MODELING GEOCHEMICAL REACTIONS IN A TYPICAL WILLISTON BASIN RESERVOIR USED FOR CO2 AND SOUR GAS STORAGE

This work reports results which were obtained during a series of laboratory experiments and numerical modeling of geochemical reactions performed by the Plains CO₂ Reduction (PCOR) Partnership. Core samples collected from various formations of the Williston Basin (North Dakota, USA) were exposed for a period of four weeks to pure supercritical carbon dioxide and a mixture of supercritical carbon dioxide (67.3 mole %) and hydrogen sulfide (32.7 mole %) at 2250 psi (155 bar) and 158°F (70°C) in 10 wt. % NaCl synthetic brine conditions. Prior to exposure, XRD and XRF mineralogical analysis demonstrated the presence of ankerite, anorthoclase, anhydrite, calcite, crystabolite, dolomite, halite, hematite, kaolinite, illite, pyrite, and quartz in the Williston Basin samples. After exposure, XRD and QEMSCAN analysis of reaction products was also performed. Some minerals displayed high reactivity with acid gas, including the conversion of anhydrite to gypsum.  Other samples showed high reactivity while exposed to pure CO₂; for instance, the pyrite was completely dissolved in brine and precipitated as a siderite and hematite later in a course of the experiment; the hematite was dissolved in brine and partially re-precipitated or converted in siderite, and the dolomite partially was converted to calcite and magnesium carbonate. The results of the laboratory experiments were compared with the numerical modeling which was performed with the Geochemist’s Workbench simulator and PHREEQC, where the thermodynamic database was adjusted with SUPRCRT92 code.