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ABSTRACT: In carbon capture and storage schemes, the carbon dioxide (CO2) can be injected at high pressures and low temperatures for permanent storage in deep reservoirs. In the North Sea, the storage sites will be predominantly sandstone. Triaxial tests on sandstone were conducted until failure using representative in-situ stresses to estimate the mechanical properties of the rock. A theoretical constitutive model, using Finite Element Modelling (FEM), describing the stress-strain behavior and damage evolution of rock during triaxial testing, is presented alongside the results of the tests. The model reproduced the experimental outcomes satisfactorily and was used to estimate the critical state mechanic parameters. Lower temperature indicated higher strength on sandstone, a decrease in the Poisson’s ratio and, consequently, damage increase. 1. INTRODUCTION Potential Carbon Capture and Storage (CCS) schemes include injecting liquid carbon dioxide (CO2) from ships into subsea formations for permanent storage. CO2 on a ship is stored at a temperature close to the triple point of CO2 at around -50°C~-57°C and exposed to almost 0.6~0.7 MPa pressure in order to keep the containment of the CO2 mixture in liquid form (Reppas et al., 2021, Wetenhall et al., 2014). The difference between the injected CO2 from a ship and the temperature of the wellbore wall, which is increasing by 30°C/km, is causing temperature fluctuations and external stresses. These loadings and their mechanical effects on the wellbore wall can be described by triaxial tests. Laboratory tests were undertaken to determine how the mechanical properties of sandstone elastic modulus E, Poisson’s ratio n, the shear modulus G and bulk modulus K of rock— are influenced by the different confining pressures and temperatures. As sandstone is found in the sub-sea basins, it has high porosity and can be a likely storage medium for CCS, it was selected as the type of rock for the triaxial testing. Three different temperatures 15°C, -5°C and -10°C were experimentally examined. The three temperatures represent a different injection scenario of CO2. Confining pressures of 12.5 MPa and 24 MPa were used to represent rock conditions at 500m depth and 1000 m depth under the North Sea, respectively.

A double porosity finite element model (FEM) is used to analyse the stability of a wellbore during carbon dioxide (CO2) injection in a carbon capture and storage (CCS) scheme. Elastoplastic deformation and continuous damage effects were considered. Attention was given to the pore and fissure pressures of the rock sur-rounding the wellbore as these can create stresses which can lead to potential fracturing. The size of the wellbore was also discussed as well as the studied depth and the different injection pressures of CO2.