The Effects of Cooling on Fine-Grained Sandstone in Relation to Wellbore Injection of Carbon Dioxide

In Carbon Capture and Storage (CCS) procedures, it is important to determine the stability of the wellbore during carbon dioxide (CO2) injection and part of this involves assessing stresses on the rock near the wellbore due to changes in temperature and pressure. To address this, this study investigated the influence of cooling on the mechanical properties of a sandstone typical of those found in the central and southern North Sea. A series of uniaxial and triaxial compression tests was conducted on dry and saturated sandstone samples to determine the effects of cooling on the strength and stiffness under different confining pressures. The elastic modulus, shear modulus, bulk modulus and Poisson's ratio were determined for three temperature conditions and three pressures representing different depths in a wellbore. Two methods, the International Society of Rock Mechanics (ISRM) and Wood's (Soil behaviour and critical state soil mechanics. Cambridge University Press, Cambridge, 1990), were used to determine the mechanical properties of the rock during the Uniaxial Compressive Strength (UCS) tests. For the triaxial test, only Wood's (1990) method was applied due to the existence of confining pressure. Microstructural analysis on thin sections of the sandstones under plane and crossed polarised light conditions in the deformed and undeformed state was conducted to elucidate deformation mechanisms and aid interpretation of experimental results. It was identified that both an increase in confinement and a reduction in temperature, increased the strength of the sandstone and reduced the Poisson's ratio. Additionally, by decreasing the temperature, especially in the UCS test the material dilated less. This is an important outcome as expanding the results to a wellbore stability problem, brittle behaviour may be more apparent and damage may occur when sub-zero injection temperatures are applied, especially at the wellbore head, where confinement is low.