SPE 124919

Measuring Hydraulic Fracture Growth in Naturally Fractured Rock R.G. Jeffrey, SPE, and A.P. Bunger, SPE, CSIRO Petroleum; B. Lecampion, SPE, Schlumberger; X. Zhang, SPE, and Z.R. Chen, SPE, CSIRO Petroleum; A. van As, Rio Tinto; D.P. Allison and W. de Beer, Golder Associates; J.W. Dudley, SPE, Shell E&P; E. Siebrits, SPE, and M. Thiercelin, SPE, Schlumberger; and M. Mainguy, SPE, Total E&P

Copyright 2009, Society of Petroleum Engineers This paper was prepared for presentation at the 2009 SPE Annual Technical Conference and Exhibition held in New Orleans, Louisiana, USA, 4–7 October 2009. This paper was selected for presentation by an SPE program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of SPE copyright.

Abstract A field experiment was carried out to measure hydraulic fracture growth in naturally fractured rock. Hydraulic fracture interactions with pre-existing natural fractures, shear zones, veins, and adjacent hydraulic fractures were measured and mapped during the project. Tiltmeter and microseismic arrays were installed to test the performance of these monitoring methods in determining the fracture geometry, which was eventually revealed by the mine-through mapping. The physically mapped fractures were oriented approximately horizontally, perpendicular to the minimum stress direction. They crossed natural fractures and shear zones, but were offset by some shear zones, most often oriented with an approximate 45° dip. The analysis of the tiltmeter data correctly predicted fractures to be horizontal. Microseismic monitoring, although a proven method for imaging hydraulic fractures, did not resolve the fracture orientation or size for conditions at the E48 Northparkes site because of a lack of recorded micro-seismic events. The hydraulic fractures grew through solid rock, along natural fractures and stepped along inclined shear zones. Proppant was distributed throughout the fractures, including in the offset portions. Initial modeling indicates higher treatment pressure and slower extension rate for a stepped 2D hydraulic fracture compared to a straight fracture. Introduction Hydraulic fracture growth through naturally fractured reservoirs presents theoretical, design, and application challenges. High treating pressure, unplanned screenout, and shorter-than-designed propped fractures are some problems that result (Thiercelin and Makkhyu 2007; Zhang and Jeffrey 2008; Beugelsdijk et al. 2000; Wu et al. 2004). An experiment to measure hydraulic fracture growth in a naturally fractured rock at a mine site was, therefore, carried out to obtain details of fracture geometry from physical mapping during and after mining. Northparkes Mines, located 300 km west of Sydney, Australia, provided the site for this experiment. The mine is developing a new copper-gold porphyry orebody called E48, which will be mined by block caving methods. The rock contains numerous veins, natural fractures, and shear zones. In 2006, prior to preconditioning, to verify fracture growth and interaction with shear zones in the rock mass, a mine-through of several hydraulic fractures placed ahead of a development tunnel was undertaken. The monitored and mined fracture project described in this paper was developed to enable additional fracture monitoring and analysis to be included in the project. The orebody was preconditioned by hydraulic fracturing in 2008 under a separate project.

Results from this work are pertinent to the application of hydraulic fracturing to stimulation of naturally fractured oil and gas reservoirs (Warpinski 1991; Settari 1988; Warpinski and Teufel 1987), to stimulation of geothermal and hot dry rock reservoirs (Sanyal et al. 2000), and to preconditioning of ore bodies prior to mining (Brown 2003; van As and Jeffrey 2002). Of special interest are the interaction of the hydraulic fractures with shear zones that exist in the rock mass, and the direct comparison of fracture geometry obtained by remote monitoring and by direct physical mapping. Description of Site Tunnel ED4 was driven as the first entry into the planned extraction level for the E48 orebody at a depth of 580 m below the surface. This tunnel was driven to the edge of the orebody as indicated by the green-colored portion shown in Fig. 1. An HQ-size (96-mm diameter) core hole (E48D102) was then drilled along the planned path of ED4. This borehole is shown as a red line in Fig. 1. An openhole straddle packer tool was used to isolate short sections along the hole and place the hydraulic

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fractures. The grey-colored tunnels shown in Fig. 1 were not present at the time of the work described in this paper. ED4 was eventually driven along the path of the borehole, exposing the hydraulic fractures, which were then mapped in detail.

A large amount of information about the

rock mass was already in mine databases while other data was measured as part of the orebody characterization process. Data from various sources are summarized in this paper and include fracture sets, in situ stress (by 3D overcoring), mechanical properties (by core testing), and rock mass permeability (by well testing). The orebody has been extensively sampled by core holes drilled from the surface, and core logging provides geology and structure data. The mine photogrammetry hardware and software system was used to obtain 3D digital stereo images after each face advance during mining of the fractures (Poropat 2006). These images were analyzed to extract coordinates of the hydraulic fractures where direct survey data was not available.

The host rock around the orebody, which also contains mineralization, consists of shallow SSE-dipping volcaniclastic sediments. These volcanics are intruded by the monzogranitic and quartz monzonite porphyries. Minor basic dikes exist throughout the deposit. The Altona Fault is a shallow thrust fault with ESE movement that cuts off the top of the orebody at a depth of 100 m or less (Crook 2006). Its alignment is consistent

with the current direction of the maximum horizontal stress as measured by overcoring (Mills 2007). Fig. 2 contains a vertical section through the E48 orebody looking east, showing the Altona Fault, the volcaniclastic sediments, and several monzonite porphyry intrusions. Nearly vertical fault zones or shear zones have been identified from core and are also shown in this figure. The fault zones contain sericite-quartz-carbonate minerals. The veining, which is prevalent in some parts of the orebody, consists of quartz and carbonate filling in predominately sub-vertical NW-trending vein sets.

Scan line mapping was performed along a 150 m long section of both the north and south walls of ED4. Data on natural fracture dip, dip direction, filling material and rock lithology were collected. A stereo plot of fracture orientation from these scan lines is shown in Fig. 3. The scan line data reveals that the vertical and sub-vertical natural fractures have a somewhat higher frequency than other fracture dips. The most frequent vertical fractures strike northwest-southeast, although the strike varies from north-south to nearly east-west.

Mechanical properties of the rocks have been measured by the mine using HQ core (63.5 mm diameter) samples. Properties measured include the uniaxial compressive strength, density, and elastic properties (Young’s modulus and Poisson’s ratio). Table 1 provides a summary of these properties for the major rock types sampled (van As 2004).

Fig. 1: A perspective view of the site, showing the microseismic accelerometers installed on and above the 9700 level to monitor the fracturing trial. The horizontal borehole E48D102 is shown in red. The grey tunnels did not exist at the time the hydraulic fracturing was carried out.

Table 1: Rock properties from core tests Lithology UCS

(MPa) Young’s Modulus

(GPa)

Poisson’s Ratio

Bulk Density kg/m3

FQS 95.7 53.8 2720 POR 136.5 51.0 0.17 2660 VCCL VBR

93.8 50.8 0.11 2750

VSS 127.4 60.0 0.22 2780

FQS – Fault Quartz Sericite alteration assemblage. POR – Monzonite Porphyry VCCL –Volcaniclastic VBR – Volcanic breccia VSS – Volcanic sandstone

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The values given in Table 1 are representative of the core sized samples. The rock mass, which includes fractures, veins, and shear zones, will be of lower modulus and strength.

The stresses at Northparkes have been measured previously

using overcoring methods. The closest measurement to E48 was located at the Lift 1 extraction level of E26, which is about 1700 m south of E48. Because stress is an important parameter for development and extraction of the orebody, a measurement was made at 143 m chainage in ED4 (at 11005E, 55213N). Table 2 summarizes the older E26 and this newer E48 stress measurement results. The principal stress vectors are plotted in Fig. 3 (based on NPK8) along with poles to the hydraulic fracture planes determined by physical mapping and from analysis of the surface tiltmeter data.

The site in ED4 was located on the 9700 m RL level of the mine, 580 m below the surface. The site at E26 was at 9800 m RL, about 450 m below the surface. Both of the measurements at the E48 site were made in the same borehole, separated by 340 mm. The minimum principal stress is oriented normal to the hydraulic fracture plane and these measurements indicate the fractures should form with a sub-horizontal orientation, which they did. The magnitude of the minimum stress is similar to the measured fracture closure pressure value of 15 MPa (from the falloff of Fracture 9). Table 2: Stress measurements at E48 and E26 by overcore method Measurement Site 1σ 2σ 3σ

Mag. (MPa)

Dip (deg)

Azimuth (deg)

Mag. (MPa)

Dip (deg)

Azimuth (deg)

Mag. (MPa)

Dip (deg)

Azimuth (deg)

E48 NPK8 41 8 290 20 11 022 12 76 165 E48 NPK9 39 10 102 24 10 010 18 76 237 E26 in BQM 23 5 141 15 15 049 12 75 248 Using a rock density of 2700 kg/m3, the vertical stress arising from the weight of the overburden at 580 m depth would be 15.7 MPa, which is consistent with the values shown in Table 2. The hydraulic fractures were physically mapped as sub-horizontal, mostly dipping south to west at about 10°. Analysis of the tilt data (Lecampion 2006) also gave a sub-horizontal fracture orientation and provided estimates of dip direction ranging from south to southeast. The stress measurements are in agreement with the dip magnitude but not the dip direction (the stress data suggests a dip direction of 345° for NPK8 and 57° for NPK9 data) of the mapped fractures. The stress measurements were located about 45 m east of Fracture 5 (Mills 2007).

Remote Monitoring The mine extended its mining microseismic array to make possible monitoring seismicity induced by the hydraulic fracturing. As part of the consortium work, surface and underground tiltmeter arrays were installed to obtain fracture orientation data for

Fig. 2: Vertical section through ore body looking east, showing geology and approximate location of tunnel ED4.

Fig. 3: Lower hemisphere concentration plot of fractures and joints mapped by scan lines along north and south walls of ED4. The principal stress vectors and poles to the hydraulic fractures (physically mapped and from tilt data inversion) are shown.

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comparison to the physical mapping. Two tube-based piezometers were also installed into surface boreholes as additional monitoring instrumentation. Microseismic Array

A total of 10 triaxial accelerometers were installed with 4 of the 10 sensors installed in boreholes drilled along the access tunnel to E48 and the rest installed in boreholes drilled from the surface. Fig. 1 shows the locations of the accelerometers. The green access tunnel system was completed as shown in this figure, with the grey tunnels yet to be driven. The red line indicates the position of borehole E48D102, from which the hydraulic fractures were placed.

The accelerometers were housed in water-proof sondes which were run on cable into 96-mm diameter boreholes. The sensors were then grouted into place by pumping cement into the hole through PVC conduit to fill the hole from the bottom up. The accelerometers installed underground were inserted into holes drilled upward into the tunnel back (roof). These instruments were cemented into place by filling the hole from the bottom up while venting air through a conduit run to the top of the hole.

The array was active during the hydraulic fracturing treatments carried out in E48D102. During the period of the fracturing, more than 8000 events were recorded. Of these, 383 events were located with acceptable accuracy, and these were subjected to further analysis. The analysis identified five clusters of events, which are shown, in plan view, in Fig. 4 (de Beer 2008). However, none of the clusters coincide in space with the locations for the hydraulic fractures. The location accuracy has been verified by observing events associated with tunnel blasting as ED4 was driven through and beyond the fractures. These blasting-induced events appropriately locate along the tunnel path. Hence, the tendency of the events to not be co-located or synchronized with the hydraulic fractures is not a result of inaccuracy in the measurements. The events that were detected occurred during and after the hydraulic fracturing injections, but were not located in the immediate vicinity of the fractures. It seems most likely that events nearer the hydraulic fractures were not recorded because of a combination of signal attenuation, with sensors located more than 100 m from the fracture initiation points, and the non-continuous mode of operation of the microseismic system. A continuous mode of operations was not possible because the system is not designed for this in terms of available bandwidth and data storage capacity. Events that were detected tended to be located around the four accelerometers installed from underground, which were also the closest sensors to the hydraulic fractures.

Tiltmeters

Both a surface and an underground array were installed at the site. Both arrays used commercial tiltmeters, which can resolve less than 5 nanoradians. The surface tiltmeter array consisted of instruments installed in 11 vertical boreholes, each 7 to 9 m deep. Each hole was cased with PVC casing, which was cemented into the hole. The instruments were placed in the bottom of the cased holes and 30/60-mesh quartz sand was placed around them to couple them to the casing-ground deformation field. The tilt data was analyzed and reported before the mine-through mapping of the hydraulic fractures took place (Lecampion 2006), and this interpretation was, therefore, not influenced by any knowledge of the fracture orientation as determined by the physical mapping. The underground array consisted of nine tiltmeters installed in vertical holes drilled into the floor of the access tunnel. Difficulty in drilling these holes resulted in the instruments being installed at 1.5 m to 2 m below the tunnel floor. The underground tiltmeter array suffered from tilt components produced by the deformation of the tunnel. This tunnel effect tilt (Harrison 1976) overwhelmed the signal from the fractures and made inversion of the data from the underground array impossible. The tiltmeters should be installed at least one tunnel diameter below the tunnel floor to avoid this problem. The surface tiltmeter array is shown in Fig. 5, superimposed onto the existing and proposed underground tunnel system.

The tilt vector plot associated with Fracture 6 is shown in Fig. 6. The inversion process for fracture treatment 6 gave a dip of 10° and a dip direction of 200°. The forward model used in the inversion assumes the fracture is a single planar feature (Lecampion et al. 2005). The offsets mapped during mining respond to internal pressure in a way that induces some shear deformation into the rock (Jeffrey et al. 2009), and this affect was not accounted for in the analysis. In addition, shearing on natural fractures in the rock mass can be expected to arise because of stress changes occurring around the hydraulic fracture. However, despite these un-accounted for sources of tilt, the analysis of the tilt data resulted in good agreement for fracture orientation with the physically mapped fractures. Analysis of Fracture 5, however, predicted a sub-vertical fracture orientation while mining revealed a sub-horizontal fracture was formed by this treatment. Such poor agreement might result from pressurization of a larger number of fracture segments with different orientation or strong shear displacement on a fault-like structure in the rock mass. Piezometers

Holes E48D59 and E48D14 each had a small nylon tube run into them using a steel cable for deployment. The cable carried the load, and the tube and cable were taped together every 5 m as they were run in. Gravel with a grain size of 2 mm to 5 mm was then placed to fill the hole to a level 100 m above the end of the nylon tube. Cement was placed on top of the gravel column to seal the piezometer tube in the hole. The piezometer in D14 was located at 380 m below the collar, and the tube end in D59 was located at 500 m. The piezometer in D59 had a check valve installed on the downhole end that only allowed flow from the hole into the tube. A pressure transducer was attached to the collar end of the nylon tube at each hole with a data logger attached to record any pressure change.

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Hydraulic Fracturing Equipment Setup The injection testing and fracturing work was carried out from a site underground, just west of the exposed face of ED4. The fracturing pumps, a small blender, and tanks were set up at the underground site. The diamond drill rig used to drill E48D102 was left set up and was used to deploy the injection rod string and straddle packer tool, which isolated a 0.55-m-long interval.

Well Testing Before any hydraulic fracturing occurred, an injection falloff well test was carried out. A single packer was set in E48D102 so that the section of hole from 51.9 m to the end of the hole at 203.9 m (152 m interval) was isolated for injection. The test injected water at an average rate of about 0.5 L/min. for 2.5 hours, followed by a 4.0 hour-long pressure falloff. The falloff portion of the test was analyzed using a Horner plot. The well test was short in duration and has a corresponding small radius of investigation, but demonstrates that the rock mass permeability is low and on the order of 0.005 md. An accurate pore pressure was not determined from the well testing, but the falloff pressure reached 1 MPa at the end of the test and was still declining. The mine is dry which is consistent with a pore pressure below 1 MPa.

Hydraulic Fracture Treatments A total of seven hydraulic fractures were created in borehole E48D102. The first one was small and ended prematurely because of a ruptured packer. Only 965 L of water were injected before the packer failed. Of the remaining six treatments, all involved injecting between 9 m3 and 15 m3 of fluid. Two of the treatments used borate crosslinked gel as the primary fracturing fluid, and the rest used water. Table 3 gives a summary of the hydraulic fracture treatments carried out. The fracture numbers associated with attempts that did not produce a fracture, because of ruptured packers or other problems, are not listed.

Plots of the data for Fractures 7

and 8 are shown in Figs. 7 and 8, respectively. All of the water-based fractures experienced some difficulty in placing the 500 to 70 μ m plastic proppant and, for Fracture 8, the 30/60-mesh (595 to 250 μ m) sand proppant. The crosslinked gel fractures accepted both propping agents without any significant pressure increase.

The locations of the fractures along the borehole are shown in Fig. 9. The circles centered on two of the fractures are about 30 m in radius. The treatment at 132 m (Fracture 5) could have potentially intersected hole D59 which contained a piezometer. The piezometer did not register any pressure increase during that treatment, which suggests that the fracture grew less than 30 m in that direction. The rapid falloff and low

Fig. 4: Clusters of microseismic events located using a hierarchical clustering algorithm. The locations of Fractures 5 and 9 are shown. Tunnels existing at the time of the fracturing work are colored green.

Fig. 5: Surface tiltmeter array (green circles) superimposed onto the underground tunnel system. Only tunnels shown in red existed at the time of the fracturing work.

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efficiency of Fracture 5 does suggest that this fracture may have intersected an open hole or a permeable fault. Hole D11 (Fig. 9) is potentially close enough to have been intersected, but was not monitored.

Closure Analysis The facture treatment falloff data from Fracture 9 are plotted in Fig. 10 to highlight closure events. The first and highest closure pressure that might be picked is about 16.8 MPa, which is higher than expected when the weight of the overburden is considered. However, this fracture treatment included red plastic proppant stages, and the first closure may be reflecting the fracture closing onto this proppant. A lower closure pressure of 15 to 14.9 MPa is more consistent with the rock mass density at E48 and is associated with a slope change in the G dP/dG curve in Fig. 10. The G-function is a dimensionless time (Castillo, 1987; Barree, 1998) that accounts for pumping and shut-in times. The closure pick in Fig. 10 is at a later G value than the

method of Barree (1998) would suggest, but agrees with the packer response and rock density data described below. According to Barree (1998), this type of flattening of the G dP/dG curve can be caused by fracture extension after shut-in, which could well have occurred at this site. The packer response observed at the start of flowback in Fractures 7 and 10 (Figs. 7 and 11) is consistent with an open fracture closing on the packers at the start of flowback (at pressures of 16.9 and 15.0 MPa) while the responses at flowback of Fractures 5 (not shown), 8 (Fig. 8), and 9 (not shown) were consistent with an already closed fracture (at pressure of 13.4, 14.1, and 13.8 MPa, respectively). This flowback data brackets closure to be between 14.1 and 15.0 MPa, and a value close to 15 MPa is consistent with the known rock density of 2700 kg/m3.

Table 3: Summary of hydraulic fracture treatments in E48D102 at Northparkes. Fracture Date

(2006) Straddle

top (m)

Fluid Start time

(hh:mm)

Volume (m3)

Notes

3 26/07 141.9 water 11:35 0.96 Top packer ruptured. 5 27/07 131.9 water 14:10 9.17 120 kg green plastic. 6 28/07 114.4 x-linked gel 14:07 10.40 275 kg red plastic. 7 29/07 94.4 x-linked gel 14:27 15.60 300 kg yellow plastic plus

500 kg 30/60-mesh sand. 8 30/07 79.4 water 9:58 15.00 75 kg green plastic plus 30

kg 30/60-mesh sand. 9 30/07 69.4 water 13:52 10.77 75 kg red plastic.

10 31/07 109.4 water 11:06 14.00 Water only fracture for microseismic generation.

The G-function plot in Fig. 10 (Fracture 9) suggests that the natural fractures were opened by the pressure in the fracture

and resulted in enhanced leakoff. These natural fracture sets closed at 17.3 to 18.4 MPa according to the analysis method described by Barree (1998). The intermediate principal stress was measured by the overcoring method as 20 to 24 MPa. Natural fractures will increase significantly in conductivity as the effective normal stress across them is decreased, even without opening. Natural fractures striking in the maximum horizontal stress direction and dipping at less then 45° or fractures that strike in the intermediate stress direction with dips of less than 21° would be potentially opened by a pressure of 18.4 MPa as illustrated by the stress circle plot in Fig. 12. The angle α in Fig. 12 is 90° and represents twice the physical angle between a plane perpendicular to the 3σ direction (a hydraulic fracture) and a natural fracture striking parallel to the 1σ direction. Likewise, θ is 42° which is twice the angle between the hydraulic fracture plane and a natural fracture striking parallel to the

2σ direction. The instantaneous shut-in pressures (ISIPs) recorded ranged from 17.3 to 23.2 MPa for the seven fracture

Fig. 6: Tilt vectors for Fracture 6, surface array.

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treatments, which suggests opening of natural fractures oriented over a larger range of values, associated with α and θ , would have been possible.

Fig. 7: Summary plot of Fracture 7 in E48D102, crosslinked gel. The packer pressure is indicated by a black line. Note the increase in packer pressure when the fracture flowback starts. This response is associated with a fracture that is opening along the axis of the hole. As this fracture closes during flowback, the packers are squeezed by the hole and the pressure in them increases.

Fig. 8: Summary plot of Fracture 8. The lack of any increase in packer pressure at flowback indicates the fracture was closed at 14.1 MPa.

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Therefore, natural fractures with a range

of orientations are likely to have contributed to the pressure-dependent leakoff response. If G at closure (Gc) is taken as about 1.8 then, using the approximate relationship (Nolte 1989), /(2 )c cG Gη= + , we find a fracture efficiency of about 47% for this treatment. Alternatively, if Gc is taken as 0.8, corresponding to the possible closure at 16.8 MPa in Fig. 10, then a low efficiency of 29 % is found. Based on the packer response at flowback, a value of closure stress close to 15 MPa, as discussed in the closure stress section above, is consistent with site conditions, giving an efficiency close to 50% for Fracture 9. This efficiency is consistent with the 30-m radius of growth implied by the lack of piezometer response and shows the strong effect that natural fracture dilation and associated pressure-

dependent leakoff can have. The analysis of the tilt data for this fracture treatment gave a volume of 6.8 m3 compared with 10.77 m3 injected, giving an efficiency of 63%. The accuracy of the tilt analysis for fracture volume depends on the overall inversion accuracy. Some poor-quality inversions, namely for Fractures 5 and 10, gave unrealistically large fracture volumes that equaled or even exceeded the injected volume.

Table 4 gives a summary of injection and closure time for each fracture, using a closure pressure value of 15 MPa to pick closure time. An estimate of fracture efficiency can then be calculated from the ratio of closure time to injection time. Fracture 5 stands out with the lowest efficiency of the fractures that used water with the other water fractures giving efficiencies in the range of 50%. Both gel fractures were much slower to close, with estimated efficiencies above 90%. The pressure in Fracture 6 was well above 15 MPa when flowback was started, but red plastic was found to have partially blocked the check valve and limited the flowback. The pressure recorded after shut-in for Fracture 6 may be unreliable because of this blockage.

Mapping of Hydraulic Fractures During and After Mining

Once the injection portion of the project was complete, the equipment was removed from the underground site, and mining of tunnel ED4 resumed. The tunnel was driven using a drill-blast-bog-support cycle. Bogging (an Australian mining colloquialism) is the process of removing broken rock using a specialized underground front-end loader. Each cycle advanced the tunnel face by approximately 4 m and required slightly more than 24 h to complete. The best time window for mapping of the hydraulic fractures was between the bogging and the support operations or between the support and drilling operations. When mapping took place between bogging and support, mine safety protocol did not permit approaching the new face past the line of existing support, which was about 4 m from the face. However, the tunnel walls and face could be washed with water and, because there was no shotcrete on the new exposed rock at that point in the development cycle, rock structures and

Fig. 9: The locations at RL 9700m of the two piezometers holes are shown along with the locations of the fracture initiation points along borehole E48D102.

Fig.10: G-function plot of falloff data from Fracture 9. The fracture closure at 15.0 MPa is picked by the break in the G dP/dG curve and because of the packer response on flow back observed at the end of several of the fracture treatments.

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hydraulic fractures were clearly revealed. During the support part of the cycle, the rock was supported by a system of rock bolts and 100 mm by 100 mm wire mesh.

The wire mesh was placed across the back (roof) of the tunnel and down each side to near the floor. Shotcrete was then sprayed over the mesh to cover the back and about half way down each side. Overspray often resulted in shotcrete covering parts of the tunnel sides below the grade line, which is a white line surveyed and painted on both sides of the tunnel at about 1.5 m above the floor. The mapping was carried out by use of a stereo photogrammetry system owned and operated by the mine, and by digital photography using an 8-megapixel DSLR camera fitted with a 17- to 85-mm zoom lens or with a 300-mm telephoto lens and an external flash unit

fitted to the camera hot shoe. This photographic mapping was supplemented by directly surveying marked points for the photogrammetry system and by directly surveying points along some of the exposed hydraulic fractures. Direct survey data were acquired by the mine surveyors using an electronic digital theodolyte, which was able to measure points along the tunnel surface to millimeter accuracy.

To obtain overall mapped-fracture geometry, we have combined the photogrammetry and the survey data for each fracture. The photogrammetry system allows coordinates of points to be determined from the stereo images produced, and this process was used whenever the hydraulic fractures could be seen in these images. Each fracture mapped in this way was then characterized by a set of coordinates associated with the points surveyed along it. The average strike and dip of each fracture was then determined by fitting a plane through these points using the method of principal

components analysis (e.g., Jolliffe 2002). Fig. 13 shows a composite view of the fractures surveyed along the tunnel by these methods. Table 5 gives a summary of the comparison between the fracture orientation determined by physical mapping and from analysis of the tilt data.

Fig. 11: Summary plot of Fracture 10. Note the small rise in packer pressure at the start of flowback, suggesting the fracture was still slightly open at 15 MPa.

Fig. 12: Mohr stress circles based on overcore stress measurements.

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Table 4: Fracture injection and closure times using 15 MPa for closure pressure. Fracture Fluid Pump

time ti, (min)

Closure time tc, (min)

Volume pumped

(m3)

Ave Rate (L/min)

tc/ti ( ρ )

Efficiency

1ρ

ρ+

5 water 27 13 8.420 311.1 0.48 0.33 6 gel 43 >600 10.630 246.5 13.95 0.93 7 gel 56 >500 15.740 285.4 8.93 0.90 8 water 46 50 14.520 302.0 1.09 0.52 9 water 34 48 10.900 320.5 1.41 0.59

10 water 58 50 14.490 245.8 0.86 0.46 Note: Efficiency calculation is based on Nolte (1989).

The distance in chainage meters is shown along the right side of the fractures in Fig. 13, corresponding to the distance to

the small diamond symbols in this figure. The chainage is the distance in meters established from a reference point located near the start of the tunnel. The collar of hole E48D102 was located at chainage -36.4 m in this system. The distance from the collar to any point in the hole is then equal to the chainage meterage plus 36.4 m. These chainage distances were indicated by markers along the sides of the tunnel and these markers and associated chainage values were used during the mapping.

The overview shown in Fig. 13 reveals that the fractures did not, in general, grow into one another. However, Fractures 7

and 8 were found in the same plane at one location near the western-most extent of Fracture 7. Fractures 6 and 7, which were both crosslinked gel treatments, were mapped over the greatest distance along the tunnel, with Fracture 6 mapped over 40 m and Fracture 7 over 25 m. These two fractures also had more plastic proppant placed into them, because adding proppant did not result in significant pressure increases. Fracture 6 disappeared from view as it rose into the roof rock at its eastern extent and Fracture 7 fell below the floor level at its western end, so these propped lengths are lower limits of actual extent. The dip and dip directions calculated by fitting a plane to the surveyed points for each fracture agree in general with the observed apparent orientation of the fractures mapped. However, fractures with significant offsets, such as Fractures 8 and 7, were made up of nearly horizontal segments and the more steeply dipping segment along the large offset. The fitted plane represents an average of these two distinct orientations. All of the fractures consisted of nearly horizontal segments which stepped up or down when they encountered and were offset by natural fractures or shear zones. The larger steps occurred in Fractures 8, 7 and 5 along shear zones that dipped at 40° to 45° to the west or southwest. Several of these offsets can be seen in Fig. 13, e.g., the offset between 35- and 45-m chainage in Fracture 8 (green) and an offset in Fracture 7 from 46- to 52-m chainage. Other offsets can be seen in the photographic record and in the survey data when the viewpoint used for Fig. 13 is moved. The tilt data analysis used a pure opening displacement discontinuity as the forward model and did not account for any shear deformation associated with fracture growth along an offset path.

Table 5: Fracture orientation from surface tilt data compared to orientation from physical mapping. Fracture Tiltmeter Mapping e3 Notes

Dip Dip Dir Dip Dip Dir 5 70° 140° 4° 267° 0.0055 Worst overall comparison 6 10° 200° 17° 187° 0.0007 Best overall comparison 7 5°-15° 130° / 350° 7° 240° 0.0044 8 10°-20° 130° to 160° 27° 269° 0.0088 Physical mapping is data poor and is affected by large offset. 9 10°-20° 120° 2° 96° 0.0009 Physical mapping is data poor.

Note: Dip is given as degrees down from horizontal and dip direction is with respect to the mine grid coordinate system which is aligned with the local state coordinate grid. The parameter e3 is a measure of the goodness-of-fit (smaller is better) of a plane through the surveyed points defining the mapped fracture.

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A small overlap and offset is seen in the photograph of Fracture 8 (Fig. 14). The overlap and offset is to the left of point 2

and consists of a step down when moving from left to right along the fracture, which corresponds to moving from north to south across the face of the tunnel. This photograph also shows Fracture 8 crossing a shear zone, with the green propped fracture visible on both sides of the greyish shear zone located between points 2 and 1.

The nature of the rock mass can be seen in this photograph and in others presented below. The rock is cut through by natural fractures and veins with many of the fractures filled by calcite and quartz and showing as lighter streaks in these photographs. Fig. 15, is a close up photograph of part of Fracture 8 where it crossed a thicker quartz-calcite filled vein. The vein is about 40-mm thick where the fracture crosses, as can be seen from the scale in the photograph which is

marked in millimeter and centimeter units. The hydraulic fracture was nearly horizontal and crossed this vein without any visible offsetting at this location. The inset photograph in Fig. 15 shows another vein crossing interaction with a 20-mm offset where, in this case Fracture 8 crossed a thin vein at 44-m chainage. Fig. 16 contains a photograph showing a portion of Fracture 8 at chainage 39.5 m where there were three sub-parallel propped fractures. Overall, branches and sub-parallel fractures represented a small percentage of the overall mapped length (approximately 10 to 15%) of the five fractures mapped.

Fracture 9 can be clearly seen in Fig. 17 at a location in the face of the tunnel. The borehole E48D102 is centered in the photograph. The hole was drilled with a 1.7° down dip along the tunnel axis to the east (into the page).

Fig. 13: A perspective 3D view of the hydraulic fractures mapped along tunnel ED4. The numbers along the south side of the tunnel indicate chainage distances corresponding to marks (indicated by grey diamonds) on the tunnel wall. The initiation point of each fracture is indicated along the borehole E48D102.

Fig. 14: Fracture 8 (green plastic) crossing a shear zone on the face of the tunnel. This photograph shows the right half of the face of the tunnel at approximately 46 m chainage. The arrows indicate the trace of the fracture with green plastic contained in it. There is no clear fracture between points 1 and 2 but the fracture may have crossed this zone either deeper into the rock or in the rock that has been excavated. Approximately 2 m of fracture extent is visible.

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Fracture 9 was initiated 2 m to the west of this location and at this location is about 95 mm above the center of the hole.

Assuming the fracture initiated at the centerline of the hole, an offset of 100 mm in 2 m is equal to an angle of 2.8°. The fracture was also nearly perfectly horizontal along its north/south extent across the tunnel face at this exposure. The fracture is following a pre-existing natural fracture here, as can be seen by the fact that several of the near-vertical light-colored veins are offset on either side of the propped hydraulic fracture.

Fig. 15: Fracture 8 crossing a carbonate-quartz vein on the south side of the tunnel at about chainage 38.5 m, near the floor. The inset photograph shows Fracture 8 crossing another vein at chainage 45 m, which resulted in a 20 mm offset.

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A second close up view of Fracture 9 is shown in Fig. 18. This photograph was taken about 2 m to the left of the one in

Fig. 17 and shows the fracture apparently terminating as it grows into a sub-vertical shear zone. Fracture 9 was also arrested by a second shear zone at the right side of the tunnel. The propped width of the fracture in Fig. 18 is seen to be slightly less than 2 mm at the well propped section, just above the “Fracture 9” label in the photograph. The fracture seems to have extended through unbroken rock at this location, judging by the more damaged nature of the rock very near the fracture plane and the lack of offsets on veins that the fracture trace is crossing. Analysis of Crossing Behavior Hydraulic fractures may arrest, cross, offset or grow into natural fractures or shear zones that they encounter (Daneshy 1974; Lamont and Jessen 1963; Cooke and Underwood 2001; Thiercelin et al. 1987). Physical mapping of fractures after mining provides direct information about the types of interactions that occur. The hydraulic fractures, as mapped, displayed a range of arresting or crossing behaviors. For example, Fracture 9 was arrested on the right (south) side of the tunnel at a shear structure and on the left (north) side at a sheared vein. The left-side arrest was also near the contact between the volcanic rock and an intruded porphyry, which might delineate a stress-change boundary.

Much of the path of each hydraulic fracture seemed to be along pre-existing natural fractures, although it was often difficult to tell if the hydraulic fracture had formed the fracture that was proppant-filled or if it had followed an existing fracture. Offsets in the path of the fractures were common, and these offsets varied in size from a few millimeters up to about 1.5 m. The offsets occurred at locations where the hydraulic fracture grew into a vein or shear zone. The larger offsets occurred at locations where an inclined shear zone cut across the fracture path at an angle. For example, the offset mapped in Fracture 8 at chainage 40- to 42-m (see Figs. 13 and 19) occurred along an inclined sheared vein that dipped at about 45° at 245°. This same fracture had a small 15- to 20-mm offset at a vertical vein at chainage 45 m.

The analysis presented here consists of a study of the effect of the offset geometry on the pressure and width distribution along a hydraulic fracture. The path of the hydraulic fracture was predefined by discretising a natural fracture with the desired path. Fluid was then injected into the center of this defined path, and the pressure and opening responses were calculated as the hydraulic fracture grew along the path that contained offsets. Fracture growth, assuming 2D plane strain conditions, was modeled using a fully coupled displacement discontinuity based research model (Zhang et al. 2005). The far-field stresses, frictional properties, injection rates, and viscosities were selected to approximate conditions at the E48 site, but the details of the offsets mapped were not modeled. The angle of the offset portion relative to the preferred fracture direction was varied from 90° to 30°. Fig. 20 shows the overall geometry of the problem considered.

Fig. 16: At chainage 39.5 m, three subparallel fractures were propped with green plastic in Fracture 8.

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The 2D model does not represent the more-or-less radial fracture geometry that is presumed to have resulted from the

treatments in D102, but the results given below are still useful in illustrating the effect of growing a hydraulic fracture along a natural fracture with offsets along its length. The injection rate was selected to represent 240 L/min into 10 m of total fracture

Fig. 17: Close-up view of the tunnel face at chainage 35.5 m (71.9 m from borehole collar) showing borehole E48D102 and part of Fracture 9 just above it. Fracture 9 was initiated from a position at chainage 33.0- to 33.55 m. The HQ-size borehole (96-mm diameter) was drilled with a 1.7° down dip along its length.

Fig. 18: North end of Fracture 9. Red plastic was found within about 50 mm of the sub-vertical sheared vein indicated and the fracture extends to the vein. The scale shown is marked in millimeter and centimeter units. Fracture growth was most likely from the right side of the figure toward the left, based on the location of the injection point.

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entry and the rock properties used are similar to the E48 rock mass properties. No attempt to model the details of mapped offsets was made.

Uniform Pressure Zero-Viscosity Case The first case considered is the opening of a fracture with offsets along it, loaded by a uniform internal pressure. A vertical confining stress of 15 MPa is applied and the stress along the fracture direction is 30 MPa, representing stress in a direction between the secondary and the maximum principal horizontal stress directions (see the inset in Fig. 21). The calculated opening displacement for this fracture geometry, subjected to 20 MPa internal pressure, is shown in Fig. 21. Even with high uniform pressure, the offset locations have restricted width compared with other parts of the fracture. Germanovich et al. (1997) calculated similar opening profiles for multi-segment fractures, but no fracture connection existed

between the individual fractures (at our offset locations) in their model. In our calculation, the 75° and 90° offset geometries do not open at the offset sites. The overall opening is not affected strongly when a uniform pressure is applied, with most of the restriction in opening occurring only at the offset location. The offsets have a low mechanical opening compliance because they are mechanically similar to short fracture segments. In addition, they are oriented so that the normal stress acting on them is higher than the minimum far-field principal stress. Non-uniform Pressure Viscous Flow Case The pressure distribution calculated for a fracture with offsets along its path, for the geometry shown in Fig. 20, is shown in Fig. 22, for the case where fluid with a viscosity of 0.001 Pa.s (water) is injected at 0.0004 m2/s (24 L/min per m of fracture height, the dimension into the page in Fig. 20) into the center of the fracture. The offsets impose local restrictions to the fracture opening. The curve for β = 0 corresponds to a straight fracture with no offsets and is provided as a reference. The increase in excess pressure between the 0° and the 30° offset case is about 0.1 MPa at a time of 17.5 seconds while, at this same time the 60° offset case predicts a pressure that is about 0.8 MPa higher than the pressure in the straight fracture, which corresponds to a minimum in the pressure curve for this geometry. The excess pressure is also dependent on the far-field deviatoric stress and on the value of the conductivity assigned to closed portions of the fracture (Jeffrey et al. 2009). The effect of the deviatoric stress is illustrated by the responses for the two 30° cases given in Fig. 22, which compare pressure and width for 2σ = 20 MPa and 2σ = 30 MPa. The higher stress results in more restriction at the offsets. The cases presented in Fig. 22 are for fractures guided through two offsets on each side of the injection point. Adding more offsets will also increase the net pressure at the wellbore. Overall, the offset fracture geometries result in higher pressure upstream of the offsets, with corresponding wider fractures there. The fracture grow rate is reduced as can be seen in Fig. 22 from the progressively longer times required for the fracture to reach the end of the guided path as β and 2σ increase. A fracture in a 3D domain may find its way around an offset, partially avoiding the restriction. However, mapped fractures can have tens of offsets of various sizes and orientations along their surfaces, and the cumulative effect of these on pressure, opening, and growth rate is expected to be significant. These 2D results, although simplified compared to 3D or real geometries, illustrate the coupled mechanical-viscous flow effect on fracturing pressure, width distribution, and growth rate that occurs during growth of hydraulic fractures with offsets.

Fig. 19: Three views of the surveyed points along the mapped fractures. The numbers refer to significant steps that occurred along the fracture paths. The fractures are coded by the colors used to mark them. From left to right, they are: Fracture 9 (red), Fracture 8 (green), Fracture 7 (yellow), Fracture 6 (red), and Fracture 5 (green). Overall, the hydraulic fractures were all sub-horizontal and dipped to the south and south west. The bounding box in the looking north view at the top is about 3.5-m high. This plot is made using a freeware viewer (Bradford 1997).

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A fracture with offsets will have a mix

of low-angle and higher-angle offsets along its path. For example, Fracture 8 had a large offset at chainage 40 to 41 m, which was along an approximate 45° dipping shear. Fracture 8 also had an approximate 90° offset at chainage 45 m associated with its crossing of a vertical vein, which is shown in the inset photograph of Fig. 15 (producing a 15- to 20-mm offset). Other offsets in this mapped fracture can be seen in Fig. 14. Offsets or steps in the fracture path were mapped in all of the mined fractures formed during this experiment and the modeling presented here suggests that fractures with offsets will require higher treating pressure to propagate, and will grow more slowly compared to planar fractures.

Conclusions The project described in this paper was designed to investigate hydraulic fracture growth in naturally fractured rock and to compare the fracture geometry inferred from microseismic and tiltmeter monitoring to the fracture geometry mapped after mining. The project also studied how and when a hydraulic fracture crosses or is blunted by a pre-existing shear zone.

The hydraulic fractures formed were sub-horizontal, dipping at an average of 10° to the west to southwest although the mapped fractures contained offsets which resulted in significant variation of the dip at different points along the fracture. The analyzed tilt data indicated that the fractures were sub-horizontal with southeast dips of 10° to 20° although one fracture was identified with a southwest dip. Nearly horizontal fractures produce tilt signals that are difficult to analyze for dip direction. The mismatch in dip direction between the tiltmeter result and the physical mapping result should, therefore, not be considered as unexpected. The tilt data did indicate sub-horizontal fracture orientation in four of the five fractures, indicating that this remote monitoring method is reliable in determining overall hydraulic fracture orientation.

The five fractures marked with colored plastic proppant grew to distances that produced some overlap of adjacent hydraulic fractures. In most cases, the fractures maintained the initial spacing dictated by their relative location along the borehole and their overall orientation. The only case of a fracture growing into the same plane as an adjacent fracture was found at the western most mapped ends of Fracture 7 chainage 39 m. Fractures 7 and 8 at this location seemed to be in the same plane (but on opposite sides of the tunnel).

The microseismic events detected could not be correlated with the fracture locations or orientations. Since the completion of the field work associated with this project, preconditioning of E48 has occurred and the microseismic data collected contained events, detected to -2.7 local magnitude, located along structures in the orebody, but not clustered around the sites of hydraulic fractures.

The micro seismic monitoring system at Northparkes Mines operates in a so-called “triggered mode.” In this mode, seismometers only transmit digitized signals of ground motion between times when a triggering criterion for a given sensor is exceeded. This is to be distinguished from a continuous ground motion recording mode in which there is a continuous transmission of recorded ground motion. A triggered system must be tuned to detect and record seismic events of interest. The small size of the events associated with hydraulic fracturing and the resultant high frequency of the waveforms do not always facilitate meeting the dual demands of triggering on larger mine production events of interest to the day-to-day operation of the mine and, at the same time, triggering on smaller events that result from hydraulic fracturing. Although the system at Northparkes is in principle capable of operation in continuous mode, the primary purpose of the microseismic system is not hydraulic fracturing, but the monitoring of seismic events associated with mine development and caving. The data storage and bandwidth demands on a system operating in a continuous ground motion mode are orders of magnitude more than those on a triggered system. Events which were recorded during the fracture times showed lower S-wave to P-wave energy ratios than what would be expected from predominantly shear-induced events, but the locations suggest that the events could be a secondary effect of the fracturing as stress was transferred away from the immediate treatment area.

The mapped hydraulic fractures consist of nearly horizontal segments with offsets at intervals along them produced as the fracture grew into and along dipping veins and shear zones. Fracture branches and sub-parallel propped sections were also mapped, making up 10 to 15% of the total fracture extent. Hydraulic fracture growth along a stepped path has been analyzed numerically, and such fractures have been shown, within the limits of 2D modeling, to grow with higher excess pressure and hence wider opening in the section of the fracture where opening is parallel to the least compressive stress. Because the

Fig. 20: Offset fracture path and parameters used in the numerical analysis.

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average opening of the fracture is increased by the presence of offsets, the fracture growth rate is slowed. In addition, the fracture opening along the offsets themselves is much reduced compared with the average opening. The physically mapped fractures exposed by mining did contain proppant material along both offset and non-offset portions of the fracture, although the higher angle offsets were less-well propped or unpropped (e.g. inset photograph in Fig. 15).

Nomenclature 1σ = Total far-field maximum principal stress, M/L2T2

2σ = Total far-field minimum (in 2D) or intermediate stress (3D), M/L2T2

3σ = Total far-field minimum (in 3D) stress, M/L2T2

ν = Poisson’s ratio η = Fracture efficiency; fracture volume divided by injected volume ρ = Ratio of closure time to injection time, Tc/Ti μ = Dynamic viscosity of injected fluid, M/L2T β = Angle of offset segment with respect to main fracture direction α = Twice the angle between natural fracture and 3σ direction

θ = Twice the angle between natural fracture and 3σ direction

λ = Coefficient of friction of interfaces and natural fractures d = Length of offset segment, L E = Young’s modulus, M/L2T2 Gc = Value of G-function at closure ISIP = Instantaneous Shut-In Pressure km = Kilometer, L Kc = Fracture toughness of the rock, M/L3/2T2 md = Millidarcy, L2 P = Pressure in fracture, M/L2T2 Q = Specified constant injection rate per unit fracture height at borehole, L2/T ti = Injection time, T tc = Elapsed time from shut-in to closure, T w0 = Pre-existing hydraulic conductivity width of fracture, L

Acknowledgments This work was sponsored by a consortium consisting of Northparkes Mines, Shell E&P, Total E&P, Schlumberger, and CSIRO. The tiltmeters were supplied and operated by Applied Geomechanics Inc. The ISSI-based microseismic array was installed and operated by Golder Associates using accelerometer sondes supplied by ES&S Pty. Ltd. We specifically acknowledge the support provided by Northparkes Mines and mine personnel during all phases of the work carried out at E48. Colin Stelzer provided valuable support during the mine-through both in collecting the photogrammetry data and in facilitating our access. Many others at the mine and at CSIRO contributed significantly to the success of this project, and we extend thanks to all of them without naming them here. The authors thank Northparkes Mines, CSIRO, Shell E&P, Total E&P, Golder Associates and Schlumberger for permission to publish.

Fig. 21: Opening displacement along a hydraulic fracture with several offsets located at 4-m intervals along it, under uniform internal pressure conditions.

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Fig. 22: Pressure and width at wellbore from numerical analysis of fracture growth along an offset fracture path.

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SI Metric Conversion Factors 1 L = 0.006290 barrels 1 m3 = 35.3146 ft3

1 m = 3.28084 ft 1 mm = 0.03937 in 1 MPa = 145.038 psi 1 GPa = 145,038 psi 1 L/s = 0.377386 barrels per minute 1 Pa.s = 0.001 cp 1 kg = 2.20462 lbm 1 μ m = 0.001 mm 1 m2 = 1.01325 x 1015 md