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 KCl. 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.
1. INTRODUCTION
Brittleness is a common term used to describe how rocks fail. The brittle regime is characterized by a stress drop when the rock is deformed after failure (beyond peak stress). It is well-known that most rocks deform in a brittle manner at low confining stresses, and become ductile above a certain stress level. Brittleness is thus detrimental to ductility (plasticity), which implies that the material can be deformed with non-declining or even increasing ability to carry load. Brittleness during compression is often associated with dilatancy, while rocks compact volumetrically when they are in the ductile regime.
Brittle vs. plastic behavior has a profound influence on borehole stability during drilling in shale, and thus on how the mud weight program should be designed for wells to facilitate efficient and safe drilling. It also impacts fracturing behavior in shales, be it during reinjection of e.g. cuttings in shale intervals, or when enabling gas production from gas shales where additional fracturing is imperative to production.
These are applications that have motivated the study presented here. Hence, our focus is on shale, which is often defined on basis of clay mineral content, or on grain size, or on fissility. Shale is a common cap rock, but also a common source rock. In oilfield language, terminology is sometimes sloppy, describing low permeability formations as shales even if they strictly should not be defined as such.
There is also no unique definition of a brittleness parameter, as pointed out already more than 35 years ago by Hucka and Das [1]. The aim of this paper is to summarize practical indices of brittleness that exist, derive some of them from laboratory data, discuss their relevance, and see how they relate to the field applications mentioned above.
2. BRITTLENESS PARAMETERS
Hucka and Das [1] listed a number of measurement principles that could serve as brittleness indicators. These were based on the conceptual understanding of brittleness as associated with “rupture or fracture with little or no plastic flow”, or as fracture formation occurring at or only slightly beyond the yield stress.
