Petrophysical quantification of Utah reservoir sandstones and cap-rocks naturally exposed to CO2 fluid fluxes by use of high-resolution Micro CT

Abstract

CO2 emission is increasing globally, resulting in a global increase in temperature and as a result climate change. CO2 storage in the subsurface is an option/ transitional measure to reduce CO2 in the atmosphere and mitigate the climate change issue. Generally, for CO2 storage to be a viable option, it must be ensured that the stored gasses should be at least 10.000 years safe in the subsurface. Simulating long-term storage of CO2 in the laboratory for 10.000 years in storage experiments is not feasible and therefore a good option is to analyze rock samples that have naturally been exposed to CO2 for such a large time frame. Such a CO2 aquifer can be found in eolian sandstones in Utah, USA. Due to the occurrence of fractures in these sandstones, CO2 bearing water has flown through these sandstones resulting in possible reaction and carbonate precipitation in the pores. In this study, the main points of interest are the determination of volume fractions of various components like porosity, carbonate precipitation close to and far from the fractures, by the use of Micro Computed Tomography (Micro CT). Micro CT results are compared to conventional lab (macroscopic) technology. The ultimate goals of this research are gaining actual insight/ data on what the result is if CO2 storage is conducted and insight and recommendation regarding the use of Micro CT technology. The first conclusion on results regards calibration. Calibration difficulties in Micro CT technique generate noise. If calculating the fraction of components, noise has no influence on the fraction result. If connectivity analysis is conducted, noise will influence the result and should therefore be filtered. Sensitivity analysis for resolution shows that 2.5 ?m is a suitable resolution to image these eolian sandstones and the smaller the samples are the higher resolution can be acquired. Lab based methods for porosity (Pycnometer, Wet Test and Hg Shell Test) generally agree with each other. The image based methods (2D and 3D) also agree with each other. The lab based methods and image based methods do not exactly agree with each other. The correlations regarding permeability, porosity and particles per volume show exponential relations. After conducting 3D grain analysis, the effect of carbonate precipitation is quantified. Because of the precipitation between grains, the grains tend to ‘deform’, which creates higher surface area and volume. The exact influence of a fracture on carbonate precipitation can be summarized to be minimal. In the case of sample car614 the influence is clearly seen, but in all other samples the fracture seems to have had no influence or overruled by other (depositional) phenomena.