Study of N2 as a Mean to Improve CO2 Storage Safety

Abstract

Current CO2 storage operations in aquifer reservoirs are naturally limited, among other parameters, by entry pressures encountered in cap-rocks, thus limiting over-pressures allowed during the storage process. The injection of nitrogen in a zone just below the cap-rock, prior CO2 injection, could be a viable protective measure to increase the storage safety by lowering the leakage risk and increasing the maximum allowable reservoir pressure.The physical background of the beneficial impact of Nitrogen on the caprock entry pressure is based on the higher N2–brine interfacial tension (IFT) compared to CO2–brine. As a maximum possible effect (for pure N2-brine systems), IFT could increase by a factor of two, yielding correspondingly to the same increase of allowable pressure. However, the N2 injection decreases the storage volume and the trade-off must be studied carefully. The IFT spread decreases rapidly with the mixing ratio of CO2 in the N2. Mixing can occur due to advective processed induced by differential absolute pressure due to CO2 injection and also due to vertical mixing due to different partial pressure. Injection placement is carefully studied carefully, especially the vertical conformance as well as saturation rarefaction.In order to study the feasibility of such approach for different storage conditions, a series of CO2 injection simulations were performed within a generic characterization framework based on dimensionless numbers. A database of dimensionless numbers governing the storage was build, using literature information. The application of an experimental design based on the minimum/maximum values found within the data-base identified a series of cases to be simulated, further reduced by a fractional approach of such design. The scenarios simulated consisted in a CO2 injection within a reservoir storage zone found at some distance from the N2 zone, just below the cap-rock, followed by a resting period during which the CO2 saturation is monitored. The N2 zone is refined, rendering possible the study of the the potential mixing and contamination with CO2. For all case members of the data base, the CO2 conformance was studied in terms of possible mixing and override of the CO2 plume. Discussion of the potential benefits and possible difficulties are addressed. Conclusions were drawn considering the possible field application of such a technique, identifying a-priori sites more suitable for such a technique, based on the dimensionless numbers characterizing them.