Numerical simulation of CO2 storage in saline aquifers

Abstract

CO2 capture and storage (CCS) has been proposed as a climate change mitigation strategy. The basic principle is to prevent CO2 which would normally be emitted from large point sources, such as power stations, from entering the atmosphere. This is achieved by capturing the CO2 at source and storing it in a location where it will be trapped and unable to enter the atmosphere. This work looks specifically at geological storage of CO2 in deep saline formations.

Dynamic simulations can be used to investigate the fundamental physical and chemical processes which occur when CO2 is injected into geological formations. They can also be used to determine the suitability of a particular site for CO2 storage. The scale of the processes being simulated is important when building a dynamic model. Here dynamic simulations have been used to explore three different aspects of geological CO2 storage in deep saline formations. The first model investigates large scale CO2 migration and pressure build up at a potential CCS site. The second model concentrates on the small scale processes of CO2 dissolution and convection. The third model attempts to accurately model both the large scale processes of CO2 injection and migration and the small scale processes of CO2 dissolution and convection.

Dynamic simulations have been used to model storage capacity, CO2 migration and pressure buildup at a potential CO2 storage site in the UK North Sea. There are large uncertainties in the input data so various models have been run using a range of parameters. The primary control on pressure buildup at the site is the permeability of the unit directly beneath it. The plume diameter is primarily controlled by the porosity and permeability of the reservoir unit. Despite uncertainties in the input data, the use of a full three-dimensional (3D) numerical simulation has been extremely useful for identifying and prioritizing factors that need further investigation.

Dissolution of CO2 into existing formation waters (brine) leads to an increase in brine density proportional to the amount of dissolved CO2. This can lead to gravitational instabilities and the formation of convection currents. Convection currents, in turn, will increase CO2 dissolution rates by removing CO2 saturated brine from the CO2-brine interface. The dissolution and subsequent convection of CO2 which has leaked through a fracture is investigated using dynamic simulations. The instigation of convection currents due to flow through a fracture increases dissolution rates. Comparison of our results with fracture flow rates shows that for typical fracture apertures dissolution from a fracture is small relative to the amount of CO2 flowing through the fracture.

Two phase flow effects and the currents caused by an advancing plume of injected CO2 can affect patterns of CO2 dissolution and convection within a reservoir. Most existing models of CO2 dissolution and convection use a static boundary layer or do not involve two phase flow effects. A radial, two phase, two component model has been built to model the injection process along with convection enhanced dissolution. The model performs well compared to analytical solutions in terms of the large scale processes of CO2 migration and pressure buildup but modelled convection is highly dependent on grid resolution. Numerical instabilities are also present. Further work is needed to increase the accuracy of the model in order to allow higher resolution modelling to be carried out and modelling of the smaller scale processes to be improved.