Capillary Trapping of CO₂ in Sandstone Using Low Field NMR Relaxometry

Abstract

Injecting carbon dioxide into geological formations for long-term storage is considered integral to reducing greenhouse gas emissions. Residual trapping of CO 2 is a primary storage mechanism, whereby CO 2 ganglia are trapped in the pore space by capillary forces. Experimental knowledge of residual trapping processes in rocks is critical to the development of safe storage strategies. Here we present a quantitative low field 1 H nuclear magnetic resonance (NMR) core flooding study of CO 2 residual trapping in three different sandstones. It was found that transverse relaxation (T 2 ) measurements were sensitive to the dissolution of paramagnetic ions from rock matrix minerals after exposure to carbonic acid; this response was observed on time scales relevant to core flooding experiments (i.e., minutes to hours). Subsequently, a brine aging protocol was designed and implemented to minimize this chemical effect, and hence, by applying the well-known T 2 -pore size relationship, changes in T 2 time distributions during core flooding could be related to displacement of brine from pores of different sizes. Comparison of T 2 distributions for partially CO 2 /brine-saturated cores to previously published data for N 2 /H 2 O systems shows an increased displacement of brine from small pores by CO 2 . Furthermore, results from cyclical brine/CO 2 injections showed an increase in the total volume of residually trapped CO 2 and an increase in trapping efficiency; compared to the results observed for N 2 /H 2 O, however, the improvement in trapping efficiency with cyclic injection was less pronounced. Potential causes for the observed differences are discussed in the context of effective N 2 and CO 2 wetting. Key Points:• NMR T 2 relaxation enhancement from paramagnetic ion dissolution from rock cores exposed to carbonic acid was measured • Cyclical brine/CO 2 injection increases trapped CO 2 volumes and trapping efficiency • Differences in the drainage of different pore size for CO 2 /brine and N 2 /water flooding suggest differences in wettability

Miscible Fluid Displacement in Rock Cores Evaluated with NMRT2Relaxation Time Measurements

“…This intriguing correlation will be explored further via a wider suite of fluids, process conditions, and porous media types. Future work will also focus on further assessment of the correlation of the values of T 2 to an approximate quantitative pore size – specifically we will further define the relaxation regime occupied by these high pressure liquid and supercritical fluids; ,, preliminary analysis suggests they are consistently just within the short-time regime where eqs and are broadly valid.…”

Quantitative measurement of Mono-Ethylene Glycol (MEG) content using low-field Nuclear Magnetic Resonance (NMR)

“…The lack of superconducting components, for instance, mitigates the need for cryogenic cooling, and so removes the economic cost and maintenance complexity associated with the constant resupply of cryogenic liquids. The associated reduction in size also significantly increases equipment portability (Johns et al, 2015), which when combined with the reduction in safety considerations which accompany both the reduced magnetic field strengths inherent to permanent magnets and the lack of cryogenic liquids employed, means that benchtop NMR systems may be more readily integrated into simple workbench environments (Sans et al, 2015;Wagner et al, 2016), and/or combined with external equipment (Connolly et al, 2019;Leutzsch et al, 2018;Robinson et al, 2020;Yang et al, 2022). Benchtop spectrometers are also largely maintenance free and can be operated under near push-button conditions, enhancing their suitability for environments without significant NMR expertise, and for high throughput screening applications (Blümich, 2016).…”

Integration of an optical FBG sensor into a nuclear magnetic resonance core flooding system

“…For instance, the laboratory NMR system is uniquely sensitive to capillary trapping parameters in rock cores, i.e. pore size, relative permeability, wettability and fluid-rock interactions [20]. The obtained knowledge from these laboratory tests is critical to the understanding of many geoscience scenarios, including hydrocarbon recovery, underground energy storage, such as hydrogen, and carbon sequestration.…”