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A field experiment was carried out to measure hydraulic fracture growth in naturally fractured rock. Hydraulic fracture interactions with pre-existing natural fractures, shear zones, veins, and adjacent hydraulic fractures were measured and mapped during the project. Tiltmeter and microseismic arrays were installed to test the performance of these monitoring methods in determining the fracture geometry, which was eventually revealed by the mine-through mapping. The physically mapped fractures were oriented approximately horizontally, perpendicular to the minimum stress direction. They crossed natural fractures and shear zones, but were offset by some shear zones, most often oriented with an approximate 45° dip. The analysis of the tiltmeter data correctly predicted fractures to be horizontal. Microseismic monitoring, although a proven method for imaging hydraulic fractures, did not resolve the fracture orientation or size for conditions at the E48 Northparkes site because of a lack of recorded micro-seismic events. The hydraulic fractures grew through solid rock, along natural fractures and stepped along inclined shear zones. Proppant was distributed throughout the fractures, including in the offset portions. Initial modeling indicates higher treatment pressure and slower extension rate for a stepped 2D hydraulic fracture compared to a straight fracture. Introduction Hydraulic fracture growth through naturally fractured reservoirs presents theoretical, design, and application challenges. High treating pressure, unplanned screenout, and shorter-than-designed propped fractures are some problems that result (Thiercelin and Makkhyu 2007; Zhang and Jeffrey 2008; Beugelsdijk et al. 2000; Wu et al. 2004). An experiment to measure hydraulic fracture growth in a naturally fractured rock at a mine site was, therefore, carried out to obtain details of fracture geometry from physical mapping during and after mining. Northparkes Mines, located 300 km west of Sydney, Australia, provided the site for this experiment. The mine is developing a new copper-gold porphyry orebody called E48, which will be mined by block caving methods. The rock contains numerous veins, natural fractures, and shear zones. In 2006, prior to preconditioning, to verify fracture growth and interaction with shear zones in the rock mass, a mine-through of several hydraulic fractures placed ahead of a development tunnel was undertaken. The monitored and mined fracture project described in this paper was developed to enable additional fracture monitoring and analysis to be included in the project. The orebody was preconditioned by hydraulic fracturing in 2008 under a separate project. Results from this work are pertinent to the application of hydraulic fracturing to stimulation of naturally fractured oil and gas reservoirs (Warpinski 1991; Settari 1988; Warpinski and Teufel 1987), to stimulation of geothermal and hot dry rock reservoirs (Sanyal et al. 2000), and to preconditioning of ore bodies prior to mining (Brown 2003; van As and Jeffrey 2002). Of special interest are the interaction of the hydraulic fractures with shear zones that exist in the rock mass, and the direct comparison of fracture geometry obtained by remote monitoring and by direct physical mapping.

SUMMARYWe present a planar three-dimensional (3D) fracture growth simulator, based on a displacement discontinuity (DD) method for multi-layer elasticity problems. The method uses a ÿxed mesh approach, with rectangular panel elements to represent the planar fracture surface. Special fracture tip logic is included that allows a tip element to be partially fractured in the tip region. The fracture perimeter is modelled in a piece-wise linear manner. The algorithm can model any number of interacting fractures that are restricted to lie on a single planar surface, located orthogonal to any number of parallel layers. The multiple layers are treated using a Fourier transform (FT) approach that provides a numerical Green's function for the DD scheme. The layers are assumed to be fully bonded together. Any fracture growth rule can be postulated for the algorithm. We demonstrate this approach on a number of test problems to verify its accuracy and e ciency, before showing some more general results.

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SUMMARYWe present a novel multigrid (MG) procedure for the efficient solution of the large non-symmetric system of algebraic equations used to model the evolution of a hydraulically driven fracture in a multi-layered elastic medium. The governing equations involve a highly non-linear coupled system of integro-partial differential equations along with the fracture front free boundary problem. The conditioning of the algebraic equations typically degrades as O(N 3 ). A number of characteristics of this problem present significant new challenges for designing an effective MG strategy. Large changes in the coefficients of the PDE are dealt with by taking the appropriate harmonic averages of the discrete coefficients. Coarse level Green's functions for multiple elastic layers are constructed using a single dual mesh and superposition. Coarse grids that are sub-sets of the finest grid are used to treat mixed variable problems associated with 'pinch points.' Localized approximations to the Jacobian at each MG level are used to devise efficient Gauss-Seidel smoothers and preferential line iterations are used to eliminate grid anisotropy caused by large aspect ratio elements. The performance of the MG preconditioner is demonstrated in a number of numerical experiments.