GEOCHEMICAL ALTERATION OF PORE STRUCTURE IN MT. SIMON SANDSTONE RESULTING FROM CONTINUOUS INJECTION OF CO2-SATURATED BRINE

The repository rock for CO₂ injection at the Illinois Basin-Decatur project site is the lower section of the Mt. Simon sandstone formation. The reservoir is composed of 76.5 wt.% quartz, 2.1 wt.% calcite, 17.3 wt.% K-feldspar, 1.1 wt.% chlorite, 0.7 wt.% illite and lesser extents of siderite, kaolinite, dolomite and marcasite. Assuming this reservoir composition is equilibrated with the aquifer brine, contact with CO₂-acidified solution should induce rapid water-rock interactions and influence the pore structure and transport properties of the system. The extent to which this couple fluid flow and chemical reactivity modifies the original porosity, permeability and degree of heterogeneity of this reservoir is unknow. In the present study, we quantify primary mineral dissolution rates characterize the secondary mineral formation, calculate the change in solid phase volumes associated with these reactions and constrain the associated change in porosity and pore structure that result in evolving physical structure of the system.

Characterization was accomplished through a series of laboratory experiments under controlled P_Total, pCO₂ and T (100 bar, 86.2 bar and 53 °C). Over 40 pore volumes were flushed through the sample at flow rates ranging from 0.08 to 5.00 mL min^-1. The acidic CO₂ rich brine solution (pH = 3.4) promoted the dissolution of K-feldspar, chloride, illite, pyrite and calcite, and the precipitation of Ca and Si -bearing phases, resulting in a net porosity increase. Pre- and post-reacted solid sample characterization was performed using X-ray computed tomography (X-ray CT), X-ray powder diffraction (XRD) and scanning electron microscopy (SEM) and coupled with time series solute chemistry to quantify the principle geochemical reactions responsible for alteration of physical structure. Imaging of the unreacted sample indicated a heterogeneous structure, including compacted layers and a wide grain size distribution. Following the flow-through experiment, SEM images of the reacted cores showed evidence of the Ca-, Fe- and Si-bearing mineral precipitation.