Effect of Water Saturation on Failure Behaviour of Mudstones Under High Pressures

ABSTRACT Water as pore fluid is an important factor that influence rock deformation and failure behaviour. This paper presents an experimental study on the effects of water saturation on the failure behaviour of mudstones at high confining pressures. Triaxial compression experiments were performed at confining pressures up to 130 MPa, on dry and water-saturated mudstone samples. Results including stress-strain curves, failure strength, and failure modes, showed that the mudstones experienced brittle, brittle-ductile transition, and ductile failure behaviour as the confining pressure increases. The results also revealed that water significantly weakened the rock, thereby reducing the failure strength and causing the rock to become ductile at lower confining pressure compare to dry conditions. Furthermore, in water-saturated conditions, the brittle-ductile transition behaviour is accompanied by shear band fractures and ductile flow, as opposed to shear fractures to shear band fractures in dry conditions. The findings of this study could provide valuable information for optimizing hydraulic fracturing techniques and improving production efficiency in unconventional oil and gas reservoirs by identifying the conditions and depths of the transition from brittle to ductile failure behavior. INTRODUCTION Crustal rocks typically contain some form of pore fluid, and most of these fluids are water (Price, 1975). Understanding how pore fluids affect the mechanical properties of rocks is crucial in solving a range of problems in geotechnical and geological applications. For example, water's significant impact on the strength and deformability of rocks has been linked to many rock engineering hazards like landslides (Iverson, 2000). Water has been shown in numerous studies to weaken the mechanical strength of various types of rock, including mudstone, sandstone, granitic rocks, and quartzite. (Cai et al., 2019; Lu et al., 2017; Wasantha and Ranjith, 2014). When rocks are saturated with water, they become weaker and more prone to failure. This is because water can act as a lubricant between the individual particles, reducing the frictional forces that hold the particles together. As a result, the rock may experience reduced strength, stiffness, and cohesion, as well as an increased risk of creep, plastic deformation, and failure rates (Brantut et al., 2013; Chen et al., 2019; Wong et al., 2016).