Capillary Trapping of CO 2 in Sandstone Using Low Field NMR Relaxometry

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.