Residual and critical saturation in geological storage of CO2 : results from field studies, pore-network modelling and laboratory experiments

Sammanfattning: Geological storage of CO2 in deep saline aquifers is a promising technology in the combat to reduce the atmospheric emissions of CO2. A critical component in this solution is the estimation of aquifer’s in situ capability to store CO2. For this, an in-depth understanding of the underlying processes is required over a wide range of scales, from the pore level where processes occur, to field scale that needs to be controlled and monitored. This Thesis is focused on residual trapping – quantitively characterized by the parameter residual gas saturation (Sgr) – which is one of the key trapping mechanisms. The overall objective is to better understand the relevant in situ phenomena that affect the stability of CO2 residual trapping over a range of scales. Important part of this are the processes controlling the residual gas remobilization that is characterized quantitively by so-called critical gas saturation (Sgc). To this end, first, numerical modelling was implemented at the field-scale to investigate the role of permeability heterogeneity and critical gas saturation in the interpretation of the collected partitioning tracer data from a pilot-scale CO2 injection experiment carried out at Heletz, Israel, 2017. With regards to this experiment, the delayed second arrival peak of the partitioning tracer could not be captured by physical processes included in presently available models, including a stochastic model of within-layer permeability heterogeneity. The results could, however, be explained by accounting for the critical gas saturation that indicates the occurrence of gas-phase remobilization driven by pressure depletion. This is the first ever field observation and demonstration of critical saturation in geological CO2 storage. The relevant fundamental pore-scale characteristics of remobilization are then investigated by means of pore-scale imaging and modeling. The results illustrate that under pressure depletion conditions (which could be caused by e.g., a leaky wellbore or a facture) remobilization of residually trapped CO2 takes place at a higher saturation than residual saturation with the difference depending on various rock and fluid properties. Furthermore, the results provide valuable insights into the pore-scale dynamics of trapped gas remobilization. A very good consistency was found between the pore-scale results and field-scale observations, which provides unique insights into the fate of CO2 residual trapping and remobilization across a wide range of scales.