In this UKCCSRC funded call project, petrophysicists at Imperial College London partnered with reservoir modelers at the British Geological Survey to provide a multiscale evaluation of CO2 flow, trapping, and storage capacity for important storage reservoirs and regions in the UK.
Rock core samples were collected from major CO2 storage regions in the Southern North Sea, the Northern North Sea, and the East Irish Sea (Figure 1). These were used to produce some of the first reservoir condition observations of relative permeability and residual trapping for these reservoirs. Numerical reservoir models were also updated and constructed for the same regions. The newly collected data on flow properties and trapping was then used in simulations evaluating the impacts of flow properties and residual trapping on CO2 storage capacity.
Taking samples from potential CO2 storage reservoirs and analyzing them in the laboratory is a field of engineering known as “petrophysics”. In this project we took a novel approach, analyzing the impact of rock texture on fluid flow and trapping at multiple scales. There were many challenges that arose over the course of the work – firstly, appropriate rock samples had to be located and collected from the British Geological Core Store (Figure 2). The core store is an invaluable resource for academic research in petrophysics as the costs of obtaining rock samples from reservoirs deep below the North Sea would be well in excess of funding available for typical research projects. As such, the rock samples must be selected from the store with great care so that the measurements provide the most meaningful information with the fewest number of samples.
In the laboratory we injected CO2 and brine through the rock samples at high pressures and moderate temperatures representative of the reservoirs from which the rock samples were taken. This requires a large suite of specialized pumps and tubing that can handle these conditions, as well as the corrosiveness of CO2-brine mixtures. These experiments were performed in the CO2 storage core analysis laboratory at Imperial College London. Additionally, we wanted to understand how rock heterogeneity impacts fluid flow and residual trapping in the reservoir. This approach comes with significant difficulty over conventional approaches because many experiments must be performed on each rock. Sometimes as many as six relative permeability experiments were performed on a single rock, in which fluid flow rates were varied so as to understand how fluids flow differently under different conditions. Ultimately, we found that the small layering present in all of the samples we collected has a very important impact on flow and trapping (Figure 3). This validated all of the time and effort taken to understand these effects. It has resulted in the most comprehensive dataset available for multiphase flow properties for CO2 storage in the United Kingdom.
Field scale reservoir simulation and dynamic capacity estimation
The next step in the project was to make use of all of the detailed petrophysical data obtained from the laboratory for modeling CO2 injection into the reservoirs. We were able to make use of already existing models of the Bunter Sandstone that had been previously constructed by the British Geological Survey. We built on this to add more detail and to incorporate the new information about fluid flow and trapping (Figure 4). Additionally, we were able to construct a new model of the South Morecambe field in the East Irish Sea, another important potential target for CO2 storage in the UK.
The results showed that at large spatial scales, and over long injection periods, the ultimate storage capacities of the regions were relatively robust against variations the relative permeability or capillary trapping curves. However, at the scale of an individual field (Figure 5) and over time scales of interest to an individual project, the flow properties have a significant impact on how quickly CO2 can be injected into a formation, and how far the CO2 plumes will move.
Future work involves making more detailed measurements of the reservoir systems and overburden layers, and using upscaling techniques to fully incorporate the measurements into reservoir scale modelling.