RESERVOIR FLUID CHARACTERIZATION: KEY TO EFFECTIVE CCUS RESERVOIR ASSESSMENT

Fluid characterization is a vital component of reservoir characterization projects. In this study we present the geochemical and isotopic results of reservoir fluids and dissolved gases from the Mississippian Madison Limestone and Pennsylvanian Weber Sandstone from a potential CCUS target on the Rock Springs Uplift in southwestern Wyoming. Results are used to evaluate brine evolution, predict geochemical fluid reactions in response to CO₂ injection and well completion practices, and investigate stacked reservoir confinement.

The brines are Na-Cl type with total dissolved solid concentrations in excess of 85,000 mg/L. Conservative analytes indicate that the evolution of the brines in both formations have been heavily influenced by evaporite dissolution, increasing the molar ratio of Br-Na-Cl. Dolomitization at depth in each reservoir results in magnesium depletion. Comparative analysis suggests that dissolution of evaporite and other minerals has had a large influence on the evolution of the formation fluids. This has resulted in increased TDS post-burial, resulting in some of the most saline formation fluids collected in Wyoming.

Simulations of CO₂ injection into the formation brines suggest a decrease in pH, resulting in CaCO₃ dissolution and CaSO₄ precipitation. These reactions could cause an increase in porosity of 1% to 3%, and increase the viability of storage. We also note significant changes to molar ratios of Br-Na-Cl, and the concentrations of HCO₃, SO₄ and volatile organic compounds before and after work over in the well-bore. The differences of solute concentrations between samples sets could be attributable to different collection methods, but are more likely recording changes to the formation fluid from work-over practices in the well-bore.

The isotopic compositions of fluids and dissolved gases were found to be unique to each formation. Rare earth element concentrations further establish distinctive fluid concentrations. Though these fluids share a similar evolution, we suggest that dissimilarities in the isotopic compositions of the brines, dissolved gases and rare earth element concentrations indicate that the target formation fluids are isolated from each other.