1% 0.8%

Table 4.1: Fracture Fluid Recipes

Rheological Consulting Services (RCS,Kilcullen, 2009) was commissioned to prepare a reportdetailing the viscosity at20◦C and the Newtonian behaviour of the fluids. The summary of thereport is included below.

The CSIRO hydraulic fracturing lab is using Glycerol/TiO2 and Glucose/TiO2mixtures to simulate fracturing fluids. As part of this study, it is important to un-derstand the rheology of the fluids, particularly whether the fluids are Newtonian ornon-Newtonian. Thus RCS undertook a rheological investigation of two samplesTF49, glucose based, and TF50, glycerine based. Although the samples displayedNewtonian behaviour the TF49 had a viscosity that is greaterthan 1000 times thatof the TF50 sample. At a shear rate of 10 s-1 the TF49 had a viscosity of 95.2 Pa.s

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and the TF50 sample had a viscosity of 0.0347 Pa.s.

5 Results

5.1 Summary of Experimental Results

The results from experiments G1 - G10 are summarized in Table5.1. Based on the analysisdetailed in Section3, we haveLs/Lmk ∼ 100 andLs/Lom ∼ 0.5, and so we expect theseexperiments to be toughness dominated but with the possibility of observable fluid lag. Thecrossing behaviour over the interface between the centre block and the surrounding blocks wasrecorded for each fracture. Interface crossing behaviour has been classified as crossing, non-crossing or partially crossing. Crossing refers to a situation where the fracture propagates acrossthe middle/outer block interface and through the surrounding granite blocks. Non-crossingrefers to a case where the fracture fluid leaks into the interface and the hydraulic fracture doesnot continue to propagate into the surrounding blocks. Partial crossing most commonly refersto a situation where a fracture has clearly initiated on the far side of the first interface, but hasnot continued to propagate through the second block.

Experiment Breakdown Horizontal Vertical Crossing Crossing DatePressure Pressure Pressure Stress Ratio(MPa) (MPa) (MPa) (Eq. 2.2) dd/mm/yy

G1 25.1 5.6 4.9 No 0.35 06/02/09G2 22.4 3.6 3.0 No 0.26 06/02/09G3 33.5 20.0 6.6 Yes 1.14 05/03/09G4 31 17.6 6.0 Yes 1.04 19/03/09G5 35 12.8 6.0 Partial 0.76 26/03/09G6 44.5 14.4 6.0 Partial 0.85 02/04/09G7 30 14.4 6.0 Partial 0.85 28/04/09G8 ≈50 12.8 4.1 No 0.85 05/05/09G9 32.8 12.8 6.0 No 0.76 04/06/09G10 ≈38 16 6.0 Partial 0.95 09/06/09

Experiment Lmk (Eq. 3.1) Lom (Eq. 3.1) Ls (Sect.3)(m) (m) (m)

G1 6.6E-4 0.17 0.075G2 6.6E-4 0.32 0.075G3 6.6E-4 0.11 0.075G4 6.6E-4 0.13 0.075G5 3.3E-4 0.10 0.075G6 6.6E-4 0.13 0.075G7 6.6E-4 0.13 0.075G8 6.6E-4 0.21 0.075G9 6.6E-4 0.13 0.075G10 6.6E-4 0.13 0.075

Table 5.1: Summary of experimental data.

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Figure5.1shows a Renshaw and Pollard plot of the experiments performed in this series alongwith those ofLlanos et al.(2006). The striking result is that the crossing behaviour appearsto be essentially independent of the friction coefficientf , depending only on the value of thecrossing stress ratioT .

1.00

10.00

Cro

ssin

g S

tres

s R

atio

(σ

h/To

+σ

v) HFNF-HFRABG Gabbro

AcrogemSandstone

GreyGranite

0.10

0.15 0.25 0.35 0.45 0.55 0.65 0.75

Cro

ssin

g S

tres

s R

atio

(

Co-efficient of Friction (ƒ)

Theoretical Threshold

No Crossing

Partial Crossing

Full Crossing

Figure 5.1: Renshaw and Pollard plot of results.

5.2 Ultrasound Monitoring

Results from the ultrasound monitoring system are summarised in Table5.2. Several difficultieswere encountered for experiments G1-G6 which were the result of either:

1. Attenuation of the incident wave: The fracture plane was too far from the transducers andall signal energy was attenuated by material damping.

2. System failures: The software versions used in experiments G1 through G5 were not ableto sample at a sufficient rate to capture hydraulic fracture growth. The pulse repition ratein experiment G6 was too fast for the DSPUT5000 pulser-receiver to maintain the 400Vthat is required to excite waves in the rock;

The second issue was rectified by commissioning new software. The first issue was rectified byintroducing hydraulic fractures closer to the transducer array. In tests G7-G10 the fracture wasdetected, however it did not (fully) cross the interface andtherefore could not be detected in theouter plates.

For example, Figure5.2 shows the time history that was recorded by one of the transducersduring experiment G10. Here, the abscissa axis represents the travel time of reflected ultrasonic

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Experiment Ultrasound Result Comments

G1 No fracturedetected.

Fracture plane too far from transducers(300mm).

G2 No fracturedetected.

Fracture plane too far from transducers(150mm).

G3 No fracturedetected.

Fracture plane too far from transducers(300mm).

G4 No fracturedetected.

Fracture plane too far from transducers(200mm).

G5 Fracture detected. Transducers focused on outer plates ofrock sample; fracture only partially

crossed inner interface; sample rate tooslow.

G6 Equipment failure. Array sampling rate too high. Pulsevoltage fully depleted before fracture

initiated.G7 Fracture detected. Array only focused on near-wellbore

region. Fracture completely grewthrough this region in between

consecutive array samples.G8 Fracture detected. Array focused on inner interfaces;

Fracture did not cross inner interface;Insufficient data.

G9 No fracturedetected.

Fracture plane too far from transducers(200mm).

G10 Fracture detected. Fracture did not cross interface.

Table 5.2: Summary of results for ultrasound monitoring system.

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Figure 5.2: Time trace signals from a single transducer comparing the arrival time of reflectedwaves with the physical location of the hydraulic fracture.

waves measured relative to the firing time of the transducer.Time trace signals from successivescans are cascaded down the ordinate axis. The appearance ofa reflected wave is clearly evidentin these plots, with the arrival time decreasing as the fracture approaches the exposure aperturefor the transducer and levels off when the fracture has completely passed. These results suggestthat the sensitivity of the transducers to detect a growing hydraulic fracture is best when thefracture has passed the centre of the transducer aperture. This conclusion also supports the useof a pulse-echo transducer configuration, as used here, compare to, for example, a reflected wavepitch-catch configuration using adjacent transducers, as each transducer can only interrogate therock material directly beneath it. Other configurations could be successful though, albeit at theexpense of reduced signal amplitude.

By considering the relative timing of pulse arrivals at eachof the transducers in the array, itis possible to construct a fracture evolution map as shown inFigure5.3. There, the locations

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of the frictional interfaces are shown as solid lines and transducer locations as circles. Therelative timing of positive detection at each of the transducer locations is indicated with theunits of seconds. Positive detection is defined as when pointB in Figure5.2 is observed inthe time trace measurements made by each transducer, and theindicated detection times aremeasured relative to positive detected at the centre transducer, which is located directly beneaththe injection wellbore.

At each snapshot in time, the transducer locations where thefracture has been positively de-tected have been shaded yellow and the series of time-lapse images provide a graphical repre-sentation of the fracture growth pattern. For this experiment (G10), it is seen that the fractureinitially has a quasi-circular footprint until it encounters the first interface on the western side ofthe block. At that point, it grows through the first interfaceand then through the first side plateuntil it and impinges on the second interface. Growth in the western direction is then arrestedand an initial penetratin of the first interface on the eastern side of the block is detected. Fur-ther growth is then observed in the north-south direction along the centre block. The fracturefootprint then grows beyond the region that is monitored by the ultrasound array.

This behaviour is support by the visual observations recorded after completion of the experiment(Figure5.4). These photographs verify the fracture crossing on the western side of the blockand an initial penetration of the first interface on the easter side of the block.

It is envisaged that later studies will attempt to significantly improve the resolution of the ultr-sound fracture mapping for better understanding of the propagation behaviour of the hydraulicfracture just prior to, during and immediately after crossing these interfaces. Future studies willutilise all transducers in the 5x5 array as used in experiments G9 and G10.

6 ConclusionsTen experiments have been completed in which hydraulic fractures have been created in Ade-laide Black Granite specimens in order to investigate the growth of a hydraulic fracture thatimpinges orthogonally on a frictional interface. Ultrasound monitoring, which was developedin conjunction with these experiments, was successful provided that the transducers were closeenough of the fracture plane. In one example, non-symmetricfracture growth about the injectionpoint was determined based on ultrasound data and corroborated with observations. The cross-ing behaviour observed in all ten experiments, combined with past experiments, contributes toan emerging picture of hydraulic fracture growth through anorthogonal natural fracture that isnot dependent on the friction coefficient of the interface. This is at odds with the basic premiseof Renshaw and Pollard(1995) and challenges the current modelling paradigm. If additionalexperiments in other rock types also give indication that the friction coefficient of the interfaceis not a parameter governing the crossing behaviour, then a new paradigm ought to be proposedin which interface plasticity plays a reduced role.

AcknowledgmentThis work was supported by CSIRO Energy Transformed Flagship and the Oil, Gas and Geother-mal Portfolio as part of a project aimed at modelling hydraulic fracturing in hot fractured rockgeothermal reservoirs.

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Figure 5.3: Fracture evolution map for experiment G10 showing the arrival time, with the unitsof seconds, of reflected waves relative to the fracture initiation. Yellow shaded transducers havepositively detected the fracture at a) t = 0s, fracture initiation; b) t = 4s; c) t = 7s; d) t = 10s; e) t= 30s; f) t = 90s. time.

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Figure 5.4: Photographs of interface surfaces on western side of the block. a) Western surfaceof inner block, b) inner and c) outer surface of interface 1, d) inner and e) outer surface ofinterface 2.

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Figure 5.5: Photographs of interface surfaces on western side of the block. a) Eastern surfaceof inner block, b) inner and c) outer surface of interface 1, d) inner and e) outer surface ofinterface 2.

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