Measurement of Diffusivity of Supercritical CO2 in Formation Brines at Reservoir Conditions for CCS Applications

Abstract CO2 diffusivity in brines plays an important role in CO2 migration/distribution in CCS processes. Pressure-decay tests are one of the most common methods to determine gas diffusion in liquids. However most of the existing studies are limited low pressure and temperature (PT) conditions. A few high PT studies with NaCl brines exist in the literature. Main objective of this work is to provide novel experimental data with supercritical CO2 and with extended range of salinities at reservoir conditions (i.e., high pressure and temperature). In this article, we extend the methodologies used in previous studies (Ratnakar and Dindoruk, 2015-SPEJ-170931-PA and 2019-SPE-181514-PA) for CO2-brine systems where we perform diffusivity experiments for CO2-brine systems containing only NaCl. The inversion technique is based on the late transient model technique developed in refs. 34 and 35while the effect of reactive nature of CO2 (forming carbonic acids during dissolution) is also discussed. In particular, The results are presented at 60°C and 1400 psi with the salinity range of 0 – 200,000ppm. The results show that the diffusivity values are aligned with the trends reported in literature. Most importantly, the work and methodology can be extended to any other reservoir conditions and formation brines. The main novelty of this work lies in the presentation of new experimental data for diffusivity of supercritical CO2 in brine at extended range of salinities and reservoir PT conditions. Additionally, the effect of reactive nature of CO2 on the inversion of the pressure-decay models are included in contrast to the existing work where such effects are not considered.