Thermo-Hydro-Mechanical analysis of rock formations subject to low temperature CO2 sequestration

Abstract

In May 2019, the UK Committee on Climate Change, recommended that the UK should aim to be net zero by 2050. To help achieve this goal, a promising approach is Carbon Capture and Storage (CCS). Potential CCS schemes include injecting liquid carbon dioxide (CO2) from ships into subsea formations for permanent storage. CO2 is stored onboard these ships at sub-zero temperatures and usually heated up to higher than 4.5℃ prior to injection. Lower injection temperatures have been suggested as these reduce energy spend. However, these temperatures could lead to damage and fracture of the rock surrounding the wellbore, which is a key component of the integrity of the storage site. In this thesis, a fully coupled Thermo-Hydro-Mechanical (THM) model considering elastoplastic behaviour of rock with continuum damage effects is presented describing the behaviour of sandstone adjacent to the wellbore wall during CO2 injection. Sandstone was selected because it is found in sub-sea basins around the UK, it has high porosity and is a likely storage medium for CCS. A macroscopic approach was adopted based on the effective stress concept, the equations of static equilibrium, the conservation of mass, momentum, and heat transfer in the fractured medium. The constitutive model was implemented using Finite Element Method coded in MATLAB and its validity was verified through comparison with thermo-hydro-mechanical models from existing literature. For elastoplastic considerations, a bounding surface model, based on critical state mechanics adopting a hardening rule, was created. Uniaxial and triaxial experimental tests were undertaken to determine the thermal effects on the mechanical properties of sandstone, while critical state parameters were estimated from a parametric study. It was identified that lower temperature increased the strength of the rock due to ice formation but decreased Poisson’s ratio, making the rock more vulnerable to fracture and damage. The full numerical model was used to investigate several representative injection scenarios and to estimate the influence of cold CO2 on the rock surrounding the wellbore. Results indicate that subzero injection temperatures decrease pore and fissure pressure, while increasing the radial effective stress leading to potential damage. Limitations on injection pressures and temperatures are suggested based on these results.