Gas shales are fine grained fissile detrital sedimentary rocks. But shale is often used as a catch-all term including different rocks with a non-negligible clay minerals content which leads to typical physical-chemical characteristics.
Several properties are generally assessed for shale gas plays (TOC, porosity…). However, regarding the access to the reservoir, drilling into the formations implies a good knowledge of their mechanical and petrographic properties. As classical cores are quite difficult to collect from deep wells, unconventional techniques using small samples are useful (Tshibangu et al. 1999). When dealing with hydraulic fracturing, the formation brittleness is to be assessed. Several estimators exist among which we propose the so named plasticity index determined from punching tests.
In the framework of a collaboration of more than fifteen years between the university and the drill bits manufacturer, samples coming from all over the World and associated with difficult drilling conditions have been collected and processed in the laboratory. This work involved thin section characterization and mechanical tests like FPMs abrasiveness, Shore hardness and punching tests. Recently, a database was then built in order to analyze the cumulated information (Descamps et al. 2013).
In the context of gas shales, this database includes various fine grained rocks like shales, claystones, fine grained sandstones (i.e. grain or crystal size below 125 microns). In this paper, two main axes are developed. First, we describe typical properties of fine grained rocks, especially when they are associated to difficult drilling environments. Then, we look to how those properties can be linked together. Particular interest will be taken on abrasiveness regarding the selection of drilling bits and on brittleness assessment in relation to hydraulic fracturing.