ABSTRACT

Spacings between hydraulic fractures placed during preconditioning of orebodies for cave mining methods and used between stages in horizontal shale gas wells for stimulation continues to be reduced. The question of the effect of the interaction between a new hydraulic fracture and one or more nearby hydraulic fractures is important for both of these applications. Here we present experimental data from both mine-through and laboratory investigations aimed at verifying model predictions of the deflection of the path of a hydraulic fracture that is influenced by a previously placed, propped hydraulic fracture. In one laboratory case, the minimum stress was zero and, consistent with the model prediction, the fracture path curved toward and coalesced with the previously-placed fracture. In another laboratory case and in the mine-through case, the stress, injection, and propping conditions were such that the model predicted that curving could be neglected. This prediction is consistent with both the laboratory and mine-through data, where in the mine-through subparallel fractures were mapped, growing at 1.25 m separation for a distance of over 15 m.

INTRODUCTION

The creation of arrays of closely spaced hydraulic fractures is an emerging technique for stimulation of horizontal wells in unconventional gas reservoirs [1, 2] and for preconditioning orebodies for block/panel caving operations [3]. In the unconventional gas applications, which are now most commonly related to shale gas plays, an array of hydraulic fractures is created from multiple perforation clusters that are placed along a horizontal well. In some cases injection is made into several or all of the perforation clusters at the same time (simultaneous fracturing), while in other cases the perforation clusters are sequentially isolated so that injection proceeds into one cluster at a time (sequential or “zipper” fracturing). The horizontal well is typically drilled sub-parallel to the minimum horizontal stress direction, thus promoting hydraulic fracture growth orthogonal to the wellbore axis. These gas industry fractures are usually driven by water containing friction reducing additives to enable high flow rates through the injection system. In light of the low viscosity fracturing fluid, proppant (typically sand) concentrations in the injected water/sand slurry are typically lower than in conventional hydraulic fracturing, but the rate of injection is usually higher and treatment duration is longer so that the total volume of proppant that is placed can be greater than for conventional hydraulic fractures [1].

Like their gas industry counterparts, arrays of hydraulic fractures that are created for preconditioning orebodies for block/panel caving operations are typically water driven. However, these fractures are most often unpropped because their eventual conductivity to fluid flow is not a primary consideration and placing proppant increases the operational complexity and costs. Often the hydraulic fractures are placed from uncased cored boreholes using inflatable packers. As in the gas industry applications, boreholes that are sub-parallel to the minimum stress direction are favored because these will experience hydraulic fracture growth that is orthogonal to the wellbore axis, in principle enabling a more closely spaced array and allowing the greatest volume of rock to be treated per borehole.

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