Improvement of the Calculation Accuracy of Acid Gas Solubility in Deep Reservoir Brines:Application to the Geological Storage of CO_2

摘要

The assessment of the short and long term consequences of CO_2 injection in aquifers requires both laboratory experiments and numerical modelling in order to better understand the various physical-chemical processes taking place.Modelling injection in a reservoir,where relatively high temperature (above 50°C),high pressure (several hundreds of bars),and high salinity (greater than that of seawater) conditions are likely to be encountered,thus requires numerical tools able to take into account the specific effects of the various electrolytes dissolved in brines,and the non-ideal behaviour of the CO_2 gaseous phase.This study evaluates the consistency of the various corrections (activity,fugacity,influence of pressure on thermodynamic constants) to be taken into account in geochemical models to meet these calculation accuracy requirements.These corrections were implemented in the thermo-kinetic modelling software SCALE2000 (Azaroual et al.,2004a) which was used to check their validity by comparing the calculation results with available experimental observations and other results from CO_2 solubility calculation models.An estimation of the relative weight of each of the corrections for a 237 g-l~! brine (60 deg C,pCO_2 = 200 bar) showed a systematic over-estimation (higher than 100%) of CO_2 solubility when either salinity (Nad equivalent) is neglected or gas is considered ideal.The error induced by the NaCl-equivalent approximation compared to real brine is lower (less than 5%).