Natural fractures in mudrocks and top seal integrity : insights from diagenesis, rock mechanics, and modeling applied to CO₂ sequestration and hydrocarbon exploration

The viability of carbon sequestration for climate change mitigation depends on both the short and long-term security of injected CO₂, which may be impacted by the coupled chemical and mechanical properties of reservoir and seal rocks. Analogs such as the Crystal Geyser/Little Grand Wash fault field site near Green River, Utah allow investigation on longer time scales than laboratory or numerical experiments and was studied to assess the potential for leakage via fracturing or capillary failure of reservoir and seal rocks altered by natural, long-term CO₂-water-rock interactions. Fracture mechanics testing using the double torsion method was first performed on a suite of naturally altered and unaltered rocks exposed at Crystal Geyser. CO₂-related alteration measurably changed fracture toughness (K [subscript IC]) and subcritical index (SCI). A schematic model based on measured K [subscript IC] and SCI values and their predicted influence on fracture pattern development, and their chemical and spatial context relative to the main fault, was developed that qualitatively matches three distinct fracture network patterns observed. Fracture toughness and subcritical index (SCI) are also sensitive to chemical environment and temperature, for example, decreasing by up to 60% and 90%, respectively, in five different sandstone samples immersed in water versus ambient conditions. Sensitivity is controlled by rock composition, grains, cements, and fabric. Aztec Sandstone, a silica-cemented subarkose is relatively insensitive to pH and salinity compared to other sandstones such as the chlorite-cemented Tuscaloosa Sandstone, a CO₂ sequestration reservoir. In general, inert grains and cements such as quartz were less sensitive to the changing chemistry than carbonates and clays. The potential for capillary failure or enhancement of top seals over long (> 10³ years) scales was also studied by mercury intrusion capillary pressure (MICP) analyses on altered shale samples from Crystal Geyser Relatively low capillary seal capacity was measured < 5 m from the fault where CO₂-alteration is most intense but then increases by over an order of magnitude before gradually declining to background levels > 100 m from the fault. Systematic variations in the petrophysical properties are largely explained by changes in pore networks due to matrix replacement with calcite observed by SEM imaging.