Experimental study of CO2–brine–rock interaction during CO2 sequestration in deep coal seams

• A series of physical and chemical process can occur in CO2 coal seam storage.
• Cap rock is an active participant in the reaction process in the coal seam storage of CO2.
• Chemical composition of the fluid and mineral alteration reflects the process of CO2–brine–rock interaction.
• Mineral sequestration of CO2 increased the security of CO2 geological sequestration.

CO2 sequestration in deep coal seams is a potential option for reducing greenhouse gas emissions. Once CO2 is injected into coal seams, sealing capability of the cap rock is critical. To investigate and quantify reactions over time between CO2, cap rocks and brine, associated with selected cap rocks of the No. 3 coalbed of the Qinshui Basin in China, batch experiments were conducted for reacting powdered rock samples (180–220 μm) with CO2 and brine, as well as CO2-free brine, at 160 °C and 15 MPa. The analysis of leachate chemistry indicated significant mobilization of major elements from dissolution of carbonate and silicate minerals in the coal measure strata. Analysis of reacted solids by XRD and SEM also revealed appreciable changes in mineralogical compositions. For lithic sandstone after reaction with CO2–brine, the contents of quartz, plagioclase, illite and chlorite increased considerably, whereas the contents of illite/smectite, biotite and kaolinite decreased more or less. The calcareous mudstone reacting with CO2–brine and CO2-free brine all showed major mineralogical alteration after 12 days of treatment. The modeling results identified key chemical processes, but they also showed that the models are not capable of covering all possible contingencies. The precipitation of carbonate minerals could also enhance the security of CO2 sequestration in deep coal seams.