Measuring Hydraulic Fracture Growth in Naturally Fractured Rock

Abstract

Abstract 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.