Abstract

We present a new definition of a brittleness index which is used as a criterion for candidate selection of rock intervals for hydraulic fracturing. The new index is a combination of material strength parameters and insitu stresses. It was derived from an analytical model of hydraulic fracturing in weak formations of varying ductility. The model is based on Mohr-Coulomb dislocations that are placed in the effective centres of the complete slip process that is distributed around the crack tip. The new brittleness index varies between 0 and 1 with the one limit to correspond to brittle propagation and the other limit to a fracture that requires infinite energy release per unit advance. The values between 0 and 1 correspond to fracture propagation of increasing ductility from brittle to small scale and finally to large scale yielding. The results are particularly interesting for predicting the propagation of axial fractures in the horizontal direction and their confinement in the vertical direction.

1. INTRODUCTION

The Brittleness index of rocks is often used as a criterion for candidate selection of rock intervals for hydraulic fracturing in shale reservoirs [1]. Several definitions for measuring the brittleness of the rocks were proposed based on different mechanical properties of rocks that are derived from the stress-strain curve or from correlations with physical properties [2, 3, 4]. An inherent problem with some proposed definitions, which are based on simple definitions that were not derived from scientific principles but from correlations that are fitted on dynamic measurements, is that they do not follow the expected trend with some varying parameters such as the confining pressure [1]. Therefore, in unconventional shale reservoirs it is important to understand how brittleness can be represented and be used for practical applications of hydraulic fracturing. An extended report and comparisons of 9 different definitions of brittleness numbers based on uniaxial, triaxial and Brazilian tests on gas shale and overburden analogues were presented in [5].

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