**1. Introduction**

Hydraulic fracturing has been applied successfully to preconditioning of hard rock at several block caving mines [1-3] and has been used to weaken a sandstone channel over a longwall panel [4]. A recent paper documents related work in China applied to control of rock bursting [5]. In addition, hydraulic fracturing has been used to induce caving in block caving operations [1] and in longwall coal mining [6]. The work described in this paper applied hydraulic fracturing to preconditioning of a strong roof rock in order to weaken it to promote earlier caving during start up of a longwall.

separated by 6.5 m. Five fractures were placed with a vertical spacing of 2.5 m and each of these was later reopened to determine intersection depths in borehole C and growth rate to boreholes A and E (Figure 1). As well as monitoring used in the first stage, temperature logging of borehole interesections in borehole C confirmed that multiple fractures were able to be

Monitoring and Measuring Hydraulic Fracturing Growth During Preconditioning of a Roof Rock…

http://dx.doi.org/10.5772/56325

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The third stage of the program was conducted at the start of the third longwall panel in an area where the overburden depth is some 20-30 m deeper than at the first site. A single injection hole and two monitoring holes confirmed that hydraulic fractures were able to be formed

This data set provides evidence for hydraulic fracture growth to more than 30 m radius at a vertical spacing between fractures of 1.25 m and 2.5 m, with non-symmetric fracture growth

Two sites were instrumented and tests were carried out to verify hydraulic fracture growth behaviour and measure the parameters needed to design the hydraulic fracture precondition‐ ing process. Figure 1a shows the two test sites and their relative location with respect to each other and to the longwall panels at the mine. Figures 1c and 1d contain scale drawings of the sites, with the fracturing and monitoring boreholes indicated. Both sites had a surface tiltmeter array installed and the tiltmeter instrument locations at the sites are indicated in the figures.

The second site was located over the start of Longwall 103 where the conglomerate lies at a depth of 162 to 181 m (see Figure 1d). The fractures at the Longwall 103 site were placed into borehole 103AA with temperature logging occurring in monitor boreholes 103ACRR and 103AB. The temperature logging served to detect the arrival of the fractures at these boreholes

The fractures at the Longwall 103 site were placed into borehole 103AA with temperature logging occurring in monitor boreholes 103ACRR and 103AB. The temperature logging served to detect the arrival of the fractures at these boreholes and to locate their vertical positions in

By placing a number of horizontal fractures through the conglomerate layer, the mechanical behaviour of the conglomerate is modified from a thick-plate behaviour to a series of much thinner stacked plates with the aim of promoting caving. For efficient preconditioning from vertical holes drilled from the surface, hydraulic fractures are required to form horizontally as shown in Figure 1b. This fracture orientation allows efficient preconditioning from a vertical

borehole because multiple fractures can be placed from each borehole.

formed parallel to each other over an extended distance.

and to locate their vertical positions in the boreholes.

measured by the offset borehole data.

**1.1. Design approach**

the boreholes.

**2. Preconditioning plan**

horizontally at this location despite the greater overburden depth.

Narrabri Coal Operations, located 28 km south of Narrabri, NSW, are extracting the Hoskis‐ sons coal seam using a 300 m wide longwall. The Digby conglomerate is 15 to 20 m thick and lies immediately above the seam. Geotechnical assessments of its potential to cave during longwall mining concluded that this conglomerate would not cave into the goaf until more than 60 m of the coal was extracted [7]. In addition, the analysis highlighted the potential for the conglomerate to pose a periodic weighting risk.

Periodic weighting occurs when the roof strata is strong enough to support itself behind the longwall face for some distance before failing suddenly as mining progresses. Failure typically occurs just ahead of the face and may cause the longwall supports to become overloaded and converge, crushing the coal on the face and posing a rock fall hazard for equipment and miners located between the face and the supports. The project described in this paper was aimed to test hydraulic fracturing as a method to precondition the conglomerate sequence and promote caving during mining.

The preconditioning test program involved initial characterisation of the in situ stresses to determine the suitability of the site for placing hydraulic fractures with a horizontal orienta‐ tion. The stress measurement work was followed by a three stage program of field trials. The first stage was aimed to confirm that hydraulic fractures were able to be formed horizontally and extended for a distance of more than 30 m from the injection hole, given the site conditions and the available equipment. The second stage was aimed to confirm that multiple hydraulic fractures placed in close vertical proximity remained essentially parallel to each other. The third stage was aimed to confirm conditions remained suitable to form horizontal fractures in a deeper area of the mine.

The field trials used an array of monitoring boreholes drilled at various distances around a central injection hole. During the first stage of the trials, five fractures were placed through the conglomerate sequence using a straddle packer system. These fractures, which were placed at a depth of 140 to 160 m, were monitored by a surface tiltmeter array, by boreholes offset 10 to 30 m from the borehole being fractured, and by stress change monitoring instruments located at 25 m and 60 m from the injection hole. Acoustic image logs of the injection hole and boreholes intersected by hydraulic fractures and core from intersected boreholes provided additional confirmation that fractures were able to be formed horizontally.

For the second stage, a second injection hole was drilled offset from the first borehole. The bottom hole locations of these two boreholes, A and J, were determined by survey to be separated by 6.5 m. Five fractures were placed with a vertical spacing of 2.5 m and each of these was later reopened to determine intersection depths in borehole C and growth rate to boreholes A and E (Figure 1). As well as monitoring used in the first stage, temperature logging of borehole interesections in borehole C confirmed that multiple fractures were able to be formed parallel to each other over an extended distance.

The third stage of the program was conducted at the start of the third longwall panel in an area where the overburden depth is some 20-30 m deeper than at the first site. A single injection hole and two monitoring holes confirmed that hydraulic fractures were able to be formed horizontally at this location despite the greater overburden depth.

This data set provides evidence for hydraulic fracture growth to more than 30 m radius at a vertical spacing between fractures of 1.25 m and 2.5 m, with non-symmetric fracture growth measured by the offset borehole data.

#### **1.1. Design approach**

**1. Introduction**

894 Effective and Sustainable Hydraulic Fracturing

caving during mining.

a deeper area of the mine.

caving during start up of a longwall.

the conglomerate to pose a periodic weighting risk.

Hydraulic fracturing has been applied successfully to preconditioning of hard rock at several block caving mines [1-3] and has been used to weaken a sandstone channel over a longwall panel [4]. A recent paper documents related work in China applied to control of rock bursting [5]. In addition, hydraulic fracturing has been used to induce caving in block caving operations [1] and in longwall coal mining [6]. The work described in this paper applied hydraulic fracturing to preconditioning of a strong roof rock in order to weaken it to promote earlier

Narrabri Coal Operations, located 28 km south of Narrabri, NSW, are extracting the Hoskis‐ sons coal seam using a 300 m wide longwall. The Digby conglomerate is 15 to 20 m thick and lies immediately above the seam. Geotechnical assessments of its potential to cave during longwall mining concluded that this conglomerate would not cave into the goaf until more than 60 m of the coal was extracted [7]. In addition, the analysis highlighted the potential for

Periodic weighting occurs when the roof strata is strong enough to support itself behind the longwall face for some distance before failing suddenly as mining progresses. Failure typically occurs just ahead of the face and may cause the longwall supports to become overloaded and converge, crushing the coal on the face and posing a rock fall hazard for equipment and miners located between the face and the supports. The project described in this paper was aimed to test hydraulic fracturing as a method to precondition the conglomerate sequence and promote

The preconditioning test program involved initial characterisation of the in situ stresses to determine the suitability of the site for placing hydraulic fractures with a horizontal orienta‐ tion. The stress measurement work was followed by a three stage program of field trials. The first stage was aimed to confirm that hydraulic fractures were able to be formed horizontally and extended for a distance of more than 30 m from the injection hole, given the site conditions and the available equipment. The second stage was aimed to confirm that multiple hydraulic fractures placed in close vertical proximity remained essentially parallel to each other. The third stage was aimed to confirm conditions remained suitable to form horizontal fractures in

The field trials used an array of monitoring boreholes drilled at various distances around a central injection hole. During the first stage of the trials, five fractures were placed through the conglomerate sequence using a straddle packer system. These fractures, which were placed at a depth of 140 to 160 m, were monitored by a surface tiltmeter array, by boreholes offset 10 to 30 m from the borehole being fractured, and by stress change monitoring instruments located at 25 m and 60 m from the injection hole. Acoustic image logs of the injection hole and boreholes intersected by hydraulic fractures and core from intersected boreholes provided additional

For the second stage, a second injection hole was drilled offset from the first borehole. The bottom hole locations of these two boreholes, A and J, were determined by survey to be

confirmation that fractures were able to be formed horizontally.

Two sites were instrumented and tests were carried out to verify hydraulic fracture growth behaviour and measure the parameters needed to design the hydraulic fracture precondition‐ ing process. Figure 1a shows the two test sites and their relative location with respect to each other and to the longwall panels at the mine. Figures 1c and 1d contain scale drawings of the sites, with the fracturing and monitoring boreholes indicated. Both sites had a surface tiltmeter array installed and the tiltmeter instrument locations at the sites are indicated in the figures.

The second site was located over the start of Longwall 103 where the conglomerate lies at a depth of 162 to 181 m (see Figure 1d). The fractures at the Longwall 103 site were placed into borehole 103AA with temperature logging occurring in monitor boreholes 103ACRR and 103AB. The temperature logging served to detect the arrival of the fractures at these boreholes and to locate their vertical positions in the boreholes.

The fractures at the Longwall 103 site were placed into borehole 103AA with temperature logging occurring in monitor boreholes 103ACRR and 103AB. The temperature logging served to detect the arrival of the fractures at these boreholes and to locate their vertical positions in the boreholes.
