**5. Conclusions**

**Figure 11.** Fracture orientation and spacing implied by intersection and temperature logging data collected in bore‐

The longwall started retreating on 12 June 2012 with a windblast management plan in place that required additional precautions to be used during mining until the caving commenced and the goaf developed. If the goaf behind the longwall face had not formed by the time the face had retreated to 25 m from the start position, additional work to induce caving was planned. However, the conglomerate caved, starting at the centre of the panel and progressing toward both gate roadways, after 24 m of retreat. This was a significant improvement over the estimated distance of more than 60 m for caving to start that was made based on modeling

Beyond the startup area for a distance of 200 m, the conglomerate was preconditioned using boreholes located on approximate 80 m centres. The intensity of fractures placed in this main part of the longwall panel was approximately 25 percent of that applied along the startup section. A 100 m wide window was then left with no preconditioning to allow comparison of the fractured and unfractured conglomerate caving behavior. Mining under this section of conglomerate demonstrated that the preconditioning reduced the intensity of the periodic weighting events, but the events that still occurred were more random under the precondi‐ tioned roof. When mining under the conglomerate that was not preconditioned, weighting events could be anticipated to occur at regular intervals of about 15 m of longwall retreat. Therefore, adjustments to the daily longwall extraction plans were made so that any slowing or halting of mining was avoided when approaching an anticipated weighting event. Using this modified mining strategy, mining was continued without using preconditioning for the

hole C during fracturing of borehole J. Fractures were placed at 2.5 m vertical spacing in borehole J.

**4. Caving behaviour**

910 Effective and Sustainable Hydraulic Fracturing

rest of Longwall 101.

studies of the untreated conglomerate.

Measurements of fracture growth, spacing and orientation at two trial sites and as the preconditioning of the Longwall 101 startup area was carried out demonstrated that the hydraulic fractures could be created that were essentially horizontal and could be extended to more than 30 m as parallel fractures. The tiltmeter data recorded during the trials and later during preconditioning, indicated dips of 2° to 20°, which provided additional assurance that the fractures were essentially horizontal, especially at sites where no other monitoring was available. But attempts to analyse the tilt data for indications of asymmetric growth proved unreliable because the dip and dip direction are coupled to the location of the centre of fracture volume.

The theory of closely spaced fracture growth, developed using a 2D numerical model has been further verified by the measurments made during this project. The theory predicts that for the conditions at the Narrabri Coal site, hydraulic fractures placed sequentually at 1.25 m along a vertical borehole will grow with negligable curving to distance of 30 m or more, allowing the conglomerate roof rock to be preconditioned and weakened by placing fractures through its thickness. This was found to be the case, based on direct measurement of fracture arrival depths in offset boreholes.

The conglomerate caved soon after the start of Longwall 101, demonstrating the effectiveness of the intensive preconditioning carried out.

Hydraulic fracturing can be used for preconditioning of strong roof sequences. When condi‐ tions allow horizontal fractures to be placed from vertical boreholes, the preconditioning can be carried out from the surface.

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[7] Medhurst, T. Narrabri Coal Pty. Ltd. Longwall support geotechnical assessment. Re‐

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[9] Mills, K. W. Interpretation of in situ stress measurements conducted at the start of longwall 1 at Whitehaven. Report by SCT Operations Pty. Ltd., March (2011). pp.

[10] Gray, I. Narrabri coal in-situ stress test (IST). Report by Sigra Pty. Ltd., 8 May (2006).

[11] Lecampion, B, & Peirce, A. Multipole moment decomposition for imaging hydraulic fractures from remote elastostatic data. Inverse Problems, (2007). doi:10.1088/

[12] Davis, P. V. Surface deformation associated with a dipping hydrofracture. Journal of

[13] Lecampion, B, Jeffrey, R, & Detournay, E. Resolving the geometry of hydraulic frac‐ tures from tilt measurements, Pure and Applied Geophysics, (2005). doi:10.1007/

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