Shale Dynamic Properties and Anisotropy Under Triaxial Loading: Experimental and Theoretical Investigations.pdf
Taken from Introduction: Clayrocks, and shales in particular, represent approximately two-third of all sedimentary rocks in shallow earth crustal rocks. In oil and gas drilling operations, shales constitute 80% of all the drilled sections, mainly because they overlie most hydrocarbon bearing reservoirs (immature shales). Furthermore, several countries are considering clayrocks as possible host lithologies for radioactive waste repository, and therefore carrying out research programs to estimate feasibility of such solution. In this trend, the french agency for radioactive waste management, ANDRA, is evaluating the reliability of the Callovo-Oxfordian layer, Jurassic in age, located in the eastern part of France (around Bure), at a depth ranging from 400 m to 700 m.
Drilling through shales is often a slow and difficult operation. During drilling, the well is filled with fluid, the role of which is to stabilize the borehole. When the drilling reaches a certain depth, there is a risk for the wellbore to collapse due to the removal of rock. Therefore, the most obvious role of the mud is to provide a hydrostatic pressure in order to compensate for the in-situ stresses. A problem may arise from the contact between the mud and the shale formation: the change of the state of stress of the wellbore walls included by physicochemical interaction between the mud and the formation pore fluid, i.e., exchange of water molecules and solute ions.