Geochemical and geomechanical alteration of sandstone reservoir rock and shale caprock under representative geological carbon sequestration conditions

Geologic Carbon Sequestration is a method of mitigating anthropogenic climate change where CO₂ is captured from point-source emissions and transported to a location where it is injected into a rock formation in the subsurface for permanent storage. The dissolution of CO₂ into reservoir brine forms carbonic acid which can react with pH-sensitive minerals, affecting the geomechanical properties of the formation. This dissertation explores the impact of geochemical reactions of low-pH brine on the geomechanical properties of sandstone reservoir rock and shale caprock. In the first study, Mt. Simon Sandstone is exposed to either N₂-saturated or CO₂-saturated brine at reservoir pressure and temperature in a batch system. The next two studies focus on acidified brine flow through first, sandstone fractures and second, shale fractures while under stress in a triaxial core holder. In all cases, the low pH of the brine was found to dissolve reactive cementing materials, resulting in a decrease of fracture toughness and increase in porosity. The batch studies also found increased fracturing in CO₂-saturated brine samples. The flow studies of fractured sandstone found increased slip occurred with exposure to acidified brine due to loss of shear strength in the fracture asperities as dolomite cements were dissolved. This study employed a novel procedure of examining shear slip following a single applied stress, which had not been seen previously in fracture literature. Finally, the shale fracture studies found surficial reaction of calcite along flow paths resulting in increased surface roughness and decreased fracture toughness. The general loss of integrity of clay- and carbonate-cemented sandstones and shale when exposed to acidic conditions like that of GSCO2 implies that these materials will have decreased shear strength and result in motion along newly created or existing fractures. These geochemical reactions must be taken into account when evaluating site suitability for sequestration, not only potential microseismic activity resulting from hydrodynamic effects.