A new experimental procedure for simulation of H2S + CO2 geological storage. Application to well cement aging

Abstract

We propose a new experimental procedure to study reactivity of H2S+CO2-brine-solid systems under high pressure and temperature conditions (up to 990 bar and 450°C). A gas loading device has been made in order to fill accurately microreactors with absolute quantity of gaseous reactants. High gas concentrations can also be reached securely. The technique of synthetic fluid inclusions has been adapted to this procedure. This technique provides an in situ sampling of fluids during experiments. After experiments, mass balance of gaseous species can be calculated and thus the molar consumption /genesis percentage. Results of well cement aging in the presence of brine (experiment 1) and in the presence of brine plus H2S + CO2 (experiment 2) are presented as an example of application of this procedure for the simulation of gas storage. The conditions of the two experiments were 500 bar, 200°C, 15 days. In brine environment (experiment 1), the cement is converted from an assemblage of tobermorite (CHS) to an asemblage of xonotlite (CHS). This transformation occurred in a pH range of 10.3-11.2. The presence of H2S-CO2 mixture (experiment 2) induces pH (from 10.3-11.2 to 8-9) and Eh (from oxidising conditions to reductive conditions) decreases. These conditions lead to the partial cement carbonation and to the sulphidation of iron bearing phases (steel and C4AF). In this case, the cement was transformed to an asemblage of slight Ca-depleted tobermorite plus calcite. The procedure allowed also to determine an H2S and CO2 molar consumption respectively of 6 and 16%. The H2S consumption is essentially related to the sulphidation of steel. The CO2 consumption predicts a reacted cement composed of 5 volumes of tobermorite for 1 volume of calcite. This estimation is coherent with the scanning electron microscope observations. Copyright © 2005, Institut français du pétrole.