THE EFFECTS OF SUPERCRITICAL CO2-WATER-ROCK INTERACTIONS ON POROSITY AND PERMEABILITY IN THE MINNELUSA FORMATION CARBONATES OF THE POWDER RIVER BASIN, WY

The interbedded sandstones and dolostones of the Minnelusa Formation in the Powder River Basin (PRB) are the primary target for CO₂ storage within the DOE-funded Wyoming CarbonSAFE Carbon Capture & Storage (CCS) Project. Despite initial geologic investigations, the carbonate units of the Minnelusa Formation lack comprehensive characterization. This project aims to quantify the potential effects of supercritical CO₂ (scCO₂)-water-rock interactions between different pore types in Minnelusa Formation carbonates. Understanding the effects of CO₂-water-rock reactions on porosity and permeability in carbonates will allow for more realistic estimations of CO₂-migration potential and storage capacity in reservoirs with varying lithologies.

Preliminary data from bulk geochemical analysis, scanning electron microscopy (SEM), optical microscopy (OM), and x-ray diffraction (XRD) confirm that the Minnelusa carbonates are intergranular dolostone composed of primary dolomite, quartz, and anhydrite with trace amounts of K-feldspar and Fe-bearing minerals. SEM and OM analyses revealed that two major pore types dominate the rock: (1) small (1 μm – 50 μm), intergranular pores and (2) large (50 μm - >250 μm), and/or vuggy pores. A synthetic Minnelusa formation water (NaCl water type, pH = 6.1, ionic strength = 0.9 mol/kg) was modeled based on available water and lithologic data from the Wyoming CarbonSAFE Phase II findings and the USGS Produced Water Database.

Three batch hydrothermal experiments will be conducted over ~1600 hours at subsurface temperature (90 °C) and pressure (270 bar). The experiments consist of 1 control, 1 closed system with water-rock-scCO₂, and 1 open system with CO₂-saturated water-rock. A time series water chemistry will be collected over the duration of the experiments to investigate the effects of scCO₂ on pH, redox conditions, and major and trace ion concentrations. Pre- and post-reaction nuclear magnetic resonance (NMR), BET, SEM, and post-reaction mercury injection capillary pressure (MICP) analyses of the rocks will provide insights into the mechanisms and degrees of change in pore size, pore-size distribution, and dissolution and/or precipitation reactions within pores. This study will provide results from the aforementioned methodology.