**1. Introduction**

The desired outcome of certain types of hydraulic fracture treatments is the creation of a system of closely spaced fractures. Forming such a closely spaced hydraulic fracture array is a highly effective technique for increasing the permeability of a rock mass [1,2] or reducing the strength of a rock mass in mining [3]. Application areas that benefit from an increase of permeability are those that value greater fluid conductivity such as tight gas extraction, in situ leaching, carbon sequestration and storage, geothermal power generation and similar activities. A reduction in the tensile and shear strength of the rock mass through the growth of hydraulic fractures is beneficial to block cave mining where earlier and more controlled caving events are preferable to uneven or irregular caving events caused by strong rock-masses not collaps‐ ing in a regular way under their own weight. In each application area, an optimal hydraulic fracturing treatment would provide the greatest alteration to the rock mass for the lowest cost. fracturing experiments by CSIRO and the material properties of this rock have been well

Three Dimensional Forms of Closely Spaced Hydraulic Fractures

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

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Four hydraulic fractures were grown in each block, as shown in Figure 1. The fractures were grown one at a time beginning with the deepest fracture and working incrementally towards

The experimental setup used CSIRO's polyaxial load frame to apply confining stresses on the blocks along the vertical and horizontal axes. These stresses were applied using stainless steel flat jacks, inflated with water and with pressure controlled by three independent ISCO 260D

Hydraulic fractures were grown by injecting fluid into an isolated section of the borehole in order to pressurize and initiate fracture growth. The Newtonian fluid consisted of a combi‐ nation of water (13.6%), glycerol (80.0%) and blue food dye (6.4%) and had a dynamic viscosity

The injection pressure was recorded upstream and downstream of a flow control (needle) valve that limits flow rate surges associated with injection system compressibility during breakdown and initial fracture growth. The temperature in the laboratory was stable at 20°C (± 1°C) and the temperature of the fracturing fluid was recorded through the tests to enable corrections

Fracture initiation at a specific point in the borehole was achieved using an injection tool similar in concept to isolation packers used in the field (Figure 1, inset). O-ring "packers" isolated the fracture initiation zone above and below ports in the tool through which the fracture fluid exited the tool. In some blocks, a circumferential notch was scribed in the borehole at the location of fracture initiation, while in other tests the fractures were grown from an un-notched

After testing, the blocks were sectioned into 15 mm thick slabs, photographically scanned, and the fracture paths digitally re-constructed. The width of these slabs determined the resolution

After being cut, the slices were finished using a surface grinder to provide a smooth and clean

face to allow the best possible observation of the hydraulic fracture paths.

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characterized and are listed in Table 1.

**Table 1.** Adelaide Black Granite material properties.

of 0.058 Pa‧s and a density of 1.21 kg/m<sup>3</sup>

for the effect of temperature on fluid viscosity.

in one direction of the final re-construction.

the top of the borehole.

syringe pumps.

borehole.

Fracture toughness KIC 2.3 MPa.m0.5 Young's modulus Ε 102 GPa Poisson's Ratio ν 0.27 Tensile Strength TS 9.4 MPa

Friction coefficient f 0.45 (coarse finish) to 0.17 (polished finish)

Crystal Size 1-10mm diameter (typical)

When creating arrays of hydraulic fractures in close proximity, stress field changes induced by initial fractures can lead to deflection in subsequent fracture paths. Path deflection can compromise the effectiveness of the treatment array because it can lead to coalescence of the fractures rather than creation of distinct fractures. Also complex fracture paths such as coalescing or curving fractures may not be appropriately accounted for in reservoir or caving models. Ineffective treatments can be costly in terms of re-initiation and re-treatment reme‐ diation works. An accurate understanding of the interaction effects between multiple hy‐ draulic fractures growing in close proximity to one another is therefore important for effective treatment design.

The 3-dimenional form of these fracture treatments can be difficult to observe in the field, and even in laboratory experiments often only a 2-dimensional cross-section of the fracture system is examined. In reality, the 3-dimensional nature of the fracture geometry is fundamental to the treatment performance in almost all industrial applications of hydraulic fracturing.

A recent numerical study [4] has identified a set of parameters controlling hydraulic fracture path deflection. The work presented here compares the results of an experimental study to the predictions of numerical modeling work with respect to the interaction effects of closely spaced hydraulic fractures. In this regard, the present work extends initial comparisons [5] to include a fully 3-dimensional characterization of the laboratory hydraulic fracture geometry.

The experimental study consists of closely spaced hydraulic fracture arrays grown in 350mm cubical sample blocks of a South Australian Gabbro. Four closely-spaced fractures were sequentially grown in each block under a variety of far-field stress and fluid injection condi‐ tions. After the experiments were completed the blocks were cut into 15 mm thick slices (serial sectioned) and the fracture paths were measured on the faces of these slices.
