The undrained shear behaviour of mudrocks is a crucial parameter when performing wellbore stability analysis and geomechanical modelling of mudrocks. Here the undrained shear behaviour of mudrocks is investigated and the different results are explained by their petrographical properties. The Kimmeridge clay formation used in the study have been sampled in two sites in UK. The two sites have undergone different burial history and different diagenetic reactions. Rock mechanical testing under high confining stresses has been performed to examine the undrained shear behaviour such as deformation properties, effective stress path, and peakand post peak strength. The geological properties of the samples as texture, mineralogy, diagenetic minerals are investigated by x-ray diffraction, various type of scanning electron microscopy and light microscopy. These results are compared with other UK mudrocks. The study shows that both the primary depositional mineralogy and the burial history influence the undrained shear behaviour. It is also seen that diagenetic reactions greatly increase both the undrained peak strength and shear stiffness for mudrocks.
Argillaceous sediments like clay, mudrock and shale make up more than tree quarters of the sediments in the sedimentary basins. A large attention is drawn to these sediments because of the problems that occurs in such sequences. However, there exists no uniform definition, classification and identification system that covers these fine-grained sediments. By definition, mudrocks and shales are referred to as rocks that contain a high amount of clay minerals. Even the term clay mineral is not clearly defined because there are several other minerals occurring in the clay size than those normally defined as clay minerals. By clay mineral the most used sedimentological definition is a group of sheet minerals or physils according to [1]. Mechanically these sediments often occur as hard soils and soft rocks [2]. In soil mechanics the properties of sediments are usually purely based on void ratio, effective stress, and maximum pre-consolidation stress, and much of the work is done on reconstituted samples in order to obtain intrisinic properties [3,4]. However several studies have reported behaviour that differ from this due to post depositional changes that creates structure e g. fabric and bonding [3,5,6]. These changes are in geological terminology called diagenesis. Diagenetic changes in mudrocks can be due to mechanical, chemical and biological processes. Biological changes are mainly a shallow process while mechanical and chemical processes will dominate when sediments are buried. The major outcome of diagentic changes within argillaceous sediments are reduction of bulk volume e.g. compaction. Mechanical compaction is reduction in volume due to effective stresses, while chemical compaction involves mineral dissolution and precipitation and is a function of mineral stability and kinetics of precipitation and mainly controlled by temperature [7]. The tests were performed at effective stress level that are an order of magnitude higher than normal for this type of surface sediments to get knowledge of the basinal behaviour of "typical" North Sea reservoir.
