A shaly look at brittleness

摘要

Brittleness is considered a key parameter for hydraulic fracturing initiation and propagation in low permeability rock, like gas shale. Plasticity, which in general increases with reduced brittleness, plays an important role in assessment of the mud weight window for borehole stability during drilling in shale. There is no unique definition of brittleness, rather a variety of index parameters that can be obtained from laboratory or field data, and which do not necessarily represent a coherent picture. This is digested here by analysis of rock mechanical test data mainly on high porosity, high clay content North Sea shale. Brittleness, defined by the amount of linear strain as part of the total strain before failure, and as the stress drop relative to peak stress after failure, was found to decrease with increasing stress level (as expected). Brittleness was also found to decrease with increasing temperature and with exposure to brine containing KC1. Experiments furthermore demonstrate that pore pressure evolution contributes strongly to undrained plasticity, whereas the drained shale response is largely brittle. In practical field situations, one needs to address the situation closely to find which definition of brittleness that is the most appropriate.