Experimental investigation of geochemical and mineralogical effects of CO(2) sequestration on flow characteristics of reservoir rock in deep saline aquifers

Interactions between injected CO(2), brine, and rock during CO(2) sequestration in deep saline aquifers alter their natural hydro-mechanical properties, affecting the safety, and efficiency of the sequestration process. This study aims to identify such interaction-induced mineralogical changes in aquifers, and in particular their impact on the reservoir rock’s flow characteristics. Sandstone samples were first exposed for 1.5 years to a mixture of brine and super-critical CO(2) (scCO(2)), then tested to determine their altered geochemical and mineralogical properties. Changes caused uniquely by CO(2) were identified by comparison with samples exposed over a similar period to either plain brine or brine saturated with N(2). The results show that long-term reaction with CO(2) causes a significant pH drop in the saline pore fluid, clearly due to carbonic acid (as dissolved CO(2)) in the brine. Free H(+) ions released into the pore fluid alter the mineralogical structure of the rock formation, through the dissolution of minerals such as calcite, siderite, barite, and quartz. Long-term CO(2) injection also creates a significant CO(2) drying-out effect and crystals of salt (NaCl) precipitate in the system, further changing the pore structure. Such mineralogical alterations significantly affect the saline aquifer’s permeability, with important practical consequences for the sequestration process.