Carbon dioxide (CO2) capture and sequestration (CCS) can play a significant role in reducing anthropogenic CO2 emissions while allowing society to slowly phase out traditional energy sources. One of the main challenge in CCS is the current absence of a clear revenue model. One idea is to use the sequestered supercritical CO2 as a working fluid for geothermal energy extraction from sedimentary reservoirs (Carbon-dioxide Plume Geothermal or CPG). Due to the large variability in density and mobility of CO2 under different temperatures, reservoir heterogeneities can give a rise to a combination of convective and conductive heat transfer. In this work, qualitative and quantitative descriptions
of the effect of reservoir properties on the performance of CPG in depleted
gas fields are provided using an example realistic depleted gas field. The primary focus is on the behaviour of the CO2 plume with regards to different reservoir properties such as porosity, permeability and thermal properties. The effect of large-scale reservoir structure, such as a presence of an aquifer, net-to-gross ratio and layering is also studied. In order to accurately model these effects, a thermal multi-component multiphase model based on a fugacity-activity Equation of State is built and validated for for pressures from 50-400 bar and temperatures from 35°C to 130°C. The developed thermodynamic model
is implemented into the Delft Advanced Research Terra Simulator. Numerous studies of 2D and 3D ensembles and sensitivity studies are carried out to examine the effects of isolated parameters on CPG performance. Results reveal that increased net-to-gross (N/G) ratio is associated with increased recovery factor. In addition, layering architecture becomes an important factor for the importance of conductive flux only at low N/G. Variations in the required pressure to sustain a production rate is associated with fluctuations
in production temperature and density due to expansive cooling. Varying reservoir properties and state also have a significant effect on brine upconing, which is detrimental to CPG performance. It appears that an increase in injection rate have a positive effect on the performance of CPG, but this should be studied in conjunction with a coupled wellbore and power plant model. Heterogeneous porosity-permeability realizations show a strong decrease in reservoir lifetime compared to their upscaled homogeneous counterparts, which is caused by a combination of preferential flow, reduced conductive flux and lower production BHP associated with the upscaled realizations. It was also found that reducing production rate delays the time of thermal breakthrough due to the combined effect of these factors.

In this study, we introduce the Delft Advanced Research Terra Simulator (DARTS), which employs the Operator-Based Linearization (OBL) approach to provide efficient and precise solutions for non-linear discretized equations, to simulate the polymer flooding process. This compositional simulation framework is feasible for describing complex physical phenomenon with high nonlinearity because detached functional operators have been developed to release the calculation load while maintaining a high degree of stability and accuracy. The main contributions and innovations are shown below: i. A two-phase and three-component chemical flooding mathematical model with some reasonable assumptions was developed to investigate the enhanced-oil-recovery (EOR) mechanism of polymer flooding. A finite volume discretization scheme and a fully implicit method (FIM) are implemented to approximate the conservation equations. The OBL approach is used to reduce the complexity of the simulation framework. The correctness of the model is verified by comparison with analytical model. ii. In addition to shear thinning effect, polymer viscoelasticity has also been taken into account. This mathematical model is able to handle high shear rates during field displacement as well as low to medium shear rates in laboratory experiments. iii. A new model for porosity reduction caused by polymer adsorption is formulated. The effect of polymer concentration on the effective pore volume is incorporated in numerical simulations based on Langmuir adsorption model.

Natural gas hydrates are often considered as a hazard in hydrocarbon production and pipeline transport, but are also recognized as a potential energy resource. Naturally occurring hydrate deposits exist under suitable conditions of low temperature and high pressure, typically in shallow marine sediments and in and below the permafrost, and may host large quantities of recoverable natural gas. Besides, the process of exchanging hydrate guest molecules potentially offers the added benefit of storage of carbon dioxide, as \ce{CO2}-hydrates are, under typical conditions, thermodynamically more stable than \ce{CH4}-hydrates. Several short-term field pilots have been undertaken, but the longer term dynamics of natural gas hydrate reservoirs remain unclear. Core-scale experiments performed at the University of Bergen were designed to provide essential data for field-scale numerical simulation of hydrate formation, dissociation and guest molecule exchange processes. Implementation of experimental findings into a simulator for hydrate-bearing formations may lead to a better understanding of the dynamics of hydrate-bearing geologic media and, in turn, to more effective strategies for commercial use of hydrate reservoirs. This thesis describes the development of an extension of the DARTS general-purpose reservoir simulator for hydrate-bearing reservoirs. The complexity of such systems can be effectively captured using the operator-based linearization approach and experimentally observed behaviour of hydrate systems can be implemented and matched with relative ease. This enables the combination of conventional conservation equations with the use of complex physics and empirical models. The existing DARTS-framework has been extended to include a kinetic description of the hydrate formation and dissociation reaction. Ultimately, a non-isothermal model has been tested for 1-D formation and dissociation cases. Comparison of results with literature shows similar behaviour and trends. The effect of water salinity has also been captured by the model, which has to be validated with experimental results. However, further development of the model is required in order for the model to be able to match measured data.