Measurement of hydraulic fracture growth in a naturally fractured orebody for application to preconditioning

Abstract

Hydraulic fracturing is applied for the preconditioning of orebodies prior to cave mining. A major project at Northparkes Mines was carried out in 2006 to investigate hydraulic fracture interaction with shear zones in the E48 orebody. The fracture growth was remotely monitored by tiltmeter and microseismic arrays. The mined fractures were then physically mapped and their geometry was compared to that inferred from the remote monitoring. The fractures were found to be subhorizontal, which was consistent with the tiltmeter and in situ stress data. Shear zones, natural fractures, and veins were crossed by the hydraulic fractures, sometimes producing an offset or step in the fracture path at the crossing site. One hydraulic fracture terminated at a shear zone in close proximity to a lithological contact that may be a stress change boundary. Numerical modelling of hydraulic fracture growth with offsets reveals that such fractures require a higher pressure to propagate and grow more slowly than a straight fracture. Results from numerical modelling indicated that the E48 caveability would prove problematic. The E48 orebody was, therefore, preconditioned using hydraulic fracturing and the footprint of the cave was enlarged. PreconditioningMethods to introduce new fractures to weaken the rock mass and enhance its caveability are needed, particularly if less fractured, stronger orebodies, some with smaller footprints, are to be reliably mined by

Hydraulic fracturing in cave mining: Opportunities for improvement

“…Then, hydraulic fractures tend to be vertically, independently the wellbore orientation (see Figure 6). In comparison, Figure 7 shows that σ1 is usually horizontal in most of cave mines in which a preconditioning technique has been applied (Hormazabal et al 2010;Catalan & Suarez 2010;Windsor et al 2006;Jeffrey et al 2010). This means that hydraulic fractures tend to propagate horizontally.…”

Una opción para el paso a la minería subterránea masiva: la actual metodología aplicada en Chile del “block caving”.

“…La aplicación de estos métodos de pre acondicionamiento del macizo rocoso en operaciones de Block Caving ha sido cuantificada por diversas operaciones en las cuales se ha podido observar ciertas ventajas de su aplicación destacando entre ellas una mejor fragmentación de la roca presente en los puntos de extracción, una sismicidad favorable y un hundimiento progresivo (Jeffrey et al 2010, Pfitzner et al 2010, Brzovic et al 2014.…”

Preconditioning by hydraulic fracturing – what does it do and what are the issues in modelling it?

“…The important aspect is that, if the numerical model is to be able to model the behaviour of the hydraulically-fractured rock mass realistically, it should be able to cater for these alternative conditions, both for initiation and for subsequent propagation of rock and rock mass fracture and failure. It would appear that the discrete fracture network approach described by Rogers et al (2010) and the synthetic rock mass approach referred to by Jeffrey et al (2010) should be applicable in taking into account the above factors for realistic modelling of the behaviour of a preconditioned rock mass.…”