**5. Conclusion**

**4. Experimental practicalities and future work**

14 **5. Experimental Practicalities and Future Work** 

13 tip segmentation [7,8].

**Fracture 3 enters altered stress field and is attracted to fracture 2** 

704 Effective and Sustainable Hydraulic Fracturing

achieved.

1

youtu.be/ZCMBWGl\_8-Y).

A significant amount of time is currently required to produce the final 3-dimensional digital images of the fractures using serial sectioning. The most time consuming steps in the process are cutting the blocks into sections and scanning and processing the fracture images. Several attempts at developing and applying existing algorithms to extract the fracture geometry from a digital image were made, but without success. In particular, for our application and in our experience, we found that line-tracing type algorithms that have been used in applications such as X-ray Computer Tomography trade off precision in the tracing for the sake of high throughput. This makes sense when one has many, very closely spaced and easily acquired cross-sectional images to analyze. However, in our case a significant time investment is required to obtain each cross-sectional image and so our priority is on accuracy rather than throughput. Hence, a manual method was finally settled on because it gave the most accurate and detailed digital fracture image from each rock slice. But, if this part of the process could be automated, significant savings in overall time needed would be

2 **Figure 10.** A 2-dimensional cross-section of Block 6 showing the deflected path of fracture 3 three (left) and a 3-dimensional reconstruction of the fracture paths in Block 6 (a video 4 animation of this fracture can be viewed at http://youtu.be/ZCMBWGl\_8-Y).

**Figure 10.** A 2-dimensional cross-section of Block 6 showing the deflected path of fracture three (left) and a 3-dimen‐ sional reconstruction of the fracture paths in Block 6 (a video animation of this fracture can be viewed at http://

5 The anomaly of fracture 3 aside, in this case the fractures are observed to propagate parallel 6 to one another, and with the fewest branches in the fracture paths of the 3 blocks we tested. 7 In fact as can be seen in the 3-dimensional reconstruction in Figure 10, when viewing the re-8 constructed cross section (to the right of the borehole) all four fractures are seen to grow 9 parallel to one another. The parallel propagation is consistent with the model predictions for 10 these stress and injection conditions [4,5]. The reduction in branches is beyond the scope of 11 the modelling, but is to be expected when the fractures are not curving since the curving is 12 associated with mixed mode loading at the crack tips that can also be associated with crack

15 A significant amount of time is currently required to produce the final 3-dimensional digital 16 images of the fractures using serial sectioning. The most time consuming steps in the 17 process are cutting the blocks into sections and scanning and processing the fracture images. 18 Several attempts at developing and applying existing algorithms to extract the fracture 19 geometry from a digital image were made, but without success. In particular, for our 20 application and in our experience, we found that line-tracing type algorithms that have been 21 used in applications such as X-ray Computer Tomography trade off precision in the tracing 22 for the sake of high throughput. This makes sense when one has many, very closely-spaced 23 and easily acquired cross-sectional images to analyze. However, in our case a significant 24 time investment is required to obtain each cross-sectional image and so our priority is on 25 accuracy rather than throughput. Hence, a manual method was finally settled on because it 26 gave the most accurate and detailed digital fracture image from each rock slice. But, if this

14 Book Title

**In the absence of fracture 4, fracture 3 re-orients to principal stress direction** 

Having a 3-dimensional representation of a hydraulic fracture is very useful, but to obtain full benefit this image should be viewed using an interactive 3-dimensional computer application. Presenting static views of the fracture, as was done in this paper, limits the benefit. Improve‐ ments to the 3-dimensional images, such as advanced highlighting and shading methods, will

In the future, in addition to running several replications of the tests described here, we see significant potential in applying serial sectioning as part of studies of fracture initiation at notches in the borehole to better understand the effect of initiation geometry on near-, intermediate- and far-field fracture geometry. A series of tests in a finer-grained rock material

be investigated as a way to improve the static images for publication.

Reconstruction of fracture paths from tested blocks gives additional insight into fracture growth and is a useful technique for extracting additional information from tested blocks. In the study presented here, the 3-dimensional reconstructed fracture images have been used to show that the curving fracture geometries resulting from sequential growth of closely spaced, interacting hydraulic fractures exhibit translational symmetry even though the hydraulic fracture growth radiated from a centrally located wellbore. While based only on a few experiments and therefore in need of further experimental confirmation, this unexpected result is important for selection of appropriate simplifying symmetry assumptions (i.e. plane strain versus radial symmetry) in numerical models.

In addition to this conclusion, which is fundamentally based on 3-dimensional considerations, these reconstructions have confirmed the consistency between model and experiment that Bunger et al. [4] draw based on a single cross section of these fractures, namely that a subse‐ quent fracture will curve towards a previous fracture when the minimum stress is zero and that this curving is suppressed when the minimum stress is sufficiently large. Also, the 3 dimensional reconstructions confirm that fracture initiation is critical to the symmetry and preponderance of branching. Therefore, cutting effective notches in the borehole that facilitate fracture initiation in the eventual plane of favored propagation have a profound impact on the hydraulic fracture geometry.

Serial sectioning and digital 3-dimensional image reconstruction is found to be an effective method to obtain a more complete understanding of fracture geometry and interaction. Once the fracture traces are assembled into a 3-dimensional model, the fracture image can be rotated in space and viewed from different directions, which allows detailed examination of the fracture geometry and spatial relationship between fractures. It is, however, time consuming and therefore relatively expensive, but no other method provides the level of detail or the ability to image fractures in large rock blocks.

### **Acknowledgements**

The authors thank both CSIRO and Newcrest Mining Limited for funding the experiments and for permission to publish. Serial sectioning comprised a portion of the final year thesis for JW at MonashUniversityunderthe supervisionofMAH.Online animationswerepreparedbyJKand Dane Kasperczyk (CSIRO Earth Sciences and Resource Engineering, Clayton, Australia).

### **Author details**

James Kear1\*, Justine White1 , Andrew P. Bunger1,2, Rob Jeffrey1 and Mir-Akbar Hessami3 [8] Leonid N. Germanovich, Dmitriy K. Astakhov, Michael J. Mayerhofer, Jacob Shlya‐ pobersky, Lev M. Ring, Hydraulic fracture with multiple segments I. Observations and model formulation, International Journal of Rock Mechanics and Mining Scien‐

Three Dimensional Forms of Closely Spaced Hydraulic Fractures

http://dx.doi.org/10.5772/56261

707

ces, Volume 34, Issues 3–4, April–June 1997, Pages 97.e1-97.e19

\*Address all correspondence to: James.Kear@csiro.au

1 CSIRO Earth Sciences and Resource Engineering, Clayton, Australia

2 Department of Civil & Environmental Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania, USA

3 Department of Mechanical and Aerospace Engineering, Monash University, Clayton, Australia

#### **References**


[8] Leonid N. Germanovich, Dmitriy K. Astakhov, Michael J. Mayerhofer, Jacob Shlya‐ pobersky, Lev M. Ring, Hydraulic fracture with multiple segments I. Observations and model formulation, International Journal of Rock Mechanics and Mining Scien‐ ces, Volume 34, Issues 3–4, April–June 1997, Pages 97.e1-97.e19

**Author details**

Pennsylvania, USA

Australia

**References**

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James Kear1\*, Justine White1

706 Effective and Sustainable Hydraulic Fracturing

\*Address all correspondence to: James.Kear@csiro.au

1 CSIRO Earth Sciences and Resource Engineering, Clayton, Australia

, Andrew P. Bunger1,2, Rob Jeffrey1

2 Department of Civil & Environmental Engineering, University of Pittsburgh, Pittsburgh,

3 Department of Mechanical and Aerospace Engineering, Monash University, Clayton,

[1] King, G. E. Thirty years of gas shale fracturing: What have we learned? In Proceed‐ ings SPE Annual Technical Conference and Exhibition. Florence, Italy. SPE 133256,

[2] Rodrigues, V. F., L. F. Neumann, D. Torres, C. Guimaraes, and R. S. Torres, 2007. Horizontal well completion and stimulation techniques - A review with emphasis on low-permeability carbonates. In Proceedings SPE Latin American and Caribbean Pe‐

[3] van As, A. and R. Jeffrey. Caving induced by hydraulic fracturing at Northparkes Mines. In Pacific Rocks 2000 - Proc. 4th North American Rock Mechanics Symposi‐ um, eds. J. Girard, M. Liebman, C. Breeds, and T. Doe. Seatle, WA: Balkema, 353–360,

[4] A. P. Bunger, X. Zhang, and R. G. Jeffrey, "Parameters Affecting the Interaction Among Closely Spaced Hydraulic Fractures," *SPE Journal*, no. March, 2012.

[5] A. P. Bunger, R. G. Jeffrey, J. Kear, X. Zhang, and M. Morgan, "Experimental Investi‐ gation of the Interaction Among Closely Spaced Hydraulic Fractures," in *American*

[6] Andrew P. Bunger, Robert G. Jeffrey and Emmanuel Detournay, "Evolution and morphology of saucer-shaped sills in analogue experiments (in Structure and em‐ placement of high-level magmatic systems )" Geological Society Special Publications

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and Mir-Akbar Hessami3

**Section 11**

**Numerical Modeling 1**
