Impact of Temperature and Pressure on CO2 Mineral Trapping
in Arkosic Reservoir with CaCl2 – type Formation Water

Abstract

The feasibility of CO2 geological storage in arkosic sandstones has been investigated by short-term physical simulation experiments using six sandstones samples from the Eocene sandstone reservoir in the Bonan Subsag, Bohai Bay Basin, East China, under a range of temperature and pressure conditions. The aqueous solution used in the experiment mimics the elemental compositions of the present oilfield formation water. The reservoir sandstone samples were ground to powders prior to the experiment. Amount of 10mL aqueous solution and 2g sandstone powders were fully mixed and assembled in a high pressure, high temperature (HPHT) batch reactor. The experimental temperatures and corresponding pressures were set to 50C/4 MPa, 100C/6 MPa and 140C/9 MPa, respectively. All experiments were carried out for 96 hours. Scanning Electron Microscopy (SEM) and energy dispersive spectroscopy (EDS) analysis of the samples after experiments shows calcite precipitated whilst feldspar partly dissolved. Feldspar dissolution is likely to promote the calcite precipitation by continuously consuming acid in the aqueous solution. To rationalize the mechanism of mineral changes in the experiments, a three-stage chemical reaction process of CO2 geological storage is proposed. In StageⅠ, CO2 dissolved and saturated in saline aqueous solution due to high pCO2, producing CO2 (aq), H+, HCO3 – and CO3 2− for the subsequent reactions. In StageⅡthe original calcite cement was dissolved rapidly due to a decreasing pH, while the dissolution rate of ankerite and feldspar remained relatively slow. As acid was consumed in this stage and pH gradually became conducive to calcite reprecipitation. In Stage Ⅲ, feldspar continued to be dissolved while calcite and kaolinite precipitated from aqueous solution. Results of Thermal Gravimetric Analysis (TGA) and Scanning Electron Microscopy (SEM) of the experiments indicates that the sample YX981 obtained the greatest amount of carbonates precipitation, which was identified that increased at high temperatures and high content of original silicates minerals, while pressure has no obvious effect on CO2 mineral trapping. In summary, Es32 member layer where sample YX981 was collected, was evaluated as a favorable CO2 mineral trapping site in Bonan Subsag.