Experimental Determination of the Effects Carbon Sequestration into Dolomite and Marl Beds of the Jefferson Formation, Montana

The Jefferson Formation is a Devonian carbonate that is currently being targeted for a pilot scale CO₂ injection project in Toole County Montana. This experiment uses samples from an analogous outcrop to experimentally determine what the consequences of CO₂ injection will be. Samples from two potential reservoir zones were quarried from an outcrop south of the drill site in Sun Canyon, Montana. One sample is from a highly porous dolomite. The second sample is from a marginally porous argillaceous carbonate.

The purpose of these experiments is to determine whether carbonate minerals (in the marl and dolomite) and susceptible silicate minerals (in the marl) undergo in-situ “weathering” reactions when exposed to the high concentrations of carbonic acid generated by the dissolution of supercritical CO₂ in formation water. The reactions of these minerals to carbonic acid has the potential to: 1.) open up pore space through dissolution; 2.) reduce pore space and/or close pore throats by precipitating new minerals, or 3.) cause little change if the reactions take place slowly.

Cores were cut and their physical, chemical, and mineralogical properties analyzed. Mineralogy was determined by thin section analysis, X-ray diffraction, and SEM. Bulk composition was measured by XRF. Experiments exposed the cores to CO₂ saturated brine in a flow-through reactor at conditions similar to those found in the subsurface (100C and 15MPa). Changes to the chemical composition of the brine were determined by withdrawing samples at regular intervals during the experiment and analyzing their contents with ion chromatography, spectrophotometry, and ICP-OES. The physical and mineralogical properties of the cores were analyzed after each experiment and compared to the initial conditions.