In recent years, the authors and co-workers have developed a 3D finite element model for coupled thermo-hydro-mechanical (THM) problems in fractured rock masses. Zero-thickness interface elements are used for taking into account explicitly the effect of fractures and discontinuities in the fluid flow as well as the effect of fluid pressure in the crack propagation. Furthermore, the use of zero-thickness elements as a discrete modelling approach for fractures and discontinuities makes it possible to account for the heat transport taking place within these elements, even when advection dominates over diffusion (high Peclet number) [1]. The model has been implemented in the finite element code DRAC5, which is equipped with fracture-based interface elements and MPI parallel capabilities [2]. The code was originally developed considering water-saturated porous medium and fractures. The new developments described in the present paper, include the extension of the original formulation to the case of two-phase (liquid and gas) flow within the porous medium and discontinuities. The liquid includes only liquid water species, while the gas phase includes water vapour and gas species. The formulation includes the equilibrium equation, the mass balance of water and gas species and the energy balance equation. The parameters of the retention and relative permeability curves for the interface elements, such as the gas entry value and the residual water saturation, are updated with the variation of the normal aperture. The new capabilities of the model are illustrated with some academic verification examples.

In Carbon Capture Storage (CCS) sites, an important element of risk to be considered is the integrity of the cement seals of the abandoned wells in the reservoir [1]. The main goal of abandonment procedure once the life of a well is completed is to provide an effective isolation of the reservoir fluids in order to reduce environmental risk of contamination. In the case that the site has been reconverted to CCS, this is even more essential to prevent CO2 leaks from the storage site. It is important to note that the cracking conditions of the well cement seal can be affected by the long term changes in pore pressures that take place after the oil exploitation activities have stopped [2]. For example, slow pressure return around extraction wells (where the pore pressure had been subject to a sustained reduction during long extraction periods) may cause a progressive reduction of the effective stresses acting on the cement casing and plug, while the opposite can happen at injection wells. And these effects may be partially modified by the overall structural response due to the volume changes implied by the effective stress changes [3]. In this paper, a preliminary study of the effects of such stress changes on the potential integrity of a 2D cross-section of the sealed oil well system (caprock-external cement sheath-steel casingcement plug) during its service-life (injection/production activities and abandonment) has been performed by means of FE method including zero-thickness interface elements to represent potential cracks. In particular, these elements are pre-inserted in the analysis in between the contacts of caprock-external cement sheath, external cement sheath-casing and casing-cement plug. The results presented show that, depending on the initial state and range of pressure evolution, the different interfaces considered may open or close in a non-trivial manner during the pressure return process. This seems to indicate the importance of considering carefully the pressure return process and subsequent effective stresses evolution in abandoned reservoirs recycled to CCS, in order to avoid that new cracks in well cement seals may lead to potential CO2 leakage in the storage site.