CO2 TRAPPING IN RESERVOIRS WITH FLUVIAL SEDIMENTARY ARCHITECTURE

A number of important candidate CO₂ reservoirs exhibit sedimentary architecture reflecting fluvial deposition. Recent studies have led to new conceptual and quantitative models for sedimentary architecture in fluvial deposits over a range of scales that are relevant to CO₂ injection and storage. We used a geocellular modelling approach to represent this multi-scaled and hierarchical sedimentary architecture. With this model, we investigated the dynamics of CO₂ plumes, during and after injection, in such reservoirs.

Capillary trapping and dissolution are primary short term processes for CO₂ immobilization in deep brine reservoirs. There are two main residual capillary trapping mechanisms: 1) CO₂ bubbles are trapped within pore spaces because of “snap off”, a process in which counter-imbibition of brine behind the advancing plume traps residual CO₂ bubbles within the intervening pore bodies; 2) trapping due to heterogeneity in the capillary entry pressure among reservoir rock types, i.e. CO₂ is pinned below local contacts between an underlying reservoir rock type with larger pores and an overlying reservoir rock type with smaller pores and thus larger entry pressure. It will be shown that the later mechanism may dominate in fluvial type reservoirs which include small-scale (cm to meter) heterogeneity in permeability and capillary pressure.

CO₂ trapping and dissolution are profoundly impacted by the type of capillary pressure curve used, i.e. Brooks-Corey or van Genuchten . This difference is especially pronounced in heterogeneous reservoirs. It will be demonstrated that CO₂ plume shape and position (simulated with Brooks-Corey or van Genuchten curves) may be significantly different a long time after CO₂ injection ceases.

The detailed petrophysical and geological parameters of any specific reservoir are typically uncertain, which motivates studies of parameter sensitivity. Sensitivity analysis on the variability of basic parameters, such as contrast in permeability, irreducible water saturation, trapping saturation, and capillary entry pressure, will be presented.